Origin of the reduced exchange bias in an epitaxial FeNi(111)/CoO(111) bilayer

Abstract

We have employed soft and hard x-ray resonant magnetic scattering and polarized neutron diffraction to study the magnetic interface and the bulk antiferromagnetic domain state of the archetypal epitaxial Ni 81 Fe 19 ͑111͒ / CoO͑111͒ exchange biased bilayer. The combination of these scattering methods provides unprecedented detailed insights into the still incomplete understanding of some key manifestations of the exchange bias effect. We show that the several orders of magnitude difference between the expected and measured value of exchange bias field is caused by an anisotropic in-plane orientation of antiferromagnetic domains. Irreversible changes in their configuration lead to a training effect. This is directly seen as a change in the magnetic half-order Bragg peaks after magnetization reversal. The antiferromagnetic domain size is extracted from the width of the ͑ 1 2 1 2 1 2 ͒ antiferromagnetic peak by both neutron and x-ray scattering and is determined to be 30 nm in size. A reduced blocking temperature as compared to the measured antiferromagnetic ordering temperature clearly corresponds to the blocking of antiferromagnetic domains. Moreover, an excellent correlation between the size of the antiferromagnetic domains, exchange bias field, and frozen-in spin ratio is found, providing a comprehensive understanding of the origin of exchange bias in epitaxial systems.

Perpendicular exchange bias in ferrimagnetic spin valves

“…S1, we show that the coercive and EB fields remain stable as a function of the loop index, demonstrating that a training effect is essentially absent. A magnetic multi-domain state in the pinning layer not only diminishes drastically the absolute value of the EB field but it also mediates a strong EB reduction upon consecutive cycling (training effect) 12,21 .…”

“…For instance, in an archetypal CoO/FM bilayer, neutron scattering has revealed the origin of reduced EB. It was shown to be related to the anisotropic domain state of the AF system 12 . Using soft X-ray techniques, the frozen and rotatable spins 12,14,15 were observed in several systems, leading to a deeper understanding of the enhanced coercivity and the origin of the EB for systems with uncompensated AF interfaces.…”

“…Yet, these two magnetic components are difficult to access experimentally with laboratory tools because of a virtually vanishing magnetization of AF layers and a small volume of the magnetic interface. Important progress has been made by involving neutron and Xray techniques to study the bulk part of the pinning layer and the interfacial magnetism [6][7][8][9][10][11][12][13] . For instance, in an archetypal CoO/FM bilayer, neutron scattering has revealed the origin of reduced EB.…”

Field and temperature-driven magnetic reversal of spin-flop coupled epitaxial Fe/MnPd bilayers

“…Despite a number of studies on the interesting individual spin behaviors, the various long-range effects, such as the lateral AF spin arrangement 9,10 and the AF bulk effect 11,12,13 , have not been fully investigated. Actually, such effects are quite critical especially in epitaxial samples where both structural coherence and magnetic ordering are optimized.…”

The Anisotropy Induced by a Magnetostriction in Exchange-Biased Two-Layer Films

“…The possibility to control the state of coupled AF/FM bilayers requires investigation of the magnetic mechanisms that could be responsible for bias effect. Many researchers [2][3][4][5][6][7] emphasize the important role of the AF domain structure in the establishing of the exchange bias. The problem of AF domains is intimately related with magnetoelastic coupling [8] and can strongly depend upon the mechanical stress that appears at the FM/AF interface due to the lattice misfit.…”