STRUCTURE, MAGNETIC PROPERTIES AND GIANT MAGNETORESISTANCE OF YMn6Sn6-xGax (x=0-0.6) COMPOUNDS

Structure, magnetic and transport properties of YMn₆Sn_6-xGa_x (0≤x≤0.6) compounds with a HfFe₆Ge₆-type structure were investigated. It was found that the Ga substitution leads to a contraction of the unit-cell volume. A transition from an antiferromagnetic to a ferromagnetic (or ferrimagnetic) state can be observed for samples (0.1≤x≤0.2) with increasing temperature. The antiferro-ferromagnetic transition for samples with x≤0.2 can also be induced by an external field. The required field is very low, and decreases with increasing Ga concentration. More Ga concentration (x≥0.3) leads to the samples being ferromagnetic in the whole temperature range below the Curie temperature. The Ga substitution weakens the interlayer magnetic coupling between the Mn spins. Corresponding to the metamagnetic transition, a magnetoresistance as large as 32% under a field of 5 T was observed at 5 K for the sample with x=0.2.     

Crystal growth and characterization of the CMR compound La1.2(Sr,Ca)1.8Mn2O7

High-quality centimeter-sized single crystals of La_1.2Sr_1.8−yCa_yMn₂O₇ (0.0y0.2) were successfully grown using a floating zone method associated with an image furnace. We present the growth conditions together with a characterization of the single crystals by means of optical and electron microscopy, EDX and ICPAES analysis, DTA-TGA measurements and redox titration, X-ray powder diffraction, Laue X-ray back-reflection and neutron diffraction. We also stress the main aspects of the complex thermodynamical and kinetic behaviors of these compounds.     

La_(1-x)Ca_xMn_(1.03)O_3外延薄膜的输运性质

本文报道了用溶胶 -凝胶 (sol- gel)方法 ,在 La Al O3 (10 0 )衬底上制备 L a1-x Cax Mn1.0 3 O3 外延薄膜的输运性质 .Ca含量 x在 0 .2～ 0 .6 2 1范围内 ,电阻率随温度的变化关系 ,是从类半导体行为向金属导电行为转变 ,在 x≥ 0 .5的 4个样品中并没有发现电荷有序绝缘体 (COI)和反铁磁绝缘体 (AFI)现象 .Tc最高达 2 75K,MR最大可达 10 4 % (x =0 .2 ,H =1.5T) .在 T >Tp 或 Tc的温区内 ,电阻率ρ∝exp(- E/ k T) ,说明载流子的迁移是以热激活方式进行的 ,热激活能为 0 .32～ 18.3me V.当 T 0 .4 ) .     

Magnetic and magnetoresistive properties of epitaxial Co/Cu/Co trilayers on Si(111)

Giant magnetoresistance of the epitaxial Co/Cu/Co trilayers grown on vicinal Si(111) was determined as a function of Cu spacer coverage in the range from 0 to 7 ML. The first maximum of giant magnetoresistance and antiferromagnetic coupling was detected at 3.0 ML coverage of the Cu spacer. The portion of antiferromagnetic coupling in the first antiferromagnetic maximum was estimated as 17%. 3D growth mode of the Cu spacer leads to the simultaneous occurrence of the ferromagnetically and antiferromagnetically coupled areas between the Co layers.     

垂直各向异性Ho3Fe5O12薄膜的外延生长与其异质结构的自旋输运

垂直磁各向异性稀土-铁-石榴石纳米薄膜在自旋电子学中具有重要应用前景.本文使用溅射方法在(111)取向掺杂钇钪的钆镓石榴石(Gd_0.63Y_2.37Sc₂Ga₃O₁₂,GYSGG)单晶衬底上外延生长了2—100 nm厚的钬铁石榴石(Ho₃Fe₅O₁₂,HoIG)薄膜,并进一步在HoIG上沉积了3 nm Pt薄膜.测量了室温下HoIG的磁各向异性和HoIG/Pt异质结构的自旋相关输运性质.结果显示,厚度薄至2 nm的HoIG薄膜(小于2个单胞层)在室温仍具有铁磁性,且由于外延应变,2—60 nm厚HoIG薄膜都具有很强的垂直磁各向异性,有效垂直各向异性场最大达350 mT;异质结构样品表现出非常可观的反常霍尔效应和“自旋霍尔/各向异性”磁电阻效应,前者在HoIG厚度小于4 nm时开始缓慢下降,而后者当HoIG厚度小于7 nm时急剧减小,说明相较于反常霍尔效应,磁电阻效应对HoIG的体磁性相对更加敏感;此外,自旋相关热电压随HoIG厚度减薄在整个厚度范围以指数方式下降,说明遵从热激化磁振子运动规律的自旋塞贝克效应是其主要贡献者.本文结果表明HoIG纳米薄膜具有可调控的垂直磁各向异性,厚度大于4 nm的HoIG/Pt异质结构具有高效的自旋界面交换作用,是自旋电子学应用发展的一个重要候选材料.     

