Showing papers on "Curie temperature published in 1973"

Relaxation polarization of PbMg 1/3Nb2/3O3(PMN)-A ferroelectric with a diffused phase transition

Abstract: Peculiarities of the dielectric properties of the compound PbMg 1/3 Nb2/3 O3(PMN) such as the relaxation character of the dielectric polarization, deviation of permeability from the Curie-Weiss law above the Curie temperature etc. have been discussed on the basis of the idea of a diffused phase transition. 1-6 A model has been proposed, in accordance with which the features of the dielectric properties PMN are explained by the relaxation of the polar regions surrounded with a non polar phase, with the number of relaxing regions dependent on temperature.

Effect of Spin Fluctuations on Itinerant Electron Ferromagnetism. II

Abstract: The theory of the effect of spin fluctuations on itinerant electron ferromagnetism, which we have developed previously, is extended to include the ferromagnetic phase. The correction to the Hartree-Fock free energy as a function of magnetization is expressed in terms of the transversal dynamical susceptibilities and is actually calculated by using a modified random phase approximation for the dynamical susceptibilities; the random phase approximation is modified so as to give a consistent static limit throughout the whole temperature range covering both below and above the Curie point. As a result, the magnetization at low temperatures shows a T 3/2 dependence due to the spin wave excitations, the Curie temperature is generally lowered from the Stoner (Hartree-Fock) value and the magnetic susceptibility above the Curie temperature shows an approximate Curie-Weiss behavior.

Pressure dependence of the Curie temperature of intermetallic compounds of iron and rare‐earth elements, Th, and Zr

Abstract: The pressure dependence of the Curie temperature (Tc) of several intermetallic compounds of iron has been determined by means of the temperature dependence of the electrical resistivity at various pressures. Pressures up to 40 kbar were applied. The value of dTc/dp was observed to be negative in the compounds R2Fe17 (R=Pr,Nd,Er, and Th), Y6Fe23, Er6Fe23, ErFe3, Th2Fe7, ThFe3, CeFe2, and ZrFe2. The Curie temperature of the compound ThFe5 is pressure independent within experimental error. In the compounds RFe2 (R=Gd,Ho,Er, and Y), the value of dTc/dp was found to be positive. The experimental data are discussed in terms of Friedel's semilocalized model and the collective electron model.

Hydrogen absorption and magnetic properties of LaCo5xNi5−5x compounds

Abstract: The lattice constants, the Curie temperature and the saturation magnetization of the pseudobinary system LaCo 5x Ni 5−5x have been determined as a function of composition. The variation of the lattice constants suggests an anisotropic magneto-volume effect accompanying the paramagnetic-ferromagnetic transition. The hydrogen absorption capacity decreases markedly with x in the series LaCo 5x Ni 5−5x . The nature of the hydride phases corresponding to the various plateau pressures of the absorption isotherms has been studied by X-ray diffraction. The hydrogen equilibrium pressure at the first hydride is strongly reduced when x increases. Dependent on x one can discern two regions where this equilibrium pressure varies as In p = Cx + q , the absolute value for C being largest for small x values.

Dielectric and optical properties of a quasi-ferroelectric PLZT ceramic

Abstract: Certain compositions within the quaternary lead-lanthanum-titanate-zirconate (PLZT) system exhibit neither a "true" ferroelectric nor a "true" paraelectric behavior and are therefore best characterized as quasi-ferroelectric. The dielectric, hysteresis, and optical properties of quasi-ferroelectric PLZT ceramics have been investigated for the representative 9/65/35 composition which was prepared as a transparent ceramic by means of a two-stage sintering-densification process. The results of these studies suggest that the quasi-ferroelectric behavior of PLZT compositions between their Curie temperature and a transition temperature T p is brought about by the existence of polar distorted microregions (polar short-range order). By applying an electric field a transition to a macroscopic ferroelectric (FE) state (polar long-range order) is enforced which is, however, not stable at zero field at T > T p . This concept, which explains the experimental observations, is compared with the possibility of a FE to anti-FE phase transition at T p . Attention is drawn to the quasi-static memory behavior of this type of material that can be realized by disconnecting the sample in its ON state from the voltage source and which might enlarge the scope of applications of quasi-ferroelectric PLZT ceramics.

