钙钛矿La0.67Ca0.33Ti0.01Mn0.99O3的磁致冷性能研究

利用固相烧结的方法制备了La0.67Ca0.33Ti0.01Mn0.99O3磁致冷材料.室温XRD分析表明该材料主要由主相的正交钙钛矿结构锰氧化物和少量La2O3杂相构成.利用VSM测量了样品在磁场下随温度变化的磁化曲线(M-T曲线)和居里温度附近的等温磁化曲线(M-H曲线),通过M-H曲线的一阶导数可得到样品的居里温度Tc(139 K),从不同温度的等温磁化曲线(M-H)计算得到材料在1.5T下的最大磁熵变△SM =0.8 J.(kg·K)-1,由M2-H/M的Arrott曲线表明La0.67Ca0.33 Ti0.01Mn0.99O3相变类型属于二级相变.由于二级相变不伴随晶格体积的变化和潜热的释放,仅是磁性材料磁畴从无序-有序的连续变化,所以二级相变的磁熵变可能较小,但是由于它是个连续的变化过程,发生相变的温区较大,造成的损耗较小.Ti4+离子的掺杂取代的是原有晶格中Mn4+离子(B位)的位置,两者之间不同的离子半径使得晶格发生Jahn-Teller 畸变,同时降低了Mn4+的浓度,使得Mn4+与Mn3+离子之间的双交换作用减弱,铁磁耦合作用减小,导致居里温度降低.该材料体系由于二级相变的温度区间较大,表明材料潜在的致冷能力较强,因此具有较好的应用前景.     

La0.67Ca0.33MnO3超巨磁阻多晶材料的制备及性质

介绍了用不同沉淀剂的共沉淀法来制备La0．67Ca0．33Mn03超巨磁阻多晶材料。通过XRD检测及R—T曲线测量研究了样品的结构性能及电输运性质。共沉淀法制备的样品基本上是单相均匀的四方结构。样品的半导体-金属转变点Tp达270K。     

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.     

La0.67Sr0.33MnO3/La0.75MnO3/La0.67Sr0.33MnO3庞磁电阻三层膜的制备及结构形貌和性能

用磁控溅射法在(001)取向的LaAlO3单晶基片上制备了La0.67Sr0.33MnO3/La0.75MnO3/La0.67Sr0.33MnO3外延三层膜.结果表明, 与La0.67Sr0.33MnO3单层膜相比较, 外延三层膜的X射线衍射图上出现3级衍射峰, 每级衍射峰由3个峰组成其中强度最高的衍射峰代表衬底的衍射;次强的代表上下两层La0.67Sr0.33MnO3膜;最弱的代表中间La0.75MnO3层.三层膜的二维原子力表面形貌为均匀的小颗粒状, 表明其三维形貌为外延岛状柱状晶.这种三层膜的巨磁电阻效应与中间层La0.75MnO3的厚度有关, 当中间层厚度为70 nm时, 其最大磁电阻值可达到32%, 这个结果优于La0.67Sr0.33MnO3 单层膜的巨磁电阻效应.其可能的原因是电子在界面上的散射、层间的交互作用, 以及点阵错配导致的内应力的综合结果.     

在La0.67Sr0.33MnO3中保持Mn3+:Mn4+=2:1的镁掺杂效应

通过测量样品的M-T曲线、M-H曲线、ESR曲线、红外光谱、拉曼光谱、ρ—T曲线和MR-T曲线，研究了双掺杂La0.67 1.33xSr0.33-1.33xMn1-xMgO3(0．00≤x≤0．25)体系的磁电性质和输运行为.对x=0．10和0．15，在温度高于Tc^onset时出现相分离；对x=0．20和0．25，在低温区出现了反铁磁(AFM)行为，x=0．05的ρ-T关系存在金属-绝缘体(MI)转变；x≥0.15的所有样品在测量温区内显示绝缘体行为：这些奇异现象用M-H关系、ESR曲线和拉曼光谱给予了很好的解释.     

