Mean field approximation and ab initio calculation of magnetic and optical properties of DyN doped with transition metals (TM: V and Cr)

The structural, optical, and magnetic properties of rare earth nitride \(\hbox {Dy}_{0.95}\hbox {TM}_{0.05}\hbox {N}\) (TM: V and Cr) diluted magnetic semiconductors have been investigated using the Korringa–Kohn–Rostoker method combined with the coherent potential approximation. In this work, the effect of GGA–SIC approximations on the electronic properties was investigated in detail. It is revealed from calculating the density of states (DOS) and optical absorption spectra that the ferromagnetic state is stable when TM introduces magnetic moments in \(\hbox {Dy}_{0.95}\hbox {TM}_{0.05}\hbox {N}\) (TM: V and Cr). Using the mean field approximation (MFA), the Curie temperature is determined by the total energy difference per V and Cr atoms between the disorder local moment (DLM) and ferromagnetic (FM) state within the ambient conditions, which becomes more important by increasing the concentration or by co-doping (V, Cr) DyN systems.     

New results on Magnetic Properties of Tin-Ferrite Nanoparticles

This paper presents new physical properties of nanocrystalline SnFe₂O₄ ferrites, synthesized by co-precipitation method. Magnetization measurement indicates superparamagnetic behavior; the blocking temperature is about 300?K. We performed X-ray diffraction, infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), Energy Dispersive Spectroscopy (EDS), UV-visible measurement, superconducting quantum interference device (SQUID) and zero-field-cooled (ZFC)/field-cooled (FC) measurements. TEM images show the high crystallinity and grain size of ferrite nanocrystals. The refinement result showed that the type of the cationic distribution over the tetrahedral and octahedral sites in the nanocrystalline lattice is a partially inverse spinel. The obtained UV?Cvis data were used to calculate the energy band gap (3.82?eV) of nanocrystalline SnFe₂O₄. On the other hand the magnetic properties of the samples; saturation magnetization (M _s ) and coercive field (H _c ) were determined using a superconducting quantum interference device (SQUID).     

High temperature magnetic properties of nanocrystalline Sn0∙95Co0∙05O2

Structural and magnetic properties of Sn_0?95Co_0?05O₂ nanocrystalline and diluted magnetic semiconductors have been investigated. This sample has been synthesized by co-precipitation route. Study of magnetization hysteresis loop measurements infer that the sample of Sn_0?95Co_0?05O₂ nanoparticle shows a well-defined hysteresis loop at 300 K temperature, which reflects its ferromagnetic behaviour. We confirmed the roomtemperature intrinsic ferromagnetic (FM) semiconductors by ab initio calculation, using the theory of the functional of density (DFT) by employing the method of Korringa–Kohn–Rostoker (KKR) as well as coherent potential approximation (CPA, explain the disorder effect) to systems. The ferromagnetic state energy was calculated and compared with the local-moment-disordered (LMD) state energy for local density approximation (LDA) and LDA–SIC approximation. Mechanism of hybridization and interaction between magnetic ions in Sn_0?95Co_0?05O₂ is also investigated. To explain the origin of ferromagnetic behaviour, we give information about total and atoms projected density of state functions.     

Cation Distribution and Magnetic Interactions in Zn-Substituted Fe(Cu)Fe2O4 Ferrites

The effect of Zn doping on magnetic properties of Fe₃O₄ and CuFe₂O₄ ferrite spinels has been studied. The general expression of saturation magnetization, the critical temperature and Curie constant (C) are obtained by the mean field theory (MFT). By considering the high-temperature series expansions (HTSE) theory and the mean field approximation for the Néel??s two-sublattice noncollinear model, we obtained the magnetic phase diagrams of Zn _x Fe_1?x Fe₂O₄, and Zn _x Cu_1?x Fe₂O₄. The Bohr magnetron is obtained for two systems. The central critical exponent ?? associated with magnetic susceptibility for two systems is obtained.     

