Variable‐Temperature EPR Study of the Semiconducting Ferromagnet EuB 6

EPR in EuB 6 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 Eu 2+ moments. The EPR behavior of the Eu 2+ 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 Eu 2+ 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 Eu 2+ 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 EuB 6 , and probably also low‐electron‐density EuB 6 , 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 Eu 2+ resonance, which cannot be attributed to crystalline‐electric‐field effects, may result from a magnetic transition, possibly involving magnetic‐polaron formation.