Complex competition between ferromagnetism and antiferromagnetism in the CMR manganites Pr1−x Ca x MnO3

The systematic investigation of the magnetic susceptibility of the CMR manganites Pr_1?x Ca _x MnO₃ versus temperature has been performed for 0.25 ≤ x ≤ 0.50. Due to the similar size of calcium and praseodymium, these results show the important role of the mixed valence of manganese upon the complex magnetic behaviour of these compounds. They demonstrate that the appearance of antiferromagnetism coincides with charge ordering, T _N = T _CO varying with x, from 250K for x = 0.50 to 225K for x = 0.35. A strong competition between ferromagnetism and antiferromagnetism is observed for 0.35 ≤ x < temperature (T > 170 K) and going through canted antiferromagnetic or weak ferromagnetic states for intermediate temperatures (70 K < T < 170 K). For 0.25 ≤ x ≤ 0.30, a strong ferromagnetic state is observed for 95 K ≤ T ≤ 150 K, with a transition to a spin glass like state below 95–110 K.     

New alkali antimonate glasses

Glass formation has been investigated in ternary systems Sb₂O₃-PbO-M₂O in which M is alkali Li, Na and K and antimony oxide is the glass former. Alkali elements were introduced as carbonates. The size of the glass forming region is enlarged as alkali ionic radius increases. Binary Sb₂O₃-M₂O glasses were obtained: (100−x) Sb₂O₃-xLi₂O, (10<x<30), (100−x) Sb₂O₃-xNa₂O, (10<x<70), and (100−x) Sb₂O₃-xK₂O, (10<x<90). In ternary systems, PbO content could reach 60 mol%. Temperatures of glass transition, T_g, onset of crystallization, T_x, and maximum of crystallization, T_p, have been measured using differential scanning calorimetry. Depending on composition, glass transition temperature ranges from 240 to 330 °C. The incorporation of alkali oxide increases T_g while lead oxide has the reverse effect. Thermal stability range (T_x-T_g) was usually between 65 and 266 °C, while no crystallization exotherm was observed in some cases. Density and thermal expansion increases as lead concentration increases. Optical transmission has been measured. The UV cut-off depends on alkali content: samples are yellow and turn green for large K and Na content. These glasses have potential applications as low phonon energy glasses for infrared transmission or active devices.     

Electron spin-lattice relaxation in noncommon metals YBa2Cu3Ox and Rb1C60

Using an original modulation technique, the electron spin-lattice relaxation have been investigated in two noncommon metals: YBa₂Cu₃O_x, high-T_c material doped with 1% Gd, and Rb₁C₆₀, linear polymer phase fulleride. In the first case, the Korringa-like temperature dependence of the Gd³⁺ longitudinal relaxation time T₁, is found forx = 6.59 in a wide temperature range 4.2 <T < 200 K, both above and below T_c = 56 K. Atx = 6.95 (T_c = 90 K), the T₁ behavior within 50 <T < 200 K is evidently affected by spin gap opening with the gap value of about 240 K. At 200 K, an unexpected acceleration of the relaxation rate takes place, suggesting some change in the relaxation mechanism. The data are discussed in terms of the Barnes-Plefke theory with allowance made for microscopic separation of the normal and superconducting phases. In Rb₁C₆₀, the evolution of the ESR line and relaxation rates have been studied accurately in the range of the metal-insulator transition (below 50 K). Interpretation is suggested which takes into account breaking down the relaxation bottleneck due to opening of the energy gap near the Fermi surface. The gap value of about 100 cm^?1 is estimated from the analysis of relaxation rates, lineshape and spin susceptibilities.     

Magnetic behaviour of U(MnxAl1−x)2 compounds

The results of magnetic measurements performed on U(Mn_xAl_1−x)₂ compounds in the temperature range 4.2K < T < 800K are reported. In the low temperature range (T < 200K), UMn₂ shows a Pauli-type paramagnetism. Above 420K a Curie-Weiss behaviour is evidenced. The magnetic properties of U(Mn_xAl_1−x)₂ compounds were analysed assuming a superposition of a temperature dependent term on a Pauli-type contribution, χ_O. The effective moments as well as the χ_O values were determined both in the low (T < 200K) and high (T > 420K) temperature range. The experimental data were discussed considering changes in the band structure and/or quenching of spin fluctuations.     

