Ferromagnetism in Fe-substituted spinel semiconductor ZnGa$_2$O$_4$

TL;DR

This paper proposes a theoretical model explaining the high-temperature ferromagnetism observed in Fe-doped ZnGa₂O₄, supported by band structure calculations and phase diagram analysis for different Fe doping sites.

Contribution

It introduces a double exchange-like model for Fe-doped ZnGa₂O₄ and explores the conditions under which ferromagnetism is stabilized in this system.

Findings

The model accounts for stable ferromagnetic phases across various doping scenarios.

High Fe²⁺ concentration at tetrahedral sites requires a two-band model for accurate description.

Both tetrahedral and octahedral Fe doping can lead to ferromagnetism in the proposed framework.

Abstract

Motivated by the recent experimental observation of long range ferromagnetic order at a relatively high temperature of 200K in the Fe-doped ZnGa$_2$O$_4$ semiconducting spinel, we propose a possible mechanism for the observed ferromagnetism in this system. We show, supported by band structure calculations, how a model similar to the double exchange model can be written down for this system and calculate the ground state phase diagram for the two cases where Fe is doped either at the tetrahedral position or at the octahedral position. We find that in both cases such a model can account for a stable ferromagnetic phase in a wide range of parameter space. We also argue that in the limit of high Fe$^{2+}$ concentration at the tetrahedral positions a description in terms of a two band model is essential. The two $e_g$ orbitals and the hopping between them play a crucial role in stabilizing the ferromagnetic phase in this limit. The case when Fe is doped simultaneously at both the tetrahedral and the octahedral position is also discussed.