Theory of ferromagnetism driven by superexchange in dilute magnetic semi-conductors

C Simserides,T Dietl,J.A Majewski,K.N Trohidou,D Niarchos,G Hadjipanayis,O Kalogirou

doi:10.1051/epjconf/20147501003

Abstract

Magnetic properties of Ga$_{1-x}$Mn$_x$N are studied theoretically by employing a tight binding approach to determine exchange integrals $J_{ij}$ characterizing the coupling between Mn spin pairs located at distances $R_{ij}$ up to the 16th cation coordination sphere in zinc-blende GaN. It is shown that for a set of experimentally determined input parameters there are no itinerant carriers and the coupling between localized Mn$^{3+}$ spins in GaN proceeds via superexchange that is ferromagnetic for all explored $R_{ij}$ values. Extensive Monte Carlo simulations serve to evaluate the magnitudes of Curie temperature $T_\mathrm{C}$ by the cumulant crossing method. The theoretical values of $T_\mathrm{C}(x)$ are in quantitative agreement with the experimental data that are available for Ga$_{1-x}$Mn$_x$N with randomly distributed Mn$^{3+}$ ions with the concentrations $0.01 \leq x \leq 0.1$.

Highlights

Dilute magnetic insulators constitute an emerging class of magnetic semiconductors in which rather than the p-d Zener mechanism [1], ferromagnetic superexchange accounts for the coupling between diluted transition metal (TM) spins [2]
To compare quantitatively the theoretical and experimental results, we assume a statistical distribution of directions corresponding to tetragonal Jahn-Teller distortions and determine the average value of the exchange energy Ji j characterizing the coupling of Mn3+ pairs at a given distance Ri j in the fcc cation sublattice
Our theoretical results on TC(x) in Ga1−xMnxN agree quantitatively with the measured values in the experimentally explored range 0.01 ≤ x ≤ 0.1 [4, 6, 22]. This agreement supports the view that ferromagnetic superexchange is the dominant coupling mechanism between Gasubstitutional Mn3+ ions in Ga1−xMnxN, leading to TC ≈ 13 K at x = 0.1

Summary

Introduction

Dilute magnetic insulators constitute an emerging class of magnetic semiconductors in which rather than the p-d Zener mechanism [1], ferromagnetic superexchange accounts for the coupling between diluted transition metal (TM) spins [2]. Contamination-free processing, and (nano)characterization [2,3,4,5] allowed the preparation of Ga1−xMnxN films with the randomly distributed Mn3+ ions up to x = 0.1 showing TC up to about 13 K [6] despite the absence of itinerant carriers. A high degree of crystallinity, a random distribution of the Mn ions, and a weak degree of compensation by residual donors were checked in these samples by a range of electron microscopy, synchrotron radiation, ion beam, optical, and magnetic resonance techniques [2, 3, 5]

Theory

Tight binding approach

Monte Carlo simulations

Results

Conclusion