Showing papers on "Curie temperature published in 1968"
TL;DR: In this paper, it was shown that there exists a composition domain for the lithium metaniobate near the stoichiometric compound Nb2O5-Li2O in the vicinity of the NbLiO3 compound.
368 citations
TL;DR: In this paper, the ferroelectric phase transition in [K-PO 4 ]-type crystals is explained by the coupling between the proton tunneling mode and the optical mode vibration of the complexes along the c-axis.
Abstract: Dynamical aspects of the ferroelectric phase transition in KH 2 PO 4 -type crystals are elucidated on the basis of the coupling between the proton tunneling mode and the optical mode vibration of the [K-PO 4 ] complexes along the c-axis. Two coupled modes are obtained within the framework of a linear theory. The mode in which both systems oscillate in phase is shown to be responsible for the ferroelectric phase transition. The isotope effect in the Curie point is explained qualitatively and the Curie constant is estimated.
326 citations
TL;DR: In this article, the dielectric constants of the materials were determined at a frequency of 0.53 GHz and at temperatures up to 800°C, and the results leave little doubt that BiFeO3 is ferroelectric or antiferroelectric.
Abstract: Solid solutions of BiFeO3 with PbTiO3, PbTi0.5Zr0.5O3, and PbZrO3 were prepared. The crystallographic data on these solutions, which are basically perovskitic, are given. The dielectric constants of the materials were determined at a frequency of 0.53 GHz and at temperatures up to 800°C. Dielectric Curie points were found in solutions containing up to 90 mole % BiFeO3. These results leave little doubt that BiFeO3 is ferroelectric or antiferroelectric. The extrapolated Curie point for BiFeO3 is above 850°C. BiFeO3 appears more likely to be ferroelectric than antiferroelectric, but the distinction between the two classifications may not be sharp.
217 citations
TL;DR: In this paper, the Curie temperature, birefringence, and phase-matching temperature of LiNbO3 are shown to vary with the stoichiometry of the melt from which the crystals are pulled.
Abstract: The Curie temperature, birefringence, and phase‐matching temperature of LiNbO3 are shown to vary with the stoichiometry of the melt from which the crystals are pulled. The range of melt stoichiometry, indexed by the Li/Nb mole ratio, was varied from 1.20 to 0.80. This Li/Nb variation changes the Curie temperature, refractive index for the extraordinary ray, and the phase‐matching temperature by 120°C,.03, and 320°C respectively. The relevance of these changes are discussed in terms of the susceptibility of LiNbO3 to index inhomogeneities.
193 citations
TL;DR: In this paper, the spin wave contribution to low temperature magnetization is considered in the limits of strong and very weak ferromagnetism, and a simple equation for the magnetic isotherms is obtained in this limit and shown to be valid over a wide temperature range.
Abstract: Calculations are given for the dependence on temperature and magnetic field strength H of the magnetization M of ferromagnetic metals treated on the basis of the itinerant electron model. The spin wave contribution to the low temperature magnetization is considered in the limits of strong and very weak ferromagnetism. For the first limit, problems related to the well-known divergence of the zero field differential susceptibility are briefly discussed. The main part of the paper is concerned with the single particle contributions to the magnetization and associated differential susceptibility for very weak ferromagnetism. A simple equation for the magnetic isotherms is obtained in this limit and shown to be valid over a wide temperature range including 0 °K and the Curie temperature. This equation implies that plots of M 2 against H / M at various temperatures in this range give a series of parallel straight lines. Recently measured isotherms for the material ZrZn 2 are analysed on the basis of the theory, and several characteristic properties of this material are obtained from the analysis.
188 citations
TL;DR: In this paper, the crystal distortion of each phase was determined and it was shown that BiMnO 3 is ferromagnetic below 103°K, while BiCrO 3 was inferred to be antiferromagnetic under 123°K accompanied with weak ferromagnetic moment.
