Magnetocaloric effect and magnetic properties of La_（0.9）Ce_（0.1）（Fe_（0.99）Mn_（0.01））_（11.6）Si_（1.4）H_（1.6） compound

Magnetocaloric effect and magnetic properties of La0.9Ce0.1(Fe0.99Mn0.01)11.6Si1.4 and its hydride La0.9Ce0.1(Fe0.99Mn0.01)11.6Si1.4H1.6 were investigated. The Curie temperature of La0.9Ce0.1(Fe0.99Mn0.01)11.6Si1.4 was increased by hydrogen absorption. XRD patterns showed that the structure of La0.9Ce0.1(Fe0.99Mn0.01)11.6Si1.4H1.6 remained NaZn13-type. The Curie temperature (TC) of the sample was increased from 174 K to 331 K. The homogeneity of the hydrogen absorption for La0.9Ce0.1(Fe0.99Mn0.01)11.6Si1.4H1.6 was proven very well by the random measurement of DSC. The magnetic entropy △SM of La0.9Ce0.1(Fe0.99Mn0.01)11.6Si1.4H1.6 had peak at 326 K. The peak value of-△SM-was 12.3 and 7.8 J/(kg.K) under magnetic field change of 0-2 T and 0-1 T,respectively,which was comparable with Gd5Si2Ge2. The negative slope and inflection point of the Arrott curve indicated that the first-order magnetic transition of La0.9Ce0.1(Fe0.99Mn0.01)11.6Si1.4 was reserved after hydrogen absorption.     

Sintering of Ce（0.8）Sm（0.2）O（1.9）

Ce0.8Sm0.2O1.9（SDC）powder was prepared with an oxalate coprecipitation route.SDC solid solutions were sintered at various temperatures ranging from 1100~1450 ℃,and characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,density measurements,and electrical conductivity measurements.The optimized processing parameters for densification were to uniaxially press the sample at 200~400 MPa and sinter it at 1350~1400 ℃ for 4 h.The density of the sintered pellets was 〉90% of the theoretical density;their soakage was 〈0.5%;and the average grain size was 1~2 μm.The conductivities of the typical sintered specimen were 0.0133 and 0.0211 S·cm-1 at 550 and 600 ℃,respectively;Its activation energy for ionic conductivity was 0.62 eV.The dense SDC bulk material could be used as the electrolyte layer of low temperature solid oxide fuel cells.     

A study on hydrogen storage and electrochemical properties of La_（0.55）Pr_（0.05）Nd_（0.15）Mg_（0.25）Ni_（3.5） （Co_（0.5）Al_（0.5））_x （x=0.0, 0.1, 0.3, 0.5） alloys

The La0.55Pr0.05Nd0.15Mg0.25Ni3.5(Co0.5Al0.5)x(x=0.0, 0.1, 0.3, 0.5) alloys were prepared by magnetic levitation melting under an Ar atmosphere, and the effects of Co and Al on the hydrogen storage and electrochemical properties were systematically investigated by pressure composition isotherms, cyclic voltammetry, Tafel polarization and electrochemical impedance spectroscopy testing. The results showed that the alloy phases were mainly consisted of (La,Pr)(Ni,Co)5, LaMg2Ni9, (La,Nd)2Ni7 and LaNi3 phases, and the cell volumes of (La,Pr)(Ni,Co)5, LaMg2Ni9, (La,Nd)2Ni7 and LaNi3 phases expanded with Co and Al element added. The hydrogen storage capacity initially increased from 1.36 (x=0) to 1.47 wt.% (x=0.3) and then decreased to 1.22 wt.% (x=0.5). The discharge capacity retention and cycle stability of the alloy electrodes were improved with the increase of Co and Al contents. The La0.55Pr0.05Nd0.15Mg0.25Ni3.5(Co0.5Al0.5)0.3 alloy electrode possessed better electrochemical kinetic characteristic.     

Structure and high-temperature electrochemical properties of La_（0.60）Nd_（0.15）Mg_（0.25）Ni_（3.3）Si_（0.10） hydrogen storage alloys

The structure and high-temperature electrochemical properties of the as-cast and annealed (940 °C, 8 h) La0.60Nd0.15Mg0.25Ni3.3Si0.10 hydrogen storage alloys were investigated. The X-ray diffraction revealed that the multiphase structure of the as-cast alloy with LaNi5 phase as the main phase was converted into a double-phase structure with La2Ni7 phase as the main phase after annealing. The surface morphology studied by scanning electronic microscope (SEM) showed that the annealed alloy had a much higher anti-corrosion ability than the as-cast alloy. Both alloys presented excellent activation characteristics at all test temperatures. The maximum discharge capacity of the as-cast alloy decreased when the test temperature increased, while the temperature almost had no effect on the annealed alloy. As the test temperature increased, the cyclic stability and charge retention of both alloys decreased, and these properties were improved significantly by annealing.     

