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

Abstract

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 angstrom). After annealing at 500 degrees 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 angstrom) and ferrimagnetic NiFe(2)O(4) (a = 8.29 angstrom) phases. Furthermore, the NiFe/as-deposited (Ni,Fe)O bilayers exhibit a strong temperature dependence of exchange bias field [H(ex) (T)] that increases linearly with decreasing temperature (H(ex) = -30Oe at 150 K). Surprisingly, no H(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 NiFe(2)O(4). 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. (C) 2009 The Japan Society of Applied Physics