Abstract: Magnetic perovskites BiMnO 3 and BiCrO 3 were synthesized under very high pressures. There are phase transitions at 500°K and 410°K at atmospheric pressure for BiMnO 3 and BiCrO 3 respectively, and the crystal distortion of each phase was determined. BiMnO 3 is ferromagnetic below 103°K, while BiCrO 3 is inferred to be antiferromagnetic below 123°K accompanied with weak ferromagnetic moment. BiCrO 3 has an abnormally large effective Bohr magneton number at room temperature phase. The distortion of BiMnO 3 decreases slightly below the ferromagnetic Curie point.
186 citations
TL;DR: In this article, the specific heat of weakly ferromagnetic and nearly-ferromagnetic metals is calculated for both phases, and numerical results are presented for spin-fluctuation contributions to the low-temperature specific heat.
Abstract: The specific heat is calculated for weakly ferromagnetic and nearly ferromagnetic metals. These systems are characterized by an enhanced susceptibility indicating large spin fluctuations. An expression for the mass enhancement on both sides of the ferromagnetic instability is presented. The specific heat just above the Curie temperature is investigated. In this region, the terms which lead to the large low-temperature mass enhancements are greatly reduced, and a new term contributes a singular specific heat at the Curie temperature. Spin-fluctuation contributions to the low-temperature specific heat are calculated for both phases, and numerical results are presented. Very large magnetic fields are shown to be effective in reducing the strongly enhanced specific heat. Calculations of the temperature dependence of the specific heat due to spin fluctuations in the paramagnetic phase are compared with the experimental results on ${\mathrm{He}}^{3}$.
183 citations
TL;DR: In this paper, the authors considered the itinerant electron model in the limit of very low exchange splitting energies and derived the magnetic isotherms giving M(H, T) to cover a wide range of fields and temperatures.
Abstract: The itinerant electron model is considered in the limit of very low exchange splitting energies. Magnetic isotherms giving M(H, T) are derived to cover a wide range of fields and temperatures. The isotherms are transformed so as to show that plots of M 2 vs H/M at different temperatures give parallel straight lines. The differential susceptibility below and above the Curie temperature is derived as a function of T. The free energy corresponding to this model is calculated and several results concerning the specific heat of very weak itinerant ferromagnets are deduced. Some properties related to spin wave excitations are also considered. New experimental data on the material ZrZn2 are considered on the basis of the theory and shown to be in reasonable agreement with it. Several characteristic parameters of this material are deduced, for example a value of the exchange splitting energy 0.040 eV, and of the effective interation between the itinerant electrons 0.35 eV. The occurrence of very weak itinerant ferromagnetism in other substances is also discussed.
176 citations
163 citations
TL;DR: In this article, a review is given of recent measurements of spinwave dispersion relations of the 3D metals by the Brookhaven neutron diffraction group using the diffraction technique and triple-axis spectrometry.
Abstract: A review is given of recent measurements of spin‐wave dispersion relations of the 3d metals by the Brookhaven neutron diffraction group using the diffraction technique and triple‐axis spectrometry. The parameters D and β in the relation ħω = Dq2(1 − βq2) have been determined at 295°K for Fe, Co, Ni, and some of their alloys. These values are compared with those obtained by thin‐film resonance and small‐angle scattering.The most extensive measurements were carried out on Fe using a triple‐axis spectrometer. The dispersion relation was measured along the three principal symmetry directions for wavevectors up to q/qmax = 0.4. The stiffness constant D as well as the linewidth of selected spin waves were studied for the temperature range between 77°K and the Curie temperature, 1042°K. Well‐defined magnons were observed up to a reduced temperature T/TC = 0.995, but not above TC.
145 citations
TL;DR: In this paper, the spontaneous polarization in gadolinium molybdate was shown to arise from an elastic instability which gives rise to a spontaneous strain in the original piezoelectric paraelectric phase.
Abstract: Ferroelectric properties in gadolinium molybdate are shown to originate from a new mechanism completely different from that in conventional ferroelectric materials. In this crystal the spontaneous polarization results from an elastic instability which gives rise to a spontaneous strain in the original piezoelectric paraelectric phase. The dielectric permittivity of the clamped crystal ($\ensuremath{\epsilon}_{33}^{x}$) is low and independent of temperature, while the elastic constant ($c_{66}^{E}$) shows a strong temperature dependence with a marked anomaly at the Curie point.
