Zinc oxide-based diluted magnetic semiconductors

Abstract

The current experimental situation on the occurrence or absence of ferromagnetism in diluted magnetic semiconductors based on wurtzite zinc oxide hosts is presented, focusing mainly on the many recent experiments which have been performed on bulk systems. Numerous reports have suggested that partial (typically less than 10 at.%) substitution of Zn²⁺ in ZnO by magnetic transition metal (tM) ions, particularly Mn²⁺ and Co²⁺, can result in samples with ferromagnetic Curie temperatures above the room temperature. Reports which cast doubt on the very existence of any kind of long range magnetic order in clean samples of Zn_1−xtM_xO at low transition metal (tM) concentrations are also appearing with increasing frequency. The confusing situation clearly calls for a critical, even subjective position to be taken on this topic. The experimental situation on bulk samples strongly favors the view that in cases when ferromagnetism is found, it is not intrinsic to Zn_1−xtM_xO.

Introduction

Magnetic insulators and magnetic semiconductors are rare, and when they do occur, possess rather low Curie temperatures. The first well-studied bulk magnetic semiconductors were spinel chalcogenides such as CdCr₂Se₄ which has a T_c of 142 K [1]. It is generally believed that in these spinel systems, ferromagnetic coupling in the absence of mediation by conduction electrons takes place because the dominant interaction is 90° superexchange [2] between d³ Cr³⁺ which is expected to be ferromagnetic. The direct exchange interaction between d³ Cr³⁺ in the B-site of the AB₂X₄ spinel structure is antiferromagnetic, but this direct exchange is weakened as a consequence of the larger size of CrX₆ polyhedra and hence greater metal–metal separation in sulfide and selenide spinels. Some other ferromagnetic semiconductors include the Jahn–Teller distorted perovskites BiMnO₃ (T_c = 105 K) [3], [4] CuSeO₃ (T_c = 26 K) [5] and YTiO₃ (T_c = 29 K) [6]. In these three perovskites, ferromagnetic ordering is thought to arise due to orbital ordering, though other subtle structural and electronic effects may play a role [7], [8], [9]. The oxide ferromagnetic semiconductor with perhaps the simplest structure is the rock-salt f⁷ compound EuO which has a Curie temperature of 77 K [10] but even in this system, the precise mechanism for ferromagnetism is a subject of some debate. Indeed, EuO is perhaps better described as a ferromagnetic semimetal; in many samples, the ferromagnetic T_c is associated with an insulator–metal transition [11].

The magnetic structure of the ordered double perovskite oxide La₂MnNiO₆ has recently been investigated [12]. This unusual material is a band ferromagnetic insulator with the ferromagnetism arising from near-180° superexchange between ${\mathrm{e}}_{\mathrm{g}}^0$ (d³ Mn⁴⁺) orbitals and ${\mathrm{e}}_{\mathrm{g}}^2$ (d⁸ Ni²⁺). Full neutron magnetic moments at 3.5 K of 3.0 μ_B on the Mn⁴⁺ and 2.0 μ_B on the Ni²⁺ are obtained, and the saturation magnetization per formula unit is nearly 5 μ_B at 5 K and 50 kOe. Ferromagnetic ordering sets in near 280 K, so if this material can be described as a true insulator/semiconductor, it would hold the record for the highest T_c of any ferromagnetic semiconductor. The structures of some of these ferromagnetic semiconductors/insulators are displayed in Fig. 1.