Heat capacity and dielectric properties of multiferroics Bi1 − x Gd x FeO3 (x = 0–0.20)

The heat capacity and the permittivity of multiferroics Bi_{1 ? x} Gd _x FeO₃ (x = 0, 0.05, 0.10, 0.15, 0.20) have been studied in the temperature range 130–800 K. It has been found that insignificant substitution of gadolinium for bismuth markedly shifts the temperature of antiferromagnetic phase transition and increases the heat capacity over a wide temperature range. It has been shown that the temperature dependence of the excess heat capacity is due to the manifestation of three-level states. Additional anomalies characteristic of the phase transitions have been revealed in the temperature dependences of the heat capacity for the compositions with x = 0.1 and 0.15 at T ≈ 680 K and T ≈ 430 K, respectively. The results of studies of the heat capacity have been discussed simultaneously with the data of structural studies.     

Thermal Physical and Dielectric Properties of Bi1 – xErxFeO3

The heat capacity and the permittivity of multiferroics Bi_{1 – x}Er_xFeO₃ (x = 0, 0.10, and 0.15) have been studied in the temperature range 130–800 K. It is found that an insignificant substitution of erbium for bismuth significantly increases the heat capacity in a wide temperature range T > 300 K. The temperature dependence of the excess heat capacity is shown to be due to manifestation of the three-level states. An additional anomaly characteristic of a phase transition has been revealed in the temperature dependences of the heat capacity and the permittivity for the compositions with x = 0.15 at T = 587 K. The results of studies are discussed in combination of the data of structural studies.

     

Dielectric properties and specific heat of Bi<Subscript>1–<Emphasis Type="Italic">x</Emphasis> </Subscript>Sm<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>FeO<Subscript>3</Subscript> multiferroics

The specific heat and dielectric permittivity of Bi_1–xSm_xFeO₃ (x = 0–0.30) multiferroics have been studied in the temperature range of 300–800 K. A slight substitution of bismuth with samarium is established to cause a considerable shift in the antiferromagnetic phase transition temperature and to an increase in the specific heat over a wide temperature range. Other anomalies typical of phase transitions have been found in the temperature dependences of the specific heat and dielectric permittivity for the compounds with x = 0.10 and 0.15 at T ≈ 735 and 495 K, respectively. The results of the studies of the specific heat have been discussed together with the data of the structural investigations.     

Heat capacity of La1−x SrxMnO3 single crystals in different magnetic states

The heat capacity of three single-crystal samples of La_1?x Sr_xMnO₃ (x=0, 0.2, and 0.3) is measured in the temperature range 4–400 K. It is found that the heat capacity undergoes abrupt changes due to the transitions from the antiferromagnetic phase to the paramagnetic phase (x=0) and from the ferromagnetic phase to the paramagnetic phase (x=0.2 and 0.3). The phonon contribution to the heat capacity and the Debye characteristic temperatures for the La_0.7Sr_0.3MnO₃ sample are determined over a wide range of temperatures. The electronic density of states at the Fermi level is evaluated. It is demonstrated that an increase in the strontium concentration x brings about an increase in the electronic density of states at the Fermi level. The contributions of spin waves to the heat capacity and the entropy are estimated under the assumption that the phonon spectrum remains unchanged upon doping with Sr.     

Heat capacity study of double perovskite-like compounds BaTi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zr<Subscript>x</Subscript>O<Subscript>3</Subscript>

The temperature dependence of the heat capacity of two compositions in the solid solution system BaTi_1?xZr_xO₃ (x = 0.25, 0.35) was measured using adiabatic calorimetry. In the T-x phase diagram, these compounds occupy positions near the crossover from conventional ferroelectric behavior to the relaxor state. Both compounds reveal diffuse heat capacity anomalies: two anomalies in the temperature ranges 250–350 and 150–200 K at x = 0.35 and one anomaly within the range ～150–320 K at x = 0.25. The results obtained are discussed together with structural and dielectric measurements.     

