Magnetic phase control by an electric field

The quest for higher data density in information storage is motivating investigations into approaches for manipulating magnetization by means other than magnetic fields. This is evidenced by the recent boom in magnetoelectronics and 'spintronics', where phenomena such as carrier effects in magnetic semiconductors and high-correlation effects in colossal magnetoresistive compounds are studied for their device potential. The linear magnetoelectric effect-the induction of polarization by a magnetic field and of magnetization by an electric field-provides another route for linking magnetic and electric properties. It was recently discovered that composite materials and magnetic ferroelectrics exhibit magnetoelectric effects that exceed previously known effects by orders of magnitude, with the potential to trigger magnetic or electric phase transitions. Here we report a system whose magnetic phase can be controlled by an external electric field: ferromagnetic ordering in hexagonal HoMnO3 is reversibly switched on and off by the applied field via magnetoelectric interactions. We monitor this process using magneto-optical techniques and reveal its microscopic origin by neutron and X-ray diffraction. From our results, we identify basic requirements for other candidate materials to exhibit magnetoelectric phase control.     

六角多铁HoMnO3的磁电性质及其研究进展

六角HoMnO3材料是近年来多铁性磁电材料研究中一个极其重要且具有代表性的材料.文章全面概述了多铁材料HoMnO3的磁电性质以及近年来国内外的相关研究进展,包括HoMnO3的低温自旋交换作用、磁性相变、外加电场诱导的磁化改变、铁电极化机理以及由于磁弹、自旋晶格耦合、自旋声子耦合作用导致的巨磁电效应等.重点评述了HoMn...     

六角多铁HoMnO3的磁电性质及其研究进展

六角HoMnO3材料是近年来多铁性磁电材料研究中一个极其重要且具有代表性的材料.文章全面概述了多铁材料HoMnO3的磁电性质以及近年来国内外的相关研究进展,包括HoMnO3的低温自旋交换作用、磁性相变、外加电场诱导的磁化改变、铁电极化机理以及由于磁弹、自旋晶格耦合、自旋声子耦合作用导致的巨磁电效应等.重点评述了HoMnO3材料磁电效应产生的物理机制,在此基础上,讨论了应用过程中存在的问题,提出了解决的可能途径.     

Observation of coupled magnetic and electric domains

Ferroelectromagnets are an interesting group of compounds that complement purely (anti-)ferroelectric or (anti-)ferromagnetic materials--they display simultaneous electric and magnetic order. With this coexistence they supplement materials in which magnetization can be induced by an electric field and electrical polarization by a magnetic field, a property which is termed the magnetoelectric effect. Aside from its fundamental importance, the mutual control of electric and magnetic properties is of significant interest for applications in magnetic storage media and 'spintronics'. The coupled electric and magnetic ordering in ferroelectromagnets is accompanied by the formation of domains and domain walls. However, such a cross-correlation between magnetic and electric domains has so far not been observed. Here we report spatial maps of coupled antiferromagnetic and ferroelectric domains in YMnO3, obtained by imaging with optical second harmonic generation. The coupling originates from an interaction between magnetic and electric domain walls, which leads to a configuration that is dominated by the ferroelectromagnetic product of the order parameters.     

具有磁电效应的A类反铁磁系统的自旋波理论

自旋波理论通常用来研究低温下各类铁磁、反铁磁的磁性质 .运用自旋波理论 ,考虑外电场作用下产生的磁电效应 ,研究了A类反铁磁系统在主要高对称性方向的自旋波频谱以及由于磁电效应而发生的改变 .发现磁电效应电场的作用相当于一个虚构的磁场 ,能够引起自旋波能谱的分裂     

Magnetization vector manipulation by electric fields

Conventional semiconductor devices use electric fields to control conductivity, a scalar quantity, for information processing. In magnetic materials, the direction of magnetization, a vector quantity, is of fundamental importance. In magnetic data storage, magnetization is manipulated with a current-generated magnetic field (Oersted-Ampère field), and spin current is being studied for use in non-volatile magnetic memories. To make control of magnetization fully compatible with semiconductor devices, it is highly desirable to control magnetization using electric fields. Conventionally, this is achieved by means of magnetostriction produced by mechanically generated strain through the use of piezoelectricity. Multiferroics have been widely studied in an alternative approach where ferroelectricity is combined with ferromagnetism. Magnetic-field control of electric polarization has been reported in these multiferroics using the magnetoelectric effect, but the inverse effect-direct electrical control of magnetization-has not so far been observed. Here we show that the manipulation of magnetization can be achieved solely by electric fields in a ferromagnetic semiconductor, (Ga,Mn)As. The magnetic anisotropy, which determines the magnetization direction, depends on the charge carrier (hole) concentration in (Ga,Mn)As. By applying an electric field using a metal-insulator-semiconductor structure, the hole concentration and, thereby, the magnetic anisotropy can be controlled, allowing manipulation of the magnetization direction.     

