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1.
Polycrystalline perovskite manganites La0.7−xEuxBa0.3MnO3(x=0.05, 0.1 and 0.15) were prepared by sol-gel method. The prepared samples remain single phase with a perovskite structure, revealed by X-ray diffraction. The structure refinement of La0.7−xEuxBa0.3MnO3(x=0.05, 0.1 and 0.15) samples was performed in the hexagonal setting of the Rc space group. The dependence of magnetization M on applied magnetic field H and temperature T was measured carefully near the Curie temperature TC for all the samples. With the increasing Eu content, both the unit cell volume and Curie temperature TC of 298 K has been detected with a maximum of magnetic entropy |ΔSMmax| for the La0.7−xEuxBa0.3MnO3 with x=0.15, reaching a value of 2.3 J/kg K when a magnetic field of 10 kOe was applied and the relative cooling power (RCP) is 46 J/kg. These results suggest that the material may be a suitable candidate as working substance in magnetic refrigeration near room temperature.  相似文献   

2.
The crystal and magnetic structures of the oxygen deficient manganites La0.7Sr0.3MnO3-d (d = 0.15, 0.20) have been studied by means of powder neutron diffraction over the 0–5.2 GPa pressure and 10–290 K temperature ranges. La0.7Sr0.3MnO2.85 exhibits a coexistence of rhombohedral and tetragonal (I4/mcm) crystal structures and below Tg ~ 50 K a spin glass state is formed. La0.7Sr0.3MnO2.80 exhibits a tetragonal (I4/mcm) crystal structure. Below Tg ~ 50 K a phase separated magnetic state is formed, involving coexistence of C-type AFM domains with spin glass domains. In both compounds the crystal structure and magnetic states remain stable upon compression. The factors leading to the formation of different magnetic states in La0.7Sr0.3MnO3-d (d = 0.15, 0.20) and their specific high pressure behavior, contrasting with that of the stoichiometric A0.5Ba0.5MnO3 (A = Nd, Sm) compounds showing pressure-induced suppression of the spin glass state and the appearance of the FM state, are analysed.  相似文献   

3.
Measurements of the basal-plane resistivity ρ a(T,H) performed on highly oriented pyrolitic graphite, with magnetic field Hc-axis in the temperature interval 2–300K and fields up to 8 T, provide evidence for the occurrence of both field-induced and zero-field superconducting instabilities. Additionally, magnetization M(T,H) measurements suggest the occurrence of Fermi surface instabilities which compete with the superconducting correlations. Fiz. Tverd. Tela (St. Petersburg) 41, 2135–2138 (December 1999)  相似文献   

4.
We carried out the heat capacity calculation of the magnetoresistance compounds EuMnO3 and Eu0.7A0.3MnO3 (where A=Ca and Sr) as a function of temperature from 5 to 100 K, using the Rigid Ion Model (RIM). The results on heat capacity for EuMnO3 and Eu0.7A0.3MnO3 (A=Ca and Sr) obtained by us are in good agreement with the measured values. Although strong electron–phonon interactions are present in these compounds but the lattice part of the specific heat also deserves proper attention. The parent compound EuMnO3 exhibits two magnetic transitions at 35 and 47 K due to weak ferromagnetic (FM) component and antiferromagnetic (AF) ordering. In addition, we have reported cohesive energy (φ), molecular force constant (f), compressibility (β), Restrahalen frequency (υ0), Debye temperature (θD) and Gruneisen parameter (γ) in the temperature range 5 K?T?100 K.  相似文献   

5.
The anisotropy of the magnetic susceptibility χ and the influence of oxygen vacancies in CuO single crystals on it are investigated. The temperature dependences of χ(T) along the a, b, and c axes in the range 60<T<600 K and the behavior of the field dependence of the magnetization σ(H) above and below the Néel temperature T N are plotted for a crystal before and after heat treatment. The χ(T) curves have the form characteristic of low-dimensional systems, which become three-dimensional when the temperature is lowered. The character of the χ(T) curves remains unchanged after annealing. Oxygen vacancies have practically no influence on the a-axis magnetic susceptibility, but they alter the absolute values of the b-and c-axis susceptibilities. The significant effects of reducing the oxygen concentration include a decrease in the magnitude of the low-temperature anomaly (increase) in χ and an increase in the minimum value of χ. The results of the calculations of the exchange parameter I/k and the g factor are discussed in terms of the Heisenberg and Ising models for a one-dimensional system. Zh. éksp. Teor. Fiz. 113, 1026–1035 (March 1998)  相似文献   

