JP3922998B2 - Magnetic memory - Google Patents

Description

ここで、Ｋ_ｕは異方性エネルギー、θは強磁性薄膜の飽和磁化Ｍ_ｓと磁化容易軸とのなす角度、αは印加磁場Ｈと磁化容易軸のなす角度である。実現する磁化方向は、自由エネルギーＥを最小にするように決定される。その結果、アステロイド曲線により磁化方向の変化が表される。さらに、磁化ベクトルに作用するトルクが−∂Ｅ／∂θで決められる。シミュレーションによって計算したトルクの特性グラフを図６に示す。図６において、横軸は角度αを表し、縦軸は最大トルクによって正規化されたトルクを表している。図６に示すトルク曲線は、印加磁場Ｈを異方性磁場Ｈ_ｋ（＝２Ｋ_ｕ／Ｍ_ｓ）で正規化した磁場ｈ（＝Ｈ／Ｈ_ｋ）の大きさをパラメータとして表したものである。パラメータｈが十分大きいときは異方性エネルギーが小さい場合に対応し、無限大では異方性がない。パラメータｈが無限大の場合のトルク曲線は正弦関数で表され、印加磁場の磁化容易軸とのなす角度αが４５度と１３５度のときに極値を取ることが図６からわかる。すなわち、この場合に磁化ベクトルは最大のトルクを受けているため、磁化の反転に最も効果的である。パラメータｈが小さくなると、最大トルクを与える印加磁場の磁化容易軸とのなす角度は、４５度より大きい角度と１３５度より小さい角度でそれぞれ極値を取る。パラメータｈが更に小さくなってｈが０．５より小さくなると、最大トルクを与える印加磁場の磁化容易軸とのなす角度は一つになり、例えば、ｈ＝０．３のときに１２０度近傍に現れることが分かる。
【００３６】
次に、本実施形態による磁気メモリの記憶層３ａの磁化反転方法を、図１（ａ）、（ｂ）を参照して説明する。
【００３７】
図１（ａ）は、本実施形態に用いられる磁気抵抗効果素子の記憶層である強磁性層３ａの上面図である。図２で説明したように、記憶層３ａの下側に書き込みワード線ＷＬが配置され、上側に書き込みビット線ＢＬが配置された構成となっている。そして、書き込みワード線ＷＬは記憶層３ａの磁化容易軸方向に配置され、書き込みビット線ＢＬは磁化困難軸方向に配置されている。
【００３８】
記憶層３ａ初期状態では、記憶層３ａの磁化は図１（ａ）に示すように、磁化容易軸方向であってかつ右を向いているとする。本実施形態においては、この記憶層３ａの磁化方向を反転させる印加磁場として、まず、記憶層３ａの上記磁化方向を基準にして第１角度θ_１を有する第１磁場Ｈ_ｅｘｔ１を作用させて記憶層３ａの磁化を第１磁場Ｈ_ｅｘｔ１の方向に回転させ、その後に、記憶層３ａの最初の磁化方向（図１（ａ）に示す右方向）を基準にして第２角度θ_２を有する第２磁場Ｈ_ｅｘｔ２を作用させ記憶層３ａの磁化を第２磁場Ｈ_ｅｘｔ２の方向に回転させ、その後、第２磁場Ｈ_ｅｘｔ２を零にすることにより、記憶層３ａの磁化を反転させるものである。
【００３９】
このとき、第１角度θ_１としては、図６のトルク曲線から分かるように、０度より大きく９０度よりも小さい角度であれば、負のトルク、すなわち図１（ａ）に示す記憶層３ａの磁化が第１磁場Ｈ_ｅｘｔ１に向かって反時計方向に回転するトルクが発生し、９０度よりも大きく１８０度よりも小さい角度で有れば、正のトルク、すなわち記憶層３ａの磁化が第１磁場Ｈ_ｅｘｔ１に向かって時計方向に回転するトルクが発生する。すなわち、第１角度θ_１が、０度より大きく９０度よりも小さい角度であっても、９０度よりも大きく１８０度よりも小さい角度であっても、記憶層３ａの磁化を回転させるトルクは発生する。
【００４０】
第１角度θ_１が０度より大きく９０度よりも小さい角度の場合は、第１角度θ_１が９０度より大きく１８０度よりも小さい角度の場合に比べて、記憶層３ａの磁化の回転角度が小さいので、記憶層３ａの初期ドメインを大きく変形させることなく磁化回転することが可能となる。しかし、第１角度θ_１が９０度より大きく１８０度よりも小さい角度の場合は、記憶層３ａの磁化の回転角度が大きいため、磁化の回転に伴い、記憶層３ａの中央部分のドメインとエッジドメインの境界領域付近に新たなドメインウォールが生じやすくなり、安定した磁化反転を行うことができない。そして、ドメインウォールが生じて残留すると、磁気抵抗効果素子のＭＲ比が悪化するという問題が発生する。
したがって、本実施形態においては、安定した磁化反転を行うために、第１角度θ_１としては０度より大きく９０度よりも小さい角度が選択される。また、第２磁場Ｈ_ｅｘｔ２を印加するときの記憶層３ａの磁化は、第１磁場Ｈ_ｅｘｔ１と磁気異方性のかねあいで釣り合った位置で安定化しているので、第１磁場Ｈ_ｅｘｔ１と磁気異方性の方向（初期磁化方向）の間にある。このため、第２磁場Ｈ_ｅｘｔ２を印加する場合は、上述したことから、第２角度θ_２と記憶層３ａの磁化方向との差が０度より大きく９０度よりも小さい角度が選択される。例えば、第２角度θ_２としては、９０度よりも大きく１８０度よりも小さい角度が選択される。なお、図６のトルク曲線から分かるように、トルクが最大になるのは記憶層３ａの磁化方向と印加磁場とのなす角度が４５度以上９０度未満の場合であるので、第１角度θ_１としては、４５度以上９０度未満であることが好ましい。
【００４１】
このような第１磁場Ｈ_ｅｘｔ１および第２磁場Ｈ_ｅｘｔ２を発生させるには、ビット線ＢＬおよびワード線ＷＬに図１（ｂ）に示す電流を流せば良い。なお、ビット線ＢＬおよびワード線ＷＬに流れる電流の向きは、図１（ａ）に示す矢印の方向を正とする。図１（ｂ）に示すように、まず、ビット線ＢＬに負の所定電流を流すとともにワード線ＷＬに正の所定電流を流す。すると、上記電流によって発生する電流磁場が合成された上記第１磁場Ｈ_ｅｘｔ１が発生する。その後、ビット線ＢＬの電流の向きを変えて、負の所定電流から正の所定電流に変化させる。すると、ビット線ＢＬおよびワード線ＷＬに流れる上記電流による電流磁場が合成された上記第２磁場Ｈ_ｅｘｔ２が発生する。
【００４２】
図１（ｂ）に示すように、本実施形態においては、ワード線ＷＬに単極パルス、ビット線ＢＬには双極パルスを印加している。この電流パルスパターンにおいて、ワード線とビット線のそれぞれのパルス高さはそれぞれによって発生される磁場が合成されたときの方向を決めており、ワード線ＷＬとビット線ＢＬのパルスのそれぞれの高さをそれぞれＩ_ｗ１、Ｉ_ｂ１とすると、合成磁場の角度θは磁化容易軸からみて関係式
ｔａｎθ＝｜Ｉ_ｗ１｜／｜Ｉ_ｂ１｜
で決められる。これらのパルス高さは同じであっても良く、同じでなくとも良い。
【００４３】
図７は図１（ａ）に示した書き込み電流パターンによる磁化反転の様子を示した特性曲線である。この特性曲線は、ＬＬＧ(Landau-Lifshitz-Gilbert)方程式に基づいて数値計算をしたものである。この計算では、記憶層として、幅が０．４μｍ、長さが０．８μｍ、厚さが３ｎｍのＮｉＦｅ薄膜（Ｍ_ｓ＝８００ｅｍｕ／ｃｍ^２、Ｋ_ｕ＝１．０×１０^４ｅｒｇ／ｃｍ^３）が用いられている。
【００４４】
ここでは、磁化容易軸から４５°方向に初期磁場を３．０ｎｓｅｃの時間だけ印加し、続いて３．０ｎｓｅｃの時間だけ１３５°方向に印加した場合を示しており、印加磁場の大きさが２０、３０、４０Ｏｅの３通りの場合をそれぞれ特性グラフｇ_１、ｇ_２、ｇ_３に示す。この特性グラフから分かるように印加磁場の大きさが２０Ｏｅでは記憶層３ａの磁化が反転せず、印加磁場の大きさが３０Ｏｅ以上では記憶層３ａの磁化が反転している。より詳細に計算すると、磁化反転するのに必要な最小の印加磁場は約２８Ｏｅであった。したがって、本実施形態による磁気メモリの記憶層３ａの磁化を反転させるには、大きさがかなり小さいスイッチング磁場でも良く、このため、少ない電流でも良いことが分かる。
【００４５】
以上説明したように、本実施形態によれば、記憶層３ａの磁化方向を基準にして０度より大きく９０度未満の第１角度を有する第１磁場を作用させ、その後９０度より大きく１８０度以下の第２角度を有する第２磁場を作用させることにより、磁気メモリを高集積化しても安定な磁化反転を引き起こすことができる。また、スイッチング磁場が小さいため、省電力化を達成することができる。
【００４６】
なお、本実施形態においては、メモリセルが磁気抵抗効果素子と選択トランジスタから構成された磁気メモリであったが、図８に示すように、ダイオード付き単純クロスポイント型のメモリセルであっても良い。すなわち、ワード線ＷＬ上にダイオード８と磁気抵抗効果素子３からなるセルを積層して形成し、磁気抵抗効果素子３上にビット線ＢＬを配置して形成した構成としても良い。この場合、ワード線ＷＬおよびビット線ＢＬは、書き込み時ばかりでなく読み出し時にも用いられる。