Phase transition and dielectric properties of Li2-x NaxGe4O9 crystals (0.2 ≤ x ≤ 0.3)

The dielectric nonlinearity of ferroelectric Li_2-x Na_xGe₄O₉ crystals (0.2 ≤ x ≤ 0.3) is measured in the region of the phase transition temperatures. The ?(T) dependences for various values of the applied dc electric field E ₌ and the ?(E ₌)_T dependences at a constant temperature are studied in the Li_2-x Na_xGe₄O₉ crystals with T _c > 300 K and T _c < 300 K. It is shown that the Landau theory for second-order phase transitions describes the dielectric properties of the crystals with T _c < 300 K and does not describe the behavior of the crystals with T _c > 300 K. The results obtained lead to the conclusion that the different properties of the crystals with T _c > 300 K are likely to be related to the changed structure of these crystals caused by a change in the ratio between the Li and Na atoms.     

Exchange bias in bulk Ni-Mn-In-based Heusler alloys

The crystal structure and exchange bias of the bulk Heusler alloys Ni₅₀Mn_50−xIn_x with 14.5?x?15.2 have been investigated using X-ray diffraction and magnetization measurements, respectively. Magnetic measurements were performed with SQUID magnetometry after samples were zero-field cooled and field cooled (FC) in positive magnetic fields up to H=50 kOe, from a temperature T=380 K. Three temperatures of the phase transitions, T₁<T_M<T_C, and a shift of the FC (50 kOe) magnetic hysteresis loops up to 120 Oe at 5 K have been detected for all samples. The exchange bias field (H_E) was almost constant for intermediate In concentrations 14.8<x<15.2, and sharply decreased to about 20 Oe on the borders of this concentration interval (x?14.5; 15.2?x). The changes of H_E have been related to changes in the ratio of T₁ to T_M: the overlapping of transitions at T₁ and T_M (for x=14.8, and 15.2) results in a decrease in H_E.     

Magnetic phase diagram of GdAg1−xZnx

The magnetic phase diagram of GdAg_1?xZn_x, an intermetallic solid solution of an antiferromagnet (GdAg: T_N = 136 K) and a ferromagnet (GdZn: T_C = 269 K), has been elaborated from magnetization measurements. The antiferromagnetic phase boundary T_N(x) first passes a broad maximum meeting the ferromagnetic phase boundary T_C(x) at x₁ = 0.575 and T₁ = 72 K, where four phases coexist. On approaching (x₁, T₁) along T_N(x) the magnetization phenomena vanish. At x₁ the phase transition still has ferromagnetic appearance but proceeds into a state without spontaneous magnetic moment. Two different ferromagnetic phases (F₁, F₂) and one ferrimagnetic phase (F₃) occur in the composition intervals 0.69 < x₁ <1, and 0.61 <x₂ < 0.69 and 0.51 < x₃ < 0.61. All phase transitions seem to be of second order except th e F₁?F₂ one at x = 0.69 which is of first order. This phase line meets the paramagnetic to ferromagnetic phase boundary T_C(x) in a multicritical point with the coordinates x_m = 0.69, T_m = 123 K.Six further mixed magnetic phases, M_{1, 2, ..., 6}, are observed between x = 0.33 and 0.6 below the antiferromagnetic branch and exhibit irreversible thermodynamic properties, such as hysteresis, below about 40 K.Assuming local magnetic interactions only between nearest and next-nearest Gd neighbours, the T_N(x) and T_C(x) phase boundaries can be described fairly well by a simple model calculation using different exchange parameters for a few relevant distributions of Ag and Zn atoms.     

