Transverse Magneto-optical Kerr Effect Research Articles

Resonant excitation of spin waves in hybrid nanostructures via frequency combs

Here we propose a new method of resonant optical excitation of spin waves in hybrid nanostructures via frequency comb. We consider a two-layer nanostructure consisting of an Au layer with a grating on the top and an yttrium iron garnet (YIG) thin film at the bottom. The plasmon evanescent fields induce time-independent magnetic polarization via the transverse magneto-optical Kerr effect (TMOKE) at the Au-YIG interface. Thus magnetic polarization in turn plays a role of a source for the excitation of the spin waves in YIG film. We show that by using the frequency comb as a source we can generate the magnetic polarization in resonance with the spin waves in YIG thus substantially increasing the magneto-optical response of the system. These results open new routes in the optical control of spintronics systems.

Giant magneto-optical Faraday effect of graphene on Co in the soft x-ray range

Using polarization analysis of linearly polarized synchrotron radiation we demonstrate the existence of a giant magneto-optical Faraday effect at the carbon $1s$ edge of single-layer graphene on Co, reaching Faraday rotation angles of $2.9\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.16em}{0ex}}\mathrm{deg}/\mathrm{mm}$. This value is of the order of those observed at the Co $3p$ and $2p$ edges. Using element-selective magnetic hysteresis curves we find that graphene on Co exhibits ferromagnetic order. The magnetism in graphene is found to be carried by and be strongly enhanced by aligned \ensuremath{\pi} orbitals of carbon atoms. It is induced by hybridization with the Co $3{d}_{z}2$ orbitals while carbon \ensuremath{\sigma} bonds show negligible magnetism due to insignificant hybridization with Co. From additional x-ray magnetic circular dichroism and transversal magneto-optical Kerr effect spectra a magnetic moment of $0.14\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ is estimated for graphene. From Faraday spectra the complete set of x-ray magneto-optical constants of graphene has been deduced which allows for future modeling of magneto-optical devices based on graphene. The strong magnetism in graphene results from hybridization of carbon ${p}_{\mathrm{z}}$ and metal $3d$ orbitals. Atoms of the graphene sublattice A, placed on top of Co, lead to strongest hybridization with Co $3{{d}_{z}}^{2}$ orbitals. Carbon atoms of sublattice B, and those of rotated graphene domains without Co atoms beneath, hybridize with each other and with $3{d}_{\mathrm{xz}}$ and $3{d}_{\mathrm{yz}}$ orbitals of neighboring Co atoms forming tilted ${p}_{\mathrm{z}}$ bonds. We show that the related reduction of A-B symmetry leads to a splitting of the spin-polarized density of conduction-band states which is responsible for the strong magneto-optical Faraday effect.

Magnetoplasmonic quasicrystals: an approach for multiband magneto-optical response

Nanostructured magneto-optical materials that sustain optical resonances provide an efficient way to control light via the magnetic field, which is of prime importance for telecommunication and sensing applications. However, their response is usually narrowband due to their resonance character. Here, we demonstrate and investigate a type of magnetoplasmonic structure, or quasicrystal, that demonstrates a unique magneto-optical response. It consists of a magnetic dielectric film covered by a thin gold layer perforated by slits, forming a Fibonacci-like binary sequence. The transverse magneto-optical Kerr effect (TMOKE) acquires controllable multiple plasmon-related resonances, resulting in a magneto-optical response over a wide frequency range. In particular, for the experimentally studied samples, the TMOKE resonances are observed in the wavelength range from λ=780 nm to λ=970 nm. Multiband TMOKE is valuable for numerous nanophotonic applications, including optical sensing, control of light, all-optical control of magnetization, etc. TMOKE spectroscopy is also an efficient tool for investigating the peculiarities of plasmonic quasicrystals.

Phase Separation in GaMnAs Layers Grown by Laser Pulsed Deposition

GaMnAs layers grown by pulsed laser deposition were studied by magneto-optical transversal Kerr effect spectroscopy, spectral ellipsometry, resistivity and the Hall effect measurements, and magnetometry. The growth regimes for magnetically inhomogeneous layers containing Mn-enriched ferromagnetic regions with different composition and sizes were established. The layers grown at a temperature of 300°C exhibit a ferromagnetic behavior at temperatures below 80 K, which is conditioned by the presence of local ferromagnetic (Ga,Mn)As regions in the paramagnetic matrix. The nature of peculiarities in the TKE spectrum of these layers is discussed.