【００４７】
（第２実施形態）
次に、本発明の第２実施形態による磁気メモリを図９（ａ）、（ｂ）を参照して説明する。本実施形態の磁気メモリは、書き込みビット線ＢＬに流す電流が異なる以外は、第１実施形態の磁気メモリと同じ構成となっている。図９（ａ）に本実施形態において、書き込みビット線ＢＬおよび書き込みワード線ＷＬに流す電流の波形図を示し、このとき発生する電流磁場による合成磁場を図９（ｂ）に示す。ワード線ＷＬに単パルス、ビット線ＢＬには三角双極パルスを印加したものである。この電流パルスのパターンにおいて、ワード線ＷＬとビット線ＢＬのそれぞれのパルス高さが、それぞれによって発生される磁場が合成されたときの方向を決めており、ワード線ＷＬとビット線ＢＬの電流パルスのそれぞれの高さを｜Ｉ_ｗ１｜、｜Ｉ_ｂ１｜とすると，合成磁場の角度は磁化容易軸からみて関係式
ｔａｎθ＝｜Ｉ_ｗ１｜／｜Ｉ_ｂ１｜
で決められる。これらのパルス高さは同じであっても良く、同じでなくとも良い。ただし、｜Ｉ_ｗ１｜は一定であることが特徴である。
【００４８】
本実施形態においては、図９（ａ）に示す電流パルスがビット線ＢＬおよびワード線ＷＬに印加されるので、図９（ｂ）に示すように、まず、記憶層３ａに第１角度θ_１を有する第１磁場Ｈ_ｅｘｔ１を作用させて、記憶層３ａの磁化を第１磁場Ｈ_ｅｘｔ１の方に回転させ、その後、第２角度θ_２を有する第２磁場Ｈ_ｅｘｔ２を作用させて記憶層３ａの磁化を第２磁場Ｈ_ｅｘｔ２の方向に回転させた後、第２磁場Ｈ_ｅｘｔ２を零にするものである。ここで、第１実施形態と同様に、安定した磁化反転を行うために、第１角度θ_１としては０度より大きく９０度よりも小さい角度が選択され、第２角度θ_２としては、９０度よりも大きく１８０度よりも小さい角度が選択される。なお、第１実施形態と同様に、第１角度θ_１としては、４５度以上９０度未満であることが好ましい。
【００４９】
また、ビット線ＢＬが負から正に直線的に遷移するので、第１磁場Ｈ_ｅｘｔ１から第２磁場Ｈ_ｅｘｔ２に変化するときに、ビット線ＢＬに流れる電流による電流磁場が零になる磁化困難軸方向に向く磁場Ｈ_ｅｘｔが現れる。そして、第１磁場Ｈ_ｅｘｔ１から第２磁場Ｈ_ｅｘｔ２に変化するときには、ワード線ＷＬに流れる電流は一定であるので、ワード線ＷＬによる電流磁場は一定となる。このため、磁場Ｈ_ｅｘｔ１、Ｈ_ｅｘｔ２、Ｈ_ｅｘｔの磁化困難軸方向の成分は一定となり、図９に示すように、磁場Ｈ_ｅｘｔ１、Ｈ_ｅｘｔ２、Ｈ_ｅｘｔを示す矢印の先端は、破線で示す直線３０に沿って動くことになる。
【００５０】
以上説明したように、本実施形態によれば、記憶層３ａの磁化方向を基準にして０度より大きく９０度未満の第１角度を有する第１磁場を作用させ、その後９０度より大きく１８０度以下の第２角度を有する第２磁場を作用させることにより、磁気メモリを高集積化しても安定な磁化反転を引き起こすことができる。また、スイッチング磁場が小さいため、省電力化を達成することができる。
【００５１】
（第３実施形態）
次に、本発明の第３実施形態による磁気メモリを図１０を参照して説明する。本実施形態の磁気メモリは、書き込みビット線ＢＬに流す電流が異なる以外は、第１実施形態の磁気メモリと同じ構成となっている。図１０に本実施形態において、書き込みビット線ＢＬおよび書き込みワード線ＷＬに流す電流の波形図を示す。ビット線ＢＬに流れる電流パルスは、その極性を変えるときには１ｎｓｅｃ以上の遷移時間幅をもって値を徐々に変えている。また、図１０の点線で示されているように、ワード線ＷＬの電流パルスが切れた後、しばらくしてビット線ＢＬの電流パルスが切れている。これは、電流パルスにより引き起こされた記憶層３ａの磁化反転を安定的に終了するためである。
【００５２】
なお、本実施形態においても、図１０に示す電流パルスから分かるように、まず、記憶層３ａに第１角度を有する第１磁場を作用させて、記憶層３ａの磁化を第１磁場の方に回転させ、その後、第２角度を有する第２磁場を作用させて記憶層３ａの磁化を第２磁場の方向に回転させた後、第２磁場を零にするものである。ここで、第１実施形態と同様に、安定した磁化反転を行うために、第１角度としては０度より大きく９０度よりも小さい角度が選択され、第２角度としては、９０度よりも大きく１８０度よりも小さい角度が選択される。なお、第１実施形態と同様に、第１角度としては、４５度以上９０度未満であることが好ましい。
【００５３】
以上説明したように、本実施形態によれば、記憶層３ａの磁化方向を基準にして０度より大きく９０度未満の第１角度を有する第１磁場を作用させ、その後９０度より大きく１８０度以下の第２角度を有する第２磁場を作用させることにより、磁気メモリを高集積化しても安定な磁化反転を引き起こすことができる。また、スイッチング磁場が小さいため、省電力化を達成することができる。
【００５４】
なお、ビット線ＢＬに流れる電流については、ビット線ＢＬに電流パルスが流れてから負極性の極値に到達するまでの遷移時間（以下、立ち上がり時間とも云う）ｔ１（図１１（ａ）参照）、負極性の極値から正極性の極値に到達するまでの遷移時間ｔ２（図１１（ｂ）参照）、正極性の極値から零に到達する遷移時間（立ち下がり時間とも云う）ｔ３（図１１（ｃ）参照）は、１ｎｓｅｃ以上であることが好ましい。
【００５５】
（第４実施形態）
次に、本発明の第４実施形態による磁気メモリを図１２（ａ）、（ｂ）を参照して説明する。本実施形態の磁気メモリは、書き込みビット線ＢＬに流す電流が異なる以外は、第１実施形態の磁気メモリと同じ構成となっている。図１２（ａ）に本実施形態において、書き込みビット線ＢＬおよび書き込みワード線ＷＬに流す電流の波形図を示し、このとき発生する電流磁場による合成磁場を図１２（ｂ）に示す。ワード線ＷＬに単パルス、ビット線ＢＬには三角双極パルスを印加したものである。
【００５６】
この図１２（ａ）に示す電流パルスは、ワード線ＷＬの電流とビット線ＢＬの電流により生成される合成磁場が、その大きさと方向を連続的に変化するように制御された例を示している。特に、ワード線ＷＬの電流とビット線ＢＬの電流の立ち上がり時間ｔ１と立ち下がり時間ｔ３のタイミングをほぼ同時に制御している。この場合に生成される磁場は、図１２（ｂ）に示されている。すなわち、初期磁場としてＨ_ｅｘｔ１で示される斜め方向で磁化容易軸と並行ではない磁場が印加され、ビット線ＢＬの電流の変化とともに合成磁場は大きさを変えるとともに、角度も磁化容易軸に対し垂直方向に変化する。ビット線ＢＬの電流がちょうど零になったところで、合成磁場は磁化容易軸に対し９０度となる。このときの磁場が、図１２（ｂ）のＨ_ｅｘｔで表されている。ビット線ＢＬの電流が更に変化すると、合成磁場も大きさを変えながら角度を変えていき、図１２（ｂ）のＨ_ｅｘｔ２で示される磁場になる。
【００５７】
図１２（ｂ）に示す第１磁場Ｈ_ｅｘｔ１と第２磁場Ｈ_ｅｘｔ２は、磁化容易軸に対して垂直方向を対称軸として対称の関係に設定することが可能である。また、対称に設定しなくともよい。
【００５８】
また別の設定として、図１２（ａ）で示したワード線ＷＬの電流パルスとビット線ＢＬの電流パルスの立下りのタイミングを同時にしなくても良い。
【００５９】
また、図１２（ａ）では三角波を示しているが、実際にはその頂点がある曲率をもつなだらかな曲線で表される、図１３に示すような電流パルスであっても良い。さらに、大きな曲率を持ち、例えば三角関数で近似されるかまたは三角関数で正確に表される電流パルスであっても良い。また、ビット線ＢＬの電流は、ピーク値を取った後、その値を一定の時間だけ保持するようなパターンであっても良い。
【００６０】
本実施形態においては、図１２（ａ）に示す電流パルスがビット線ＢＬおよびワード線ＷＬに印加されるので、図１２（ｂ）に示すように、まず、記憶層３ａに第１角度θ_１を有する第１磁場Ｈ_ｅｘｔ１を作用させて、記憶層３ａの磁化を第１磁場Ｈ_ｅｘｔ１の方に回転させ、その後、第２角度θ_２を有する第２磁場Ｈ_ｅｘｔ２を作用させて記憶層３ａの磁化を第２磁場Ｈ_ｅｘｔ２の方向に回転させた後、第２磁場Ｈ_ｅｘｔ２を零にするものである。ここで、第１実施形態と同様に、安定した磁化反転を行うために、第１角度θ_１としては０度より大きく９０度よりも小さい角度が選択され、第２角度θ_２としては、９０度よりも大きく１８０度よりも小さい角度が選択される。なお、第１実施形態と同様に、第１角度θ_１としては、４５度以上９０度未満であることが好ましい。