The magnetic susceptibility of Hg−xMnxSe for small x

The magnetic susceptibility of Hg_1?xMn_xSe semimetals for x ? 0.031 and 2.3 ? T ? 250 K is presented. The samples show Curie-Weiss behaviour in several temperature regions. S_eff at high temperatures exceeds that at low temperatures. θ_p is antiferromagnetic (θ_p < 0) for all x at low T, but θ_p is positive at high T for low x. The coupling mechanism considered is long-range and caused by virtual band transitions across the zero-gap.     

Universal conductivity variation in glassy Zr-M alloys

Temperature dependence (3–300 K) of the electrical conductivity in a number of amorphous Zr_1?xM_x alloys (M = Cu, Ni, Co and Fe, 0.19 < x < 0.71) has been analysed in some detail. Like in some other alloys with a high electrical resistivity, the conductivity varies as T at lower temperatures (T < 80 K) and √T at higher temperatures. A new feature observed is that the ratio of the coefficients of a low temperature T and a high temperature √T conductivity variation is practically constant for allalloys. Therefore a universal conductivity-temperature curve can be constructed for all amorphous Zr_1?xM_x alloys with the resistivity higher than 150 μohms cm. These results are consistent with the effects of incipient localisation and indicate that the electron-phonon coupling determines the conductivity variation.     

EPR observation of the spin pair correlation above Tc in lightly alkali metal-doped C60 fullerene

The existence of two temperatures: T_p-Cooper pairing and T_c-Bose–Einstein condensation in high temperature superconductors has been stipulated in a lightly potassium-doped C₆₀ by Magnetically Modulated Microwave Absorption. This doping level corresponds to the carrier density greater than the critical one: x>x₁. In case of rubidium lightly doped C₆₀, where the carrier density x was smaller than the critical one: x<x₁, anomalous EPR temperature dependence was observed. The characteristic temperature of bound electron pair formation T_p≈65 K and the energy gap 2Δ/k=30 K were estimated from the temperature dependence of the EPR signal intensity in non-superconducting state. These results suggest that the liquid fermions–liquid bosons transition can be observed as the opening of the spin gap at temperature T_p postulated in Micnas–Ranninger–Robaszkiewicz theory.     

Electrical behavior of some silver-doped lanthanum-based CMR materials

With a view to understand the structural, magnetic and electrical properties of La_1−xAg_xMnO₃ (x=0.05-0.3), a series of samples were prepared by polyvinyl alcohol (PVA) gel route. It has been found that both the metal-insulator and ferro- to paramagnetic transition temperatures after increasing up to the composition x=0.20, are found to remain constant thereafter. The electrical resistivity vs. temperature plot of the sample x=0.10 is found to exhibit an insulating behavior below 36 K, while the sample, x=0.20 exhibits two peaks, and the observed behavior is explained on the basis of the phase separation model. The low-temperature (T<T_P), electrical resistivity data were analyzed by a theoretical model, ρ=ρ₀+ρ₂T²+ρ_4.5T^4.5, indicating the importance of grain/domain boundary effects, electron-electron and two-magnon scattering processes. The low-temperature resistivity data (T<50 K) were fitted to an equation, which is based on the combined effect of weak localization, electron-electron and electron-phonon scattering.     

The thermoelectric power in AgxTiS2 and AgxNiPS3. Part II. The ionic component of the thermoelectric power