Spin-galvanic effect

There is much recent interest in exploiting the spin of conduction electrons in semiconductor heterostructures together with their charge to realize new device concepts. Electrical currents are usually generated by electric or magnetic fields, or by gradients of, for example, carrier concentration or temperature. The electron spin in a spin-polarized electron gas can, in principle, also drive an electrical current, even at room temperature, if some general symmetry requirements are met. Here we demonstrate such a 'spin-galvanic' effect in semiconductor heterostructures, induced by a non-equilibrium, but uniform population of electron spins. The microscopic origin for this effect is that the two electronic sub-bands for spin-up and spin-down electrons are shifted in momentum space and, although the electron distribution in each sub-band is symmetric, there is an inherent asymmetry in the spin-flip scattering events between the two sub-bands. The resulting current flow has been detected by applying a magnetic field to rotate an optically oriented non-equilibrium spin polarization in the direction of the sample plane. In contrast to previous experiments, where spin-polarized currents were driven by electric fields in semiconductor, we have here the complementary situation where electron spins drive a current without the need of an external electric field.     

Direct electronic measurement of the spin Hall effect

The generation, manipulation and detection of spin-polarized electrons in nanostructures define the main challenges of spin-based electronics. Among the different approaches for spin generation and manipulation, spin-orbit coupling--which couples the spin of an electron to its momentum--is attracting considerable interest. In a spin-orbit-coupled system, a non-zero spin current is predicted in a direction perpendicular to the applied electric field, giving rise to a spin Hall effect. Consistent with this effect, electrically induced spin polarization was recently detected by optical techniques at the edges of a semiconductor channel and in two-dimensional electron gases in semiconductor heterostructures. Here we report electrical measurements of the spin Hall effect in a diffusive metallic conductor, using a ferromagnetic electrode in combination with a tunnel barrier to inject a spin-polarized current. In our devices, we observe an induced voltage that results exclusively from the conversion of the injected spin current into charge imbalance through the spin Hall effect. Such a voltage is proportional to the component of the injected spins that is perpendicular to the plane defined by the spin current direction and the voltage probes. These experiments reveal opportunities for efficient spin detection without the need for magnetic materials, which could lead to useful spintronics devices that integrate information processing and data storage.     

多铁性异质结研究进展

多铁异质结中的磁电耦合效应是凝聚态物理和材料物理的研究热点之一.相比单相的多铁材料,多铁异质结中界面处的自旋、电荷、轨道以及晶格之间存在着复杂的相互作用,导致出现一些新的物理现象,使得其在新一代的存储器、传感器、微波等领域中具有重要的应用前景.文章介绍近年来在多铁异质结方向取得的进展,着重介绍实现电场对磁性控制的场效应、应变效应、交换偏置效应等,以及磁场对多铁性的调控,从而获得很大的磁电耦合效应;分析了多铁隧道结及其磁电耦合效应,其集成了传统铁电隧道结和铁磁隧道结的优势,可大幅度提高单个存储单元存储状态,从而提高存储密度.最后提出当前面临的问题和对未来的展望.     

铁磁/有机系统的自旋扩散漂移性质研究王

基于自旋扩散漂移方程和欧姆定律, 理论研究了电场对铁磁/有机半导体界面的电流自旋极化性质的影响. 考虑到有机半导体内特殊的载流子以及电场对其自旋扩散长度的影响, 计算了界面处的电流自旋极化率. 结果表明, 高电场可以使界面处的电流自旋极化率得到有效提高. 同时还进一步研究了电场下有机半导体中极化子比率、自旋相关界面电阻等因素对电流自旋极化的影响.     

