Showing papers on "Curie–Weiss law published in 2006"
TL;DR: In this paper, it was shown that the magnetic moment of half-metallic ferromagnets can be calculated from the generalized Slater-Pauling rule, and empirically that the Curie temperature of Co2-based Heusler compounds can be estimated from a seemingly linear dependence on the magnetic moments.
Abstract: A concept is presented serving to guide in the search for materials with high spin polarization. It is shown that the magnetic moment of half-metallic ferromagnets can be calculated from the generalized Slater-Pauling rule. Furthermore, it was found empirically that the Curie temperature of Co2-based Heusler compounds can be estimated from a seemingly linear dependence on the magnetic moment. As a successful application of these simple rules, it was found that Co2FeSi is, actually, the half-metallic ferromagnet exhibiting the highest magnetic moment and the highest Curie temperature measured for a Heusler compound.
223 citations
TL;DR: The magnetic moment of Co2FeSi was measured to be about 6μB at 5 K and the Curie temperature was measured with different methods to be (1100±20)K.
Abstract: Co2FeSi crystallizes in the ordered L21 structure as proven by x-ray diffraction and Moβbauer spectroscopy. The magnetic moment of Co2FeSi was measured to be about 6μB at 5 K. Magnetic circular dichroism spectra excited by soft x-rays were taken to determine the element-specific magnetic moments of Co and Fe. The Curie temperature was measured with different methods to be (1100±20)K. Co2FeSi was found to be the Heusler compound as well as the half-metallic ferromagnet with the highest magnetic moment and Curie temperature.
168 citations
TL;DR: In this article, the authors used simulation techniques with atomistic resolution to show that this conventional micromagnetic approach fails for higher temperatures since they find two effects which cannot be described in terms of the LLG equation: (i) enhanced damping when approaching the Curie temperature and (ii) a magnetization magnitude that is not constant in time.
Abstract: In conventional micromagnetism magnetic domain configurations are calculated based on a continuum theory for the magnetization. This theory assumes that the absolute magnetization value is constant in space and time. Dynamics is usually described with the Landau-Lifshitz-Gilbert (LLG) equation, the stochastic variant of which includes finite temperatures. Using simulation techniques with atomistic resolution we show that this conventional micromagnetic approach fails for higher temperatures since we find two effects which cannot be described in terms of the LLG equation: (i) an enhanced damping when approaching the Curie temperature and, (ii) a magnetization magnitude that is not constant in time. We show, however, that both of these effects are naturally described by the Landau-Lifshitz-Bloch equation which links the LLG equation with the theory of critical phenomena and turns out to be a more realistic equation for magnetization dynamics at elevated temperatures.
158 citations
TL;DR: In this paper, a relativistic generalization of the disordered local moment theory was proposed to describe the variation of magnetic anisotropy with temperature, T,i n metallic ferromagnets.
Abstract: We present a first-principles theory of the variation of magnetic anisotropy, K, with temperature, T ,i n metallic ferromagnets. It is based on relativistic electronic structure theory and calculation of magnetic torque. Thermally induced local moment magnetic fluctuations are described within the relativistic generalization of the disordered local moment theory from which the T dependence of the magnetization, m, is found. We apply the theory to a uniaxial magnetic material with tetragonal crystal symmetry, L10-ordered FePd, and find its uniaxial K consistent with a magnetic easy axis perpendicular to the Fe/Pd layers for all m and proportional to m 2 for a broad range of values of m. This is the same trend that we have previously found in L10-ordered FePt and which agrees with experiment. We also study a magnetically soft cubic magnet, the Fe50Pt50 solid solution, and find that its small magnetic anisotropy constant K1 rapidly diminishes from 8 eV to zero. K1 evolves from being proportional to m 7 at low T to m 4 near the Curie temperature. The accounts of both the tetragonal and cubic itinerant electron magnets differ from those extracted from single ion anisotropy models and instead receive clear interpretations in terms of two ion anisotropic exchange.
153 citations
TL;DR: In this paper, the authors synthesized polycrystalline Sr2IrO4 and measured its magnetic susceptibility, electrical resistivity, specific heat, Seebeck coefficient, and thermal conductivity.
