Curie–Weiss Temperature as the Descriptor of the Change in the Thermopower of Layered Magnet NiCl2– x Br x ( x

Abstract

We explored how the thermopowers of magnetic insulators are affected by ferromagnetism and antiferromagnetism by studying the magnetic structure and thermopower of the layered magnet NiCl2–xBrx (x = 0.0–2.0). Bulk NiCl2 (x = 0) and bulk NiBr2 (x = 2) are isostructural magnetic insulators, but the NiCl2 monolayer exhibits ferromagnetism, while the NiBr2 monolayer shows spin-frustrated antiferromagnetism. To characterize the magnetic structure of NiCl2–xBrx, we determined its spin exchanges by DFT calculations and calculated its magnetic susceptibility using Monte Carlo simulations with the calculated spin exchanges to obtain the Curie–Weiss temperature θCW. Our study shows that the maximum thermopower, Smax, of NiCl2–xBrx increases steadily with increasing the θCW of NiCl2–xBrx, hence showing that Smax increases with increasing θCW. Thus, the Curie–Weiss temperature is the descriptor of the change in the thermopower of layered magnet NiCl2–xBrx, and the monolayer ferromagnetism leads in NiCl2 to a higher thermopower than the monolayer antiferromagnetism in NiBr2. Implications of this finding were discussed.