Showing papers on "Curie temperature published in 1974"

Ferroelectric and antiferroelectric properties of (Na 0.5Bi 0.5)TiO3-SrTiO3 solid solution ceramics

Abstract: Phase transitions in the solid solution Sr χ(Na 0.5Bi 0.5)l-χTiO3 have been studied by dielectric, pyroelectric and D-E characteristic measurements. An intermediate antiferroelectric phase has been observed in the composition range of 0 ⩽ χ ⩽ 0.5 of this system. These compositions undergo a ferroelectric-antiferroelectric-paraelectric sequence of transitions at elevated temperatures. The antiferroelectric phase transition temperature and Curie point decrease monotonically with increasing SrTiO3 concentration.

A new ferroelectric: La2Ti2o7

Abstract: Ferroelectricity was observed in a lanthanum titanate single crystal (La2Ti2O7), a monoclinic crystal of the space group C22-P21. The Curie temperature was found to be about 1500°C. It was determined by a D-E hysteresis loop at room temperature that the spontaneous polarization Ps = 5 μC/cm2 and that the coercive field Ec = 45 kV/cm. Ferroelectric domains were observed using a crystal etched by an aqueous solution of nitric acid. The La2Ti2O7crystal showed strong piezoelectric and linear electro-optic effects: electromechanical coupling factor k22 = 0.29; half-wave voltage [E l]δ/2 = 2000 V at a wavelength of 6328 A.

The diffuse phase transition in potassium strontium niobate

Abstract: The diffuse, paraelectric/ferroelectric, phase transition of Potassium Strontium Niobate is discussed. It is found that the Curie point is strongly dependent on the K and Sr content. Several possible causes of the diffuse phase transition (DPT) are discussed. An empirical relationship describing the behaviour of the inverse dielectric constant in the transition region, is presented. A statistical analysis of the DPT is described in terms of microscopic composition variations within the crystal arising from non-stoichiometry. Plots of the polar dielectric constant and spontaneous polarization against temperature, derived from this model, compare well with the experimentally measured quantities.

Exchange optics in Gd-doped EuO

Abstract: This paper is concerned with magneto-optical properties of EuO single crystals, doped with up to 5-at.% Gd. The optical absorption and the Faraday rotation have been measured above and below the magnetic-ordering temperatures and in various magnetic fields. In addition the reflectivity between 13 eV and 250 \ensuremath{\mu}m has been observed and the Curie temperature has been determined. The measurements permit the evaluation of the magnetic short-and long-range order. The exchange interaction between free electrons and $4f$ spins can be determined and is found to decrease with increasing doping. The analysis of the optical data permits the derivation of the carrier concentration, their mobility, and temperature dependence. The carrier concentration shows an exchange-induced ionization near ${T}_{C}$ for nondegenerate samples and the mobility has a sharp minimum at ${T}_{C}$. The transport parameters also reflect the fact that the electron-$4f$-spin interaction decreases with increasing Gd concentration, probably owing to electrostatic-shielding effects.

Calculations of Stable Domain Radii Produced by Thermomagnetic Writing

Abstract: Calculations are performed to determine the stable radius of a cylindrically symmetric domain nucleated in magneto-optical films during thermomagnetic writing with a laser beam. A critical bound on domain size is calculated which determines whether or not a domain of given radius, once nucleated, will be stable. The analysis shows that for a ferromagnetic material such as MnAlGe, the domain dimensions can grow beyond the local region of material that is heated above the Curie temperature. For ferrimagnetic thin films having a compensation point T comp , stability depends on the difference between ambient and compensation temperatures, ΔT = T a − T comp . With ΔT ≈ 0, wall energy dominates and the critical radius can be calculated from R c = σ/(2MH c ).

Magneto-volume effects in rare-earth transition metal intermetallics

Abstract: The pressure derivative of the Curie temperature, the compressibility, and the thermal expansion of several representative members of the group of R -Co and R -Fe intermetallics ( R stands for rare-earth elements Y, Th, and in one case Zr) has been studied. The results derived for the volume dependence of T c agree rather well with the relation ( d \ln T_{c})/(d \ln V) = a+b/T_{c}^{2} in which, for both series of compounds (Co and Fe), the constant a equals about -5/3. This result favors an interpretation of the magnetic properties of these compounds in terms of the collective electron model rather than one in terms of localized moments. For some compounds the thermal expansion above room temperature has been studied. A considerable reduction of the thermal expansion has been observed near T c for those compounds for which dT_{c}/dp is large and negative (invar effect).

