Surface-induced low-field instability of antiferromagnetic multilayers

Abstract

We discuss the surface-induced low-field instability of the antiferromagnetic phase of magnetic multilayers. The threshold field is calculated analytically for multilayers of arbitrary thickness containing an even number of layers. We show that the threshold is given by ${H}_{\mathrm{SSF}}=\sqrt{{H}_{e}{H}_{a}{+H}_{a}^{2}},$ where ${H}_{e}$ and ${H}_{a}$ are the effective exchange and anisotropy fields. The effective anisotropy field ${H}_{a}$ may include both uniaxial and fourfold crystalline anisotropy. Numerical simulations of the equilibrium phases, based on a self-consistent effective field method, are used to obtain the magnetization pattern. We find that thick uniaxial multilayers display a three-stage transition from the antiferromagnetic to the field-aligned phases, whereas in thin multilayers the transition is from the antiferromagnetic to a nearly spin-flop structure, which gradually aligns with the applied field. If the films composing the multilayer have uniaxial and crystalline anisotropy, the magnetization profile in the multilayer and the nature of the transition depend on the relative values of the uniaxial and crystalline anisotropies.