Showing papers on "Curie temperature published in 1970"

Soft ferroelectric modes in lead titanate.

Abstract: The ferroelectric phase transition in PbTi${\mathrm{O}}_{3}$ has been investigated by the neutron inelastic scattering technique. As is the case with BaTi${\mathrm{O}}_{3}$, the soft optic mode at the zone center condenses when the temperature is lowered to the Curie point at 490\ifmmode^\circ\else\textdegree\fi{}C. In contrast to BaTi${\mathrm{O}}_{3}$, however, the lowest transverse optic modes along [100] are well defined (not overdamped), except for the phonons with very small $q$ values. When this optic branch approaches the acoustic branch near the Curie temperature, effects of the "coupling" are clearly observed. Intensities of the soft modes were measured at several zone centers in the ferroelectric phase. Observed values are in qualitative agreement with calculations based upon the mode motions deduced from the atomic positions in the ferroelectric phase.

Magnetic Properties of some Cubic Rare‐Earth‐Iron Compounds of the Type RFe2 and RxY1−xFe2

Abstract: Magnetic measurements in fields up to 30 kOe and for temperatures between 4.2° and 1000°K have been performed for cubic Laves phase compounds RFe2 (R=Gd, Tb, Dy, Ho, Er, and Y) and RxY1−xFe2 (R=Gd, Tb, and Er). The Curie temperatures and saturation moments at 4.2°K are given. A compensation in the temperature dependence of the magnetization is only observed for the various compounds in a certain range of x values. Conditions for the occurrence of a compensation point are derived. For the compounds RxY1−xFe2 the iron moment remains constant for 0≤x≤0.8. Above x=0.8 the iron moment increases strongly. The variation of the Fe moments with x is discussed in terms of a nonlocalized moment associated with the iron ions. The moments shown by Er and Tb in ErFe2 and TbFe2 are both 8.3 μB. For most of the compounds RFe2 the lattice constants are given.

The Growth of Gamma Prime Precipitates in Aged Ni-Ti Alloys

Abstract: The kinetics of growth of the γ′ precipitate in a Ni-8.74 wt pct Ti alloy were studied by magnetic analysis and transmission electron microscopy. The variation of the titanium content of the nickel-rich matrix as a function of aging time was studied by measuring the ferromagnetic Curie temperature of alloys aged at 692°, 593°, and 525°C. The kinetics of this process accurately obeyed the predictions of the Lifshitz-Wagner theory of diffusion controlled coarsening after relatively short aging times at all aging temperatures. Dark-field transmission electron microscopy was used to measure the particle-size distributions and the average particle sizes of samples aged for various times at 692°C. The kinetics of particle growth also obeyed the time law predicted by the Lifshitz-Wagner theory within the limits of experimental error. Additional analysis of the data provided a value of approximately 21 erg per sq cm for the interfacial free energy of the γ′-matrix interface, and a value for the diffusion coefficient of titanium in nickel which is in very good agreement with an independently determined value. The distribution of γ′ particle sizes was found to be significantly broader than the theoretical distribution of the Lifshitz-Wagner theory. It is suggested that this is due to the relatively large lattice parameter mismatch between γ′ and the Ni−Ti matrix. The results and conclusions of this study are critically compared with those of other investigations.

Ising‐Model Spin Correlations on the Triangular Lattice. IV. Anisotropic Ferromagnetic and Antiferromagnetic Lattices

Abstract: A detailed discussion of pair correlations ω2(r) = 〈σ0σr〉 between spins at lattice sites 0 and r on the axes of anisotropic triangular lattices is given. The asymptotic behavior of ω2(r) for large spin separation is obtained for ferromagnetic and antiferromagnetic lattices. The axial pair correlation for the ferromagnetic triangular lattice has the same qualitative behavior as that for the ferromagnetic rectangular lattice: There is long‐range order below the Curie point TC and short‐range order above. It is shown that correlations on the anisotropic antiferromagnetic triangular lattice must be given separate treatment in three different temperature ranges. Below the Neel point TN (antiferromagnetic critical point), the completely anisotropic lattice exhibits antiferromagnetic long‐range order along the two lattice axes with the strongest interactions. Spins along the third axis with the weakest interaction are ordered ferromagnetically. Between TN and a uniquely located temperature TD, there is antiferro...

