Effective pair exchange interactions between Mn atoms in III-V and group-IV diluted magnetic semiconductors are determined from a two-step first-principles procedure. In the first step, the self-consistent electronic structure of a system is calculated for a collinear spin structure at zero temperature with the substitutional disorder treated within the framework of the coherent-potential approximation. The effective exchange pair interactions are then obtained in a second step by mapping the total energies associated with rotations of magnetic moments onto an effective classical Heisenberg Hamiltonian using the magnetic force theorem and one-electron Green functions. The formalism is applied to Ga 1Ϫx Mn x As alloys with and without As antisites, and to Ge 1Ϫx Mn x alloys recently studied experimentally. A detailed study of the behavior of pair exchange interactions as a function of the distance between magnetic atoms as well as a function of the concentrations of the magnetic atoms and compensating defects is presented. We have found that due to disorder and the half-metallic character of the system the pair exchange interactions are exponentially damped with increasing distance between the Mn atoms. The exchange interactions between Mn atoms are ferromagnetic for distances larger than the ones corresponding to the averaged nearest-neighbor Mn-Mn distance. The pair exchange interactions are also reduced with increasing concentrations of the Mn atoms and As antisites. As a simple application of the calculated exchange interactions we present mean-field estimates of Curie temperatures.

Recent studies have demonstrated the potential of antiferromagnets as the active component in spintronic devices. This is in contrast to their current passive role as pinning layers in hard disk read heads and magnetic memories. Here we report the epitaxial growth of a new hightemperature antiferromagnetic material, tetragonal CuMnAs, which exhibits excellent crystal quality, chemical order and compatibility with existing semiconductor technologies. We demonstrate its growth on the III-V semiconductors GaAs and GaP, and show that the structure is also lattice matched to Si. Neutron diffraction shows collinear antiferromagnetic order with a high Néel temperature. Combined with our demonstration of room-temperatureexchange coupling in a CuMnAs/Fe bilayer, we conclude that tetragonal CuMnAs films are suitable candidate materials for antiferromagnetic spintronics.

We use the density-functional calculations to investigate the compositional dependence of the lattice constant of (Ga,Mn)As containing various native defects. The lattice constant of perfect mixed crystals does not depend much on the concentration of Mn. The lattice parameter increases if some Mn atoms occupy interstitial positions. The same happens if As antisite defects are present. A quantitative agreement with the observed compositional dependence is obtained for materials close to a complete compensation due to these two donors. The increase of the lattice constant of (Ga,Mn)As is correlated with the degree of compensation: the materials with low compensation should have lattice constants close to the lattice constant of GaAs crystal.PACS numbers: 71.15. Ap, 71.20.Nr, 71.55.Eq Diluted magnetic III-V semiconductors (DMS), such as Ga 1−x Mn x As, combine semiconducting and ferromagnetic properties [1,2,3,4] and are attractive for applications in spin electronics. These materials have been extensively studied in the last years, both experimentally and theoretically.There is, however, still not much known about the details of the crystal structure of these materials and about the incorporation of Mn atoms. It is generally believed that in well defined samples the volume of the MnAs precipitates is reduced to zero, and that Mn simply substitutes for the host cation in a tetrahedral (zinc-blende or wurtzite) crystal structure. Only recently it was suggested [5,6] and experimentally proved [7] that a portion of Mn occupies interstitial rather than substitutional positions in the zinc-blende lattice of (Ga,Mn)As. The interstitial Mn atoms act as double donors [5,6,8,9], in contrast to Mn atoms in the substitutional positions that are known to be acceptors.Almost unnoticed remains the surprising fact that the lattice constant of (Ga,Mn)As increases with increasing concentration of Mn [10]. According to the atomic radii [11], Mn atoms are smaller (R Mn = 1.17Å) than Ga atoms (R Ga = 1.25Å) and, in the simplest approximation, the lattice constant should be expected to decrease rather than to increase. This is also a result of a recent theoretical study [12] of the structure of zinc-blende α-MnAs. According to these calculations the lattice constant of α-MnAs is smaller then the lattice constant of GaAs.On the other hand, the lattice constant of GaAs is well known to increase in the presence of As antisite defects [13,14]. The MBE-grown GaAs crystals may contain up to 1 atomic percent of these defects and a large amount of the antisite defects is expected also in (Ga,Mn)As [15]. Being donors, they have an important role in the compensation of Mn acceptors. It was also shown recently [16] that formation energy of an As antisite defect in (Ga,Mn)As decreases remarkably with the increasing content of Mn and that the concentration of As antisites should be correlated with the concentration of Mn atoms. This indirect mechanism, i.e., the increasing number of the As antisites due to the addition of Mn, could be a possible...

