Fabrication of epitaxial heterostructures with halrf-metallic ferromagnets and development of spin-controlled devices

本研究的目的是利用钴基Heusler合金（Co_2YZ）的半金属丰度来开发自旋电子器件。制备了Co_2YZ薄膜或Co_2YZ薄膜与MgO势垒的完全外延磁隧道结（MTJs），其界面具有突兀和极其光滑的特点。此外，制备的Co_2YZ/ mgo基MTJs在室温下具有较高的隧道磁阻（TMR）比。主要研究结果总结如下：1)开发了基于Heusler合金的MTJ器件技术。该器件技术具有以下特点：a) MTJ三层结构中的所有层均为外延单晶结构；b) Co_2YZ (Co_2Cr_<0.6>Fe_<0.4>Al （CCFA), Co_2MnSi （CMS）或Co_2MnGe）薄膜与MgO隧道势垒之间的界面为突然的原子平面界面；c) MgO势垒下方的CMS薄膜界面区域未被氧化。2)制备的CCFA/MgO/Co_<50>Fe_<50> MTJs在室温下的TMR比高达109%（在4.2 K时为317%）。3)制备了以CMS薄膜为上下电极并具有MgO势垒的全外延交换偏置MTJs。上部CMS电极沉积后，随原位退火温度（T_a）的增加，在RT和4.2 K下的TMR比均有所增加。此外，在RT下（4.2 K时为683%），TMR比率高达179%。综上所述，由Co_2YZ薄膜和MgO势垒组成的外延单晶异质结构在利用co基Heusler合金的半金属性的自旋电子器件中非常有前途。

The purpose of the present study was to develop spintronic devices utilizing the half-metallicity of Co-based Heusler alloys (Co_2YZ). Fully epitaxial magnetic tunnel junctions (MTJs) with a Co_2YZ thin film or Co_2YZ thin films and a MgO barrier featuring abrupt and extremely smooth interfaces were fabricated. Furthermore, high tunnel magnetoresistance (TMR) ratios were demonstrated at room temperature (RT) for fabricated Co_2YZ/MgO-based MTJs. The main results are summarized as follows.1) The Heusler alloy-based MTJ device technology was developed. This device technology features the followings: a) all layers in the MTJ trilayer structures are epitaxial and single-crystalline, b) abrupt and atomically flat interfaces between a Co_2YZ (Co_2Cr_<0.6>Fe_<0.4>Al (CCFA), Co_2MnSi (CMS), or Co_2MnGe) thin film and a MgO tunnel barrier, and c) the interface region of CMS thin films underneath a MgO barrier is not oxidized.2) The fabricated CCFA/MgO/Co_<50>Fe_<50> MTJs demonstrated a high TMR ratio of 109 % at RT (317% at 4.2 K).3) Fully epitaxial exchange-biased MTJs with CMS thin films as both lower and upper electrodes and with a MgO barrier were fabricated.The TMR ratios at both RT and 4.2 K increased with increasing the in-situ annealing temperature (T_a) just after the deposition of the upper CMS electrode. Furthermore, a high TMR ratio of 179% at RT (683% at 4.2 K) was demonstrated.In summary, it was demonstrated that epitaxial, single-crystalline heterostructures consisting of Co_2YZ thin films and a MgO barrier are highly promising for spintronic devices that utilize the half-metallicity of Co-based Heusler alloys.