Thermodynamic properties and resistivity of the ferromagnetic semiconductor EuC2

Abstract

EuC2 is a ferromagnet with a Curie temperature of TC ≃ 15 K. It is semiconducting with the particularity that the resistivity drops by about five orders of magnitude on cooling through TC, which is therefore called a metal–insulator transition. In this paper, we study the magnetization, specific heat, thermal expansion and the resistivity around this ferromagnetic transition on high-quality EuC2 samples. At TC we observe well-defined anomalies in the specific heat cp(T) and thermal expansion α(T) data. The magnetic contributions of cp(T) and α(T) can satisfactorily be described within a mean-field theory, taking into account the magnetization data. In zero magnetic field, the magnetic contributions of the specific heat and thermal expansion fulfil a Grüneisen scaling, which is not preserved in finite fields. From an estimation of the pressure dependence of TC via Ehrenfest's relation, we expect a considerable increase of TC under applied pressure due to a strong spin–lattice coupling. Furthermore, the influence of weak off-stoichiometries δ in EuC2±δ was studied. It is found that δ strongly affects the resistivity, but hardly changes the transition temperature. In all these aspects, the behaviour of EuC2 strongly resembles that of EuO.