Maximum magnetic entropy change modulated toward room temperature in perovskite manganites La0.7−xNdx(Ca,Sr)0.3MnO3

With Nd³⁺ doping and Ca²⁺, Sr²⁺ modulating in the sol–gel technique, a series of polycrystalline perovskite samples La_0.7?xNd_x(Ca,Sr)_0.3MnO₃ (x = 0, 0.05, 0.1, 0.15, 0.20, 0.25) was prepared, their maximum magnetic entropy changes were tuned to room temperature (ΔS_H = ?1.47 J/kg K at 298 k for La_0.45Nd_0.25(Ca,Sr)_0.3MnO₃), an enhancement of the maximum magnetic entropy change (ΔS_H = ?1.89 J/kg K at 315 k) and its refrigerant capacity (about 45.3 J/kg) had also been obtained under 9 kOe magnetic field variation for La_0.55Nd_0.15(Ca,Sr)_0.3MnO₃ contrast to La_0.7(Ca,Sr)_0.3MnO₃.     

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.     

Colossal magnetoresistance in La0.7Ca0.3−xHgxMnO3 (0⩽x⩽0.2) system over wide temperature range

The divalent substitutions in rare-earth manganites create quenched disorders; those are responsible for the modification of magnetoelectronic properties through competition of two phases. In this regards, the substitution of divalent Hg is rarely studied. Here, we present the results on effect of Hg²⁺ substitution in LCMO manganites. We have synthesized polycrystalline samples with nominal compositions La_0.7Ca_0.3?xHg_xMnO₃ (0?x?0.2) by the standard solid-state reaction method. The X-ray diffraction data revealed monophasic nature without a signature of Hg related or any other impurity phase. The ac susceptibility measurement showed variation in the magnetic transition temperature as well as the transition width. The electrical resistivity measurement showed variation in metal-insulator transition temperature. The magnetoresistance (MR) measurements were carried out at 5 T magnetic field. The samples show varying magnitude of magnetoresistance as high as 74.48%. The distinct feature seen with Hg substitution is the increase in the magnitude of MR and temperature range over which MR value is sustained. It also improves the microstructure of the samples.     

Electron-phonon coupling through the orthorhombic to rhombohedral phase transition in La2/3(Ca1−xSrx)1/3MnO3 manganites

We present a systematic study of the Raman modes of a ferromagnetic series of manganites, La_2/3(Ca_1?xSr_x)_1/3MnO₃ with 0?x?1, that presents two different regimes for the variation of the ferromagnetic order temperature, T_C, with Sr doping. This change occurs for x～0.5 when the temperature of the orthorhombic, Pbnm, to rhombohedral, R-3c, structural phase transition, T_t, coincides with T_C. We have analyzed the evolution of the frequencies and widths of the observed Raman modes as a function of Sr doping and temperature. At room temperature, features of the Pbnm structure are detected for compounds up to x=0.6, over the orthorhombic phase limit x=0.45 at 300 K. Octahedra bending modes behave as expected with doping while the tilt mode, which is related to Mn–O–Mn angles and therefore to electronic conduction mechanisms, presents different behaviors in both structures. The tilt frequency is much less sensitive to Mn–O–Mn angle in R-3c than in Pbnm structure indicating a reduction of electron-phonon coupling. Its width presents an anomalous behavior both as a function of doping and temperature with unexpectedly large width in the rhombohedral phase.     

La0.7Ca0.3MnO3超大磁电阻材料正电子湮没研究

对La0.7Ca0.3MnO3材料样品在77K至室温范围的一系列温度, 测量了正电子寿命谱和多普勒展宽谱.结果表明在居里温度附近正电子平均寿命和多普勒线形参数S出现明显变化, 反映了此类化合物晶格结构的不稳定性.本文对此进行了讨论.     

Ultrafast optical probes of dynamic behavior in La0.7Sr0.3MnO3/YBa2Cu3O7 − δ/La0.7Sr0.3MnO3 heterostructure

Ultrafast pump-optical probe spectroscopy was used to analyze carriers dynamics behavior in La_0.7Sr_0.3MnO₃/YBa₂Cu₃O_{7 ? δ}/La_0.7Sr_0.3MnO₃ heterostructure. Our results show the pump signal ΔR/R for higher laser power (such as 21 and 41 mW), below T _c , first goes positive, then crosses zero and goes negative, before relaxing back to equilibrium over a time scale of a few ten picoseconds. We extract the characteristic relaxation time of the different process by fitting the data at these powers with a three-exponential decay. For higher laser power, the long characteristic relaxation time are obtained, which implies the competition between FM order and SC order in the La_0.7Sr_0.3MnO₃/YBa₂Cu₃O_{7 ? δ}/La_0.7Sr_0.3MnO₃ heterostructure.     