On the 90° Exchange Interaction between Cations (Cr3+, Mn2+, Fe3+ and Ni2+) in Oxides

Abstract: Some empirical relations between the bond angle (of M–O 2- –M) and the asymptotic Curie temperature (\(\varTheta\)) or Neel temperature ( T N ) are derived from data for oxides containing Cr 3+ , Mn 2+ , Fe 3+ or Ni 2+ ions. Further, these relations seem to be explained through assuming the presence of two types of exchange interactions; one is a 90° superexchange interaction which obeys the rule proposed by Kanamori and Goodenough, the other a direct exchange interaction which is sensitive to the overlap between neighboring two d e orbitals. In oxides containing Cr 3+ or Mn 2+ the above two types of exchange are effective, while in oxides containing Fe 3+ or Ni 2+ only the former (superexchange) effective.

Lithium tantalate single crystal stoichiometry

Abstract: Accurate lattice parameter, hydrostatic density and Curie temperature measurements have been made on lithium tantalate single crystals grown from different melt compositions. Crystal compositions were determined by comparing crystal Curie temperatures with those determined from well reacted ceramic specimens of known composition. These measurements showed that a stoichiometric melt produces nonstoichiometric crystals and that the congruently melting composition is 49.0 mole % Li2O. The resulting defect model for nonstoichiometric lithium tantalate is Li[1−(5−n)x]Ta(1+x)nxe−O3 where n≃2 and the existence region is 46.0 to 50.4 mole % Li2O at room temperature.

Low- and high-temperature oxidation of titanomagnetite in relation to irreversible changes in the magnetic properties of submarine basalts

Abstract: Re-examination of the sintering process of Akimoto, Katsura, and Yoshida indicates that titanomagnetite made according to this process does not undergo low-temperature oxidation to a single-phase cation-deficient spinel (γ-titanomaghemite) but instead oxidizes and unmixes to Mtss and Ilss at about 350–400°C, and Mtss, Ht, rutile, and pseudobrookite at about 600°C. Since this unmixing (high-temperature oxidation) reduces the Ti content in the spinel phase, the saturation magnetization Js and Curie point increase up to 4 μB (Bohr magnetons at 0°K) and 580°C, respectively, and the lattice parameter decreases down to 8.39 A. From experimental results on synthesized titanomagnetite and titanomaghemite, we conclude that the irreversible increase in Js and Curie point of some submarine basalts during heating in air or in vacuo is the result of unmixing of Ti-rich titanomagnetite and/or breakdown of γ-titanomaghemite into Mtss, Ht, pseudobrookite, and rutile. This conclusion is supported by microscopic obervation and chemical and X-ray analysis.

Stacking‐Fault Model for Stoichiometry Deviations in LiNbO3 and LiTaO3 and the Effect on the Curie Temperature

Abstract: Several arrangements are suggested to explain deviations from stoichiometry in LiNbO3 in a recent report by Lerner, Legras, and Dumas. These arrangements are considered to be unlikely, and an alternative mechanism in terms of partial stacking faults is proposed for LiNbO3 and LiTaO3. A microscopic statistical theory of ferroelectricity, which is outlined, can predict the variation of the Curie temperature with changes in the bonding forces and atomic masses. Using this approach, the stacking‐fault model is consistent with the observed variation of Curie temperature with composition.

Dielectric and Thermal Study of K2SeO4 Transitions

Abstract: The behaviours of the various dielectric properties and specific heat of potassium selenate, K 2 SeO 4 , have been studied with particular reference to the transitions at 129.5°K and 93°K. Below 93°K the crystal is ferroelectric along the c axis. The spontaneous polarization and coercive field determined from the hysteresis loop are 6.5×10 -8 coul/cm 2 and 60 V/cm respectively at 80°K. The curve of dielectric constant vs temperature obeys the Curie-Weiss law above the Curie point and the Curie constants for warming and cooling processes are 27°K and 30°K respectively. The Curie temperature varies almost linearly with the biasing field with the coefficient of 0.19°/kV/cm. Effects of the γ-radiation on the dielectric behaviour are observed. Discontinuity of the specific heat of about 2.3 cal/mol·deg has found at the transition of 129.5°K. Anomaly at the transition of 93°K is very small. The transition energy and the entropy change are estimated to be of the order of 0.5 cal/mol and 5×10 -3 cal/mol·deg of le...

