Showing papers on "Curie temperature published in 1972"

Permittivity of Strontium Titanate

Abstract: The permittivity of single‐crystal single‐domain strontiumtitanate has been measured in detail in the [001], [011], and [111] directions, as a function of temperature (from 4.2 to 300 °K), electric field (from −23 000 to +23 000 V/cm, and frequency (from 1 kHz to 50 MHz). The free energy of the crystal is determined as a function of polarization with temperature as a parameter. The Curie‐Weiss law is satisfied in the temperature range 60–300 °K, giving a Curie temperature of 30 ± 2 °K for the three crystal orientations. The Lyddane‐Sachs‐Teller (LST) relation is satisfied for temperatures between 30 and 300 °K and for electric fields between 0 and 12 000 V/cm. A generalized LST relation is used to calculate the permittivity of strontiumtitanate from zero to optic frequencies. Two active optic modes are important. The lower‐frequency mode is attributed mainly to motion of the strontium ions with respect to the rest of the lattice, while the higher‐frequency active mode is attributed to motion of the titanium ions with respect to the oxygen lattice. The restoring forces that act on the Ti ions begin to "harden" when these ions are displaced approximately 0.002 A from their equilibrium positions.

Magnetic Properties of Oxidized (Cation-Deficient) Titanomagnetites (Fe, Ti, _??_)43O4

Abstract: A comprehensive study of the magnetic properties of nonstoichiometric (cation-deficient) titanomagnetites is described. Measurement of the saturation magnetization of synthetic samples provides information about the mechanism by which increasingly nonstoichiometric states evolve. It is concluded that in the spinel structure the inherent availability for oxidation of Fe2+ ions on tetrahedral sites is about 20% of that of Fe2+ ions on octahedral sites. Curie temperature increases with degree of oxidation, showing that the increasing vacancy concentration is offset by the greater number of strongly interacting Fe3+ ions. Comparison of calculated and observed Curie temperatures of highly oxidized compositions indicates that such materials have nonideal u-parameters. Thermomagnetic curves of the oxidized samples are of the “anomalous” type, but none was found having compensation temperatures. Coercivity and anisotropy decrease with decreasing Fe2+ ion concentration. Finally, contours of equal cell edge and Curie temperature are constructed for spinels in the FeO-TiO2-Fe2O3 ternary system. These contours may be useful in the identification of naturally occurring samples, provided they do not contain appreciable amounts of other cations. An extrapolated value of 695°C is found for the Curie temperature of γ-Fe2O3.

The precision determination of the lattice parameters and the coefficients of thermal expansion of BiFeO3

Abstract: The lattice parameters of BiFeO3 were determined with the Straumanis method. At 25–13±0.02 °C, the hexagonal parameters are ah = 5.5799±0.0003, and ch = 13.8670±0.0005 A. The temperature dependence of the lattice parameters in the range 20–325 °C is given by the equations: ah = 5.5764 A + 6.06 × 10−5t, and ch = 13.8620 A + 2.10 × 10−4t. In the range of 344–838 °C, the lattice parameters obey the following equations: ah = 5.5946 + 6.83 × 10−5 t, and ch = 13.7251 + 9.05 × 10−4 t −12.503 × 10−7 t2 + 9.40 × 10−10 t3 − 3.57 × 10−13 t4. By extrapolation of the angular separation of the 11.0 and the 10.4 reflections, the electrical Curie temperature was determined to be 845±5 °C.

Ferroelectric and optical properties of Pb5Ge3O11 and its isomorphous compound Pb5Ge2SiO11

Abstract: The temperature characteristics of dielectric constants, spontaneous polarization, thermal‐expansion coefficients, indices of refraction, and optical rotatory power of Pb5Ge3O11 single crystals have been investigated in detail from room temperature up to above the Curie temperature, 177°C. From the results of these measurements, the following constants were obtained: linear thermal‐expansion coefficients (αa)F=7.75×10−6/deg°C and (αc)F=7.79×10−6/deg°C (ferroelectric phase), (αa)P=13.8×10−6/deg°C and (αc)P=13.4×10−6/deg°C (paraelectric phase); quadratic electro‐optic constants g33T=0.47×108 cm4 /C2, g13T=0.37×108 cm4 /C2; and electrogyration coefficient γ33 =8.7 cm2/C. An isomorphous compound Pb5(Ge2O7)(SiO4) was synthesized and was found to be ferroelectric with T c=60°C and Ps=1.7 μC/cm2 at room temperature. Ferroelectric properties and electrogyration coefficient were studied in the solid‐solution system Pb5(Ge2O7)(GeO4)1−x (SiO4)x.

