This focus issue of New Journal of Physics is devoted to the materials science of dilute magnetic semiconductors (DMS). A DMS is traditionally defined as a diamagnetic semiconductor doped with a few to several atomic per cent of some transition metal with unpaired d electrons. Several kinds of dopant–dopant interactions can in principle couple the dopant spins leading to a ferromagnetic ground state in a dilute magnetic system. These include superexchange, which occurs principally in oxides and only between dopants with one intervening oxygen, and double exchange, in which dopants of different formal charges exchange an electron. In both of these mechanisms, the ferromagnetic alignment is not critically dependent on free carriers in the host semiconductor because exchange occurs via bonds. A third mechanism, discovered in the last few years, involves electrons associated with lattice defects that can apparently couple dopant spins. This mechanism is not well understood. Finally, the most desirable mechanism is carrier-mediated exchange interaction in which the dopant spins are coupled by itinerant electrons or holes in the host semiconductor. This mechanism introduces a fundamental link between magnetic and electrical transport properties and offers the possibility of new spintronic functionalities. In particular electrical gate control of ferromagnetism and the use of spin polarized currents to carry signals for analog and digital applications. The spin light emitting diode is a prototypical device of this kind that has been extensively used to characterize the extent of spin polarization in the active light emitting semiconductor heterostructure.

The prototypical carrier mediated ferromagnetic DMS is Mn-doped GaAs. This and closely related narrow gap III–V materials have been very extensively studied. Their properties are generally quite well understood and they have led to important insights into fundamental properties of ferromagnetic systems with strong spin-orbit coupling. They have also led to the demonstration of a wide range of novel phenomena including some, like tunneling anisotropic magnetoresistance, which have subsequently been achieved in metal ferromagnetic systems. However despite considerable effort over many years the maximum Curie point achieved in (Ga,Mn)As is still less than 200 K. So unless some major new breakthrough is achieved these materials are unlikely to be of use for practical spin electronics technologies.

In 2000, Dietl et al [1] published a seminal paper in which mean field theory was used to predict which of the common diamagnetic semiconductors would exhibit a Curie point above ambient if doped with 5 at.% Mn and a hole concentration of 3.5 × 10²⁰ cm⁻³. Of the many host semiconductors simulated, only ZnO and GaN were predicted to exhibit a critical temperature in excess of 300 K. Since 2000, high-T_c DMS research has proliferated in both experimental and theoretical arenas. Many papers have been published containing claims of new DMS materials based largely on limited film growth, powder diffraction, and magnetometry. In these papers, a film which exhibits a hysteretic SQUID or VSM loop at 300 K and phase purity with only the host semiconductor detected by XRD are often claimed to be true ferromagnetic DMSs. Many of these papers are flawed because the criteria for a well-defined DMS are much more extensive. These include: (i) a random dopant distribution, (ii) a well-known and preferably unique charge state and preferentially a unique local structural environment for the dopant, (iii) a demonstrated coupling of the dopant spin to the host band structure, leading to spin polarization of the majority carriers, and (iv) a rational dependence of the saturation magnetization and Curie point on the magnetic dopant and carrier concentrations. Implicit in this list is that trivial causes of ferromagnetism, such as magnetic contamination and magnetic secondary phase formation, are eliminated. Yet, in many papers, the authors have not carried out the necessary control experiments and materials characterization to convincingly eliminate these possibilities. The former includes the growth of films without the magnetic dopant and the associated demonstration of the absence of ferromagnetism. Magnetic secondary phase formation is particularly problematic because in order to inject enough magnetic dopant to generate appreciable magnetization and spin polarization, one must often exceed the solid solubility of the dopant in the host. If the dopant is itself ferromagnetic in its elemental state, or if unintended magnetic products nucleate, spurious ferromagnetism will occur. Moreover, it is often a major analysis challenge to detect secondary phases when they consist of only a few per cent of the dopant; element specific spectroscopies such as x-ray absorption have been invaluable in this task. Powder diffraction is not sufficiently sensitive for this level of analysis.

Against this backdrop, this focus issue of New Journal of Physics now appears. The editors' principal goal in soliciting papers has been to encourage investigators to submit work in which the necessary experiments have been done to allow the material to be adequately characterized. This collection contains a mix of experimental and theoretical papers, and many different types of materials are covered. This focus issue thus constitutes a snapshot in time of a fast-moving and fascinating field of materials physics.

