Analysis and control of spin polarization of half-metallic ferromagnets by the use of runneling junctions

为了阐明影响铁磁体界面自旋极化的因素，研究了Co_<0.9>Fe_<0.1>/SrTiO_3/La_<0.7>Sr_<0.3>MnO_3 （LSMO）的隧道结。通过4.2 K下的结磁电阻（MR）评价和高分辨率卢瑟福后向散射技术，发现界面降解的原因是SrTiO_3势垒中的氧缺乏。根据MR偏置电压(V)依赖性推导出的Co_<0.9>Fe_<0.1>界面的局部态密度与第一原理计算结果吻合较好，表明Co_<0.9>Fe_<0.1>界面是高质量的界面。在LSMO结中，MR效应在200 K左右的温度下消失，远低于360 K的铁磁转变温度T_c，这表明在该温度下LSMO界面的自旋极化降至零。作为LSMO的替代品，研究了高T_c半金属铁磁体Sr_2FeMoO_6和Fe_3O_4的基本性质。对在Ar+H_2气氛中溅射生长的Sr_2FeMoO_6薄膜，研究了晶格失配引起的应变及其弛豫过程对薄膜磁化和输运性能的影响，并通过晶格匹配缓冲层技术实现了高质量的全磁化薄膜。这表明Sr_2FeMoO_6薄膜在基态时具有半金属性。考虑到Sr_2FeMoO_6的高T_c (42O K)，期望在RT下具有0.7的高自旋极化。此外，Fe_3O_4还获得了具有Verway转变的高质量超薄膜。为了实现高t_n结，绝缘阻挡材料如A1N被证明是有前途的。

Tunnel junctions of Co_<0.9>Fe_<0.1>/SrTiO_3/La_<0.7>Sr_<0.3>MnO_3 (LSMO) were investigated in order to elucidate the factors affecting the spin polarization of interfaces of ferromagnets. Through the evaluation of junction magnetoresistance (MR) at 4.2 K and the high resolution Rutherford backscattering technique, it is found that the origin of inter facial degradation is the oxygen deficiencies in the SrTiO_3 barriers. The local densities of states at the Co_<0.9>Fe_<0.1> interface deduced from the bias voltage (V) dependence of MR conincide well with the results of the first principle calcualtion, indicating that the Co_<0.9>Fe_<0.1> interface is of high quality. In the LSMO juncttions, the MR effects disappear at temperatures of around 200 K well below the ferromagnetic transition temperature T_c of 360 K, which indicates that the spin polarization of LSMO interface goes down to zero at the temperatures.As an alternative of LSMO, high T_c half-metallic ferromagnets Sr_2FeMoO_6 and Fe_3O_4 were investigated with respect to their fundamental properties. For thin films of Sr_2FeMoO_6, which were grown by sputtering in an Ar+H_2 gas atmosphere, the effects of lattice-mismatch induced strain and its relaxation process on the magnetization and transport properties have been elucidated, and high quality films with a full magnetization have been realized by measns of a lattice-matched buffer layer technique. This strongly indicates the half metalicity of Sr_2FeMoO_6 films at the ground state. Considering the high T_c (42O K) of Sr_2FeMoO_6, a high spin polarlization of 0.7 at RT is expected. Moreover, as for Fe_3O_4, high quality urtra thin films showing Verway transition have been obtained. For realization of high T_cjunctions, insulating barrier materials such as A1N is shown to be promising.