Pressure Effect on the Curie Temperatures of Transition Metals and Alloys

Abstract: We determined the variation of the Curie temperature with pressure for iron, cobalt, nickel, six iron-nickel alloys, and five cobalt-nickel alloys. A permeability method in zero magnetic field has been used in a belt-type apparatus at pressures up to 90 kbar and temperatures up to 1400 \ifmmode^\circ\else\textdegree\fi{}K. The pressure shifts measured for the Curie temperatures of iron and cobalt are zero; for nickel it is positive and markedly nonlinear. The Curie temperatures of face-centered-cubic iron-nickel alloys decrease very rapidly with pressure when the nickel content is low and show a progressively smaller decrease when nickel is added. They begin to increase when the nickel concentration is over 68 wt%. The existence of maxima on the curves of the Curie temperatures versus pressure or volume seems to be definitely established. pressure increases the Curie temperature of cobalt-nickel alloys in all cases, but here also in a nonlinear way. For strong ferromagnets with a few carriers a band theory has been previously developed. The shift of the Curie temperature is then given by $\frac{d\ensuremath{\theta}}{\mathrm{dP}}=\frac{5}{3}k\ensuremath{\theta}$ ($k=\mathrm{compressibility}$). For pure nickel and nickel-cobalt alloys with a cobalt content less than about 50 wt% good agreement with experiment is obtained at low pressures, but not at higher pressures. The Curie temperature shift of Invar-type alloys has been previously calculated using a weak-itinerant-electron-ferromagnet band theory. In this case we have been able to predict and observe a parabolic decrease of the Curie temperature with pressure: ${\ensuremath{\theta}}^{2}={\ensuremath{\theta}}_{0}^{2} (1\ensuremath{-}\frac{P}{{P}_{0}})$. In order to explain the shifts for the different fcc alloys of nickel with iron, use is made of a model where $\ensuremath{\gamma}$-iron is supposed to have two electronic levels and which gives account of many properties of these alloys at normal pressure.

The magnetic properties of rare earth-Pd3 phases

Abstract: Magnetic measurements from 1.5 to 300 K on the complete series of Cu3Au-type R Pd3 phases have been made. LaPd3 and LuPd3 are diamagnetic at room temperature. CePd3 shows Pauli-type paramagnetism indicative of a nonmagnetic virtual bound state. PrPd3 and NdPd3 are affected at low temperatures by the presence of the crystal field. The magnetic properties of SmPd3 and EuPd3 are determined by the energy levels of the lowest multiplet. GdPd3 and (possibly) TbPd3 become antiferromagnetic at 7.5 and 2.5 K respectively. DyPd3-ErPd3 obey the Curie law. The magnetic properties of these phases are discussed in relation to crystal field effects and a possible band structure.

Magnetic Properties of the System SrCo 1− x Fe x O 3− y

Abstract: Samples of the system SrCo 1- x Fe x O 3- y (0≤ x ≤1, 0≤ y ≤0.5) have been prepared under varying oxygen pressure and temperature and their crystal-lographic and magnetic properties have been studied. SrCoO 2.5 with brown-millerite type structure (high temperature phase) is an antiferromagnet with the Neel temperature of 570 K and the Co 3+ -O 2- -Co 3+ superexchange interaction is strongly antiferromagnetic. SrCoO 3 and SrCo 1- x Fe x O 3 ( x <0.9) with the cubic perovskite structure (high temperature phase) are ferromagnetic with the Curie temperature at about 200 K and the magnetization decreases rapidly with increasing x near SrFeO 3 whose magnetic structure is helical.

Application of the Weiss molecular field theory to the B‐site spinel

Abstract: A molecular field treatment of the B-site spinel was used to determine the ordering temperatures and the asymptotic Curie temperature in terms of the exchange integrals. The theory was developed from a 16-sublattice model and took into account interactions between an atom and its nearest to fourth-nearest neighbors. Experimental values of the ordering temperatures and the asymptotic Curie temperature were then used to determine the exchange integrals for the systems Hg1−xCdx Cr2 S4, CdCr2S4(1−y)Se4y, and Zn1−xCdxCr2Se4.

