Development of magnetic tunnel junctions using Heusler alloy and observation of spin-injected magnetization reversal

1. 利用高质量的Heusler合金电极和MgO晶体势垒制备的磁隧道结在室温和低温下分别达到217%和753%的大隧道磁阻（TMR）。MTJ的结构为Co2MnSi/MgO/CoFe。观察到的TMR比在使用Heusler合金电极的MTJs中是最高的。此外，对MTJ的电导电压特性的详细研究表明，大TMR的起源是通过晶体MgO势垒的相干隧穿过程。利用（Co_<50>Fe_<50>）_<100-x>B_x （x= 20,25,30，厚度d=2 nm）的开关层，成功地制备了CoFeB/MgO/CoFeB-MTJs的自旋注入磁化反转。结果表明，开关电流密度与铁磁层的磁化强度、阻尼常数和自旋极化有关，与理论预测一致，减小磁化强度和阻尼常数对降低开关电流密度是非常有效的。用FMR技术系统地研究了各种铁磁薄膜的磁阻尼常数。此外，还建立了一种估计MTJs阻尼常数的测量技术。结果发现，2nm厚的CoFeB薄膜的阻尼常数是本体CoFeB的5倍，并且开关层的相邻层影响开关层的阻尼常数。

1. Fabrication of magnetic tunnel junctions using epitaxial Heusler alooy electrodes Large tunnel magneto-resistance (TMR) ratios of 217% at room temperature and 753% at low temperature have been successfully observed in a magnetic tunnel junction (MTJ) using a high quality Heusler alloy electrode and an MgO crystalline barrier. The structure of the MTJ was Co2MnSi/MgO/CoFe. This observed TMR ratio is highest among the MTJs using Heusler alloy electrodes. Moreover, conductance-voltage property of the MTJ investigated in detail indicates that the origin of the large TMR was coherent tunneling process through the crystalline MgO barrier.2. Micro-fabrication of MTJs and Observation of spin-injected magnetization reversal Spin-injected magnetization reversal has been successfully observed in the micro-fabricated CoFeB/MgO/CoFeB-MTJs using (Co_<50>Fe_<50>)_<100-x>B_x (x=20, 25, 30, thickness d=2 nm) switching layers. We found from this result that the switching current density depended on magnetization, damping constant and spin polarization of the ferromagnetic layer as expected by theory and reducing of magnetization and damping constant was quite effective to decrease the switching current density.3. Magnetic damping constant α Magnetic damping constants in various ferromagnetic thin films were investigated systematically by FMR technique. Moreover, a measurement technique for estimation of damping constants in MTJs has been established. As a result, we found that the damping constant of 2nm thick CoFeB film was five times larger than that of bulk CoFeB and the neighboring layer of the switching layer influence the damping constant of the switching layer.