【００６１】
また、ビット線ＢＬが負から正に直線的に遷移するので、第１磁場Ｈ_ｅｘｔ１から第２磁場Ｈ_ｅｘｔ２に変化するときに、ビット線ＢＬに流れる電流による電流磁場が零になる磁化困難軸方向に向く磁場Ｈ_ｅｘｔが現れる。そして、第１磁場Ｈ_ｅｘｔ１から第２磁場Ｈ_ｅｘｔ２に変化するときには、ワード線ＷＬに流れる電流は一定であるので、ワード線ＷＬによる電流磁場は一定となる。このため、磁場Ｈ_ｅｘｔ１、Ｈ_ｅｘｔ２、Ｈ_ｅｘｔの磁化困難軸方向の成分は一定となり、図９に示すように、磁場Ｈ_ｅｘｔ１、Ｈ_ｅｘｔ２、Ｈ_ｅｘｔを示す矢印の先端は、破線で示す直線３０に沿って動くことになる。
【００６２】
以上説明したように、本実施形態によれば、記憶層３ａの磁化方向を基準にして０度より大きく９０度未満の第１角度を有する第１磁場を作用させ、その後９０度より大きく１８０度以下の第２角度を有する第２磁場を作用させることにより、磁気メモリを高集積化しても安定な磁化反転を引き起こすことができる。また、スイッチング磁場が小さいため、省電力化を達成することができる。
【００６３】
なお、上記第１乃至第４実施形態においては、電流パルスパターンは説明を簡単にするために、ステップ数が１または２の場合のみを記述した。しかし、本発明ではそれに限ることなく、複数ステップを持つことが可能である。この場合には、例えば上記の三角関数パルスを複数ステップをもつ階段パルスの合成として表すことができ、本発明の磁気メモリに用いられるデジタル回路に好適である。
【００６４】
なお、パルスの幅、高さと磁化反転の関係について、文献（Physical Review Letters，81（1998），pp．4512）では
【数２】

であらわされる関係式が成り立つことが示されている。ここで、τはスイッチング時間、Ｂ_pulse（τ）はパルスの発生磁場、Ｂ_cは磁気記憶層の保磁力に対応する磁束、Ｓ_wはスイッチング係数である。消費電力の観点からは、パルス幅が短くなるに伴い消費電力が大きくなるため、パルス幅はできるだけ長いほうが好ましい。
【００６５】
これらのことから、この関係式によりパルス幅が１ｎｓｅｃより小さくなると磁化反転に必要なパルス高さが急激に大きくなることが示されるので、パルス幅は １ｎｓｅｃ以上であることが好ましい。
【００６６】
図４または図５に示す二重トンネル接合で、大きさが ０．４×１．２μｍ２のものについて、ＢＬにパルス電流を繰り返し流して“０”と“１”を交互に書き込んだときの読み出しの出力電圧をオシログラフに表示したときの一例を図１４に示す。この図１４には２チャンネルの出力が示されており、第１チャネルはパルス幅５ｎｓｅｃ、パルス立ち上がり時間ｔ１と立ち下がり時間ｔ３が共に１ｎｓｅｃ未満、パルス高さ２．５Ｖに対応した出力、第２チャンネルは、パルス幅５ｎｓｅｃ、パルス立ち上がり時間ｔ１と立ち下がり時間ｔ３が共に３ｎｓｅｃ、パルス高さが２．５Ｖに対応した出力を示している。なお、パルス高さが２．５Ｖより高いと、１ｎｓｅｃ未満でも読み出しは可能となるが、昇圧回路が複雑化することになる。
【００６７】
この図１４に示すように、パルスの幅と高さを一定にして、立ち上がり時間ｔ１と立ち下がり時間ｔ２に対する出力の依存性を調べてみると、図１４の第１チャネルに示すようにビット線ＢＬに書き込み電流を流しているにもかかわらず、立ち上がり時間ｔ１および立下り時間ｔ３が共に１ｎｓｅｃ未満では読み出し出力信号に変化が見られない。すなわち、書き込みができていない。一方、立ち上がり時間ｔ１および立下り時間ｔ３を共に３ｎｓｅｃにした場合には、図１４の第２チャネルに示すように、書き込み電流に対応して“０”と“１”の出力電圧変化が正しく見られる。
【００６８】
一般に、正しく書き込みできるために要求される書き込み電流の立ち上がり・立下り時間は、書き込み電流パルス高さに依存する。しかし、例えば、１ｎｓｅｃ以上であれば正しく書き込みができることが、図１４に示されている。よって、書き込み電流パルスについて、立ち上がり時間と立ち下がり時間に実際上は制限があり、１ｎｓｅｃ以上であることが好ましく、３ｎｓｅｃ以上であることがより好ましい。なお、上限値としては、５０ｎｓｅｃ〜１００ｎｓｅｃの範囲にあることが好ましい。
【００６９】
なお、記憶層として、少なくとも二つの強磁性層を含み、それらの間に介在する非磁性層からなる多層膜において、上記の強磁性層の間に反強磁性結合を含むものを用いても良い。この場合、二つの強磁性層は、その磁気モーメントまたは厚さが異なっており、反強磁性的結合により磁化が逆方向を向いている。このため、実効的に互いに磁化が相殺し、記憶層全体としては、磁化容易軸方向に小さな磁化を持った強磁性体と同等と考えることができる。この記憶層のもつ磁化容易軸方向の小さな磁化の向きと逆向きに磁場を印加すると、各強磁性層の磁化は、反強磁性結合を保ったまま反転する。このため、磁力線が閉じていることから反磁場の影響が小さく、記憶層のスイッチング磁場は、各強磁性層の保磁力により決まるので、小さなスイッチング磁場で記憶層の磁化の反転が可能になる。
【００７０】
なお、上記実施形態においては、外部磁場を２段階または連続的に変化させていたが、３段階以上の多段階のステップで変化させるように構成しても良い。この場合は、例えば、第１磁場として記憶層の磁化容易軸に対して３０度の角度をなす磁場を印加し、続いて第２磁場を、上記磁化容易軸に対して７０度、すなわち第１磁場に対して４０度の角度となるように印加し、更に続けて、第３磁場を磁化容易軸から１２０度、すなわち第２磁場から５０度の角度となるように印可する。このように多段階で磁場を印加すると、トルクは最大ではないが、実効的に大きな値を持ちかつ変化の角度が小さいために、磁化の変化が滑らかに起こるようになり、安定した磁化反転をすることができる。なお、上記第１乃至第３磁場の角度は、それぞれ同じ間隔をなしていても良いし、異なっていても良い。また、３段階に磁場を変化させることに限らず、４段階以上のステップで磁場を変化させても良い。この場合、第１磁場の角度は０度より大きく９０度より小さい角度であり、最終段の磁場の角度は９０度より大きく１８０度より小さい角度であれば良い。
【００７１】
【発明の効果】
以上述べたように、本発明によれば、磁気メモリを高集積化しても安定な磁化反転を引き起こすことができる。
【図面の簡単な説明】
【図１】本発明の第１実施形態による磁気メモリの磁化反転方法を説明する図。
【図２】第１実施形態による磁気メモリのメモリセルの構成を示す断面図。
【図３】第１実施形態による磁気メモリの構成を示すブロック図。
【図４】１重の強磁性トンネル接合を有する強磁性トンネル接合素子の一具体例の構成を示す断面図。
【図５】２重の強磁性トンネル接合を有する強磁性トンネル接合素子の他の具体例の構成を示す断面図。
【図６】記憶層を単磁区モデルで表したときの磁化ベクトルに作用するトルクの特性を示す図。
【図７】記憶層の磁化を反転する磁場の大きさのシミュレーション結果を示す図。
【図８】ダイオード付き単純クロスポイント型のメモリセルを備えた磁気メモリの構成を示すブッロク図。
【図９】本発明の第２実施形態による磁気メモリの磁化反転方法を説明する図。
【図１０】本発明の第３実施形態による磁気メモリの磁化反転方法を説明する図。
【図１１】書き込みビット線に印加される電流パルスの遷移時間を説明する図。
【図１２】本発明の第４実施形態による磁気メモリの磁化反転方法を説明する図。
【図１３】書き込みビット線に印加される電流パルスの他の例を示す図。
【図１４】電流パルスの立ち上がり・立ち下がり時間と、磁気抵抗効果素子の出力電圧との関係を説明する図。
【図１５】微小強磁性体の磁気的構造を説明する図。
【符号の説明】
２ メモリセル
３ 磁気抵抗効果素子
３ａ 記憶層
３ｂ 絶縁層
３ｃ 基準層
４ 引き出し電極
５ 接続プラグ
６ 選択トランジスタ
６ａ、６ｂ ソース・ドレイン領域
２１ ビット線駆動回路
２３ ワード線駆動回路
２５ センスアンプ
ＢＬ 書き込みビット線
ＢＬ_Ｒ 読み出しビット線
ＷＬ 書き込みワード線
ＷＬ_Ｒ 読み出しワード線
Ｈ_ｅｘｔ１ 第１磁場
Ｈ_ｅｘｔ２ 第２磁場
θ_１ 第１角度
θ_２ 第２角度[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic memory.
