Variation of magnetic structure with order in the Ni3Mn superlattice

Abstract

The magnetic domain structure has been studied extensively by transmission electron microscopy techniques in the Ni₃Mn alloy, which becomes strongly magnetic with increasing atomic order. It has been found that the configurations of intermediate equilibrium long-range order consist of a mosaic structure of long-range ordered single non-contiguous magnetic domain rods in a disordered non-magnetic matrix, similar to that observed in the Alnico V structure. Magnetic domain walls are therefore not observed in these alloys. The fully ordered alloy, on the other hand, as well as alloys possessing homogeneous configurations of imperfect long-range order, show a well-defined magnetic domain wall structure. In addition some complex magnetic domain wall configurations in the vicinity of grain and twin boundaries have been observed and analysed. Long tails which are exhibited by the magnetization—temperature curves for alloys possessing imperfect order, and which are eliminated as the order becomes perfect, have also been studied. These tails have been discussed in terms of a model in which the Curie temperature of the alloy is determined by the individual Curie temperatures associated with each magnetic spin, which in turn depends on the nature of its nearest-neighbour spins. An important feature of this aspect of the investigation is that magnetic domains are observed at temperatures well within these tails, suggesting that the Curie temperature determined by extrapolation techniques is significantly lower than the true Curie temperature. Lastly, extensive measurements of the manner in which the magnetic remanence is affected by order have also been made, and are discussed in detail.