TL;DR: In this paper, the dielectric constants of Bi2/3TiO3, PbZn1/3Nb2/ 3O3 or their derivatives have been obtained and the aging rate of frequency constant does not exceed 0.1%/time decade.
Abstract: PbTiO3 ceramics with high density and high resistivity have been prepared by adding less than 5 mol % of Bi2/3TiO3, PbZn1/3Nb2/3O3 or their derivatives. These ceramics have dielectric constants of about 200, and can be poled at 200°C under a d. c. field of 40 to 70 kV/cm. The piezoelectric coupling factors k15, k33 and k31 amount to 0.43, 0.35 and 0.068, respectively. The value of k33 is nearly constant up to the Curie temperature of about 500°C. The aging rate of frequency constant does not exceed 0.1%/time decade. The present ceramics can be used as stable piezoelectric elements in high temperature and high frequency.
TL;DR: In this article, an analysis of experimental methods and the design of an appropriate torquemeter made it possible to improve by several orders of magnitude the accuracy of anisotropy measurements.
Abstract: An analysis of experimental methods and the design of an appropriate torquemeter made it possible to improve by several orders of magnitude the accuracy of anisotropy measurements. The experiments performed on nickel between 100°K and the Curie point allowed, for instance, the determination of the eighth‐order anisotropy constant K3 within a few percent. The anisotropy constants K1, K2, and K3 are found to be negative in this whole temperature range. An explanation is given of the main discrepancies in published data between the numerous determinations of anisotropy constants by static methods. The influences of both anisotropy of energy and anisotropy of magnetization are separated and measured as functions of applied magnetic field and temperature in the above‐mentioned range. This interpretation of torque measurements has been confirmed by a direct determination of anisotropic magnetization of nickel at room temperature, and an absolute measurement of the saturation magnetization of nickel at 20°C has ...
TL;DR: The effect of the Eu metal on the formation of these crystals and its role in forcing the trivalent rare earth oxide into the rock-salt lattice is discussed in this article.
Abstract: The ferromagnetic transition temperature (TC) of EuO has been increased from 69° to 135°K by selectively doping to increase the electrical conductivity Three different chemical systems have been studied: the EuO‐Eu, the EuO‐Eu‐RE2O3, and the EuO‐RES systems (RE is a rare earth other than europium) Compositions in the latter system were multiphase because the solubility of the rare‐earth sulfides in EuO is very small and it was necessary to add about 10% to cause significant Curie‐temperature increase Single crystals have been grown from ternary compositions in the EuO‐Eu‐RE2O3 system with magnetizations at 77°K as high as 170 G cm3/g and ferromagnetic Curie temperatures (TC) close to the paramagnetic θ values, a condition not observed in the other doped europium chalcogenides The effect of the Eu metal on the formation of these crystals and its role in “forcing” the trivalent rare‐earth oxide into the rock‐salt lattice is discussed From Kerr measurements and the optical absorption data it is shown that the magnitude of the rotation of the doped material at 80°K is equal to that of pure EuO at 18°K This makes this material attractive for use in magneto‐optical devices operating at nitrogen temperature
TL;DR: In this paper, the spontaneous magnetization σ and the Curie constant Cmol were measured on a synthetic crystal of CoS2 and they were compatible when they assume Co is in divalent and low-spin state.
Abstract: Magnetic properties of FeS2 and CoS2 having pyrites structure are studied. Magnetic susceptibility of a good natural pyrites (FeS2) crystal is almost temperature‐independent down to 4.2°K. The constant susceptibility is explained by the Van Vleck paramagnetic term and diamagnetic one of the core. This suggests iron in pyrites is in divalent and low‐spin state contrary to the assumption of L. R. Walker et al. that iron in pyrites is in a trivalent state from the Mossbauer isomer‐shift data. CoS2 is a ferromagnet of a Curie temperature of 130°K. The spontaneous magnetization σ and the Curie constant Cmol are measured on a synthetic crystal of CoS2. We obtained the values, σ=44.4 emu/g and Cmol=0.37 emu·deg. The values are compatible when we assume Co is in divalent and low‐spin state. This fact accords with the result of magnetic measurement of FeS2.
