Ionic liquid gating control of magnetic anisotropy in Ni0.81Fe0.19 thin films

Voltage control magnetism is one of the most energy efficient pathway towards magnetoelectric (ME) device. Ionic liquid gating (ILG) method has already shown impressive manipulation power at the IL/electrode interface to influence the structure, orbital as well as spin of the electrode materials. As key material in anisotropy magnetoresistance sensor and spin valve heterostructure, the permalloy Ni_0.81Fe_0.19 was utilized as the electrode to investigate the ILG induced magnetic anisotropy change. In this work, we realized magnetic anisotropy control in Au/[DEME]⁺[TFSI]^-/Ni_0.81Fe_0.19 (2.5 nm)/Ta heterostructure via ILG caused electrostatic doping. This is evidenced in situ reversible ferromagnetic field (H_r) shift with electron spin resonance (ESR) spectrometer. Aiming at the question whether the charge accumulation at the ionic liquid interface is the main control mechanism at low voltage, we carefully tested the relationship between the change of resonance field and the amount of surface charge. It was found that these two had a good linear relationship between −1 V and +1 V. Defining the linear parameter as A whose value is 28.7 mT m²/Col. Unlike previously reported chemical regulation of Co, this article used ionic liquids to physically regulate NiFe, which has not been studied in the previous ionic liquid regulation. And NiFe has a narrower resonance line width for easy reference to microwave devices. In addition, It also has a stronger ferromagnetic signal than Co, which can be more easily detected as a sensor device. Therefore, this system is more promising. The ILG control NiFe may lead to a new kind of magnetoelectric sensor devices and path a new way to low energy consumption spintronics.     

Theory of the magnetoelectric effect in ferromagnetic-piezoelectric heterostructures

A theory of the magnetoelectric effect in ferromagnetic-piezoelectric bilayer structures is considered for platelike samples. The magnetoelectric voltage coefficient is expressed through the parameters characterizing the magnetic and piezoelectric phases. It is shown that the magnetoelectric voltage coefficient considerably increases in the region of electromechanical resonance. The thickness ratio between the ferromagnetic and piezoelectric phases at which the magnetoelectric voltage coefficient is maximum is determined. The calculated magnetoelectric voltage coefficients for Permendur-PZT (lead zirconate titanate) structures are presented and compared with the experimental data.     

磁电异质结及器件应用

磁电异质结是由铁磁和铁电材料通过连接层耦合而成,其磁电效应来源于铁电相的压电效应和铁磁相的磁致伸缩效应.相对于颗粒混相磁电复合材料,层状磁电异质结材料具有更高的磁电耦合系数和更低的介电损耗,使得其在磁场传感器、能量收集器、天线以及存储器等领域都有着巨大的应用前景.本综述重点总结了磁电异质结材料的发展历程以及相关应用领域的最新进展,最后评述了磁电异质结材料发展的挑战和前景展望.     

Voltage control magnetism and ferromagnetic resonance in an Fe19Ni81/PMN-PT heterostructure by strain

Voltage control magnetism has been widely studied due to its potential applications in the next generation of information technology. PMN-PT, as a single crystal ferroelectric substrate, has been widely used in the study of voltage control magnetism because of its excellent piezoelectric properties. However, most of the research based on PMN-PT only studies the influence of a single tensile (or compressive) stress on the magnetic properties due to the asymmetry of strain. In this work, we show the effect of different strains on the magnetic anisotropy of an Fe₁₉Ni₈₁/(011) PMN-PT heterojunction. More importantly, the (011) cut PMN-PT generates non-volatile strain, which provides an advantage when investigating the voltage manipulation of RF/microwave magnetic devices. As a result, a ferromagnetic resonance field tunability of 70 Oe is induced in our sample by the non-volatile strain. Our results provide new possibilities for novel voltage adjustable RF/microwave magnetic devices and spintronic devices.     

