Curie-Weiss Constant Research Articles

Study of Fe 1−xV xBO 3 system magnetization

The measurements of the magnetization of Fe 1− x V x BO 3 ( x = 0 , 0.1 3 , 0.3 , 0.9 5 , 1 ) solid solutions as a function of temperature and field have been made. The critical magnetic Curie and Neel temperatures and spontaneous magnetic moments have been determined. The weak ferromagnetic behavior has been observed and the cant angle of the magnetic sublattices, as well as the exchange H ex and Dzyaloshinsky H D fields have been found for the Fe 0.87V 0.13BO 3 sample. The magnetic susceptibility appeared to obey the Curie–Weiss law for all concentrations x. The Curie–Weiss constants C and the paramagnetic Curie temperatures θ have been determined. The effective magnetic moment μ eff has been calculated for VBO 3.

The pressure influence on the incommensurate–commensurate ferroelectric phase transition in [4-NH 2C 5H 4NH][SbCl 4

The dielectric properties of the [4-NH 2C 5H 4NH] SbCl 4 (abbreviated as 4-APCA) crystal were investigated under hydrostatic pressure up to 300 Mpa. The pressure–temperature phase diagram was given. The paraelectric–ferroelectric phase transition (II→III) temperature ( T c) increases linearly with increasing pressure with a slope d T c/d p=21×10 −2 K/MPa. The pressure dependence of Curie–Weiss constants has been evaluated also. In the paraelectric phase (II) the Curie constant ( C +) was pressure dependent whereas the C − constant over the ferroelectric phase (III) was almost constant. The results are interpreted in terms of improper and displacive type phase transition model with a soft phonon at a zone boundary.

The Microstructural, Dielectric, Pyroelectric and Ferroelectric Properties of SrBi2(Nb1-xVx)2O9(0 ≤ x ≤ 0.3) Ceramics

Layered SrBi2(Nb1−x V x )2O9 (SBVN) ceramics with x ranging from 0 to 0.3 (30 mol%) were fabricated via the conventional sintering technique. A remarkable transformation in the microstructure of SBN ceramic was encountered as the V2O5 content increased from 3–30 mol%. The average grain size of the SBN ceramic containing V2O5 was found to increase with increase in V2O5 content. The dielectric constant (ϵ r ) as well as the dielectric loss (D) increased with increase in grain size (6 μm–17 μm). The ferroelectric to paraelectric transition temperature (T c ) shifted to higher temperature with increasing grain size. The pyroelectric coefficient was positive at 300 K and increased with increasing grain size. Interestingly, the SrBi2(Nb0.7V0.3)2O9 (x = 0.3) ceramics with a grain size of 17 μm exhibited higher remnant polarization (P r ) and lower coercive field (E c ) than those with grains of 7 μm. The Curie-Weiss constant that was obtained for these ceramics was in line with that of displacive type ferroelectrics.

Electrical properties of sol–gel derived pyrochlore-type bismuth magnesium niobate Bi 2(Mg 1/3Nb 2/3) 2O 7 thin films

A new type of Bi-based relaxor, Bi 2(Mg 1/3Nb 2/3) 2O 7 (BMN), has been investigated to understand nature of relaxor behavior. The BMN thin films have been successfully fabricated by a sol–gel spin coating method on Pt/Ti/SiO 2/Si(1 0 0) substrate. Dielectric properties of BMN thin films were investigated in the temperature range of 20–750 °C at frequencies of 10 2–10 7 Hz. The BMN thin film exhibits a characteristic diffused phase transition; the decreased dielectric constant and the increased dielectric constant maximum temperature with increasing frequency. The temperature dependent reciprocal dielectric constant ( 1 / ε ′ ) deviates from the Curie–Weiss law in the temperature range over the dielectric constant maximum temperature ( T m ) . The diffuseness of the phase transition of BMN thin film was estimated by calculating the difference between T m and T cw (starting temperature following the Curie–Weiss law). For BMN thin film, the Curie–Weiss temperature Θ , the Curie–Weiss constant c, the temperature T cw and the temperature difference T cw - T m measured at frequency of 10 3 Hz are 912, 1.15×10 5, 969 and 49 K, respectively. The dielectric relaxation obeys the Vögel–Fulcher relationship with the freezing temperature of 770 K.

