Magnetic phases in ordered Ni 3Mn

Abstract

The magnetic domain structure of ferromagnetic MnNi alloys in the composition range near Ni 3Mn has been observed by means of Lorentz microscopy. The domains in the alloys containing 26 and 27 at .% Mn in a thermally demagnetized state consist of small closure domains; the ordered regions are separated by excess Mn atoms. When demagnetized by an alternating field, the domains are extended perpendicular to the direction of the demagnetizing field, the domain width being a few microns. In the alloys containing 23 and 24 at.% Mn, the ferromagnetic phase develops in different ways for different ordering temperatures. In the case of annealing at 420°C, the magnetization is quite homogeneous and the usual 180° and 90° walls with faint ripples are observed. In the case of annealing at 470°C, the magnetic phase develops inhomogeneously. After annealing at 480°C, no magnetic domains are observed; the Curie temperature is at room temperature or lower.