磁子转矩翻转磁矩研究

磁子是自旋波量子化的准粒子。磁子具有在绝缘磁性材料中无热耗散、低阻尼、长距离传输自旋的优势,避免了因电荷流动而产生焦耳热,可以克服日益显著的器件发热问题,因此磁子器件在低功耗信息存储与计算领域具有潜在应用前景。文章首先介绍了自旋波和磁子的概念,磁子具有的优势和研究价值;然后总结了磁子在铁磁和反铁磁绝缘体中输运以及新型磁子器件方面的最新研究结果;最后详细介绍了室温下实现磁子转矩驱动磁矩翻转的最新研究工作。这些工作对发展磁子学,实现低功耗、高速磁子型器件及应用具有较为重要的现实意义。     

激发态电荷转移有机体的多铁性研究

随着人们对多铁性的深入了解,越来越多不同类型的有机多铁材料被合成出来.激发态电荷转移有机体的电荷转移网络是由一个提供电子的分子(给体donor,D~+)和一个接受电子的分子(受体acceptor,A~-)有序排列后构成的.D~+A~-长程有序排列,其激发态(激子)具有较长寿命和±1/2自旋,这是产生室温铁电性和铁磁性的根本原因.激发态容易受外场刺激,因此光照、磁场、电场、应力等能够很好地调控这类材料的铁电极化、磁矩和相应的磁电耦合系数.激发态电荷转移有机体不仅大大丰富了室温多铁材料体系,而且可以为开发新型多功能电子器件提供材料基础和技术储备.     

稀磁半导体--自旋和电荷的桥梁

稀磁半导体可能同时利用载流子的自旋和电荷自由度构造将磁、电集于一体的半导体器件．尤其是铁磁半导体材料的出现带动了半导体自旋电子学的发展．室温铁磁半导体材料的制备，半导体材料中有效的自旋注入，以及自旋在半导体结构中输运和操作已成为目前半导体自旋电子学领域中的热门课题．稀磁半导体呈现出强烈的自旋相关的光学性质和输运性质，这些效应为人们制备半导体自旋电子学器件提供了物理基础．     

铁磁共振磁交换力显微镜

原子间的自旋相互作用力对原子级别磁性纳米构造体的表面磁性质的理解是极为重要的. 磁交换力显微镜是测量表面自旋作用力的重要手段, 但它的缺点一是需要加外部强磁场, 二是不能分离表面形貌和自旋信息, 这就导致材料表面受外部磁场的影响, 而且表面形貌和自旋信息之间相互影响, 使自旋间的相互作用力的检测和成像研究受到限制.为了解决上述问题, 利用原子力显微镜, 并采用微波照射的方法, 根据铁磁共振原理, 分别独立提取磁性材料表面形貌和自旋信息, 称之为铁磁共振磁交换力显微镜, 理论和实验结果表明此方法可以有效地分离两种信息. 铁磁共振磁交换力显微镜可以促进对原子级磁性材料机能的理解以及磁性相关科学领域的进步, 特别是对自旋电子元件的发展有很大的促进作用, 是新世纪高度信息化社会不可缺少的测量技术. 关键词： 原子力显微镜 磁交换力显微镜 自旋 铁磁共振     

Spin accumulation in coupled quantum dots with ferromagnetic and superconducting electrodes

We theoretically investigate the spin accumulation in two parallel coupled quantum dots (QDs) with ferromagnetic and superconducting electrodes. Due to the ferromagnetic lead, the spin accumulation appears on the resonance of Andreev reflection. The spin accumulation in each of the two QDs can be controlled by the gate voltage. The sign of the spin accumulation is also controllable by tuning the bias. Furthermore, tuning the magnetic flux can exchange the amplitude of the spin accumulation in the two QDs. The Aharonov-Bohm oscillation effects also provides a way to control the spin accumulation of each QD.     

Domain-wall induced phase shifts in spin waves

We study the interaction between two important features of ferromagnetic nanoparticles: magnetic domain walls and spin waves. Micromagnetic simulations reveal that magnetostatic spin waves change their phase as they pass through domain walls. Similar to an Aharonov-Bohm experiment, we suggest to probe this effect by splitting the waves on different branches of a ring. The interference of merging waves depends on the domain walls in the branches. A controlled manipulation of spin-wave phases could be the first step towards nanoscaled ferromagnetic devices performing logical operations based on spin-wave propagation.     

磁性隧道结自旋极化电子的隧穿特性

铁磁金属间通过中间层的自旋极化电子隧穿产生的磁性耦合,在自旋电子器件中有许多潜在的应用.考虑由一平面磁性势垒层隔开的两铁磁性金属电极构成的磁性隧道结,针对中间层形成的矩形势垒,在近自由电子模型的基础上,计算零偏压下的隧穿电导、自旋极化率和隧穿磁阻比率,分析势垒层特性、分子场强弱、分子场相对取向等对隧道结自旋极化电子隧穿特性的影响.计算结果对自旋电子器件的设计具有一定的指导意义.     