Temperature-dependent hyperfine interactions in Fe2B.

Abstract: Moessbauer absorption spectra have been observed for Fe2B from 290 to 1175 K. A change in the electric quadrupole interaction was observed from 460 to 480 K and interpreted as a result of the rotation of the easy axis of magnetization as a function of temperature. The effective magnetic hyperfine field is defined as a function of temperature. A discontinuity in the isomer shift of 0.018 (plus or minus 0.010) mm/sec was found across the ferromagnetic transition.

Raman Spectrum of PbZrO3

Abstract: Laser-induced Raman spectra were obtained for powders and multidomain single crystals of PbZrO3 at room temperature. Factor-group analysis was used to interpret the spectra on the basis of the space group C2v8-Pba 2. Raman spectra were also observed for single crystals of PbZrO3 at several other temperatures between 76° and 520°K and related to crystal structural changes. No first-order Raman bands were noted above the Curie temperature, indicating a phase transformation to a cubic zirconate structure. The interpretation of Raman spectra for thermally decomposed PbZrO3 is discussed.

Polymorphism and penferroelectricity in PLZT ceramics

Abstract: The interaction of structure with optical and electrical properties in lanthanum-doped lead zirconate-titanate (PLZT) ceramics is complex. An interpretation is presented which shows the critical dependence of structure and, hence, properties on the state of polarization, the temperature, and the La content. To explain the behavior of PLZT ceramics two new concepts are presented: polymorphism and penferroelectricity. Polymorphism occurs when either electrical or mechanical ordering fields generate a hybrid crystal structure in which an individual grain attains a crystal structure dependent on its orientation with respect to the field. Penferro-electricity refers to the condition in which material is noncubic and polar but with such a low spontaneous polarization that no domains exist. The justification for applying these concepts to PLZT rests on the existing structural, optical, and dielectric data supplemented by new X-ray evidence and new data on the temperature dependence of optical and electrical properties. Emphasis is placed on materials with 65/35 Zr/Ti ratio and 6 < X ≤ 8 where X is the atom % La substituted for Pb. Analysis of existing data disclosed 1) a lack of X-ray evidence for the morphotropic boundary predicted by average remanent birefrigence and planar coupling and 2) a discrepancy in the position of the phase transition located by the Curie point and by planar coupling coefficient. The new X-ray, optical insertion loss, dielectric and piezo-electric data show conclusively that the phase equilibria in 65/35 PLZT depend strongly on the state of polarization. In thermally annealed ceramics a state of penferroelectricity exists between the cubic paraelectric state and the rhombohedral ferroelectric state. Rhombohedral ferroelectric material with 6 < X < 8 transforms to polymorphic material with very strong ferroelectric properties during poling. Because of the large crystal distortions which accompany poling, the optical transmission in the poled state is much reduced. This behavior is the basis for many of the useful device applications of PLZT in this compositional range.

Magnetic properties of CsCl-type rare earth-magnesium compounds

Abstract: The magnetic properties of rare earth-magnesium compounds with the CsCl structure have been determined in the temperature range 4.2–300 K with applied magnetic fields up to 18 kOe. There is a change in sign of the asymptotic Curie temperatures in going from compounds with light rare-earth elements to those with heavy rare-earth elements. The temperature dependence of the magnetization in the magnetically ordered region points to antiferromagnetism for the former compounds and to ferromagnetism for the latter compounds. For all compounds investigated the lattice constants have been determined.

Magnetoabsorption in Single-Crystal HgCr 2 Se 4

Abstract: The optical absorption spectra of HgCr 2 Se 4 single crystals in the intrinsic transition region were measured as a function of magnetic field at a number of fixed temperatures. The absorption edge, 0.80 eV, at room temperature shifted to 0.27 eV at liquid helium temperature. The temperature dependence of the absorption edge was nonlinear and remarkable around the Curie temperature. The absorption edge also shifts to the lower energy side with the increasing magnetic field. The field dependence of the edge shift in weaker magnetic field was larger at the Curie temperature than it in the higher temperature region. The temperature and magnetic field dependence of the absorption edge was compared to a calculated spin correlation function in spinels with the nearest neighbour exchange interaction between localized Cr +3 ions. Other kind of exchange interaction would be required to explain the temperature dependence of the absorption edge.

