Magnetic properties and magnetocaloric effect of Sr-doped Pr0.7Ca0.3MnO3 compounds

In this work, we pointed out that Sr substitution for Ca leads to modify the magnetic and magnetocaloric properties of Pr_0.7Ca_0.3-xSr_xMnO₃ compounds. Analyzing temperature dependence of magnetization, M(T), proves that the Curie temperature (T_C) increased with increasing Sr content (x); T_C 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 Pr_0.7Ca_0.3-xSr_xMnO₃ compounds has been investigated through their temperature and magnetic field dependences of magnetic entropy change ΔS_m(T,H) and the change of the specific heat change ΔC_P(T,H). Under an applied magnetic field change of 10 kOe, the maximum value of -ΔS_m is found to be about 3 J/kg·K, and the maximum and minimum values of ΔC_P(T) calculated to be about ±60 J/kg·K for x = 0.3 sample. Additionally, the critical behaviors of Pr_0.7Ca_0.3-xSr_xMnO₃ compounds around their T_C 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.     

Structural and magnetic phase transitions in Pr0.7Ca0.3MnO3 at high pressures

JETP Letters - The crystal structure and Raman spectra of Pr0.7Ca0.3MnO3 manganite at high pressures of up to 30 GPa and the magnetic structure at pressures of up to 1 GPa have been studied. A...     

Structural and magnetic phase transitions occurring in Pr0.7Sr0.3MnO3 manganite at high pressures

The crystal and magnetic structures and the vibrational spectra of Pr_0.7Sr_0.3MnO₃ manganite are studied within the pressure range up to 25 GPa by methods of X-ray diffraction and Raman spectroscopy. Neutron diffraction studies have been performed at pressures up to 4.5 GPa. The magnetic phase transition from the ferromagnetic phase (T _C = 273 K) to the A-type antiferromagnetic phase (T _N = 153 K) is found at P ≈ 2 GPa. This transition is characterized by a broad pressure range corresponding to the phase separation. The Raman spectra of Pr_0.7Sr_0.3MnO₃ measured under high pressures significantly differ from the corresponding spectra of the isostructural doped A_{1 ? x} A′ _x MnO₃ manganites, (where A is a rare-earth ion and A′ is an alkaline-earth ion) with the smaller average ionic radius 〈r _A〉 of A and A′ cations. Namely, the former spectra do not include clearly pronounced stretching phonon modes. At P ～ 7 GPa, there appears the structural phase transition from the orthorhombic phase with the Pnma space group to the orthorhombic high-pressure phase with the Imma symmetry. In the vicinity of the phase transition, anomalies in the pressure dependences of the lattice parameters, unit cell volume, and phonon frequencies corresponding to the characteristic lattice vibration modes are observed.     

Manganite Pr0.7Ca0.3MnO3 films obtained by extraction pyrolysis

The magnetic properties of manganite Pr_0.7Ca_0.3MnO₃ films, obtained for the first time by the extraction pyrolysis method, have been investigated. The effect of the synthesis temperature of the magnetic properties of the manganite films is demonstrated.     

Transport properties of Pr0.7Ca0.3MnO3/Nb:SrTiO3 heterojunctions

Current–voltage (J–V) characteristics of epitaxial hetero-junctions composed of Pr_0.7Ca_0.3MnO₃ and Nb:SrTiO₃ were studied under forward and reversed bias conditions. Detailed analysis showed that the J–V characteristics of these heterojunctions can be well-fitted by the thermally-assisted tunnelling model. While the dielectric constant of Nb:SrTiO₃ extracted under the forward bias was about one order of magnitude smaller than that of bulk SrTiO₃, the value obtained under reverse bias was very close to that of the bulk SrTiO₃. The result can be explained by the existence of an interface layer on the Nb:SrTiO₃ substrate with a smaller effective dielectric constant. The current finding suggested that the properties of interface layer should be taken into account in order to accurately simulate the J–V characteristics of such heterojunctions.     

