Effects of thermomagnetic treatments on the magnetism of Cu–Al–Mn alloys

Abstract

Measurements of the magnetic properties of Cu–Al–Mn alloys are presented. We studied three alloys with compositions of the DO3 + L21 two-phase field occurring at low temperatures. In all studied alloys, annealing at intermediate temperatures promotes the development of magnetic properties due to the formation of ferromagnetic L21 particles in a paramagnetic DO3 matrix. The evolution of magnetization with temperature was measured. Upon heating, the magnetization disappears, which is due to demagnetization and dissolution of the L21 precipitates. After subsequent cooling, the Mn-rich alloys regain some magnetization, while the sample with the lowest Mn content has very low magnetization. The magnetic hysteresis measurements show that after cooling, the saturation magnetization decreases and the coercive field increases. The application of an external magnetic field during cooling appears to change the volume fraction and size of the L21 precipitates. The results concerning the formation of the L21 ferromagnetic phase by spinodal decomposition and its demagnetization and dissolution at temperatures above the miscibility gap are discussed. The differences found after the two thermomagnetic treatments are related to the possibility of different kinetics for each composition and to heat treatments with and without applied magnetic fields.