Magneto-optical Kerr and Faraday effects in bilayer antiferromagnetic insulators

Control and detection of antiferromagnetic topological materials are challenging since the total magnetization vanishes.Here we investigate the magneto-optical Kerr and Faraday effects in bilayer antiferromagnetic insulator Mn Bi2Te4.We find that by breaking the combined mirror symmetries with either perpendicular electric field or external magnetic moment,Kerr and Faraday effects occur.Under perpendicular electric field,antiferromagnetic topological insulators(AFMTI)show sharp peaks at the interband transition threshold,whereas trivial insulators show small adjacent positive and negative peaks.Gate voltage and Fermi energy can be tuned to reveal the differences between AFMTI and trivial insulators.We find that AFMTI with large antiferromagnetic order can be proposed as a pure magneto-optical rotator due to sizable Kerr(Faraday)angles and vanishing ellipticity.Under external magnetic moment,AFMTI and trivial insulators are significantly different in the magnitude of Kerr and Faraday angles and ellipticity.For the qualitative behaviors,AFMTI shows distinct features of Kerr and Faraday angles when the spin configurations of the system change.These phenomena provide new possibilities to optically detect and manipulate the layered topological antiferromagnets.

Determination of the magnetic anisotropy constant of Cu/Fe/SiO_2/Si by a magneto-optical Kerr effect susceptometer

The magneto-optical Kerr effect susceptometry technique is proposed to determine the uniaxial magnetic anisotropy （UMA） constant Ku. The magnetic properties of Cu/Fe/SiO2/Si grown by dc magnetron sputtering were investigated. The in-plane uniaxial magnetic anisotropy was probed by the magneto-optical Kerr effect （MOKE）. The value of UMA, Ku = 2.5 x 103 J/m3, was simulated from the field dependence of ac susceptibility along the hard axis according to the Stoner-Wohlfarth （S-W） model, which is consistent with Ku = 2.7~ 103 J/m3 calculated from the magnetic hysteresis loops. Our results show that the magneto-optical Kerr effect susceptometry can be employed to determine the magnetic anisotropy constant owing to its high sensitivity.

Resonant magneto-optical Kerr effect induced by hybrid plasma modes in ferromagnetic nanovoids

With nanovoids buried in Co films, resonant structures were observed in spectra of polar magneto-optical Kerr effect（MOKE）, where both a narrow bandwidth and high intensity were acquired. Through changing the thickness of Co films and the lattice of voids, different optical modes were introduced. For a very shallow array of voids, the resonant MOKE was induced by Ag plasma edge resonance, for deeper ones, hybrid plasma modes, such as void plasmons in the voids, surface lattice plasmons between the voids, and the co-action of them, etc. resulted in resonant MOKE. We found that resonant MOKE resulted from the void plasmons resonance which possesses the narrowest bandwidth for the lowest absorption of voids. The simulated electromagnetic field（EF） distribution consolidated different effects of these three optical modes on resonant MOKE modulation. Such resonant polar MOKE possesses high sensitivity, which might pave the way to on-chip MO devices.

The subwavelength tuned magneto-optical Kerr effect in L1_0-FePt films with perpendicular magnetic anisotropy

For L10-FePt films with strong perpendicular anisotropy covered by arrays of hexagonal close-packed polystyrene spheres （PSSs）, fine structures are observed in magneto-optical Kerr rotation spectra in the visible spectral range. The reflection minima are found to be located at the same wavelengths as the Kerr rotation peaks. The Kerr rotation enhancement is attributed to the excitation of both the surface plasmon polariton in the dielectric PSS/metal interface and the guide waves （guide mode） in the PSS array. The two-dimensional PSSs/SiO2/FePt system exhibiting a tunable magneto-optical Kerr effect and a high perpendicular magnetic anisotropy will be helpful for designing and fabricating magneto-optics devices.

Surface Plasmon and Fabry-Perot Enhanced Magneto-Optical Kerr Effect in Graphene Microribbons

A single sheet of graphene exhibits the ability to turn polarization of light by several degrees in modest magnetic fields. Here we demonstrate that giant angle rotation in graphene in the terahertz range can be realized and further increased by the introduction of surface plasmon and constructive Fabry Perot interference with the supporting substrate. The maximum Kerr rotation angle is up to 15° in a single layer of graphene ribbons at 6 TPIz for the applied magnetic field 4 T. Such a magnification in magneto-optical Kerr effect can be realized in a fairly large incident angle.

