The magnetoresistance effect in a nanostructure with the periodic magnetic barriers

The magnetoresistance (MR) effect is theoretically investigated in a periodic magnetically modulated nanostructure, which can be realized experimentally by depositing periodic parallel ferromagnetic strips on the top of a heterostructure. We find that there exists a significant conductance difference for electrons through the parallel (P) and antiparallel (AP) magnetization configurations, which results in a considerable magnetoresistance effect. We also find that the magnetoresistance effect depends not only on the temperature but also on the number of the periodic magnetic barriers.     

Magnetoresistance effect in a hybrid ferromagnetic/semiconductor nanostructure

We propose a Magnetoresistance device in a magnetically modulated two-dimensional electron gas, which can be realized experimentally by the deposition of two parallel ferromagnetic strips on the top and bottom of a semiconductor heterostructure. It is shown that there exists a significant transmission difference for electrons through the parallel and antiparallel magnetization configurations of such a device, which leads to a considerable magnetoresistance effect. It is also shown that the magnetoresistance ratio of the device depends greatly on the magnetic strength difference in the two delta barriers of the system.     

Electron interaction and ‘memory’ effects in the presence of mixed disorder

We have studied the effect of electron–electron interaction in the presence of mixed disorder on the conductivity and Hall effect of a high-mobility two-dimensional electron gas in a GaAs/AlGaAs heterostructure. A parabolic, negative, temperature-dependent magnetoresistance (MR) and temperature-dependent Hall effect are observed. We show that these effects can be explained in terms of the interaction theory. In addition, a temperature independent, positive MR is observed. This classical MR is also shown to be a consequence of the mixed disorder.     

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.     

Magnetic depopulation of subbands and universal conductance fluctuations in quasi-one dimensional GaAsAlGaAs heterostructures

Narrow conducting channels have been fabricated in the two dimensional electron gas in a GaAsAlGaAs heterostructure, using a recently developed shallow mesa etch technique. Four terminal high field magnetoresistance measurements at temperatures down to 2 K have been performed on samples with etched width between 8 μm and 0.5 μm. The Shubnikov-de Haas oscillations are studied, and clear evidence is presented for magnetic depopulation of 1-dimensional subbands. The data for the 0.5 μm wide channel are in satisfactory agreement with a simple analysis based on a parabolic confinement potential. Large sidewall depletion effects are found, in agreement with our recent analysis of the weak field negative magnetoresistance. We also observe irregular structure in the magnetoresistance. The dependence of the effects on the orientation of the magnetic field shows that the magnetoresistance arises from the orbital motion of the 2-dimensional electrons. The magnitude and typical field scales of the aperiodic structure are in agreement with predictions based on the theory of universal conductance fluctuations.     

顺磁性La2/3Sr1/3MnO3层对Bi0.8Ba0.2FeO3薄膜多铁性能的影响

界面效应在提升异质结构材料的多铁性能方面有着重要的作用. 本文采用脉冲激光沉积技术在SrTiO₃(STO)基片上制备了Bi_0.8Ba_0.2FeO₃(BBFO)/La_2/3Sr_1/3MnO₃(LSMO)异质结. X-射线衍射图谱表明异质结呈现单相外延生长, 利用高分辨透射电镜进一步证实了BBFO为四方相结构. X-射线光电子能谱证实异质结中只存在Fe³⁺ 离子, 没有产生价态的变化, 揭示了异质结铁电和铁磁性的增强与BBFO/LSMO的界面有关. 同时, 测试了磁电阻(MR)和磁介电(MD), 当磁场强度为0.8 T, 温度为70 K时, MR约为-42.2%, MD约为21.2%. 并且发现在180 K时出现磁相的转变. 实验结果揭示出异质界面效应在提升材料的多铁性和磁电耦合效应方面具有超常的优点, 是加快多铁材料实际应用的有效途径.     

Giant magnetoresistance effect in hybrid ferromagnetic/semiconductor nanosystems

We theoretically investigate the giant magnetoresistance (GMR) effect in general magnetically modulated semiconductor nanosystems, which can be realized experimentally by depositing two parallel ferromagnetic strips on the top of a heterostructure. Here the exact magnetic profiles and arbitrary magnetization direction of ferromagnetic strips are emphasized. It is shown that a considerable GMR effect can be achieved in such nanosystems due to the significant transmission difference for electrons tunneling through parallel and antiparallel magnetization configurations. It is also shown that the magnetoresistance ratio is strongly influenced by the magnetization direction of ferromagnetic strips in nanosystems, thus possibly leading to tunable GMR devices.     

