Showing papers on "Curie–Weiss law published in 2021"

Modified Curie-Weiss law for j eff magnets

Abstract: In spin-orbit-coupled magnetic materials, the usually applied Curie-Weiss law can break down. This is due to potentially sharp temperature dependence of the local magnetic moments. We therefore propose a modified Curie-Weiss formula suitable for analysis of experimental susceptibility. We show for octahedrally coordinated materials of ${d}^{5}$ filling that the Weiss constant obtained from the improved formula is in excellent agreement with the calculated Weiss constant from microscopic exchange interactions. Reanalyzing the measured susceptibility of several Kitaev candidate materials with the modified formula resolves apparent discrepancies between various experiments regarding the magnitude and anisotropies of the underlying magnetic couplings.

Curie–Weiss behavior of the low-temperature paramagnetic susceptibility of semiconductors doped and compensated with hydrogen-like impurities

Abstract: For the first time, a quantitative model of the Curie–Weiss behavior of a low-temperature paramagnetic susceptibility of electrically neutral donors in n-type diamagnetic covalent semiconductors is proposed. The exchange interaction between nearest two neutral donors was calculated with the use of the Heitler–London model. In this model, we take into account the change in the thermal ionization energy of donors due to the shift of the bottom of the conduction band to the bandgap with doping and compensation. The energy of the exchange spin–spin interaction between electrons localized on donors is calculated as a function of the donor concentration and the degree of their compensation by acceptors. The broadening of the donor band due to the Coulomb interaction of the nearest impurity ions was taken into account. We considered crystals of n-type germanium doped with arsenic up to the concentration close to the insulator–metal phase transition (Mott transition) and compensated with gallium. The compensation ratio K is the ratio of the concentration of compensating acceptors KN to the concentration of doping donors N. The model predicts a change in the sign of the Curie–Weiss temperature from minus to plus (a transition from the antiferromagnetic to ferromagnetic local ordering of electron spins on donors) for K ≈ 0.15–0.3, reaching its maximum positive values of ≈1.3 K for K ≈ 0.5 with the following decrease (a transition to paramagnetism) for K > 0.85. The calculated behavior of the paramagnetic susceptibility of donors is consistent with the experimental data for compensated n-Ge:As,Ga samples close to the Mott transition.

Energy density and storage capacity of La3+ and Sc3+ co-substituted Pb(Zr0.53Ti0.47)O3 thin films

Abstract: We studied the energy density and storage capacity properties of rare-earth modified lead zirconate titanate thin films. Highly oriented thin films of (PbZr0.53Ti0.47)(1−y)(LaxSc1−x)yO3 wherein; [for y = 0 and x =0 viz PL0] and, [for y = 0.1 and x = 0.2, 0.4, 0.6 and 0.8 viz PL2, PL4, PL6 and PL8 respectively] abbreviated as PL10x have synthesized on MgO (100) substrate by the pulsed laser deposition technique. The higher proportion of lanthanum increased the broadening of dielectric permittivity and dielectric maxima that shifted to higher temperatures with increasing frequency, signifying the relaxor-type behavior of these films. The value of the relaxation parameter varies from γ = 1.69 for PL6 and 1.95 for PL8 that was estimated from the linear fit of the modified Curie-Weiss law indicating the relaxor nature satisfying Vogel-Fulcher relation. Furthermore, we achieved enhanced spontaneous polarization of the fabricated thin films. Slim loop hysteresis was observed on tuning lanthanum and scandium and the estimated recovered energy density (Ure) is 51.15 J cm−3 and 26.54 J cm−3 with efficiency (η) of 47.38% and 65.88% respectively for PL6 and PL8 thin films. The high dielectric permittivity, high breakdown strength, and enhanced energy storage density of thin films could make it promising materials for memory, power electronics, and energy storage applications.