Easy directions of magnetization in ternary R 2 (Co, Fe) 17 phases

Abstract: The magnetocrystalline symmetry and the Curie temperature of six families of quasi-binary rare earth-cobalt-iron intermetallic phases were studied. Investigated were the phases R 2 (Co 1-x Fe x ) 17 of the light rare-earth elements R = Ce, Pr, Nd, Sm, and Y and also of Ce-rich mischmetal (MM). In the Sm system, the terminal compound Sm 2 Co 17 has an easy c axis and the easy-axis symmetry persists up to x \approx 0.5 . The Nd alloys have an easy basal plane for all values of x. In the other systems, small iron substitutions change the easy-plane behavior of the R 2 Co 17 to an easy-axis anisotropy. The easy-axis ranges are x \approx 0.05 to 0.45 for Ce, x \approx 0.2 to 0.6 for Pr, x \approx 0.1 to 0.5 for Y, and x \approx 0.1 to 0.45 for MM. The Curie temperatures are high, between 600°C and 940°C, in these ranges. It is concluded that some of the alloys investigated are promising candidate materials for use in improved "rare earth-cobalt" magnets.

Stabilization effects in piezoelectric lead titanate zirconate ceramics

Abstract: The effect of stabilizing the properties of piezoelectric Pb(ZrTi)O3-ceramics by addition of the oxides of Mn, Cr or U is studied. Some ordering of the polarization is found to be preserved after heating the material above the Curie temperature. A model for the mechanism is presented, the basic idea of which is the trapping of charge carriers compensating the grain surface charges caused by the spontaneous polarization.

Magnetic Characteristics of Some Binary and Ternary 2–17 Compounds

Abstract: The magnetic anisotropy of R2Co17 and R2Co17−xTx compounds with R=Y, Ce, Pr, Nd, Th and T=Mn, Fe has been investigated. The directions of easy magnetization were determined by x‐ray studies on oriented powders. The several R2Co17 compounds, except for Ce2Co17, have an easy basal plane; the behavior of Ce2Co17 could not be clarified. Replacement of Co by Mn or Fe changes the easy magnetization direction from the basal plane to the c axis. Anisotropy fields were measured on magnetically aligned powders. The values for the anisotropy fields range between 12 and 24 kOe. Curie temperatures and saturation magnetization data for R2Co17−xTx compounds with T=Mn, Ru and R=Ho, Y are reported. The decrease of the magnetization with increasing x observed in the system Y2Co17−xTx is accounted for by assuming either a reduced Co moment or an antiparallel coupling of the T transition‐metal spin. Similar considerations hold for Ho2Co17−xTx ternaries.

Elastic and Piezoelectric Properties of Ferroelectric 5PbO · 3GeO2 Crystals

Abstract: The elastic, piezoelectric, and dielectric constants of a new ferroelectric crystal 5PbO · 3GeO2 have been measured. The measurements involve mostly the use of thickness modes in small plates of various crystallographic orientations. The electromechanical coupling factors turn out to be small, which makes this material unattractive for ultrasonic applications. The temperature characteristics of some piezoelectric constants are presented. The piezoelectric constant d31 vanishes above the Curie point (177 °C), while d22 remains constant above and below the Curie point.

Specific heat of a three dimensional Ising ferromagnet above the Curie temperature. II

Abstract: For pt. I see abstr. A37614 of 1967. High temperature series expansions for the specific heat of the Ising model of a ferromagnet are given for the face-centred cubic, body-centred cubic, and simple cubic lattices. From a numerical study it is concluded that the critical index ( alpha ) is lattice independent and that in three dimensions alpha approximately=1/8. A numerical representation of the specific heat in the range TC

Paramagnetic Sheet at the Surface of the Heisenberg Ferromagnet EuO

Abstract: Measurements of the spin polarization of photoelectrons from (100) surfaces of "pure" and doped EuO monocrystals reveal a paramagnetic surface sheet existing at temperatures far below the Curie temperature of the bulk. The extra electrons that are introduced on doping produce an increase of the molecular field in the surface. ${\mathrm{Eu}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}\mathrm{O}$ crystals exhibit the highest degree of spin polarization (\ensuremath{\sim} 80%) observed in a photoemission experiment so far.

Magnetic Properties of Cr 3 Te 4 in Ferromagnetic Region

Abstract: Single crystals of Cr 3 Te 4 with the pseudo-NiAs type structure have been made by chemical transport method in order to investigate the anomaly below 80 K. The saturation moment at 4.2 K is equal to 1.79±0.03 µ B per Cr 1- x Te (x=0.25), and the Curie temperature is 316 K. The magnetization only in the direction to the a -axis shows the anomaly that the saturation field increases abruptly below 80 K, which corresponds to the Neel point proposed by Andresen. But on the contrary to his result, the easy axis of the moment is parallel to the c -axis in the whole ferromagnetic region and antiferromagnetic order appears only in the c -plane. By a spin echo method, two nuclear magnetic resonance absorption lines of Cr 53 -nuclei are observed at 45.3 and 57.5 MHz at 4.2 K. The resonance frequencies increase remarkably below 80 K. This fact may support subsidiarily the existence of the antiferromagnetic order below 80 K.

