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 eté etudiés dans les échantillons de EuB6 ayant des densités electroniques très variées, au‐dessus de la température Curie. Raies étroites a g = 2,000 ± 0,002 ont eté observées dans les échantillons massifs et elles sont attribuées aux electrons de conduction qui sont fortement couplés aux moments de Eu2+. La RPE de Eu2+ se comporte dans un façon exceptionnelle dans les échantillons massifs et dans les échantillons de densité electronique basse. Aux températures suffisamment hautes pour éviter les fluctuations critiques de spin, le champ d'échange augmente linéairement avec la température. Au‐dessous de 21 K, la RPE de Eu2+ se partage en deux, et le partage augmente comme la température decroit. La ligne a bas champ est considérablement plus intense que la ligne a haut champ, qui devient imperceptible près de 10 K. Plusieurs modèles pour l'interaction d'échange entre les moments de Eu2+ sont examinés, et on a con