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\nPolarised Neutron Diffraction to study the magnetic interface and the bulk\nantiferromagnetic domain state of the archetypal epitaxial\nNi$_{81}$Fe$_{19}$(111)/CoO(111) exchange biased bilayer. The combination of\nthese scattering tools provides unprecedented detailed insights into the still\nincomplete understanding of some key manifestations of the exchange bias\neffect. We show that the several orders of magnitude difference between the\nexpected and measured value of exchange bias field is caused by an almost\nanisotropic in-plane orientation of antiferromagnetic domains. Irreversible\nchanges of their configuration lead to a training effect. This is directly seen\nas a change in the magnetic half order Bragg peaks after magnetization\nreversal. A 30 nm size of antiferromagnetic domains is extracted from the width\nthe (1/2 1/2 1/2) antiferromagnetic magnetic peak measured both by neutron and\nx-ray scattering. A reduced blocking temperature as compared to the measured\nantiferromagnetic ordering temperature clearly corresponds to the blocking of\nantiferromagnetic domains. Moreover, an excellent correlation between the size\nof the antiferromagnetic domains, exchange bias field and frozen-in spin ratio\nis found, providing a comprehensive understanding of the origin of exchange\nbias in epitaxial systems.\n