Curie-Weiss Relation Research Articles

Large magnetocaloric effect in [formula omitted] (RE = Gd, Tb and Dy) series of compounds

We report on the low-temperature physical properties of RE8Pd24Ga (RE = Gd, Tb and Dy) compounds studied by means of thermodynamic and electronic transport measurements. The three compounds exhibit a putative antiferromagnetic ordering. In the paramagnetic region, their magnetic susceptibility follows the Curie-Weiss relation with effective magnetic moments close to those expected for the respective RE3+ ions. Gd8Pd24Ga and Dy8Pd24Ga undergo another magnetic phase transitions in the ordered state. In fairly small magnetic fields, the magnetization of Tb8Pd24Ga and Dy8Pd24Ga reveals a metamagnetic-like transition that bears a first-order character. All three compounds studied exhibit magnetocaloric effect of the magnitude similar to those reported for materials applicable as magnetic refrigerants.

Correlation of dielectric dispersion with distributed Curie temperature in relaxor ferroelectrics

The fascinating dielectric property of relaxor ferroelectrics is investigated on the basis of phase-transition dynamics within the mean-field theory. With a power-law distribution of the Curie temperature for the second order phase transition ferroelectrics, the complex dielectric permittivity is derived. The result shows that the modified Curie–Weiss relation in the paraelectric phase follows the power-law distribution directly. The significant conclusion is that the “rediscovered” distributed Curie temperature can describe the universal dielectric dispersion for both real part and imaginary part of dielectric permittivity with temperature and frequency in relaxor ferroelectrics, and it conforms well to experimental results.

Li1.5Rb0.5MoO4: Ferroelectric properties and characterization of phase transitions by Raman spectroscopy

The new Li1.5Rb0.5MoO4 compound has been synthesized by solid state reaction route, which crystallized at room temperature in the monoclinic system (space group P21) with the lattice parameters are a = 15.3157Å, b = 14.3226Å, c = 9.0217Å, β = 97.741°, and V = 1960.97Å3. Differential scanning calorimetric (DSC) disclosed two endothermic peaks at T1 = 393K and T2 = 410K. The temperature dependence of the dielectric constant at different frequency proved that this compound is ferroelectric below approximately 412K. Temperature dependent variation of the dielectric constant showed a diffused phase transition which can be well described by fitting the modified Curie-Weiss relation, 1ε′−1ε′r,max=(T−Tmax)γC. Raman scattering studies (200–1100cm−1) were performed in the temperature range 353–503K. The analysis of the wavenumber, intensity and the half-width based on the order–disorder model allowed to obtain information relative to the thermal coefficient and the activation energy near the transitions phase.

Structure properties and relaxor characteristics of the phases transformation in BaTi0.5(Fe0.33Mo0.17)O3 perovskite ceramic

The effect of replacing titanium by iron and molybdenum in the B site on the structural and physical properties of BaTi0.5(Fe0.33Mo0.17)O3 polycrystalline sample was investigated by X-ray diffraction, scanning electron microscopy (SEM) as well as dielectric characterizations. Crystal phase, microstructure, and dielectric property of the ceramic were examined. A single hexagonal perovskite structure with space group P63/mmc was obtained at 1400 °C and stabilized at room temperature. The microstructural study of the sintered pellets revealed that the plate-like grains are the typical grain morphologies in this ceramic. The temperature dependence of the dielectric properties was investigated in the frequency range 1 kHz to 1 MHz. Three dielectric relaxations were observed in the present ceramic at the temperature ranges of 330–473 K, 473–550 K and 650–800 K with a maximum in the dielectric permittivity (ε’r ∼ 3518 at 443 K at 1 KHz, ε’r ∼4335 at 502 K at 1 KHz and ε’r ∼11,331 at 749 K at 1 KHz) that shifted to a higher temperature with increasing frequency. Temperature dependent variation of the dielectric constant showed a diffused phase transition which can be well described by fitting the modified Curie–Weiss relation, (1/ε‘r–1/ε‘r,max)=(T–Tm)γ/C.