Magnetic Coupling in La0.67 Ca0.33 MnO3／La0.67 Sr0.33 CoO3／La0.67 Ca0.33MnO3 Trilayer Films

The perovskite La0.67 Ca0.33 MnO3/La0.67 Sr0.33 CoO3/La0.67 Ca0.33 MnO3 trilayers were fabricated by a facing-target sputtering technique and their magnetotransport properties were investigated. The magnetoresistance is dependent on spacer thickness and dramatically decreases when La0.67 Sr0.33 CoO3 layer is thick enough because of its short-circuiting effect. Different from La0.67 Ca0.33 MnO3 single layer, trilayer films with thin La0.67 Sr0.33 CoO3 spacer have the enhanced metal-semiconductor transition temperature (TMS) of La0.67 Ca0.33 MnO3 layers. The magnetic coercivity Hc shows a nonmonotonic behavior with changing the spacer layer thickness at 230 K. The waist-like hysteresis indicates that there is an indirect exchange coupling between the top and bottom Lao.67 Ca0.33 MnO3 layers across the spacer La0.67 Sr0.33 CoO3 layer.     

改善钙钛矿材料室温下磁电阻效应的有效途径

将Ag2O与La0.67Ca0.25Sr0.08MnO3均匀混合经高温烧结后形成La0.67Ca0.25Sr0.08MnO3/xAg两相复合体系。随Ag掺入量的增加,样品的电阻率明显下降,磁化强度有小量下降,TC及电阻率的峰值温度TP没有明显变化。304K温度下,从x=0.25样品中得到最大磁电阻效应,磁电阻比约为41%,分别是La0.67Ca0.25Sr0.08MnO3和La0.67Sr0.33MnO3样品的1.6及10倍。磁电阻的本征效应和传导电子在界面处的自旋相关散射作用是产生室温下增强磁电阻效应的主要原因。结果说明,利用离子的掺杂效应调整居里温度到室温附近,再结合金属/钙钛矿复合体系界面属性丰富的特性来制备庞磁电阻材料是提高室温磁电阻有效的途径,这对应用研究具有十分重要的意义。     

Spin Dependent Hopping in Ti Doped La0.67 Ca0.33 MnO3

A systematic investigation of the magnetic and transport properties of Ti doped La0.67Ca0.33MnO3 was reported. The Ti substitution for Mn ions results in a reduction in ferromagnetism and conductivity. The metal-insulator transition temperature is close to Curie temperature which decreases from 274 to 82 K as x increases from 0 to 0.17. The most important effect of Ti doping is to introduce spin clusters in the samples due to the distortion of local lattice and the inhomogeneous magnetic structure induced primarily by the random distribution of Mn ions. A maximum magnetoresistance ratio as large as 90% in 1 T at 122 K was obtained for the sample with x =0. 055, which is four times larger than that obtained for LCMO sample at 272 K. There is a remarkable field-history dependent MR in the cooling process for the doped samples while such phenomenon disappears in the warming run. The resistivity follows well the variable range hopping behavior in paramagnetic state. Both the size effect and spin dependent hopping of carriers between the spin clusters should be considered in this system.     

Anisotropic Magnetoresistance of Perovskite La_(2/3)Ca_(1/3)MnO_3/YBa_2Cu_4O_8/La_(2/3)Ca_(1/3)MnO_3 Trilayer Films

Magneticanisotropyhasbeenfoundinmanyex oticmaterials,suchasmetallicsuperlatticeswithgi antmagnetoresistanceeffectsandthehightempera turesuperconductingoxideYBa2Cu3O7 δ[1～4].The anisotropycouldfindnovelusesinnewdeviceappli cations.Agoodexampleisthehig…     

Thickness Effect on Electronic Transport in Single Crystal La0.67 Ca0.33 MnO3 Thin Films

Lattice strain in thin films induced by film-sub-strate mismatchis one source for variant magnetic andresistive behaviors especiallyfor the manganite perovs-kites for which the lattice effects are wellestablished[1~5].It has been also reported that low-fi…     

