Thermal Annealing Effects on the Structural, Magnetic, and Magnetotransport Properties of NiFe/(Ni,Fe)O Bilayers

概要

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In this study, the exchange bias and magnetoresistance of NiFe/(Ni,Fe)O bilayers were investigated. Transmission electron microscopy has shown that the (Ni,Fe)O single layer is a pure rock-salt solid solution ($a= 4.24$ Å). After annealing at 500 °C for 2 h, the (Ni,Fe)O film undergoes a phase transformation from the pure antiferromagnetic (Ni,Fe)O into ferromagnetic Ni ($a= 3.53$ Å) and ferrimagnetic NiFe2O4 ($a= 8.29$ Å) phases. Furthermore, the NiFe/as-deposited (Ni,Fe)O bilayers exhibit a strong temperature dependence of exchange bias field [$H_{\text{ex}}(T)$] that increases linearly with decreasing temperature ($H_{\text{ex}}= -30$ Oe at 150 K). Surprisingly, no $H_{\text{ex}}$ was observed in the NiFe/annealed (Ni,Fe)O bilayers, even at 150 K. This indicates that the AF (Ni,Fe)O phase provides stronger exchange coupling than the ferrimagnetic NiFe2O4. Magnetotransport measurements have shown that these NiFe/(Ni,Fe)O bilayers exhibit anisotropic magnetoresistance (AMR). The NiFe/as-deposited (Ni,Fe)O bilayers exhibit a higher MR ratio than the NiFe/annealed (Ni,Fe)O bilayers, which suggests stronger anisotropic electron scattering between the NiFe and (Ni,Fe)O interfaces in the annealed system.
2009-07-25