Transport and Magnetic Properties of Eu and Sr Doped Manganite Compound La0.7Ca0.3MnO3

The effect of simultaneous increase in carrier density and size-disorder on the transport and magnetic properties of La_0.7Ca_0.3MnO₃ has been investigated by studying the (La_{0.7 − 2x} Eu _x )(Ca_0.3Sr _x )MnO₃ (0.05 ≤ x ≤ 0.2) (LECSMO) compounds. These compounds have been compared with standard La_{1 − x} Ca _x MnO₃ (0.3 ≤ x ≤ 0.5) (LCMO) in which carrier density alone varies. In LECSMO, the insulator-metal transition temperature (T _p) decreases from 180 K for x = 0.05 to 80 K for x = 0.15 sample vis-à-vis ∼240 K for x = 0.35 to ∼225 K for x = 0.45 in LCMO system. Similarly, the Curie temperature (T _C) in LECSMO, decreases from 205 K for x = 0.05 to 75 K for x = 0.2 sample against 240 K for x = 0.35 to ∼220 K for x = 0.45 in LCMO system. Also, in LCMO the T _C and T _p are coincident whereas, in LECSMO system, with increasing x there is an increasing disparity between the two. At 5 K, in the metallic region, a large MR (>90%) is observed in x = 0.15 sample and is discussed in terms of phase segregation and inter-grain magnetoresistance.     

室温下Pr0.7Ca0.3MnO3薄膜的瞬态光响应特性

在室温下利用波长532 nm,脉冲宽度7 ns的纳秒脉冲激光研究了不同电压和激光能量密度作用下Pr0.7Ca0.3MnO3薄膜的瞬态光响应特性.在激光能量密度为275.16 mJ/cm2时,其最大电阻变化率达到92.3%,响应时间约36 ns.室温下电压变化对薄膜的光响应特性影响不大,而诱导光能量密度的影响则很明显,能量密度越大,电阻变化越大,响应时间越短,并且电阻变化和响应时间均与激光能量密度呈非线性关系.这种光响应来源于薄膜中的光致非稳态绝缘体-金属相变,有望在新型光电器件上获得应用.     

Magnetic and magnetooptical properties of polycrystalline films of La0.7Sr0.3MnO3

Magnetic circular dichroism (MCD) is studied in comparison with the magnetic behavior of polycrystalline films of La_0.7Sr_0.3MnO₃ (LSMO) deposited on a single crystal of zirconium oxide stabilized by yttrium (YSZ). It is found that the bands observed in MCD spectra are characterized by different temperature dependences.     

Magneto-transport properties of La0.7Ca0.3MnO3/SrTiO3/La0.7Ce0.3MnO3 tunnel junction

Hole-doped rare-earth manganite La_0.7Ca_0.3MnO₃ and the electron-doped manganite La_0.7Ce_0.3MnO₃ both show a metal-insulator transition around 250 K associated with a ferromagnetic transition and colossal magnetoresistance. In an earlier publication we have reported the rectifying characteristic of La_0.7Ca_0.3MnO₃/SrTiO₃/La_0.7Ce_0.3MnO₃ tunnel junction at room temperature, showing that it is possible to fabricate a diode out of the polaronic insulator regime of doped manganites. Here we report the magneto-transport properties of such a tunnel junction above and below the metal-insulator transition. We show, from the large positive magnetoresistance of the tunnel junction at low temperature, that La_0.7Ce_0.3MnO₃ could be a minority spin carrier ferromagnet. The implication of this observation is discussed.     

