Monte Carlo Study of the Core-Shell Phase Transitions in Nanostructure Systems

We present in this work the effects of the external magnetic field, the exchange anisotropy, the coupling exchange interaction and the temperature on the magnetic properties of the nanostructure systems. More exactly, the effect of the core-shell spin Ising systems has been also examined. A number of favorable phenomena such as the wetting magnetic and the layering transitions have been investigated. The Monte Carlo simulations have been used to investigate the behavior of these nanostructure systems and to obtain the interesting results presented in the form of the phase diagrams.     

Kinetic Monte Carlo and density functional study of hydrogen diffusion in magnesium hydride MgH2

In this work, we propose a model to investigate the hydrogen storage ab/desorption kinetic properties for the pure MgH₂ using Kinetic Monte Carlo (KMC) simulations. To do such computations, the activation energies for different elementary processes have been estimated. Moreover, the different thermodynamical quantities of interest are determined by performing the ab-initio calculations. Then, we discuss the hydrogen diffusion in magnesium hydride (such as adsorption, diffusion and desorption). More precisely, we study the effect of each elementary mechanism on the diffusion that we characterize through the density distribution of hydrogen atoms including filling ratios, diffusion time, temperature and pressure. Among others, we show that all elementary mechanisms are needed to reproduce a behavior of ab/desorption which correlates well with the experimental results reported in literature. In particular, at high temperature and pressure, the results of simulations indicate that the studied material involves slow kinetics.     

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.     

Magnesium vacancies and hydrogen doping in MgH2 for improving gravimetric capacity and desorption temperature

Performing an ab initio analysis, we inspect the effect of magnesium vacancies and hydrogen doping on the magnesium hydride (MgH₂). The Korringa – Kohn – Rostoker method integrated with the coherent potential approximation is used to perform our calculations. In particular, we find that the gravimetric capacity of MgH₂ increases from 7.658 to 9.816 wt% when the concentrations of magnesium vacancies and hydrogen dopant atoms increase from 0 to 10%. Concretely, the results reveal that the magnesium vacancies and the hydrogen doping have a beneficial effect on the hydrogen storage properties of the hydride by decreasing its desorption temperature and stability. This decrease can be explained on the one hand by the diminution of the number of Mg atoms that establish strong bonds with H atoms, and on the other hand by using the density of states, which indicates that when the concentrations increase, the Mg and H states shift to the conduction band. We also obtain that the value of the desorption temperature can be controlled by varying the concentrations of magnesium vacancies and hydrogen dopant atoms from 4.2 to 5.8% in order to reach the optimum range 289–393 K for the practical use of fuel cell vehicles.     

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.     

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 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.     

Magnetic Properties of Zn0.8(Fe0.1,Co0.1)O Diluted Magnetic Semiconductors: Experimental and Theoretical Investigation

Structural and magnetic properties of Zn_0.8(Fe_0.1, Co_0.1)O bulk diluted magnetic semiconductor have been investigated using X-ray diffraction (XRD) and magnetic measurements. TEM (Transmission Electron Microscopy) images confirmed the high crystallinity and grain size of Zn_0.8(Fe_0.1,Co_0.1)O powder, the samples were characterized by energy dispersive spectroscopy (EDS) to confirm the expected stoichiometry. This sample has been synthesized by co-precipitation route. The study of magnetization hysteresis loop measurements infers that the bulk sample of Zn_0.8(Fe_0.1,Co_0.1)O shows a well-defined hysteresis loop at T _c (200?K) temperature, which reflects its ferromagnetic behavior. Hydrogenation treatment was used for the control of phase separation. Based on first-principles spin-density functional calculations, using the Korringa?CKohn?CRostoker method (KKR) combined with the coherent potential approximation (CPA), the ferromagnetic state energy was calculated and compared with the local-moment-disordered (LMD) state energy. The mechanism of hybridization and interaction between magnetic ions in Zn_0.8(Fe_0.1,Co_0.1)O is also investigated.     

Ferromagnetism from Acceptors of Native Point Defects in (Zn, Mn)O Doped Systems

In this paper, we represent a theoretical study of the (Zn, Mn)O doped system with native point defects of ZnO as oxygen interstitials (O_i) and zinc vacancies (V_Zn). Under these defects, it has been shown that the ground state can be converted from spin glass to ferromagnetic phase, by performing ab initio density functional theory calculations relying on the Korringa?CKohn?CRostoker coherent potential approximation (KKR-CPA) method employing the local density approximation (LDA) with the parameterization by Morruzi, Janak, and Williams. The stability of magnetism in the (Zn, Mn)O doped system with different native point defects has been discussed. We find that acceptor-like defects (that is, O_i, O_Zn, and V_Zn) may cause the enhancement of the ferromagnetic characteristics in ZnMnO with increasing T _c . Based on the theoretical results, we suggest that the native point defects have a key role with respects to the FM properties. Using the mean field approximation, the Curie temperature in our studied model is estimated.