Modulation of Spin Dynamics via Voltage Control of Spin‐Lattice Coupling in Multiferroics

Motivated by the most recent progresses in both magnonics (spin dynamics) and multiferroics fields, this work aims at magnonics manipulation by the magnetoelectric coupling effect. Here, voltage control of magnonics, particularly the surface spin waves, is achieved in La_0.7Sr_0.3MnO₃/0.7Pb(Mg_1/3Nb_2/3)O₃‐0.3PbTiO₃ multiferroic heterostructures. With the electron spin resonance method, a large 135 Oe shift of surface spin wave resonance (≈7 times greater than conventional voltage‐induced ferromagnetic resonance shift of 20 Oe) is determined. A model of the spin‐lattice coupling effect, i.e., varying exchange stiffness due to voltage‐induced anisotropic lattice changes, has been established to explain experiment results with good agreement. Additionally, an “on” and “off” spin wave state switch near the critical angle upon applying a voltage is created. The modulation of spin dynamics by spin‐lattice coupling effect provides a platform for realizing energy‐efficient, tunable magnonics devices.     

Reentrant Spin Glass and Large Coercive Field Observed in a Spin Integer Dimerized Honeycomb Lattice

2D magnetic materials with dimerized honeycomb lattices can be treated as mixed-spin square lattices, in which a quantum phase transition may occur to realize the Bose–Einstein condensation of magnons under reachable experimental conditions. However, this has never been successfully realized with integer spin centers. Herein, a spin integer (S = 2) dimerized honeycomb lattice in an iron(II)-azido compound [Fe(4-etpy)₂(N₃)₂]_n (FEN, 4-etpy = 4-ethylpyridine) is realized. Morphology characterization by transmission electron microscopy, scanning electron microscopy, and atomic force microscopy spectroscopies show that the thinnest place of the sample is ≈13 nm, which is equal to ten layers of the compound. In contrast to the common magnetic properties of long-range magnetic ordering, Mössbauer and polarized neutron scattering studies reveal that FEN exhibits a reentrant spin glass behavior owing to competing ferro- and antiferromagnetic exchange-coupling interactions within the lattice. Two spin glass phases with disparate canting angles are characterized at 39 and 28 K, respectively. By using Curély's model, two exchange-coupling constants (J₁ = +35.8 cm⁻¹ and J₂ = −3.7 cm⁻¹) can be simulated. Moreover, a very large coercive field of ≈1.9 Tesla is observed at 2 K, making FEN a “very hard” van der Waals 2D magnetic material.     

Binary Ferromagnetic Nanostructures: Fabrication,Static and Dynamic Properties

The fabrication of large area binary magnetic nanostructures made from one or two ferromagnetic materials (Ni and Ni₈₀Fe₂₀) is reported using self‐aligned shadow deposition technique. The static and dynamic properties are characterized using magneto‐optical Kerr effects (MOKE) and broadband ferromagnetic resonance spectroscopy. Compared with Ni₈₀Fe₂₀ nanomagnets made using a conventional lithographic technique from identical resist templates, tunable magnetization switching is observed with a marked increase in the coercive field and more adjustable dynamic response for the Ni₈₀Fe₂₀/Ni₈₀Fe₂₀ and Ni/Ni₈₀Fe₂₀ binary structures. The results are validated by direct domain imaging using magnetic force microscopy and micromagnetic simulations.     