Pb5(Ge,Si)3O11 Ferroelectrics

Abstract: The compound Pb5Ge3O11 is an optically active ferroelectric with a transition temperature near 177°C. Solid solutions containing up to 62% Si replacing Ge were prepared from the melt. Increasing the Si content lowers the melting point, Curie point, and coercive field. Permittivity measurements on single crystals indicate a simple second-order phase change.

Band structure and optical properties of tetragonal BaTiO3

Abstract: The electronic band structure of ferroelectric BaTiO3 is determined according to the Slater and Koster interpolation scheme (1954) with nonorthogonal orbitals. The effects of a phase transition on the energy levels are studied. Some integrated functions, such as the density of states, the joint density of states and the imaginary part of the dielectric constant epsilon 2( omega ), are deduced from the band scheme. The moments of the optical spectrum related to epsilon 2( omega ) are computed and the results related to the empirical relations concerning the behaviour of the dielectric constant, with a constant optical frequency conductivity model. For T

Quadratic Electro-Optic Effect in the System Pb(Zn1/3Nb2/3)O3-PbTiO3

Abstract: Quadratic electro-optic effects in the system Pb(Zn1/3Nb2/3)O3–PbTiO3 (up to 40 mole percent PbTiO3) are measured by applying dc electric field in the temperature region above Curie temperature. The crystals are prepared by a flux method with PbO. For Pb(Zn1/3Nb2/3)O3, the values of g coefficients are g11-g12=0.011 m4/C2 and g44=0.007 m4/C2 at λ=632.8 mµ. With the increase of PbTiO3 mole percent, the g values increase, approaching 0.06 m4/C2 at PbTiO3. Some discussions are presented on the optical properties of the crystals.

Spontaneous Polarization Measurement in NaNO 2

Abstract: Spontaneous polarization of NaNO 2 was determined by reversing P s with a relatively low d c field of from 60 V/cm to 12 kV/cm and measuring the polarization reversal charge with an electrometer and a recorder. A clear jump in P s of 5.8 µC/cm 2 was observed at the Curie point T c . The dielectric constant e 22 in the ferroelectric phase obeys the Curie-Weiss law very well over a wide temperature range and P s is proportional to (1- T / T 0 ) 0.20±0.02 over 25°C below T c , where T 0 is the ferroelectric Curie-Weiss temperature. Below room temperature P s does not vary appreciably. The saturation polarization is estimated to be 11.9 µC/cm 2 . The electronic contribution to P s is discussed on the basis of a point-dipole approximation. It is probable that about one third of the total spontaneous polarization is contributed from the electronic polarization. The experimental technique is described in detail.

Pyroelectric and Optical Properties of Potassium Lithium Niobate

Abstract: The measurements were made on the dielectric constant, refractive index, electro-optic coefficient and spontaneous polarization of the crystals (K2O)0.30(Li2O)0.70-x(Nb2O5)x with x=0.53~0.55. The spontaneous polarization was experimentally determined by the pyroelectric current method. The pyroelectric current was superposed by an anomalous absorption current above 200°C, which could be reduced by deep polishing of the crystal surface after the poling, or discriminated from the former by the observation of the time change of the current. An ordinary temperature dependence of the spontaneous polarization was observed, and the result gave no evidence of a positive temperature gradient of Ps near the room temperature reported before. The birefringences were explained by the quadratic Kerr effect biased by the spontaneous polarization, and the quadratic electro-optic coefficient gc=0.09 m4/C2 was obtained. With this gc value the linear electro-optic effect could not be explained except in the vicinity of the Curie temperature.

The pressure dependence of the Curie temperature of rare earth-cobalt compounds

Abstract: The pressure dependence of the Curie temperature (Tc) was determined up to 40 kbar for the compounds R2Co17 (R=Nd, Gd, Er, Y and Th), RCo5 (R=Gd, Y and Th), R2Co7 (R=Gd and Y), RCo3 (Gd, Y and Pr) and RCo2 (R=Gd and Tb). Single phase compounds with off-stoichiometric composition were included in this investigation. The experimental data are discussed in terms of the collective electron model. With few exceptions the pressure derivative of Tc of the various compounds can be represented by a general relation of the form dTc/dP=5/3 kappa Tc-5/6 kappa B/Tc with B=3.5*106K2 and Tc ranging from 400-1200K. For the compounds Y2Co17, YCo5, YCo3, YCo2, GdCo5, ThCo5 and Th2Co17 the compressibility kappa was determined.

