A Comparative First-Principles Study of Fe-, Co- and FeCo-Doped ZnO with Wurtzite and Zinc Blende Structures

First-principles study of the electronic and magnetic properties of zinc-blende and wurtzite structures of Fe-, Co-, and FeCo-doped ZnO is presented. It is found that after doping, this diamagnetic material becomes ferromagnetic and half-metallic. It is also shown that the half-metallicity may be obtained for ZnFeO, ZnCoO, and ZnFeCoO. The analysis of the spin density reveals that the ferromagnetic phase is due to the ferromagnetic coupling between the p?Cd states. The effects of Fe on the magnetic properties of ZB and WZ Fe-doped ZnO compound have been investigated with the GGA calculations. In order to understand the role of Fe atom in the ferromagnetism, the density of states both in the presence and absence of Co doping, were calculated. The obtained results show the presence of coupling between Co and Fe atoms through the spin-split impurity band exchange mechanism. More importantly, the calculations show that the magnetic moment changes sensitively with the type of structure of ZnO, zinc-blende, or wurtzite. A discussion by comparing the results obtained in this study and the experimental results reported in the literature of similar systems show a very good agreement.     

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.     

Electronic and Magnetic Theoretical Investigation of Antiferromagnetically ErRh Layers

Self-consistent ab initio calculations, based on the DFT (Density Functional Theory) approach and using the FLAPW (Full potential Linear Augmented Plane Wave) method, are performed to investigate both electronic and magnetic properties of the ErRh layers. Polarized spin and spin–orbit coupling are included in calculations within the framework of the antiferromagnetic state between two sites of Er atoms. Magnetic moments considered to lie along axes are computed. The data obtained from the ab-initio calculations are then used as input for the high-temperature series expansions (HTSEs) calculations to compute other magnetic parameters. The exchange integrals between the magnetic atoms in the same sites are given by using mean field theory. The HTSEs of the magnetic susceptibility of ErRh antiferromagnetic spin-S through two model: Heisenberg and XY for ErRh layers, are studied to sixth order series in β=1/k _B T. The critical exponent γ associated with the magnetic susceptibility is deduced for the two models.     

Origin of Magnetism from Native Point Defects in ZnO

Based on the first-principle calculations by using the Korringa?CKohn?CRostoker coherent potential approximation (KKR-CPA) method in connection with the local density approximation (LDA), we study theoretically the electronic and magnetic properties of different point defects in ZnO, which are Zinc interstitials (Zn_i), Zinc antisites (Zn_O), Oxygen interstitials (O_i) and Oxygen antisites (O_Zn) defects in ZnO. The supercell calculations were also performed using the full potential local-orbital (FPLO) band structure scheme. This work presents detailed information about total and local density of states at some concentrations of these defects; the stability of the ferromagnetic state compared with the spin-glass state is investigated by comparing calculating their total energy. The results show on one hand that Zn_i and Zn_O produce a shallow donor bellow the bottom of the conduction band (CB), while O_i and O_Zn produces the shallow acceptors above the top of the valence band (VB), and moment magnetic; on other hand that the ferromagnetic state is more stable than the spin-glass in Oxygen interstitials (O_i) and vice versa for oxygen antisites (O_Zn) of native point defects in ZnO. The other native point defects (Zn_i, Zn_O, V_O, and V_Zn) have a zero magnetic moment. The results show that the Curie temperature increases with the concentration of interstitial oxygen.     

Magnetic Properties of Fe/Cr Layers Studied by Monte Carlo Simulations

The magnetic properties of antiferromagnetic Fe/Cr layers have been studied using Monte Carlo simulations within the Ising model framework. The considered Hamiltonian includes the exchange interactions between Cr–Cr, Fe–Fe and Cr–Fe and the external magnetic field. The thermal magnetizations of Fe/Cr layers are computed for a fixed size. The critical temperature is deduced. In addition, the magnetizations versus the external magnetic field are also established.     

Magnetic Properties of SrO Doped with 3d Transition Metals

Based on the density functional theory and using the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) method, we study the (Sr, TM)O doped systems where TM = V, Cr, Mn, Fe, Co, and Ni atoms. In particular, we start first by relaxing the parameters of the corresponding structures. Then we discuss its electronic structures, magnetic stabilities, and half-metal properties using 3d transition metals. Among others, it has been shown that doping with Cr, Mn, Fe, and Co, the ferromagnetic phase can be stabilized using a double exchange mechanism. Moreover, we find that the half-metallic properties of these compounds are formed due to a large exchange splitting and the delocalized properties of the majority spin e _g state and the minority spin t _eg states.     

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.     

Effect of rare earth on physical properties of Na0.5Bi0.5TiO3 system: A density functional theory investigation

Na_0.5(Bi_3/4RE_1/4)_0.5TiO₃ (RENBT, RE = Nd, Gd, Dy, and Ho) compounds were investigated in the framework of first-principles calculations using the full potential linearized augmented plane wave (FP-LAPW) method based on the spin-polarized density functional theory implemented in the WIEN2k code. Combined charge density distribution and Ti K-edge X-ray absorption spectra reveal that the RENBT compositions with high polarization values are accompanied by a higher TiO₆ distortion, DyNBT, and NdNBT compounds. The effect of the rare-earth elements on the polarization is confirmed experimentally with the collection of the hysteresis loops. The investigation of the electronic properties of the compounds highlights the emergence of a magnetization owing to the 4f orbital effect of the rare-earth elements. Besides, the investigation of the chemical ordering shows a short-range chemical ordering for the pure composition and an increased A-site disorder for dysprosium doped NBT system. The increased disorder may speak for increased relaxor properties in the RE doped compositions.