Controlling Chiral Spin States of a Triangular‐Lattice Magnet by Cooling in a Magnetic Field

Magnetic materials with a non‐collinear and non‐coplanar arrangement of magnetic moments hosting a nonzero scalar spin‐chirality exhibit unique magnetic and spin‐dependent electronic transport properties. The spin chirality often occurs in materials where competing exchange interactions lead to geometrical frustrations between magnetic moments and to a strong coupling between the crystal lattice and the magnetic structure. These characteristics are particularly strong in Mn‐based antiperovskites where the interactions and chirality can be tuned by substitutional modifications of the crystalline lattice. This study presents evidence for the formation of two unequal chiral spin states in magnetically ordered Mn_3.338Ni_0.651N antiperovskite based on density functional theory calculations and supported by magnetization measurements after cooling in a magnetic field. The existence of two scalar spin‐chiralities of opposite sign and different magnitude is demonstrated by a vertical shift of the magnetic‐field dependent magnetization and Hall effect at low fields and from an asymmetrical magnetoresistivity when the applied magnetic field is oriented parallel or antiparallel to the direction of the cooling field. This opens up the possibility of manipulating the spin chirality for potential use in the emerging field of chiral spintronics.     

Strain-Controlled Spin Wave Excitation and Gilbert Damping in Flexible Co2FeSi Films Activated by Femtosecond Laser Pulse

The dynamic response of magnetic order to optical excitation at sub-picosecond scale has offered an intriguing alternative for magnetism manipulation. Such ultrafast optical manipulation of magnetism has become a fundamental challenging topic with high implications for future spintronics. Here, this study demonstrates such manipulation in Co₂FeSi films grown on flexible polyimide substrate, and demonstrates how the magneto-optical interaction can be modified by using strain engineering which in turn triggers the excitation of both dipolar and exchange spin waves modes. Furthermore, Gilbert damping and spin-orbit coupling in Co₂FeSi can both be tuned significantly by altering the magnitude and type of applied strain, suggesting an appealing way to manipulate spin wave propagation. These results develop the optical manipulation magnetism into the field of spin wave dynamics, and open a new direction in the application of spin orbitronics and magnonics devices using strain engineering.     

Dirac Cone Spin Polarization of Graphene by Magnetic Insulator Proximity Effect Probed with Outermost Surface Spin Spectroscopy

The effects of the proximity contact with magnetic insulator on the spin‐dependent electronic structure of graphene are explored for the heterostructure of single‐layer graphene (SLG) and yttrium iron garnet Y₃Fe₅O₁₂ (YIG) by means of outermost surface spin spectroscopy using a spin‐polarized metastable He atom beam. In the SLG/YIG heterostructure, the Dirac cone electrons of graphene are found to be negatively spin polarized in parallel to the minority spins of YIG with a large polarization degree, without giving rise to significant changes in the π band structure. Theoretical calculations reveal the electrostatic interfacial interactions providing a strong physical adhesion and the indirect exchange interaction causing the spin polarization of SLG at the interface with YIG. The Hall device of the SLG/YIG heterostructure exhibits a nonlinear Hall resistance attributable to the anomalous Hall effect, implying the extrinsic spin–orbit interactions as another manifestation of the proximity effect.     

Enhanced Spin Seebeck Effect in Monolayer Tungsten Diselenide Due to Strong Spin Current Injection at Interface

The spin current is significantly limited by the spin‐orbit interaction strength, material quality, and spin‐mixing conductance at material interfaces. Such limitations lead to spin current decay at the interfaces, which severely hinders potential applications in spin‐current‐generating thermoelectric devices. Thus, methodical studies on the enhancement of spin currents are indispensable. Herein, a novel approach for enhancing the spin current injected into a normal metal, Pt, using interface effects with a ferromagnetic insulator, yttrium iron garnet (YIG), is demonstrated. This is accomplished by inserting atomically thin monolayer (ML), tungsten diselenide (WSe₂) between Pt and YIG layers. A comparative study of longitudinal spin Seebeck effect (LSSE) measurements is conducted. Two types of ML WSe₂ (continuous and large‐area ML WSe₂ and isolated ML WSe₂ flakes) are used as intermediate layers on YIG film. Notably, the insertion of ML WSe₂ between the Pt and YIG layers significantly enhances the thermopower, V_LSSE/ΔT by a factor of approximately 5.6 compared with that of the Pt/YIG reference sample. This enhancement in the measured LSSE voltages in the Pt/ML WSe₂/YIG trilayer can be explained by the increased spin‐to‐charge conversion at the interface owing to the large spin‐orbit coupling and improved spin mixing conductance with the ML WSe₂ intermediate layer.     

