Mechanism for enhanced unidirectional spin-glass behavior in layered Mn-Ni/Co structures

Abstract

Magnetization measurements on compositionally modulated structures (CMS) consisting of alternating layers of Mn and Co or Ni show spin-glass behavior for CMS of equal layer thicknesses of less than 25 Å for Ni and 15 Å for Co. Some of the Mn-Ni CMS are found to have large uniaxial and unidirectional anisotropy fields of the order of 1500 Oe. We propose a mechanism for the origin of these anisotropy fields. The d conduction electron polarization (dCEP) is known to be responsible for the antiferromagnetic coupling between the localized moments on adjacent Mn atoms and the ferromagnetic coupling between Mn-Ni or Co and Ni-Ni or Co-Co pairs. The character (AF or FM) of the dCEP coupling is determined by the density of itinerant d electrons in the vicinity of the atoms. Since Mn, Ni, and Co have different d conduction electron densities in their vicinity, any grouping of three or more atoms containing both Mn and Ni/Co atoms will have an anisotropic unidirectional dCEP distribution associated with it. Thus the anisotropic character of the dCEP in an inhomogeneous mixture of Mn with Ni or Co provides a mechanism for producing unidirectional spin-glass behavior. From simple geometric considerations the layering of Mn and Co/Ni intrinsically causes preferred directions for the anisotropy of the dCEP which, as observed, can give rise to large unidirectional fields in the CMS. This same mechanism should also be operative in any random alloy system although it will be enhanced in the layered structures of CMS films.