共沉淀法制备纳米多晶La2／3Sr1／3MnO3及其低场磁电阻性质

利用共沉淀法制备了纳米多晶La2/3Sr1/3MnO3粉体,并研究了相应烧结陶瓷的相结构、显微形貌、电性质以及磁电阻效应.样品中存在高密的晶界,对电子的疏运过程产生明显的散射作用,从而使得样品具有较大的电阻值,并且只有当烧结温度大于800℃时才出现金属绝缘体温度相变.样品的磁电阻是低场磁电阻效应,源于晶界处电子的自旋极化隧穿.烧结温度越低的样品,其磁电阻越大.     

(1-x)La_(0.7)Ca_(0.3)MnO_3/xCoFe_2O_4复合体系的电子输运和磁电阻特性的研究

本文采用两步法制备了(1-x)La0.7Ca0.3MnO3/xCoFe2O4复合样品,对样品的电磁输运特性的测量表明,随着CFO含量的增加,复合样品电阻率增大,且出现双峰,而比饱和磁矩却先减小后增大;另外在样品中观察到了低温增强的磁电阻效应,这一低场磁电阻效应与颗粒晶界处传导电子所受到的自旋极化隧穿和散射有关.     

Co-SiO2颗粒膜的巨磁电阻效应

采用离子束溅射方法在玻璃基片上制备了一系列的Co-SiO2颗粒膜样品,并对样品的巨磁电阻效应进行了研究.在Co35(SiO2)65(体积百分比)颗粒膜样品中,观测到室温下近4%的巨磁电阻效应.研究了不同基片温度对巨磁电阻效应的影响并发现,随着基片温度的升高样品的巨磁电阻效应下降.根据样品的电阻率-温度关系曲线分析,在铁磁金属-非磁绝缘介质颗粒膜中,除了电子自旋相关隧穿效应外,可能还存在其他的导电机制.     

利用金属掩模法制备钉扎型磁性隧道结

利用金属掩模法和Ir22Mn78合金反铁磁钉扎层，制备了四种钉扎型的Py/Al2O3/Py，Py/Al2O3/Co，Co/Al2O3/Py和Co/Al2O3/Co磁性隧道结，坡莫合金的成分为Py＝Ni79Fe21.例如：利用狭缝宽度为100?μm的金属掩模，直接制备出室温隧穿磁电阻比值为17.2%的磁性隧道结Co/Al2O3/Co，其结电阻为76Ω,结电阻和结面积的积矢为76×10_{4Ωμm2，自由层的偏转场为1114?A/m，并且在外加磁场0.1114A·m-1之间时室温磁电阻比值 关键词： 磁性隧道结 隧穿磁电阻 磁随机存储器 金属掩模}     

La0.5Sm0.2Sr0.3MnO3/x（Sb2O3）体系电输运性质及MR温度稳定性

用固相反应法制备La0.5Sm0.2Sr0.3MnO3/x(Sb2O3)(x=0.00,0.02,0.04,0.05,0.075,0.10,0.15)系列样品,通过X射线衍射(XRD)谱、电阻率-温度(ρ～T)曲线、ρ～T拟合曲线、磁电阻-温度(MR～T)曲线,研究了该体系的电输运性质及MR的温度稳定性.所有样品的电输运性质都表现出绝缘体-金属相变,相变温度很高(312K)且基本保持不变,随Sb2O3复合量增大,电阻率迅速增大,类金属导电可以用ρ=ρ0+AT2公式拟合,表明导电机制是电子-电子相互作用,x=0.075的样品,在200～320K温区磁电阻基本保持不变,MR的温度稳定性是晶界引起的隧穿磁电阻与钙钛矿颗粒体相本征磁电阻竞争的结果.     

制备压力对La2/3Sr1/3MnO3的磁电阻效应的影响

 研究了制备压力对纳米块状样品La_2/3Sr_1/3MnO₃的结构、磁学和电学性质的影响。结果表明：样品的晶粒尺寸随制备压力的增加而变小，且不同样品的磁电阻效应不同。低场磁电阻效应在整个实验温区都随制备压力的升高而变弱，这主要是由制备压力使样品晶粒界面连接更紧密所导致；T<200 K时，高场磁电阻效应随制备压力的升高而变强，T>200 K时，高场磁电阻效应随制备压力的升高反而变弱，这主要是由制备压力改变样品的晶粒尺寸所引起。     