The EMF of the isothermal cells: Ag/AgI/Ag_xTiS₂: 0<x<1, T=150–200°C/Ag_xNiPS₃: 0<x<3, T=150–350°C has been measured. From the EMF-x curves the existence ranges of the 2-phase (stage I and II) regions ?0.16<x<0.32 for the Ag/Ag_xTiS₂ system at 190°C; 0.20 < x < 0.50 and 1 < x < 2 for the Ag/Ag_xNiPS₃ system at 400°C - have been determined. The results are sustained by X-ray diffraction and electrical conductivity measurements. From the EMF-T curves the partial enthalpy (ΔH?_Ag) and entropy (ΔS?_Ag) of dissolution of silver in the Ag_xSSE (solid solution electrode) materials were obtained. In the case of Ag_xTiS₂, ΔH?_Ag has a low absolute value, while ΔS?_Ag is distinctly positive. The EMF of the Ag/Ag_xNiPS₃ system also has a positive temperature coefficient. Furthermore, the ionic component of the thermoelectric power, ΔE/ΔT, of the thermogalvanic cells: Ag/AgI/Ag_xSSE/AgI/Ag Ag_xTiS₂: 0 < x < 1, T = 150–200°C( T ) (T+ΔT) Ag_xNiPS₃: 0 < x < 1, T= 150–350°C has been measured. The kinetically important heat of transport of silver ions in the Ag_xSSE materials has been determined in two ways: first from the dependence of the ionic Seebeck coefficient (?_Ag+) on reciprocal temperature; and second from direct calculation, using the data for ?_Ag+ and ΔS?_Ag. The heat of transport is much smaller than the activation enthalpy for Ag⁺-conduction, indicating a high ionic polaron binding energy in these materials.     

Two-band BCS mechanism of superconductivity in a Ba(Fe0.9Co0.1)2As2 high-temperature superconductor

Terahertz and infrared spectra of the conductivity, σ(ν), and dielectric constant, ?(ν), of a Ba(Fe_0.9Co_0.1)₂As₂ film (T _c = 20 K) have been analyzed together with previous specific-heat and angular resolved photoelectron spectroscopy data. It has been shown that the spectra σ(ν) and ?(ν) of Ba(Fe_0.9Co_0.1)₂As₂ in the superconducting phase at T = 5 K, as well as the magnetic field penetration depth, can be described well using the standard Bardeen-Cooper-Schrieffer (BCS) model with an additive contribution of electron and hole bands. It has been found that the measured temperature dependence of the magnetic field penetration depth in a wide temperature range 5 K < T < T _c can be described only with the introduction of interband pairing interaction. The coupling constant of electron and hole bands, λ_{1, 2} = 0.1, as well as the temperature dependences of superconducting gaps in the electron and hole subsystems, has been determined using the model of two-band superconductivity developed earlier for MgB₂.     

X-ray, Mössbauer, and dielectric studies of the Co1 − x Ni x Cr2O4 ceramic system

The temperature dependences of the dielectric constant, ?(T), dielectric loss tangent, tanδ(T), and thermostimulated depolarization currents of (Co_{1 ? x} Ni _x )Cr₂O₄:y ⁵⁷Fe₂O₃ samples with 0.2 ≤ x ≤ 0.6 and 0.01 ≤ y ≤ 0.04 exhibit anomalies at temperatures of T ₁ ≈ 220 K and T ₂ ≈ 240 K that indicate a transition to an ordered ferroelectric state at temperatures below T ₂. Also observed at the same temperatures are abrupt changes in the isomer shift and quadrupole splitting for two Mössbauer spectral doublets of a sample with x = 0.2.     

Thermoelectric power of potassium doped lanthanum manganites at low temperatures

The temperature-dependent resistivity and thermoelectric power of monovalent (K) doped La_1−xK_xMnO₃ polycrystalline pellets (x=0.05, 0.10 and 0.15) between 50 and 300 K are reported. K substitution enhances the conductivity of this system. Curie temperature (T_C) also increases from 260 to 309 K with increasing K content. In the paramagnetic region (T>T_C), the electrical resistivity is well represented by adiabatic polaron hopping, while in the ferromagnetic region (T<T_C), the resistivity data show a nearly perfect fit for all the samples to an expression containing, the residual resistivity, spin-wave and two-magnon scattering and the term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. Small polaron hopping mechanism is found to fit well to the thermoelectric power (S) data for T>T_C whereas at low temperatures (T<T_C) in ferromagnetic region (S_FM), S_FM is well explained with the spin-wave fluctuation and electron–magnon scattering. Both, resistivity and thermopower data over the entire temperature range (50–300 K) are also examined in light of a two-phase model based on an effective medium approximation.     

