Exchange anisotropy of ferromagnetic/antiferromagnetic bilayers: Intrinsic magnetic anisotropy of antiferromagnetic layer and single spin ensemble model

The origin of the magnetic anisotropy of the antiferromagnetic (AF) layer and the role of it in the magnetization process of exchange coupled ferromagnetic/antiferromagnetic bilayers are discussed. Through the magnetic torque analysis of a pseudo-single crystalline Ni-Fe/Mn-Ni bilayer and a polycrystalline Ni-Fe/Mn-Ir bilayer, the magnetocrystalline anisotropy of the antiferromagnet is strongly suggested to be the origin of the magnetic anisotropy of the antiferromagnetic (AF) layer. The single spin ensemble model is newly introduced for polycrystalline bilayers, taking into account the two-dimensionally random distribution of the magnetic anisotropy axes of the AF grains. The mechanism of a well-known experimental fact, the reversible induction of the exchange anisotropy along desirable directions by field cooling procedure, is successfully elucidated with the new model.