Magnetocaloric Effect in Colossal Magnetoresistance Material（ La0.6 Dy0.1 ） Sr0.3 MnO3

The magnetocaloric effect was discovered in 1881by Warburg[1]. When a magnetic field is applied to amagnetic material, the unpaired spins are aligned par allel to the magnetic field, which lowers the magneticentropy and causes the sample to heat up. I…     

Transport Property of La0.67-xSmxSr0.33MnO3 at Heavy Samarium Doping （0.40≤x≤0.60）

The influence of heavy samarion （Sm） doping （0.40≤x≤0.60） on magnetic and electric properties of La0.67-xSmxSr0.33MnO3 was investigated by measuring the magnetization-temperature （M - T） curves, magnetization-magnetic density （ M - H） curves, resistivity-temperature （ρ- T） curves and magnetoresistivity-temperature （ MR - T） curves of the samples under different temperatures. It is found that, form from long-range ferromagnetic order to spin-cluster glass with the increase of Sm doping amount, the samples transstate and anti-ferromagnetic state; and when x = 0.60, the transport property becomes abnormal under magnetic background; and the magnetic structure changes and extra magnetic coupling induced by doping leads to colossal magnetoresistance effect. The transport mechanism of metallic conduction at low temperature is mainly electron-magneton interaction and can be fitted by the formula ρ = ρ0 ＋ AT^4.5, and the insulatorlike transport mechanism on high temperature range is mainly the function of variable-range hopping and can be fitted by the formula ρ = ρ0exp（T0/T）^1/4. In the formulas above, p is resistivity, T is temperature, and A, ρ0, T0 are constants.     

Magnetic Properties and Colossal Magnetoresistance of La0.7Sr0.3MnO3 Materials Doped with Fe

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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.     

镱的高掺杂对La0.67Sr0.33MnO3体系磁电性质的影响

研究了La0.67-xYbxSr0.33MnO3(x=0.40,0.50,0.60)体系的M-T曲线、M-H曲线、ρ-T曲线和MR-T曲线.实验结果表明:随着Yb掺杂的增加,体系从长程铁磁有序向自旋团簇玻璃态和反铁磁状态转变;高掺杂时的输运行为在其磁背景下发生异常,Yb掺杂引起的磁结构的变化将导致CMR效应.     

La_(0.67)Ca_(0.33)Mn_(1-x)T_xO_3低场下的大磁阻效应

研究了在低磁场 H≤ 0 .8T区间下的材料 L a0 .6 7Ca0 .33Mn1 - x Tx O3( T=Fe或 Ni)中的磁阻效应 ,发现在L a0 .6 7Ca0 .33Mn1 - x Fex O3中用铁替代锰降低了磁阻比 ,但改善了磁阻随磁场变化的线性度。用少量的镍替换锰 ( x=0 .1)时 ,可得到较好的低场高磁阻变化特性     

Large Magnetic Entropy Effect in La2／3Ca1／3MnO3

The magnetocaloric effect in the colossal magnetoresistance material La2/3Ca1/3MnO3 was studied. From the measurements of temperature dependence of magnetization in various magnetic fields, the large magnetic entropy change associated with the ferromagnetic-paramagnetie transition was discovered. This result suggests that perovskite manganites are suitable candidates as working substance in magnetie refrigeration technology.     

Research of Colossal Magnetoresistance in La0.67Ca0.33Mn1-xCrxO3 (0.00≤x≤0.15)

The extraordinary colossal magnetoresistance (CMR) behavior in was found that the substitution with Cr on Mn sites introduces an additional bump in zero-field resistivity. With increasing Cr content, this additional bump grows up drastically while the original resistivity peak associated with magnetic order transition diminishes gradually. Under the applied magnetic field, both bumps of resistivity are deeply compressed, which leads to the appearance of two peaks in CMR response. As a result, the temperature range of CMR response is significantly broadened, spanned from the lowest to near room temperature. These results suggest that Mn-site element substitution could be a potent way of tuning CMR response.     

La0.67-xDyxSr0.33MnO3(0.00≤x≤0.30)中的晶格效应

用固相反应法在La0．67-xDyxSr0．33MnO3中进行了Dy掺杂研究。结果发现，随着Dy掺入量的增加，样品的金属一绝缘体相变温度逐渐降低，对应的峰值电阻渐渐增大，居里温度下降，样品的磁电阻效应迅速增加，对Dy掺杂的作用可以用晶格效应来解释。