(La0.7Sr0.3MnO3)m(BiFeO3)n超晶格间隔的La0.7Sr0.3MnO3三明治结构制备及表征

采用射频磁控溅射的方法在SrTiO3(001,基片上制备了(La0.7Sr0.3MnO3)m(BiFeO3)n超晶格间隔的La0.7Sr0.3MnO3三明治结构.X射线衍射分析证明(La0.7Sr0.3MnO3)m(BiFeO3)n具有明显的超晶格结构.电流垂直于薄膜表面测得的电阻-温度关系表明.La0.7Sr0.3MnO3)m(BiFeO3)n超晶格薄膜在290 K有金属-绝缘体转变,略低于单层La0.7Sr0.3MnO3薄膜的转变温度.电流在0.01-10 mA范围内,观察到薄膜的峰值电阻随电流增大而减小,峰值变化率远大于单层La0.7Sr0.3MnO3薄膜,且随着超晶格周期厚度的增加而增大.低温下,电流-电压曲线表明其导电机制应主要为空间载子限制,且显示较大的电压偏置,表现出肖特基结的特性.     

Structural and magnetic characterisation of the perovskite oxides La0.7Ca0.3−x Na x MnO3

X-ray powder diffraction (XRD) and magnetic measurements were performed in order to investigate the effect of Na⁺ ion substitution for Ca²⁺ ions on the crystallographic structure, the character of magnetic ordering, and the effect of transition temperature in La_0.7Ca_0.3−x Na _x MnO₃ manganites series (0 ⩽ × ⩽ 0.2). All samples crystallise in an orthorhombic structure with the Pnma space group. We have found a strong dependence of structural and magnetic properties on the cation-size disorder parameter σ ². The temperature dependence of magnetization of all samples obeys the Bloch T ^3/2 law. The values of the spin wave constant at low temperature B increase with the increase of x and the Curie temperature decreases. It is concluded that the substitution of Ca by Na⁺ ions causes a decrease in total exchange integral Aof the samples.      

Superparamagnetism and spin-glass-like behavior in (La1−xTbx)2/3Ca1/3MnO3 Perovskite

The magnetic and magnetotransport properties of a series of (La_1−xTb_x)_2/3Ca_1/3MnO₃ oxides have been investigated. Substitutions ranging from x=0.0 to 0.4 have been used. In the largest 〈r_A〉 region, only ferromagnetism is observed. As 〈r_A〉 decreases, there is a transition from ferromagnetic to spin-glass order. As 〈r_A〉 decreases further, the ferromagnetism will completely disappear and a direct transition from superparamagnetic to spin-glass order will occur on cooling.     

A位Eu掺杂对La0.7Ca0.3MnO3电输运的影响

采用传统的固相反应法制备出了高质量的La_(0.7)Ca_(0.3)MnO_3(其中x=0.00,0.05,0.10,0.15,0.20)系列样品,并对其进行了XRD,电阻-温度测量.实验结果表明,随着Eu掺杂量的增加,样品的金属-绝缘体转变温度朝低温附近移动,峰值电阻增加.通过对A位平均离子半径和尺寸无序度的计算,我们发现,由Eu掺杂导致A位平均离子半径〈rA〉减小,尺寸无序度σ2增大,使晶格畸变加剧,从而削弱了eg巡游电子在Mn3 和Mn4 之间的跃迁,同时促进了自旋极化子的形成.我们还对样品电阻-温度曲线的高温部分进行了拟合,对于x=0.00的样品,曲线可以用非绝热近似下的小极化子模型拟合;当掺杂量为x=0.05,0.10,样品的导电机理符合变程跳跃模型;进一步加大掺杂量,当x=0.15,0.20时,样品的电输运行为可以用晶格极化子与自旋极化子共存来解释.     

