Magnetic coupling properties of rare-earth metals (Gd, Nd) doped ZnO: First-principles calculations

The electronic structure and magnetic coupling properties of rare-earth metals (Gd, Nd) doped ZnO have been investigated using first-principles methods. We show that the magnetic coupling between Gd or Nd ions in the nearest neighbor sites is ferromagnetic. The stability of the ferromagnetic coupling between Gd ions can be enhanced by appropriate electron doping into ZnO Gd system and the room-temperature ferromagnetism can be achieved. However, for ZnO Nd system, the ferromagnetism between Nd ions can be enhanced by appropriate holes doping into the sample. The room-temperature ferromagnetism can also be achieved in the n-conducting ZnO Nd sample. Our calculated results are in good agreement with the conclusions of the recent experiments. The effect of native defects (V-Zn, V-O) on the ferromagnetism is also discussed. (C) 2009 American Institute of Physics. [DOI 10.1063/1.3176490]

Mn-AlInN: a new diluted magnetic semiconductor

Mn ions have been incorporated into MOCVD grown Al1-x In (x) N/GaN thin films by ion implantation to achieve the room temperature ferromagnetism in the samples. Magnetic characterizations revealed the presence of two ferromagnetic transitions one has Curie points at similar to 260 K and the other above room temperature. In-diffusion of indium caused by the Mn implantation leads to the partition of AlInN epilayer into two diluted magnetic semiconductor sub-layers depending on the Mn concentration. The Curie temperature of 260 K is assigned to the layer having lower concentration, whereas T (c) above room temperature is assumed to be associated to the layer having higher Mn concentration.

Anisotropy of magnetic properties in ferrimagnetic ytterbium iron garnet under high magnetic fields

The magnetic anisotropy in ytterbium iron garnet (YbIG) is theoretically investigated under high magnetic fields (up to 160 kOe). According to the crystal field effect in ytterbium gallium garnet (YbGaG), a detailed discussion of crystal-field interaction in YbIG is presented where a suitable set of crystal-field parameters is obtained. Meanwhile, the influences of nine crystal-field parameters on the crystal-field energy splitting are analyzed. On the other hand, considering the ytterbium-iron (Yb-Fe) superexchange interaction of YbIG, the spontaneous magnetization is calculated at different temperatures for the [111] direction. In particular, we demonstrate that the Wesis constant lambda is the function of 1/T in YbIG. In addition, the field dependences of the magnetization for the [110] and [111] directions are theoretically described where a noticeable anisotropy can be found. Our theory further confirms the great contribution of anisotropic Yb-Fe superexchange interaction to the anisotropy of the magnetization in YbIG. Moreover, our theoretical results are compared with the available experiments.

Strain and magnetic anisotropy of as-grown and annealed Fe films on c(4x4) reconstructed GaAs (001) surface

Fe films with the different thicknesses were grown on c(4x4) reconstructed GaAs (001) surfaces at low temperature by molecular-beam epitaxy. Well-ordered bcc structural Fe epitaxial films are confirmed by x-ray diffraction patterns and high-resolution cross-sectional transmission electron microscopy images. A large lattice expansion perpendicular to the surface in Fe film is observed. In-plane uniaxial magnetic anisotropy is determined by the difference between magnetizing energy along [110] and [110] directions, and the constant of interfacial uniaxial magnetic anisotropy is calculated to be 1.02x10(-4) J m(-2). We also find that magnetic anisotropy is not obviously influenced after in situ annealing, but in-plane strain is completely changed.

Structural and Magnetic Properties of Sm Implanted GaN

The structural and magnetic properties of Sm ion-implanted GaN with different Sm concentrations are investigated. XRD results do not show any peaks associated with second phase formation. Magnetic investigations performed by superconducting quantum interference device reveal ferromagnetic behavior with an ordering temperature above room temperature in all the implanted samples, while the effective magnetic moment per Sm obtained from saturation magnetization gives a much higher value than the atomic moment of Sm. These results could be explained by the phenomenological model proposed by Dhar et al. [Phys. Rev. Lett. 94(2005) 037205, Phys. Rev. B 72(2005) 245203] in terms of a long-range spin polarization of the GaN matrix by the Sm atoms.