On randomly dilute ferromagnetic models

Abstract: The paper considers the I(1/2), H(1/2), and H( infinity ) ferromagnetic models in which there are nonmagnetic impurities distributed at random on the sites of the underlying physical lattice. High temperature power series expansions, which are presented, are analysed by conventional methods to find the Curie temperature Tc(p) and susceptibility exponent gamma (p), where p is the concentration of magnetic elements. The series are analysed for p>1, as well as p<1, thus making contact with the nonselfintersecting chain problem. At p=1, gradients of TC(p)/TC(1) against p, are found, for the face centred cubic lattice, to be 1.05+or-0.02 for I(1/2), 1.15+or-0.01 for H( infinity ) and 1.36+or-0.03 for H(1/2). For the plane triangular lattice, the initial gradient for the I(1/2) model is 1.45+or-0.05. The behaviour found for gamma (p) is rejected as physically implausible, and the implications of its anomalous behaviour are discussed.

Actinide pnictides and chalcogenides. I. study of magnetic ordering and ordered moments in uranium monochalcogenides by neutron diffraction

Abstract: The magnetic ordering and ordered magnetic moments in uranium monochalcogenides (US, USE and UTE) was investigated by neutron diffraction. These measurements were undertaken to clarify the conflicting evidence from magnetization measurements. Measurements on powdered samples of USe and UTe show that these compounds are simple ferromagnets with the moments probably directed along the rhombohedral axis as found in a single crystal of US. Compared with an ordered magnetic moment (at 4.2 K) of 1.70+0.03 mu B in US, the values 2.0+or-0.1 mu B and 2.2+or-0.1 mu B were found in USe and UTe, respectively. These values are significantly higher than the values measured by magnetization due to the large magnetic anisotropy associated with the rhombohedral distortion and also due to conduction electron polarization effects.

Structural, Magnetic, and Magneto‐Optic Properties of Ti‐Substituted MnBi Films

Abstract: Structural, magnetic, and magneto‐optic properties of Mn1−xTixBi films (0≤x≤0.2) prepared by a vacuum‐evaporation technique are reported. The crystallographic structure and the order of phase transformations of unsubstituted MnBi films were not altered by Ti. Both low‐temperature (LT) phase and quenched high‐temperature (HT) phase films were oriented with the hexagonal c axis and the magnetization normal to the film plane. The Curie temperature of HT films was decreased by Ti. There is a strong effect of Ti on the kinetics of the crystallographic phase transformations in the films. Particularly, the speed of the HT‐phase‐to‐LT‐phase transformation is reduced by up to three orders of magnitude, depending on the Ti concentration and the temperature. At room temperature the specific Faraday rotation of LT (HT) phase films decreased with increasing Ti concentration from 9.8 × 105 deg/cm (4.3 × 105 deg/cm) for x = 0 to 3.8 × 105 deg/cm (1.6 × 105 deg/cm) for x = 0.18, measured with light of 632.8‐nm wavelength...

Magnetism of rare-earth elements, alloys, and compounds

Abstract: The magnetic properties of the rare-earth metals and a number of the alloy and compound systems formed with rare earths are discussed. Emphasis is placed on current experimental results of bulk magnetic measurements and related transport phenomena and their bearing on theoretical studies. Topics covered include exchange interactions, magnetization processes, crystal field effects and anisotropy, magnetostriction, spin structures and their field dependence, specific heat, optical effects, resistivity, magnetoresistivity, and other transport properties affected by the magnetic order. Examples are drawn from a number of material systems including ionic compounds, solid solution compounds (e.g., rare-earth magnetic semiconductors), intermetallic compounds (permanent magnet materials, etc.), and bubble domain materials in addition to the rare-earth elements and intra-rare-earth alloys. Tables are presented giving the spin structures, ordering temperatures, saturation and effective moments, and other properties of the elements and specific compounds.

Nonlinear Optical Properties of Ferroelectric Lead Titanate

Abstract: Optical transmission, refractive‐index, and nonlinear optical properties of single domain PbTiO3 have been measured between room temperature and the Curie temperature Tc. The second‐harmonic coefficient d31 is found to be the highest for any room‐temperature ferroelectric. Since the birefringence is small (approximately 0.008), phase‐matched SHG is not possible, but nondegenerate three‐frequency mixing should be possible. The Miller δ coefficients are analyzed in terms of Jerphagnon's formulation and poor agreement is observed.