Ferromagnetism and strong correlations in metals

Abstract: A single band description of ferromagnetism in 3d transition metals is given based on a recent formal theory of strong correlations by the author. Correlations are assumed to be important leading to 'Hubbard split bands' and localized moments. Correlations and magnetic effects of the system are calculated selfconsistently. An effective Heisenberg Hamiltonian is obtained. The magnetic excitations of the system are spin waves, their dispersion relation is derived. The spin wave stiffness constant is compared with earlier theories. At the Curie temperature Tc long range magnetic order disappears, localized moments and short range order, however, still exist above Tc. Comparison with experiment is made.

Contribution of Fe2+ ions to the magnetocrystalline anisotropy constant K1 of Fe3-xTixO4 (0<x<0.1)

Abstract: The magnetocrystalline anisotropy constant K1 of Fe3O4, Fe2.96Ti0.04O4 and Fe2.9Ti0.1O4 has been measured between room temperature and the Curie point using the torque method. The Fe3+ contribution to the anisotropy in this temperature range has been calculated using the theory of Wolf (1957) and the relative sublattice magnetizations determined from Mossbauer-effect spectra. The specific Fe2+ contribution, K2B1 is then found to vary with composition x and temperature T as K12B(x,T)= (0.2-3.5X) * 10-2-(0.24+3.0x)(Tc-T)10/3 * 10-8 cm-1/ion.

Photoconductivity in certain ferroelectrics

Abstract: Certain peculiarities of photoconductivity in ferroelectrics are discussed. On the example of the ferroelectric SbSI, the nature of anomalies in electrical conductivity and photoconductivity in the Curie temperature region is discussed. For the first time the photoconductivity in certain ferroelectric niobates and the ferroelectric 5PbO. 3GeO2 was studied

Specific heat and resistivity of gadolinium near the Curie point in external magnetic fields

Abstract: A modification of the ac calorimetric technique has been used to test the proportionality of the specific heat and temperature derivative of the basal-plane resistivity of gadolinium through the Curie point. Because of a large temperature-dependent background to the resistivity derivative, a direct comparison of the critical contributions to the two quantities was impossible. It was found, however, that the background was insensitive to applied magnetic fields so that a direct comparison of the deviation of the specific heat at a finite field from its zero-field value with the similar deviation of the resistivity derivative demonstrated the proportionality of the two quantities. In applied fields below 585 Oe, a "kink point" was observed in the specific heat below ${T}_{C}$ which moved to lower temperatures with increasing fields. Such an effect has been predicted to occur at fields and temperatures for which the internal magnetic field vanishes. The specific heat in several fields above 585 Oe was compared with the predictions of scaling laws and with the linear approximation to the parametric equation of state. Reasonable agreement between the data and the specific-heat scaling function predicted by the linear model could only be obtained by using an unrealistic value of the critical exponent describing the zero-field specific heat.

Specific heat and resistivity of iron near its Curie point

Abstract: The specific heat and the temperature derivative of electrical resistivity of Fe have been measured simultaneously using an ac technique. The results for Fe demonstrate that the magnetic specific heat and the temperature derivative of the magnetic contribution to the resistivity are proportional both above and below the Curie point. The critical exponents are found to be $\ensuremath{\alpha}={\ensuremath{\alpha}}^{\ensuremath{'}}=\ensuremath{-}0.120\ifmmode\pm\else\textpm\fi{}0.01$.

Oxides and Hydroxides

Abstract: Oxidic minerals can only be recognized differential thermal analytically by the appearance of structural or magnetic changes. The high-low inversion of quartz (~573° C) has been for many years a means of calibrating the temperature, as well as for calorimetric measurements (following a proposal of Faust), but neither the inversion temperature (= ti) nor the heat of reaction (3.1 cal/g after Faust) can be used for calibration, since the ti can vary by a great temperature interval (Keith and Tuttle; Smykatz-Kloss, 1970, see III-8), and the heat of reaction is dependent on the degree of disorder of the crystals. Grim and Rowland have measured 565° for the ti of authigenically formed quartz crystals of soils. The ti-variation of the SiO2 minerals cristobalite and tridymite is still much greater than the ti-variation of quartz crystals (see III-7). Magnetites oxidize in two steps to Fe2O3, reflecting two very strong exothermic effects with peaks at 380 and 580° C. Shape and temperatures of these peaks are dependent on the grain size (Egger), and on some other factors (Kirsch). A weak endothermic effect at 680° C points out the Curie point of Fe2O3 (Egger); the temperature of this peak varies with different contents of titanium and manganese (compare with III-3). In DTA curves of hematite Peters obtained a broad endothermic deflection between 400 and nearly 700° C with a small endothermic peak at 675° C, which probably points out the Curie point of the Fe2O3.

Magnetic Properties of Ho-Co and Ho-Fe Amorphous Films

Abstract: Polar-Faraday-rotation and magnetization measurements on evaporated amorphous films of Ho-Co and Ho-Fe show that the principle mechanism responsible for differences in the ${T}_{C}$ of the crystalline and amorphous states is the altered electronic configuration of the transition metal, rather than local anisotropy. Additionally we found that the films possess uniaxial anisotropy.