Dielectric properties and specific heat of the (1 ? x)PbNi1/3Nb2/3O3-xPbTiO3 ferroelectric ceramics

The temperature dependences of the permittivity and specific heat of a mixed (1 ? x)PbNi_1/3Nb_2/3O₃-xPbTiO₃ system (where x = 0.3, 0.4, 0.5) have been studied over a wide temperature range 290?C700 K. It has been shown that the composition with x = 0.3 undergoes a diffuse phase transition at T _m ?? 315 K. A specific heat anomaly that is characteristic of the phase transition has been revealed at T ?? 315 K in all the compositions under study.     

Specific heat of YBa2Cu3O7−x from 4.2 to 60 K

The specific heat of superconducting oxide compound, YBa₂Cu₃O_{7 ?x} , is studied using a quasi-adiabatic calorimeter from 4.2 to 60 K. The analysis of the specific heat data below 15 K gives a value of 17 mJ/mole K² for the electronic heat capacity coefficient. The value ofθ _D(0) is determined to be 397±8 K. The variation ofθ _D with temperature was calculated in the temperature range 4.2 to 60 K.     

Study of the thermal properties of Ba1−x SrxTiO3 thin films by the ac hot-probe method

The heat capacity and heat conductivity of Ba_1−x Sr_xTiO₃ (x=0.2, 0.5, 0.8) polycrystalline films 1.5–2.0 μm thick on a massive substrate have been studied by the ac hot-probe method for three-layer systems (conducting probe-dielectric film-substrate) at temperatures ranging from 100 to 400 K. It is found that the thermal properties exhibit anomalies in the phase transition range. __________ Translated from Fizika Tverdogo Tela, Vol. 42, No. 10, 2000, pp. 1839–1841. Original Russian Text Copyright ? 2000 by Davitadze, Kravchun, Strukov, Taraskin, Gol’tsman, Lemanov, Shul’man.     

BCS-like behaviour in the superconducting state of itinerant antiferromagnetic CrRe alloys

Measurements of the heat capacity and the magnetic susceptibility have revealed BCS-like behaviour in the superconducting state of itinerant antiferromagnetic Cr_1?xRe_x alloys for x = 0.30) and 0.26. The thermodynamic quantities, such as electronic heat capacity and thermodynamic critical field have been reproduced with the BCS theory with the energy gap Δ = (1.76 ± 0.05)k_BT_s, where T_S is the superconducting transition temperature for the corresponding system: T_S = 3.61 K (2.35 K) for x = 0.30 (0.26).     

Effect of carbon substitution on the specific heat of Fe80B20−xCx (0≤x≤8)

The specific heat of ferromagnetic metallic glasses Fe₈₀B_20?xC_x (0≤x≤8) has been measured in the temperature range between 1.5 and 10 K. It is found that the electronic specific heat coefficient is independent of the carbon concentration x. The Debye temperature has a broad peak around x = 4. Both these results are in sharp contrast to the case of FeB binary metallic glasses and could be qualitatively understood in terms of magnetic and structural properties.     

Magnetocaloric properties of LaFe13−x−yCoxSiy and commercial grade Gd

The magnetocaloric properties of three samples of LaFe_13−x−yCo_xSi_y have been measured and compared to measurements of commercial grade Gd. The samples have (x=0.86, y=1.08), (x=0.94, y=1.01) and (x=0.97, y=1.07) yielding Curie temperatures in the range 276-288 K. The magnetization, specific heat capacity and adiabatic temperature change have been measured over a broad temperature interval. Importantly, all measurements were corrected for demagnetization, allowing the data to be directly compared. In an internal field of 1 T the maximum specific entropy changes were 6.2, 5.1 and 5.0 J/kg K, the specific heat capacities were 910, 840 and 835 J/kg K and the adiabatic temperature changes were 2.3, 2.1 and 2.1 K for the three LaFeCoSi samples respectively. For Gd in an internal field of 1 T the maximum specific entropy change was 3.1 J/kg K, the specific heat capacity was 340 J/kg K and the adiabatic temperature change was 3.3 K. The adiabatic temperature change was also calculated from the measured values of the specific heat capacity and specific magnetization and compared to the directly measured values. In general an excellent agreement was seen.     