Co SiO2颗粒膜的巨磁电阻效应

采用离子束溅射方法在玻璃基片上制备了一系列的Co－SiO\-2颗 粒膜样品,并对样品的巨磁电阻效应进行了研究.在Co35(SiO\-2)65(体积 百分比)颗粒膜样品中,观测到室温下近4%的巨磁电阻效应.研究了不同基片温度对巨磁电 阻效应的影响并发现,随着基片温度的升高样品的巨磁电阻效应下降.根据样品的电阻率温度关系曲线分析,在铁磁金属- 非磁绝缘介质颗粒膜中,除了电子自旋相关隧穿效应外, 可能还存在其他的导电机制. 关键词：     

新半金属Fe2ScO4磁电性能的第一原理计算

应用基于密度泛函理论的第一原理赝势法设计了具有尖晶石结构的新半金属材料Fe2ScO4和FeSc2O4,并对它们进行了几何结构优化．详细计算并分析了它们Fe2ScO4和FeSc2O4的分子磁矩、电子结构等磁电性能,并与Fe3O4的磁电性能进行了比较．结果表明,Fe2ScO4和FeSc2O4均是新发现的典型的铁磁性II B型半金属,而Fe3O4则具有亚铁磁性．Fe2ScO4的分子磁矩为7.14 μB,远大于Fe3O4的4.0 μB和FeSc2O4的3.96 μB．Fe2ScO4具有较高分子磁矩的主要原因是在O2p和Fe3d杂化轨道作用下,Fe3d电子高度自旋极化并且局域化．Fe2ScO4中心离子的平均电子结构近似为,A位Sc：Sc+3s23p43d2和B位Fe：Fe2+t2g3"eg2"t2g#． 通过分析,预测Fe2ScO4比Fe3O4和FeSc2O4具有更大的室温磁电阻以[Ca24Al28O64]4+·4O-(C12A7-O-)为催化剂,在流动反应器中研究了苯羟基化合成苯酚的转化率以及苯酚的选择性．苯的转化率随反应温度增加而增加,苯酚的选择性与温度及反应物的组成有关．此外还通过XRD、EPR和FT-IR对催化剂的结构,表面及内部物种进行了考察．结果表明,C12A7-O-的电正性骨架结构在反应前后几乎没有任何差别,样品内部有部分O-和O2-在反应后转化为OH-．中性物种及负离子中间体分别由Q-MS和TOF-MS所检测．     

LaB0.5Mn0.5O3(B=Co,Fe)的磁性及电输运特性的研究

采用固相烧结工艺制备了钙钛矿锰氧化物LaCo0.5Mn0.5 O3和LaFe0.5 Mn0.5O3,系统研究了样品的磁性、电输运特性和磁电阻效应.室温下LaCo0.5M0.5O3和LaFe0.5Mn0.5O3均呈现顺磁行为.电子顺磁共振得到LaFe0.5Mn0.5O3样品的朗德因子g=1.9661,与Fe3+离子(g=...     

钙钛矿锰氧化物La0.7Sr0.3MxMn1-xO3(M=Cr,Fe)的巨磁电阻效应与磁性

研究了溶胶 -凝胶法制备氧化物巨磁电阻材料的工艺 ,制备了La0 .7Sr0 .3 CrxMn1-xO3 (x =0 ,0 .10 ,0 .15 )和La0 .7Sr0 .3 FexMn1-xO3 (x =0 .0 5 ,0 .10 ,0 .16 )两系列的单相钙钛矿锰氧化物多晶样品 ,并研究了Cr ,Fe替代La0 .7Sr0 .3 MnO3 中部分Mn后对其结构、磁性和巨磁电阻性质的影响 .观察到La0 .7Sr0 .3 Cr0 .15Mn0 .85O3 和La0 .7Sr0 .3 Fe0 .0 5Mn0 .95O3 两个样品的电阻 温度曲线都出现了双峰 .定性讨论了可能产生双峰的机制 .随Cr(或Fe)替代量的增加 ,材料的居里温度很快下降 ,铁磁性减弱 ,导电性降低 ,巨磁电阻效应增强 .但与Fe掺杂相比 ,相同数量的Cr掺杂对材料的影响要小 .     

