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[en] The magnetic properties of spinel GeNi2-2xCo2xO4 systems in the range 0 ≤ x ≤ 1 are studied by mean Geld theory and high-temperature series expansions. The nearest neighbouring and the next-neighbouring super-exchange interactions J1(x) and J2(x) are evaluated for these systems in the range 0 ≤ x ≤ 1, by using the first theory. The intra-planar and the inter-planar interactions and the exchange energy are deduced. The second theory is applied in the spinel GeNi2-2xCo2xO4 systems, combined with the Padé approximants method, to determine the magnetic phase diagrams (Tn versus dilution x) in the range 0 ≤ x ≤ 1. The obtained theoretical results are in agreement with experimental data obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility γ and the correlation lengths v are deduced. (condensed matter: electronic structure, electrical, magnetic, and optical properties)
[en] Using the mean field theory, we investigate the effect of the random crystal-field on the spin-3/2 Blume-Capel model. Several new features are found including the appearance of new order phase with the magnetization m=1 at low temperature and consequently a rich ground-state phase diagram. At finite temperature, new types of phase diagrams appear. Furthermore, we show that at low temperature a first-order transition line terminated at isolated critical points, between the ferromagnetic phases. A reentrant phenomenon is also discussed
[en] Using mean field theory and high-temperature series expansions (HTSEs), extrapolated with the Pade approximants method, the effect of Zn doping on magnetic properties of NiFe2O4 ferrite spinel has been studied. The nearest neighbour super-exchange interactions for intra-site (JAA, JBB) and inter-site (JAB) of the ZnxNi1-xFe2O4 ferrites spinels, in the range 0≤x≤1, have been computed using the probability approach, based on Moessbauer data. The paramagnetic Curie-Weiss temperature θ and the Curie temperature TC are calculated as a function of Zn concentration. The critical exponent γ associated with magnetic susceptibility is calculated. The spin correlation functions intra-plane and inter-plane have been also computed and compared with exchange couplings. The obtained theoretical results are in good agreement with experimental ones obtained by magnetic measurements and Moessbauer spectroscopy.
[en] A mean-field approximation is used to study the effects of random crystal field on the critical behaviour of decorated ferrimagnetic Ising model, in which the two magnetic atoms A and B have spins σA=12 and SB=1, respectively. The results indicate that there may exist some interesting phenomena in the system, such as the appearance of a new ferrimagnetic phase, namely partly ferrimagnetic phase, and the possibility of one or two compensation temperatures. Re-entrant phenomena can be seen for appropriate ranges of crystal field. Phase diagrams and magnetization curves are investigated in details
[en] The exchange interactions and the magnetic exchange energies are calculated by using the mean field theory and the probability law of Zn1-xMnxCr2O4 nanoparticles. The high-temperature series expansions have been applied in the spinels Zn1-xMnxCr2O4 systems, combined with the Pade approximants method, to determine the magnetic phase diagram, i.e. TC versus dilution x. The critical exponent associated with the magnetic susceptibility (γ) is deduced. The obtained value of γ is insensitive to the dilution ratio x and may be compared with other theoretical results based on the 3D Heisenberg model.
[en] The susceptibility of body-centered cubic lattice (bcc) antiferromagnetic spin-1/2 through two model: Ising and XY films consisting of n=2,3,4,5, and 6 interacting layers is studied by high-temperature series expansions. Sixth order series in x=βJ(β=1/(kBT) ) have been obtained for free-surface boundary conditions. The high temperature series expansion of the magnetic susceptibility series has been investigated by using the Pade approximant method. The Neel temperatures TN(n) as a function of the number of n spin layers in the films is obtained. The critical exponent γ associated with the magnetic susceptibility is deduced for Ising and XY models. The shifts of the critical temperatures from the bulk value [(TN(∞))/(TN(n))-1] can be described by a power law n-λ with λ=1/(ν), where ν is the correlation length exponent.
[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 magnetic properties of diluted spinels Zn1-xMxFe2O4 (M = Mg, Ni) systems have been studied by mean field theory, probability law and high-temperature series expansions method in the range 0 ≤ x ≤ 1. The exchange interactions are calculated by the first and second theory, respectively. The magnetic phase diagrams of Zn1-xMxFe2O4 (M = Mg, Ni) are calculated by the high-temperature series expansions, combined with the Pade approximants method. The critical exponent associated with the magnetic susceptibility (γ) is deduced.
[en] The nearest neighbour J1(x) and the next-neighbour super-exchange J2(x) interactions are evaluated by using the mean field theory for Mg1-xBxO (B=Co and Ni) systems. The magnetic energy E(x) is obtained. A magnetic phase diagram of the Mg1-xBxO (B=Co and Ni) solid solutions with 0≤x≤1 is drawn by high-temperature series expansions (HTSE) combined with the Pade approximants method (PA). The critical exponents associated with the magnetic susceptibility (γ) and with the correlation length (ν) are deduced in order phase.
[en] By ab-initio calculation we show that the (Ga,Fe)N ground state may be changed from anti-ferromagnetic to ferromagnetic by acceptor defect like Ga vacancies. The electronic structures are calculated by using the Korringa-Kohn-Rostoker (KKR) method combined with coherent potential approximation (CPA). We show that we can increase the magnetic moment of Fe in p-type GaN by oxygen co-doping. Mechanism of exchange interactions between magnetic ions in p-type (Ga,Fe)N is also studied. The effect of external magnetic field on the electronic structure of (Ga, Fe)N and p-type (Ga, Fe)N is investigated.