[0002]
[Prior art]
In recent years, a magnetic memory using a magnetoresistive effect element exhibiting a giant magnetoresistive effect has been proposed for a memory cell, and in particular, a magnetic memory using a ferromagnetic tunnel junction element having a ferromagnetic tunnel junction as the magnetoresistive effect element. Attention has been gathered.
[0003]
The ferromagnetic tunnel junction is mainly composed of a three-layer film of a first ferromagnetic layer / insulating layer / second ferromagnetic layer, and a current flows through the insulating layer. In this case, the junction resistance value changes in proportion to the cosine of the relative angle of magnetization of the first and second ferromagnetic layers. Therefore, the resistance value takes a minimum value when the magnetizations of the first and second ferromagnetic layers are parallel, and takes a maximum value when the magnetization is antiparallel. This is called the tunnel magnetoresistance (TMR) effect. It has been reported that the resistance value change due to the TMR effect reaches 49.7% at room temperature (see, for example, Non-Patent Document 1).
[0004]
In a magnetic memory including a ferromagnetic tunnel junction element as a memory cell, the magnetization of one of the first and second ferromagnetic layers is fixed as a magnetization reference layer, and the other ferromagnetic layer is used as a magnetization storage layer. To do. In this cell, information is stored by associating binary information “0”, “1” with the magnetization arrangement of the magnetization reference layer and the magnetization storage layer in parallel or antiparallel. In writing the record information, the magnetization of the magnetization storage layer is reversed by a magnetic field generated by passing a current through a write wiring provided separately for the memory cell. Reading is performed by passing a current through the ferromagnetic tunnel junction element and detecting a resistance change due to the TMR effect. A magnetic memory is configured as a large-capacity memory by arranging a large number of such memory cells.
[0005]
As for the actual configuration, for example, a switching transistor is arranged for each cell and a peripheral circuit is incorporated so that any one cell can be selected, as in a DRAM (Dynamic Random Access Memory). There has also been proposed a method in which a ferromagnetic tunnel junction element is incorporated together with a diode at a position where a word line and a bit line intersect (see, for example, Patent Documents 1 and 2).
[0006]
Now, considering the case of high integration of a magnetic memory using a ferromagnetic tunnel junction element as a memory cell, the size of the memory cell is reduced, and the size of the ferromagnetic layer of the ferromagnetic tunnel junction element constituting the memory cell is also reduced. Inevitably smaller. Such a magnetic thin film having a finite size generally has a complicated magnetic domain structure composed of a plurality of magnetic domains as a magnetic state. In particular, when the memory layer is a rectangular memory cell, it is known that there is a so-called edge domain having magnetization oriented in a direction different from the central portion in the region of the end in the long axis direction (for example, Non-Patent Document 2). If there is such an edge domain region, the magnetization of the entire memory cell is lowered, and as a result, the resistance change rate of the TMR effect is lowered. Moreover, since the change of the magnetic structure pattern in the magnetization reversal is complicated, it not only causes noise, but also increases the coercive force and increases the switching magnetic field.
[0007]
In order to prevent a complicated magnetic domain structure, a memory cell in which the shape of the storage layer is asymmetric with respect to the easy axis of magnetization, particularly a parallelogram has been proposed (see Patent Document 3). . When the memory cell has a memory layer having such a shape, the edge domain region is reduced, and a substantially single magnetic domain is formed over the entire memory cell.
[0008]
On the other hand, as a method for preventing the change of the complicated magnetic structure pattern in the magnetization reversal described above as much as possible, it may be possible to fix the edge domain by adding a structure that applies a hard bias to the end of the memory cell. (For example, see Patent Documents 4 and 5).
In addition, by providing an “H” or “I” shape with a portion protruding in a direction perpendicular to the easy magnetization axis of the cell, the edge domain can be stabilized and a complex magnetic domain can be formed. Is proposed (see, for example, Patent Document 6).
[0009]
Further, when a magnetic memory using a ferromagnetic tunnel junction element as a memory cell is highly integrated, the size of the memory cell is reduced, and the size of the ferromagnetic material constituting the memory cell is inevitably reduced. In general, as the ferromagnetic material becomes smaller, its coercive force increases. Since the magnitude of the coercive force is a measure of the magnitude of the switching magnetic field necessary for reversing the magnetization, this means an increase in the switching magnetic field. Therefore, when writing bit information, a larger current must be passed through the write wiring, which brings about undesirable results such as an increase in power consumption and a shortened life of the wiring. Therefore, reducing the coercivity of the ferromagnetic material used in the memory cell of the magnetic memory is an important issue in the practical application of the highly integrated magnetic memory.
[0010]
In order to reduce coercive force, the storage layer includes at least two ferromagnetic layers, and in a multilayer film composed of a nonmagnetic layer interposed therebetween, includes antiferromagnetic coupling between the ferromagnetic layers. It has been proposed to use one (see Patent Documents 7, 8, and 9). In this case, the two ferromagnetic layers have different magnetic moments or thicknesses, and their magnetizations are opposite to each other due to antiferromagnetic coupling. For this reason, the magnetizations effectively cancel each other, and the entire storage layer can be considered equivalent to a ferromagnetic material having a small magnetization in the direction of the easy axis of magnetization. When a magnetic field is applied in a direction opposite to the direction of small magnetization in the easy axis direction of the storage layer, the magnetization of each ferromagnetic layer is inverted while maintaining antiferromagnetic coupling. For this reason, since the lines of magnetic force are closed, the influence of the demagnetizing field is small, and the switching magnetic field of the storage layer is determined by the coercive force of each ferromagnetic layer, so that the magnetization of the storage layer can be reversed by a small switching magnetic field.
[0011]
When there is no interlayer coupling between the magnetic layers, there is an interaction due to magnetostatic coupling due to the leakage magnetic field from the magnetic layer. In this case, however, the switching magnetic field is reduced as in the case of the interlayer coupling. It is known (see Non-Patent Document 3). However, when there is no interlayer coupling between magnetic layers and only magnetostatic coupling exists, the magnetic structure formed by magnetization is unstable, and the squareness ratio in the hysteresis curve or magnetoresistive curve is small, resulting in large magnetic properties. It is difficult to obtain a resistance ratio, and it is not preferable to use it as a magnetoresistive effect element.
[0012]
[Non-Patent Document 1]
Appl. Phys. Lett. 77, 283 (2000)
[Patent Document 1]
US Pat. No. 5,640,343
[Patent Document 2]
US Pat. No. 5,650,958
[Non-Patent Document 2]
J. App. Phys. 81, 5471 (1997)
[Patent Document 3]
Japanese Patent Laid-Open No. 11-26344
[Patent Document 4]
US Pat. No. 5,748,524
[Patent Document 5]
JP 2000-100153 A
[Patent Document 6]
US Pat. No. 6,205,053
[Patent Document 7]
Japanese Patent Laid-Open No. 9-25162
[Patent Document 8]
Japanese Patent Application No. 2001-156358
[Patent Document 9]
US Pat. No. 5,953,248
[Non-Patent Document 3]
24th Annual Conference of Japan Society of Applied Magnetics 12aB-3, 12aB-7, 24th Annual Conference of Japan Society of Applied Magnetics p. 26, 27
[0013]
[Problems to be solved by the invention]
As described above, avoiding complication of the magnetic domain of the storage layer and stabilizing the edge domain are indispensable elements for obtaining a large and low-noise output signal. However, generally, in the case where the shape of the storage layer is a parallelogram, the magnetic domain structure is simplified and becomes almost a single magnetic domain, so that the coercive force or the switching magnetic field is increased.