Abstract: We have synthesized polycrystalline Sr2IrO4 and measured its magnetic susceptibility, electrical resistivity, specific heat, Seebeck coefficient, and thermal conductivity. The magnetic susceptibility χ(T) shows a ferromagnetic transition at 250 K while the behaviour above the transition temperature is well described by a Curie–Weiss fit with a small effective moment μeff = 0.33 μB and a paramagnetic Curie–Weiss temperature, θCW = +251 K, consistent with previous studies on this compound. However, specific heat, Seebeck coefficient, and thermal conductivity are all dominated by the phonon contribution and show no anomalies at the ferromagnetic transition. Electrical resistivity, unlike the single crystal, shows a huge increase, three orders of magnitude, with decreasing temperature. The temperature dependence of resistivity is logarithmic at high temperatures (210 K
80 citations
TL;DR: In this paper, the electron paramagnetic resonance (EPR) and optical absorption spectra of 0.5 mol% CuO doped with mixed alkali glasses have been investigated.
Abstract: Electron Paramagnetic Resonance (EPR) and optical absorption spectra of 0.5 mol% CuO doped $xLi_{2}O-(30-x)Na_{2}O-{69.5}B_{2}O_{3}$ (5 \leq x \leq 25) mixed alkali glasses have been investigated. The EPR spectra of all the investigated samples exhibit resonance signals characteristic of $Cu^{2+}$ ions in octahedral sites with tetragonal distortion. It is found that the spin-Hamiltonian parameters do not vary much with x. It is interesting to observe that the number of $Cu^{2+}$ ions participating in resonance (N) and its paramagnetic susceptibility (χ) exhibits the mixed alkali effect with composition. It is observed that the temperature dependence of paramagnetic susceptibility (χ) obeys Curie–Weiss law. The paramagnetic Curie temperature $(\theta_p)$ is negative for the investigated sample, which suggests that the copper ions are antiferromagnetically coupled by negative super exchange interactions at very low temperatures. A broad band corresponding to the transition $(^{2}B_{1g}\longrightarrow^{2}B_{2g})$ in the optical absorption spectrum shows a blue shift with x. By correlating the EPR and optical absorption data, the molecular orbital coefficients $\alpha^{2}$ and $\beta^{2}_{2}$ have been evaluated. It is interesting to observe that the optical band gap $(E_{opt})$ and Urbach energies $(\Delta{E})$ exhibit the mixed alkali effect. The theoretical values of optical basicity $({\Delta}_{th})$ have also been evaluated.
77 citations
TL;DR: In this paper, generalized gradient approximation (GGA)+U calculations for LaCoO3 confirm the existence of a stationary state composed of a mixture of the low spin (LS) and high spin (HS) Co3+ ions in a 1:1 ratio, which is of insulating character.
Abstract: Generalized gradient approximation (GGA)+U calculations for LaCoO3 confirm the existence of a stationary state composed of a mixture of the low spin (LS) and high spin (HS) Co3+ ions in a 1:1 ratio, which is of insulating character. At low temperatures, this state is located about 25 meV above the homogeneous LS ground state if CoLS–O–CoHS bond length optimization is taken into account. The energy difference decreases with the lattice dilatation and the LS+HS(1:1) phase with bond length optimization becomes stable at some intermediate temperature. These results, together with previous electron spin resonance evidence for HS excitations in the LS ground state, lead to the conclusion that the diamagnetic–paramagnetic transition in LaCoO3 at 50–150 K is caused by a gradual population of HS Co3+ ionic states, provided that neighbouring sites are in the LS state. Consistency between experimental data for the paramagnetic susceptibility and anomalous thermal expansion is achieved for an HS–LS energy difference of 16 meV that corresponds to 180 K. With increasing HS population, the antiferromagnetic (AF) interactions HS–LS–HS become effective. The LS+HS(1:1) phase is saturated above 150 K and the susceptibility acquires a simple Curie–Weiss behaviour in which AF interactions are characterized by θ = −160 K.
76 citations
TL;DR: In this article, the role of Al substitution in variation of magnetic properties of NiFe2O4 is studied and the confirmation of single phase spinel structure and the evaluation of lattice constant have been worked out from X-ray powder data and location of band position has been studied using IR spectra of the present series.
53 citations
TL;DR: In this paper, a rather high dependence of the dielectric permittivity on the magnetic field in La2∕3Ca1∕ 3MnO3 was reported.