非对称条形纳磁体的铁磁共振频率和自旋波模式

通过建立微波激励下的非对称条形多铁纳磁体的微磁模型,研究了倾斜角和缺陷角对该形纳磁体的铁磁共振谱和自旋波模式的影响.通过对微磁仿真得到的动态磁化数据进行分析发现,非对称条形纳磁体倾斜角度增加,铁磁共振频率随之增加,而这一现象与纳磁体的缺陷角度无关.倾斜角不变,非对称条形纳磁体的铁磁共振频率与缺陷角度呈单调递增关系,并且不同缺陷角度纳磁体的自旋波模式显示出极大的差异.非对称条形纳磁体与矩形纳磁体相比,它的自旋波模式局部化,具体为非对称条形纳磁体的自旋波模式不对称且高进动区域存在于边缘,表现为非对称边缘模式.倾斜角改变导致纳磁体内部退磁场变化,引起纳磁体边缘模式的移动,而中心模式对倾斜角的变化并不敏感.最后,对建立的模型在高频微波磁场激励下的磁损耗进行了分析,验证了模型的可靠性.这些结论说明缺陷角和倾斜角可用于纳磁体自旋波模式和铁磁共振频率的调谐,所得结果为可调纳磁微波器件的设计提供了重要的理论依据和思路.     

Optimal magnetoelectric coupling performance of Terfenol-D/PZT composites in resonance state under bias magnetic field

Laminated magnetoelectric (ME) composites with various thickness ratios were optimized, fabricated and experimentally investigated in this work. The Terfenal-D/PZT specimens with optimal thickness ratio between the magnetostrictive phase and piezoelectric phase, and two other values were tested for their ME coupling performance. The coupling voltage output increases linearly with the increase of DC bias magnetic field. The ME voltage coefficient increases more than 100 times in the resonance state for the optimal laminate. The DC bias magnetic field affects the ME voltage coefficient significantly, and also has little effect on the resonant frequency. The strength of AC magnetic field also slightly affects the ME voltage coefficient in resonance state, but does not affect the resonant state under which the same DC magnetic field is required. The experimental results can help understand the coupling performance of ME composite under bias magnetic field and prompt the application of ME devices.     

Space-time parity violation and magnetoelectric interactions in antiferromagnets

The properties of antiferromagnetic materials with violated space-time parity are considered. Particular attention is given to the bismuth ferrite BiFeO₃ ferroelectric magnet. This material is distinguished from other antiferromagnets in that the inversion center is absent in its crystal and magnetic structures. This circumstance gives rise to diversified and unusual properties, namely, to the appearance of a spatially modulated spin structure and to the unique possibility of the linear magnetoelectric effect coexisting with a weak ferromagnetic moment. The magnetic-induced phase transitions accompanied by the suppression of the modulated spin structure and appearance of a number of new and unusual effects are considered. These are the linear magnetoelectric effect and the appearance of a toroidal moment and a weak ferromagnetic moment of the magnetoelectric nature.     

2-2型磁电复合薄膜CoFe2O4/Pb(Zr0.53Ti0.47)O3静磁耦合

运用溶胶-凝胶法在Pt/Ti/SiO₂/Si基片上旋涂制备了2-2型CoFe₂O₄/Pb(Zr_0.53Ti_0.47)O₃磁电复合薄膜．制备的磁电薄膜结构为基片/PZT/CFO/PZT*/CFO/PZT,通过改变中间层PZT*溶胶的浓度,改变磁性层间距以及静磁耦合的大小．SEM结果表明,复合薄膜结构致密,呈现出界面清晰平整的多层结构．制备的复合薄膜具有较好的铁电与铁磁性能．实验还研究了静磁耦合对薄膜磁电性能的影响,结果表明,随着复合薄膜磁性层间距的减小,静磁耦合效应的增加,磁电电压系数有逐渐增大的趋势.     

Magnetoelectric coupling induced multistate magnetization

Complex oxides provide an ideal playground for exploring the interplay among the fundamental degrees of freedom: structural (lattice), electronic (orbital and charge), and magnetic (spin). With trends toward device miniaturization, there is growing interest in combining electronic and magnetic properties into multifunctional thin-film materials for potential applications in novel versatile devices. Here, we report growth of ferromagnetic nanostructures on ferroelectric ($1?x$)[Pb(Mg_1/3Nb_2/3)O₃]???x[PbTiO₃] (PMN-PT) single crystals. With careful control of material composition and application of electric field direction and amplitude, we realized stabilization of both monoclinic and orthorhombic phases in the PMN-PT single crystals with the intrinsic rhombohedral phase. Multistate magnetization modulation has been realized through phase transformation control in the ferroelectric component. This finding promises multistate information recording through magnetoelectric effect.     

