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

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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.