Dependence of interface motion on pressure in ferroelectrics

The motion of the paraelectric–ferroelectric interface in KNbO3 at a hydrostatic pressure is studied in the framework of the mean-field theory. The kink solution is applied to the calculations of the width and velocity of the interface at different pressures. The calculations are based on the experimental data for the Curie–Weiss constant and the parameter of the Ginzburg-Landau expression for the free energy. The response of the interface motion to the hydrostatic pressure and the tricritical point is investigated nonempirically.

Magnetocaloric effect in pyrochlore antiferromagnetGd2Ti2O7

An adiabatic demagnetization process is studied in Gd2Ti2O7, a geometrically frustrated antiferromagnet on a pyrochlore lattice. In contrast to conventional paramagnetic salts, this compound can exhibit a temperature decrease by a factor of ten in the temperature range below the Curie-Weiss constant. The most efficient cooling is observed in the field interval between 120 and 60 kOe corresponding to a crossover between saturated and spin-liquid phases. This phenomenon indicates that a considerable part of the magnetic entropy survives in the strongly correlated state. According to the theoretical model, this entropy is associated with a macroscopic number of local modes remaining gapless till the saturation field. Monte Carlo simulations on a classical spin model demonstrate good agreement with the experiment. The cooling power of the process is experimentally estimated with a view to possible technical applications. The results for Gd2Ti2O7 are compared to those for Gd3Ga5O12, a well-known material for low temperature magnetic refrigeration.

Magnetic susceptibility of intermetallic compounds in the Er-In system at high temperatures

The temperature dependence of the magnetic susceptibility χ(T) of intermetallic compounds of Er with In in a wide temperature range from 20 to 1600°C inherent in the solid state and melting process of the materials under review was studied for the first time by the Faraday method. For all test compounds, the temperature dependence was found to be approximated by the Curie-Weiss linear law both in a solid and liquid state. The Curie paramagnetic temperature, Curie-Weiss constants, and effective magnetic moment numbers per Er atom were calculated by computer processing of the χ−1(T) data, using the least-squares technique. A semiempirical estimation of the indirect exchange interaction parameter showed the test compounds to be characterized by exchange interactions of the Ruderman-Kittel-Kasuya-Yosida type.

Regularities of Variation of the Curie-Weiss Law Parameters in Pb(Ti,Zr)O3- and NaNbO3-Based Solid Solution Systems

A complex morphology of the morphotropic phase transition region in ternary solid solution systems assigned by lead zirconate-titanate and a binary natrium niobate—lead titanate system has been established. A nonmonotonic behavior of basic characteristics (the dielectric permittivity maximum temperature T m , the temperature T 0 and the Curie-Weiss constant C w and the difference of temperatures T m − T 0) inside the morphotropic region at solid solutions under study was revealed and may be related with the existence of an intermediate (pseudocubic) phase in it.

Dielectric Properties of SbSI-Modifed in Phase Transition Region

For SbSI-modifed (SbSI:0.1%Cl) X-Ray investigations at room temperature have been carried out. The lattice parameters a, b, c have been determined. Measurements of temperature/frequency dependence of electric permitivities of SbSI-modifed as well as hysteresis loops have been carried out. The Curie-Weiss constant has been calculated. A weak dispersion of dielectric permittivity has been observed.

Dielectric properties and phase transition behavior of xPMN-(1 - x)PZT ceramic systems

The dielectric properties and phase transition behavior of the pseudo-ternary xPb(Mg1/3Nb2/3)O3-(1 − x)Pb(Zr,Ti)O3 solid solution system were investigated as a function of the Pb(Mg1/3Nb2/3)O3 (PMN) content and Ti/Zr ratio for selected compositions. The investigations have demonstrated a general trend in broadening of the phase transition and increasing diffusivity with increasing PMN content. For the morphotropic phase boundary (MPB) compositions, the dielectric permittivity maximum, its temperature (T m) and the Curie-Weiss constant were found to decrease with increasing Mg1/3Nb2/3 concentration. When a Ti/Zr ratio was constant and equal to 53/47, temperature-dependent investigations demonstrated that the dielectric parameters involved in a modified Curie-Weiss law increase monotonically with increasing PMN content and T m moves toward room temperature with average rate of ≈ −4.1°C/mol% as well. A phase transition in 0.5PMN-0.5Pb(Zr0.47Ti0.53)O3 and 0.25PMN-0.75Pb(Zr0.60Ti0.40)O3 ceramic systems exhibited a diffused behavior with a characteristic frequency dependence of T m. From pyroelectric measurement, an unusual spontaneous polarization behavior at about 215 K is reported for some MPB compositions.