Reference

[1]   Dietl T, Ohno H, Matsukura F, Cibert J and Ferrand D 2000 Science 287 1019

Focus on Dilute Magnetic Semiconductors Contents

The structural, optical and magnetic properties and anomalous Hall effect of InMnP:Zn epilayers
Yoon Shon, Sejoon Lee, D Y Kim, T W Kang, Chong S Yoon, Eun Kyu Kim and Jeong Ju Lee

Magnetic reversal under external field and current-driven domain wall motion in (Ga,Mn)As: influence of extrinsic pinning
K Y Wang, A C Irvine, J Wunderlich, K W Edmonds, A W Rushforth, R P Campion, C T Foxon, D A Williams and B L Gallagher

Huge tunnelling anisotropic magnetoresistance in (Ga,Mn)As nanoconstrictions
A D Giddings, O N Makarovsky, M N Khalid, S Yasin, K W Edmonds, R P Campion, J Wunderlich, T Jungwirth, D A Williams, B L Gallagher and C T Foxon

Lithographically and electrically controlled strain effects on anisotropic magnetoresistance in (Ga,Mn)As
E De Ranieri, A W Rushforth, K Výborný, U Rana, E Ahmad, R P Campion, C T Foxon, B L Gallagher, A C Irvine, J Wunderlich and T Jungwirth

Structure and magnetism of cobalt-doped ZnO thin films
M Ivill, S J Pearton, S Rawal, L Leu, P Sadik, R Das, A F Hebard, M Chisholm, J D Budai and D P Norton

Role of charge carriers for ferromagnetism in cobalt-doped rutile TiO₂
T Fukumura, H Toyosaki, K Ueno, M Nakano and M Kawasaki

Ab-initio study of exchange constants and electronic structure in diluted magnetic group-IV semiconductors
Silvia Picozzi and Marjana Ležaić

Phase coherent transport in (Ga,Mn)As
D Neumaier, K Wagner, U Wurstbauer, M Reinwald, W Wegscheider and D Weiss

Hydrogen interstitials-mediated ferromagnetism in Mn_xGe_1-x magnetic semiconductors
Xin-Xin Yao, Shi-Shen Yan, Shu-Jun Hu, Xue-Ling Lin, Chong Han, Yan-Xue Chen, Guo-Lei Liu and Liang-Mo Mei

Electronic structures of magnetic semiconductors FeCr₂Se₄ and Fe_0.5Cu_0.5Cr₂Se₄
B I Min, Seung Su Baik, H C Choi, S K Kwon and J-S Kang

Investigation of pure and Co²⁺-doped ZnO quantum dot electronic structures using the density functional theory: choosing the right functional
Ekaterina Badaeva, Yong Feng, Daniel R Gamelin and Xiaosong Li

Magnetic properties of sol-gel-derived doped ZnO as a potential ferromagnetic semiconductor: a synchrotron-based study
N R S Farley, K W Edmonds, A A Freeman, G van der Laan, C R Staddon, D H Gregory and B L Gallagher

Local electronic structure of Cr in the II–VI diluted ferromagnetic semiconductor Zn_1-xCr_xTe
M Kobayashi, Y Ishida, J I Hwang, G S Song, A Fujimori, C S Yang, L Lee, H-J Lin, D J Huang, C T Chen, Y Takeda, K Terai, S-I Fujimori, T Okane, Y Saitoh, H Yamagami, K Kobayashi, A Tanaka, H Saito and K Ando

Lack of ferromagnetism in n-type cobalt-doped ZnO epitaxial thin films
T C Kaspar, T Droubay, S M Heald, P Nachimuthu, C M Wang, V Shutthanandan, C A Johnson, D R Gamelin and S A Chambers

XMCD studies on Co and Li doped ZnO magnetic semiconductors
Thomas Tietze, Milan Gacic, Gisela Schütz, Gerhard Jakob, Sebastian Brück and Eberhard Goering

Ferromagnetic semiconductors and the role of disorder
B W Wessels

An extensive comparison of anisotropies in MBE grown (Ga,Mn)As material
C Gould, S Mark, K Pappert, R G Dengel, J Wenisch, R P Campion, A W Rushforth, D Chiba, Z Li, X Liu, W Van Roy, H Ohno, J K Furdyna, B Gallagher, K Brunner, G Schmidt and L W Molenkamp

Local structural, magnetic and magneto-optical properties of Mn-doped SiC films prepared on a 3C–SiC(001) wafer
Wenhong Wang, Fumiyoshi Takano, Hironori Ofuchi and Hiro Akinaga

Effects of proton irradiation on the magnetic properties of GaGdN and GaCrN
J K Hite, K K Allums, G T Thaler, C R Abernathy, S J Pearton, R M Frazier, R Dwivedi, R Wilkins and J M Zavada