Giant volume magnetostriction and colossal magnetoresistance in La0.7Ba0.3MnO3 at room temperature

In single-crystal La_0.7Ba_0.3MnO₃, giant volume magnetostriction was observed for the first time to reach 2.54 × 10^?4 at room temperature and a still larger level of 4 × 10^?4 at the Curie point T _C = 310 K in a magnetic field of 8.2 kOe. At the same temperatures and magnetic field, this effect is complemented by a colossal magnetoresistance of 15.2 and 22.7%, respectively. The volume magnetostriction ω and magnetoresistance Δρ/ρ follow similar patterns in the proximity of T _C; namely, ω and Δρ/ρ are negative, maxima are observed in the |ω|(T) and |Δρ/ρ|(T) curves, and the ω and Δρ/ρ isotherms do not saturate in the highest fields applied. These phenomena are assigned to the fact that, in the above composition, there exists a two-phase magnetic (ferromagnetic-antiferromagnetic) state induced by strong s–d exchange.     

Effect of strontium deficiency on the transport and magnetic properties of Pr0.7Sr0.3MnO3

Ceramic samples of Pr_0.7Sr_0.3−x□_xMnO₃ with x⩽0.2 have been investigated by various techniques including ⁵⁵Mn nuclear magnetic resonance, SQUID magnetometry, resistivity and magnetoresistance measurements. On increasing x, the samples remain ferromagnetic at a low temperature with a decreasing Curie temperature, but a metal–insulator transition is observed. Besides, a positive magnetoresistance (MR) at very low temperatures with respect to a negative colossal MR at high temperatures is observed in the Pr_0.7Sr_0.2□_0.1MnO₃ sample.     

Structural,Magnetic, and Magneto-Caloric Properties of Cu-Substituted Nd0.67Ba0.33MnO3 Manganites

Physics of the Solid State - The crystal structure and magnetic properties of polycrystalline Nd0.67Ba0.33Mn1 – xCuxO3 (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.10) manganites...     

Nonlinear optical spectroscopy of (La0.6Pr0.4)0.7Ca0.3MnO3 manganite

The electronic structure of doped orthorhombic (La_0.6Pr_0.4)_0.7Ca_0.3MnO₃ manganite is studied using the second optical harmonic generation method. The analysis of the frequency and polarization dependence of the intensity related to the second harmonic demonstrates that the applied magnetic field stimulates a transition of spin-polarized electrons from the d orbitals of Mn ions to the p orbitals of O₂ ligands. The manifestation of such transition in the orthorhombic RMnO₃ manganite with the distorted crystal lattice is attributed to the dipole-forbidden electronic transitions arising in the applied magnetic field. The mechanism of magnetooptical activity induced by the charge transfer between Mn and O ions is discussed.     

Pr0.7(Sr1-xCax)0.3MnO3多晶体系中的小极化子性质

采用基于柠檬酸体系的溶胶-凝胶法制备了Pr0.7(Sr1-xCax)0.3MnO3系列的多晶块材, 同时还用脉冲激光沉积技术(PLD)在SrTiO3(100)衬底上外延生长了同一系列的薄膜, 系统研究了它们的晶格结构和电输运行为. 多晶和薄膜样品都具有正交晶格结构, 电输运行为在居里温度TC以上的高温顺磁相都很好地符合了绝热小极化子模型. 用绝热小极化子模型对两者的电阻率温度曲线进行拟合, 多晶和薄膜样品的拟合结果在定性上是一致的. 随着Ca掺杂量的增加, 极化子跳迁能Ehop变大,ρ0先减小后增大. 实验结果说明, 只要多晶样品制备的质量好, 缺陷、晶界的影响可以大大降低, 多晶样品的电输运行为也能表现出本征的小极化子性质.     

Magnetic properties of La0.7Sr0.3MnO3 manganite films obtained by extraction pyrolysis

The structural and magnetic properties of manganite films obtained for the first time by extraction pyrolysis have been experimentally investigated. It has been established that at annealing temperatures T ≈ 600 K the films are fine-grained and have spin-glass-like magnetic properties. Prolonged annealing at T ≥ 700 K results in grain overgrowth and ferromagnetic behavior.     