TL;DR: In this paper, a small amount of Ba or Sr is substituted for Pb in Pb(Mg1/3 Nb2/3)O3-PbTiO3 -Pb2rO3, and the morphotropic boundary and the compositions which show the highest planar coupling coefficient and dielectric constant shift slightly toward the decreasing Pb TiO3 content.
Abstract: When a small amount of Ba or Sr is substituted for Pb in Pb(Mg1/3 Nb2/3)O3-PbTiO3-Pb2rO3, the morphotropic boundary and the compositions which show the highest planar coupling coefficient and dielectric constant shift slightly toward the decreasing PbTiO3 content. The tetragonality of Pb(Mg1/3Nb2/3)O3-PbTiO3 and Pb(Mg1/2 Nb2/3)-O3-PbTiO3-PbZrO3 ceramics decreased with increasing Ba or Sr content. The lattice parameter (α axis) in the rhombohedral or pseudocubic phase increased with the increase of Ba but decreased with the increase of Sr substitution. Although the Curie temperature was lowered with the increase of Ba or Sr, the dielectric constants of the ceramics were increased. The dielectric and piezoelectric properties of the ternary compositions near the morphotropic boundary were improved through selection of sub-stituent and base composition. A planar coupling coefficient of 0.66 and a low Young's modulus were obtained with substitution of 5 mole % Ba. A dielectric constant greater than 3500 and a planar coupling of 0.63 can be obtained by substituting 5 mole % Sr.
TL;DR: In this paper, the electron-phonon interaction of two bands is considered in small-gap semiconductors and the Helmholtz free energy of the system is obtained on the basis of the electron spectrum renormalized by the interaction.
Abstract: Displacive phase transitions are considered in small-gap semiconductors due to the electron-phonon interaction of two bands. The Helmholtz free energy of the system is obtained on the basis of the electron spectrum renormalized by the interaction. The temperature dependent low-symmetry lattice distortion, renormalized frequencies of the active “soft”; optical branch in both phases, the Curie temperature, etc., are obtained. By means of the Green's function method combinative vibronic frequencies of the system are found. The dielectric susceptibility which is essentially determined by active vibrations is calculated and the general picture of similar (ferroelectric) phase transitions is discussed.
[Russian Text Ignored].
TL;DR: In this article, various ferromagnetic quantities, Curie temperature, spin susceptibility, etc., for fcc Ni-alloys are calculated in the band model and the Invar effect for Fe-Ni alloys is discussed.
TL;DR: In this paper, the magnetic properties of the series of pseudo-binary compounds YFeuCo2-u have been examined in an attempt to obtain further information concerning the nature of the transition-metal moment in the cubic Laves-phase rare-earth -transition-metal compounds.
Abstract: The magnetic properties of the series of pseudo-binary compounds YFeuCo2-u have been examined in an attempt to obtain further information concerning the nature of the transition-metal moment in the cubic Laves-phase rare-earth - transition-metal compounds. The variation of the observed moment throughout the series can be understood on the basis of a non-localized moment associated with the transition-metal ion. The variation of the Curie temperature may be associated with the variation of the lattice parameter between the terminal compounds.
TL;DR: In this article, the angular distribution characteristic of the small-angle scattering of neutrons by long-wavelength spin waves has been observed at temperatures up to and above the Curie temperature in iron and nickel.
Abstract: Angular distributions characteristic of the small-angle scattering of neutrons by long-wavelength spin waves have been observed at temperatures up to and above the Curie temperature in iron and nickel. For temperatures below 09Tc the spin-wave stiffness D, the spin-wave scattering cross section and the spin-wave lifetime have been obtained directly from the measured angular distributions, and the results are compared with the predictions of spin-wave interaction theories. For temperatures up to 042Tc in iron a good fit to the variation of D can be obtained using an expression of the form D0-D1T2-D2T 5/2 based on the itinerant electron model, but above 042Tc the fit becomes progressively poorer and D decreases faster than the fitted expression. The cross section for spin-wave scattering shows a temperature dependence additional to that contained in the thermodynamic factor alone; for iron at temperatures up to 07Tc this additional dependence has the form {1-(030±008)T/Tc} and is stronger than the variation with predicted by Marshall and Murray in a calculation of the effects of kinematic interactions between spin waves in the Heisenberg model. The observed spin-wave lifetimes have the same form of temperature dependence as is obtained from a nearest-neighbour Heisenberg model calculation by Cooke and Gersch. At temperatures above 09Tc the observed angular distributions were corrected for critical scattering using calculated cross sections for the latter due to Villain. D is found to fall slowly on passing through the Curie temperature in both iron and nickel, and is still of the order of a quarter of the room temperature value when the spin-wave scattering intensity falls below the threshold of observability.
