Dependence of exchange bias in NiFe/NiO bilayers on film thickness

Abstract

Here we report on the effect of the ferromagnetic (FM) and antiferromagnetic (AF) films thicknesses on the exchange bias field in a FM/AF bilayer. For this, a series of NiFe($t_{\textrm{NiFe}}$)/NiO($t_{\textrm{NiO}}$) bilayers were grown by DC magnetron sputtering onto commercial Si(001) wafers. Magneto-optical hysteresis loops were used as probes to measure the exchange-bias field, and the coercivity field, as functions of the in-plane angle, $\varphi_H$, and the films' thicknesses, $t_{\textrm{NiFe}}$ and $t_{\textrm{NiO}}$. The in-plane symmetry of the exchange field and coercivity display unidirectional and uniaxial anisotropies, with angular dependences different from the simple $\cos \varphi_H$ and $cos^2 \varphi_H$, respectively. These symmetries are intrinsically sensitive to the thickness of both NiFe and NiO layers. With respect to the FM layer thickness, the exchange bias and coercivity field follow the usual 1/$t_{\textrm{NiFe}}$, while the dependence on the thickness of the AF layer is more complicated, and is characterized by a critical behavior.