[0014]
By adding a hard bias structure to fix the edge domain at the edge of the memory cell, it is possible to suppress the complicated change in the magnetic structure pattern during magnetization reversal, but this also increases the coercivity. To do. At the same time, it is necessary to add another structure to fix the edge domain, which is not suitable for high density required for a large capacity memory or the like.
[0015]
In the case where the shape of the memory layer is “H” or “I” type, it is necessary to enlarge the protruding portion in order to make the effect of the protruding shape effective. In this case, the area occupied by the memory cell substantially increases, and it is difficult to achieve high integration required for a large capacity memory.
[0016]
As described above, reducing the switching magnetic field necessary for reversing the magnetization of the storage layer is an indispensable element in realizing a magnetic memory. In order to reduce this switching magnetic field, it has been proposed to use a multilayer film including antiferromagnetic coupling via a nonmagnetic metal layer as the storage layer. However, in a small ferromagnet placed in a small memory cell used in a highly integrated magnetic memory, for example, when the width of the minor axis is about several microns to submicron, at the end of the magnetization region It is known that a magnetic structure different from the magnetic structure of the central part of the magnetic material is generated due to the influence of the demagnetizing field. Such an end magnetic structure is called an edge domain (see, for example, J. App. Phys. 81, 5471 (1997)). An example of such a magnetic structure is shown in FIG. 15, in which magnetization occurs in the direction according to the magnetic anisotropy in the central portion of the magnetization region, but magnetization in a direction different from the central portion at both ends. Occurs. The magnetic structure of the microferromagnet 40 includes a C-type structure shown in FIG. 15A and an S-type structure shown in FIG.
[0017]
As described above, in a minute magnetic body used in a memory cell of a highly integrated magnetic memory, the influence of an edge domain generated at the end portion is large, and the change of the magnetic structure pattern during magnetization reversal becomes complicated. As a result, the coercive force is increased and the switching magnetic field is increased. For this reason, stable magnetization reversal cannot be obtained.
[0018]
The present invention has been made in view of the above circumstances, and an object thereof is to provide a magnetic memory capable of causing stable magnetization reversal even when highly integrated.
[0019]
[Means for Solving the Problems]
A magnetic memory according to an aspect of the present invention includes a memory cell including a magnetoresistive effect element having a storage layer that changes the direction of magnetization in accordance with an external magnetic field, a magnetic field, and a magnetization direction of the magnetoresistive effect element. When reversing, a first magnetic field having a first angle greater than 0 degrees and smaller than 90 degrees is applied to the storage layer with reference to the magnetization direction of the storage layer, and further has a second angle greater than the first angle. And a magnetic field generation mechanism that causes the second magnetic field to act on the memory layer.
[0020]
The first angle is preferably not less than 45 degrees and less than 90 degrees.
[0021]
The second angle is preferably larger than 90 degrees and smaller than 180 degrees.
[0022]
The first and second magnetic fields generated from the magnetic field generation mechanism are disposed in the direction of the hard axis of the magnetoresistive element and the first wiring disposed in the direction of the easy axis of the magnetoresistive element. Preferably, the current magnetic field is generated by the current flowing through the second wiring, the current flowing through one of the first and second wirings is constant, and the direction of the current flowing through the other wiring is changed.
[0023]
The first and second magnetic fields generated from the magnetic field generation mechanism are disposed in the direction of the hard axis of the magnetoresistive element and the first wiring disposed in the direction of the easy axis of the magnetoresistive element. A current magnetic field generated by a current flowing through the second wiring, and a transition time from when the current reaches one extreme value of negative polarity or positive polarity, from one extreme value to the other extreme value. It is preferable that both the transition time up to and the transition time from the other extreme value to zero are 1 nsec or more.
[0024]
Note that the storage layer of the magnetoresistive element may have a structure in which the first and second ferromagnetic layers are stacked via a nonmagnetic layer.
[0025]
The magnetic field generated from the magnetic field generation mechanism may be configured to change along a continuous curve.
[0026]
The magnetoresistive element may be a ferromagnetic tunnel junction element.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described with reference to the drawings.
[0028]
(First embodiment)
A magnetic memory according to a first embodiment of the present invention will be described with reference to FIGS.
[0029]
The configuration of the magnetic memory according to the present embodiment is shown in FIG. The magnetic memory according to this embodiment includes a memory cell 2 arranged in a matrix, a plurality of write bit lines BL, and a plurality of read bit lines BL. _R A plurality of write word lines WL and a plurality of read word lines WL. _R A bit line driving circuit 21, a word line driving circuit 23, and a sense amplifier 25. The memory cell 2 includes a magnetoresistive effect element 3 and a selection transistor 6. The magnetoresistive effect element 3 of each memory cell 2 has one end connected to the corresponding write bit line BL and the other end connected to one of the source and drain of the selection transistor 3. The select transistor 6 of each memory cell 2 has a corresponding read word line WL. _R The other of the source and drain is selected by the read bit line BL _R Connected to. The bit line driving circuit 21 causes a current to flow through the write bit line BL at the time of writing, and the read bit line BL at the time of reading. _R A voltage is applied to the write bit line BL. The word line drive circuit 23 causes a current to flow through the write word line WL at the time of writing, and the read word line WL at the time of reading. _R By applying a predetermined voltage to the word line WL, _R The selection transistor 6 connected to is turned on. Further, the sense amplifier 25 reads the bit line BL for reading when reading data stored in the memory cell. _R The current flowing between the write bit lines BL is measured.
[0030]
A detailed configuration of the memory cell 2 according to the present embodiment is shown in FIG. The memory cell 2 includes a magnetoresistive effect element 3 provided at an intersection of a corresponding write bit line BL and a corresponding write word line WL, and a selection transistor 6 for selecting the magnetoresistive effect element 3 at the time of reading. It has. One end of the magnetoresistive effect element 3 is connected to the corresponding write bit line BL, and the other end is connected to one of the source and drain of the selection transistor 6 via the lead electrode 4 and the connection plug 5. The gate of the selection transistor 6 is a read word line WL. _R Doubles as Further, the read bit line BL not shown in FIG. _R Connected to.
[0031]
In addition, as shown in FIG. 4, the magnetoresistive element 3 used in the present embodiment includes a ferromagnetic layer 3a serving as a storage layer, an insulating layer 3b serving as a storage layer, and a ferromagnetic layer serving as a reference layer having fixed magnetization directions This is a ferromagnetic tunnel junction element having a ferromagnetic tunnel junction structure in which 3c is laminated. As shown in FIG. 5, the magnetoresistive element 3 is a double ferromagnetic tunnel junction, that is, an insulating layer 3b on both sides of a ferromagnetic layer 3a serving as a memory layer. ₁ 3b ₂ A ferromagnetic layer 3c serving as a reference layer via ₁ 3c ₂ A ferromagnetic tunnel junction element having a double ferromagnetic tunnel junction structure in which is stacked may be used, or a normal magnetoresistive element may be used.
[0032]
Next, before describing the magnetization reversal method of the storage layer 3a of the magnetic memory according to the present embodiment, first, using a relatively simple model, a magnetic field is applied to the storage layer 3a facing the easy axis direction. The torque that is sometimes generated will be described.
[0033]
Of the magnetoresistive effect elements used in the present embodiment, attention is focused on a ferromagnetic layer serving as a storage layer. For example, the ferromagnetic layer has a width of 0.1 μm to 1.0 μm, a length of 0.1 μm to 3.0 μm, and a thickness of 1.0 nm to 10 nm. In a ferromagnetic thin film of this size, it can be considered that the magnetization is mostly in the direction of the easy axis of magnetization. Therefore, the entire ferromagnetic layer forms a single magnetic domain, and a single magnetic domain model that expresses magnetization by one vector can be used. In practice, the material, structure, shape, and magnetism of the ferromagnetic layer may vary greatly, but even in such a case, the single domain model can be treated as a basic model.
[0034]
Now, the free energy E of a single domain model having uniaxial anisotropy is expressed as follows.
