Coexistence of different magnetic moments in CeRuSn probed by polarized neutrons

TL;DR

This study uses polarized neutron diffraction to reveal that CeRuSn contains coexisting magnetic Ce3+ and non-magnetic Ce(4-delta)+ ions, with their distribution influenced by temperature-induced structural changes.

Contribution

It provides the first detailed analysis of how different Ce valence states coexist and are spatially distributed in CeRuSn, linked to its structural transformations.

Findings

Ce ions exist in different valence states at different temperatures.

Localized 4f states are close to Ce3+ configuration.

Valence states correlate with Ce-Ru interatomic distances.

Abstract

We report on the spin densities in CeRuSn determined at elevated and at low temperatures using polarized neutron diffraction. At 285 K, where the CeRuSn crystal structure, commensurate with the CeCoAl type, contains two different crystallographic Ce sites, we observe that one Ce site is clearly more susceptible to the applied magnetic field whereas the other is hardly polarizable. This finding clearly documents that distnictly different local environment of the two Ce sites causes the Ce ions to split into magnetic Ce3+ and non-magnetic Ce(4-delta)+ valence states. With lowering the temperature, the crystal structure transforms to a structure incommensurately modulated along the c axis. This leads to new inequivalent crystallographic Ce sites resulting in a re-distribution of spin densities. Our analysis using the simplest structural approximant shows that in this metallic system Ce ions co-exist in different valence states. Localized 4f states that fulfill the third Hund's rule are found to be close to the ideal Ce3+ state (at sites with the largest Ce-Ru interatomic distances) whereas Ce(4-delta)+ valence states are found to be itinerant and situated at Ce sites with much shorter Ce-Ru distances. The similarity to the famous alpha-gamma transition in elemental cerium is discussed.