Negative spin polarization of Fe3O4 in magnetite/manganite-based junctions

Epitaxial oxide trilayer junctions composed of magnetite (Fe3O4) and doped manganite (La0.7Sr0.3MnO3) exhibit inverse magnetoresistance as large as -25% in fields of 4 kOe. The inverse magnetoresistance confirms the theoretically predicted negative spin polarization of Fe3O4. Transport through the barrier can be understood in terms of hopping transport through localized states that preserve electron spin information. The junction magnetoresistance versus temperature curve exhibits a peak around 60 K that is explained in terms of the paramagnetic to ferrimagnetic transition of the CoCr2O4 barrier.     

Giant room-temperature magnetoresistance in polycrystalline Zn(0.41)Fe(2.59)O4 with alpha-Fe2O3 grain boundaries

A tunneling-type magnetoresistance (MR) as large as 158% is observed at T = 300 K in a polycrystalline Zn0.41Fe2.59O4 sample, in which the Zn0.41Fe2.59O4 grains are separated by insulating alpha-Fe2O3 boundaries. The huge room-temperature MR is attributed to the high spin polarization of Zn(0.41)Fe(2.59)O4 grains and antiferromagnetic correlations between magnetic domains on both sides of the insulating alpha-Fe2O3 boundary. The MR exhibits strong temperature dependence below 100 K and its magnitude is enhanced to reach 1280% at 4.2 K, which may arise from the Coulomb blockade effect.     

Finding universal correlations between cationic disorder and low field magnetoresistance in FeMo double perovskite series

We search for general patterns that explain the low field magnetoresistance at low temperatures in the system A(2-x)A'xFeMoO6. The observed linear dependence of the low field magnetoresistance with the saturation magnetization for the series is related to the antisite disorder at the Fe and Mo sites. This is explained in terms of a spin dependent crossing of intragranular barriers originated from the presence of antiferromagnetic SrFeO3 patches that naturally develop when antisite disorder occurs in the double perovskite. The presence of a moderate level of antisite disorder is at the very root of low field magnetoresistance although effects such as disorder distribution, connectivity, or morphology add their contribution.     

Effect of a SiO2 coating on the magnetic properties of Fe3O4 nanoparticles

In this work the effect of a SiO2 coating on the magnetic properties of Fe3O4 nanoparticles obtained by the sol-gel method is analyzed. Two sets of samples were prepared: Fe3O4 nanoparticles and Fe3O4@SiO2 core-shell composites. The samples display the characteristic spinel structure associated with the magnetite Fe3O4 phase, with the majority of grain sizes around 5-10 nm. At room temperature the nanoparticles show the characteristic superparamagnetic behavior with mean blocking temperatures around 160 and 120 K for Fe3O4 and Fe3O4@SiO2, respectively. The main effect of the SiO2 coating is reflected in the temperature dependence of the high field magnetization (μ(0)H = 6 T), i.e. deviations from the Bloch law at low temperatures (T < 20 K). Such deviations, enhanced by the introduction of the SiO2 coating, are associated with the occurrence of surface spin disordered effects. The induction heating effects (magnetic hyperthermia) are analyzed under the application of an AC magnetic field. Maximum specific absorption rate (SAR) values around 1.5 W g(-1) were achieved for the Fe3O4 nanoparticles. A significant decrease (around 26%) is found in the SAR values of the SiO2 coated nanocomposite. The different heating response is analyzed in terms of the decrease of the effective nanoparticle magnetization in the Fe3O4@SiO2 core-shell composites at room temperature.     

Large magnetothermoelectric power in Co/Cu, Fe/Cu and Fe/Cr multilayers

We report and discuss experimental data on the thermoelectric power of magnetic multilayers. Measurements of the thermoelectric power of Fe/Cr, Co/Cu and Fe/Cu multilayers have been carried out in the temperature range 4K < T < 150 K magnetic fields perpendicular to the layers. All specimens were found to exhibit pronounced magnetothermoelectric power (MTEP) effects correlating with their giant negative magnetoresistance. The main difference between the MTEP and the magnetoresistance is in their temperature dependence. Whereas the magnetoresistance is a decreasing function of temperature, the MTEP, at least in Co/Cu and Fe/Cu multilayers, is very small at low temperature and increases rapidly above 30–40 K. We ascribe this high temperature part of the MTEP to spin-dependent electron-magnon scattering and we propose a theoretical model.     

