Magnetization dynamics of synthetic MRAM free layers: coupled precession and ultra fast magnetization reversal strategies

本工作的目的是在10ps的时间尺度上研究微观合成反铁磁体(Syaf)细胞的磁化动力学。SIAF由两个铁磁薄膜FM1、FM2组成，由超薄的非磁交换耦合层(CL)隔开，导致两个铁磁体的磁化强度M1、M2呈反平行排列。这种Syaf现在用作磁随机存取存储器(MRAM)和磁逻辑中的存储层(或自由层)。到目前为止，关于Syaf自由层器件的最大写入速率的重要问题还没有得到回答。Syaf自由层电池的磁化反转速度有多快？为了回答这个问题，我们研究了Syaf中两个铁磁体的磁化强度M1、M2的耦合进动动力学及其对器件参数的依赖关系。为了得到Syaf动力学的完整图景，从时间分辨磁输运测量得到了两个铁磁体FM1、FM2的磁化强度M1、M2的时间分辨轨迹。为此，开发了一种新的样品设计--由夹在磁隧道结和自旋阀之间的Syaf组成。它允许第一次在全电装置中独立地推导出两个铁磁体FM1、FM2的磁化动力学。这项实验允许完全跟踪亚微米磁存储单元中Syaf的磁化动力学。研究了不同样品参数(交换耦合强度、自旋补偿等)下的动力学行为，并与数值模拟结果进行了比较。对最近提出的超快进动Syaf MRAM反转方案进行了测试。

The aim of this work is to study the magnetization dynamics of microscopic synthetic antiferromagnets (SyAF) cells on the 10 ps time scale. SyAF consist of two ferromagnetic thin films FM1, FM2 separated by an ultra thin non-magnetic exchange coupling layer (CL) which induces an antiparallel alignment of the magnetization M1, M2 of the two ferromagnets. Such SyAF are now used as the storage layer (or ¿free layer¿) in magnetic random access memories (MRAM) and magnetic logic. So far, the important question of the maximum write rate of a SyAF free layer device has not been answered. How fast can the magnetization of a SyAF free layer cell be reversed? To answer this question the coupled precessional dynamics of the magnetization M1, M2 of the two ferromagnets in the SyAF and their dependence on device parameters is studied. To get a complete picture of the SyAF dynamics the time resolved trajectories of the magnetization M1,M2 of both ferromagnets FM1, FM2, are derived from time resolved magneto transport measurements. For this purpose a novel sample design - consisting of a SyAF sandwiched between a magnetic tunnel junction and a spin valve ¿ is developed. It allows for the first time to derive the magnetization dynamics of both two ferromagnets FM1, FM2 independently in an all electrical setup. This experiment allows to fully track the magnetization dynamics of the SyAF in sub micron magnetic memory cells. The dynamics are studied for different sample parameters (exchange coupling strength, spin compensation, etc¿) and are compared to numerical simulations. Recently proposed ultra fast precessional SyAF MRAM reversal schemes are tested.