Abstract: We report a rather high dependence of the dielectric permittivity on the magnetic field in La2∕3Ca1∕3MnO3 The variation is maximum at around 270K, little above the Curie temperature TC, and it reaches a 35% under only 05T We attribute this phenomenon to the space-charge or interfacial polarization produced between the insulator and the metallic regions segregated intrinsically in the material above TC
48 citations
43 citations
TL;DR: In this article, the conductivity activation energy becomes larger by increasing either the Gd content or the oxygen vacancies, and it is shown that the conductivities increase with the number of oxygen vacancies.
TL;DR: In this article, the temperature dependence of magnetic susceptibility of mixed valence manganite was investigated and the spin-stiffness constant D obtained from the Bloch constant was found to increase linearly with the number of spins.
TL;DR: In this article, the magnetic and dielectric properties of the ruthenium double perovskites La 2 MRuO 6 (M = Mg, Co, Ni, and Zn) were investigated.
Abstract: Magnetic and dielectric properties of the ruthenium double perovskites La 2 MRuO 6 (M = Mg, Co, Ni, and Zn) were investigated. The magnetization measurements for M = Co and Ni showed the existence of magnetic order at 20-30 K. Though the oxides with M = Zn and Mg exhibit a deviation from the Curie-Weiss law, magnetic order was not clearly observed. The result of La 2 ZnRuO 6 was different from that previously reported, in which a ferromagnetic transition was found at around 165 K. The AC dielectric measurements for M = Co and Ni showed large dielectric constants (typically larger than 1000) at around room temperature, suggesting both the formation of short-ranged polar regions and the magnetic origin of large dielectric constant. In addition, two peaks were found for the temperature dependence of the tan δ component for La 2 NiRuO 6 . The behavior suggests the existence of two different polar regions.
TL;DR: The double perovskite with a non-magnetic element, such as Sb, occupying one half of the B positions of the pervskite structure was synthesized by solid-state reaction in air as discussed by the authors.
TL;DR: In this article, 59Co NMR studies on magnetically oriented powder samples of Co oxide superconductors, NaxCoO2·yH2O, with Tc~4.7 K were carried out, and the abplane Knight shift in the normal state was estimated from the magnetic field dependence of second-order quadrupole shifts at various temperatures.
Abstract: We report 59Co NMR studies on magnetically oriented powder samples of Co oxide superconductors, NaxCoO2·yH2O, with Tc~4.7 K. From the two-dimensional powder pattern in the NMR spectrum, the ab-plane Knight shift in the normal state was estimated from the magnetic field dependence of the second-order quadrupole shifts at various temperatures. Below 50 K, the Knight shift shows a Curie–Weiss-like temperature dependence, similarly to the bulk magnetic susceptibility χ. From the analysis of the so-called K–χ plot, the spin and the orbital components of K and the positive hyperfine coupling constant were estimated. The onset temperature of the superconducting transition in the Knight shift does not change much in an applied magnetic field up to 7 T, which is consistent with the reported high upper critical field Hc2. The Knight shift at 7 T shows an invariant behaviour below Tc. No coherence peak just below Tc was observed in the temperature dependence of the nuclear spin–lattice relaxation rate 1/T1 in either case (NMR, nuclear quadrupole resonance). We conclude that the invariant behaviour of the Knight shift below Tc and unconventional behaviours of 1/T1 may indicate spin triplet superconductivity with p- or f-wave symmetry.
TL;DR: In this article, the magnetic properties of Zn2VO(PO4)2 have been investigated by means of magnetic susceptibility χ(T) and specific heat Cp(T), performed on polycrystalline samples.
Abstract: We have reinvestigated the magnetic properties of Zn2VO(PO4)2 by means of magnetic susceptibility χ(T) and specific heat Cp(T) measurements performed on polycrystalline samples. At high temperatures χ(T) follows a Curie–Weiss law with the effective moment expected for a V4+, S = 1/2 ion, while a rounded maximum in χ(T) at 6.95 K and a hump in Cp(T) around 4.5 K indicate the onset of antiferromagnetic correlations at low temperatures. Finally, a kink in χ(T) and a well defined mean-field anomaly in Cp(T) at TN = 3.7 K evidence a transition into an ordered antiferromagnetic state. A comparison of χ(T) and of the magnetic contribution Cpmagn(T) to the specific heat with theoretical predictions indicates that the magnetic lattice of this compound corresponds to a square lattice rather than the spin chain proposed in an earlier report. The results of quantum Monte Carlo calculations for a square lattice agree very well down to TN with Cpmagn(T) extracted from our experiment, while finite size calculations overestimate the specific heat in the region of the maximum in Cpmagn(T).