Effect of Ti doping on electronic and magnetic properties of Sm0.55Sr0.45Mn1- xTixO3 (0.0 ≤ x ≤ 0.2)

Abstract: In this work, tetravalent Ti doping effect on structural, electrical, and magnetic properties of disordered Sm0.55Sr0.45Mn1- xTixO3 (0.0 ≤ x ≤ 0.2) system was investigated. Unlike the metallic nature of resistivity below Curie temperature TC in parent compound, the Ti-doped composition exhibits only semiconducting-type behavior throughout the measured temperature range. The electrical transport results above TC can be explained within the framework of variable range hopping (VRH) and small polaron hopping (SPH) models. Furthermore, magnetization measurements show drastic reduction of TC, Curie Weiss temperature (θCW) and effective magnetic moment (µeff) due to replacement of Ti4+ at Mn4+ site, which can broke exchange coupling of Mn3+/Mn4+ network, destroy the long-range ferromagnetic order.

Local Central Limit Theorem for Multi-group Curie–Weiss Models

Abstract: We study a multi-group version of the mean-field Ising model, also called Curie–Weiss model. It is known that, in the high-temperature regime of this model, a central limit theorem holds for the vector of suitably scaled group magnetisations, that is, for the sum of spins belonging to each group. In this article, we prove a local central limit theorem for the group magnetisations in the high-temperature regime.

First-principles investigation of Nd(Fe,M)12 (M = K--Br) and Nd(Fe,Cr,Co,Ni,Ge,As)12: Possible enhancers of Curie temperature for NdFe12 magnetic compounds

Abstract: We investigate the effects of various dopants (M = K--Br) on the Curie temperature of the magnetic compound NdFe12 through first-principles calculations. Analysis by the Korringa--Kohn--Rostoker method with the coherent potential approximation reveals that doping the Fe sites with optimal concentrations of Ge and As is a promising strategy for increasing the Curie temperature. To search over a wider space, we also perform Bayesian optimization. Out of over 180,000 candidate compositions, co-doped systems with Co, Ge, and As are found to have the highest Curie temperatures.

Limit Theorems for Multi-Group Curie-Weiss Models via the Method of Moments

Abstract: We define a multi-group version of the mean-field or Curie-Weiss spin model. For this model, we show how, analogously to the classical (single-group) model, the three temperature regimes are defined. Then we use the method of moments to determine for each regime how the vector of the group magnetisations behaves asymptotically. Some possible applications to social or political sciences are discussed.

Semiclassical Approximation for the Curie – Weiss Model

Abstract: The paper is devoted to the construction of spectral series and the estimation of the approximation accuracy for the operator of the Curie – Weiss model. In the course of work, the operator is reduced to a tridiagonal form in the subspace of the original space, then to a second-order difference equation. The admissibility of reducing an operator to a subspace is presented. It is shown that the difference equation can be considered in the discrete semiclassical approximation. In the obtained classical system, the dependence of the turning points on the model parameters is investigated. The asymptotics of the spectrum of the Curie-Weiss operator is calculated and the accuracy of the approximation is estimated.

