Showing papers on "Curie–Weiss law published in 2018"

Semicircle Law for Generalized Curie–Weiss Matrix Ensembles at Subcritical Temperature

Abstract: Hochstattler et al. (J Theor Probab 29:1047–1068, 2016) showed that the semicircle law holds for generalized Curie–Weiss matrix ensembles at or above the critical temperature. We extend their result to the case of subcritical temperatures for which the correlations between the matrix entries are stronger. Nevertheless, one may use the concept of approximately uncorrelated ensembles that was first introduced in Hochstattler et al. (2016). In order to do so, one needs to remove the average magnetization of the entries by an appropriate modification of the ensemble that turns out to be of rank 1, thus not changing the limiting spectral measure.

Coexistence of Stable Limit Cycles in a Generalized Curie-Weiss Model with Dissipation

Abstract: In this paper, we modify the Langevin dynamics associated to the generalized Curie–Weiss model by introducing noisy and dissipative evolution in the interaction potential. We show that, when a zero-mean Gaussian is taken as single-site distribution, the dynamics in the thermodynamic limit can be described by a finite set of ODEs. Depending on the form of the interaction function, the system can have several phase transitions at different critical temperatures. Because of the dissipation effect, not only the magnetization of the systems displays a self-sustained periodic behavior at sufficiently low temperature, but, in certain regimes, any (finite) number of stable limit cycles can exist. We explore some of these peculiarities with explicit examples.

Spin State of Cobalt and Electrical Transport Mechanism in MgCo2O4 System

Abstract: MgCo2O4 samples were synthesized by inverse co-precipitation method. The formation of a single-phase spinel structure was confirmed by X-ray diffraction measurements and Fourier-transform infrared spectroscopy. The samples crystallized in a face-centered cubic structure with Fd-3m space group as revealed from the Rietveld refinement of X-ray diffraction data. Magnetic measurements carried out in a broad temperature range of 5–300 K showed antiferromagnetic to paramagnetic phase transition (Neel temperature) observed at 101 K. Magnetic susceptibility data fitted using the Curie Weiss law and effective Bohr magnetic moment (μeff) for Co atoms was determined. Calculated μeff comes out to be 3.05 μB. These results were correlated to the spin states of Co3+ atoms. A small hysteresis in the field-dependent magnetization MH loop taken at 5 K indicates the existence of weak ferromagnetism in this system. The electrical resistivity measurement in the temperature range 77–750 K displayed the semiconducting-like behavior for this system.

Magnetic Properties of Some Tellurite Glasses

Abstract: The AC magnetic susceptibility in the range 5–130 K of the tellurite glass systems: TeO2–MnO2–ZnO–PbO and TeO2–MnO2–V2O5–Fe2O3 was measured and analyzed. The investigations of the AC magnetic susceptibility facilitated the determination of the molar susceptibility, paramagnetic magnetic susceptibility, paramagnetic Curie temperature, and magnetic entropy changes of the tellurite glasses. The results clarified that the temperature dependence of the magnetic susceptibility deviated from the Curie law and the increase of the small negative values of Curie temperature indicated negative interchange interactions between the antiferromagnetically coupled manganese ions within the present glass network. The magnetic moments evaluated from susceptibility measurements of the glasses show the predominance of the Mn2+ valence state than Mn3+ valence state of MnO2.

Enhanced magnetic moment and curie temperature due to co-substitution in Heusler alloys Fe2−xCoxMnSi

Abstract: Magnetic full Heusler alloys are extremely important for spintronics because of their half-metallicity. Since they can be synthesized without any rare-earth elements, they are interesting for applications based on their magnetic behavior also. Hence solid solutions of the two Heusler alloys Fe2MnSi and Co2MnSi, Fe2−xCoxMnSi with 0 ≤ x ≤ 0.4 have been synthesized so that their magnetic moment as well as Curie temperature can be tuned. The structural characterization using a combination of X-ray diffraction and Rietveld refinement of the structure shows that all the alloys are single phase with a cubic Fm $$\overline {3}$$ m structure. The lattice parameter does not change with Co-substitution in all the alloys. The magnetization studies as a function of temperature clearly shows a paramagnetic to ferromagnetic transition in all the alloys with the Curie temperature increasing from 220 K for x = 0 to 580 K for x = 0.4. It is found that the re-entrant antiferromagnetic transition observed in x = 0 alloy at 62 K can be suppressed either with a substitution of Co for Fe or by increasing the external magnetic field. The isothermal magnetization shows an extremely soft behavior with low remanence in all the alloys. The saturation magnetic moment is found to increase from 2.5 μB for x = 0 to 3.6 μB for x = 0.4 at 100 K with a simultaneous increase in the magnetocrystalline anisotropy energy density constant from 1.3 × 105 to 3.3 × 105 Jm−3. These results clearly show the capability of magnetic properties tunability of Heusler alloys with substitution.

Disorder can enhance large self-sustained oscillations in dissipative Curie-Weiss models

Abstract: We modify the spin-flip dynamics of a Curie-Weiss model with dissipative interaction potential (Dai Pra, Fischer and Regoli (2013)) by adding a site-dependent i.i.d. random magnetic field. The purpose is to analyze how disorder affects the time-evolution of the observables in the macroscopic limit. Our main result shows that, whenever the field intensity is sufficiently strong, a periodic orbit may emerge through a global bifurcation in the phase space, giving origin to a large-amplitude rhythmic behavior.

Crystal structures and property characterization of two magnetic frustration compounds

Abstract: We report the structure and physical properties of two quasi-two-dimensional triangular antiferromagnetic materials, Co0.66Al2Se3.53 and Ni0.61Al2Se3.55, which show highly magnetically frustrated characters. Powder X-ray diffractions demonstrate that Co0.66Al2Se3.53 and Ni0.61Al2Se3.55 possess identical space group of P-3m1 with lattice parameters a = 3.8089(1) A, c = 12.676(1) A and a = 3.7880(1) A, c = 12.650(1) A, respectively. Analyzing the susceptibility data of Co0.66Al2Se3.53 reveal a Curie Weiss temperature of −216 K, and a spin-freezing transition temperature of 4.5 K, giving a frustration index f = −θcw/Tf ≈ 48. Ni0.61Al2Se3.55 possesses an effective moment of 2.38 µB, a Curie–Weiss temperature of −62 K with no sign of spin-freezing transition down to 2 K. The AC susceptibility data of Co0.66Al2Se3.53 suggest a spin glass-like transition, but no intersite mixing between Co2+ and Al3+ was observed from the X-ray photoelectron spectroscopy measurements.

Dynamics of quantum measurements employing two Curie-Weiss apparatuses

Abstract: Two types of quantum measurements, measuring the spins of an entangled pair and attempting to measure a spin at either of two positions, are analysed dynamically by apparatuses of the Curie-Weiss type. The outcomes comply with the standard postulates.

Critical Regime in a Curie-Weiss Model with two Groups and Heterogeneous Coupling

Abstract: We discuss a Curie-Weiss model with two groups in the critical regime. This is the region where the central limit theorem does not hold any more but the mean magnetization still goes to zero as the number of spins grows. We show that the total magnetization normalized by $N^{3/4}$ converges to a non-trivial distribution which is not Gaussian, just as in the single-group Curie-Weiss model.