Variable‐Temperature EPR Study of the Semiconducting Ferromagnet EuB6

EPR in EuB6 samples of widely separated electron density has been studied above the Curie temperature at X band. In bulk samples, narrow signals at g = 2.000 ± 0.002 have been observed and attributed to conduction electrons that are strongly coupled to the Eu2+ moments. The EPR behavior of the Eu2+ resonance is unusual in both bulk and low-electron-density samples. At temperatures high enough to avoid critical spin fluctuations, i.e. high temperatures, the exchange field increases linearly with temperature. Below 21 K, the Eu2+ resonance splits into two lines, and the splitting increases with decreasing temperature, with the low-field line being considerably more intense than the high-field line, which is unobservable near 10 K. Several models for the exchange interaction between the Eu2+ moments are examined, and it is found that to a good approximation the RKKY interaction can account for the magnitude of the exchange field at high temperatures in bulk EuB6, and probably also low-electron-density EuB6, but that another interaction, quite possibly the BR interaction, makes the dominant contribution to the temperature dependence of the high-temperature exchange field. The BR model can also account for the measured Weiss constant at high temperatures, but the RKKY model fails to predict the correct low-temperature magnetic behavior. The splitting of the Eu2+ resonance, which cannot be attributed to crystalline-electric-field effects, may result from a magnetic transition, possibly involving magnetic-polaron formation. Des spectres RPE a X-bande ont ete etudies dans les echantillons de EuB6 ayant des densites electroniques tres variees, au-dessus de la temperature Curie. Raies etroites a g = 2,000 ± 0,002 ont ete observees dans les echantillons massifs et elles sont attribuees aux electrons de conduction qui sont fortement couples aux moments de Eu2+. La RPE de Eu2+ se comporte dans un facon exceptionnelle dans les echantillons massifs et dans les echantillons de densite electronique basse. Aux temperatures suffisamment hautes pour eviter les fluctuations critiques de spin, le champ d'echange augmente lineairement avec la temperature. Au-dessous de 21 K, la RPE de Eu2+ se partage en deux, et le partage augmente comme la temperature decroit. La ligne a bas champ est considerablement plus intense que la ligne a haut champ, qui devient imperceptible pres de 10 K. Plusieurs modeles pour l'interaction d'echange entre les moments de Eu2+ sont examines, et on a constate que d'une bonne approximation l'interaction RKKY peut expliquer aux temperatures hautes la grandeur du champ d'echange dans EuB6 massif, et probablement dans EuB6 de densite electronique basse. Neanmoins une autre interaction (probablement I'interaction BR) fait la contribution dominante a la dependence du champ d'echange de la temperature, pour les temperatures hautes. Le modele BR peut expliquer aussi la constante Weiss aux temperatures hautes, mais le modele RKKY faillit a preire correctement la tenue magnetique a basse temperature. Le partage de la RPE de Eu2+, qui ne peut pas ětre attribue aux effects du champ electrique cristallin, peut resulter d'un transition magnetique, probablement entrainant la formation du polaron magnetique.