Co-Ti共掺杂M型六角钡铁氧体磁电耦合研究

磁电多铁性材料兼具电极化及磁性，可用于构建新型传感器和高密度存储器件．但对于单相多铁性材料，大多工作温度低且所需磁场强度较高，无法满足实用化需求．近来发现六角铁氧体可明显提高这两方面特性，但还需提高其工作温度及电阻率．我们采用Co-Ti元素对M-型钡六角铁氧体进行大剂量共掺杂，制备一系列的铁氧体陶瓷BaFe12-2xCoxTixO19 (x=0~4)．同时利用物理性能综合测试仪（PPMS）及其他检测仪器搭建了一套磁电测试系统，并结合LabVIEW软件进行编程，对样品的磁性、电性、磁介电和磁电耦合特性进行系统表征．结果表明，Co-Ti共掺杂可显著改变M型钡六角铁氧体的矫顽场及饱和磁化强度．同时，这种掺杂可使漏电流降低到3个量级（掺杂量x=2）．值得一提的是，我们在掺杂量为x=2的样品中观测到室温下的磁介电效应，同时在100K以下观测到明显的磁致铁电极化，且其电极化方向可以被磁场反转．该结果在探索新型多铁六角铁氧体及推进其应用化进程具有一定的意义．     

三终端量子点环中的自旋极化输运

利用非平衡态格林函数方法,研究了一个存在局域Rashba自旋轨道耦合作用的三电极量子点环结构中的电子输运性质.结果发现,Rashba自旋轨道耦合作用引起的自旋相关的量子干涉效应能够在电极中产生自旋流.这种自旋流的大小、方向以及自旋极化度等性质可以通过纯电学手段改变系统参数来加以调控.在适当选择这些参数时,电极中甚至可以产生完全自旋极化流或纯自旋流.这些效应说明我们所研究的系统可用来设计纯电学的自旋流产生装置.     

Minority spin condensate in the spin-polarized superfluid 3He A1 phase

The magnetic properties of (3)He in its various phases originate from the interactions among the nuclear spins. The spin-polarized 'ferromagnetic' superfluid (3)He A(1) phase (which forms below 3 mK between two transition temperatures, T(c1) and T(c2), in an external magnetic field) serves as a material in which theories of fundamental magnetic processes and macroscopic quantum spin phenomena may be tested. Conventionally, the superfluid component of the A(1) phase is understood to contain only the majority spin condensate, having energetically favoured paired spins directed along the external field and no minority spin condensate having paired spins in the opposite direction. Because of difficulties in satisfying both the ultralow temperature and high magnetic field required to produce a substantial phase space, there exist few studies of spin dynamics phenomena that could be used to test the conventional view of the A(1) phase. Here we develop a mechanical spin density detector that operates in the required regime, enabling us to perform measurements of spin relaxation in the A(1) phase as a function of temperature, pressure and magnetic field. Our mechanical spin detector is based in principle on the magnetic fountain effect; spin-polarized superfluid motion can be induced both magnetically and mechanically, and we demonstrate the feasibility of increasing spin polarization by a mechanical spin filtering process. In the high temperature range of the A(1) phase near T(c1), the measured spin relaxation time is long, as expected. Unexpectedly, the spin relaxation rate increases rapidly as the temperature is decreased towards T(c2). Our measurements, together with Leggett-Takagi theory, demonstrate that a minute presence of minority spin pairs is responsible for this unexpected spin relaxation behaviour. Thus, the long-held conventional view that the A(1) phase contains only the majority spin condensate is inadequate.     

Adjusting the voltage step-up ratio of a magnetoelectric composite transformer

A magnetoelectric composite transformer is proposed. The voltage step-up ratio can be adjusted by an applied magnetic field based on the direct and converse magnetoelectric effects. The nonlinear relationship between the voltage step-up ratio and magnetic field is caused by the nonlinear relationship of the magnetoelectric effect in magnetic field.     

