Exchange Anisotropy in Cu-Mn and Ag-Mn Alloys

Cu-Mn and Ag-Mn alloy specimens of about 25 atomic % Mn were cooled to 4.2°K in a magnetic field, and it was found that their hysteresis loops measured parallel to this field were displaced from their symmetrical positions about the origin. The displacement of the loops decreases monotonically with increasing temperature, and its disappearance is accompanied by large hysteresis losses. These unusual properties are similar to those previously reported for disordered Ni-Mn alloys and are also attributed to exchange anisotropy interactions between small regions of ferromangetic and antiferromagnetic order. It is proposed that these regions arise from statistical composition fluctuations in the alloys and the existence of both ferromagnetic and antiferromagnetic interactions between Mn atoms. The essence of this microscopic exchange anisotropy model is illustrated for a square lattice representation of these alloys. The magnetic transformation temperatures are identified with anomalies in the $\text{1/χ}$ vs $T$ curves; these temperatures are considerably higher than those at which the exchange anisotropy effects disappear.