Antiferromagnetic Kondo lattice to intermediate valence transition in Ce(Au1−xNix)2Si2 ([formula omitted

Transition effects from magnetically ordered Kondo lattice (KL) behaviour in CeAu2Si2 to intermediate valence (IV) behaviour in CeNi2Si2 have been investigated by substitution of Au with Ni. These investigations were done through measurements of the lattice parameters, electrical resistivity (ρ(T)), magnetisation (M(μ0H)) and magnetic susceptibility (χ(T)) on the Ce(Au1−xNix)2Si2 (0≤x≤1) series. Lattice parameters as derived from XRD measurements deviate from Vegard's rule around x=0.6–0.8. ρ(T) data indicate KL behaviour in the presence of a crystal field for x≤0.6, the occurrence of magnetic ordering for x≤0.2 and valence fluctuation for x≥0.8. χ(T) data at high temperatures, follow the Curie–Weiss relation for alloys in the concentration 0≤x≤0.6 (KL region) and give effective magnetic moment values μeff close to that expected for free Ce3+ ion. The low temperature χ(T) data indicate the onset of antiferromagnetic ordering for x≤0.2. For alloys in the concentration range 0.8≤x≤1, the χ(T) behaviour is reminiscent of intermediate valence (IV) systems. M(μ0H) measurements indicate metamagnetic behaviour for alloys in the concentration range 0≤x≤0.1.

Magnetic, electrical and thermodynamic properties of RE 8Pd 24In

Investigations of magnetic, transport and thermodynamic properties of RE 8Pd 24In (RE=Pr, Nd, Gd, Tb, Dy, Ho, Er and Tm) are reported through measurements of X-ray diffraction, electrical resistivity ( ρ ( T ) ), magnetic susceptibility ( χ ( T ) ), magnetization ( σ ( μ 0 H ) ) and heat capacity ( C p ( T ) ). These ternary compounds have the cubic AuCu 3-type structure. Compounds with RE=Gd and Tb order antiferromagnetically (AF) and exhibit magnetic transitions observed in their χ ( T ) and C p ( T ) . ρ ( T ) results for the Gd and Tb compounds indicate a minimum above the magnetic phase transition temperature T N , and an increase in ρ ( T ) upon further cooling as a precursor of AF order at lower temperatures. χ ( T ) data at high temperatures follow the Curie–Weiss relation for all compounds and give effective moment values close to those expected for the free RE 3+-ion. Enhanced values of the electronic contribution to the heat capacity were observed for Gd 8Pd 24In and Tb 8Pd 24In.

Dielectric and Piezoelectric Properties of 10% KF-Doped BaTiO3 Ceramics

10% KF-doped barium titanate powders, Ba0.9K0.1TiO2.9F0.1, were synthesized through a sol–gel process. The powders, calcined at 650 °C, consist of cubic crystalline particles of ∼70 nm in length; the particle size increases to ∼200 nm as the firing temperature increases to ca. 800 °C, above which F2 begins to evaporate. Dense ceramics were fabricated by the spark plasma sintering (SPS) method; the average grain size is ∼2 µm in lengths. The ceramics, well annealed at 1,000 °C in an O2 gas flow, have good dielectric and piezoelectric properties; the piezoelectric d33 value is 230 pC/N at room temperature. At the ferroelectric Curie temperature TC = 47 °C, the dielectric constant and loss tan δ are 10,000 and <5% at 10 kHz, respectively. The Curie–Weiss relation holds in the fully disordered cubic and ordered rhombohedral phases, showing the second order 1:2 relation. Below 10 kHz, large dielectric dispersion caused by a domain-wall motion appears at the temperature range of -50 to 107 °C. Some discussions are made for these dielectric properties of the ceramics.

Critical properties of symmetric nanoscale metal–ferroelectric–metal capacitors

The size, surface and interface effects on the magnitude and stability of spontaneous polarization in a symmetric nanoscale ferroelectric capacitor were studied by analyzing its evolutionary trajectory based on a thermodynamic model. Analytic expressions of the Curie temperature, spontaneous polarization, critical thickness and the Curie–Weiss relation were derived, taking into account the effects of the depolarization field, built-in electric field, interfaces and surfaces. Our results show that the critical properties are not only functions of the ambient temperature, misfit strain and electromechanical boundary conditions, but also depend on the characteristics of electrodes, surfaces and interfaces, through the incomplete charge compensation, near-surface variation of polarization and work function steps of ferroelectric–electrode interfaces, which are adjustable.

