Charge dynamics and "ferromagnetism" of A1-xLaxB6 (A=Ca and Sr)

TL;DR

This study investigates La-doped alkaline-earth hexaborides, revealing they are simple doped semimetals without intrinsic ferromagnetism, and surface effects may have caused previously reported magnetic properties.

Contribution

It provides new experimental evidence that challenges the intrinsic ferromagnetism in La-doped hexaborides, emphasizing the importance of surface effects and sample purity.

Findings

A1-xLaxB6 behaves as a doped semimetal.

No evidence of intrinsic ferromagnetism was found after surface correction.

Surface paramagnetic moments can mimic ferromagnetic signals.

Abstract

Ferromagnetism has been reported recently in La-doped alkaline-earth hexaborides, A1-xLaxB6 (A=Ca, Sr, and Ba). We have performed the reflectivity, Hall resistivity, and magnetization measurements of A1-xLaxB6. The results indicate that A1-xLaxB6 can be regarded as a simple doped semimetal, with no signature of an excitonic state as suggested by several theories. It is also found that the surface of as-grown samples (10 micrometer in thickness) has a different electronic structure from a bulk one, and a fairly large number of paramagnetic moments are confined in this region. After eliminating these paramagnetic moments at the surface, we could not find any evidence of an intrinsic ferromagnetic moment in our samples, implying the possibility that the ferromagnetism of A1-xLaxB6 reported so far is neither intrinsic.