Change from Metastable Cubic to Stable Tetragonal Form of Submicron Barium Titanate

Abstract: The change from metastable cubic to stable tetragonal form was investigated on the submicron BaTiO3 which was prepared by the hydrolysis of titanium tetraisopropoxide in barium hydroxide solution and by crystallization in the glass of BaO–TiO2–SiO2–Al2O3 system. The precipitated BaTiO3 was found to be the aggregates of minute grains of about 100 A, to have large amount of strain and to absorb about 6% of OH and alcoholic radicals, some of them absorbed chemically. It is in a metastable cubic form. After the heating above 800 °C, it changed to stable tetragonal form in association with weight loss and decrease in lattice strain. The samples heated at 800–900 °C did not show the Curie point on the DTA curve although they were tetragonal. The samples heated at 1300 °C had the Curie point about 122 °C. The glass-devitrified BaTiO3 was also cubic and had large lattice strain. In order to change to tetragonal form, the heating above 1000 °C was needed. The metastable cubic form of submicron BaTiO3 seemed to be ...

Magnetization studies on Tm 2 Fe 17-x Co x and Tm 2 Fe 17-x Al x compounds

Abstract: Magnetization studies were carried out on Tm 2 Fe 17-x Co x and Tm 2 Fe 17-x Al x compounds in the temperature range 4.2 K to 1200 K. Tm 2 Fe 17 changes its easy axis of magnetization from the basal plane to c-axis below 85 K. This transition occurs at higher temperatures as cobalt or aluminum is substituted for iron. The \underline{c} lattice parameter goes through a maximum as a function of x in the Tm 2 Fe 17-x Co x ternaries. The results are interpreted in terms of distance dependence of the magnetic interactions and a competition between the sublattice magnetic anisotropy. Tm 2 Fe 3 Co 14 has an anisotropy field of 38 kOe and a potential energy product of 43 MGOe at room temperature.

Magnetic and Magnetoelastic Properties of Highly Magnetostrictive Rare Earth‐Iron Laves Phase Compounds

Abstract: The rare earth‐Fe2 (RFe2) compounds possess huge magnetic anisotropies and magnetostrictions at room temperature. This, coupled with a relatively high Curie temperature and magnetic saturation, makes these alloys attractive for various applications, including magnetostrictive transducers and permanent magnets. A variety of measurements on single crystals, polycrystals and the amorphous forms of the RFe2 alloys are reported. These include magnetization, magnetic anisotropy, magnetostriction, elastic moduli, and resistivity. We find saturation moments (from single crystal measurements) substantially higher than polycrystalline values previously reported. For ErFe2, we calculate the T = 0 anisotropy constant K1(0) to be −5.4 × 108 ergs/cm3. Torque measurements on DyFe2 at room temperature yield K1 = 2 × 107 ergs/cm3, a large value for a cubic material at room temperature. We report our measured values for the magnetostriction of RFe2 compounds, where R = Sm, Tb, Dy, Er and Tm. Notable are TbFe2 and SmFe2 which stand out as giants with room temperature values of magnetostriction, λ, greater than 2000 ppm. A maximum magnetostriction/anisotropy ratio, λ/K1, was found for the pseudobinary compound TbxDy1−xFe2 with x≅0.3. Rapidly sputtered (amorphous) TbFe2 and DyFe2 have high magnetizations at absolute zero, although they are not as magnetic as the corresponding polycrystals at room temperature. A striking feature is the high coercivity (∼30 kOe) at low temperatures. Materials with anisotropic hysteresis loops and intrinsic coercivities > 3 kOe were obtained after heat treatments in a magnetic field.

Grain size effects on the ferroelectric-paraelectric transition, the dielectric constant, and the lattice parameters in lanthana-substituted lead titanate

Abstract: The effect of the grain size on unit-cell dimensions and dielectric properties has been studied in ceramic lanthana-substituted lead titanate of the general composition Pb1-αxLaxTiO3+x (15-α) with α = 120 and x = 007 (PLT7) and x = 018 (PLT18) The grain size ranges from 06 to 11 µm and the effects are most pronounced on the smaller sizes The tetragonal distortion, characterized by the ratio c/a of the unit-cell dimensions c and a, decreases by decreasing grain size This effect becomes relatively smaller by decreasing temperature A decrease of the grain size leads to a decrease of the maximum value of the dielectric constant e' at the Curie temperature T'c and to an increase of e' at lower temperatures The Curie temperature shifts to a lower temperature while the phase transition is less sharp with decreasing grain size The results can be discussed in terms of an average value of internal compression stresses in the direction of the c-axis of the crystal