Room-temperature ferromagnetism and in-plane magnetic anisotropy characteristics of nonpolar GaN:Mn films

Diluted magnetic nonpolar GaN Mn films have been fabricated by implanting Mn ions into nonpolar aplane (1 1 (2) over bar 0) p-type GaN films and a subsequent rapid thermal annealing process. The ferromagnetism properties of the films were studied by means of superconducting quantum interference device (SQUID). Clearly in-plane magnetic anisotropy characteristics of the sample at 10 K were revealed with the direction of the applied magnetic field rotating along the in-plane [0 0 0 1]-axis. Moreover, obvious ferromagnetic properties of the sample up to 350 K were detected by means of the temperature-dependent SQUID. (C) 2009 Elsevier B.V. All rights reserved.

Electric-tunable magneto resistance effect in a two-dimensional electron gas modulated by hybrid magnetic-electric barrier nanostructure

The electric-tunable spin-independent magneto resistance effect has been theoretically investigated in ballistic regime within a two-dimensional electron gas modulated by magnetic-electric barrier nanostructure. By including the omitted stray field in previous investigations oil analogous structures, it is demonstrated based on this improved approximation that the magnetoresistance ratio for the considered structure can be efficiently enhanced by a proper electric barrier up to the maximum value depending on the specific magnetic suppression. Besides, it is also shown the introduction of positive electrostatic modulation can effectively overcome the degradation of magnetoresistance ratio for asymmetric configuration and enhance the visibility of periodic pattern induced by the size effect, while for an opposite modulation the system magnetoresistance ratio concerned may change its sign. (C) 2009 Elsevier B.V. All rights reserved.

Influence of growth temperature on structural and magnetic characterization of (In, Cr)As quantum dots

(In, Cr)As ferromagnetic semiconductor quantum dots (QDs) were grown by molecular beam epitaxy on GaAs (001) substrates. The growth temperature effects on structure and magnetism of the QDs were investigated systematically. The Cr(2+)3d(4) states and quantum confined effect are assumed to play an important role in the room-temperature ferromagnetism of (In, Cr)As QDs. (C) 2009 Elsevier B.V. All rights reserved.

First-Principles Study of Magnetic Properties of 3d Transition Metals Doped in ZnO Nanowires

The defect formation energies of transition metals (Cr, Fe, and Ni) doped in the pseudo-H passivated ZnO nanowires and bulk are systematically investigated using first-principles methods. The general chemical trends of the nanowires are similar to those of the bulk. We also show that the formation energy increases as the diameter of the nanowire decreases, indicating that the doping of magnetic ions in the ZnO nanowire becomes more difficult with decreasing diameter. We also systematically calculate the ferromagnetic properties of transition metals doped in the ZnO nanowire and bulk, and find that Cr ions of the nanowire favor ferromagnetic state, which is consistent with the experimental results. We also find that the ferromagnetic coupling state of Cr is more stable in the nanowire than in the bulk, which may lead to a higher T (c) useful for the nano-materials design of spintronics.

Systematic investigation on the influence of the As-4 flux on the magnetic property of (In,Cr)As quantum dots

We study the structure, optical and magnetic characteristics of self-assembled (In,Cr) As diluted magnetic semiconductor quantum dots as a function of the As-4 flux. Increasing the surface energy by increasing the As4 pressure leads to a smaller number of larger dots for a higher As-4 flux. The remanent magnetization measured at 5K also increases with increasing As-4 flux, which is attributed to the enhancement of the effective Cr content due to the As-4-rich condition. We explore the possibility of tailoring magnetism by controlling the As-4/In flux ratio without changing the Cr concentration. Furthermore, extremely low-density QDs have also been successfully grown. Copyright (C) EPLA, 2008

The structural, morphological and magnetic characteristics of Mn-implanted nonpolar a-plane GaN films

Dilute magnetic nonpolar GaN films have been fabricated by implanting Mn into unintentionally doped nonpolar a-plane GaN films at room temperature, and a subsequent rapid thermal annealing. The X-ray diffraction analysis shows that after rapid thermal annealing the peak of the GaN X-ray diffraction curve shifts to a lower angle, indicating a slight expansion of the GaN crystal lattice. Atomic force microscopy analysis shows that the annealing process does not change the morphology of the sample greatly. Magnetic property analysis indicates that the as-annealed sample shows obvious ferromagnetic properties. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Magnetic properties of ZnO-doped cobalt ferrite