Impact of thermal strain on the dielectric constant of sputtered barium strontium titanate thin films

Barium strontium titanate thin films were deposited by sputtering on Pt/SiO2 structures using five different host substrates: magnesium oxide, strontium titanate, sapphire, silicon, and vycor glass. These substrates were chosen to provide a systematic change in thermal strain while maintaining the same film microstructure. All films have a weakly textured microstructure. Temperature dependent dielectric measurements from 100–500 K determined that decreasing thermal expansion coefficient of the host substrate (i.e., larger tensile thermal strain) reduced the film dielectric permittivity. The experimentally determined Curie–Weiss temperature decreased with increasing tensile thermal strain and the Curie–Weiss constant increased with tensile strain as predicted by Pertsev et al. [J. Appl. Phys. 85, 1698 (1999)].

Dielectric properties of kdp-adp mixed crystals in the vicinity of ferroelectric phase transitions

Abstract

A study of the paramagnetic to ferromagnetic transition in the optimally doped CMR compound La0.65Ca0.35MnO3 and its dependence on oxygen mass

We present a study of the paramagnetic to ferromagnetic transition in the CMR compound La0.65Ca0.35MnO3 and its dependence on magnetic field and oxygen mass. The transition is characterized by two temperatures, the thermodynamic transition temperature at Tc, obtained from specific heat and thermal expansion data, and the resistive transition obtained from the resistivity maximum. The resistive transition occurs well within the paramagnetic range. The magnetic susceptibility in the paramagnetic range is isotope dependent up to 400 K. The magnitude of the Curie-Weiss constant indicates the presence of small clusters of about 4–5 unit cells. The resistive transition occurs when the percolation limit for these clusters is reached.

Grain-Boundary Effect on the Curie-Weiss Law of Ferroelectric Ceramics and Polycrystalline Thin Films: Calculation by the Method of Effective Medium

The influence of grain boundaries on the dielectric properties of ferroelectric ceramics and polycrystalline thin films is described theoretically by the method of effective medium. Grain boundaries are modeled by low-permittivity (“dead”) layers, which do not exhibit ferroelectric instability. The effective permittivity of a polycrystalline material is calculated in the paraelectric regime above the transition temperature. The calculations are based on the solution of electrostatic problem for a spherical dielectric inclusion separated from the surrounding dissimilar matrix by a low-permittivity interface layer. For isotropic bulk ceramics, an analytic expression is derived for the effective permittivity as a function of the grain size, dead-layer thickness, and its permittivity. Temperature dependence of the aggregate dielectric response is calculated for BaTiO3 (BT) ceramics of different grain sizes and found to be in good agreement with measurements. It is shown that grain boundaries not only renormalize the Curie-Weiss temperature and constant, but may also cause deviations from the Curie-Weiss law. For BT polycrystalline thin films grown on dissimilar substrates, numerical calculations of the effective dielectric constants are performed, taking into account both the grain-boundary and substrate effects on the film anisotropic dielectric response. Theoretical predictions are compared with the grain size dependence of the permittivity of BT films grown on Pt-coated Si.

Study of (Sr, Pb)TiO3 ceramics on dielectric and physical properties

(Sr1−xPbx)TiO3 (x=0.2, 0.25 and 0.3) ceramics were prepared by the conventional mixed oxide method. Their dielectric permittivity, and loss tangent values were measured with and without DC bias field, thermal expansion and microstructure were observed. Three compositions showed similar Curie–Weiss constant, ca. 1×105°C and thermal expansion coefficient, 10×10−6°C−1. Transition temperatures of the compositions were, −60, −25 and 10°C, respectively, with loss tangent values of less than 0.1% measured at room temperature and at 10 kHz. Dielectric tunabilities of, 3, 15, and 70% were measured on x=0.2, 0.25 and 0.3 under 20 kV/cm bias field at 10 kHz and at room temperature.

