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 4.2 K induced by field cooling (FC). It is found that the FC-induced anisotropy field H-K, and coercivity H-C, 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 degrees to 180 degrees and back to 0 degrees in various stationary fields above and below H-K and the parallel component is measured (the longitudinal magnetization component M-L). From the analysis of the angular dependence of M-L, 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 (theta(c)) relative to H, the unidirectional anisotropy field turns rigidly with the sample, while above theta(c), the coupled regions become unstable and magnetically rearrange such that a unidirectional anisotropy is induced along H. The value of theta(c) varies from region to region. An increase in H-K and H-C 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. (C) 2013 Elsevier B.V. All rights reserved.