Surface Plasmon Resonance Enhanced the Transverse Magneto-optical Kerr Effect in One-dimensional Magnetoplasmonic Nanostructure

The properties of the optics and transverse magneto-optical Kerr effect (TMOKE) of one-dimensional magnetoplasmonic nanostructures are experimentally investigated. It shows that the resonant dips of the reflectance spectra and the enhancement of the TMOKE of the designed structures are attributed to the excitation and the coupling of the localized surface plasmon (LSP) and surface plasmon polariton (SPP) modes. Moreover, the insertion of the nonmagnetic dielectric SnO2 layer into the quadrilayer structure of Ag/Co/SnO2/Ag can not only pronouncedly enhance the TMOKE signals of the sample but also prompt their resonant positions to generate a blueshift. It has been demonstrated that the enhancement of the TMOKE signal and the blueshift of the resonant wavelength of the TMOKE stems from the coupling of SPP of the two different interfaces of the silver film.

Element-selective investigation of femtosecond spin dynamics in NiPd magnetic alloys using extreme ultraviolet radiation

We studied femtosecond spin dynamics in ${\mathrm{Ni}}_{x}{\mathrm{Pd}}_{1\ensuremath{-}x}$ magnetic thin films by optically pumping the system with infrared (1.55 eV) laser pulses and subsequently recording the reflectivity of extreme ultraviolet (XUV) pulses synchronized with the pump pulse train. XUV light in the energy range from 20 to 72 eV was produced by laser high-harmonic generation. The reflectivity of XUV radiation at characteristic resonant energies allowed separate detection of the spin dynamics in the elemental subsystems at the ${M}_{2,3}$ absorption edges of Ni (68.0 and 66.2 eV) and ${N}_{2,3}$ edges of Pd (55.7 and 50.9 eV). The measurements were performed in transversal magneto-optical Kerr effect geometry. In static measurements, we observed a magnetic signature of the Pd subsystem due to an induced magnetization. Calculated magneto-optical asymmetries based on density functional theory show close agreement with the measured results. Femtosecond spin dynamics measured at the Ni absorption edges indicates that increasing the Pd concentration, which causes a decrease in the Curie temperature ${T}_{\text{C}}$, results in a drop of the demagnetization time ${\ensuremath{\tau}}_{\text{M}}$, contrary to the ${\ensuremath{\tau}}_{\text{M}}\ensuremath{\sim}1/{T}_{\text{C}}$ scaling expected for single-species materials. This observation is ascribed to the increase of the Pd-mediated spin-orbit coupling in the alloy.

TMOKE as efficient tool for the magneto-optic analysis of ultra-thin magnetic films

Ultra-thin magnetic dielectric films are of prime importance due to their applications for nanophotonics and spintronics. Here, we propose an efficient method for the magneto-optical investigation of ultra-thin magnetic films which allows one to access their state of magnetization and magneto-optical properties. It is based on the surface-plasmon-polariton-assisted transverse magneto-optical Kerr effect (TMOKE). In our experiments, sub-100 nm-thick bismuth-substituted lutetium iron-garnet films covered with a plasmonic gold grating have been analyzed. The excitation of surface plasmon-polaritons provides resonance enhancement of TMOKE up to 0.04 and makes it easily detectable in the experiment. For films thicker than 40 nm, the TMOKE marginally depends on the film thickness. A further decrease in the film thickness diminishes TMOKE since for such thicknesses the surface plasmon-polariton field partly penetrates inside the non-magnetic substrate. Nevertheless, the TMOKE remains measurable even for few-nm-thick films, which makes this technique unique for the magneto-optical study of ultra-thin films. Particularly, the proposed method reveals that the off-diagonal components of the magnetic film permittivity tensor grow slightly with the reduction of the film thickness.

