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

## TL;DR

This paper reveals that Ce- and Yb-based Kondo-lattice ferromagnets predominantly order along their magnetically hard axis, a common but previously underestimated phenomenon, largely unaffected by structural or magnetic variations.

## Contribution

It demonstrates that ordering along the hard magnetic axis is the typical behavior in these systems, challenging prior beliefs and suggesting a universal aspect of their magnetic properties.

## Key findings

- Order along the hard axis is common in Ce- and Yb-based Kondo ferromagnets.
- Kondo coherence maximum occurs just above the Curie temperature.
- Transition shows non-mean-field behavior influenced by anisotropy.

## 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.