Results 1 - 10 of 33
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[en] Zinc ferrite nanocrystals were synthesized from metal chloride precursors via chemical co-precipitation method, using different synthesis conditions. Characterization measurements including X-ray diffraction (XRD), transmission electron microscopy (TEM) and super conductiong quantum device (SQUID) were used to study the influence of precursor's concentration and reaction time on the crystalline structure, average sizes and magnetic properties of zinc ferrite nanoparticles. The transmission images show spherical, homogenous shape and particle size ranging from 16 to 22 nm. DC magnetization (2-300 K) measurements reveal a superparamagnetic behavior for the ZnFe2O4 samples with a blocking temperature in the range of 18-24 K. Our results demonstrate that magnetic properties of magnetic particles can be largely modified by just changing the reaction condition such as concentration and reaction time, which might be a useful way to design novel magnetic materials. (paper)
[en] Magnetocaloric effect on SrFe12O19 ceramic have been studied using Monte Carlo simulation. The thermal magnetization, dM/dT, magnetic entropy, and the specific heat of SrFe12O19 ceramic are obtained for several magnetic fields. The temperatures dependence of the magnetic entropy and of the adiabatic temperature for a several magnetic field have been obtained. The field dependence of relative cooling power (RCP) of SrFe12O19 ceramic has been determined for a several magnetic fields. The magnetic hysteresis cycle of SrFe12O19 ceramic has been obtained for a several temperatures. The obtained values are close to the experimental values. The transition paramagnetic to ferromagnetic is found at the Curie temperature. The second phase transition is also obtained around the Curie temperature.
[en] The first-principles density functional calculation is used to investigate the electronic structures and magnetic properties of Mn-doped and N-co-doped ZnO nanofilms. The band structure calculation shows that the band gaps of ZnO films with 2, 4, and 6 layers are larger than the band gap of the bulk with wurtzite structure and decrease with the increase of film thickness. However, the four-layer ZnO nanofilms exhibit ferromagnetic phases for Mn concentrations less than 24% and 12% for Mn-doping performed in the whole layers and two layers of the film respectively, while they exhibit spin glass phases for higher Mn concentrations. It is also found, on the one hand, that the spin glass phase turns into the ferromagnetic one, with the substitution of nitrogen atoms for oxygen atoms, for nitrogen concentrations higher than 16% and 5% for Mn-doping performed in the whole layers and two layers of the film respectively. On the other hand, the spin-glass state is more stable for ZnO bulk containing 5% of Mn impurities, while the ferromagnetic phase is stable by introducing the p-type carriers into the bulk system. Moreover, it is shown that using the effective field theory for ferromagnetic system, the Curie temperature is close to the room temperature for the undamped Ruderman—Kittel—Kasuya—Yoshida (RKKY) interaction
[en] CoFe2O4 and MnFe2O4 ferrite nanoparticles were prepared by the co-precipitation method. The structural evolutions of the nanophase have been studied. The refinement result showed that the type of cationic distribution over the tetrahedral and octahedral sites in the nanocrystalline lattice is a partially inverse spinel. The morphology of the samples has been determined using transmission electron microscopy and the magnetic properties of the samples are given. The gap energy is calculated using the Korringa–Kohn–Rostoker method combined with a coherent potential approximation and using optical measurements for CoFe2O4 and MnFe2O4. This study has been extended theoretically to the mixed spinel ferrites, FeCoMnxFe1−xO4. Indeed, the saturation magnetization, the critical temperature, exchange interactions and Curie constant for the cation distribution (Fe3+)A (Co2+Fe1-x3+Mnx3+)B O42-, have been calculated. The high temperature series expansion combined with the Padé approximant are used to determine the critical temperature and critical exponent (γ) associated with the magnetic susceptibility of mixed spinels. (paper)
[en] In this work, the hydrogen storage properties of the Mg-based hydrides, i.e., Mg1−x Mx H2 (M = Ti, V, Fe, 0 ≤ x ≤ 0.1), are studied using the Korringa—Kohn—Rostoker (KKR) calculation with the coherent potential approximation (CPA). In particular, the nature and concentrations of the alloying elements and their effects are studied. Moreover, the material's stability and hydrogen storage thermodynamic properties are discussed. In particular, we find that the stability and the temperature of desorption decrease without significantly affecting the storage capacities
[en] In this work, using the ab-initio calculations, we have investigated the phantom magnetism when the diamagnetic solids, carbon and nitrogen with d doped CdTe. We have applied in these calculations the combination between the Korringa-Kohn-Rostoker and coherent potential approximation method within the local density approximation and generalized gradient approximation (GGA). In this study, the doped compound presents a metallic behavior for both the approximations characterized by a small moment of about 0.299/0.326 and 0.249/0.266 μ for 24% of C and N, respectively. The polarization has shown a low and decreasing value from 43.73/59.56 to 0.29/2.26% for 9% and 24% of C impurity concentration, respectively. Unlike for the case of N, this parameter varies from 76.7/72.86 to 85.29/83.63% for 9% and 24% concentration, respectively. In addition, we have determined the mechanism of ferromagnetic coupling for the C- and N-doped CdTe. Furthermore, the stability of the compound is investigated by comparing the energy difference between the spin glass and ferromagnetic states. It is found that below the percolation threshold, contrary to the case of doping by N except for 20% using GGA, the C impurities lead to the most ferromagnetic stable phase. While the system changes its stability above this threshold when doped by the C impurities. Finally, we have estimated and discussed the Curie temperature using the mean field approximation.