Spin filter type transformation in Sn-phthalocyanine

We have investigated the electronic transport properties of Sn-phthalocyanine (SnPc) by using nonequilibrium Green's functions in combination with the density functional theory. The results show that, the spin-up/spin-down PDOS peak at 0.74 eV of SnPc will firstly move toward to the Fermi level and then be away from the Feimi level as the Sn atom is pulled out. A transform from spin-down filter to spin-up filter can be observed in parallel configuration due to spin flipping introduced by structural reorganization. While in anti-parallel configuration, the band structures of the two electrodes will play the dominant role in the electron properties of SnPc. The spin filter type conversion can be realized by pulling the Sn atom out of the SnPc or flipping the magnetic field of one electrode.     

量子点接触中的自旋过滤特性

利用可调节自旋过滤器模型,首次计算并讨论了磁场和电子跃迁能量间隔变化对量子点接触结构中自旋电子过滤特性的影响。研究发现,磁场和电子跃迁能量间隔的变化引起了自旋电子隧穿概率和隧穿电导都呈现出量子台阶效应,磁场的增加使电子的回旋频率和电子的Zeeman能级分裂同时加强,从而导致量子点接触结构中的横向约束加强,而自旋过滤效应明显减弱;当磁场一定时,电子跃迁能量间隔越小,电子的自旋过滤效应越明显。电子跃迁能量间隔改变的同时,也改变了鞍形势的势垒形状和自旋过滤的灵敏度。对于不同的材料,同时考虑磁场和电子跃迁能量间隔的作用可以找到自旋过滤器的最佳过滤效果。尤为重要的是过滤器的结构可以用标准的电子束技术很容易得到,所以研究结论为设计新型自旋过滤器提供了理论依据,具有广阔的应用前景和潜在的商业价值。此外,使用朗道因子值较高的材料作自旋过滤器的衬底,可以进一步提高过滤器的性能。     

NbIr2B2 and TaIr2B2 – New Low Symmetry Noncentrosymmetric Superconductors with Strong Spin–Orbit Coupling

Superconductivity was first observed more than a century ago, but the search for new superconducting materials remains a challenge. The Cooper pairs in superconductors are ideal embodiments of quantum entanglement. Thus, novel superconductors can be critical for both learning about electronic systems in condensed matter and for possible application in future quantum technologies. Here two previously unreported materials, NbIr₂B₂ and TaIr₂B₂, are presented with superconducting transitions at 7.2 and 5.2 K, respectively. They display a unique noncentrosymmetric crystal structure, and for both compounds the magnetic field that destroys the superconductivity at 0 K exceeds one of the fundamental characteristics of conventional superconductors (the “Pauli limit”), suggesting that the superconductivity may be unconventional. Supporting this experimentally based deduction, first-principle calculations show a spin-split Fermi surface due to the presence of strong spin–orbit coupling. These materials may thus provide an excellent platform for the study of unconventional superconductivity in intermetallic compounds.     

Utilization of the Antiferromagnetic IrMn Electrode in Spin Thermoelectric Devices and Their Beneficial Hybrid for Thermopiles

The thermoelectric effect in various magnetic systems, in which electric voltage is generated by a spin current, has attracted much interest owing to its potential applications in energy harvesting, but its power generation capability has to be improved further for actual applications. In this study, the first instance of the formation of a spin thermopile via a simplified and straightforward method which utilizes two distinct characteristics of antiferromagnetic IrMn is reported: the inverse spin Hall effect and the exchange bias. The former allows the IrMn efficiently to convert the thermally induced spin current into a measurable voltage, and the latter can be used to control the spin direction of adjacent ferromagnetic materials. It is observed that a thermoelectric signal is successfully amplified in spin thermopiles with exchange‐biased IrMn/CoFeB structures, where an alternating magnetic alignment is formed using the IrMn thickness dependence of the exchange bias. The scalable signal on a number of thermopiles allowing a large‐area application paves the way toward the development of practical spin thermoelectric devices. A detailed model analysis is also provided for a quantitative understanding of the thermoelectric voltages, which consist of the spin Seebeck and anomalous Nernst contributions.     

