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An empirical multiorbital (spd) tight binding (TB) model including magnetism and spin-orbit coupling is applied to calculations of magnetic anisotropy energy (MAE) in CoPt L1_0 structure. A realistic Slater-Koster parametrisation for single-element transition metals is adapted for the ordered binary alloy. Spin magnetic moment and density of states are calculated using a full-potential linearized augmented plane-wave (LAPW) ab initio method and our TB code with different variants of the interatomic parameters. Detailed mutual comparison of this data allows for determination of a subset of the compound TB parameters tuning of which improves the agreement of the TB and LAPW results. MAE calculated as a function of band filling using the refined parameters is in broad agreement with ab initio data for all valence states and in quantitative agreement with ab initio and experimental data for the natural band filling. Our work provides a practical basis for further studies of relativistic magnetotransport anisotropies by means of local Green's function formalism which is directly compatible with our TB approach., Comment: 10 pages, 4 figures

We present an experimental and theoretical study of magnetocrystalline anisotropies in arrays of bars patterned lithographically into (Ga,Mn)As epilayers grown under compressive lattice strain. Structural properties of the (Ga,Mn)As microbars are investigated by high-resolution X-ray diffraction measurements. The experimental data, showing strong strain relaxation effects, are in good agreement with finite element simulations. SQUID magnetization measurements are performed to study the control of magnetic anisotropy in (Ga,Mn)As by the lithographically induced strain relaxation of the microbars. Microscopic theoretical modeling of the anisotropy is performed based on the mean-field kinetic-exchange model of the ferromagnetic spin-orbit coupled band structure of (Ga,Mn)As. Based on the overall agreement between experimental data and theoretical modeling we conclude that the micropatterning induced anisotropies are of the magnetocrystalline, spin-orbit coupling origin., Comment: 11 pages, 18 figures

The X-ray absorption near-edge structure (XANES) at the Mn K-edge in the (Ga,Mn)As magnetic semiconductor was simulated using the full potential linearized augmented plane wave (FLAPW) method including the core-hole effect. The calculations were performed in the supercell scheme for a substitutional and two tetrahedral interstitial Mn sites. The resulting pre-edge absorption structures show sharp distinction between the spectra simulated for the substitutional and interstitial Mn defects which is determined mainly by the second nearest neighbor ligands. A single feature is obtained for the substitutional Mn impurity, whereas two peaks appear for both types of interstitial defects. An interpretation of the pre-edge features was proposed., Comment: 5 pages, 4 figures

We present a theoretical survey of magnetocrystalline anisotropies in (Ga,Mn)As epilayers and compare the calculations to available experimental data. Our model is based on an envelope function description of the valence band holes and a spin representation for their kinetic-exchange interaction with localised electrons on Mn ions, treated in the mean-field approximation. For epilayers with growth induced lattice-matching strains we study in-plane to out-of-plane easy-axis reorientations as a function of Mn local-moment concentration, hole concentration, and temperature. Next we focus on the competition of in-plane cubic and uniaxial anisotropies. We add an in-plane shear strain to the effective Hamiltonian in order to capture measured data in bare, unpatterned epilayers, and we provide microscopic justification for this approach. The model is then extended by an in-plane uniaxial strain and used to directly describe experiments with strains controlled by postgrowth lithography or attaching a piezo stressor. The calculated easy-axis directions and anisotropy fields are in semiquantitative agreement with experiment in a wide parameter range.

