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 SSF ϭͱH e H a ϩH 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.

Stability of ferrimagnetic multilayers

“…The critical field for instability of the AF order in finite multilayers has been calculated analytically, combining the simultaneous effect of uniaxial and four-fold crystalline anisotropy of the F layers. It was shown that the relative orientation of the anisotropy easy directions may affect the nature of the low field phase transition [8]. However, for finite multilayers [5,7,8] where H E and H A are the effective interlayer exchange field and the anisotropy field, in agreement with the prediction in the semi-infinite geometry [4].…”

“…where for the two choices of m z mentioned above, we have used aZ bðHhC a 1 bC 1=2Þ; bZKb/2 and cZ ðGhC a 2 C b=2Þ: Notice that M 4 has the same structure of the corresponding matrix used in the study of the stability of AF multilayers [8], with a redefinition of the parameters a, b and c for the ferrimagnetic case. From the structure of M 4 , it is clear how to obtain M 2 and also how to proceed in order to obtain matrices for larger values of N. All the algebraic manipulations used previously [8] to determine the critical field which produces instability of the AF order of AF multilayers are also valid for ferrimagnetic multilayers.…”

“…It was shown that the relative orientation of the anisotropy easy directions may affect the nature of the low field phase transition [8]. However, for finite multilayers [5,7,8] where H E and H A are the effective interlayer exchange field and the anisotropy field, in agreement with the prediction in the semi-infinite geometry [4]. Field induced phase transitions of AF multilayers have also been examined from different perspectives, exploring other theoretical methods to obtain the magnetic ground state of the multilayer in the presence of external field [9].…”

“…This study has been done for AF multilayers [8]. In the case of an AF multilayer [8], the off-diagonal elements of M N correspond to the exchange coupling of neighboring layers. The diagonal elements of M N alternate between two values correspond to positive and negative Zeeman energies.…”

“…AF multilayers [4,5,[7][8][9] consist in the stacking of equivalent F layers (or atomic planes). In this paper, we consider ferrimagnetic multilayers, consisting of an AF stacking alternating two different ferromagnetic layers (F 1 and F 2 ).…”