Magnetic substitution in [formula omitted

The effects of substituting moment bearing RE = Gd , Tb, and Dy in CeIn 3 are reported through X-ray diffraction (XRD), electrical resistivity ( ρ ( T ) ), magnetic susceptibility ( χ ( T ) ) and magnetization ( σ ( μ 0 H ) ) measurements. XRD results for all compositions of the ( Ce 1 ‐ x RE x ) In 3 systems indicate a cubic AuCu 3 -type structure. ρ ( T ) results indicate evolution from dominantly coherent Kondo lattice to metallic behaviour with increase in RE content. χ ( T ) data at high temperatures follow the Curie–Weiss relation for all alloy compositions and give effective moment values μ eff which increase gradually from the expected value of 2.54 μ B for the Ce 3 + -ion to the expected values of the RE 3 + -ion . All three alloy systems exhibit antiferromagnetism and low temperature ρ ( T ) and χ ( T ) data were used to find the Néel temperature T N as a function of x. σ ( μ 0 H ) data indicate metamagnetic behaviour for DyIn 3 and ( Ce 0.5 Tb 0.5 ) In 3 .

Thermomagnetic properties of the Finland trityl radical

The Finland trityl paramagnet is characterized by magnetic susceptibility and a new form of quantitative electron paramagnetic resonance (EPR) that utilizes a superconducting quantum interference device (SQUID) as a detection method. This radical is of interest due to its use as a dynamic nuclear polarization agent as well as a potential magnetic refrigerant and quantum computing bit. The SQUID-EPR data show that the EPR linewidth of a concentrated trityl powder decreases dramatically from 4.4 to 1.4 mT as the temperature is increased from 1.8 to 10 K. The quantitative nature of SQUID-EPR is used to thermodynamically quantify the EPR energy transfer times and saturated fractions. At 95 GHz and 1.8 K, only 40% of the spins are in resonance at the onset of saturation. Conventional dc magnetic susceptibility over 1.8-150 K indicates an S=12 Curie-Weiss relationship with little long range interaction. Magnetization versus applied field at 1.8 and 4 K fits a Brillouin function with >80% electronic polarization at a normalized field of gmu(B)mu(0)HkT approximately 3. These results provide information required for theoretical modeling and engineering of the trityl radical for a wide range of applications.

Structure and magnetism of the β-Mn–Co solid-solution phase

The crystal structure of the β-Mn 1− t Co t solid-solution phase (0 ≤ t ≤ 0.40) has been studied with powder neutron (10 and 298 K) and single-crystal X-ray (150 K) diffraction methods. The lattice-constant ( a) isotherms at 10, 150, and 298 K go through flat maxima between t = 0.10 and 0.25. Up to t = 0.25 all Co is found to substitute at the T1 ( T1 = Mn and/or Co) site of the β-Mn crystal structure (position 8 c of space group P4 132) whereas for t = 0.40 also the T2 site (position 12 d) is partly occupied by Co (some 0.2 Mn + 0.8 Co occupancy of the T2 site). The variable positional parameters x (for T1) and y (for T2) exhibit remarkably small variations with composition ( t) and temperature. The present low-temperature powder neutron-diffraction data confirm the earlier finding that the β-Mn 1− t Co t phase does not exhibit conventional co-operative magnetic ordering. However, the appearance of diffuse scattering in the low-temperature diffraction patterns is clearly generated by short-range ordering of magnetic moments, which owing to the atomic arrangement of the β-Mn-type structure becomes geometric frustrated. The temperature dependence of the magnetic susceptibility for β-Mn 1− t Co t is re-measured. Neglecting β-Mn itself (which exhibits virtually temperature-independent paramagnetism), our magnetic susceptibility curves above some 80 K for t = 0.15, 0.25, and 0.40 can with good-will be described by the Curie–Weiss relation, indicating antiferromagnetic correlations at low temperatures. However, the thus involved paramagnetic moments and Weiss constants must indeed be stamped as unphysically large.

Surface tension and size effect in ferroelectric nanotubes

Using a thermodynamic model, linear stability analysis is performed on the associateddynamic Ginsburg–Landau equation for a ferroelectric nanotube. Analytic expressions ofthe transition temperature and Curie–Weiss relation are derived. The ferroelectricproperties of the tube are found to generally depend on its dimensions through effects ofthe surface tension and near-surface eigenstrain relaxation. Our results also show that thetransition temperature and polarization are both enhanced due to the dominance of theeffective radial pressures induced by the surface tension, resulting in a remnantpolarization and coercive field that may become larger than the bulk values.