Galvanomagnetic properties of the Al1−x Six nonequilibrium substitutional solid solutions

Galvanomagnetic characteristics of the A_1−x Si_x solid solutions (x<12 at. %) have been studied within a broad range of temperatures (1.8–290 K) and magnetic fields (up to 15 T). An anomaly in the concentration dependence of the Hall coefficient R _H (x,T=290 K) has been revealed near the boundary of absolute instability (x<8.5 at. %) of compounds in the Al_1−x Si_x series. The variation of the Hall coefficient and of the magnetoresistance in the A_1−x Si_x series at low (T<77 K) temperatures is analyzed within models taking into account the anisotropy in conduction-band electron scattering. Fiz. Tverd. Tela (St. Petersburg) 41, 3–10 (January 1999)     

Stoichiometry,electrical properties,and electronic structure of Cr1.70Nb2.30Se10

Both powder and single-crystal X-ray investigations show that Cr_{1 + x}Nb_3?xSe₁₀ which belongs to the FeNb₃Se structure type, exists only in a narrow range of stoichiometry close to x=0.70. In contrast to the Fe analog, Cr_1.70Nb_2.30Se₁₀ undergoes no disticnt metal-insulator transition; the resistivity and the thermoelectric power remain of the same order as the temperature is lowered from 300 K to 4.2 K. The thermoelectric powers S of Fe_{1 + x}Nb_3?xSe₁₀ (0.25<x<0.40) and FeVNb₂Se₁₀, however, follow such a temperature T dependence as $\text{S∝}\text{1}\text{T}$ even at room temperature. On the basis of light-binding band calculations, these observations are interpreted in terms of the contribution of Cr or Fe d-orbitals to electron conduction.     

Spin state of Co<Superscript>3+</Superscript> ions in EuBaCo<Subscript>2–<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>5.5–δ</Subscript> in the paramagnetic temperature range

The magnetic studies of EuBaCo₂O_5.50, EuBaCo_1.93O_5.40, and EuBaCo_1.90O_5.36 have been performed. Layered cobaltites EuBaCo_2–x O_5.5–δ demonstrated the ferrimagnetic ordering in the temperature range 200 K < T < 650 K. The ferromagnetic Curie temperature T _C almost did not change as the number of Co³⁺–O–Co³⁺ bonds decreased. The contributions of the ferro- and antiferromagnetic interactions were nearly the same and determine the relationship T _C > T _N and T _C < T _N. At T > T _C, the antiferromagnetic order was retained in the EuBaCo1.90O5.36. As temperature increased, EuBaCo_2–xO_5.5–δ conserved the IS state in pyramids, and gradual LS → IS → HS transitions occurred in octahedron sites. The increase in the effective magnetic moment μeff in EuBaCo_1.90O_5.36 at T > 580 K demonstrated the transition of Co³⁺ ions to the HS state at lower temperatures as compared to the EuBaCo₂O_5.50 stoichiometric composition, and the structural distortions and the increase in the unit cell volume favored to this transition.     

Influence of carbon concentration on structural, magnetic and electrical transport properties for antiperovskite compounds AlCxMn3

The structural, magnetic and transport properties of the antiperovskite AlC_xMn₃ (1.0≤x≤1.4) are investigated. It is found that the lattice parameter a increases monotonously with nominal carbon concentration x. The Curie temperature T_C increases with increasing x from 1.0 to 1.1 and then decreases with further increasing x. The highest T_C value is 364 K, about 70 K higher than that of stoichiometric AlCMn₃ reported previously. This may be attributed to a competition between the lattice expansion and the strong Mn 3d-C 2p hybridization. Below 100 K, the resistivity can be well described as ρ(T)=ρ₀+AT², corresponding to the electron-electron scattering. A increases with x, suggesting certain changes in the electronic structure, e.g. carrier density. Above 250 K, all ρ(T) curves depart from the linear dependence on temperature and seem to take on a tendency towards saturation.