Field induced unidirectional anisotropy in NiMn and NiMnPt alloys

Abstract

We report the characterization of the magnetic properties of polycrystalline disks of Ni72Mn28 and Ni(72−x)Mn28Ptx (x=1.0, 4.0, and 10.0) at 4.2K induced by field cooling (FC). It is found that the FC-induced anisotropy field HK, and coercivity HC, are strongly enhanced by the addition of Pt impurities. The remanent magnetization in the direction of the initial applied field (in the disk plane) for each samples can be rotated from 0° to 180° and back to 0° in various stationary fields above and below HK and the parallel component is measured (the longitudinal magnetization component ML). From the analysis of the angular dependence of ML, we show that these results can be accounted for by the coexistence of Mn(Ni)-rich and/or Mn(Ni)-deficient nanoscale regions coupled antiferromagnetically. It is found that the unidirectional anisotropy originates from interfacial exchange interactions between these regions. Up to some critical angle rotation (θc) relative to H, the unidirectional anisotropy field turns rigidly with the sample, while above θc, the coupled regions become unstable and magnetically rearrange such that a unidirectional anisotropy is induced along H. The value of θc varies from region to region. An increase in HK and HC with increasing Pt content can be attributed to the increase in the number of antiferromagnetically coupled regions of decreasing size. In addition, it is found that some ordered single-domain clusters (presumably Ni3Mn compound) are distributed within the disordered Ni or Mn-rich regions.