TL;DR: Rutile-type Co-doped SnO(2) nano-crystal powders are prepared by the wet chemical method and the magnetic moment is found to decrease with increasing Co content.
Abstract: Rutile-type Co-doped SnO2 nano-crystal powders are prepared by the wet chemical method. The magnetic moment is found to decrease with increasing Co content. There is hysteresis between field cooled (FC) and zero field cooled (ZFC) magnetization when the temperature is below 70 K. These magnetic behaviours represent antiferromagnetic interaction between Co ions. By measuring the magnetization as a function of temperature, the magnetic interaction between Co ions is evaluated qualitatively using the Curie-Weiss law. The value of the effective exchange integral J(1)/k(B) is about -62 K, indicating a very strong antiferromagnetic superexchange interaction between Co ions.
TL;DR: In this paper, a survey on the low-temperature region of the Zn-Mn-O phase diagram is presented, focusing on cubic spinels with a nominal composition Mn3−xZnxO4 with x = 1.6 and 1.7.
TL;DR: In this article, single crystals of a novel ruthenate, Ca 2 Ru 2 O 7, were obtained and an X-ray diffraction study on a single crystal revealed that this material crystallizes in a pyrochlore structure with a lattice parameter, a = 10.197 A.
Abstract: Single crystals of a novel ruthenate, Ca 2 Ru 2 O 7 , were obtained. An X-ray diffraction study on a single crystal revealed that this material crystallizes in a pyrochlore structure with a lattice parameter, a = 10.197 A. The magnetic susceptibility above 30 K is the summation of a Curie–Weiss contribution and a constant term independent of temperature. The effective moment per Ru atom is only 0.36 µ B , one order of magnitude smaller than that expected from a localized spin model with S =3/2 for Ru 5+ . Below 23 K, the localized spins freeze in a spin-glass state. The resistivity at room temperature is 2 ×10 -3 Ω·cm, comparable to that in metallic, highly correlated oxides.
TL;DR: In this paper, the bulk magnetic, electron paramagnetic resonance, and magneto-optical properties of a magnetic molecule with antiferromagnetically coupled tetrahedral in a diamagnetic molybdenum matrix were investigated.
Abstract: We investigate the bulk magnetic, electron paramagnetic resonance, and magneto-optical properties of ${{\mathrm{Ni}}_{4}{\mathrm{Mo}}_{12}}$, a magnetic molecule with antiferromagnetically coupled tetrahedral ${\mathrm{Ni}}^{\mathrm{II}}$ in a diamagnetic molybdenum matrix. The low-temperature magnetization exhibits steps at irregular field intervals, a result that cannot be explained using a Heisenberg model even if it is augmented by magnetic anisotropy and biquadratic terms. Allowing the exchange and anisotropy parameters to depend on the magnetic field provides the best fit to our data, suggesting that the molecular structure (and thus the interactions between spins) may be changing with applied magnetic field.
TL;DR: In this article, plate-like single crystals of magnetic semiconductor FeIn 2 Se 4 were grown with a chemical vapour transport technique and X-ray powder diffraction analyses suggest that the compound crystallize in the hexagonal structure with space group P3m1.
Abstract: Plate-like single crystals of magnetic semiconductor FeIn 2 Se 4 were grown with a chemical vapour transport technique. The X-ray powder diffraction analyses suggest that the compound crystallize in the hexagonal structure with space group P3m1. We have performed dc magnetization measurements at different magnetic fields on the diluted magnetic semiconductor FeIn 2 Se 4 . Low field magnetizations measurements shows irreversibility in the DC magnetization, as evidenced by field cooled and zero field cooled measurements below 17 K, suggesting a spin-glass like behaviour. The high-temperature susceptibility data follow a typical Curie–Weiss law with θ =−183±2 K which suggest the presence of predominant antiferromagnetic interactions with high degree of frustration. The randomness and frustration necessary for spin-glass behaviour are explained in a manner compatible with the cation and charge ordering present in the material.
