Coercivity of bulk anisotropic nanocomposite Sm(CoFeTi)8–10 magnets

Abstract

The present study investigates the effects of nonmagnetic metal additions on Sm(CobalFe0.2Ti0.05MxB0.01)z (x=0–0.04 and z=8–10) based melt-spun alloys. M=Nb, Ga, Cu, and Al were selected to form anisotropic nanocomposite bulk magnets via hot pressing followed by a hot deformation. The effects of Nb, Ga, Cu, and Al substitutions on intrinsic coercivity Hci and possible mechanisms behind the improvement in Hci, such as phase formations, anisotropy field HA, and microstructure were investigated. Experimental results indicated an over 20%–50% enhancement in Hci after hot deformation (HD) with Nb, Ga, and Cu additions for magnets with z=8 and z=10. Especially with the Nb addition, the Hci improved from 8.7 to 12 kOe for the z=8 magnet, and from 5 to 11 kOe for the z=10 magnet. An unusual enhancement in Hci, from 9 kOe after hot pressing (HP) at 700 °C to 11 kOe after HD at 850 °C, was observed in the Nb-doped magnet with z=10. Our analysis on possible mechanisms behind the improvement in Hci indicated that Ga and Cu atoms tend to accompany and thus likely to stabilize a 1:5H phase, which possesses a higher anisotropy field HA, resulting in an improvement of Hci. Nb atoms tend to localize at the grain boundaries of (1:5H or 2:17R) phases and form a Nb-rich CoFe phase with average sizes around 50 nm. This phase is believed to suppress grain growth during HP and HD resulting in a stronger Hci. A subgrain structure (∼50 nm) within the apparently larger grains (a few hundred nanometers) of 1:5H or 2:17R phases was observed in the Nb-doped HD-magnet of z=10. The formation of these subgrains could be the mechanism responsible for increased Hci values after HD. Al atoms did not show any particular location preference and positive effect on Hci. The highest (BH)max of 13.4 MG Oe was obtained on anisotropic magnets of Sm(Co0.74Fe0.2Ti0.05B0.01)8.