Kondo-lattice ferromagnets and their peculiar order along the magnetically hard axis

Abstract

We show that Ce- and Yb-based Kondo-lattice ferromagnets order mainly along the magnetically hard direction of the ground state Kramers doublet determined by crystalline electric field (CEF). Here we argue that this peculiar phenomenon, that was believed to be rare, is instead the standard case. Moreover, it seems to be independent on the Curie temperature $T_{\mathrm{C}}$, crystalline structure, size of the ordered moment and type of ground state wave function. On the other hand, all these systems show the Kondo coherence maximum in the temperature dependence of the resistivity just above $T_{\mathrm{C}}$ which indicates a Kondo temperature of a few Kelvin. An important role of fluctuations is indicated by the non-mean-field like transition in specific heat measurements as well as by the suppression of this effect by a strong Ising-like anisotropy. We discuss possible theoretical scenarios.

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• Magnetic field-induced softening of spin waves and hard-axis order in Kondo-lattice ferromagnet CeAgSb${}_2$
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• Importance of anisotropic interactions for hard-axis/plane ordering of Ce-based ferromagnets
• Physical properties of $R$Co${}_2$Al${}_8$ ($R=$ La, Ce, Pr, Nd and Sm) single crystals: An emerging structure-type for anisotropic Kondo lattice studies
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• Distinct effect of Kondo physics on crystal field splitting in electron and spin spectroscopies