TL;DR: In this article, the one-dimensional Ising model is investigated by generalizing the Bethe approximation, which, in this case, gives exact solutions, and the energy, specific heat and the zero field susceptibility for S = 1, 3/2 and 2 except the susceptibility for s = 2 are calculated exactly and compared with the results of Suzuki et al..
Abstract: The one-dimensional Ising model is investigated by generalizing the Bethe approximation, which, in this case, gives exact solutions. The energy, specific heat and the zero field susceptibility for S =1, 3/2 and 2 except the susceptibility for S =2 are calculated exactly and compared with the results of Suzuki et al. . As a direct application of this method, the special lattice called the Bethe lattice is treated, and the Curie temperature of the lattice for S =1 is obtained. This method is also applied to solve the one-dimensional Ising model with the second neighbor interactions as well as the first neighbor ones, and the energy and specific heat for S =1/2 are calculated.
TL;DR: In this article, the magnetic properties of nine compounds represented by the formula Ln2Ni17 (Ln=Y or a lanthanide) are reported and the nickel moment is 0.27 μB, which is in very close agreement with 0.25 μB.
Abstract: Magnetic characteristics of nine compounds represented by the formula Ln2Ni17 (Ln=Y or a lanthanide) are reported. The Y, Sm, and Lu compounds are ferromagnetic; the others are ferrimagnetic. Curie temperatures range from 601° to 641°K. The nickel moment is 0.27 μB, which is in very close agreement with 0.25 μB, the value expected if all Ln valence electrons are transferred into the Ni 3d band. Magnetization of the Y2Ni17 and Lu2Ni17 compounds is lost in two stages as temperature is increased. Ferrimagnetism of heavy lanthanide compounds is indicated by the magnitude of their saturation moments and the form of the magnetization‐temperature curves. The magnetic coupling is the same as that observed in similar and related Ln–Co and Ln–Fe compounds, i.e., the Ln–Ni coupling is antiferromagnetic and ferromagnetic for heavy and light lanthanides, respectively. From the Curie temperature it is inferred that the Ni–Ni interaction is dominant. This is in contrast with the situation in Ni‐containing Laves and Hauc...
TL;DR: In this article, the magnetic properties of PrAl2 and ErAl2 have been investigated by neutron-diffraction and susceptibility measurements on powder samples of the above compounds; the results show that the spontaneous mag...
Abstract: The magnetic properties of PrAl2 and ErAl2 have been investigated by neutron‐diffraction and susceptibility measurements on powder samples of the above compounds. The susceptibility data show that both compounds obey the Curie‐Weiss law; the effective moment per molecule in the paramagnetic region is 3.5±0.05 μB for PrAl2 and 9.2±0.1 μB for ErAl2. The neutron‐diffraction measurements confirm that these compounds are ferromagnetically ordered below a Curie temperature of 34°K for PrAl2 and 14°K for ErAl2; the respective saturated ordered moments at the rare earth atom sites are 2.94±0.05 μB and 8.3±0.3 μB. The magnetic form factor data from the ErAl2 coherent magnetic reflections are in good agreement with similar data obtained by other investigators on an Er single crystal. The magnetic scattering intensity of the (111) reflection from PrAl2 was measured just below the Curie temperature TC in order to determine the long‐range magnetic‐order temperature dependence. The results show that the spontaneous mag...
TL;DR: The ferromagnetic Curie temperature of EuO films has been increased by selective doping with trivalent rare-earth oxides as mentioned in this paper, and the electrical resistivity, which has a broad peak at ≈100°K, decreases markedly to ≈10−1 Ω·cm with doping.