TL;DR: In this paper, it was shown that the magnetic scattering function in the paramagnetic phase above the Curie temperature, TC, follows a simple double Lorentzian behavior in both momentum and energy.
Abstract: The recent twenty years have witnessed significant progress in the basic understanding of the dynamical magnetic properties in metallic and strongly correlated materials, mainly due to the progress in inelastic neutron scattering (INS) techniques and polarization analysis. One of the major achievements in this discipline is, in fact, the finding that the magnetic scattering in ferromagnetic metals shows a universal behavior independent of the itinerancy of the magnetic moment. The dynamic susceptibility in the ordered state at low temperatures is well described by spin-wave excitation. In addition, the magnetic scattering function in the paramagnetic phase above the Curie temperature, TC, follows a simple double Lorentzian behavior in both momentum and energy. Then, the energy scale of the spin fluctuations can be expressed in terms of the ratio of the strength of the magnetic correlations defined by the molecular field Curie temperature with respect to TC. The magnetic fluctuations in incommensurate antiferromagnets, i.e., in metallic Cr or in the high temperature superconductors of doped La2CuO4 compounds, show, on the other hand, a rather complicated dynamical structure factor. In contrast to ferromagnetic metals and localized antiferromagnets, the longitudinal fluctuations seem to play an important role deep in the ordered state of Cr.
TL;DR: In this paper, a spinel-type CuCr 2 Te 4 has been synthesized successfully and the heat capacity exhibits a sharp peak due to the ferromagnetic-phase transition with the Curie temperature T C = 326 K.
TL;DR: In this paper, the stationary self-polarization field of a thin film in an open circuit is analytically solved for temperatures near the para-/ferroelectric transformation within the Ginzburg-Landau theory.
Abstract: The stationary self-polarization field of a thin film in an open circuit is analytically solved for temperatures near the para-/ferroelectric transformation within the Ginzburg-Landau theory. For second-order ferroelectrics, or first-order ferroelectrics with a sufficiently large elastic self-energy of the transformation strain, the solution is real and stable, from which the corresponding electric susceptibility of the film can be derived. A Curie-Weiss-type relation of the permittivity is obtained for both the supercritical and subcritical temperature regimes near the transition. In the paraelectric state, the Curie parameter of the thin film is found to be independent of its thickness, whereas in the ferroelectric state, its magnitude decreases rapidly with decreasing film thickness.
TL;DR: In this article, the effects of applied magnetic fields on the α/γ phase boundary in the Fe-Si system in the paramagnetic state were examined using the CALPHAD (calculations of phase diagrams) method and the Redlich-Kister polynomials were proposed to account for compositional and temperature dependence of the contribution to the applied field in the range over which the Curie-Weiss law is obeyed.
Abstract: The CALPHAD (calculations of phase diagrams) method is used to examine the effects of applied magnetic fields on the α/γ phase boundary in the Fe–Si system in the paramagnetic state. The reported susceptibility data for pure Fe is first re-evaluated. The contributions to the total Gibbs energy of the ferrite (α) and austenite (γ) from the external fields are calculated based on the Curie–Weiss law and the re-evaluated susceptibility data. The Fe–Si phase diagram on the Fe-rich side as a function of applied field is calculated using the Thermo-Calc™ package. With increasing field strength, the γ loop shrinks monotonically; that is, the α/γ-Fe transition temperature increases while that for γ/δ-Fe transition decreases, albeit more slowly. Finally, in conformance with the existing CALPHAD databank, Redlich–Kister polynomials are proposed to account for the compositional and temperature dependence of the contribution to the total Gibbs energy from the applied field in the paramagnetic state in the range over which the Curie–Weiss law is obeyed.
TL;DR: In this article, the effects of open shell cations on magnetic, optical and carrier transport properties were examined for layered wide bandgap semiconductors MCuFCh (M=Sr, Eu, Ch=S, Se).
TL;DR: In this article, polycrystalline hexagonal Yttrium Manganites (YMnO 3 ) were prepared by solid-state route and X-ray studies revealed the formation of homogenous and single phase material in hexagonal structure.