[en] Self-consistent ab initio calculations, based on the density functional theory (DFT) and using the full potential linear augmented plane wave (FLAPW) method, are performed to investigate both electronic and magnetic properties of the MnS layers. Polarized spin and spin—orbit coupling are included in the calculations within the framework of the antiferromagnetic state between two adjacent Mn layers. Magnetic moments considered to lie along axes are computed. Obtained data from ab initio calculations are used as input data for the high temperature series expansion (HTSE) calculations to compute other magnetic parameters. The zero-field high temperature static susceptibility series of the spin-4.39 nearest-neighbour Heisenberg model on centred face cubic (FCC) and lattices is thoroughly analysed by a power series coherent anomaly method (CAM). The exchange interactions between the magnetic atoms, the Néel temperature, and the critical exponent associated with the magnetic susceptibility are obtained for the MnS layer. (condensed matter: electronic structure, electrical, magnetic, and optical properties)
[en] In this work, we have studied the electronic, electrical and optical properties of HoMn2O5 using first-principles density functional theory within the generalized gradient approximation (GGA). The electrical conductivity decreases with increasing temperature and it exhibits metal-like behavior. The maximum conductivity values as a function of relaxation time reach 11,44.1022 (Q-1m-1s-1) at 300 K. The optical calculation presents this material a good absorbing light in the visible region which is good for many optoelectronic and photovoltaic applications. (paper)
[en] Based on first-principles spin-density functional calculations, using the Korringa-Kohn-Rostoker method (KKR) combined with the coherent potential approximation (CPA), we investigated the magnetic and half-metallic properties of Mn-doped p-type ZnO and the mechanism which control these properties. Mn-doped ZnO is anti-ferromagnetic spin-glass state, but it becomes half-metallic ferromagnetic upon holes doping. The electronic structure, total magnetic moment of Zn0.8Mn0.2O1-yNy and magnetic moments of Mn and N in Zn0.8Mn0.2O1-yNy are calculated for different holes (y) concentrations. In this paper we address the origin of half-metallic and ferromagnetic properties as controlled and oriented by the nature of hybridization of the Mn (3d) state and host p(N) states. The band structure has been used to explain the strong ferromagnetism observed in Zn0.8Mn0.2O0.1N0.9. We applied magnetic fields to Mn and we calculated the spin magnetic moments of Mn and N. We show that the spin alignments of Mn atoms and the interlocking N atoms can be shown as Mn(↑)-N(↓)-Mn(↑), indicating that ferromagnetism is mediated through the RKKY or double exchange interaction between the carriers and Mn atoms. We show that for weak holes concentrations the ferromagnetism is due to the double exchange interaction, and for higher holes concentrations the RKKY exchange interaction, mediated by mobile holes, strongly oscillates with distance. Finally, we propose a damped or undamped RKKY interaction model to describe the exchange coupling constants Jij between the local moments Mni and Mnj
[en] The exchange interactions of the nearest-neighbor exchange constant between tetrahedral and octahedral sublattices (JAB(x)), nearest-neighbor exchange constant inside tetrahedral sublattice (JAA(x)) and nearest-neighbor exchange constant inside octahedral sublattice (JBB(x)) in cobalt and zinc chromites are calculated using the probability distribution. The Curie-Weiss temperature and the critical temperature are deduced using the mean field and the high temperature series expansion theories in ZnxCo1-xCr2O4. The critical exponent associated with the magnetic susceptibility (γ) is deduced for CoCr2O4. (authors)