Electrical and magnetic properties of the Kondo system [formula omitted

Measurements of electrical resistivity ρ ( T ) , magnetoresistivity MR and magnetic susceptibility χ ( T ) are reported for the pseudo-ternary alloys ( Ce 1 - x Y x ) Pt 2 Si 2 . ρ ( T ) results reveal coherent Kondo scattering above 80% Ce concentration and a rapid evolution to single-ion Kondo scattering with an increase in Y concentration x. MR measurements on Ce alloys in the single-ion Kondo region are analyzed on the basis of calculations by Schlottmann for the Bethe-ansatz solution in the context of the Coqblin–Schrieffer model. The analysis yields values of the Kondo temperature T K and the effective moment of the Kondo ion μ K . χ ( T ) data above 120 K follow the Curie-Weiss relation for all the investigated concentrations and give effective moment values μ eff in close agreement with μ eff = 2.54 μ B expected for the Ce 3 + -ion.

Structural and Electrical Characterization of Ferroelectric SrBi2Nb2O9 Single Crystals Grown by High-Temperature Self-Flux Solution

High-quality SrBi2Nb2O9 (SBN) single crystals were grown from a melt using a high-temperature self-flux solution method and Bi2O3 added with B2O3 as a flux. A suitable thermal profile involving slow cooling rates allowed growing large and translucent SBN crystals exhibiting platelet morphology with typical size ∼5 × 5 mm 2 and thickness approximately 400 μm. X-ray diffraction revealed a dominant (001)-orientation of the major face of the platelet crystals and edges oriented parallel to the [110] directions. The dielectric properties were evaluated along the ab-plane and in the c-axis direction. The ferro-paraelectric phase transition was observed at T C = 440°C with Curie-Weiss relationship above T C . The anisotropy of dielectric permittivity, i.e., the ratio between permittivity in the ab-plane and along c-axis was about 10 at T C . The obtained results are used to discuss the observed correlations between anisotropy, crystalline orientation, and electrical properties.

Phase-transition temperature and character ofCd2Nb2O7

The complicated dielectric spectrum of ${\mathrm{Cd}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$ around the ``paraelectric-ferroelectric'' (PF) phase transition is greatly simplified due to the suppression of the multiple dielectric relaxation processes under a dc electric field. The PF transition is revealed under a dc electric field and characterized by the Curie-Weiss relation. The results show that the PF phase transition is of second order. The dc electric field influence on the dielectric relaxation modes and PF phase transition for both single crystals and ceramics is discussed.

Specific heat, susceptibility, magnetotransport and thermoelectric power of the Kondo alloys(Ce1−xLax)Cu5In

The influence of substituting Ce with La in the Kondo compoundCeCu5In has been investigated using x-ray diffraction, specific heat(Cp), electricalresistivity (ρ(T)), magnetoresistivity (MR) and thermoelectric power (TEP) measurements.Cp ofCeCu5In andthe (Ce1−xLax)Cu5In alloys shows enhanced values at low temperatures. Between 10 and 20 K the equationCp/T = γconv+βT2 fits the data forall alloys with γconv scaling linearly with Ce concentration. Below 10 K a pronounced upturn inCp values occurs. Susceptibility data above 100 K follow the Curie–Weiss relation and give effective momentμeff values in fair agreementwith that of a Ce3+-ion. The ρ(T) studies illustrate the evolution from Kondo lattice to single-ion Kondo behaviour with increase in La contentin the (Ce1−xLax)Cu5In alloy series. MR measurements on Ce dilute alloys are interpreted withinthe single-ion Bethe ansatz description and values of the Kondo temperatureTK are calculated. A compressible Kondo lattice model has been used to describe the decrease inTK and inTmρ (the temperature atwhich a maximum in ρ(T) occurs for the coherent dense Kondo alloys) with decrease in Ce concentration for these alloys.The TEP is positive and shows a maximum at approximately 45 K for several investigated(Ce1−xLax)Cu5In alloys.