Фазовое разделение в слоях GaMnAs, сформированных импульсным лазерным осаждением

AbstractGaMnAs layers grown by pulsed laser deposition were studied by magneto-optical transversal Kerr effect spectroscopy, spectral ellipsometry, resistivity and the Hall effect measurements, and magnetometry. The growth regimes for magnetically inhomogeneous layers containing Mn-enriched ferromagnetic regions with different composition and sizes were established. The layers grown at a temperature of 300°C exhibit a ferromagnetic behavior at temperatures below 80 K, which is conditioned by the presence of local ferromagnetic (Ga,Mn)As regions in the paramagnetic matrix. The nature of peculiarities in the TKE spectrum of these layers is discussed.

Au/NiFe/M(Au, MoS2, graphene) trilayer magnetoplasmonics DNA-hybridized sensors with high record of sensitivity.

The demonstration of biosensors based on the surface plasmon effect holds promise for future high-sensitive electrodeless biodetection. The combination of magnetic effects with surface plasmon waves brings additional freedom to improve sensitivity and signal selectivity. Stacking biosensors with two-dimensional (2-D) materials, e.g., graphene (Gr) and MoS2, can influence plasmon waves and facilitate surface physiochemical properties as additional versatility aspects. We demonstrate magnetoplasmonic biosensors through the detuning of surface plasmon oscillation modes affected by magnetic effect via the presence of the NiFe (Py) layer and different light absorbers of Gr, MoS2, and Au ultrathin layers in three stacks of Au/Py/M(MoS2, Gr, Au) trilayers. We found minimum reflection, resonance angle shift, and transverse magneto-optical Kerr effect (TMOKE) responses of all sensors in the presence of the ss-DNA monolayer. Very few changes of ∼5×10-7 in the ss-DNA's refractive index result in valuable TMOKE response. We found that the presence of three-layer Gr and two-layer MoS2 on top of the Au/Py bilayer can dramatically increase the sensitivity by nine and four times, respectively, than the conventional Au/Co/Au trilayer. Our results show the highest reported DNA sensitivity based on the coupling of light with 2-D materials in magnetoplasmonic devices.

45° sign switching of effective exchange bias due to competing anisotropies in fully epitaxial Co3FeN/MnN bilayers

We report an unusual angular-dependent exchange bias effect in ferromagnet/antiferromagnet bilayers, where both ferromagnet and antiferromagnet are epitaxially grown. Numerical model calculations predict an approximately 45° period for the sign switching of the exchange-bias field, depending on the ratio between magnetocrystalline anisotropy and exchange-coupling constant. The switching of the sign is indicative of a competition between a fourfold magnetocrystalline anisotropy of the ferromagnet and a unidirectional anisotropy field of the exchange coupling. This predicted unusual angular-dependent exchange bias and its magnetization switching process are confirmed by measurements on fully epitaxial Co3FeN/MnN bilayers by longitudinal and transverse magneto-optic Kerr effect magnetometry. These results provide a deeper understanding of the exchange coupling phenomena in fully epitaxial bilayers with tailored materials and open up a complex switching energy landscape engineering by anisotropies.

Magneto-optical spectroscopy of diluted magnetic semiconductors GaMnAs prepared by ion implantation and further impulse laser annealing

Ga1−xMnxAs layers prepared by ion implantation and subsequent pulsed laser annealing with the planned Mn concentrations of x = 0.01–0.08 have been studied using the magneto-optical transversal Kerr effect (TKE) and spectral ellipsometry. The spectral dependences of the diagonal and nondiagonal components of the permittivity tensor (PT), as well as the spectrum of magnetic circular dichroism (MCD) have been calculated for the layers. The obtained spectra of the diagonal PT components show that the layers under study maintain the zinc-blende crystal structure of the parent GaAs semiconductor. All studied samples reveal a strong TKE response at low temperatures with a dependence of an effective Curie temperature (at which TKE appears) on the Mn concentration. A number of extrema in the low-temperature TKE and MCD spectra are close to the energies of transitions in the Γ and L critical points of the parent semiconductor band structure that confirms the intrinsic ferromagnetism of the Ga1−xMnxAs layers. The MCD spectra shape and its change with Mn concentration increasing are discussed on a base of the valence-band model of ferromagnetism in DMS.

