Positive Exchange Bias in GdFe/NiCoO Thin Films

Thin films of GdFe/NiCoO are one of the few systems that exhibit positive exchange bias [1-4]. In this study, we show that the positive bias in Gd$_{x}$Fe$_{1-x}$/NiCoO is sensitive to the GdFe composition and the field cooling sequence. In particular, the hysteresis loops are often bifurcated due to the existence of multiple phases: a low anisotropy phase with a single reversal in small fields and a higher anisotropy phase with a single or double loop, depending on the GdFe stoichiometry. In Fe-rich samples, increasing the cooling field causes the low anisotropy phase to shift from negative to positive bias and the double-loop high anisotropy phase to shift toward negative bias. In Gd-rich samples, the low anisotropy phase is always positively biased and the single-loop high anisotropy phase is always negatively biased for all cooling field strengths. These behaviors are a result of the parallel and antiparallel couplings between different magnetic phases of GdFe with the NiCoO layer. [1] J. Nogues\textit{, et al.}, Phys. Rev. Lett. \textbf{76}, 4624 (1996). [2] S. Mangin\textit{, et al.}, Phys. Rev. B \textbf{68}, 140404 (2003). [3] X. Ke\textit{, et al.}, Appl. Phys. Lett. \textbf{84}, 5458 (2004). [4] D. Z. Yang\textit{, et al.}, Phys. Rev. B \textbf{71}, 144417 (2005).