Study of magnetization reversal and anisotropy of single crystalline ultrathin Fe/MgO (001) film by magneto-optic Kerr effect

The magnetization reversal process of Fe/MgO （001） thin film is investigated by combining transverse and longi- tudinal hysteresis loops. Owing to the competition between domain wall pinning energy and weak uniaxial magnetic anisotropy, the typical magnetization reversal process of Fe ultrathin film can take place via either an ＂l-jump＂ process near the easy axis, or a ＂2-jump＂ process near the hard axis, depending on the applied field orientation. Besides, the hysteresis loop presents strong asymmetry resulting from the variation of the detected light intensity due to the quadratic magneto-optic effect. Furthermore, we modify the detectable light intensity formula and simulate the hysteresis loops of the Kerr signal. The results show that they are in good agreement with the experimental data.

Magneto-optical Kerr Effect of Mono-layer NiX_(2)(X=Cl,Br,I):A Density Functional Theory Study

The full-potential linearized augmented plane wave plus local orbital method is utilized for exploring the electronic,magnetic,and magneto-optical properties of the NiX_(2)(X=Cl,Br,and I)single layer.The first-principles calculation demonstrates that these compounds are ferromagnetic indirect semiconductors,and the energy band gaps of NiX_(2)for X=Cl,Br,and I are 3.888,3.134,and 2.157 eV,respectively.The magnetic moments of Ni atoms in NiX_(2)monolayer are 1.656,1.588,1.449μB,and their magneto-crystalline anisotropy energies are 0.167,0.029,0.090 meV,respectively.Based on the macro-linear response theory,we systematically studied the influences of the external magnetic field and out-of-plane strain on the magneto-optical Kerr effect(MOKE)spectrum of the NiX_(2)single layer.It is found that,when the external magnetic field is perpendicular to the sample plane,the value of the Kerr rotation angle reaches the maximum,and the single-layer NiI_(2)material has a Kerr rotation angle of 1.89°at the photon energy of 1.986 eV.Besides,the Kerr rotation spectrum of NiCl_(2)and NiBr_(2)monolayers redshift as the out-of-plane strain increases,while NiI_(2)monolayer blueshifts.Accurate computation of the MOKE spectrum of NiX_(2)materials provides an opportunity for applications of 2D magnetic material ranging from sensing to data storing.

Magneto-optical Kerr effect in Co_xAg_(1-x) nanostructured granular films

A series of CoxAg1-x nanostructured granular films have been prepared by ion beam sputtering and annealed at 100, 250, 400, 5001. The optical constants, the complex dielectric func-tion and magneto-optical Kerr parameters were measured at room temperature. For the samples hav-ing low Co compositions, the strong Kerr effect enhancement is observed around the Ag plasma edge. With an increase in annealing temperature, the resonance-like peaks appear in the Kerr rotation, and the peak positions are shifted toward the low photon energy region. From the analysis of the numerical calculations, it is concluded that the magneto-optical Kerr effect enhancement (MOKE) is due to the presence of the steep plasma edge.

Frequency dependence of magnetization and giant magneto impedance effect of amorphous wires

The frequency dependence of magnetization process and giant magneto impedance （GMI） effect of Co-rich melt-extracted amorphous wires was studied by Kerr effect and impedance analyzer, respectively. It is demonstrated that the transverse Kerr intensity and the corresponding GMI response increase with increasing frequency, which contributes to the upgraded skin effect. However, the skin depth has a slothful trend with frequency when it is up to the megahertz range, which gives rise to the transformation of magnetization. The process is much more sensitive to the direct current magnetic field and the sensitive change of the circular permeability, and GMI response is observed as its consequence. This proves that the evolution of circumferential magnetization and the corresponding permeability with the direct current magnetic field is the essence of GMI response, and a much more sensitive magnetization promises a better GMI response.

Improving the sensitivity of DC magneto-optical Kerr effect measurement to 10^(-7)rad/√Hz

A high-sensitivity DC magneto-optical Kerr effect(MOKE)apparatus is described in this Letter.Via detailed analysis on several dominatingnoise sources,we have proposed solutions that significantly lower the MOKE noise,and a sensitivity of 1.5×10^(-7)rad/√Hz is achieved with long-term stability.The sensitivity of the apparatus is tested by measuring a wedgeshaped Ni thin film on SiO_(2)with Ni thickness varying from 0 to 3 nm.A noise floor of 1.5×10^(-8)rad is demonstrated.The possibility of further improving sensitivity to 10^(-9)rad via applying AC modulation is also discussed.