This work reports theoretical and experimental study of the X-ray absorption near-edge structure (XANES) at the Mn K-edge in (Ga,Mn)As diluted magnetic semiconductors. The spectra have been calculated from the first-principles using FLAPW including the core-hole effect, a special attention has been paid to consequences of coexistence of Mn impurities in substitutional and tetrahedral interstitial positions. We have performed quantitative component analysis of experimental spectra collected on the (Ga,Mn)As samples before/after annealing and etching, with the aim to determine the proportion of Mn impurity configurations. Comparison of the experimental data with theoretical computations indicates that even after annealing and etching some Mn atoms still reside in interstitial sites, although the concentration of interstitial defects has been reduced by annealing., Comment: 8 pages, 10 figures

We demonstrate dynamic voltage control of the magnetic anisotropy of a (Ga,Mn)As device bonded to a piezoelectric transducer. The application of a uniaxial strain leads to a large reorientation of the magnetic easy axis which is detected by measuring longitudinal and transverse anisotropic magnetoresistance coefficients. Calculations based on the mean-field kinetic-exchange model of (Ga,Mn)As provide microscopic understanding of the measured effect. Electrically induced magnetization switching and detection of unconventional crystalline components of the anisotropic magnetoresistance are presented, illustrating the generic utility of the piezo voltage control to provide new device functionalities and in the research of micromagnetic and magnetotransport phenomena in diluted magnetic semiconductors., Comment: Submitted to Physical Review Letters. Updates version 1 to include a more detailed discussion of the effect of strain on the anisotropic magnetoresistance

The magnetoresistance of the MBE-grown GaAs/AlGaAs superlattice with Si-doped barriers has been measured in tilted magnetic fields in the as-grown state, and after brief illumination by a red-light diode at low temperature, T is approximately 0.3 K. A remarkable illumination-induced modification of magnetoresistance curves has been observed, which indicates a significant change of the superlattice Fermi surface topology. Analysis of magnetoresistance data in terms of the tight-binding model reveals that not only electron concentration and mobility have been increased by illumination, but also the coupling among 2D electron layers in neighboring quantum wells has been reduced., Comment: 7 pages, 6 figures, accepted for publication in Phys. Rev. B

The body of research on (III,Mn)V diluted magnetic semiconductors initiated during the 1990's has concentrated on three major fronts: i) the microscopic origins and fundamental physics of the ferromagnetism that occurs in these systems, ii) the materials science of growth and defects and iii) the development of spintronic devices with new functionalities. This article reviews the current status of the field, concentrating on the first two, more mature research directions. From the fundamental point of view, (Ga,Mn)As and several other (III,Mn)V DMSs are now regarded as textbook examples of a rare class of robust ferromagnets with dilute magnetic moments coupled by delocalized charge carriers. Both local moments and itinerant holes are provided by Mn, which makes the systems particularly favorable for realizing this unusual ordered state. Advances in growth and post-growth treatment techniques have played a central role in the field, often pushing the limits of dilute Mn moment densities and the uniformity and purity of materials far beyond those allowed by equilibrium thermodynamics. In (III,Mn)V compounds, material quality and magnetic properties are intimately connected. In the review we focus on the theoretical understanding of the origins of ferromagnetism and basic structural, magnetic, magneto-transport, and magneto-optical characteristics of simple (III,Mn)V epilayers, with the main emphasis on (Ga,Mn)As. The conclusions we arrive at are based on an extensive literature covering results of complementary ab initio and effective Hamiltonian computational techniques, and on comparisons between theory and experiment., Comment: 58 pages, 49 figures Version accepted for publication in Rev. Mod. Phys. Related webpage: http://unix12.fzu.cz/ms/

Full quantum-mechanical description of electrons moving in 3D structures with unidirectional periodic modulation subject to tilted magnetic fields requires an extensive numerical calculation. To understand magneto-oscillations in such systems it is in many cases sufficient to use the quasi-classical approach, in which the zero-magnetic-field Fermi surface is considered as a magnetic-field-independent rigid body in k-space and periods of oscillations are related to extremal cross-sections of the Fermi surface cut by planes perpendicular to the magnetic-field direction. We point out cases where the quasi-classical treatment fails and propose a simple tight-binding fully-quantum-mechanical model of the superlattice electronic structure., Comment: 8 pages, 7 figures, RevTex, submitted to Phys. Rev. B