6.
The real and imaginary parts of the surface impedance Z s =R s +iX s of YBa2Cu3O6.95 single crystals are measured at a frequency ω/π=9.4 GHz. The quantities R s (T) and X s (T) are linear functions of temperature for T<0.3T c (T c =93.5 K). A maximum of R s (T) and a plateau of X s (T) are observed in the interval 35<T<65 K. Our experimental data, just as all recent measurements of R s (T) in YBa2Cu3O6.95 single crystals in the temperature range 0<T<1.3T c , are described well in a two-fluid model which assumes electron-phonon scattering of quasiparticles. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 12, 893–898 (25 June 1997)  相似文献   

7.
《Current Applied Physics》2018,18(11):1280-1288
In this work, we pointed out that Sr substitution for Ca leads to modify the magnetic and magnetocaloric properties of Pr0.7Ca0.3-xSrxMnO3 compounds. Analyzing temperature dependence of magnetization, M(T), proves that the Curie temperature (TC) increased with increasing Sr content (x); TC value is found to be 130–260 K for x = 0.0–0.3, respectively. Using the phenomenological model and M(T,H) data measured at several applied magnetic field, the magnetocaloric effect of Pr0.7Ca0.3-xSrxMnO3 compounds has been investigated through their temperature and magnetic field dependences of magnetic entropy change ΔSm(T,H) and the change of the specific heat change ΔCP(T,H). Under an applied magnetic field change of 10 kOe, the maximum value of -ΔSm is found to be about 3 J/kg·K, and the maximum and minimum values of ΔCP(T) calculated to be about ±60 J/kg·K for x = 0.3 sample. Additionally, the critical behaviors of Pr0.7Ca0.3-xSrxMnO3 compounds around their TC have been also analyzed. Results suggested a coexistence of the ferromagnetic short- and long-range interactions in samples. Moreover, Sr-doping favors establishing the short-range interactions.  相似文献   

8.
This paper reports on a study of the resistance and differential magnetic susceptibility χ ac of lead embedded in nanosized glass pores with a diameter of ∼7 mm, which was performed at temperatures of 6–300 K and magnetic fields of up to 6 T. The field dependence of the resistance R(H) and the temperature dependences of the real, χ″(T), and imaginary, χ″(T), parts of magnetic susceptibility reveal indications of superconducting phase transitions associated with the volume and surface superconductivity of Pb nanopar ticles. The measurements of the field dependence of resistance have been used to set up the H c -T c phase diagram and to carry out a comparison with the study of the heat capacity performed on the same samples.  相似文献   

9.
Within a generalized non-relativistic Fermi-liquid approach we have found general analytical formulae for phase-transition temperatures T c,1(n, H) and T c,2(n, H) (which are nonlinear functions of density, n, and linear of magnetic field, H) for phase transitions in spatially uniform, dense, pure neutron matter from normal to superfluid states with spin-triplet p-wave pairing (similar to anisotropic superfluid phases 3He - A1 and 3He - A2) in steady and homogeneous sufficiently strong magnetic field (but |μ n |HE c < ɛ F (n), where μ n is the magnetic dipole moment of a neutron, E c is the cutoff energy and ɛ F (n)is the Fermi energy in neutron matter). General formulae for T c,1,2(n,H) are valid for arbitrary parameterization of the effective Skyrme forces in neutron matter. We have used for definiteness the so-called SLy2, Gs and RATP parameterizations of the Skyrme forces with different exponents in their power dependence on density n (at sub- and supranuclear densities) from the interval 0.7 n 0n < n c (Skyrme)< 2 n 0, where n 0 =0.17 fm−3 is the nuclear density and n c (Skyrme)is the the critical density of the ferromagnetic instability in superfluid neutron matter. These phase transitions might exist in the liquid outer core of magnetized neutron stars.  相似文献   

10.
The galvanomagnetic properties of single-crystal samples with various isotopic boron compositions have been investigated for the first time for the normal state of superconductor LuB12 (T c ≈ 0.44 K). Precision measurements of the resistivity, Hall coefficient, and magnetic susceptibility have been performed over a wide temperature range of 2–300 K in magnetic fields up to 80 kOe. A change of the charge transport regime in this nonmagnetic compound with metallic conduction is shown to occur near T* ≈ 50−70 K. As a result, a sharp peak with significantly different amplitudes for Lu10B12 and Lu11B12 is recorded in the temperature dependences of the Hall coefficient R H(T) near T*. A significant (about 10%) difference (in absolute value) of the Hall coefficients R H for the Lu10B12 and Lu11B12 compounds at helium and intermediate temperatures has been found and the patterns of behavior of the dependence R H(H) for T < T* in an external magnetic field H ≤ 80 kOe for Lu10B12 and Lu11B12 are shown to differ significantly. Analysis of the Curie-Weiss contribution to the magnetic susceptibility χ(T) leads to the conclusion about the formation of magnetic moments μeff ≈ (0.13−0.19)μB in each unit cell of the fcc structure of LuB12 compounds with various isotopic compositions. The possibility of the realization of an electronic topological 2.5-order transition near T* and the influence of correlation effects in the 5d-band on the formation of a spin polarization near the rare-earth ions in LuB12 is discussed.  相似文献   