Fe位Al掺杂对Sr2FeMoO6磁结构和磁输运性质的影响

研究了Sr₂Fe_1-xAl_xMoO₆(0≤x≤0.30) 系列多晶样品的磁学和输运性质.室温X射线衍射谱图的精修结果显示Al³⁺掺杂没有改变样品的晶格结构，但提高了Sr₂FeMoO₆晶格的阳离子有序度.5K时样品的磁化曲线说明平均单位分子饱和磁矩随着Al含量的增加而下降，但平均单位Fe离子磁矩却逐渐提高.磁化曲线的拟合结果显示样品内反铁磁相互作用对饱和磁矩的贡献随着Al含量的增加而下降，说明一定量的Fe离子被Al替代后，抑制了样品内Fe—O—Fe反相边界的形成，从而提高了Sr₂FeMoO₆晶格的阳离子有序度和平均单位Fe离子磁矩.对饱和磁矩的分析表明非磁性Al³⁺离子掺杂会形成无磁相互作用的Mo—O—Al—O—Mo区，可以将原来较大的Mo—O—Fe亚铁磁区分割成许多小的区域，并且使这些亚铁磁区间的磁耦合作用变弱，从而提高了低场磁电阻效应.阳离子有序度的提高使来源于自旋相关电子在反相边界处散射的高场磁电阻明显降低，导致了样品的磁电阻在x=0.15时达到了最大值. 关键词： 2FeMoO₆')" href="#">Sr₂FeMoO₆ 掺杂 磁结构 磁输运性质     

Polystyrene/Fe3O4 magnetic emulsion and nanocomposite prepared by ultrasonically initiated miniemulsion polymerization

Ultrasonically initiated miniemulsion polymerization of styrene in the presence of Fe3O4 nanoparticles was successfully employed to prepare polystyrene (PS)/Fe3O4 magnetic emulsion and nanocomposite. The effects of Fe3O4 nanoparticles on miniemulsion polymerization process, the structure, morphology and properties of PS/Fe3O4 nanocomposite were investigated. The increase in the amount of Fe3O4 nanoparticles drastically increases the polymerization rate due to that Fe3O4 nanoparticles increase the number of radicals and the cavitation bubbles. Polymerization kinetics of ultrasonically initiated miniemulsion polymerization is similar to that of conventional miniemulsion polymerization. PS/Fe3O4 magnetic emulsion consists of two types of particles: latex particles with Fe3O4 nanoparticles and latex particles with no encapsulated Fe3O4 nanoparticles. Fe3O4 nanoparticles lower the molecular weight of PS and broaden the molecular weight and particle size distribution. Thermal stability of PS/Fe3O4 nanocomposite increases with the increase in Fe3O4 content. PS/Fe3O4 emulsion and nanocomposite exhibit magnetic properties. PS/Fe3O4 magnetic particles can be separated from the magnetic emulsion by an external magnetic field and redispersed into the emulsion with agitation.     

Magnetic property and magnetoresistance in Fe/ITO multilayers

Giant magnetoresistance was found in DC magnetron sputtering Fe/ITO multilayers. The magnetic properties, electrical properties and magnetoresistance were investigated. A critical temperature is found around 50 K where the temperature dependence of resistivity and magnetoresistance ratio exhibit an abruptly change. The temperature dependence of resistance is found to obey Mott's 1/4 law for low temperature. The max magnetoresistance ratio of 2.0% and 6.7% is found at room temperature and 12.5 K, respectively. The increase of magnetoresistance ratio at low temperature is due to the decrease of spin-mixing effect.     

Description of the Colossal Magnetoresistance of La 1.2 Sr 1.8 Mn 2 O 7 Based on the Spin-Polaron Conduction Mechanism in the Ferromagnetic Temperature Range

We have analyzed the resistance of La1.2Sr1.8Mn2(1 – z)O7 single crystal in magnetic fields from 0 to 90 kOe in the ferromagnetic temperature range. The observed magnetoresistance of La1.2Sr1.8Mn2O7 is described based on the spin-polaron conduction mechanism. The magnetoresistance is determined by the change in the sizes and magnetic moment directions of magnetic inhomogeneities (polarons). It is shown that the colossal magnetoresistance is ensured by an increase (along the magnetic field) of the polaron linear size. It is found using the method for separating the contributions of different conduction mechanisms to the magnetoresistance that the contribution to the magnetoresistance from the orientation mechanism at 80 K in low magnetic fields is close to 50%. With increasing magnetic field, this contribution decreases and becomes small in fields exceeding 30 kOe. The comparable contributions to the conductivity from the orientational and spin-polaron mechanisms unambiguously necessitate the inclusion of both conduction mechanisms in the magnetoresistance calculations. We have calculated the temperature variation of the polaron size (in relative units) in zero magnetic field and in a magnetic field of 90 kOe.