Dielectric relaxation and order-parameter dynamics in lawsonite

The results of dielectric measurements ( f 510 kHz‐400 MHz) of the natural mineral lawsonite are presented in the region of the ferroelectric transition ( T2’124 K). The dielectric behavior is characterized by a small value of the Curie-Weiss constant ( CCW52220 K) typical for order-disorder ferroelectrics. This is consistent with the fact that the origin of ferroelectricity in lawsonite is related to the dynamical ordering of protons. In the paraelectric phase the dielectric dispersion is characterized by a Debye relaxation with a single relaxation time t P54.2310 210 (T2T2) 21 . A broadening in the dielectric dispersion occurs in the paraelectric phase only in a narrow temperature range (’5 K) close to T2 and can be associated with disordering processes in the lattice affected by large polarization clusters. The dielectric relaxation in the ferroelectric phase includes the order parameter dynamics and the domain wall dynamics. The relaxational domain wall dynamics is characterized by an extremely broad distribution of relaxation times. The characteristic parameters of the dielectric relaxation are determined.

Synthesis and Sintering of Nanocrystalline Barium Titanate Powder under Non-Isothermal Conditions. Part 6. Structure, Grain Boundaries, and Dielectric Properties of Barium Titanate Obtained by Various Sintering Methods

The grain boundary structure of barium titanate obtained by controlled-rate sintering and high-pressure sintering (HPS), and the dependence of dielectric properties on grain size and consolidation method were studied. It was shown that sintering without the application of pressure leads to a diffusion-controlled formation of equilibrium grain boundaries with minimal impedance factor, which minimally decrease the dielectric constant of the ceramics. HPS results in the formation of non-equilibrium grain boundaries which have a large free volume, and which substantially decrease the dielectric constant. The Curie-Weiss constant was analyzed from the viewpoint of a matrix structural model, and a «brick-wall» model.

Dielectric and ferroelectric properties of phosphoric acid doped triglycine sulfate single crystals

Single crystals of triglycine sulfate (TGS) grown with 25–50 mol% phosphoric acid dopant in solution were investigated for their dielectric, ferroelectric and mechanical hardness properties. A 25 mol% of H 3PO 4 in solution yielded crystals with a large area in the ac plane without any seed crystal. The room temperature (300 K) dielectric constant (ε′) and tan δ in the frequency range 0.1–100 kHz were found to increase with increasing phosphoric acid concentration. A maximum shift of 2.5 K in the phase transition temperature ( T c ) is observed for crystals grown with 50 mol% of H 3PO 4. Systematic lowering of Curie–Weiss constant ( C f ) in ferroelectric phase and an increase in the paraelectric phase ( C p ) indicates domain wall clamping effects in the doped samples. The changes in the spontaneous polarization (2.6–3.2 μC cm −2) with phosphoric acid doping are insignificant, but the high coercive fields ( E c =5 kV cm −1), and distorted loops indicate the presence of stresses and inhomogeneities. The activation energy (Δ E) from temperature dependent d.c. conductivity measurements is lower in both the ferroelectric and the paraelectric phase for doped crystals indicating a strong influence of dopants on the conduction process. The microhardness of the doped crystals is lower in comparison to undoped TGS.

Effects of Deuteration on Displacive-Type Ferroelectric Phase Transition of LiTlC 4H 4O 6·H 2O

The effects of deuteration on the ferroelectric phase transition of LiTlC 4 H 4 O 6 ·H 2 O (LTT) have been studied through dielectric measurements. The phase transition temperature T c decreases with increasing concentration of deuterium in the crystal, and vanishes in the highly deuterated crystal. On the other hand, the Curie-Weiss constant C remains almost constant with deuteration. The decreasing and vanishing of T c with a finite C in deuterated LTT is attributed to the phase transition mechanism of displacive-type.

Domain wall splitting and creation of the fine domain structure

The study of the movement of the paraelectric-ferroelectric interphase boundary in (Ba,Sr)TiO 3 with concentration change is provided in the framework of the mean-field theory. The analytical solution for the parameters of motion of the interphase boundary is applied to the calculations of the splitting of domain walls in (Ba,Sr)TiO 3 for different concentrations of Sr. The calculations are based on the experimental data for the Curie–Weiss constant and for the parameters of the Landau–Ginzburg expression for the free energy.