Magnetic properties and low temperature X-ray studies of the weak ferromagnetic monoclinic and trigonal chromium tellurides Cr 5Te 8

Abstract

Monoclinic and trigonal Cr 5Te 8 show a transition into the ferromagnetic state with a Curie temperature T c, which sensitively depends on the actual composition. Monoclinic samples exhibit a lower T c despite their higher Cr content. This observation is explained on the basis of less effective ferromagnetic superexchange in the monoclinic compounds and the larger number of Cr atoms being antiferromagnetically coupled. Magnetization experiments performed at 5 K demonstrate that the compounds saturate already at rather low magnetic fields. In addition, small values are estimated for the coercitive field H c as well as for the remanence magnetization both being typical for weak ferromagnetic materials. The values for the saturation magnetization amount to about 72% and 65% for monoclinic and trigonal Cr 5Te 8, respectively. These low values can partially be explained on the basis of antiferromagnetically coupled Cr(III) d 3 centers. An additional spin canting is assumed to fully account for the reduced saturation moments. Above 300 K the susceptibilities follow a Curie–Weiss law with large positive values for the Weiss constant and magnetic moments in accordance with a Cr 3+ 3 d 3 spin configuration. Low temperature X-ray investigations reveal unusual thermal expansion of the a as well as b lattice parameters of the monoclinic and of the a lattice parameter of the trigonal sample, respectively. Near T c the slope of the thermal expansions changes significantly.