The antiferromagnetic-ferromagnetic transition in iron-rhodium alloys

Abstract: The critical field required to induce this transition in Fe-Rh alloys close to the equiatomic composition has been measured using pulsed magnetic fields up to 280 kOe. Measurements of the thermal expansion and high field magnetization have also been carried out. It is found that application of Kittel's theory of exchange inversion to the results leads to serious numerical discrepancies. A four sublattice model is developed which indicates the importance of the excess entropy of the ferromagnetic over the antiferromagnetic phase in exciting the antiferromagnetic-ferromagnetic transition. The exchange iteration β between iron sublattices is small and negative and, although its lattice parameter dependence is large and positive, there is no suggestion of exchange inversion corresponding to a change in sign of β at the antiferromagnetic-ferromagnetic transition. Thus the ferromagnetic state is stabilized by the excess entropy even in the presence of antiferromagnetic interactions and this stability is maintained right up to the Curie temperature

MnBi Films: High‐Temperature Phase Properties and Curie‐Point Writing Characteristics

Abstract: Manganese bismuth films can exist in two crystallographic phases. The Curie temperatures, magneto‐optical, and magnetic properties of films prepared at the quenched high‐temperature phase were measured and compared with those of the films at the normal low‐temperature phase. The low Curie temperature of 180°C makes the quenched films more attractive for optical memory applications. Laser Curie‐point writing experiments have been performed on both types of films at various operating temperatures. The results can be explained on the basis of the physical properties of the two phases of MnBi films.

Diffusion potentials in BaTiO3 and the theory of ptc materials

Abstract: We present two methods for calculating the potential profiles of depletion layers in ferroelectric materials. The simplified method is valid for materials possessing a high permittivity and with the electric field parallel to the polarization. The second method is valid for low permittivity materials as well and can be adapted to any orientation. We applied these methods to a study of the 001-001 interfacial barrier on semiconducting BaTiO3. Below the Curie point the calculations are in agreement with the resistivity-temperature behavior of doped, air-sintered ceramics. The theory does not predict a large resistance discontinuity at the Curie point. We discuss several refinements which may account for this discrepancy. The most plausible seems to be the piezoelectric charge compensation resulting from lattice distortion and grain boundary clamping.

On New Ferromagnetic Intermetallic Compounds PtMnSn and PtMnSb

Abstract: Crystal structures and magnetic properties of Pt-Mn-Sn and Pt-Mn-Sb alloys have been investigated by means of X-ray analysis, magnetic analysis and nuclear magnetic resonance. It is found that the alloys with the stoichiometric compositions PtMnSn and PtMnSb are ferromagnetic and that the former has a structure of the CaF 2 (Cl) type while the latter an ordered structure of the MgAgAs (Cl b ) type. Pt 1.01 Mn 0.99 Sn 1.00 and Pt 1.01 Mn 1.00 Sn 1.00 alloys have lattice parameters of 6.263 and 6.201 A at room temperature, ferromagnetic Curie temperatures of 360° and 582°K, numbers of Bohr magnetons of 3.65 and 4.14 µ B /mol at 0°K and the internal magnetic field of 204.5 and 220.2 kOe at Mn 55 nucleus at 77°K, respectively.

Optical Constants of the Eu Chalcogenides Above and Below the Magnetic Ordering Temperatures

Abstract: The optical constants of the Eu chalcogenides between 1 and 6 eV have been obtained by means of a reflection technique for single crystals and transmission measurements for evaporated thin films. The films have been cooled below the magnetic transition temperature and the transmission was measured with linearly and circularly polarized light in external magnetic fields. In addition the Faraday effect of thin films in the energy range of 1 to 3.5 eV has been observed. Below the Curie point the lowest energy absorption peak is split into three polarization‐dependent maxima and the absorption peak between 4 and 5 eV into a doublet. The data suggest transitions from the localized 4f7 states into the spin‐split states, respectively, 4f6(7FJ)5d(t2g and 4f6(7FJ)5d(eg). The specific Faraday rotation of EuS at 2.2 eV is 550 000°/cm, the largest rotation yet observed for a nonmetallic compound.

Transport properties of the europium chalcogenides

Abstract: Transport properties of pure and doped Eu chalcogenides are reviewed to determine the mechanisms responsible for the anomalous behavior near Tc, the Curie temperature. It is found that, whereas the scattering theory of simple metals accounts for the behavior of materials containing impurities in excess of 2 × 1020 cm-3, several models for transport have been proposed for smaller concentrations. The impurity hopping model appearsto be consistent with the data for very dilute systems.

