Magnetic Constitution of Topologically Trivial Thermoelectric PbTe:Cr

Abstract

In this paper we report on detailed temperature and magnetic field dependence of m agnetization of IV-VI semiconductor PbTe doped with mixed valence transition metal Cr$^{2+/3+}$. The material is studied solely by an integral superconducting quantum interference device magnetometer in order to quantitatively determine the contribution of single substitutional Cr$^{3+}$ as well as of various Cr-Te magnetic nanocrystals, including their identification. The applied experimental procedure reveals the presence of about $10^{19}$~cm$^{-3}$ paramagnetic Cr$^{3+}$ ions formed via self-ionization of Cr$^{2+}$ resonant donors. These are known to improve the thermoelectric figure of merit parameter zT of this semiconductor. The magnetic finding excellently agrees with previous Hall effect studies thus providing a new experimental support for the proposed electronic structure model of PbTe:Cr system with resonant Cr$^{2+/3+}$ state located (at low temperatures) about 100 meV above the bottom of the conduction band. Below room temperature a ferromagnetic-like signal points to the presence of Cr-rich nanocrystalline precipitates. Two most likely candidates, namely: Cr$_2$Te$_3$ and Cr$_5$Te$_8$ are identified upon dedicated temperature cycling of the sample at the remnant state. As an ensemble, the nanocrystals exhibits (blocked) superparamagnetic properties. The magnetic susceptibility of both n- and p-type PbTe in the temperature range $100 < T < 400$~K has been established. These magnitudes are essential in proper accounting for the high temperature magnetic susceptibility of PbTe:Cr.