Magnetic states and structural transformations in Sm(Co,Cu)5 and Sm(Co,Fe,Cu)5 permanent magnets

Abstract

We have studied the stability of RCo5−xCux (R = Y, Sm) compounds with respect to phase separation. First principles density functional calculations imply that (i) decomposition into two phases having different x is energetically favourable and (ii) both the stable x values and the Cu atomic site preferences depend on the magnetic state of the alloys. Guided by this result, we studied the structure and magnetic properties of different Sm(Co,Cu)5 and Sm(Co,Fe,Cu)5 alloys. Separation into two chemically dissimilar Sm(Co,Cu)5 phases is typical for the as-made Sm(Co,Cu)5 alloys. We also observed in different alloys a universal correlation between the room-temperature coercivity and the magnetic state at the temperature of annealing. The coercivity increases significantly if annealed 100–140°C below the Curie temperature; in particular, for SmCo2.25Fe0.75Cu2, the room-temperature coercivity increases from 12.3 to 37.3 kOe. The possibility of different magnetic state-dependent structure transformations is discussed. The experimental results do not support the spinodal decomposition theory, so we suggest that the coercivity increase might be caused by a change in preferred atomic site occupancies.