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

TL;DR

This study investigates how valence fluctuations in Ce-based intermetallic magnets enhance intrinsic coercivity through increased magnetic anisotropy, driven by charge-transfer effects between 4f and 3d electrons.

Contribution

It reveals that valence fluctuations induce a significant increase in coercivity by enhancing magnetic anisotropy energy via 4f-3d charge transfer, a novel mechanism in these magnets.

Findings

Enhanced coercivity observed at specific compositions (x=0.3, 0.4).

Charge-transfer process depends on magnetization direction and valence state crossover.

Intrinsic pinning linked to anomalously strong magnetic anisotropy energy.

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.