Effect of Pressure on Curie Temperature of Calcogenide Spinels CuCr2X4(X=S, Se and Te)

Abstract: The pressure coefficient of the Curie temperature of CuCr 2 X 4 was obtained from the result of measurement of the variation with pressure in the self inductance of search coil wounded on the specimen vessel. The coefficients, d T c /d p , were founded to be -11.3×10 -4 °Kkg -1 cm 2 for CuCr 2 S 4 , -4.1×10 -4 °Kkg -1 cm 2 for CuCr 2 Se 4 and nearly zero for CuCr 2 Te 4 , respectively. If the main exchange interaction in a pair of Cr ions is thought to be the ferromagnetic superexchange of the Cr-X-Cr type and the antiferromagnetic direct interaction of the Cr-Cr type, it can be seen from the results of the present experiment that the direct Cr-Cr interaction is more sensitive to lattice compression than the superexchange Cr-X-Cr. The magnetic properties of those compounds are also reported.

Magnetic Studies of C1 b -Compounds CuMnSb, PdMnSb and Cu 1− x (Ni or Pd) x MnSb

Abstract: Magnetic properties of manganese compounds with C1 b -type crystal structure are studied. CuMnSb (lattice parameter a =6.09 5 A) is antiferromagnetic with a Neel temperature of 55°K and an asymptotic Curie temperature of -160°K. The paramagnetic Bohr magneton number per Mn is 5.4 7 . PdMnSb ( a =6.24 6 A) is ferromagnetic with a Curie temperature of 500°K and the magnetic moment per Mn is 3.9 5 µ B . Magnetic studies are also carried out for the compounds in which Ni or Pd are substituted for Cu in CuMnSb, resulting in a conclusion that the concentration of conduction electrons is very important in determining the magnetic interaction. Qualitative interpretation of experimental results is given based on the theory of resonance scattering by Friedel, Caroli and Blandin.

Elastic Anomaly of Ba2NaNb5O15

Abstract: The temperature characteristics of elastic, piezoelectric, dielectric, and optical constants of Ba2NaNb5O15 have been measured from room temperature up to the Curie point. The shear elastic compliance related to the tetragonal principal axes (TS66) is found to become large near the transition point from tetragonal to orthorhombic phase. A phenomenological approach is tried, in which the free energy is expanded in power series of the polarization and strain, and it becomes clear that this tetragonal‐orthorhombic transition is little dependent of the behavior of spontaneous polarization but that elastic instability is the main cause of this phase transition. The elastic, piezoelectric, dielectric, and optical constants in the orthorhombic phase are derived from the constants in the tetragonal phase by the biased effect of the spontaneous shear strain to the elastic higher‐order terms in the free energy.

Nuclear Magnetic Resonance in Heusler Alloys: Ni 2 MnSn, Co 2 MnSn and Ni 2 MnSb

Abstract: The internal magnetic fields of Mn and Sn nuclei in Ni 2 MnSn, Co 2 MnSn and Ni 2 MnSb were measured with the spin-echo N. M. R. method, as well as their thermo-magnetic curves. The Curie temperatures measured of Ni 2 MnSn and Ni 2 MnSb were different from Castellitz's data, and a sluggish change of the magnetization in Ni 2 MnSb was observed, near the Curie temperature. The NMR signals of Mn nuclei were observed in Co 2 MnSn, Ni 2 MnSn and Ni 2 MnSb. The internal magnetic fields were 352.3, 312.7 and 285 kOe, respectively. An unexpected resonance signal appeared in Ni 2 MnSb and was interpreted as originating from Mn atoms occupying non-regular sites. The resonance signals of Sn nuclei were also observed in Co 2 MnSn and Ni 2 MnSn. The internal magnetic fields were 153.8 and 150.4 kOe, respectively. The difference in the internal magnetic fields of the above compounds from those of Cu 2 MnSn can be tentatively attributed to the difference in the conduction electron polarization by RKKY interaction.

A neutron diffraction study of ferroelectric KFCT, K4Fe(CN)6.3D2O, above the Curie temperature

Abstract: A neutron diffraction study of deuterated potassium ferrocyanide trihydrate (KFCT) has been made above the ferroelectric Curie temperature. The monoclinic form, space group A2/a (No. 15), has a = 9.42, b = 17.01, c = 9.38 A, and β= 90.03° at 20°C. Since all of the large crystals of KFCT were twinned by a 90° rotation about the b axis, data from both overlapped and nonoverlapped reflections were used in the determination of the structure. Refinement by a modified least-squares method that accounted for the overlapped data led to a weighted R value on F2 (3678 reflections) of 4.8%. The corresponding R on F is 4.9%. The disordered water molecules are of two types: one is disordered into two positions related by a twofold axis parallel to b: the other is disordered into two positions that are related by an approximate 90° rotation in the plane of the molecule. Both types of water molecules show O–D ⋯O or O–D⋯N bonding. The Fe(CN)4−6 octahedra are regular and the potassium ions are loosely surrounded by O and N atoms. This analysis has elucidated the role of the water molecules in the ferroelectric switching mechanism. The [{\bar 1}01] ferroelectric axis in this crystal can be explained on the basis of the dipole moment and orientation of the water molecule dipoles.

