IrMn 교환결합층을 갖는 스핀밸브막의 자기적특성과 열적안정성에 관한 연구
(A) Study on Thermal Stability and Magnetic Properties in Spin Valve Films with IrMn Exchange Biased Layers
IrMn 교환결합층 스핀밸브막 자기적특성;
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To determine thermal stability and magnetic properties in spin valve films with IrMn exchange biased layers, exchange biasing properties of bilayers containing Co, CoFe, and NiFe were investigated by varying thickness and annealing temperatures. In order to understand the thermal degradation of magnetoresistance (MR) of spin valves containing Co, CoFe, and NiFe, we have studied relationships between changes in the magnetic structure and magnetotransport properties upon annealing. The MR measurements were made at annealing temperature (T_(ANN)) and room temperature (T_(RT)) upon subsequent cooling. The remagnetization processes in the spin valve films were observed by means of visualizing the Magneto-optical indicator film (MOIF) technique with digital imaging processer. We found that the exchange bias field is inversely proportional to 1) the magnetization, 2) the thickness of the pinned layer, and 3) the grain size of antiferromagnetic layer. In addition, the exchange bias field is related to the interfacial exchange energy difference, which is expected to depend on the surface roughness. The MR ratio at annealing temperature (T_(ANN)) decreased linearly with temperature increase and the MR properties disappeared at about 260℃. The MR ratio at room temperature (T_(RT)) after annealing was essentially not changed up to 260℃. However, the MR ratio rapidly decreased above 260℃. MOIF pictures of as-deposited spin valve containing NiFe showed the formation and motion of domain walls separating large domains in the free film, unlike the spin valves containing Co and CoFe in which the magnetization reversal is due to the nucleation, growth, and annihilation of many small domains. MOIF images of spin valve containing NiFe after annealing at 390℃ show regions of antiferromagnetically coupled ferromagnetic layers, which appeared in ferromagnetically coupled area after procedure of cooling from annealing, and it revealed that increasing annealing temperature leads to the change in the exchange coupling between two ferromagnetic layers through Cu space layer. This result is in accordance with magnetoresistance ratio, which changes as a function of an applied magnetic field. Based on the analysis of the MR and MOIF data obtained, it is concluded that pinning interactions, that are not strongly affected by the exchange bias and enhanced coercivity of pinned ferromagnetic layer, only weakly depend on the annealing temperature.