[0035]
[Expression 1]

Where K _u Is the anisotropic energy, θ is the saturation magnetization M of the ferromagnetic thin film _s And α is an angle formed between the applied magnetic field H and the easy magnetization axis. The magnetization direction to be realized is determined so as to minimize the free energy E. As a result, a change in the magnetization direction is represented by an asteroid curve. Further, the torque acting on the magnetization vector is determined by -∂E / ∂θ. A torque characteristic graph calculated by simulation is shown in FIG. In FIG. 6, the horizontal axis represents the angle α, and the vertical axis represents the torque normalized by the maximum torque. The torque curve shown in FIG. 6 shows that the applied magnetic field H is an anisotropic magnetic field H. _k (= 2K _u / M _s ) Normalized magnetic field h (= H / H _k ) As a parameter. When the parameter h is sufficiently large, this corresponds to the case where the anisotropic energy is small, and when it is infinite, there is no anisotropy. It can be seen from FIG. 6 that the torque curve when the parameter h is infinite is expressed by a sine function and takes an extreme value when the angle α between the applied magnetic field and the easy axis of magnetization is 45 degrees and 135 degrees. That is, in this case, since the magnetization vector receives the maximum torque, it is most effective for reversal of magnetization. When the parameter h is reduced, the angle formed between the applied magnetic field that gives the maximum torque and the easy magnetization axis takes an extreme value at an angle larger than 45 degrees and an angle smaller than 135 degrees. When the parameter h is further decreased and h is smaller than 0.5, the angle formed by the magnetization easy axis of the applied magnetic field that gives the maximum torque becomes one, for example, near 120 degrees when h = 0.3. You can see it.
[0036]
Next, the magnetization reversal method of the storage layer 3a of the magnetic memory according to the present embodiment will be explained with reference to FIGS. 1 (a) and 1 (b).
[0037]
FIG. 1A is a top view of a ferromagnetic layer 3a that is a storage layer of the magnetoresistive effect element used in this embodiment. As described with reference to FIG. 2, the write word line WL is disposed below the storage layer 3a, and the write bit line BL is disposed above. The write word line WL is disposed in the easy axis direction of the storage layer 3a, and the write bit line BL is disposed in the hard axis direction.
[0038]
In the initial state of the storage layer 3a, the magnetization of the storage layer 3a is assumed to be in the direction of the easy axis of magnetization and to the right as shown in FIG. In the present embodiment, as an applied magnetic field that reverses the magnetization direction of the storage layer 3a, first, the first angle θ with respect to the magnetization direction of the storage layer 3a. ₁ A first magnetic field H having _ext1 To cause the magnetization of the storage layer 3a to change to the first magnetic field H. _ext1 And then the second angle θ with reference to the initial magnetization direction of the storage layer 3a (the right direction shown in FIG. 1A). ₂ Second magnetic field H having _ext2 Acting on the magnetization of the memory layer 3a to the second magnetic field H _ext2 And then the second magnetic field H _ext2 Is reversed to reverse the magnetization of the storage layer 3a.
[0039]
At this time, the first angle θ ₁ As can be seen from the torque curve in FIG. 6, if the angle is greater than 0 degrees and smaller than 90 degrees, the negative torque, that is, the magnetization of the storage layer 3a shown in FIG. _ext1 When a torque that rotates counterclockwise toward is generated and has an angle that is greater than 90 degrees and less than 180 degrees, the positive torque, that is, the magnetization of the storage layer 3a is the first magnetic field H. _ext1 A torque is generated that rotates clockwise toward. That is, the first angle θ ₁ However, even if the angle is larger than 0 degree and smaller than 90 degrees, or the angle larger than 90 degrees and smaller than 180 degrees, a torque for rotating the magnetization of the storage layer 3a is generated.
[0040]
First angle θ ₁ Is an angle greater than 0 degree and smaller than 90 degrees, the first angle θ ₁ Since the rotation angle of magnetization of the memory layer 3a is smaller than when the angle is larger than 90 degrees and smaller than 180 degrees, it is possible to rotate the magnetization without greatly deforming the initial domain of the memory layer 3a. However, the first angle θ ₁ When the angle is larger than 90 degrees and smaller than 180 degrees, the rotation angle of magnetization of the storage layer 3a is large, so that the rotation of the magnetization causes a new area near the boundary region between the domain of the central portion of the storage layer 3a and the edge domain Domain wall is likely to occur, and stable magnetization reversal cannot be performed. When the domain wall is generated and remains, there arises a problem that the MR ratio of the magnetoresistive effect element is deteriorated.
Therefore, in the present embodiment, in order to perform stable magnetization reversal, the first angle θ ₁ As an angle, an angle larger than 0 degree and smaller than 90 degrees is selected. The second magnetic field H _ext2 Is applied to the first magnetic field H. _ext1 The first magnetic field H is stabilized at a position balanced by the balance of magnetic anisotropy and magnetic anisotropy. _ext1 And the direction of magnetic anisotropy (initial magnetization direction). For this reason, the second magnetic field H _ext2 Is applied, the second angle θ ₂ And an angle between which the difference between the magnetization direction of the storage layer 3a is larger than 0 degree and smaller than 90 degrees. For example, the second angle θ ₂ As the angle, an angle larger than 90 degrees and smaller than 180 degrees is selected. As can be seen from the torque curve in FIG. 6, the torque is maximized when the angle between the magnetization direction of the storage layer 3a and the applied magnetic field is 45 degrees or more and less than 90 degrees. ₁ Is preferably 45 degrees or more and less than 90 degrees.
[0041]
Such a first magnetic field H _ext1 And the second magnetic field H _ext2 In order to generate the current, the current shown in FIG. 1B may be supplied to the bit line BL and the word line WL. Note that the direction of the current flowing through the bit line BL and the word line WL is positive in the direction of the arrow shown in FIG. As shown in FIG. 1B, first, a negative predetermined current is supplied to the bit line BL and a positive predetermined current is supplied to the word line WL. Then, the first magnetic field H in which the current magnetic field generated by the current is synthesized. _ext1 Will occur. Thereafter, the direction of the current of the bit line BL is changed to change from a predetermined negative current to a predetermined positive current. Then, the second magnetic field H in which the current magnetic field by the current flowing through the bit line BL and the word line WL is synthesized. _ext2 Will occur.
[0042]
As shown in FIG. 1B, in this embodiment, a unipolar pulse is applied to the word line WL and a bipolar pulse is applied to the bit line BL. In this current pulse pattern, the pulse height of each of the word line and the bit line determines the direction when the magnetic fields generated by them are combined, and the respective heights of the pulses of the word line WL and the bit line BL. Each I _w1 , I _b1 Then, the angle θ of the composite magnetic field is a relational expression as seen from the easy axis of magnetization.
tan θ = | I _w1 | / | I _b1 ｜
It is decided by. These pulse heights may or may not be the same.
[0043]
FIG. 7 is a characteristic curve showing the state of magnetization reversal by the write current pattern shown in FIG. This characteristic curve is a numerical calculation based on the LLG (Landau-Lifshitz-Gilbert) equation. In this calculation, a NiFe thin film having a width of 0.4 μm, a length of 0.8 μm, and a thickness of 3 nm (M _s = 800emu / cm ² , K _u = 1.0 × 10 ⁴ erg / cm ³ ) Is used.
[0044]
Here, a case is shown in which the initial magnetic field is applied in the 45 ° direction from the easy axis for 3.0 nsec, and then applied in the 135 ° direction for 3.0 nsec. , 30 and 40 Oe for each of the characteristic graphs g ₁ , G ₂ , G ₃ Shown in As can be seen from this characteristic graph, the magnetization of the storage layer 3a is not reversed when the applied magnetic field is 20 Oe, and the magnetization of the storage layer 3a is reversed when the applied magnetic field is 30 Oe or more. When calculated in more detail, the minimum applied magnetic field required for magnetization reversal was about 28 Oe. Therefore, it can be understood that a considerably small switching magnetic field may be used to reverse the magnetization of the storage layer 3a of the magnetic memory according to the present embodiment, and therefore a small current may be used.
[0045]
As described above, according to the present embodiment, the first magnetic field having the first angle greater than 0 degree and less than 90 degrees is applied with reference to the magnetization direction of the storage layer 3a, and then greater than 90 degrees and 180 degrees. By applying a second magnetic field having the following second angle, stable magnetization reversal can be caused even if the magnetic memory is highly integrated. Further, since the switching magnetic field is small, power saving can be achieved.
[0046]
In this embodiment, the memory cell is a magnetic memory composed of a magnetoresistive effect element and a selection transistor. However, as shown in FIG. 8, it may be a simple cross-point type memory cell with a diode. . That is, a configuration in which a cell including the diode 8 and the magnetoresistive element 3 is stacked on the word line WL and the bit line BL is disposed on the magnetoresistive element 3 may be employed. In this case, the word line WL and the bit line BL are used not only for writing but also for reading.
[0047]
(Second Embodiment)
Next, a magnetic memory according to the second embodiment of the present invention will be described with reference to FIGS. The magnetic memory of this embodiment has the same configuration as the magnetic memory of the first embodiment except that the current flowing through the write bit line BL is different. FIG. 9A shows a waveform diagram of currents flowing through the write bit line BL and the write word line WL in this embodiment, and FIG. 9B shows a combined magnetic field generated by the current magnetic field generated at this time. A single pulse is applied to the word line WL, and a triangular bipolar pulse is applied to the bit line BL. In this current pulse pattern, the pulse height of each of the word line WL and the bit line BL determines the direction when the magnetic fields generated by each of them are combined, and the current pulse of the word line WL and the bit line BL. Each height | I _w1 |, | I _b1 If │, the angle of the synthetic magnetic field is a relational expression as seen from the easy axis of magnetization.
tan θ = | I _w1 | / | I _b1 ｜
It is decided by. These pulse heights may or may not be the same. However, | I _w1 It is a feature that | is constant.