Crystallography,magnetic properties and magnetocaloric effect in Gd4(BixSb1−x)3 alloys

A study of magnetic and thermal properties has been carried out on the alloys from the Gd₄(Bi_xSb_1−x)₃ series with x=0, 0.25, 0.5, 0.75, and 1. All of the alloys are ferromagnetic below their respective Curie temperatures which vary from 266 K for x=0 to 332 K for x=1.0. The magnetocaloric effect calculated from the temperature and magnetic field dependencies of the magnetization and heat capacity is moderate when compared to that of other materials, which order in the same temperature range. Both the magnetic ordering and the magnetocaloric effect peak temperatures increase nearly linearly with the increasing Bi content. Experimental magnetocaloric effect data obtained from two different measurement techniques are in excellent agreement.     

Heat capacity and the magnetocaloric effect in Pr0.6Sr0.4Mn1–x Fe x O3 manganite

The heat capacity and the magnetocaloric effect of Pr_0.6Sr_0.4Mn_1–xFe_xO₃(x = 0 and 0.1) manganite have been studied in the temperature range 80–350 K and magnetic fields to 18 kOe. The magnetocaloric effect is estimated using two independent methods: the method of magnetic field modulation (direct method) and from the data on the heat capacity in magnetic field and without magnetic field (indirect method). The substitution of Fe atoms for Mn atoms (x = 0.1) shifts T _C by 167 K to lower temperatures; in this case, the magnetocaloric effect (MCE) is changed insignificantly in magnetic field 18 kOe with ΔS _M = 2.05 and 2.31 J/kg K for x = 0 and 0.10, respectively.

     

Disorder induced ferromagnetism in the Cu-doped molybdate SrMo1−xCuxO3 (0≤x≤0.20)

The effect of Cu-doping at Mo-site on structural, magnetic, electrical transport and specific heat properties in molybdates SrMo_1−xCu_xO₃ (0≤x≤0.2) has been investigated. The Cu-doping at Mo-site does not change the space group of the samples, but decreases the structural parameter a monotonously. The magnetic properties change from Pauli-paramagnetism for x=0 to exchange-enhanced Pauli-paramagnetism for x=0.05 and 0.10, and then ferromagnetism for x=0.15 and 0.20. All samples exhibit metallic-like transport behavior in the whole temperature range studied. The magnitude of resistivity increases initially to x=0.10 and then decreases with increasing Cu-doping concentration. The results are discussed according to the electron localization due to the disorder effect induced by the random distribution of Cu at Mo site in the samples. In addition, the temperature dependence of specific heat for the Cu-doped sample has also been studied.     

Dependence of the heat capacity of La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ag<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> manganites on the Ag content

The heat capacity of La_1?x Ag _x MnO₃ manganites with x = 0.1, 0.15, and 0.2 is measured in the temperature range 77–350 K. An analogy between the effect of doping and the effect of a magnetic field on the temperature dependence of heat capacity of the La_1?x Ag _x MnO₃ system is revealed. As lanthanum is replaced by silver, the volume fraction of the ferromagnetic phase increases, while, in the paramagnetic state, the Jahn-Teller distortions are eliminated. The results of the aforementioned measurements suggest that the phase transition near the Curie point is caused by the competition between the Coulomb and exchange electrostatic interactions. The comparison of the concentration dependences of T _C for La_1?x Sr _x MnO₃ and La_1?x Ag _x MnO₃ points to good potentialities of the latter system from the viewpoint of applications.     

Effect of carbon doping on thermodynamic behavior of MgB2

The thermodynamic behavior of carbon doped MgB₂ has been studied using a rigid ion model (RIM). The model potential consists of the long-range Coulomb, the short-range repulsive and the van der Waals interactions. This model has successfully explained the cohesive and thermodynamic properties of Mg(B_1−xC_x)₂ (x=0.0, 0.02, 0.05, 0.075, 0.1, 0.2). The properties studied are the cohesive energy, molecular force constant, Restrahlen frequency, compressibility, Debye temperature and Gruneisen parameter. Our results on Restrahlen frequency and Debye temperature are in reasonably good agreement with the available experimental data. In addition, we have computed the specific heat C_p for Mg(B_1−xC_x)₂ (x=0.2) as a function of temperature T in the range 16 K?T?1000 K. We have also shown the variation of specific heat C_p with doping concentration at room temperature (300 K). The calculated specific heat C_p for Mg(B_1−xC_x)₂ (x=0.2) in the temperature range 16 K?T?22 K for which experimental results are available, agrees pretty well with the experimental data.     