Lunar rock magnetism

Abstract: The relationship between the magnetization and temperature in a high constant magnetic field for a temperature range between 5 K and 1100 K was examined for Apollo 11, 12 and 14 lunar materials. The average value of Curie point temperature is (768.2 ± 3.5)°C for the lunar igneous rocks and (762.5 ± 3.4)°C for the lunar fines and breccias. A tentative conclusion about the ferromagnetic substance in the lunar materials would be that Fe is absolutely dominant with a slight association of Ni and Co, and probably Si also, in the lunar native irons. The antiferromagnetic phase of ilmenite and the paramagnetic phase of pyroxenes are considerably abundant in all lunar materials. However, a discrepancy of observed magnetization from a simulated value based on known magnetic elements for the temperature range between 10 and 40 K suggests that pyroxene phase represented by (M x Fe1-x ) SiO3 (whereM = Ca2+, Mg2+, etc and 0 ≤x ≤1/4) also may behave antiferromagnetically. Magnetic hysteresis curves are obtained at 5 K and 300 K, and the viscous magnetic properties also are examined for a number of lunar materials. The superparamagnetically viscous magnetization has been experimentally proven as due to fine grains of metallic iron less than 200 A in mean diameter. The viscous magnetization is dominant in the lunar fines and breccias which is classified into Type II, while it is much smaller than the stable magnetic component in lunar igneous rocks (Type I). The superparamagnetically fine particles of metallic iron are mostly blocked at 5 K in temperature; thus coercive force (H c ) and saturation remanent magnetization (I R ) become much large at 5 K as compared with the corresponding values at 300 K. Strongly impact-metamorphosed parts of lunar breccias have an extremely stable NRM which could be attributed to TRM. NRM of the lunar igneous rocks and majority of breccias (or clastic rocks) are intermediately stable, but their stability is considerably higher than that of IRM of the same intensity. This result may imply that some mechanism which causes an appreciable magnitude of NRM and the higher stability, such as the shock effect, may take place on the lunar surface in addition to TRM mechanism for special cases. A particular igneous rock (Sample 14053) is found to have an unusually strong magnetism owing to a high content of metallic iron (about 1 weight percent), and its NRM amounts to 2 × 10−3 emu/g. The abundance of such highly magnetic rocks is not known as yet but it seems that the observed magnetic anomalies on the lunar surface could be related to such highly magnetized rock masses.

A New Ternary Compound in Rare Earth-Cobalt-Boron System

Abstract: A new ternary boride RCo12B6 with a rhombohedral structure was synthesized by arc-melting in R-Co-B system, where R is a rare earth ion. From magnetic measurements, it was found that CeCo12B6 had a Curie temperature at 130°–140°K and exhibited a Curie–Weiss behavior in the paramagnetic region above 140°K.

Magnetic properties of Clb compounds; CoVSb,CoTiSb and NiTiSb

Abstract: Experimentally CoVSb is found to be a ferromagnetic compound with a ferromagnetic and a paramagnetic Curie temperature of 58 K and 75 K, respectively. The saturation magnetic moment per formula at 0 K is 0.18µ B . and the paramagnetic moment per formula 1.26µ B . The relations of the Curie temperature and magnetic moment between ferromagnetic and paramagnetic states are very different from that of usual ferromagnetic materials. CoTiSb and NiTiSb are found to be temperature independent paramagnetic compounds with the susceptibility per gram of 5×10 -7 and 6×10 -7 emu/g, respectively. It should be a noticeable fact that CoVSb has a spontaneous magnetic moment while CoTiSb and NiTiSb have not.

On the ferrite-austenite equilibrium in the Fe-Cu system

Abstract: The thermodynamics of ferrite containing dissolved copper is analyzed theoretically, taking into account the effect of copper on the Curie temperature. By combination with experimental information on the solubility of copper in austenite, this model is used to predict the solubility of copper in ferrite. The result shows good agreement with experimental data but indicates that the data should be extrapolated in a manner different from that which has been previously suggested. The effects of copper and cobalt on the relative stability of the α and γ phases in iron-rich alloys are compared. It is concluded that their difference is mainly caused by the difference in their effects on the Curie temperature of α-iron.

Bibliography of Magnetic Materials and Tabulation of Magnetic Transition Temperatures

Abstract: Tabulation of Curie Temperatures.- Tabulation of Neel Temperatures.- Addendum.- Curie Temperatures.- Neel Temperatures.- References.- Recent Reviews, Conferences, and Compilations on Magnetic Materials.- Monochromator and Analyzing Crystals.