Dielectric permittivity dynamics of Ba1−x SrxTiO3 epitaxial films (x=0.75): Microstructure and depolarization effects

Trilayer epitaxial heterostructures including metal oxide electrodes (SrRuO3, 200 nm) and a sandwiched dielectric layer (Ba0.25Sr0.75TiO3, 700 nm) were grown by laser ablation on (001)LaAlO3 substrates. The maximum permittivity of the Ba0.25Sr0.75TiO3 layer (ɛ′/ɛ0≈3700) was obtained at T M =160 K and an external electric field E≈106 V/m. The ɛ′(T) dependence for the Ba0.25Sr0.75TiO3 layer in the paraelectric phase is well fitted by the Curie-Weiss relation, with the Curie constant and the Weiss temperature differing only insignificantly from the corresponding bulk values. The change in the permittivity of the Ba0.25Sr0.75TiO3 layer induced by the application of a ±2.5 V bias voltage to the electrodes reached as high as 85%. The electric-field dependence of the polarization retained clearly pronounced saturated hysteresis loops up to temperatures 10–15 K above T M .

Electronic and Magnetic Properties Due to Co Ions in La0.9Sr0.1Fe1−xCoxO3

Variations in physical properties due to Co ions in La0.9Sr0.1Fe1−xCoxO3 (x≤0.5) in which short-range ferromagnetic orders are expected to work predominantly at a rather large x have been investigated so as to elucidate Co states. dc and ac measurements indicate that a hopping process of small polarons dominates electronic conduction, but there is a large reduction in hopping energy when x increases from 0.1 to 0.5. This reduction is related to the structural transition induced by an increase in Co content. A spin-glass phase due to short-range ferromagnetic orders is recognized for x=0.5 in magnetic measurements. A Curie–Weiss relation realized for x=0.5 and comparison of the thermopower extrapolated experimentally at T→∞ with magnitudes estimated theoretically deduce that all or most of Co ions in La0.9Sr0.1Fe1−xCoxO3 are HS Co3+ and IS Co4+ ions.

Magnetic and electrical properties of the Kondo system CePt 2(Si 1− xGe x) 2

Magnetic susceptibility χ( T), electrical resistivity ρ( T) and magnetoresistivity of CePt 2(Si 1− x Ge x ) 2 alloys (0⩽ x⩽1) are reported. For χ( T) of these, the Curie–Weiss relationship is followed with effective moments in close agreement with μ eff =2.54 μ B expected for Ce 3+. A maximum in ρ( T) is observed at a temperature T max∼ T K∼exp(−1/∣ JN( E F)∣). From the decrease in T max with increasing x it is deduced that ∣ JN( E F)∣ x=0 =0.11±0.02.

Dielectric response of epitaxial (100)SrTiO 3 films between electrodes of SrRuO 3 or high- Tc superconducting YBa 2Cu 3O 7− δ

Epitaxial heterostructures (001)YBa 2Cu 3O 7− δ ‖(100)SrTiO 3‖ (001)YBa 2Cu 3O 7− δ , (001)YBa 2Cu 3O 7− δ ‖(100)SrTiO 3‖ (100)SrRuO 3 and (100)SrRuO 3 ‖(100)SrTiO 3‖ (100)SrRuO 3 were grown by laser ablation on polished substrates of (100) oriented (LaAlO 3) 0.3+(Sr 2AlTaO 6) 0.7. Replacing YBa 2Cu 3O 7− δ electrodes by SrRuO 3 ones was accompanied by a 60% decrease in width of the rocking curve for the (200)SrTiO 3 X-ray reflection and pronounced, up to three times, increase of the permittivity of the ferroelectric layer at T<100 K and a decrease in high frequency losses. The temperature dependence of the permittivity for the SrTiO 3 layer in the (100)SrRuO 3 ‖(100)SrTiO 3‖ (100)SrRuO 3 heterostructure was well represented by a Curie–Weiss relation ( T=80–300 K, f=1–100 kHz). The smallest loss factor, 0.002–0.02, in the temperature range 10–300 K was measured for the (100)SrRuO 3 ‖(100)SrTiO 3‖ (100)SrRuO 3 capacitance heterostructure ( f=1–100 kHz). At T>200 K, the conductance G of the SrTiO 3 layer sandwiched between YBa 2Cu 3O 7− δ electrodes followed well the dependence log G∼(− φ L/ kT), where φ L≈0.08 eV.