Phase stratification in the Nd1 − x BaxCoO3 − δ (0.3 ≤ x ≤ 0.54) system

Slowly cooled Nd_{1 ? x} Ba_xCoO_{3 ? δ} samples were two-phase in the concentration interval 0.3 ≤ x ≤ 0.46. One of the phases had O-orthorhombic lattice distortions (Pbnm) characteristic of ferromagnetic samples with x ≤ 0.3, and the other phase had tetragonal distortions (P4/mmm) characteristic of samples with x ≥ 0.46. Tetragonal distortions were caused by ordering of Nd³⁺ and Ba²⁺ ions. Samples with ordered neodymium and barium ions (Nd_{1 ? y} Ba_{1 + y} Co₂O_{6 ? γ} at ?0.08 ≤ y ≤ 0.08) experienced metal-dielectric and orientation magnetic phase transitions.     

Spin-reorientation, ferroelectricity, and magnetodielectric effect in YFe(1-x)Mn(x)O3(0.1 ≤ x ≤ 0.40)

We report the observation of magnetoelectric and magnetodielectric effects at different temperatures in Mn-substituted yttrium orthoferrite, YFe(1-x)Mn(x)O(3)(0.1 ≤ x ≤ 0.40). Substitution of Mn in antiferromagnetic YFeO(3)(T(N) = 640 K) induces a first-order spin-reorientation transition at a temperature, T(SR), which increases with x whereas the Néel temperature (T(N)) decreases. While the magnetodielectric effect occurs at T(SR) and T(N), the ferroelectricity appears rather at low temperatures. The origin of magnetodielectric effect is attributed to spin-phonon coupling as evidenced from the temperature dependence of Raman phonon modes. The large magnetocapacitance (18% at 50 kOe) near T(SR) = 320 K and high ferroelectric transition temperature (～115 K) observed for x = 0.4 suggest routes to enhance magnetoelectric effect near room temperature for practical applications.     

Magnetic ordering and magnetoresistive effect in La1−x Srx(Mn1−y Mey)O3 perovskites (Me = Nb, Mg)

Perovskites of composition La_1?x Sr_x(Mn_1?x/2Nb _x/2)O₃ and La_0.49Sr_0.51(Mn_1?y Nb_y)O₃ have been synthesized and investigated. The substitution of nonmagnetic niobium ions for manganese was shown to lead to a transition from the metallic into the insulating state due to a decrease in the number of dissimilar (different-valence) manganese atoms in the lattice. In spite of the high resistivity, the niobium-containing perovskites exhibit a large magnetoresistive effect and ferromagnetic ordering. Small additions of Nb⁵⁺ to La_0.49Sr_0.51MnO₃ stimulate the transition from the antiferromagnetic into the ferromagnetic state, whereas the substitution of Mg²⁺ for Mn stabilizes the antiferromagnetic state. It is supposed that the ferromagnetism in the insulating perovskites at hand is due to the positive superexchange of the Mn³⁺-O-Mn³⁺ type, and the magnetoresistive effect owes to the intergranular transfer of spin-polarized charge carriers and the suppression of magnetic nonuniformities by an applied magnetic field near T _C.     

Nd0.3Sr0.7Mn1-xCrxO3(0.01≤x≤0.15)的低温热导率研究

报道Nd0.3Sr0.7Mn1-xCrxO3(0.01≤x≤0.15)的热导率在10~300K温区内随温度和Cr含量的变化关系.在所测温区,随着Cr掺杂的增加,样品热导率值整体上逐渐减小,但在x=0.15时,出现反弹.热导率值在绝缘体-金属转变附近有很大提高,这个提高随Cr含量的增加逐渐被压制,直到x=0.15消失.分析指出热导率在绝缘体-金属转变附近的提高是由声子热导率和自旋相关热导率两部分贡献的;Cr掺杂通过电荷、晶格和自旋相互作用影响和制约了热导率随温度的变化.结果表明体系中自旋对热导率的调制作用是不可忽略的.     

Dielectric and piezoelectric properties of (1−x)Ba0.7Sr0.3TiO3−xBa0.7Ca0.3TiO3 perovskites

Dielectric and piezoelectric properties of (1?x)Ba_0.7Sr_0.3TiO₃?xBa_0.7Ca_0.3TiO₃ (BST?xBCT) (x=0.2–0.9) perovskite ceramics have been investigated. BCT has fully incorporated into BST lattice, forming a complete perovskite solid solution, whose lattice constant χ decreases almost linearly with increase in x from 0.2 to 0.4, while showing an anomalous expansion at 0.4<x≤0.6. This, together with the deviation of tetragonal–orthorhombic phase transition temperature (T_O–T) from the linear relation T_O–T (K)=?103.7x+239.3 at x=0.5, suggests that a small amount of Ca²⁺ has substituted for Ti⁴⁺. Curie temperature T_C increases linearly with increase in x from 0.2 to 0.9, which is mainly contributed to the increase of the Ba/Sr ratio. The calculated degree of relaxation (γ) is in the range of 1.41–1.53, indicating that the BST–xBCT ceramics are ferroelectric materials with diffuse phase transition. Strain and piezoelectric constant (d₃₃) decrease with increasing x, whereas planar electromechanical coefficient (k_p) reaches a maximum (17.0%) at x=0.6.     