Pr0.7Ca0.15Ba0.15MnO3 manganite: Electron paramagnetic resonance,conductivity and susceptibility

We present a systematic experimental study of single phase Pr_0.7Ca_0.15Ba_0.15MnO₃ manganites. Ferromagnetic insulating state is observed in wide temperature range between the Curie point and peak resistivity temperature. Two approaches for the analysis of the electron paramagnetic resonance (EPR) line-width (ΔH) were considered in details. The first approach is based on the proportionality between the temperature dependencies of resistivity and inverse ΔH. The second approach is based on the model where ΔH∝1/Tχ_dc (χ_dc is a static magnetic susceptibility). For the studied Pr_0.7Ca_0.15Ba_0.15MnO₃ manganites the resistivity, associated with the grain boundary scattering, follows Mott law; the EPR line-width, suggested to be an intrinsic property, is well described with polaron hopping model.     

Dielectric response of PVC polymer loaded with Ba0.3Na0.7Ti0.3Nb0.7O3 ceramic powder

Dielectric response of poly(vinyl chloride), (PVC), loaded with different amount of Ba_0.3Na_0.7Ti_0.3Nb_0.7O₃ (BNTN) ceramic powders was investigated in frequency range 100 Hz–1 MHz and temperature range 100–450 K. Ceramic solid solution of barium titanate and sodium niobate with composition Ba_0.3Na_0.7Ti_0.3Nb_0.7O₃ was obtained from BaCO₃, TiO₂, Na₂CO₃ and Nb₂O₅ by conventional method. Powders were prepared by grinding of ceramics. The obtained ceramics, used to produce BNTN–PVC composites, are characterized by the relaxor behaviour with a broad peak of dielectric permittivity ε′ at T _m ≈ 230 K. The microstructure of the powders was observed and the grain size was estimated using scanning electron microscope Hitachi S-4700. The EDS analysis confirms the qualitative and quantitative chemical composition of powders and ceramics. The BNTN–PVC composite samples of 0-3 connectivity were prepared from ceramic and polymer powders by hot-pressing method. The dielectric response of the composites displays features originated from the PVC polymer modified by those of BNTN ceramics. The relaxation time of the α-process of PVC obeys the Vogel–Fulcher law and decreases with increasing volume fraction of the ceramics.     

Transport properties in La0.7-xGdxSr0.3MnO3 system

The influence of Gd doping at La-site on the electrical transport properties and the colossal magnetoresistance of La0.7-xGdxSr0.3MnO3 (x = 0.00, 0.10, 0.15, 0.20, 0.30, 0.40, 0.50, 0.60, and 0.70) is studied. The experimental results indicate that the transport properties exhibit abnormal behavior under high doping condition. For x = 0.50, we find that a transition from metal to insulator occurs after the occurrence of insulator-metal transition near Tc, which seldom occurs in ABO3 structure. For samples x = 0.60 and 0.70, it exhibits insulator behavior far above Tc. These abnormal behaviors are attributed to different magnetic background, i.e. the system undergoes a transition from long range ferromagnetic order to the cluster-spin glass state and further to antiferromagnetic order.     

Magnetic Properties of Sm0.7Ho0.3Fe3(BO3)4

The magnetic properties of a ferroborate single crystal of substituted composition Sm_0.7H_0.3Fe₃(BO₃)₄ with competing Sm-Fe and Ho-Fe exchange interactions are studied. The measured properties and effects are interpreted in terms of a general theoretical approach based on the molecular field approximation and calculations using the crystal field model for a rare-earth ion. The experimental temperature dependences of the initial magnetic susceptibility in the temperature range 2?C300 K, the anomalies in the magnetization curves for B ?? c and B ?? c in fields lower than 1.2 T, and the field and temperature dependences of magnetization in fields lower than 9 T are described. The crystal field parameters and the parameters of the R-Fe and Fe-Fe exchange interactions are determined during the interpretation of the experimental data.