Estimation of the cross section of neutron scattering by spin waves in thin ferromagnetic layers

Inelastic neutron scattering by magnetic excitations in thin ferromagnetic films has not been observed so far owing to the small cross section of the interaction of neutrons with spin waves. To increase the probability of inelastic magnetic scattering, it has been proposed to implement three-layer structures in which the neutron wave functions exhibit resonant enhancement in a ferromagnetic layer. The cross section of neutron scattering by spin waves in the regime of the resonant enhancement of the neutron wave function has been estimated.     

Perspectives of electrically generated spin currents in ferromagnetic materials

Spin-orbit coupling enables charge currents to give rise to spin currents and vice versa, which has applications in non-volatile magnetic memories, miniature microwave oscillators, thermoelectric converters and Terahertz devices. In the past two decades, a considerable amount of research has focused on electrical spin current generation in different types of nonmagnetic materials. However, electrical spin current generation in ferromagnetic materials has only recently been actively investigated. Due to the additional symmetry breaking by the magnetization, ferromagnetic materials generate spin currents with different orientations of spin direction from those observed in nonmagnetic materials. Studies centered on ferromagnets where spin-orbit coupling plays an important role in transport open new possibilities to generate and detect spin currents. We summarize recent developments on this subject and discuss unanswered questions in this emerging field.     

Mode selective control of flow turbulence

We study spin transport in normal/ferromagnetic/normal/ferromagnetic.../normal graphene superlattices, which can be realized by putting a series of magnetic insulator bars on top of a graphene sheet. Owing to magnetic proximity effect, local exchange splittings will be induced in the graphene sheet, effectively forming a magnetic graphene superlattice. The spin polarization of tunneling conductance and the magneto resistance (MR) exhibit oscillatory behavior with the gate voltage. The superlattice structure leads to an enhanced spin polarization and MR ratio, making the magnetic graphene superlattice become very promising in spintronics applications.     

基于PbMg_(1/3)Nb_(2/3)O_3-PbTiO_3压电单晶的磁电复合薄膜材料研究进展

电子信息技术的迅速发展对磁电功能器件的微型化、智能化、多功能化以及灵敏度、可靠性、低功耗等都提出了更高的需求,传统的块体磁电功能材料已日渐不能满足上述需求,而层状磁电复合薄膜材料同时具有铁电性、铁磁性和磁电耦合等多种特性,因此能满足上述需求且有望应用于新一代磁电功能器件.层状磁电复合材料不仅具有非常丰富的物理现象和效应,而且在弱磁探测器、多态存储器、电写磁读存储器、电场可调低功耗滤波器、移相器、天线等微波器件中也具有广阔的应用前景,因而受到材料科学家和物理学家广泛的关注和研究.在层状磁电复合材料中,功能薄膜/铁电单晶异质结因其制备简单、结构设计和材料选择灵活以及电场调控方便和有效,最近十余年引起了越来越多的研究人员的兴趣.目前,以具有优异铁电和压电性能的(1-x)PbMg_(1/3)Nb_(2/3)O_3-PbTiO_3(PMN-PT)单晶作为衬底,构建功能薄膜/PMN-PT异质结已成为国内外多铁性复合薄膜材料研究领域的重要方向之一.相比于其他国家,我国科学家无论在发表的文章数量还是在文章被引用次数方面都处于领先地位,表明我国在功能薄膜/PMN-PT单晶异质结方面的研究卓有成效.迄今为止,研究人员已构建了锰氧化合物/PMN-PT、铁氧体/PMN-PT、铁磁金属/PMN-PT、稀磁半导体/PMN-PT、发光材料/PMN-PT、二维材料/PMN-PT、多层薄膜/PMN-PT、超导薄膜/PMN-PT等多种类型的异质结,在理论研究和实验方面都取得了丰富的研究成果.本文对基于PMN-PT压电单晶的磁电复合薄膜材料的研究进展进行了总结:简要介绍了与功能薄膜/PMN-PT异质结相关的研究论文发表现状;介绍了PMN-PT单晶在准同型相界附近的相图和应变特性;按照功能薄膜材料所属的体系对异质结进行了分类,并选取部分代表性的研究成果,介绍了材料的磁电性能和内涵的物理机制;最后就目前有待解决的问题和未来可能的应用方向进行了总结和展望.