La0.7Ca0.3MnO3中的双交换作用与小极化子效应

本文研究了La0.7Ca0.3MnO3多晶样品的输运性质、电子顺磁共振(EPR)谱随温度的变化规律.实验发现,材料电阻率、热电势在居里温度附近各形成一个尖锐的峰,而在居里温度上、下有着完全不同的变化规律,EPR谱结果也与之完全吻合.可以认为在居里温度以下双交换作用使eg电子退局域而减小电阻率;而在居里温度以上由Jahn-Teller晶格畸变形成的小极化子使eg电子局域化而增大电阻率.进而证实是双交换作用和小极化子效应共同导致了CMR效应的出现.     

La0.7Ca0.3Mn1-xCrxO3的激光超声研究

运用激光超声的方法测定了五种不同掺杂的巨磁锰氧化物La0.7Ca0.3Mn1-xCrxO3(0.01≤x≤0.60)室温(300 K)下的超声纵波声速.结合热导率的数据,对声速随掺杂浓度的变化原因进行了深入的分析.得到在顺磁绝缘态Cr3 离子的掺入造成了晶格局部涨落的变化,低掺杂使得晶格局部涨落增强,声子U散射弛豫时间减小.     

Magnetic dilution and electronic nature in La0.7Pb0.3Mn1−xZnxO3

We have studied the magnetic dilution and electronic nature of Zn doping on the Mn site in the colossal magnetoresistant material La_0.7Pb_0.3MnO₃ (x≤0.3). Small non-magnetic Zn²⁺ doping tends to separate the system into ferromagnetic clusters to weaken the long-range ferromagnetic order and to reduce the Curie temperature. The spin polarizability of the x=0–0.3 samples is estimated to be 0.97–1.00, indicating that the x=0–0.3 samples are the spin polarized materials in which the conductivity is dominated by single-spin charge carriers. Small doping (x≥0.1) induces the metal–insulator transition and destroys the metallic state with long-range ferromagnetic order.     

Mn位W掺杂对La0.3Ca0.7MnO3体系磁结构的影响

通过对La0.3Ca0.7Mn1-xWxO3(x=0.00,0.04,0.08,0.12,0.15)多晶样品M-T曲线、M-H曲线及ESR谱的测量,研究了Mn位W掺杂对电荷有序体系La0.3Ca0.7MnO3磁结构的影响.结果表明,当掺杂量为0.00≤x≤0.08时,体系存在电荷有序(CO)相,AFM/CO态共存于相变温度以下,电荷有序温度TCO随着W掺杂量的增加而增加;x=0.04时,样品在低温下为FM相与AFM/CO相共存,在CO相建立前、后均有FM从PM中分离出来;当x≥0.12时,CO态融化,在极低温度下存在顺磁-铁磁(PM-FM)相变.     

钙钛矿结构La0.7Ca0.3MnO3-δ外延薄膜中氧的扩散

利用脉冲激光淀积方法，我们制备了外延La0．7Ca0．3MnO3-δ薄膜．实验发现，在30Pa氧分压下制备的薄膜晶格中仅有少量氧空位．可以通过高真空条件下的热处理(热处理温度为780℃)减少样品晶格中的氧含量．室温x射线衍射分析表明，在780℃通过高真空热处理的La0．7Ca0．3MnO3-δ外延薄膜结构不发生改变．通过延长高真空热处理时间可以逐渐减少外延薄膜样品晶格中的氧含量，这反映在样品晶格常数有系统变化及样品绝缘体-金属(I-M)转变温度降低．对样品电阻率．温度曲线的拟合表明，样品晶格中的氧含量是均匀的，这与X射线衍射分析的结果是一致的．通过长时间高真空条件下的热处理可以使样品的I-M转变温度低于10K，而短时间氧气氛中的充氧热处理可以使转变温度回复．这说明样品晶格中的氧扩散出和充入是有区别的．在同一外延薄膜样品可以反复通过高真空条件下的高温脱氧热处理和充氧热处理来改变样品中氧含量，这将方便于将薄膜性能与氧含量的关系进行比较、分析和讨论．     

Ultrafast Photoelectric Effects in Heterojunctions of La0.7Sr0.3MnO3 and Si

Ultrafast photoelectric effects have been observed in p-n heterojunctions of La0.7Sr0.3MnO3(LSMO)/Si and LSMO/SrTiO3_/Si for the first time. The rise time was about 1 ns and the full width at half maximum was about 2ns for the photovoltaic pulse when the heterojunction was irradiated by a laser of -25 ps pulse duration and 1064 nm wavelength. The photovoltaic sensitivity was as large as 435 mV//mJ for a 1064 nm laser pulse. No such pulse was observed with irradiation from a pulsed 10.6 μm CO2 laser.