The cobalt ferrites with chemical composition Co1+xZnxFe2-2xO4 (r=0.0, 0.1, 0.2, 0.4) were obtained with conventional solid reaction. The ZnO-doped samples have lower lattice constant than CoFe2O4 by adjusting Co ions to the octahedral sites. The results show that doping ZnO could extremely improve the magnetic properties. In comparison with pure CoFe2O4, the little ZnO-doped sample has higher permeability and much lower coercivity at the condition of a little decrease of magnetization saturation. Sample with x=0.1 shows evident magnetostrictive effect at the magnetic field of 30-60 mT while pure cobalt ferrite sample does not, though the saturation magnetostriction decreases. These indicate that ZnO-doping improves the magnetostrictive sensitivity of the cobalt ferrites and have potential applications in magnetoelectric devices and magnetic detector.

Nitrogen defects and ferromagnetism in Cr-doped dilute magnetic semiconductor AlN from first principles

High Curie temperature of 900 K has been reported in Cr-doped AlN diluted magnetic semiconductors prepared by various methods, which is exciting for spintronic applications. It is believed that N defects play important roles in achieving the high-temperature ferromagnetism in good samples. Motivated by these experimental advances, we use a full-potential density-functional-theory method and supercell approach to investigate N defects and their effects on ferromagnetism of (Al,Cr)N with N vacancies (V-N). We investigate the structural and electronic properties of V-N, single Cr atom, Cr-Cr atom pairs, Cr-V-N pairs, and so on. In each case, the most stable structure is obtained by comparing different atomic configurations optimized in terms of the total energy and the force on every atom, and then it is used to calculate the defect formation energy and study the electronic structures. Our total-energy calculations show that the nearest substitutional Cr-Cr pair with the two spins in parallel is the most favorable and the nearest Cr-V-N pair makes a stable complex. Our formation energies indicate that V-N regions can be formed spontaneously under N-poor condition because the minimal V-N formation energy equals -0.23 eV or Cr-doped regions with high enough concentrations can be formed under N-rich condition because the Cr formation energy equals 0.04 eV, and hence real Cr-doped AlN samples are formed by forming some Cr-doped regions and separated V-N regions and through subsequent atomic relaxation during annealing. Both of the single Cr atom and the N vacancy create filled electronic states in the semiconductor gap of AlN. N vacancies enhance the ferromagnetism by adding mu(B) to the Cr moment each but reduce the ferromagnetic exchange constants between the spins in the nearest Cr-Cr pairs. These calculated results are in agreement with experimental observations and facts of real Cr-doped AlN samples and their synthesis. Our first-principles results are useful to elucidate the mechanism for the ferromagnetism and to explore high-performance Cr-doped AlN diluted magnetic semiconductors.

Growth Parameter Dependence of Structural Characterizations of Diluted Magnetic Semiconductor (Ga, Cr)As

We report the influence of growth parameters and post-growth annealing on the structural characterizations and magnetic properties of (Ga, Cr)As films. The crystalline quality and magnetic properties are sensitive to the growth conditions. The single-phase (Ga, Cr)As film with the Curie temperature of 10 K is synthesized at growth temperature T-s = 250 degrees C and with nominal Cr content x = 0.016. However, for the films with x > 0.02, the aggregation of Cr atoms is strongly enhanced as both T. and x increase, which not only brings strong compressive strain in the epilayer, but also roughens the surface. The origin of room-temperature ferromagnetism in (Ga, Cr)As films with nanoclusters is also discussed.

Spin precession induced by an effective magnetic field in a two-dimensional electron gas

We theoretically study the spatial behaviors of the spin precession in a two-dimensional electron system with spin-orbit interaction. Through analysis of interaction between the spin and the effective magnetic field in the system, we obtain the general conditions to generate a persistent spin helix and predict a persistent spin helix pattern in [001]-grown quantum wells. Particularly, we demonstrate that the phase of spin can be locked to propagate in a quantum well with SU(2) symmetry.