The Effect of Quantum Transverse Anisotropy on Multilayer Transitions in a Spin-3/2 Blume–Capel Model with RKKY Interaction

We have studied the effect of quantum transverse anisotropies on the multilayer transition and magnetic properties of a spin-3/2 Blume–Capel model of a system formed by two magnetic multilayer materials, of different thicknesses, separated by a nonmagnetic spacer of thickness and RKKY interaction. Using mean-field theory, it is found that the multilayer magnetic order–disorder transition temperature depends strongly on the value of the transverse anisotropy. The multilayer transition temperature decreases when increasing the transverse anisotropy. Furthermore, there exists a critical quantum transverse anisotropy Δ _xL beyond which the separate transitions occur in the two magnetic layers. The critical transverse anisotropy Δ _xL increases on increasing the magnetic crystal field and/or the thickness of the magnetic multilayers N. On other hand, the Δ _xL decreases on increasing the Fermi level and/or the nonmagnetic thickness M.     

Electronic Structure and Magnetic Properties of Doped GaAs (Zinc Blende) with Double Impurities and Defects

Using ab-initio calculations based on Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) method in connection with the local density approximation (LDA), we study theoretically the electronic and magnetic properties of a system based on GaAs material. These properties have been studied, with different point defects in GaAs, which are Gallium interstitials (Ga_i), Gallium antisites (Ga_As), Gallium vacancies (V_Ga), Arsenic interstitials (As_i) and Arsenic antisites (As_Ga), Arsenic vacancies (V_As), and in-doped or codoped system Ga_1?x Co _x As and Ga_1?y?z Co _y Fe _z As, respectively. This work presents detailed information about total and local density of states at different concentrations of these defects and doped elements; the stability of the ferromagnetic state compared with the spin-glass state has been discussed. The result of electronic properties shows that codoped GaAs material with Fe and Co become ferromagnetic, also doped GaAs with Co or Fe with a defect (hole in Ga) is stable in the ferromagnetic state. The Curie temperature is estimated from the total energy difference between the Disorder Local Moment (DLM) and the (FM) state by using a mapping on the Heisenberg model in mean field approximation.     

Effect of Grain Size on Magnetic Properties of ZnO Doped with Nickel Single Impurities

The purpose of our study is to find a suitable material to be used in spintronic applications and to find the relation between the parameters of deposition by spray pyrolysis technic (temperature, concentration of Ni doping) and the ferromagnetic properties (Curie temperature, magnetic moment). The nickel-doped zinc oxide, Zn.1?x.NixO (x = 0.01, 0.03, 0.05), and diluted magnetic semiconductors (DMSs) are synthesized by the spray pyrolysis technic. The results of The X-ray diffraction (XRD) of prepared substrates confirm the incorporation of the dopants into the ZnO lattice structure. The spin-polarized electronic properties has been found and was investigated in detail by using the density-functional theory (DFT), the local density approximation (LDA), and The Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA). As result, nickel doping brings up a half-metallic appearance due to the hybridization between the 3d state of Nickel impurities and the oxygen 2p state. The mechanism of the interatomic exchange has been explained as being a p-d double exchange.     

Subsampling-based HMC parameter estimation with application to large datasets classification

This paper presents a contextual algorithm for the approximation of Baum’s forward and backward probabilities, which are extensively used in the framework of Hidden Markov chain models for parameter estimation. The method differs from the original algorithm by taking into account only a neighborhood of limited length and not all the data in the chain for computations. It then becomes possible to propose a bootstrap subsampling strategy for the computation of forward and backward probabilities, which greatly reduces computation time and memory saving required for EM-based parameter estimation. Comparative experiments regarding the neighborhood size and the bootstrap sample size are conducted by mean of unsupervised classification error rates. Practical interest of such an algorithm is then illustrated through the segmentation of large-size images; classification results confirm the validity and the accuracy of the proposed algorithm while greatly reducing computation and memory requirements.