Low temperature specific heat and magnetic properties of V/FE and V/FE/H alloys

Low temperature specific heats of V_1?xFe_x (0?x?0.34) and V_1?xFe_xH_n (x = 0.02; 0.05; 0.1; 0.15 a 0 ? n ? 0.98) were investigated between 1.5 and 16 K.In V/Fe alloys temperature independent band paramagnetism was observed at iron concentrations below 20at%; at higher iron contents local magnetic moments occur and contribute magnetic terms to the low temperature specific heat. According to Schröder [14] these contributions can be explained by thermal agitation of superparamagnetic clusters of the Fe atoms with local moments. In the temperature range of the specific heat measurements the magnetic terms are dependent on temperature (contrary to Schröder) and can be described by Planck-Einstein functions. A separation of the magnetic contributions from the lattice and electronic term of heat capacity could be obtained in a satisfying way by least squares fitting with three Planck-Einstein functions.In “nonmagnetic” V/Fe alloys with less than 20at% Fe absorption of hydrogen generates local magnetic moments and superparamagnetic contributions. These are, therefore, connected primarily more with the band electron concentration n_e, than with the iron content X_Fe At the highest n_e, values, n_e >0, large negative deviations from the regular course of the heat capacity plot are observed; these can be related to mictomagnetic behavior (freezing temperature of spin-glass state above the range of measurements, T_F > 16 K).After correction for the superparamagnetic contributions there remains in the Fe containing alloys an appreciable enhancement of the electronic heat coefficient γ, up to a factor of about 2 when compared with the iron-free V/H samples. This effect is interpreted as resulting from mass enhancements of the itinerant electrons by electron-magnon (and-paramagnon) interactions.     

Heat capacity of the Pb<Subscript>5</Subscript>(Ge<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript>x</Subscript>)<Subscript>3</Subscript>O<Subscript>11</Subscript> ferroelectric system

The temperature dependences of the molar heat capacity at constant pressure, C_p, of Pb₅(Ge_1?xSi_x)₃O₁₁ crystals with x=0, 0.39, and 0.45 in the range 5–300 K, as well as of their permittivity, dielectric losses, and the pyroelectric effect, have been measured. Experimental data on the temperature behavior of the heat capacity are presented in the form of a sum of two Debye and one Einstein terms, C_p(T)=0.405C_D1(Θ_D1=160 K, T)+0.53C_D2(Θ_D2=750 K, T)+0.046C_E(Θ_E=47 K, T). Besides a peak in the region of the ferroelectric Curie point T_c=450 K for crystals with x=0, the temperature dependences of the heat capacity did not reveal any other pronounced anomalies.     

Low-energy excited states of 3d transition metal ions in zinc selenide

The results of studying the impurity heat capacity of Zn_1?x M _xSe (M = Cr²⁺, Fe²⁺, Ni²⁺, Mn²⁺) solid solutions in the temperature range 1.8–20.0 K are presented. A heat-capacity method is described and applied for the measurement of the intracenter-transition energy in these systems. The role of the Jahn-Teller effect in the formation of low-energy excited states of 3d ions in ZnSe is discussed.     

Effect of Cd impurity on the electrical properties of a-Se80Te20

Temperature dependence of dark and photoconductivity is studied in amorphous films of Se₈₀Te_20−xCd_x in the temperature range (300 K to 380 K) with a view to see the effect of Cd impurity on a-Se₈₀Te₂₀ binary alloy. It is observed that, at low concentration of Cd (x = 0.5), the dark and photoconductivity increase at all temperatures. However, at higher concentration of Cd (x = 10), an appreciable reduction in these parameters occurs in the same temperature range. The photosensitivity (σ_ph/σ_d) remains unchanged at x = 0.5 but decreases quite significantly at x = 10. The results are explained in terms of impurity doping at x = 0.5 and alloying effect at x = 10.