We perform a theoretical study of the magnetism induced in transition metal dioxides ZrO2 and TiO2 by substitution of the cation by a vacancy or an impurity from the groups 1A or 2A of the periodic table, where the impurity is either K or Ca. In the present study both supercell and embedded cluster methods are used. It is demonstrated that the vacancy and the K-impurity leads to a robust induced magnetic moment on the surrounding O-atoms for both the cubic ZrO2 and rutile TiO2 host crystals. On the other hand it is shown that Ca-impurity leads to a non magnetic state. The native O-vacancy does not induce a magnetic moment in the host dioxide crystal.

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We present ab initio calculations of total energies of Mn atoms in various interstitial positions. The calculations are performed by the full-potential linearized plane-wave method. The minimum energy is found for tetrahedral T(As4) position, but the energy of the T(Ga4) site differs by only a few meV. The T(Ga4) position becomes preferable in the p-type materials. In samples with one substitutional and one interstitial Mn the Mn atoms tend to form close pair with antiparallel magnetic moments. We also use the spin-splitting of the valence band to estimate the exchange coupling Jpd for various positions of Mn. It is the same for the substitutional and T(As4) position and it is only slightly reduced for the T(Ga4) position. The hybridization of Mn d-states with six next-nearest neighbors of the interstitial Mn explains the insensitivity of Jpd to the position of Mn.Comment: 6 pages, 3 figures, 3 tables, submitted to the Physical Review

A first-principles investigation of the anomalous ferromagnetism of a quasi-one-dimensional Co chain at the Pt(111) step edge is reported. Our calculations show that the symmetry breaking at the step leads to an easy magnetization axis at an odd angle of ∼20 • towards the Pt step, in agreement with experiment [P. Gambardella et al., Nature 416, 301 (2002)]. Also, the Co spin and orbital moments become noncollinear, even in the case of a collinear ferromagnetic spin arrangement. A significant enhancement of the Co orbital magnetic moment is achieved when modest electron correlations are treated within LSDA+U calculations. PACS numbers: 75.30.Gw, 75.75.+a, 75.10.Lp Exploring magnetism in the one-dimensional (1D) limit has been a great challenge for many years. Only recently, Gambardella et al.[1] succeeded to observe ferromagnetism of monatomic Co wires decorating the Pt(997) surface step edge. By exploiting the elementselectivity of the x-ray magnetic circular dichroism (XMCD), the existence of long-range ferromagnetic order on Co was demonstrated below 15 K [1,2]. Although theoretically the Mermin-Wagner theorem [3] forbids longrange 1D ferromagnetic order at non-zero temperatures, ferromagnetism in 1D can be stabilized by a large magnetic anisotropy energy, which creates barriers effectively blocking thermal fluctuations. The significance of such blocking mechanism was recognized earlier for the occurrence of long-range magnetic order in 2D systems [4,5].The experiments of Gambardella et al. revealed novel magnetic properties of monatomic Co wires at Pt step edges. An unexpected magnetocrystalline anisotropy was observed: the easy magnetization axis was directed along a peculiar angle of +43 • towards the Pt step edge and normal to the Co chain. The magnetocrystalline anisotropy energy (MAE) was estimated to be substantial, of the order of 2 meV/Co atom [1]. In addition, a considerable enhancement of the Co orbital magnetic moment M L ≈ 0.7 µ B -as compared to the bulk Co M L value of 0.14 µ B -was deduced from XMCD experiments.In this paper we report a first-principles investigation of the anomalous ferromagnetism of a monatomic Co wire at the Pt(111) surface step edge, using state-of-the-art electronic structure calculations. We focus on the intriguing features of the quasi-1D Co wire, i.e., the easy axis rotated away from the (111) surface normal, the enhanced Co orbital moment and huge estimated MAE.The key outcomes of our study are (i) the ab initio calculation of an easy axis at an odd angle rotated towards the Pt step edge and (ii) the prediction of an intrinsic noncollinearity between spin and orbital magnetic moments of both the ferromagnetic Co wire and Pt substrate. The origin of this novel magnetic behavior, which is to our knowledge not present in known 2D and 3D itinerant ferromagnets is explained to be a consequence of the magnetic symmetry lowering at the surface step edge [6]. Our calculations furthermore yield a MAE of the order of 4 meV/Co atom, and-using the LSDA+U approach-a Co orbital mo...

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