Current Opinion in Solid State and Materials Science
Zinc oxide-based diluted magnetic semiconductors
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 Tc 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 (Tc = 105 K) [3], [4] CuSeO3 (Tc = 26 K) [5] and YTiO3 (Tc = 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 Tc 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 (d3 Mn4+) orbitals and (d8 Ni2+). Full neutron magnetic moments at 3.5 K of 3.0 μB on the Mn4+ and 2.0 μB on the Ni2+ are obtained, and the saturation magnetization per formula unit is nearly 5 μB 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 Tc of any ferromagnetic semiconductor. The structures of some of these ferromagnetic semiconductors/insulators are displayed in Fig. 1.
Section snippets
Diluted magnetic semiconductors
Recent interest in magnetic semiconductors has actually focused not on concentrated systems such as the ones described so far, but dilute systems where typically magnetic transition metal ions partially substitute main group cations in traditional zinc blende or wurtzite semiconductors [**13]. The structures of these semiconductor hosts are depicted in Fig. 2. It is in such materials, prepared as appropriate thin-film architectures, that electrical manipulation of magnetism, or magnetic
Studies on substituted ZnO
It is well known that the equilibrium solubility of Mn2+ and Co2+ in ZnO is significant at temperatures above 800 °C and that metastable phases with up to 10% or more of these cations substituting for Zn2+ can be prepared [34], [35]. The first experiment seeking possible ferromagnetism in Mn-substituted ZnO was performed by Fukumura et al. [36] who grew thin films by pulsed laser deposition with an x value in Zn1−xMnxO as large as 0.36. These authors found no evidence for ferromagnetic ordering.
Conclusions
In conclusion, studies on carefully characterized bulk samples of transition metal substituted zinc oxide seem to suggest a complete absence of magnetic ordering for low substitution levels. The reports of ferromagnetism seem to always be rather weak, often many orders of magnitude smaller than the expected spin-only saturation moments. The samples that are ferromagnetic are often so at levels small enough that no other technique than magnetization itself is able to detect whether the
Acknowledgements
The National Science Foundation Chemical Bonding Center (NSF) is acknowledged for support of this work through Grants CHE04-34567 (Chemical Bonding Center) and DMR04-49354 (Career Award). I thank my collaborators Priya Gopal, Gavin Lawes, Art Ramirez, Aditi Risbud, Nicola Spaldin, Susanne Stemmer for their many and varied inputs.
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