Abstract: The ferromagnetic Curie temperature of EuO films has been increased by selective doping with trivalent rare‐earth oxides. The electrical resistivity, which depends upon the temperature and has a broad peak at ≈100°K, decreases markedly to ≈10−1 Ω·cm with doping. A typical ferromagnetic Curie temperature, determined from magneto‐optic measurements in zero applied field, is ∼135°K. Maximum longitudinal Faraday rotation occurs at 0.65 μ with a specific rotation of +1×105 deg/cm for an incidence angle of 20 deg. At 0.84 μ the Faraday rotation reverses sign and reaches a negative maximum at 0.93 μ. The longitudinal Kerr rotation also has two maxima at 0.58 μ and 0.83 μ with double rotations of ≈4°. The transverse Kerr effect depends on the wavelength in the same manner and has maximum values of +0.4 and −0.3, respectively. There is a strong optical absorption band centered at 0.6 μ (at 300°K) with α≈1.2×105 cm−1 which shifts toward longer wavelengths at lower temperatures accompanied by a small decrease in α. ...
TL;DR: The magnetic properties of hexagonal CeAl3 and orthorhombic CeAl4 between 2.2 and 300°K are reported and discussed in detail in this paper, where it is attributed to a combination of crystal field and exchange interactions.
TL;DR: In this article, the intrinsic magnetisations of the ferromagnetic iron-chromium and iron-vanadium sigma phase alloy s have been augmented by measurements of paramagnetic susceptibility above the Curie temperature.
TL;DR: The anomalous volume magnetostriction characteristic of Invar alloys was investigated as a link in the Invar-problem in this paper, where the variation of volume magnetization with temperature was measured on the specimens subjected not only to annealing but also to cold-work; especially the behaviours just below and beyond the ferromagnetic Curie temperature were observed.
Abstract: The anomalous volume magnetostriction characteristic of Invar alloys was investigated as a link in the Invar-problem. The variation of volume magnetostriction with temperature was measured on the specimens subjected not only to annealing but also to cold-work; especially the behaviours just below and beyond the ferromagnetic Curie temperature were observed. The measurements of Mossbauer effect for the alloys were also carried out as a function of temperature, and the influence of external magnetic field on the Mossbauer spectra was examined. It was confirmed that an Invar-type alloy contains a considerable amount of the magnetization caused by the application of the field or an assembly of states of predominance of the para-process, the origin of which may be ascribed to the internal disturbances caused around the α-phase embryos embeded in the γ-matrix, due to the transitionary process towards an equilibrium of the two-phase transformation.
TL;DR: In this article, a magnetic phase with an anisotropy field of more than 200 kOe at 77°K and a Curie point of 240°K was reported.
Abstract: Composites made up of MnBi needles embedded in bismuth single crystals are grown from eutectic melts by both Czochralski and zone‐melting techniques. The galvanomagnetic properties of the composite are determined mainly by the bismuth matrix. Magnetic measurements reveal a magnetic phase with an anisotropy field of more than 200 kOe at 77°K and a Curie point of 240°K. This phase is believed to be a modification of MnBi different from the two already known. The solubility of Mn in solid Bi is concluded to be less than 30 wt ppm.
TL;DR: The magnetic susceptibility of powdered FeGe, cubic B 20 phase, has been studied in the temperature range 4.2°K - 520°K as mentioned in this paper, and the results indicate ferromagnetic ordering with a Curie point of (280±2)°K.
TL;DR: A high-temperature x-ray study of pure iron has shown that a small but easily detectable, lattice parameter anomaly, representing a decrease in the mean linear thermal expansion coefficient, occurs over a narrow range of temperature (similar 35 degc) about the Curie point.
Abstract: A high-temperature x-ray study of pure iron has shown that a small, but easily detectable, lattice parameter anomaly, representing a decrease in the mean linear thermal expansion coefficient, occurs over a narrow range of temperature (similar 35 degc) about the Curie point. The total volume change associated with the ferromagnetic-paramagnetic transition in pure iron obtained from the experimental measurements gave values in good agreement with those calculated directly from volume magnetostriction data, although for an alternative method of calculation the agreement was poor.