贵金属/半导体异质结中的光致自旋电流效应研究

自旋电子材料因能同时对电子的自旋和电荷两个自由度实施操控,在构筑以低功耗、超高速、大容量和超宽带为特征的新一代信息处理技术中展现出巨大的应用潜力.然而,通过掺杂过渡金属元素和稀土离子而形成的传统稀磁半导体和钙钛矿锰氧化物往往因结构缺陷导致的居里温度不高、自旋磁矩和自旋极化率偏低等不足,阻碍了自旋电子材料的商业化应用.近年来,在高纯半导体上沉积贵金属薄膜所形成的贵金属/半导体异质结中,通过使用偏振光激发该类异质结可产生纯自旋电流.这种基于逆自旋霍尔效应(ISHE)、可在室温下运行的、非接触和非破坏型的自旋极化激励方法理论上可获得高于50%自旋极化率,引起了人们的广泛关注.文章主要介绍光致自旋电流形成机制和测试方法,以及入射光圆偏振度、光强、入射角度等参数对光致自旋注入效率的调控机理,介绍杂质介导和声子介导对光致自旋输运的贡献,最后提出增强光致自旋电子极化率的可行方案,可为揭示自旋载流子产生、注入和输运相关的自旋动力学核心科学问题以及研制高性能自旋电子器件提供有益的参考.     

外电场对层状复合材料磁电效应的影响

以Landau唯象热力学理论为基础，求得了与电场相关的压电相状态方程，结合磁致伸缩相状态方程和一定的边界条件，研究了外电场对磁电电压系数的影响．结果显示：用来极化铁电层的外电场对磁电作用有很大的影响，随着外电场的增大，横向和纵向的磁电电压系数将明显减小，从而有效分析了磁电电压系数实验值小于理论值的可能原因．     

T型有机器件自旋注入效率的提高

考虑到有机半导体中特殊的载流子电荷自旋关系,建立了一个自旋注入有机半导体的简单的T型器件模型,运用自旋扩散理论计算得出了此模型的电流白旋极化率并与铁磁／有机半导体异质结构的注入效率进行了比较．理论计算发现T型模型中通过调节分支电流的大小会使自旋极化率较铁磁／有机半导体模型有明显的提高,并讨论了极化子比率、外加电场、自旋相关界面电阻以及有机半导体电导率等因素对电流自旋极化性质的影响．     

浓磁半导体材料的制备、磁性和自旋极化的输运

成功制备出过渡金属元素含量高的Zn_1-xCo_xO、Ti_1-xCo_xO₂、(In_1-xCo_x)2O3铁磁半导体(浓磁半导),发现这些氧化物浓磁半导体具有高于室温的居里温度、高自旋极化率、巨磁光克尔效应等优良材料特性。还对浓磁半导的输运性质进行了系统的实验和理论研究,提出了自旋依赖的电子变程跃迁理论模型。这些用新方法制备的氧化物浓磁半导体,不同于常规的稀磁半导体,有望成为高效自旋注入源和半透明磁光新材料。     

Magnetic monopoles in spin ice

Electrically charged particles, such as the electron, are ubiquitous. In contrast, no elementary particles with a net magnetic charge have ever been observed, despite intensive and prolonged searches (see ref. 1 for example). We pursue an alternative strategy, namely that of realizing them not as elementary but rather as emergent particles-that is, as manifestations of the correlations present in a strongly interacting many-body system. The most prominent examples of emergent quasiparticles are the ones with fractional electric charge e/3 in quantum Hall physics. Here we propose that magnetic monopoles emerge in a class of exotic magnets known collectively as spin ice: the dipole moment of the underlying electronic degrees of freedom fractionalises into monopoles. This would account for a mysterious phase transition observed experimentally in spin ice in a magnetic field, which is a liquid-gas transition of the magnetic monopoles. These monopoles can also be detected by other means, for example, in an experiment modelled after the Stanford magnetic monopole search.     

Spin entropy as the likely source of enhanced thermopower in Na(x)Co2O4

In an electric field, the flow of electrons in a solid produces an entropy current in addition to the familiar charge current. This is the Peltier effect, and it underlies all thermoelectric refrigerators. The increased interest in thermoelectric cooling applications has led to a search for more efficient Peltier materials and to renewed theoretical investigation into how electron-electron interaction may enhance the thermopower of materials such as the transition-metal oxides. An important factor in this enhancement is the electronic spin entropy, which is predicted to dominate the entropy current. However, the crucial evidence for the spin-entropy term, namely its complete suppression in a longitudinal magnetic field, has not been reported until now. Here we report evidence for such suppression in the layered oxide Na(x)Co2O4, from thermopower and magnetization measurements in both longitudinal and transverse magnetic fields. The strong dependence of thermopower on magnetic field provides a rare, unambiguous example of how strong electron-electron interaction effects can qualitatively alter electronic behaviour in a solid. We discuss the implications of our finding--that spin-entropy dominates the enhancement of thermopower in transition-metal oxides--for the search for better Peltier materials.