Study on non-strain single crystal MR junctions using half metallic ferromagnetic thin films.

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The methods to fabricate the high quality and single crystal half metallic perovskite ferromagnetic (La_<0.7>Sr_<0.3>MnO_3 (LSMO), Sr_2FeMnO_6 (SFMO), Sr_2CrReO_6(SCRO)) films were established, where the magnetron sputtering method was employed. The conditions to make the non-strain single crystal TMR junctions using the high quality perovskite oxides are followings. 1) The misfit between substrate and film should be minimized. 2) In order to minimize the mismatching, furthermore, it is better that an intermediate layer such as Ba_<0.4>Sr_<0.6>TiO_3 is grown between substrate and film. 3) The oxygen partial pressure is important in cooling process after growth of the film. Too small partial pressure deletes the oxygen atoms in film and too much pressure make the non ferromagnetic oxides layer. The pressure should be optimized. 4) In the case of argon gas milling in the process of fabrication of MR devices, the film was damaged by the strain and the deletion of oxygen atoms from film su … More rface layer (about 40nm). Sometimes, the electrical resistance was extremely increased at low temperatures due to the damage. The damage was recovered by the annealing above 900℃ in the oxygen atmosphere. It is important to anneal in the fabrication process of MR devices. We optimized the above conditions. Then, the TMR ratios of 64% for LSMO/SrTiO_3/LSMO, 14% for SFMO/ SrTiO_3/Co and 114% for SrCrReO/MgO/Co junctions were observed. The spin polarization of the ferromagnetic half metals was measured by the Andreev reflection method using NbN superconductor layer, inserting an insulator layer between NbN and the half metal. The methods to fabricate the high quality single crystal films or the preferred oriented films of Heusler alloys such as Co_2MnGe, Co_2MnGa, and Co_2MnSi were established. TMR junctions composed by (100) or (110) preferred oriented Co_2MnGe film was fabricated on a Si substrate. The origin of half metallic property of Co-based Heusler alloys or non half metallic property of Ni-based Heusler alloys was discussed by the band calculations and experiments. Less