Non-linear quasi-optical cell with polarized nuclear target (PNT) material

A resonance electromagnetic structure (a Fabry-Perot resonator) with a plane-parallel sample of the polarized nuclear target (PNT) material of a paramagnetic complex Na HMBA (Cr^V) is considered under condition of dynamic polarization of nuclear by millimeter waves. Non-linear effects observed at the electron spin resonance (ESR) in such structure have been studied. The results of the experiment with the quasi-optical resonance cell of the radiospectrometer designed for the investigation of the magnetic resonance in the millimeter wavelengths range are given. A hysteresis of the transfer coefficient of the cell in the ESR-line has been found while slow passing the ESR-line by retuning of the static magnetic field. The deformation of the form of non-stationary signals passed through the cell has been found.     

Observation of Uniaxial Strain Tuned Spin Cycloid in a Freestanding BiFeO3 Film

Bismuth ferrite (BiFeO₃) possesses a non-collinear spin order while the ferroelectric order breaks space inversion symmetry. This allows efficient electric-field control of magnetism and makes it a promising candidate for applications in low-power spintronic devices. Epitaxial strain effects have been intensively studied and exhibit significant modulation of the magnetic order in bismuthBiFeO₃, but tuning its spin structure with continuously varied uniaxial strain is still lacking at this moment. Herein, in situ uniaxial tensile strain is applied to a freestanding BiFeO₃ film by mechanically stretching an organic substrate. A scanning nitrogen-vacancy (NV) microscopy is applied to image the nanoscale magnetic order in real space. The strain is continuously increased from 0% to 1.5% and four images under different strains are acquired during this period. The images show that the spin cycloid tilts by ≈12.6° when strain approaches 1.5%. A first principle calculation is processed to show that the tilting is energetically favorable under such strain. The in situ strain applying method in combination with scanning NV microscope real-space imaging ability paves a new way in studying the coupling between magnetic order and strain in BiFeO₃ films.     

自旋对晶体内弱耦合磁极化子性质的影响

本文采用线性组合算符和微扰法研究极性晶体中弱耦合磁极化子的性质。在计及电子在反冲效应中发射和吸收不同波矢的声子之间的相互作用时,讨论电子的自旋对极性晶体中弱耦合磁极化子基态能量的影响。结果表明,在不同电子自旋状态下,磁极化子的基态能量Ｅ０随磁场Ｂ的增大而减小;电子自旋能量与Ｅ０之比随Ｂ的增加而增加。     

基于自旋阀结构的磁传感器的研究

自旋阀结构的发现为磁电子学以及磁传感器的研究揭开了新的一页。基于自旋阀结构的磁传感器由于具有灵敏度高、功耗小、高集成度等优点,因此在传感器工业中具有广泛的应用前景。本文介绍了基于自旋阀结构的磁传感器的研究方法。首先介绍了自旋阀结构及其特性,然后介绍了基于自旋阀结构的磁性薄膜的制备方法和结构优化,其次介绍了基于自旋阀薄膜的磁传感器芯片的制造工艺,最后介绍了基于惠斯通电桥结构的自旋阀磁传感器芯片。     

Applications of electron paramagnetic resonance to studies of defects in insulators and semiconductors

