# Rare earth magnetism in CeFeAsO: A single crystal study

## TL;DR

This study provides a detailed single-crystal analysis of CeFeAsO, revealing its magnetic anisotropy, phase transitions, and the nature of Ce magnetic ordering, challenging previous assumptions about the influence of Fe magnetic order on Ce ions.

## Contribution

First high-quality single-crystal growth of CeFeAsO enabling anisotropic magnetic property studies, with insights into Ce magnetic order and crystal electric field effects.

## Key findings

- High residual resistivity ratio indicating crystal quality
- Large easy-plane magnetic anisotropy observed
- Ce magnetic order occurs at T_N = 3.7 K, with reduced Zeeman splitting below 15 K

## Abstract

Single crystals of CeFeAsO, large enough to study the anisotropy of the magnetic properties, were grown by an optimized Sn-flux technique. The high quality of our single crystals is apparent from the highest residual resistivity ratio, RRR = 12, reported among undoped RFeAsO compounds (R=rare earth) as well as sharp anomalies in resistivity, specific heat, C(T), and thermal expansion at the different phase transitions. The magnetic susceptibility chi(T) presents a large easy-plane anisotropy consistent with the lowest crystal electric field doublet having a dominant Gamma_6 character. Curie-Weiss like susceptibilities for magnetic field parallel and perpendicular to the crystallographic c-axis do not reveal an influence of a staggered field on the Ce site induced by magnetic ordering of the Fe. Furthermore, the standard signatures for antiferromagnetic order of Ce at T_N = 3.7 K observed in chi(T) and C(T) are incompatible with a Zeeman splitting Delta = 10 K of the CEF ground state doublet at low temperature due to the Fe-magnetic order as previously proposed. Our results can be reconciled with the earlier observation by assuming a comparatively stronger effect of the Ce-Ce exchange leading to a reduction of this Zeeman splitting below 15 K.