Electron spin resonance study of EuB 6- xC x: Correlation between electronic localization and transport properties

Abstract

EuB 6− x C x carbide borides with a CaB 6-type structure were prepared in the range 0 ⩽ x ⩽ 0.20. Magnetic and transport measurements performed on single crystals show that pure EuB 6 is ferromagnetic below T c = 13.7 K. It is semiconducting in the paramagnetic region with an intrinsic small band gap estimated to be 0.05–0.1 eV; an electronic transition to a semimetal occurs below T c . The carbon-doped EuB 6 samples are degenerate semiconductors in which both the carrier concentration and the antiferromagnetic interactions increase with carbon content. These carbide borides were studied using electron spin resonance in the X band at temperatures above the magnetic ordering temperature. A comparison at 300 K of the peak-to-peak linewidth ΔH p- p (the first derivative of the Eu 2+ resonance line) in both the Eu 2+B 6− x C x and the Eu 2+ 1− x Gd 3+ x B 6 systems shows that only x 2 electrons are delocalized in the conduction band for x carbon atoms present in the unit cell. This observation is well confirmed by Hall effect measurements. Furthermore a weak and extremely narrow line which appears (superimposed on the Eu 2+ resonance line) only when x ≠ 0 involves an electronic localization for the remaining x 2 electrons introduced. They are likely to form pairs.