Electron paramagnetic resonance (EPR) which is the absorption of electromagnetic waves in the microwave frequency domain has been used for many years to study magnetic dipoles in crystals subjected to magnetic fields. These magnetic dipoles arise from electrons in the material whose intrinsic spin is exposed due to processes such as doping with transition metal ions, doping with ions whose spin is not locked up with chemical bonding and irradiation with bond breaking energy. The sensitivity of the EPR technique depends on such experimental parameters as, incident microwave power, size of the sample, quality of the microwave cavity, number of paramagnetic spins participating in the absorption, width of the EPR resonance line, temperature of the sample and relaxation time of the spin system to the lattice. The experimental spectrum is described by parameters which by the application of basic quantum mechanics can be related to properties of the wave function of the electron spin in the crystal environment such as bonding directions to neighboring ions in the crystal lattice. Some examples of this analysis will be presented to show how EPR is used in the study of defects in Si and SiO₂.     

自旋晶体管的最新研究进展

自旋晶体管是指利用电子自旋自由度构建的在结构上类似于传统半导体晶体管的三端自旋器件。对基于自旋劈裂的磁双极型自旋晶体管、基于热电子输运的自旋晶体管和基于Rashba效应的自旋晶体管的最新研究动态进行了评述,并对其发展前景做了展望。     

SiC半导体二维自旋磁极化子能量的磁场效应

在考虑声子之间相互作用和电子自旋的情况下,应用么正变换和线性组合算符法研究了电子自旋对SiC半导体弱耦合二维自旋磁极化子能量磁场效应的影响。数值计算给出了下列结果:电子自旋使自陷能分裂为二,且随磁场B增加其分裂间距增大;电子自旋能量与电子在磁场中的Landau基态能之比恒为0.23;电子自旋能量与自能之比小于电子自旋能量与自陷能之比,但非常接近,它们随B增强而近似线性增大,当B为0和10T时,它们分别为0和0.008;电子自旋能量与声子之间相互作用能量之比也随B增加而线性增大,当B为0和7.949T时,其比值分别为0和1。该结果有助于设计和研制自旋场效应晶体管,自旋发光二极管和自旋共振隧道器件等。     

纳米尺寸自旋阀中电流驱动的自旋波发射

提出了在纳米赝自旋阀中的电流感应自旋传输矩的磁动力学描述,成功地解释了在磁纳米多层结构中的电流驱动微波发射和电流感应磁化翻转现象。自旋流极化由在电导匹配时的自旋流和化学势连续性边界条件决定。自旋矩的纵向和横向分量在自旋阀的电流驱动微波发射和电流感应磁化翻转现象中扮演了不同的角色:纵向自旋矩分量决定了电流感应磁化翻转(CIMS)效应,而横向自旋矩是自旋波发射(SWE)效应所不可缺少的。根据这一理论,由LLG方程自然得到自旋波发射的双模,分别为横向自旋矩引发的X和Y方向的振动,并引起磁多层电阻以频率2w或w(进动频率)随时间变化。磁场和自旋流共同决定了自旋波发射的频率和功率,这一理论预言了某种特殊的磁多层结构,如磁层相互垂直的结构,将具有大得多的微波发射效率,这一结论已经被实验所证实。     

Dzyaloshinskii-Moriya Torque-Driven Resonance in Antiferromagnetic α-Fe2O3

The high-frequency optical mode of α-Fe₂O₃ is examined, and it is reported that Dzyaloshinskii−Moriya (DM) interaction generates a new type of torque on the magnetic resonance. Using a continuous-wave terahertz interferometer, the optical mode spectra is measured, where the asymmetric absorption with a large amplitude and broad linewidth is observed near the magnetic transition point, Morin temperature (T_M ≈ 254.3 K). Based on the spin wave model, the spectral anomaly is attributed to the DM interaction-induced torque, enabling to extract the strength of DM interaction field of 4 T. This work opens a new avenue to characterize the spin resonance behaviors at an antiferromagnetic singular point for next-generation and high-frequency spin-based information technologies.