Magnetoresistance of Electrons Channelled by Microscopic Magnetic Field Modulation

We report the magnetoresistance of two-dimensional electron gas, which is made of GaAs based epitaxial mul-tilayers and laterally subjected to a periodic magnetic field. The modulation field is produced by an array of submicrometre ferromagnets fabricated at the surface of the heterostructure. The magnetoresistance of about 20% is found at low temperature 80K. The measurement is in quantitative agreement with semiclassical simulations, which reveal that the magnetoresistance is due to electrons trapped in snake orbits along lines of zero magnetic field.     

From semiclassical transport to quantum Hall effect under low-field Landau quantization

The crossover from the semiclassical transport to the quantum Hall effect is studied by examining a two-dimensional electron system in an AlGaAs/GaAs heterostructure. By probing the magneto-oscillations, it is shown that the semiclassical Shubnikov-de Haas (SdH) formulation can be valid even when the minima of the longitudinal resistivity approach zero. The extension of the applicable range of the SdH theory could be due to the damping effects resulting from disorder and temperature. Moreover, we observed plateau-plateau transition-like behavior with such an extension. From our study, it is important to include positive magnetoresistance to refine the SdH theory.     

Spin-dependent transport for a two-dimensional electron gas with magnetic barriers

The spin-dependent conductance and magnetoresistance ratio (MRR) for a semiconductor heterostructures consisting of two magnetic barriers with different height and space have been investigated by the transfer-matrix method. It is shown that the splitting of the conductance for parallel and antiparallel magnetization configurations results in tremendous spin-dependent MRR, and the maximal MRRs reach 5300\% and 3800\% for the magnetic barrier spaces W=81.3 and 243.9~nm, respectively. The obtained spin-filtering transport property of nanostructures with magnetic barriers may be useful to magnetic-barrier-based spintronics.     

Giant magnetoresistance and hysteretic effects in hybrid semiconductor/ferromagnet devices

We have investigated ballistic magnetoresistance effects in a two dimensional electron gas subjected to a periodic magnetic field that alternates in sign. The magnetic field was produced by a submicron ferromagnetic grating, made of either nickel or cobalt stripes, which was fabricated at the surface of the heterostructure. We observe giant magnetoresistance effects due to the channelling of electrons along lines of zero magnetic field orientated perpendicular to the current. Our semiclassical model accounts in great detail for all features in the magnetoresistance.     

Spin Seebeck effect and spin Hall magnetoresistance in the Pt/Y_3Fe_5O_(12) heterostructure under laser-heating

In the previous study of longitudinal spin Seebeck effect(LSSE), the thermal gradient was often generated by inserting the sample between the cool bath and the hot bath. For practical use, this method is too cumbersome to be easily integrated into modern electrical circuits. Since the laser can be easily focused into a small region, it will be more convenient and friendly to the integrated circuit. In this paper, we systematically investigate the LSSE and spin Hall magnetoresistance(SMR) of the Pt/Y_3 Fe_5 O_(12) heterostructure under focused laser-heating. We find that the extremely large voltage of inverse spin Hall effect(VISHE) can be obtained by reducing the diameter of laser or increasing the number of light spots.Meanwhile, even under the illumination of the ultraviolet light which will excite the electron from the valence band to the conduction band in yttrium iron garnet(YIG), the magnitude of SMR is nearly constant. It indicates that the spin transport behavior of the adjacent Pt is independent of the electron configuration of YIG. The laser-heating method to generate LSSE will be very promising for modern integrated electronic circuits and will promote the application of spin caloritronics in practice.     

Laterally patterned high mobility two-dimensional electron gases obtained by overgrowth of focused ion beam implanted Al1−xGaxAs

The use of focused ion beam implantation doping of an inverted GaAs/Al_1−xGa_xAs heterostructure during a growth interruption allows for the lateral modulation of the heterostructure doping. Hence, laterally patterned two dimensional electron gases (2DEGs) are obtained with no further processing steps required. We have performed the direct writing of a 2DEG with a Hall-bar pattern, such that only the application of ohmic contacts was necessary and the sample surface remained unharmed otherwise. The 2DEG has an electron density of 3.6×10¹¹ cm⁻² and an electron mobility of 4.8×10⁵ cm²/V s, as determined by magnetotransport measurements. A conventional mesa-etched Hall-bar with almost identical electronic properties has also been studied. Different behaviour of the longitudinal as well as the transversal magnetoresistance for the two Hall-bars is observed and can be concluded to be due to a different confinement potential.     

Giant magnetoresistance effect in hybrid ferromagnetic-Schottky-metal and semiconductor nanosystem

We report on a theoretical investigation of the giant magnetoresistance (GMR) effect in hybrid ferromagnetic-Schottky-metal and semiconductor nanosystem. Experimentally, this GMR device can be realized by the deposition of two ferromagnetic (FM) stripes and one Schottky normal metal (NM) in parallel way on the top of a semiconductor GaAs heterostructure. The GMR effect emanates from the significant transmission difference for electrons tunneling through parallel and antiparallel magnetization configurations of the device, and its magnetoresistance ratio (MR) can reach the order of ¹⁰⁶%. Furthermore, it is also shown that the MR of the device depends strongly on the relative location of the Schottky NM stripe between two FM stripes.     