Magnetic Properties of Undoped and Bi doped LaMnO3

Abstract: Perovskite materials have wide range of applications in the area of spintronics and thermoelectricity. Among manganites LaMnO3 is a well studied simple perovskite system due to its interesting magnetic and transport properties.Most of the investigation is concentrated on the properties of A site hole- doped(A=Ca,Sr,Ba etc) LaMnO3.Previous studies report that undoped LaMnO3 as an antiferromagnetic (AFM) insulator having Mn3+ oxidation state.It arises from the superexchange AFM interactions between Mn3+ ions.Here ,we have synthesized LaMnO3 and La1-xBixMnO3(x=0.1,0.2) from La2O3 ,Bi2O3 and MnO2 through three step heating process by conventional solid state route. It crystallizes in orthorhombic phase with space group pnma,structural and magnetic properties were also investigated with increasing Bi content.The saturating hysteresis loops confirm ferromagnetic (FM) nature with transition temperature TC=241.66K and have high magnetization with small coercive field for LaMnO3. From Curie Weiss plot at 500Oe gives C-W temperature(θ) as 233.68K ,the positive value of θ is another justification for the FM material.The ferromagnetic order may be due to the evolution of oxygen stoichiometry,during synthesize the material can be oxidized, leading to the presence of both Mn3+ and Mn4+ results double exchange ferromagnetic interactions between Mn3+ and Mn4+ ions[1]. There is a peak in the zero field cooled (ZFC) magnetization at a temperature (Tp) 43.20K.,the presence of cusp in ZFC is an indication of glassy nature.Epitaxial LaMnO3 films [2] exhibit a similar peak in previous report and identified as glassy nature.The magnetization versus temperature measurement is carried out at different field confirms the ferromagnetic ordering.The variation of oxygen content plays a major role in the magnetic properties of manganites.Generally hole doping at the cationic La site with Sr,Ba,Ca generates unusual magnetic properties like colossal magneto resistance (CMR) and ferromagnetic property in LaMnO3. La site doped with Bi results in noticeable magnetic properties,it has similar ionic radius and valency as that of La.It belongs to the orthorhombic crystal symmetry with pnma space group, similar to undoped LaMnO3.The transition temperature (TC) changes with Bi substitution,TC =157.16K for x=0.1 and TC=104.06K for x=0.2.At low temperatures it shows ferromagnetic nature and above TC transforms into paramagnetic phase. The linear fit of the 1/χ vs T gives positive CW temperatures for doped system.From previous reports Bi substitution act like divalent ions similar to Ba or Sr [3].LaMnO3 with x=0.1 concentration of Bi shows minute splitting in ZFC –FC curve at low temperature at 500Oe field.There is a visible bifurcation in the ZFC-FC curve for x=0.2 doped LaMnO3 and observed a cusp in the ZFC magnetization at low temperature. The bifurcation and the cusp are the primary features of glassy like behaviour and also it shows a spin glass [4] like behavior in Bi doped LaMnO3.The synthesized undoped and Bi doped LaMnO3 exhibit ferromagnetic nature instead of antiferromagnetic behavior,which is confirmed from various magnetic measurements and there exist exotic magnetic property like glassy nature.

Fluctuations of the Magnetization in the p-Spin Curie-Weiss Model

Abstract: In this paper we study the fluctuations of the magnetization in the p-spin Curie–Weiss model, for $$p \geqslant 3$$ . We provide a complete description of the asymptotic distribution of the magnetization in the p-spin Curie–Weiss model, complementing the well-known results in the 2-spin case. Our results unearth various new phase transitions, such as the existence of a certain ‘critical’ curve in the parameter space, where the limiting distribution of the magnetization is a discrete mixture, with local Gaussian fluctuations around each of the atoms. The number of atoms (mixture components) is either two or three depending on the sign of one of the parameters and the parity of p. Another interesting revelation is the existence of certain ‘special’ points in the parameter space where the magnetization converges to a non-Gaussian limiting distribution at rate $$N^{\frac{1}{4}}$$ .

Anomalous magnetic behavior of BaTiO3 and La0.7Ca0.3MnO3 composite system

Abstract: We report dc magnetization study of a composite made of well known ferroelectric material BaTiO3 (BTO) and a colossal magnetoresistance material (CMR) La0.7Ca0.3MnO3(LCMO). The phase purity of the parent compounds and the composite are confirmed from Rietveld refinement of the XRD pattern. The insignificant change of the spin state of the composite with respect to parent ingredient has been confirmed from same values of Curie constant (C) and Curie Weiss temperature (θC). A significant reduction of the magnetic moment of the composite below Curie temperature is explained in terms of magnetoelectric coupling induced magnetic anisotropy inside LCMO present in the composite and this magnetic anisotropy is observed to be wakened at lower temperature.