Hydrostatic pressure effect on the metal-insulator transition in LaO0.7Ca0.3Mn1−x(Fe/Ge)xO3 perovskites

Abstract

The temperature dependences of the resistivity of La_0.7Ca_0.3Mn_1?xFe_xO₃ and La_0.7Ca_0.3Mn_1?xFe_xO₃ (0 < × < 0.04) mixed crystals were studied under hydrostatic pressures up to 15kbar. The substitution of Fe for Mn results in an increase of the resistivity and a continuous decrease of the metal-insulator transition temperature T_mi while the substitution of Ge for Mn leads to a more complicated T_mi(x)-curve. In all cases T_mi shifts under pressure with a rate between 1.6 and 2.9K/kbar and a correlation between T_mi and its pressure derivative dT_mi/dP is observed which is in accordance with the general trend of dT_mi/dP versus T_mi as derived for other manganites and is discussed in terns of a competition between superexchange and double exchange.     

La0.7Ca0.3Mn1-xCrxO3 (0.01≤x≤0.60)的低温热导

测量了锰位掺Cr系列巨磁阻多晶样品La0 .7Ca0 .3Mn1-xCrxO3(0 .0 1≤x≤ 0 .6 0 )的热导 ,实验温区 77～ 30 0K .随着Cr掺杂浓度x的增加 ,热导率κ(T)在所测温区内受到的压制变大 ,但当x =0 .6 0时 ,热导值却有较大的回升 ,显示了Cr掺入对晶格结构局部涨落的影响 .随温度降低 ,低掺杂样品在MI(金属 绝缘体 )转变点附近热导率κ(T)由dκ/dT >0变成dκ/dT<0 ,而高掺杂样品无MI转变 ,热导率随温度降低而减小 ,即dκ/dT >0 ,我们认为这是由载流子处于局域或巡游状态时与晶格的相互作用不同而导致的 .     

On-line phase transition in La1− x Sr x MnO3 (0.28 ≤ x ≤ 0.36) perovskites through ultrasonic studies

La_{1? x} Sr _x MnO₃ perovskite manganite materials with different compositions (x?=?0.28, 0.31 and 0.36) have been prepared employing solid-state reaction technique. On-line evaluation of ultrasonic velocities and longitudinal attenuation of the above samples has been done over a wide range of temperatures using the transmission method. The observed anomalies in velocities, attenuation and elastic moduli reveal the occurrence of lattice softening and hardening near Curie temperature. The observed dramatic hardening in sound velocities and softening in attenuation are correlated with the phase transition, i.e. ferromagnetic to paramagnetic. The increase in magnitude of maximum velocity with change in Sr content at T _c indicates the existence of linear magnetostriction effect. The elastic moduli study elucidates the observations made from the above-mentioned studies. The variation in the ultrasonic velocities, longitudinal attenuation and its derived parameters help us to understand the competitions between ferromagnetism and paramagnetism.     

High-temperature thermoelectric characteristics of B-site substituted Yb0.1Ca0.9Mn1−x Nb x O3 system (0≤x≤0.1)

The thermoelectric performance of Yb_0.1Ca_0.9MnO₃ doped with Nb⁵⁺ at B-site is investigated in this paper. It is found that there is a phase transition from O-type to O*-type orthorhombic structure with increasing of Nb doping content, which indicates that the structure distortion becomes more seriously. Since the electron–phonon interaction can be enhanced by the structure distortion, the small polaron formation is promoted in Yb_0.1Ca_0.9Mn_1?x Nb _x O₃ with increasing Nb content. In the whole measured temperature range, the electrical conductivity can be fitted very well by the adiabatic small polaron hopping model. The activated energy E _α is ascending with increasing Nb content. The temperature dependence of Seebeck coefficient S of Yb_0.1Ca_0.9Mn_1?x Nb _x O₃ shows that the S is basically inversive to the charge carrier concentration. S(T) can be fitted well by the Cutler and Mott model, which indicates that the density of state around the Fermi level is strongly affected by Nb-doping at B-site. It is contrary to those of CaMnO₃ and RE_0.1Ca_0.9MnO₃, when Nb content x>0.05, the |S| and σ show a same tendency of the temperature dependence.     

Magnetocaloric effect in La0.65−xEuxSr0.35MnO3

The magnetocaloric properties for the Eu-doped La_0.65?xEu_xSr_0.35MnO₃ samples with x = 0.05, 0.15, 0.20, and 0.30 upon 0.05T magnetic field have been investigated. It is found that the Eu doping in this system decreases the magnetocaloric properties lightly. Moreover, the results of Eu doping clearly indicate that the magnetocaloric effect in this system is tunable, which is beneficial for manipulating magnetocaloric refrigeration that occurs in various temperature ranges. This makes the La_0.65?xEu_xSr_0.35MnO₃ samples potential candidates for practical applications. A complete characterization of the magnetic properties of this material aids to the understanding required for the technological exploitation of such materials, and it suggests La_0.65?xEu_xSr_0.35MnO₃ perovskite as the promising magnetic refrigerant.     