[0048]
In the present embodiment, since the current pulse shown in FIG. 9A is applied to the bit line BL and the word line WL, as shown in FIG. 9B, first, the first angle θ is applied to the storage layer 3a. ₁ A first magnetic field H having _ext1 To cause the magnetization of the storage layer 3a to change to the first magnetic field H. _ext1 And then the second angle θ ₂ Second magnetic field H having _ext2 To cause the magnetization of the storage layer 3a to change to the second magnetic field H. _ext2 After rotating in the direction of the second magnetic field H _ext2 Is zero. Here, as in the first embodiment, in order to perform stable magnetization reversal, the first angle θ ₁ Is selected as an angle greater than 0 degrees and smaller than 90 degrees, and the second angle θ ₂ As the angle, an angle larger than 90 degrees and smaller than 180 degrees is selected. As in the first embodiment, the first angle θ ₁ Is preferably 45 degrees or more and less than 90 degrees.
[0049]
Further, since the bit line BL transitions linearly from negative to positive, the first magnetic field H _ext1 To second magnetic field H _ext2 , The magnetic field H directed in the hard axis direction where the current magnetic field due to the current flowing through the bit line BL becomes zero _ext Appears. And the first magnetic field H _ext1 To second magnetic field H _ext2 Since the current flowing through the word line WL is constant, the current magnetic field generated by the word line WL is constant. For this reason, the magnetic field H _ext1 , H _ext2 , H _ext The component in the direction of the hard axis of magnetization is constant, and as shown in FIG. _ext1 , H _ext2 , H _ext The tip of the arrow indicating the arrow moves along a straight line 30 indicated by a broken line.
[0050]
As described above, according to the present embodiment, the first magnetic field having the first angle greater than 0 degree and less than 90 degrees is applied with reference to the magnetization direction of the storage layer 3a, and then greater than 90 degrees and 180 degrees. By applying a second magnetic field having the following second angle, stable magnetization reversal can be caused even if the magnetic memory is highly integrated. Further, since the switching magnetic field is small, power saving can be achieved.
[0051]
(Third embodiment)
Next, a magnetic memory according to a third embodiment of the present invention will be described with reference to FIG. The magnetic memory of this embodiment has the same configuration as the magnetic memory of the first embodiment except that the current flowing through the write bit line BL is different. FIG. 10 shows a waveform diagram of currents flowing through the write bit line BL and the write word line WL in the present embodiment. When changing the polarity of the current pulse flowing through the bit line BL, the value gradually changes with a transition time width of 1 nsec or more. Further, as indicated by the dotted line in FIG. 10, after the current pulse of the word line WL is cut off, the current pulse of the bit line BL is cut off after a while. This is for stably ending the magnetization reversal of the storage layer 3a caused by the current pulse.
[0052]
In this embodiment as well, as can be seen from the current pulse shown in FIG. 10, first, the first magnetic field having the first angle is applied to the storage layer 3a, and the magnetization of the storage layer 3a is moved toward the first magnetic field. Thereafter, the second magnetic field having a second angle is applied to rotate the magnetization of the storage layer 3a in the direction of the second magnetic field, and then the second magnetic field is made zero. Here, as in the first embodiment, in order to perform stable magnetization reversal, an angle larger than 0 degree and smaller than 90 degrees is selected as the first angle, and larger than 90 degrees as the second angle. An angle smaller than 180 degrees is selected. As in the first embodiment, the first angle is preferably 45 degrees or more and less than 90 degrees.
[0053]
As described above, according to the present embodiment, the first magnetic field having the first angle greater than 0 degree and less than 90 degrees is applied with reference to the magnetization direction of the storage layer 3a, and then greater than 90 degrees and 180 degrees. By applying a second magnetic field having the following second angle, stable magnetization reversal can be caused even if the magnetic memory is highly integrated. Further, since the switching magnetic field is small, power saving can be achieved.
[0054]
As for the current flowing through the bit line BL, a transition time (hereinafter also referred to as a rise time) t1 (see FIG. 11A) from when a current pulse flows through the bit line BL until reaching a negative extreme value. Transition time t2 (see FIG. 11 (b)) from the negative extreme value to the positive extreme value, transition time t3 (also referred to as a fall time) from the positive extreme value to zero In FIG. 11C, it is preferable that the time is 1 nsec or more.
[0055]
(Fourth embodiment)
Next, a magnetic memory according to a fourth embodiment of the present invention is described with reference to FIGS. The magnetic memory of this embodiment has the same configuration as the magnetic memory of the first embodiment except that the current flowing through the write bit line BL is different. FIG. 12A shows a waveform diagram of currents flowing through the write bit line BL and the write word line WL in this embodiment, and FIG. 12B shows a combined magnetic field generated by the current magnetic field generated at this time. A single pulse is applied to the word line WL, and a triangular bipolar pulse is applied to the bit line BL.
[0056]
The current pulse shown in FIG. 12A shows an example in which the combined magnetic field generated by the current of the word line WL and the current of the bit line BL is controlled so as to change continuously in magnitude and direction. Yes. In particular, the timings of the rise time t1 and fall time t3 of the current of the word line WL and the current of the bit line BL are controlled almost simultaneously. The magnetic field generated in this case is shown in FIG. That is, the initial magnetic field is H _ext1 A magnetic field that is not parallel to the easy axis is applied in an oblique direction, and the combined magnetic field changes in magnitude as the current of the bit line BL changes, and the angle also changes in a direction perpendicular to the easy axis. When the current of the bit line BL becomes just zero, the combined magnetic field is 90 degrees with respect to the easy axis. The magnetic field at this time is H in FIG. _ext It is represented by When the current of the bit line BL further changes, the synthesized magnetic field also changes its angle while changing its magnitude, and the H in FIG. _ext2 The magnetic field indicated by
[0057]
First magnetic field H shown in FIG. _ext1 And second magnetic field H _ext2 Can be set in a symmetric relationship with the axis perpendicular to the easy axis of symmetry as the axis of symmetry. Moreover, it is not necessary to set symmetrically.
[0058]
As another setting, the falling timing of the current pulse of the word line WL and the current pulse of the bit line BL shown in FIG.
[0059]
Although FIG. 12A shows a triangular wave, a current pulse as shown in FIG. 13 represented by a gentle curve having a certain curvature may actually be used. Further, it may be a current pulse having a large curvature, for example, approximated by a trigonometric function or accurately represented by a trigonometric function. Further, the current of the bit line BL may be a pattern in which after taking a peak value, the value is held for a certain time.
[0060]
In this embodiment, since the current pulse shown in FIG. 12A is applied to the bit line BL and the word line WL, as shown in FIG. 12B, first, the first angle θ is applied to the storage layer 3a. ₁ A first magnetic field H having _ext1 To cause the magnetization of the storage layer 3a to change to the first magnetic field H. _ext1 And then the second angle θ ₂ Second magnetic field H having _ext2 To cause the magnetization of the storage layer 3a to change to the second magnetic field H. _ext2 After rotating in the direction of the second magnetic field H _ext2 Is zero. Here, as in the first embodiment, in order to perform stable magnetization reversal, the first angle θ ₁ Is selected as an angle greater than 0 degrees and smaller than 90 degrees, and the second angle θ ₂ As the angle, an angle larger than 90 degrees and smaller than 180 degrees is selected. As in the first embodiment, the first angle θ ₁ Is preferably 45 degrees or more and less than 90 degrees.
[0061]
Further, since the bit line BL transitions linearly from negative to positive, the first magnetic field H _ext1 To second magnetic field H _ext2 , The magnetic field H directed in the hard axis direction where the current magnetic field due to the current flowing through the bit line BL becomes zero _ext Appears. And the first magnetic field H _ext1 To second magnetic field H _ext2 Since the current flowing through the word line WL is constant, the current magnetic field generated by the word line WL is constant. For this reason, the magnetic field H _ext1 , H _ext2 , H _ext The component in the direction of the hard axis of magnetization is constant, and as shown in FIG. _ext1 , H _ext2 , H _ext The tip of the arrow indicating the arrow moves along a straight line 30 indicated by a broken line.
[0062]
As described above, according to the present embodiment, the first magnetic field having the first angle greater than 0 degree and less than 90 degrees is applied with reference to the magnetization direction of the storage layer 3a, and then greater than 90 degrees and 180 degrees. By applying a second magnetic field having the following second angle, stable magnetization reversal can be caused even if the magnetic memory is highly integrated. Further, since the switching magnetic field is small, power saving can be achieved.
[0063]
In the first to fourth embodiments, the current pulse pattern is described only when the number of steps is 1 or 2 for the sake of simplicity. However, the present invention is not limited to this, and it is possible to have a plurality of steps. In this case, for example, the above trigonometric function pulse can be expressed as a composition of stepped pulses having a plurality of steps, which is suitable for a digital circuit used in the magnetic memory of the present invention.