一维金属光栅嵌入磁性介质纳米结构下的横向磁光克尔效应的增强

本文在一维金属光栅嵌入磁性介质的体系中实现了横向磁光克尔效应的增强.通过最优化金属光栅的嵌入深度来有效激发磁性介质中的波导模式与金属条带上的局域等离激元模式,从而使横向磁光克尔效应的响应得到巨大增强.本文提出了一种用于增强横向磁光克尔效应的新型等离激元微纳结构,这种结构可以应用于高性能磁光器件的设计.

Tuning the magnetic anisotropy of Co Fe B grown on flexible substrates

The magnetic properties of CoFeB thin films grown on flexible polyimide substrates were investigated using a magneto-optical Kerr effect magnetometer. In-plane uniaxial magnetic anisotropy was observed in the virgin state. The strain induced by bending the flexible substrate was applied on the sample to change the magnetic properties of CoFeB.The strain induced uniaxial magnetic anisotropy changed linearly with the deformation by about 8.41 × 10^4erg/cm^3 at 1%of deformation. Our results prove the magnetic properties of CoFeB grown on flexible polyimide substrate can be tuned effectively by bending, which could be important for future flexible spintronics.

Rapid screening of magnetic properties in several Fe-X-Ni systems via combinatorial materials chip method

Fe-X-Ni(X=Cr,W and V)combinatorial thin-film(∼100 nm thick)materials chips covering the full composition range of ternary systems were fabricated.The crystal structure distribution was mapped by micro-beam X-ray diffractometers(XRD)and the magnetic hysteresis loops over the chip were characterized by a high-throughput magneto-optical Kerr effect(HT-MOKE)system to establish the composition-phase-magnetic properties relationships.The results showed that saturation magnetization for all systems has a strong dependency on alloying composition,and decreases with increasing dopped elements content as a general trend.Although the trend of saturation magnetization in bulk is in good agreement with that from thin films,all bulk samples show almost no coercivity,attributable to the much smaller grain size,and stronger texture in thin-film samples.Comparing the Fe-X-Ni systems under a similar condition,in the out-of-plane,Cr alloying obtained the largest coercivity(∼400 mT)followed by W alloying(∼300 mT)and then V alloying(∼200 mT).We suggest that alloying with different elements leads to the diverse orientation and crystallinity of the fcc phase resulting in different magnetic properties.Meanwhile,the effect of heat treatment on magnetic properties indicates that saturation magnetization is more closely related to the duration of heat treatment.

Combining two-photon lithography with laser ablation of sacrificial layers:A route to isolated 3D magnetic nanostructures

Three-dimensional(3D)nanostructured functional materials are important systems allowing new means for intricate control of electromagnetic properties.A key problem is realising a 3D printing methodology on the nanoscale that can yield a range of functional materials.In this article,it is shown that two-photon lithography,when combined with laser ablation of sacrificial layers,can be used to realise such a vision and produce 3D functional nanomaterials of complex geometry.Proof-of-principle is first shown by fabricating planar magnetic nanowires raised above the substrate that exhibit controlled domain wall injection and propagation.Secondly,3D artificial spin-ice(3DASI)structures are fabricated,whose complex switching can be probed using optical magnetometry.We show that by careful analysis of the magneto-optical Kerr effect signal and by comparison with micromagnetic simulations,depth dependent switching information can be obtained from the 3DASI lattice.The work paves the way for new materials,which exploit additional physics provided by non-trivial 3D geometries.

