Influence of temperature on critical fields in ZnCr2Se4

The complex ac dynamic magnetic susceptibility was used to study the influence of temperature on critical fields in polycrystalline ZnCr2Se4 spinel. An anti ferromagnetic order with a Neel temperature T-N = 20.7 K and a strong ferromagnetic exchange evidenced by a positive Curie-Weiss temperature theta(CW) = 55.1 K were established. An increasing static magnetic field shifts TN to lower temperatures while a susceptibility peak at T-m in the paramagnetic region-to higher temperatures. The non-zero and negative values both of the second and third harmonics of susceptibility suggest only a parallel spin coupling in ferromagnetic clusters in the range between the Neel and Curie-Weiss temperatures. Below T-N the magnetic field dependence of susceptibility, chi(ac)(H), shows two peaks at critical fields H-c1 and H-c2. The values of H-c1 decrease slightly with temperature while the values of H-c2 drop rapidly with temperature. The strong changes of H-c2 temperature induced are mainly responsible for a spin frustration of the re-entraint type in the spinel under study. (C) 2009 Elsevier B.V. All rights reserved.