[0064]
The relationship between pulse width and height and magnetization reversal is described in the literature (Physical Review Letters, 81 (1998), pp. 4512).
[Expression 2]

It is shown that the relational expression expressed by Where τ is the switching time and B _pulse (Τ) is the pulse generation magnetic field, B _c Is the magnetic flux corresponding to the coercivity of the magnetic memory layer, S _w Is a switching coefficient. From the viewpoint of power consumption, since the power consumption increases as the pulse width becomes shorter, the pulse width is preferably as long as possible.
[0065]
From these facts, this relational expression shows that the pulse height required for magnetization reversal increases rapidly when the pulse width becomes smaller than 1 nsec. Therefore, the pulse width is preferably 1 nsec or more.
[0066]
Reading when double tunnel junction shown in FIG. 4 or 5 having a size of 0.4 × 1.2 μm 2 is repeatedly written with “0” and “1” by repeatedly applying a pulse current to BL An example when the output voltage is displayed on an oscillograph is shown in FIG. FIG. 14 shows the output of two channels. The first channel has an output corresponding to a pulse width of 5 nsec, a pulse rise time t1 and a fall time t3 of both less than 1 nsec, and a pulse height of 2.5 V. The channel shows an output corresponding to a pulse width of 5 nsec, a pulse rise time t1 and a fall time t3 of 3 nsec, and a pulse height of 2.5V. If the pulse height is higher than 2.5V, reading is possible even if it is less than 1 nsec, but the booster circuit becomes complicated.
[0067]
As shown in FIG. 14, when the pulse width and height are made constant and the output dependency on the rise time t1 and fall time t2 is examined, the bit line as shown in the first channel of FIG. In spite of the writing current flowing through BL, if the rise time t1 and the fall time t3 are both less than 1 nsec, no change is seen in the read output signal. That is, writing is not possible. On the other hand, when both rise time t1 and fall time t3 are set to 3 nsec, as shown in the second channel of FIG. 14, the output voltage changes of “0” and “1” are correctly observed corresponding to the write current. It is done.
[0068]
In general, the rise / fall time of the write current required for correct writing depends on the write current pulse height. However, for example, FIG. 14 shows that the data can be written correctly if it is 1 nsec or more. Therefore, the write current pulse has practically limited rise time and fall time, preferably 1 nsec or more, and more preferably 3 nsec or more. In addition, as an upper limit, it is preferable to exist in the range of 50nsec-100nsec.
[0069]
As the memory layer, a multilayer film including at least two ferromagnetic layers and a nonmagnetic layer interposed between them may be one including antiferromagnetic coupling between the above ferromagnetic layers. . In this case, the two ferromagnetic layers have different magnetic moments or thicknesses, and their magnetizations are opposite to each other due to antiferromagnetic coupling. For this reason, the magnetizations effectively cancel each other, and the entire storage layer can be considered equivalent to a ferromagnetic material having a small magnetization in the direction of the easy axis of magnetization. When a magnetic field is applied in a direction opposite to the direction of small magnetization in the easy axis direction of the storage layer, the magnetization of each ferromagnetic layer is inverted while maintaining antiferromagnetic coupling. For this reason, since the lines of magnetic force are closed, the influence of the demagnetizing field is small, and the switching magnetic field of the storage layer is determined by the coercive force of each ferromagnetic layer, so that the magnetization of the storage layer can be reversed with a small switching magnetic field.
[0070]
In the above embodiment, the external magnetic field is changed in two steps or continuously. However, the external magnetic field may be changed in three or more steps. In this case, for example, a magnetic field that forms an angle of 30 degrees with respect to the easy axis of the storage layer is applied as the first magnetic field, and then the second magnetic field is 70 degrees with respect to the easy axis of magnetization, that is, the first magnetic field. The magnetic field is applied at an angle of 40 degrees, and then the third magnetic field is applied at an angle of 120 degrees from the easy axis, that is, an angle of 50 degrees from the second magnetic field. When magnetic fields are applied in this way, the torque is not the maximum, but since it has an effective large value and the angle of change is small, the change in magnetization occurs smoothly and stable magnetization reversal is achieved. can do. Note that the angles of the first to third magnetic fields may be the same interval or different. Further, the magnetic field is not limited to changing in three steps, and the magnetic field may be changed in four or more steps. In this case, the angle of the first magnetic field is larger than 0 degree and smaller than 90 degrees, and the angle of the magnetic field in the final stage may be larger than 90 degrees and smaller than 180 degrees.
[0071]
【The invention's effect】
As described above, according to the present invention, stable magnetization reversal can be caused even if the magnetic memory is highly integrated.
[Brief description of the drawings]
FIG. 1 is a view for explaining a magnetization reversal method of a magnetic memory according to a first embodiment of the present invention.
FIG. 2 is a cross-sectional view showing the configuration of a memory cell of the magnetic memory according to the first embodiment.
FIG. 3 is a block diagram showing the configuration of the magnetic memory according to the first embodiment.
FIG. 4 is a cross-sectional view showing the configuration of a specific example of a ferromagnetic tunnel junction element having a single ferromagnetic tunnel junction.
FIG. 5 is a cross-sectional view showing the configuration of another specific example of a ferromagnetic tunnel junction device having a double ferromagnetic tunnel junction.
FIG. 6 is a diagram showing characteristics of torque acting on a magnetization vector when a storage layer is represented by a single domain model.
FIG. 7 is a diagram showing a simulation result of the magnitude of a magnetic field for reversing the magnetization of a storage layer.
FIG. 8 is a block diagram showing a configuration of a magnetic memory including a simple cross-point type memory cell with a diode.
FIG. 9 is a view for explaining a magnetization reversal method of a magnetic memory according to a second embodiment of the present invention.
FIG. 10 is a view for explaining a magnetization reversal method of a magnetic memory according to a third embodiment of the present invention.
FIG. 11 is a diagram for explaining the transition time of a current pulse applied to a write bit line.
FIG. 12 is a view for explaining a magnetization reversal method of a magnetic memory according to a fourth embodiment of the present invention.
FIG. 13 is a diagram showing another example of a current pulse applied to a write bit line.
FIG. 14 is a diagram for explaining the relationship between the rise / fall time of a current pulse and the output voltage of the magnetoresistive element.
FIG. 15 is a view for explaining the magnetic structure of a microferromagnetic material.
[Explanation of symbols]
2 Memory cells
3 Magnetoresistive effect element
3a memory layer
3b Insulating layer
3c Reference layer
4 Lead electrode
5 Connection plug
6 Select transistor
6a, 6b Source / drain regions
21 Bit line drive circuit
23 Word line drive circuit
25 sense amplifier
BL Write bit line
BL _R Read bit line
WL Write word line
WL _R Read word line
H _ext1 1st magnetic field
H _ext2 Second magnetic field
θ ₁ 1st angle
θ ₂ Second angle

Claims (7)

A memory cell having a magnetoresistive element having a storage layer that changes the direction of magnetization according to an external magnetic field;
When generating a magnetic field and reversing the magnetization direction of the magnetoresistive effect element, a first magnetic field having a first angle greater than 0 degrees and smaller than 90 degrees with respect to the magnetization direction of the storage layer is applied to the storage layer. A magnetic field generating mechanism for causing a second magnetic field having a second angle larger than 90 degrees and smaller than 180 degrees to act on the storage layer;
A magnetic memory comprising:   The magnetic memory according to claim 1, wherein the first angle is not less than 45 degrees and less than 90 degrees. The first and second magnetic fields generated from the magnetic field generation mechanism are a first wiring arranged in the easy magnetization axis direction of the magnetoresistive effect element and a first wiring arranged in the hard magnetization axis direction of the magnetoresistive effect element. A current magnetic field generated by a current flowing through two wirings, wherein a current flowing through one of the first and second wirings is constant, and a direction of a current flowing through the other wiring changes. Item 3. The magnetic memory according to Item 1 or 2 . The first and second magnetic fields generated from the magnetic field generation mechanism are a first wiring arranged in the easy magnetization axis direction of the magnetoresistive effect element and a first wiring arranged in the hard magnetization axis direction of the magnetoresistive effect element. 2 is a current magnetic field generated by the current flowing through the wiring, and the transition time from the zero to one of the extreme values of the negative polarity or the positive polarity reaches the other extreme value from the one extreme value. transition time to, and the transition time to reach zero from the other extreme, the magnetic memory according to any one of claims 1 to 3, characterized in that both at 1nsec more. The storage layer of the magnetoresistive element, according to any one of claims 1 to 4 the first and second ferromagnetic layers via a nonmagnetic layer is characterized in that it has a laminated structure Magnetic memory. The magnetic memory according to any one of claims 1 to 5 field generated from the magnetic field generating mechanism is characterized by changes along the continuous curve. It said magnetoresistive element, a magnetic memory according to any one of claims 1 to 6, characterized in that a ferromagnetic tunnel junction element.

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