Special Issue "Ferromagnetic Semiconductors"

Guest Editor
Prof. Dr. Wolfgang Nolting
Humboldt-University at Berlin, Institute of Physics / Chair: Solid State Theory, Newtonstr. 15, D-12489 Berlin, Germany
Website: http://tfk.physik.hu-berlin.de/~nolting
E-Mail:

Dear Colleagues,

In the last few years diluted ferromagnetic semiconductors have experienced a dramatic upsurge of interest due to their very promising potential for technological applications, on the one hand, and being attractive from a fundamental physics point of view, on the other. These materials might be able to integrate data processing (semiconductor technique: electron charge) and storage (ferromagnetism technique: electron spin) on a single chip. The simultaneous exploitation of charge and spin is known as “spintronics”. Undoubtedly ferromagnetism in such semiconducting materials would open the door for exciting microelectronic device applications provided the following non-trivial boundary conditions were fulfilled: (1) The Curie temperature should clearly exceed room temperature, (2) the charge carriers should react sensitively on changes in the magnetic state, and (3) the material should retain its excellent semiconductor properties. The simultaneous achievement of these objectives has been in the last years and continues to be the main goal of intense experimental as well as theoretical research on (diluted) ferromagnetic semiconductors.
The classical ferromagnetic semiconductors EuO and EuS have been investigated for several decades and are considered as rather well understood. For application, however, they do not come into question because of too low transition temperatures and only poor semiconductor properties. On the other hand, however, they may help to work out the basic physics of ferromagnetic semiconductors. More promising for future applications are (III,V) semiconductors doped with magnetic ions such as the prototypical Mn-doped GaAs with Curie temperatures well above 100K. Both localized magnetic moments and itinerant charge carriers are provided by the magnetic ion. Since the quality of the samples and their magnetic properties seem to be closely connected material science of growth and defects plays an important role with respect to spintronics aspects of such materials.
From a basic theoretical point of view the interplay between electronic structure, exchange interaction and moment disorder with respect to electric, magnetic and transport properties must be understood. By proper modelling and reliable many-body evaluation of the decisive magnetic correlations as well as “ab initio” calculations of real materials one can hope to get a better understanding of the fundamental physics of the ferromagnetism that occurs in (diluted) ferromagnetic semiconductors and of the prerequisites necessary for getting sufficiently high Curie temperatures.

Prof. Dr. Wolfgang Nolting
Guest Editor

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• ferromagnetic local-moment systems
• carrier-induced ferromagnetism
• magnetic polaron
• disorder and magnetic stability
• Curie temperature
• super-exchange
• spintronics
• spin-dependent transport
• electronic correlations
• nanomagnetism

Type of Paper: Review
Title: Coherent Potential Approach to the Carrier States of Diluted Magnetic Semiconductors
Author: Masao Takahashi
Affiliation: Kanagawa Institute of Technology, 1030 Shimo-Ogino, Atsugi-shi, 243-0292, Japan; E-Mail: taka@gen.kanagawa-it.ac.jp
Abstract: The recent theoretical study of diluted magnetic semiconductors (DMS's) using the dynamical coherent potential approximation (dynamical CPA) is briefly reviewed. The model calculation was performed for three typical cases of A_1-xMn_xB-type DMS's: cases with strong and moderate exchange interactions in the absence of nonmagnetic potentials, and the case with strong attractive nonmagnetic potentials in addition to moderate exchange interaction. The results of the density of states (DOS), local density of states (local DOS), carrier spin polarization, the spin-coupling strength, the optical band edge and the electric resistivity are presented to clarify the nature and properties of the carrier states. The mechanism of carrier-induced ferromagnetism of Ga_1-xMn_xAs is also discussed.

Title: Ferromagnetism in Magnetic Semiconductor without Doping Element or Doped With Nonmagnetic Element
Author: J.B. Yi
Affiliation: Department of Materials Science and Engineering, National University of Singapore, 119260, Singapore; E-Mail: mseyj@nus.edu.sg
Abstract: In this work, the research and progress of unconventional oxide ferromagnetic semiconductor without doping element or doped with non-magnetic element will be reviewed. The review mainly covers aspects of the magnetism of pure oxide semiconductor host, such as ZnO, TiO₂, SnO₂, etc; the magnetism of light element doped oxide magnetic semiconductor (carbon doped ZnO system is as an example) and non-magnetic transition metal doped oxide magnetic semiconductors. Theoretical and experimental Investigations of the mechanisms of these unconventional magnetic semiconductors will be reviewed and discussed.

Type of Paper: Review
Title: Element Specific Versus Integral Structural and Magnetic Properties of Gd:GaN and Co:ZnO Probed With Hard X-ray Absorption Spectroscopy
Author: Andreas Ney
Affiliation: Universität Duisburg-Essen, Experimentalphysik, ME 343 Lotharstr. 1, D-47057 Duisburg, Germany; E-Mail: andreas.ney@uni-due.de
Abstract: Dilute magnetic semiconductors (DMS) are envisioned as sources of spin-polarized carriers for future semiconductor devices which simultaneously utilize spin and charge of the carriers. The hope of discovering a DMS with ferromagnetic order up to room temperature (RT) still motivates research on suitable DMS materials. Two candidate wide-band gap DMS are Gd:GaN and Co:ZnO. We have used hard x-ray absorption spectroscopy (XAS) and in particular x-ray linear dichroism (XLD) and x-ray magnetic circular dichroism (XMCD) to study both DMS materials with element specifity and compare these findings with results from integral SQUID magnetometry as well as electron paramagnetic resonance (EPR).

Type of Paper: Article
Title: Voltage-Controlled Ferromagnetic Ordering in MnGe Nanodots
Authors: Faxian Xiu, Igor V. Ovchinnikov, Kin Wong and Kang L. Wang
Affiliation: University of California, Los Angeles, USA; E-Mail: iovchinnikov@ucla.edu
Abstract: Here, we discuss possible pathways to the experimental realization of the voltage controllability of ferromagnetic ordering, - the founding property for the next-generation low-power spintronics nanodevices, - and, in particular, the special role of the dilute-magnetic-semiconductors as the most likely material basis. We report on our latest experimental achievements in the voltage-manipulation of the ferromagnetism in MnGe nano-dots and demonstrate the experimental capacity of pushing the Curie temperature further up above the room temperature, required for the
technological applicability of the phenomenon.

Type of Paper: Review
Title: Rare-Earth Nitrides: Intrinsic Ferromagnetic Semiconductors
Authors: B.J. Ruck, H.J. Trodahl, C. Meyer, J. Cezar, J.H. Richter, F. Natali, N.O.V. Planck
Affiliation: School of Chemical and Physical Sciences, Victoria University of Wellington, Box 600, Wellington, New Zealand; E-Mail: Ben.Ruck@vuw.ac.nz (B.J.R.)
Abstract: The rare-earth nitride series contains intrinsic ferromagnetic semiconductors, attractive for long mean-free path spintronics devices. They show strongly contrasting magnetic properties across a series with crystal structures that permit heteroepitaxial growth. However, despite enormous theoretical interest, their propensity for N vacancy formation and for oxidation has left their properties largely unexplored experimentally. We report thin-film growth, band structures and ferromagnetic properties of SmN and GdN. These indirect-gap semiconductors have band edges comprising majority-spin states; both electrons and holes carry that majority spin. Coupled with the thousand-fold contrast in their coercive fields this pair has special promise for a MRAM element.
Keywords: Rare-earth nitride; ferromagnetic semiconductor; spintronics; band structure