Evaluation of curie constants of ferroelectric crystals from pyroelectric response

Abstract: A method is described for determining the Curie constant (Cc) of ferroelectric crystals below the Curie temperature (Tc) from the pyioelectric response. It is shown from Devonshire's phenomenological theory of ferroelectricity that the material parameter (p/ϵ) of the pyroelectric response is given by p/ϵ =PS/Cc , where Ps is the spontaneous polarization. This relationship is tested in detail for Sr0.5Ba0.5Nb2O6 crystals. The value of Cc obtained from the Curie-Weiss law, ϵ = CC /(T - Tc ), which is valid above Tc , is compared to the value of Cc obtained below Tc for SBN and other ferroelectric crystals and reasonable agreement is observed.

Proton spin-lattice relaxation by critical polarization fluctuations in KH2PO4

Abstract: The observed anomalous decrease in the proton spin-lattice relaxation timeT1 on approaching the Curie point in a rather pure KH2PO4 single crystal is explained by magnetic dipolar coupling to the ferroelectric mode. The isolated “non-interacting” O−H...O proton flipping time is estimated from theT1 data as τ=0.66·10−12 sec for the paraelectric phase and τ=2.24·10−12 sec for the ferroelectric phase, in good agreement with the results obtained from other methods.

The crystal and magnetic structure of UPTe and UAsTe studied by neutron diffraction

Abstract: Neutron-diffraction study confirmed the occurrence of an UGeTe type of structure in UPTe and UAsTe. The parameters of uranium and X (X = P, As) atoms were refined using the room-temperature neutron data. Below the Curie point, the magnetic moment alignment was found to be along the fourfold axis, in accordance with previous magnetization measurements. The magnetic moment of the uranium ion at liquid helium temperature was determined to be 1.44 and 1.59 mu /sub BETA / in UPTe and UAsTe, respectively. (auth)

Effects of high nonstoichiometry on EuO properties

Abstract: Electric, magnetic, and electron paramagnetic resonance (EPR) measurements have been performed on thin EuO films prepared either by reactive evaporation of Eu at 400°C or by oxidation of a Eu metal thin film at 200–250°C. The films obtained by reactive evaporation have electrical and magnetic properties very similar to those reported for EuO single crystals of various degrees of nonstoichiometry. The films prepared by oxidation of Eu films at 200–250°C are characterized by a much higher‐valued Curie temperature (Tc≃148 K), while their resistivity is in the range 10−2−1 Ω cm. The EPR signal can be attributed to Eu spins, which shows that in these films there exist fluctuations of Eu doping due to nonstoichiometry. We analyzed the high concentration of oxygen vacancies in the bound magnetic polaron (BMP) model. A high level of defects and oxygen vacancies, inhomogeneously distributed in the film, can account for both their electrical and magnetic properties.

Magnetocrystalline anisotropy in the cubic TbAl2 compound

Abstract: The isothermal magnetization of a TbAl, single crystal has been measured along the main symmetry directions in static magnetic fields up to 150 kOe and in the temperature range from 1.5 to 300 K. (111) is the easy direction of magnetization a t all field strengths and for a11 temperatures below the Curie point T, = 105 K. The spontaneous magnetization along (111) follows a Brillouin law; however, there is a strong magnetization anisotropy. By neglecting magnetostriction, the thermal variations of the anisotropy parameters of 4th and 6th order have been obtained: They are of the same type as those anticipated by Callen and Callen. Nous avons mesure les variations isothermes de l'aimantation d'un monocristal de Tb AI, suivant les principales directions cristallographiques dans des champs statiques atteignant 150 kOe, entre 1,5 et 300 K. L'axe (111) est de facile aimantation b toutes temperatures infkrieures b la tempkrature de Curie T, = 105 K. Selon cet axe la variation thermique de l'aimantation spontanke obkit b une loi de Brillouin; il apparait cependant une forte ani-sotropie de l'aimantation. E n n6gligeant la magn6tostriction, les variations thermiques des constantes d'anisotropie d'ordres 4 et 6 ont pu atre obtenues: elles sont du type de celles pr6vues par Callen et Callen.

Magnetic Properties of Cementite (Fe1-xMex)3C, (Me; Cr or Ni)

Abstract: Magnetization and Mossbauer absorption spectra have been measured on cementite substituted partially with Cr or Ni. The magnetic moment increases slightly with the increase of nickel concentration, while it decreases very abruptly from 1.9 µ B in Fe 3 C to 0.4 µ B in (Fe 0.799 Cr 0.201 ) 3 C. The internal field and the Curie temperature also decrease abruptly for the Cr-substituted cementite. The observed experimental results strongly indicate that the magnetic properties of the (Fe 1- x Me x ) 3 C system are closely analogous to those of γ Fe-Ni Invar alloy.