Intrinsic coercivity induced by valence fluctuations in $4f$-$3d$ intermetallic magnets

Abstract

Temperature dependence of magnetization curves of well homogenized samples of Ce(Co${}_{1-x}$Cu${}_x$)${}_5$ ($0\le x\le 0.7$), a family of representative $4f$-$3d$ intermetallic magnets found in rare-earth permanent magnets, is measured. A remarkable enhancement of intrinsic coercivity is observed with $x=0.3$ and $x=0.4$, persisting to higher temperatures. This experimental observation is theoretically attributed to an effect of electronic correlation among $4f$-electrons. That is, an intrinsic pinning happens originating in an anomalously enhanced magnetic anisotropy energy contributed by an order of magnitude stronger charge-transfer process between $4f$-electrons and $3d$-electrons, than the conventional crystal field effects. It is demonstrated that the $4f$-$3d$ charge-transfer process depends on the direction of magnetization in the middle of a crossover of the valence state of Ce between CeCu${}_5$ with robust Ce${}^{3+}$ and CeCo${}_5$ with the mixed valence state.

7 pages, 4 figures