X-ray study on thermal expansion of ferroelectric KH2PO4

Abstract: The lattice parameters of KH2PO4 at various temperatures between 23.4 and —169.3 °C have been measured by using an accurate X-ray method recently developed by the authors. Specifical emphasis was laid on their temperature dependence near the ferroelectric Curie point. Small but distinct thermal hysteresis was found to occur at the onset of the para-electric-ferroelectric phase transformation. The results of temperature variation of the lattice parameters are in fairly good agreement with those of other authors except in the vicinity of the Curie point. It is noticeable that a pure shear x in a plane perpendicular to the ferroelectric axis, which directly reflects the spontaneous polarization, begins to decrease more rapidly than Ps as temperature approaches the Curie point.

Magnetic Properties of Amorphous Pd–Si Alloys Containing Iron

Abstract: Magnetic properties of amorphous FexPd80−xSi20 alloys (x=0.5–7) obtained by rapid quenching from the liquid state are studied between 1.5° and 300°K and in fields up to 8.40 kOe. The amorphous alloys are paramagnetic above the characteristic temperature Td. The observed magnetic moments can be explained by postulating a simple magnetic domain consisting of one Fe atom and its Pd nearest neighbors. This is consistent with the magnetic moments assigned to each Fe atom (μFe=3.0μB) and to each Pd atom (μPd=0.20μB). Below Td superparamagnetic clusters exist in the amorphous alloys. Ferromagnetism is found in amorphous Fe7Pd73Si20 alloys with a Curie temperature of 28°K. Evidence is discussed which demonstrates that the d‐d spin exchange interaction is weaker in the amorphous alloys than in the corresponding crystalline alloys.

Magnetic properties of the rare earth hydroxides.

Abstract: The rare‐earth hydroxides, R(OH)3 with R–La to Yb and Y form a series of simple magnetic crystals isostructural with the hexagonal rare‐earth trichlorides. Compared with most ionic rare‐earth crystals they have relatively small lattice parameters, and one may therefore expect magnetic cooperative effects at readily accessible temperatures. In this paper we report the first series of magnetic measurements on small, hydrothermally grown single crystals (1 to 10 mg) which show that Tb(OH)3, Dy(OH)3, and Ho(OH)3 order ferromagnetically at 3.72°, 3.50°, and 2.55°K, respectively, while Nd(OH)3 and Gd(OH)3 undergo more complex antiferromagnetic transitions near 1.7° and 2.0°K. Er(OH)3 remains paramagnetic down to 1.2°K. Magnetization measurements on Tb(OH)3, Dy(OH)3, and Ho(OH)3 in fields up to 14 kG give saturation moments (corrected for Van Vleck temperature‐independent paramagnetism) of 1350 emu/cc (9.0 μB/ion), 1418 emu/cc (9.6 μB/ion), and 1121 emu/cc (7.6 μB/ion) parallel to the c axis and almost zero perp...

Magnetic Studies of the Alloy System SnTe–MnTe

Abstract: Magnetic ordering and superconductivity have been investigated in the system Sn0.97‐xMnxTe for 0.00001≤x≤0.16 over the temperature range 0.02≤T≤300°K. Reciprocal Hall coefficient, 1/Re, for all these samples was ∼1021 cm−3. Samples with x>0.005 showed ferromagnetic ordering with the Curie temperatures varying linearly with x. For 0.0001

Magnetic properties of gadolinium ortho-aluminate

Abstract: An account is given of specific heat, magnetic susceptibility and magnetic moment measurements on gadolinium ortho-aluminate, GdAlO$_{3}$, at temperatures in the liquid helium range. This compound orders antiferromagnetically at 3.87 K. In the antiferromagnetic state, its properties can be accounted for satisfactorily as those of an array of S-state magnetic ions with isotropic magnetic properties, coupled together by exchange interaction between nearest neighbours and subject to a 'single ion' anisotropy force due to the crystalline electric field. Because the structure approximates closely to a simple cubic lattice of Gd$^{3+}$ ions, magnetic dipole effects have little influence on the ordering. The exchange and anisotropy forces are calculated from the experimental data, and the measured properties are compared with various theoretical predictions for a simple two sublattice antiferromagnet. The variation of the specific heat in the critical region close to the Neel point is compared with that of other compounds undergoing magnetic transitions at low temperatures, and a consistent pattern of behaviour is found.