Spin fluctuation effects in nearly antiferromagnetic vanadium and chromium diborides

Abstract

The first transition series diborides are isomorphous compounds from ScB 2 to MnB 2. They all show metallic behaviour and are paramagnetic up to VB 2; CrB 2, on the opposite, is antiferromagnetic ( T N = 86 K) with a small magnetic moment value in the ordered state which suggests the occurrence of itinerant spin magnetism. Heat capacity measurements on V 1− x Cr x B 2 show a sharp maximum of the electronic specific heat for x ∼ 0.8, correlated with the onset of long-range antiferromagnetic ordering as given by magnetic susceptibility measurements. The existence of local effects cannot explain by itself the observed specific heat enhancement. The existence of similar phenomena in the isoelectronic Mo 1- x Cr x B 2 alloys suggests that large amplitude spin fluctuations contribute to the renormalization of the one electron energies in a significant way. The free energy of the electron gas is computed in the R.P.A. Only intraatomic interactions are taken into account. It is shown that various phenomenological models are capable of yielding a significant spin fluctuation effect.