Effects of Atomic Ordering on the Curie Temperature of Fe 3 Pt

Abstract: The ferrromagnetic Curie temperatures were measured for ordered and disordered phases of Fe 1- x Pt x (0.25≤ x ≤0.30). The effect of atomic ordering on the Curie temperature was discussed by a pair-interaction model. It was concluded that Fe-Fe pair interaction is negative when only the nearest neighbor pair interactions were taken into account.

Single‐crystal growth and piezoelectric properties of lead potassium niobate

Abstract: Large single crystals of lead potassium niobate were grown by the method of pulling from a melt, and their piezoelectric properties were preliminarily investigated. The crystals have ferroelectric tungsten bronze structure with a Curie point of about 460°C. Spontaneous polarization lies perpendicular to the c axis as in ferroelectric PbNb2O6. Particularly large electromechanical coupling factors were measured; i.e., 0.69 ± 0.03, 0.73 ± 0.03, and 0.59 ± 0.01 for k15, k24, and kt, respectively. Temperature coefficients of the fundamental resonant frequencies in y‐ and z‐cut plates were found to be <30 ppm around room temperature.

Ferromagnetism of Amorphous Iron Alloys

Abstract: It is well known that ferromagnetism appears in 3d metals (Fe, Co and Ni) and alloys whose average outer electron concentration, N, ranges from about 6.5(Fe-Cr) to 10.6 (Ni-Cu). The saturation magnetization of these metals and alloys is expressed by the so-called Slater Pauling curve. The crystal structure of these alloys in ordinary phases is generally body centered cubic or face centered cubic for N 8.7 respectively. There is some dincontinuity between the saturation moment of bcc alloys and that of fcc alloys. The Curie temperature is much more sensitive to the crystal structure. There are interesting topics for Invar alloys in a region close to the phase boundary between fcc and bcc phase.1 The magnetic properties of metals and alloys are necessarily affected by their crystal structures.

Resistive anomalies at the critical point of isotropic ferromagnets

Abstract: the positions of these levels. In addition to those reported here, we have performed a variety of high-field measurements on single crystals of both light and heavy rare-earth metals, and a full account of these investigations will appear in due course. We gratefully acknowledge fruitful discussions with F. A. Muller, G. de Vries, B. D. Rainford, R. J. Elliott, and M. Blume. P. Touborg and A. C. Moleman kindly assisted with the preparation and orientation of some of the crystals used . in this study, This work is part of the research program of Stichting voor Fundamenteel Ondersoek der Materie which is financially supported by the Nederlandse Organisatie voor Zuiver Wetenschappelijk Qndersoek, and also benefitted from financial support through the NATO Research Grant Program.

Spontaneous polarization of Gd2 (MoO4)3

Abstract: The spontaneous electric polarization Ps has been measured for Gd2(MoO4)3 over the temperature range from 4.2 °K to the Curie point. Near 4 °K the maximum Ps is slightly less than 0.29 μC/cm2 with a very low temperature coefficient. An electric field enforced phase transition from paraelectric to ferroelectric at temperatures just above zero‐field Tc demonstrates the first‐order nature of the phase change. Better agreement was found near Tc between Ps and the relation (T − T1)1/3 than between Ps and a Landau‐type function.

Magnetic Properties of the System Fe2P-Mn2P

Abstract: Magnetic measurements have been made of the system Fe x Mn 2- x P. With x >1.35 and x 1.35 is ferromagnetic if x is close to 2, but becomes antiferromagnetic as x decreases, even though the paramagnetic Curie temperature remains positive. The hexagonal phase with x <0.65 is antiferromagnetic with the Neel temperature T N at 140 K for x =0 (Mn 2 P). The orthorhombic phase is also antiferromagnetic with T N at 320 K for x =1 (FeMnP).