Anomalous temperature dependence of coercivity in rare earth cobalt magnets

Abstract

We report on the anomalous nonmonotonic temperature dependence of coercivity H c( T) which has been observed in bulk-hardened 2:17 magnets. This anomalous effect appears to be universal for all R(Co,Fe,Cu,Zr) z magnets (R=Sm and Y) with different amount of Cu having a cellular (or cellular-like) microstructure consisting of R 2Co 17 cells surrounded by the RCo 5 cell boundary phase. Cu substitutes mostly in the RCo 5 cell boundary phase leading to dilution of its intrinsic magnetic properties including the anisotropy field and Curie temperature. The anomalous behavior in H c( T) is explained by both the domain wall pinning and nucleation models. Domain wall pinning explains the anomalous behavior as a temperature-induced transition at a critical temperature T cr from a low H c repulsive DW pinning ( γ 1:5⪢ γ 2:17, γ is domain wall energy density) to a high H c attractive pinning ( γ 2:17⪢ γ 1:5). The nucleation model can be used in the case of complete magnetic isolation of the 2:17 cells. This condition is expected to be fulfilled at any temperature above RT in the Cu-rich magnets (nonmagnetic R(Co, Cu) 5 cell boundaries), and only above the T c of the Cu substituted 1:5 phase in the magnets with lower Cu content ( T c(1:5)< T c(2:17)).