11.
The low-temperature thermal and magnetic-resonance properties of a monoclinic KDy(WO4)2 single crystal are investigated. It is established that a structural phase transition takes place at T c=6.38 K. The field dependence of the critical temperature is determined for a magnetic field oriented along the crystallographic a and c axes. The initial part of the H-T phase diagram is plotted for Ha. The prominent features of the structural phase transition are typical of a second-order Jahn-Teller transition, which is not accompanied by any change in the symmetry of the crystal lattice in the low-temperature phase. The behavior of C(T) in a magnetic field shows that the transition goes to an antiferrodistortion phase. An anomalous increase in the relaxation time (by almost an order of magnitude) following a thermal pulse is observed at T>T c(H), owing to the structural instability of the lattice. A theoretical model is proposed for the structural phase transition in a magnetic field, and the magnetic-field dependence of T c is investigated for various directions of the field. Fiz. Tverd. Tela (St. Petersburg) 40, 750–758 (April 1998)  相似文献   

12.
Hybrid herostructures comprising an YBa2Cu3O x (YBCO) high-temperature superconductor (HTS) layer and Nb/Au low-temperature superconductor (LTS) bilayer (with critical HTS and LTS temperatures T c and Tc, respectively), separated by a thin (d M = 5–20 nm) interlayer of LaMnO3, La0.7Ca0.3MnO3, or La0.7Sr0.3MnO3 manganite have been studied. The electric resistance and magnetic properties of individual (evaporated directly onto the substrate) manganite films and related hybrid herostructures have been measured. Based on quasi-classical equations, analytical expressions for the conductivity of herostructures at TTc are obtained in the case of a low-transparency superconductor/manganite interface. It is established that the conductivity of heterostructures is determined by the proximity effect (related to the penetration of a condensate wavefunction from the Nb/Au bilayer to manganite) and depends strongly on interface transparency. At low temperatures (TT c′), the conductivity peaks are found at voltages determined by the exchange field of the manganite interlayer. At T c′ < T < T c, conductivity features at nearly zero bias voltages are observed, which are related to the superconductivity of the YBCO electrode.  相似文献   

13.
A theory describing the magnetic properties of a two-band superconductor with a varying charge carrier density is constructed. The upper critical field H c2(ab) parallel to the ab plane and field H c2(c) parallel to the c axis are determined in the entire temperature range 0 < T < T c . A considerable increase in upper critical field H c2(ab) as compared to H c2(c) because of strong anisotropy of the system is detected. Anisotropy of coefficient γ H = H c2(ab) / H c2(c) is obtained as a function of temperature for pure MgB2 and as a function of the chemical potential in the case when Mg and B atoms are replaced with other chemical elements. A correlation between the variation in the superconducting transition temperature upon an increase in the chemical potential and critical magnetic fields H c2(ab) and H c2(c) is observed. The effect of doping on magnetic anisotropy is also determined.  相似文献   

14.
Magnetization measurements were performed on a lanthanum manganite La0.9Sr0.1MnO3 single crystal in the temperature interval 4.2–300 K and magnetic field interval 50 Oe-55 kOe in two sample cooling regimes: 1) cooling down to 4.2 K in a high (55 kOe) magnetic field, and 2) cooling in a “zero” field. It is shown that the temperature dependences of the magnetization M(T) are substantially different in these regimes. Pronounced anomalies of M(T) were observed at temperatures T*=103 K and T c =145 K. The first anomaly is attributed to a structural transition, while the second one corresponds to a ferromagnet-paramagnet phase transition. The magnetization of a La0.9Sr0.1MnO3 single crystal in the cooling regimes studied shows typical “spin-glass” behavior. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 39–43 (10 July 1998)  相似文献   

15.
Direct microwave absorption was studied as a function of magnetic field up to 8 kG and in a temperature range between 10 K and 150 K in Y1Ba2Cu3O7−x. Large microwave absorption was observed below Tc over a wide magnetic field range. The microwave power absorption is approximately described by I=A(T) H+C(T), where A(T) and C(T) are temperature dependent constants. A(T) is proportional to the square root of the microwave power, while C(T) shows a different behaviour. We propose different origins for A(T) and C(T), namely fluxons in grains and weak Josephson links. Moreover, also flux trapping was observed in the superconducting state. Possible mechanisms for the absorption are discussed.  相似文献   

