Yuji Ishii

Improved tunneling magnetoresistance in interface engineered (La,Sr)MnO3 junctions

Spin tunnel junctions were fabricated and characterized with various insulating barriers and interface structures employing (La,Sr)MnO3 (LSMO) as the ferromagnetic electrodes. Junctions with LaAlO3 barriers exhibited systematically larger tunnel magnetoresistance (TMR) (TMR ratio ∼230% at 10K) than those with SrTiO3 barriers (∼50%). When two unit cells of nondoped LaMnO3 are inserted between LSMO and SrTiO3 at both interfaces in the SrTiO3-barrier junction, the TMR was also significantly enhanced to ∼170%. These results, including the temperature dependence of TMR, qualitatively agree with the characteristics of the local magnetization at the interface that was evaluated by magnetization-induced second harmonic generation (MSHG) for the corresponding interface structures. However, slight deviations appear as a systematic suppression of TMR for all the junctions at high temperatures compared with MSHG results. Also, TMR results appear to be more degraded than MSHG results for the SrTiO3-barrier junctions. ...

Perovskite manganite magnetic tunnel junctions with enhanced coercivity contrast

We have fabricated (La,Sr)MnO3 (LSMO)-based magnetic tunnel junctions with Ru-doped LSMO (LSMRO) film used as one of the electrodes. The Ru doping enhances the coercivity of LSMO, resulting in a sharp steplike tunnel magnetoresistance (TMR) response with a high contrast in the coercive field (Hc) for magnetization reversal. Although the difference of the Hc between the top LSMRO and the bottom LSMO electrodes decreases with increasing the temperature, it is large enough to give a clear TMR response even at a relatively higher temperature region of around 200K. By changing the dimensions of the top LSMRO electrode, the Hc is found to have no dependence on the geometric aspect variations of the top electrode but slightly on the junction area.

Precipitate-free films of La1−xSrxMnO3 grown on the substrates with artificial step edges

We have fabricated precipitate-free La0.7Sr0.3MnO3 (LSMO) films by locating precipitate nucleation at artificial step edges patterned on SrTiO3 substrates and absorbing excess species of off-stoichiometry by getter effect. The precipitate-free and atomically flat area of about 10μm width can be obtained along the step edges by pulsed-laser deposition at a substrate temperature Ts of ∼900°C. Thermally activated processes are shown to be involved in such chemical reactions between the precipitates and surface migrating species. LSMO∕SrTiO3∕LSMO tunnel junctions fabricated by this method showed an excellent uniformity of the junction resistances.

Negative spin polarization in (La,Sr)CoO3 probed by a magnetic tunnel junction with (La,Sr)MnO3

By using perovskite (La,Sr)MnO3 (LSMO)-based magnetic tunnel junctions with LaAlO3 barrier, we have experimentally revealed the spin state of (La,Sr)CoO3 (LSCO) that was employed as the counterelectrode of the junctions. Inverse tunnel magnetoresistance (TMR) up to −14% was observed at 10K in low-bias measurement. This indicates that t2g↓ electrons are the majority at the Fermi level in the LSCO band structure, taking into account the half-metallic character of LSMO. The temperature dependence of the interfacial spin polarization of LSCO as deduced from the TMR ratio indicates the reduction of the Curie temperature at the interface by 70K. The TMR ratio exhibits an asymmetric bias dependence in qualitative agreement with the calculated spin band structure of LSCO.