磁场下合成Fe3O4粉体的隧道磁阻

通过在半金属Fe₃O₄合成过程中外加磁场的方法，改变样品粒子的表面结晶状态和晶格缺陷，研究了由此引起的Fe₃O₄输运性质的变化.合成的Fe₃O₄粉体的主要导电机理均为自旋极化隧穿和高阶跃迁电导，电阻随温度升高成指数降低，电阻与电压显示了非线形相关性，磁阻与磁场的关系为蝴蝶形，是典型的隧道磁阻特征.与没有外加磁场时合成的样品比较，外加磁场合成的样品显示了更低的电阻和更高的磁阻.     

Co掺杂对铁磁金属La0.8Sr0.2MnO3磁电阻影响机理

通过对La_0.8Sr_0.2Mn_1-yCo_yO₃(y≤02)饱和磁矩和输运的测量,研究了Co对La_0.8Sr_0.2MnO₃的磁电阻影响机制.结果表明,在La_0.8Sr_0.2Mn_1-yCo_yO₃（y≤02）中Co³⁺离子是低自旋态.由于Mn³⁺—O—Co³⁺—O—Mn³⁺类型的磁交换与Mn³⁺-Mn⁴⁺离子间双交换作用相比较弱,Curie温度T_C附近的磁电阻随着Co掺杂量的增加而降低.与此相反,由于Co²⁺离子与e_g巡游电子的反铁磁交换耦合作用,低温区间的磁电阻随着Co掺杂量的增加而升高.     

Fitting of transport measurements in polycrystalline La2/3Ca1/3MnO3

Using a previous qualitative explanation to describe the transport properties of polycrystalline La_2/3Ca_1/3MnO₃ thick films, we achieved a good fit of the temperature dependence of the resistance R(T). Depending on the sample, we have observed different metal–insulator (MI) transitions while the magnetic behavior is always similar. Small regions of depleted T_c adjacent to the grain boundary could have an important resistance contribution without affecting the magnetic properties in an appreciable manner. In this work, we achieve a quantitative explanation for the different transport behaviors that we have observed experimentally.     

磁场下合成Fe3O4粉体的隧道磁阻

通过在半金属Fe₃O₄合成过程中外加磁场的方法,改变样品粒子的表面结晶状态和晶格缺陷,研究了由此引起的Fe₃O₄输运性质的变化.合成的Fe₃O₄粉体的主要导电机理均为自旋极化隧穿和高阶跃迁电导,电阻随温度升高成指数降低,电阻与电压显示了非线形相关性,磁阻与磁场的关系为蝴蝶形,是典型的隧道磁阻特征.与没有外加磁场时合成的样品比较,外加磁场合成的样品显示了更低的电阻和更高的磁阻. 关键词： 磁阻 隧穿 表面 晶格缺陷     

Isotope effect for transport and magnetic properties of La 0.35 Pr 0.35 Ca 0.3 MnO 3 thin films

The effect of ¹⁶ O → ¹⁸ O isotope substitution on electrical resistivity, magnetoresistance, and ac magnetic susceptibility was studied for La_0.35Pr_0.35Ca_0.3MnO₃ epitaxial thin films deposited onto LaAlO₃ and SrTiO₃ substrates. For the films on LaAlO₃, the isotope substitution resulted in the reversible transition from a metal-like to insulating state. The applied magnetic field ( H ≥ 2 T) transformed the sample with ¹⁸O back to the metallic state. The films on SrTiO₃ remained metallic at low temperatures for both ¹⁶O and ¹⁸O, but the shift of the resistivity peak corresponding to onset of metallic state exceeded 63 K after ¹⁶ O → ¹⁸ O substitution. The temperature dependence of both resistivity and magnetic susceptibility was characterized by hysteresis, especially pronounced in the case of the films on LaAlO₃. Such a behavior gives certain indications of the phase separation characteristic of interplay between ferromagnetism and charge ordering. Received 11 February 2000 and Received in final form 13 September 2000