16.
A magnetic phase transition in carbon-doped (0.1 and 0.7 at. %) Fe70Ni30 Invar alloys was investigated by the method of depolarization of a transmitted neutron beam and by small-angle scattering of polarized neutrons. It is shown that for both alloys, two characteristic length scales of magnetic correlations coexist above T c. Small-angle scattering by critical correlations with radius R c is described well by the Ornstein-Zernike (OZ) expression. The longer-scale (second) correlations, whose size can be estimated from depolarization data, are not described by the OZ expression, and hypothetically can be modeled by a squared OZ expression, which in coordinate space corresponds to the relation 〈M(r)M(0)〉∝exp(−r/R d), where R d is the correlation length of the second scale. The temperature dependence of the correlation radius R c was obtained: R c ∝ ((TT c)/Tc)ν , where ν≈2/3 is the critical exponent for ferromagnets, over a wide temperature range up to T c exp , at which the correlation radius becomes constant and equals its maximum value R c(T c)=R c max . The maximum correlation radius established (R c max =140 Å and 230 Å for the first and second alloys, respectively) characterizes the length-scale of the fluctuation for which the appearance of critical correlations first results in the formation of a ferromagnetic phase, and the phenomenon itself exhibits a “disruption” of the second-order phase transition at T=T c exp , as a result of which a first-order transition arises. Temperature hysteresis was also detected in the measured polarization of the transmitted beam and intensity of small-angle neutron scattering in the alloy above T c, confirming the character of this magnetic transition as a first-order transition close to a second-order transition. Zh. éksp. Teor. Fiz. 112, 2134–2155 (December 1997)  相似文献   

17.
The magnetocaloric effect ΔT has been studied by a direct method in two samples of the manganite Sm0.55Sr0.45MnO3, namely, a single crystal (sample A) and a ceramic sample (sample C). The temperature dependences of the ΔT effect of both samples exhibit a maximum at T max = 143.3 K for the sample A and T max = 143 K for the sample C. In these maxima, the values of the ΔT effect are 0.8 and 0.4 K in the magnetic field H = 14.2 kOe for the samples A and C, respectively. In addition, the ΔT(T) curve of the sample A has a minimum at T min = 120 K, in which ΔT = −0.1 K. The maximum value of the ΔT effect increases with an increase in the magnetic field H in the range of magnetic fields up to 14.2 kOe, and the rate of this increase at H > 8 kOe is higher than that at H < 8 kOe. These features of the ΔT effect are explained by the presence of ferromagnetic and antiferromagnetic A- and CE-type clusters in the samples.  相似文献   

18.
The temperature dependences of the intense magnetocaloric effect ΔT AD(T, H) and the heat capacity C p (T) of the (La0.4Eu0.6)0.7Pb0.3MnO3 manganite are directly measured using adiabatic calorimetry. The experimental dependences ΔT AD(T) are in satisfactory agreement with those calculated from the data on the behavior of the magnetization. The factors responsible for the absence of an anomaly in the experimental temperature dependence of the heat capacity C p (T) in the range of the magnetic phase transition are discussed.  相似文献   

19.
对Eu1-xSrxMnO3 (ESMO, x=0—1)体系的结构和磁性进行了系统的研究,结果表明Sr的掺入使EuMnO3反铁磁母体的磁结构发生巨大的变化.通过磁化和电输运测量,深入探讨了高掺杂浓度Eu0.4Sr0.6MnO3和Eu0.3Sr0.7MnO3关键词: 1-xSrxMnO3体系')" href="#">Eu1-xSrxMnO3体系 Sr掺杂 可变程跳跃模型  相似文献   

20.
The crystal and magnetic structures of manganite Pr0.7Ba0.3MnO3 have been studied at high pressures of up to 5.1 GPa and temperatures from 10 to 300 K by means of the neutron diffraction. At normal pressure and a temperature T C = 200 K, a ferromagnetic state forms in Pr0.7Ba0.3MnO3. At high pressures P ≥ 1.9 GPa and T < T N ≈ 153 K, a new antiferromagnetic state of A-type have been observed. Under high pressure, the Curie temperature T C increases with the characteristic quantity dT C/dP ≈ 2.4 K/GPa. A possible reason for the appearance of an A-type antiferromagnetic phase in Pr0.7Ba0.3MnO3 at high pressures may be anisotropic uniaxial compression of oxygen octahedra along the b axis of the orthorhombic structure.  相似文献   

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