EuS/Ta异质结的极大磁电阻效应

在铁磁/超导异质结中,铁磁体的交换场通过近邻效应将导致超导体准粒子态密度的塞曼劈裂.基于该效应,在外磁场不强的情况下,通过外加磁场可以有效地调节铁磁/超导界面处的交换作用,从而实现超导体在正常态和超导态之间转换,产生极大磁电阻.本文利用脉冲激光沉积方法制备了EuS/Ta异质结并研究了其电磁特性.Ta在3.6 K以下为超...     

Sensing domain wall pinning in the longitudinal magnetoresistance of a two-dimensional electron gas

We investigate the sensing of domain wall pinning in thin Co wires positioned on top of a two-dimensional electron gas (2DEG) heterostructure by measuring the longitudinal resistance of the 2DEG as the magnetic field is swept, in an analogy to the Barkhausen effect. For comparison, we also measure the magnetoresistance of the ferromagnetic film in the same device in a subsequent sweep. Compared to the Hall measurements, the longitudinal measurement has the advantage of sensing magnetic activity over longer lengths, while compared to the measurement of the magnetoresistance in the ferromagnetic wire, it offers complementary information related to the pinning and unpinning of the domain wall, due to its sensitivity only to the out-of-plane magnetic field component.     

Effect of Chemicals on Chemical Mechanical Polishing of Glass Substrates

We investigate the effect of chemicals on chemical mechanical polishing （CMP） of glass substrates. Ceria slurry in an ultra-low concentration of 0.25 wt. % is used and characterized by scanning electron microscopy. Three typical molecules, i.e. acetic acid, citric acid and sodium acrylic polymer, are adopted to investigate the effect on CMP performance in terms of material removal rate （MRR） and surface quality. The addition of sodium acrylic polymer shows the highest MRR as well as the best surface by atomic force microscopy after CMP, while the addition of citric acid shows the worst performance. These results reveal a mechanism that a long-chain molecule without any branches rather than small molecules and common molecules with ramose abundant-electron groups is better for the dispersion of the slurry and thus better for the CMP process.     

Electromodulation of the magnetoresistance in diluted magnetic semiconductors based heterostructures

We study the properties of heterostructures formed by two layers of a diluted magnetic semiconductor separated by a nonmagnetic semiconductor layer. We find that there is a RKKY-type exchange coupling between the magnetic layers that oscillates between ferromagnetic and antiferromagnetic as a function of the different parameters in the problem. The different transport properties of these phases make that this heterostructure presents strong magnetoresistive effects. The coupling can be also modified by an electric field. We propose that it is possible to alter dramatically the electrical resistance of the heterostructure by applying an electric field. Our results indicate that in a single gated sample the magnetoresistance could be modulated by an electrical bias voltage.     

La1.3Sr1.7Mn2O7/SrTiO3-Nb异质结的整流和光伏特性

采用脉冲激光沉积法在SrTiO₃:0.7%Nb(100)单晶衬底上生长了La_1.3Sr_1.7Mn₂O₇(LSMO)薄膜, 并 研究了LSMO/SrTiO₃-Nb异质结的输运性质和光伏效应. 研究发现, 异质结具有良好的整流特征和明显的光生伏特效应. 在532 nm激光辐照下, 光生电压随温度升高先增大后减小, 并且在150 K达到最大值400 mV, 此温度点与LSMO薄膜发生金属-绝缘体转变的温度一致, 这表明异质结的光生伏特效应受LSMO薄膜内部的输运特征调控. 进一步, 从光生电压随时间的变化曲线中分析发现, 上升沿符合一阶指数函数, 这与载流子的迁移过程相关; 而下降沿符合二阶指数函数, 这与结两侧载流子的外部回路中和以及材料内部的电子-空穴湮灭有关. 值得注意的是, 上升沿和下降沿的时间常数均随着温度先增大后减小, 且最大值均出现在LSMO薄膜的金属-绝缘转变温度.     

Optical properties of a magneto-donor in a quantum dot

The temperature-dependent magnetoresistance effect is investigated in a magnetically modulated two-dimensional (2D) electron gas (2DEG) which can be realized by depositing two parallel ferromagnets on top of a 2DEG electron gas. In the resonant tunnelling regime the transmission for the parallel and antiparallel magnetization configurations shows a quite distinct dependence on the longitudinal wave vector of the incident electrons. This leads to a very large magneto resistance ratio with a strong temperature dependence.