Abstract: Polycrystalline hexagonal Yttrium Manganites (YMnO 3 ) were prepared by solid-state route. X-ray studies revealed the formation of homogenous and single phase material in hexagonal structure. Dielectric, impedance, electrical, magnetic and magnetoelectric measurements were performed on the YMnO 3 ceramics. Dielectric and impedance studies show relaxations near 200°C. Complex spectroscopic analysis, in the temperature range of 30–300°C from 1 kHz to 1 MHz, showed non-Debye relaxations. Ac and dc electrical measurements were extracted to understand the transport mechanism: Ac conductivity data is found to obey ‘Universal’ power law with exponent 0.8. Low temperature magnetization measurements were performed to understand the magnetic nature of the sample. Room temperature magnetoelectric (ME) measurements were also performed and corroborated with the magnetization data.
TL;DR: In this paper, the results of 121,123 Sb-nuclear quadrupole resonance (NQR) and 139 La-NMR measurements on a filled skutterudite antimonide LaFe 4 Sb 12 were reported.
Abstract: We report the results of 121,123 Sb-nuclear quadrupole resonance (NQR) and 139 La-NMR measurements on a filled skutterudite antimonide LaFe 4 Sb 12 in order to investigate the magnetic properties at low temperatures from a microscopic point of view. The nuclear spin–lattice relaxation time T 1 of Sb nuclei deviates from the relation T 1 T = constant above 4.2 K, where 1/ T 1 T has a Curie–Weiss temperature dependence 1/ T 1 T = C /( T +θ) with θ∼30 K. The temperature dependence of the Knight shift of 139 La nuclei, which is related to the susceptibility at q = 0, is scaled to that of 1/ T 1 T above 40 K. This relation strongly suggests that ferromagnetic fluctuations are predominant in LaFe 4 Sb 12 . We also point out that LaFe 4 Sb 12 is situated close to the ferromagnetic instability due to the small Weiss temperature in the Curie–Weiss behavior of 1/ T 1 T and the Knight shift.
TL;DR: In this article, it has been shown that the superconductivity of Na 0.3 CoO 2 · y H 2 O is consistent with the anisotropy of the hyperfine coupling constant A spin estimated from the K-χ plot.
Abstract: Studies of the NMR Knight shift K of Na 0.3 CoO 2 · y H 2 O have been carried out in detail. The suppression of K by the occurrence of the superconductivity reported previously by the present authors in both magnetic field directions perpendicular and parallel to the c -axis has been confirmed, indicating that the Cooper pairs are in the singlet state. The anisotropy of the suppression amplitudes is consistent with the anisotropy of the hyperfine coupling constant A spin estimated from the K –χ plot. It has also been found that even samples that do not exhibit a significant the Curie–Weiss-like increase of the uniform magnetic susceptibility χ with decreasing temperature T , exhibit superconducting transition, which indicates that the superconducting Na 0.3 CoO 2 ·1.3H 2 O is not necessarily in the region of proximity of the ferromagnetic phase. It has also been confirmed that the superconducting transition temperature T c of the samples prepared by mixing Na 0.7 CoO 2 , H 2 O (or D 2 O) and bromine for 4...
TL;DR: In this paper, the magnetic field, temperature, and neutron-polarization dependence of the small-angle neutron scattering intensity in the soft magnetic iron-based nanocomposite Nanoperm was analyzed.
Abstract: We present results for the magnetic-field, temperature, and neutron-polarization dependence of the small-angle neutron scattering intensity in the soft magnetic iron-based nanocomposite Nanoperm $({\mathrm{Fe}}_{89}{\mathrm{Zr}}_{7}{\mathrm{B}}_{3}\mathrm{Cu})$. An unusual ``clover-leaf-shaped'' intensity distribution on the detector is attributed to the dipolar stray fields around the nanosized iron particles, which are embedded in an amorphous magnetic matrix of lesser saturation magnetization. The dipole field induces spin disorder, correlating the spin misalignment of neighboring particles and matrix over several particle spacings. The clover-leaf-shaped anisotropy is observed over a wide range of applied magnetic field and momentum transfer. It persists up to several hundred degrees Kelvin above the Curie temperature of the matrix phase, indicating that some degree of magnetic coupling persists even when the matrix is paramagnetic.