Study of the critical-fields and the thermal broadening in polycrystalline Ag$_2$FeGeSe$_4$ semiconducting compound

Abstract

We show that a phenomenological model based on sublattice magnetization describes the temperature and field dependent magnetism in the Ag$_2$FeGeSe$_4$ semiconductor compound with wurtz-stannite-type structure. The model successfully finds the antiferromagnetic (AF), spin flop (SF) and paramagnetic (P) phases for all magnetization curves below the Néel temperature. The Langevin classical function instead of the Brillouin one is used in the analysis of the phase transitions to take into account the randomness of the magnetic moments given the polycrystalline nature of the samples. The critical-fields and the thermal broadening of the phase transitions were also found. The model was tested in the Ba$_3$Cu$_3$In$_4$O$_{12}$ and Ba$_3$Cu$_3$Sc$_4$O$_{12}$ compounds, and was successfully identified AF, SF and P phases in these materials.