We present a theoretical study of the infrared magneto-optical properties of ferromagnetic (III,Mn)V semiconductors. Our analysis combines the kinetic exchange model for (III,Mn)V ferromagnetism with Kubo linear response theory and Born approximation estimates for the effect of disorder on the valence band quasiparticles. We predict a prominent feature in the ac-Hall conductivity at a frequency that varies over the range from 200 to 400 meV, depending on Mn and carrier densities, and is associated with transitions between heavy-hole and light-hole bands. In its zero frequency limit, our Hall conductivity reduces to the $\vec k$-space Berry's phase value predicted by a recent theory of the anomalous Hall effect that is able to account quantitatively for experiment. We compute theoretical estimates for magnetic circular dichroism, Faraday rotation, and Kerr effect parameters as a function of Mn concentration and free carrier density. The mid-infrared response feature is present in each of these magneto-optical effects., Comment: 11 pages, 5 figures

Recent materials research has advanced the maximum ferromagnetic transition temperature in semiconductors containing magnetic elements toward room temperature. Reaching this goal would make information technology applications of these materials likely. In this article we briefly review the status of work over the past five years which has attempted to achieve a theoretical understanding of these complex magnetic systems. The basic microscopic origins of ferromagnetism in the (III,Mn)V compounds that have the highest transition temperatures appear to be well understood, and efficient computation methods have been developed which are able to model their magnetic, transport, and optical properties. However many questions remain., Comment: 4 pages, 4 figures, review, to be published in Curr. Appl. Phys

The far-infrared absorption in two-dimensional electron layers subject to magnetic field of general orientation was studied theoretically. The Kubo formula was employed to derive diagonal components of the magneto-conductivity tensor of two-dimensional electron single-layers and double-layers. A parabolic quantum well was used to model single-layer systems. Bilayer systems were represented by a pair of tunnel-coupled, strictly two-dimensional quantum wells. Obtained results were compared to experimental data., Comment: 8 pages, 11 figures, Latex, iopart, Semi-Mag 15

We report on a theoretical study of dc transport coefficients in (Ga,Mn)As diluted magnetic semiconductor ferromagnets that accounts for quasiparticle scattering from ionized Mn$^{2+}$ acceptors with a local moment $S=5/2$ and from non-magnetic compensating defects. In metallic samples Boltzmann transport theory with Golden rule scattering rates accounts for the principle trends of the measured difference between resistances for magnetizations parallel and perpendicular to the current. We predict that the sign and magnitude of the anisotropic magnetoresistance can be changed by strain engineering or by altering chemical composition., Comment: 4 pages, 2 figures

We present a theory of the infrared conductivity and absorption coefficients of metallic (III,Mn)V ferromagnetic semiconductors. We find that the conductivity is dominated by inter-valence-band transitions that produce peaks at $\hbar \omega \sim 220 {\rm meV}$ and obscure the broadened Drude peak. We demonstrate that transverse f-sum rule measurements can be used to extract accurate values for the free carrier density, bypassing the severe characterization difficulties that have till now been created by the large anomalous Hall effect in these materials., Comment: 5 pages, 3 figures. Model parameters in figures of previous version did not match caption. Web-page link corrected

We present a theoretical survey of ferromagnetic transition temperatures in cubic (III,Mn)V semiconductors based on a model with $S=5/2$ local moments exchange-coupled to itinerant holes in the host semiconductor valence band. Starting from the simplest mean-field theory of this model, we estimate the $T_c$ enhancement due to exchange and correlation in the itinerant-hole system, and the $T_c$ suppression due to collective fluctuations of the ordered moments. We show that high critical temperatures in these ferromagnetic semiconductors require both the large magnetic susceptibility contribution from the valence band's heavy holes and the large spin stiffness that results from strong spin-orbit coupling in the valence band. Our calculations demonstrate that this model for the ferromagnetism of these systems is fully consistent with the room-temperature ferromagnetism reported for Mn doped nitrides., Comment: 4 pages, 2 figures, 1 table; web-link in Ref.16 corrected, Ref. 18 added