Anomalous temperature dependence of coercivity and reversal mechanism in bulk-hardened rare earth-cobalt magnets

Abstract

The “anomalous” nonmonotonic temperature dependence of coercivity, reported in Sm–Zr–Co–Cu magnets, has also been observed in bulk-hardened Y–Zr–Co–Fe–Cu alloys with a similar microstructure. The phenomenon appears to be universal for all R–Co magnets (R=rare earth) having a microstructure consisting of R2Co17 cells surrounded by the RCo5 phase. The effect of R and Cu on the temperature dependence of coercivity cannot be simply explained by traditional domain-wall pinning model based on the difference in a domain wall energy. Possibility that the coercivity is controlled by nucleation of reversed domains in magnetically isolated R2Co17 cells is discussed.

Kinetical analysis of the heat treatment procedure in SmCo5 and other rare-earth transition-metal sintered magnets

“…In this situation, there are islands of Sm 2 (Co, Cu) 17 phase separated by a paramagnetic Sm(Co, Cu) 5 . Then, these magnets would be better classified as nanocrystalline nucleation-controlled magnets [11,37].…”

Coercivity Analysis in Sm(CoFeCuZr)z Magnets with Abnormal Temperature Behavior

“…The nucleation hypothesis has been considered as a probable coercivity mechanism in the Sm(Co,Fe,Cu,Zr) z magnets for the region above the Curie temperature of Sm(Co,Cu) 5 , 550 C. In this case, the 2 : 17 phase would be totally insulated by a paramagnetic phase, and the models that suppose pinning would not be valid any longer [21]. There is another possibility deserving to be discussed: Could the Sm(Co,Fe,Cu,Zr) z magnets, at least partially, be nucleation controlled over the whole range of temperatures?…”

Exchange through nonmagnetic insulating matrix

“…An alternative explanation is a thermal decoupling of the grains above the Curie temperature of the grain-boundary phase. 8,9 In fact, these two mechanisms are complementary rather than exclusive and have a common origin, namely that intersublattice exchange is unable to suppress rare-earth intramultiplet excitations at temperatures comparable to T C . 3,10 Second, embedding magnetic particles or clusters in a nonmagnetic matrix tends to enhance the coercivity and may be used to fabricate artificially structured permanent magnets.…”