Superconducting Transition Temperature in YBa2Cu4O8/La2/3 Ca1/3 MnO3 / YBa2 Cu4O8 Heterostructure

YBa2Cu4O8/La2/3 Ca1/3 MnO3/YBa2Cu4O8 （ Y-124/LCMO/Y- 124） heterostructure was prepared by facing-target sputtering technique. The oscillatory superconducting transition temperature was observed when the thickness of LCMO d L is larger than critical thickness d L^CR. The metal-insulator transition temperature can only be detected at d L 〉 d L^CR. The dependence on the spacer layer in LCMO/Y-124 systems suggests strongly the interplay of ferromagnetic and superconducting couplings.     

Ag掺杂对La0.8Ca0.2MnO3电输运性质及磁电阻的影响

用固相反应法制备La1-x(Ca1-yAgy)xMnO3(y=0.0,0.2,0.4,0.6,0.8)系列样品,通过X射线衍射(xRD)谱,电阻率-温度(p-T)曲线,磁电阻-温度(MR-T)曲线,研究了在A位同时掺入一价、二价元素而保持Mn3+/Mn4+比值(摩尔比n(A)/n(B))不变的La1-x(Ca1-yAgy)xMnO3体系A位离子半径＜rA＞及A位离子的无序度σ2对电输运性质及磁电阻的影响.结果表明:所有样品的绝缘体,金属相变温度基本不变,用A位离子半径＜rA＞及A位离子的无序度对电输运性质影响的竞争给予解释;在0.8T磁场下,y=0.0样品在133～26K温区MR基本保持23％以上,y=0.6样品在209 ～ 131 K温跨区MR都在23％以上,在如此宽温区产生如此大的MR有利于MR的实际应用;MR的温度稳定性的机制是本征磁电阻与隧穿磁电阻竞争的结果.     

Magnetic properties and magnetocaloric effects of manganite (La_0.8Ho_0.2)_2/3Ca_1/3MnO₃ and (La_0.5Ho_0.5)_2/3Ca_1/3MnO₃

We reported the magnetic properties and magnetocaloric effects(MCE) of(La0.8Ho0.2)2/3Ca1/3MnO3 and(La0.5Ho0.5)2/3Ca1/3MnO3 nanoparticles by sol-gel technique.With this method,we were able to obtain the samples with particle diameters ranging from 50 to 200 nm.In the(La1-xHox)2/3Ca1/3MnO3 compound,an external magnetic field induced a magnetic transition from an paramagnetic phase to a ferromagnetic phase above Ts=105-135 K,leading to magnetocaloric effects.The maximum value of ΔSM was 1.19 J/(kg·K) at 100 K and 2.03 J/(kg·K) at 152 K for a magnetic field change of 5 T.Because both samples had large relative cooling power(RCP) and wide δTFWHM,the study on systems with the(La1-xHox)2/3Ca1/3MnO3-related magnetic transitions may open an important field in searching good magnetic materials.     

High Temperature LFMR in Yttrium Doped Perovskite Manganites

Porous ceramic samples of Y doped perovskite manganites were prepared. In these samples, the transition from high temperature paramagnetic insulator to low temperature ferromagnetic metal as well as the low field magnetoresistance (LFMR) effect at the low temperature is similar to that in dense samples. Opposite to that in dense samples, LFMR effect in porous sample is observed at the high temperature close to the peak of MR-T curves. The results suggest that the high temperature LFMR effect and the applicable colossal magnetoresistance (CMR) materials could be obtained by controlling the microstructures of this class of perovskite manganites.     

La_(0.5)Sm_(0.2)Sr_(0.3)MnO_3/Ag_x二相复合体的结构及磁电阻

用固相反应法制备La0.5Sm0.2Sr0.3MnO3/(Ag2O)x/2(x=0.00,0.04,0.08,0.25,0.30)样品,通过X射线衍射谱线(XRD),扫描电子显微镜(SEM)照片及SEM能谱(EDS),ρ-T曲线,研究样品的输运行为及磁电阻效应。结果表明:少量掺杂时Ag全部挥发。掺杂量较多时,挥发后多余的Ag主要以金属态包覆在母体颗粒的表面,使体系形成两相复合体。掺Ag为30%摩尔比时,样品的电阻率较低掺杂样品的电阻率降低一个数量级,在300K、0.5T磁场下,磁电阻明显增强,达到9.4%,这与颗粒母体界面结构的改善有关,也与材料电阻率的降低有关。