硅材料的场致线性电光效应

从经典的极化理论出发,分析了直流电场、低频调制电场和光波电场共同存在时硅材料折射率的变化,从理论上揭示了场致线性电光效应的物理实质。以近本征硅材料为样品,采用金属-绝缘体-半导体样品结构,搭建了由塞纳蒙(Senarmont)补偿器改进成的横向电光调制系统。在硅材料空间电荷区内观测到显著的线性电光调制效应,系统的半波电压小于170 V,从实验上直接证实了硅材料中内建电场诱导的场致线性电光效应的存在。此外还观测到由克尔效应引起的二次电光调制信号,以及由场致光整流效应引起的、随线偏振光的方位角的二倍余弦变化的电信号。实验结果与经典极化理论的预期完全一致,也间接证实了硅材料中场致线性电光效应的存在。

衰减全反射磁镜偏频激光陀螺的理论分析

通过分析常规磁镜在红光波长下制造激光陀螺技术上不可行的原因 ,提出了一种新型的磁镜偏频激光陀螺结构 ,其核心是利用全内反射结构和掺杂YIG薄膜的横向克尔磁光效应作为偏频技术去克服闭锁的衰减全反射磁镜。

双横向电光克尔效应

理论分析了克尔(Kerr)介质在两个横向电场作用下的电光调制特性,并由此确定了具有双横向电光克尔效应的光学材料种类。采用折射率椭球方法分析可知,各向同性克尔介质和一些晶体在横向外加电场作用下,其电光相位延迟与外加电场幅值的平方成正比,与电场方向无关;而其电光双折射主轴方位角与外加电场的方向角成正比,与电场幅值无关,称这种电光调制特性为双横向电光克尔效应。理论分析结果表明,排除兼有泡克耳斯(Poekels)效应和电致旋光效应的材料,立方晶系432,m3m点群和六角晶系6/mmm点群的晶体以及所有各向同性光学介质具有双横向电光克尔效应。目前已有克尔常数较大的陶瓷玻璃材料研制成功,将有力地促进双横向电光效应在光学各领域中的广泛应用。

基于Au/Ce···YIG/TiN结构的磁光表面等离激元共振及折射率传感器研究

提出了一种基于Au/Ce:YIG/TiN结构的磁光表面等离激元共振器件(MOSPR)。通过构建Au周期纳米盘、Ce:YIG薄膜和TiN薄膜三层结构,实现Au纳米盘局域表面等离激元共振(LSPR)和TiN/Ce:YIG界面传播型表面等离激元共振耦合,显著降低了LSPR的散射损耗,并实现了磁光效应的显著增强。MOSPR的横向磁光克尔效应(TMOKE)信号的绝对值达0.21。应用这一器件制备传感器,借助磁光氧化物的强磁光效应,可以显著提高LSPR传感器的品质因数(FoM)。基于TMOKE谱进行传感,器件的FoM可达2192.4586RIU-1。该研究为高灵敏度、高FoM LSPR器件的制备提供了一种新思路。

Relationship between microstructure and magnetic domain structure of Nd-Fe-B melt-spun ribbon magnets

The relation between the microstructure, observed using an electron probe microanalyzer, and the domain structure, observed using a Kerr microscope, was established to evaluate the effects of hot rolling and the addition of Ti-C on the c-axis orientation and the magnetization process of hot-rolled Nd-Fe-B-Ti-C melt-spun ribbons. The addition of Ti-C promotes the c-axis orientation and high coercivity in the ribbons. Elemental mapping suggests a uniform elemental distribution; however, an uneven distribution of Ti was observed in an enlarged grain with Ti-enriched points inside the grain. The reversal domains that nucleated at the Ti-enriched point inside the grain cause low coercivity.

Vector analysis of electric-field-induced antiparallel magnetic domain evolution in ferromagnetic/ferroelectric heterostructures

Electric field(E-field)control of magnetism based on magnetoelectric coupling is one of the promising approaches for manipulating the magnetization with low power consumption.The evolution of magnetic domains under in-situ E-fields is significant for the practical applications in integrated micro/nano devices.Here,we report the vector analysis of the E-field-driven antiparallel magnetic domain evolution in FeCoSiB/PMN-PT(011)multiferroic heterostructures via in-situ quantitative magneto-optical Kerr microscope.It is demonstrated that the magnetic domains can be switched to both the 0°and 180°easy directions at the same time by E-fields,resulting in antiparallel magnetization distribution in ferromagnetic/ferroelectric heterostructures.This antiparallel magnetic domain evolution is attributed to energy minimization with the uniaxial strains by E-fields which can induce the rotation of domains no more than 90°.Moreover,domains can be driven along only one or both easy axis directions by reasonably selecting the initial magnetic domain distribution.The vector analysis of magnetic domain evolution can provide visual insights into the strain-mediated magnetoelectric effect,and promote the fundamental understanding of electrical regulation of magnetism.