Low temperature magnetic properties of nearly- or non-stoichiometric alfa-MnS

Manganese(II) sulfide crystallizes into three different polymorphs (alpha-, beta-, and gamma-MnS); out of these, alfa-MnS (alabandite) is considered as the most stable polymorph and widespread mineral in terrestrial rocks as well as in various meteorites . In our study, nearly- or non-stoichiometric manganese(II) sulfides (alfa-MnS) have been synthesized. The resulting products have been prepared both by solvothermal processes as given by Biswas et al. [2007] and using a direct thermal synthesis from sulfur and manganese powders. In both cases, the prepared samples have been characterized by an electron microprobe analysis (EMA) and X-ray diffraction (XRD). In all alabandite samples, the Néel temperature can be observed in narrow temperature range about 155 K as it is expected. The low temperature evolutions of magnetic response show a rapid increase in magnetization values around 40 K in all synthetic samples as well as in natural terrestrial alabandite polycrystalline material. This observed anomalous behavior at low temperature can be ascribed to the magnetic transition from a high-temperature antiferromagnetic state to low-temperature ferromagnetic similar as reported in earlier studies by Petrakovskii et al. [2001] for slightly non-stoichiometric alfa-MnxS ( x = 1.05). In order to quantify the magnetic response of our alabandite samples in the suggested ferromagnetic state hysteresis measurements were done at 5 K in the external magnetic field of 7 T. There is a variation in hysteresis parameters observed among samples of various origin. However as a first order estimate of the upper threshold of the coercivity is found to be 200 mT, the saturation magnetization reaches up to 1.6 Am2/kg, and saturation remanent magnetization of 0.7 Am2/kg is observed. Based on saturation magnetization alabandite with slightly non-stoichiometric composition can reach below 40 K roughly hundred times lower saturation magnetization values compared to metallic iron and has significant coercivity.