Comparative study of magnetoresistance and magnetization in nano-oxide specular and nonspecular MnIr/CoFe/Cu/CoFe spin valves from 10 to 300 K

Abstract

The temperature and magnetic field dependences of the magnetoresistance ͑MR͒, superconducting quantum interference device magnetization M, its electrical resistance R and temperature derivative dR/dT ͑10-300 K͒ are reported for nonspecular CoFe/Cu/CoFe spin valves and specular spin valves formed by nano-oxidation of the pinned and free CoFe layers. The MR(T) increases linearly with decreasing temperature on both spin valves, and data extrapolation converges to zero MR practically at the same Curie temperature. At temperatures below ϳ175 K the specular spin valve MR(H) curves present two anomalous bumps not seen in the nonspecular curves. Also, for the nonspecular spin valve a clear relation is visible between M and MR curves, which is not the case for the specular spin-valve curves. In the specular spin valve, dR/dT presents an anomalous enhancement after Tϳ150 K, which is discussed in terms of electron scattering in the nano-oxide layers. The application of a saturating magnetic field suppresses most of the anomaly, indicating its magnetic origin.

Training effect in specular spin valves

“…Notice that M͑H͒ cycles could not reveal any sign of such open loop behavior in our samples, being an indication of the smallness of the lost magnetization. 38 In contrast, GMR measurements, being very sensitive to small deviations from parallelism between pinned and free layers, provide an excellent tool to qualitatively probe even a small interfacial magnetization loss. The spin-glass-like nature of the AFM spin structure was also reported in ␥-Fe 2 O 3 coated Fe nanoparticles, 18,39 wherein a nonsaturation of the magnetic moment was observed in fields up to 5 T; in CoO nanoparticles, 40,41 where the origin of exchange bias was attributed to pinning of the FM magnetization to a spin-glass-like state of uncompensated AFM spins with high anisotropy; and in exchange bias FeNi/FeMn bilayers, 42 wherein the obtained magnetic properties were similar to those of disordered interfaces.…”

Transport and magnetic properties of the Er5Si4 compound

“…Inset: thermopower derivative near the structural transition (O: orthorhombic phase; M: monoclinic phase; PM: paramagnetic phase).experimental details on thermopower, magnetization and electrical resistivity measurements can be found in refs [7,8]…”

Distribution of blocking temperatures in nano-oxide layers of specular spin valves

“…Because both MR and H exch in fact vary with decreasing T cool even down to 15 K, we conclude that the NOL has a broad range of local ͑i͒ blocking temperatures T B ͑i͒, which may arise from different oxide-grain sizes 7 ͑on which the exchange field and blocking temperature are known to depend 8 ͒. 3 The existence of a distribution of blocking temperatures leads each NOL region to start contributing to exchange bias at different temperatures. Therefore, as field cooling runs are performed with successive lower T cool , more NOL regions become AFM-ordered in the ͑H 0 -dependent͒ direction of M pl , and their contribution to the measured H exch is noticed.…”

“…For H 0 ϳ −500 Oe, the magnetization of the pinned layer ͑M pl ͒ switches from positive ͑→͒ to negative ͑←͒ between 320 and 15 K. 3 Thus, M pl is not aligned along the easy axis when crossing T B , and at least part of the interfacial spins of the oxide layer are oriented out of the easy axis. This prevents full antiparallel ͑ ͒ alignment at free-layer switching, giving a low MR ratio.…”