Evolution of structural,magnetic and magnetocaloric properties in Sn-doped manganites La0.57Nd0.1Sr0.33Mn1−x Sn x O3 (x = 0.05–0.3)

Structural and magnetic properties of manganites series La_0.57Nd_0.1Sr_0.33Mn_1?x Sn _x O₃ with (0.05 ≤ x ≤ 0.30) have been investigated, and the critical exponents and magnetocaloric effect are studied around the room temperature, to shed light on Sn substitution influence. A solid-state reaction method was used in the preparation. A structural study using Rietveld refinement of XRD patterns indicates rhombohedral structure with R \( \overline{3} \) c space group for (0.05 ≤ x ≤ 0.20) and shows the existence of a secondary phase attributed to the neodymium tin oxide (Nd₂Sn₂O₇) pyrochlore for x = 0.3. The variation of the magnetization (M) vs. temperature (T), under an applied magnetic field of 0.05 T, reveals a ferromagnetic–paramagnetic transition at the Curie temperature T _C. In addition, it was discovered that increasing the tin content leads to a reduction in magnetization and a lowering of T _C from 282 K (x = 0.05) to 158 K (x = 0.20) with increasing Sn substitution. The samples exhibit the characteristics of spin/cluster-glass state which is evident from (zero-field-cooled and field-cooled) magnetization vs. temperature curves. Indeed, the thermal evolution of magnetization in the ferromagnetic phase at low temperature varies as T ^3/2, in accordance with Bloch’s law. The spin-stiffness constant D obtained from the Bloch constant was determined. A large magnetocaloric effect has been observed in both samples (x = 0.05 and x = 0.10): the maximum entropy change, \( \left| {\varDelta S_{\text{M} }^{\text{peak}} } \right| \) , reaches the highest value of 3.22 J/kg K under a magnetic field change of 5 T with a RCP value of 56 J/kg for x = 0.10 composition. This opens an interesting opportunity to this compound to compete with materials which work as magnetic refrigerants near room temperature. Besides, we show that the samples follow the conventional behavior of a second-order ferromagnetic transition. This was possible by investigating the critical behavior at the transition region by adopting the modified Arrott plot method. The values of the critical exponents (β, γ, δ and n) are determined and they are between those predicted by the three-dimensional Heisenberg model.     

The influence of disorder on magnetocaloric effect and the order of transition in ferromagnetic manganite (La1−x Nd x )2/3(Ca1−y Sr y )1/3MnO3

Magnetocaloric effect and order of transition in (La_1?x Nd _x )_2/3(Ca_1?y Sr _y )_1/3MnO₃, prepared by conventional solid-state reaction, have been investigated. Using Banerjee criterion, we demonstrate first-order transition for (J1) and (J2 ) as well as second-order transition for (J3 ), (J4 ), and (J5 ) samples. The ΔS _M ^max is ranging between 9.18 Jkg^?1 K^?1 and 4.87 when Nd and Sr content changes leading to relative cooling power (RCP) varying between 330 and 229.35 J/kg. Both ΔS _M ^max and the RCP are found sensitive to the disorder σ ². The universal behavior obtained from ΔS variation curves confirmed the first-order transition for (J1) and (J2 ) samples and second-order transition for (J3), (J4), and (J5 ) samples obtained by Banerjee criterion. All samples with second-order phase transition exhibit inhomogeneous character estimated from local exponent n.     

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.

     

Properties of ceramic manganite (La0.65Sr0.35)1 − x Mn1 + x O3 ± Δ (x = 0, 0.1, 0.2) sintered at a temperature of 1500°C

It has been shown for the first time that the structure of (La_0.65Sr_0.35)_{1 ? x} Mn_{1 + x} O_{3 ± Δ} (x = 0, 0.1, 0.2) ceramics sintered at 1500°C substantially depends on the content of superstoichiometric Mn. It has been found that the average grain size increases from 10 to >100 μm with increasing x from 0 to 0.2. It has been revealed that grains of the ceramic samples sintered from powders with excess Mn have an internal nanoscale layered structure. A correlation has been revealed between the size and structure of grains and and the magnetoresistive properties.     

Ionic and electronic conductivity in CaTi1−xFexO3−δ (x=0.1–0.3)

Electrical conductivity measurements on CaTi_1?xFe_xO_3?δ (x=0.1, 0.2, 0.3) were performed on polycrystalline pressed and sintered tablets using the van der Pauw four point method in controlled atmospheres. The results were interpreted to reflect n-type, ionic and p-type conductivity at different oxygen partial pressures. An increasing iron content increases the number of oxygen vacancies and increases the ionic conductivity at high temperatures, but also increases the tendency of ordering, which suppresses the ionic conductivity at more moderate temperatures. These findings are in accordance with the phase diagram of the system CaTiO₃-CaFeO_2.5 based on X-ray and Mössbauer studies.