Enhancement of magneto-optical effects by a single 1D all dielectric resonant grating

We demonstrate theoretically the enhancement of the four first-order magneto-optical effects using an all-dielectric 1D resonant grating. The modeled structure was designed for the Faraday effect at normal incidence at a wavelength of 1.55 µm for which the calculated improvement is shown to be one hundred times the off-resonance values. At 1° incidence, polar and longitudinal Kerr effects were also improved, showing polarization rotations up to 14°. For the same incidence conditions, transverse magneto optical Kerr effect was also improved, leading to a reflectance difference of 12% for a reflectance intensity of 40%. These results are interesting for the development of magnetic-field sensors capable of detecting of several components of the field.

Enhanced Transverse Magneto-Optical Kerr Effect in Magnetoplasmonic Crystals for the Design of Highly Sensitive Plasmonic (Bio)sensing Platforms

We propose a highly sensitive sensor based on enhancing the transversal magneto-optical Kerr effect (TMOKE) through excitation of surface plasmon resonances in a novel and simple architecture, which consists of a metal grating on a metal magneto-optical layer. Detection of the change in the refractive index of the analyte medium is made by monitoring the angular shift of the Fano-like resonances associated with TMOKE. A higher resolution is obtained with this technique than with reflectance curves. The key aspect of the novel architecture is to achieve excitation of surface plasmon resonances mainly localized at the sensing layer, where interaction with the analyte occurs. This led to a high sensitivity, S = 190° RIU–1, and high performance with a figure of merit of the order of 103, which can be exploited in sensors and biosensors.

Voltage-driven magneto-optical Kerr effect in a glass/Au/NiFe/dielectric/WS_2magneto-plasmonic structure

The current study was undertaken to design a voltage-driven magneto-plasmonic device analog to a field effect transistor (FET) but alternatively proposed for a transverse magneto-optical Kerr effect (TMOKE) response. The suggested structure is a glass/Au/NiFe/dielectric/WS2 multilayer with very similar constituted layers to the substrate/metal-gate/dielectric/conductive-channel FET design. Here, a Au/NiFe bilayer is of metal that Au is individually working as a plasmonic excitation layer and NiFe is a ferromagnetic layer for a TMOKE response. The WS2 monolayer is a conductive-channel layer separated from the metal-gate layer via a dielectric layer. The WS2 refractive index and absorption/reflection can be controlled upon applying voltage to the Au/NiFe metal-gate bilayer. Light reflection and TMOKE responses from this structure in a Kretschmann configuration were determined based on a transfer matrix method. Significant change in TMOKE was predicted as the result of large changes in the refractive index of the WS2 monolayer versus voltage as recently reported by Yu et al. [Nano Lett.17, 3613 (2017)NALEFD1530-698410.1021/acs.nanolett.7b00768]. Dielectric layers having different thicknesses were examined to increase the TMOKE response and reduce the reflectance. The results offer design options for achieving miniaturized electrically driven magneto-optical response elements.

Magneto-optical reflection spectroscopy on graphene/Co in the soft x-ray range

The existence of ferromagnetic ordering in graphene on cobalt is demonstrated by means of resonant magnetic reflection spectroscopy exploiting the transversal magneto-optical Kerr-effect (T-MOKE). Using linearly polarized synchrotron radiation in the soft x-ray range with energies spanning the carbon 1s edge, the π- and σ- bonds of graphene were excited individually, showing that magnetism in graphene is carried by the π – orbitals. Magnetic signals were detected over a wide energy range from 257 – 340 eV with a T-MOKE peak value of 1.1 % at the π – resonance energy near 285 eV. By comparison with corresponding spectra measured at the 2p edges of the Co substrate, a large induced magnetic moment of 0.14 μB was derived for graphene. Individual hysteresis curves monitored at the Co 2p and C 1s edges show that the carbon magnetism is induced by the Co substrate.

Giant enhancement of the transverse magneto-optical Kerr effect through the coupling of ɛ -near-zero and surface plasmon polariton modes

We demonstrate a concept for the giant enhancement of the transverse magneto-optical Kerr effect (TMOKE), with amplitudes reaching the maximum theoretical values of $\ifmmode\pm\else\textpm\fi{}1$. The concept exploits the strong electromagnetic field localization in $\ensuremath{\varepsilon}$-near-zero metamaterials to excite surface plasmon resonances with no need of a prism or grating coupler, thus opening routes for magneto-optical devices amenable to miniaturization. A demonstration of the capability of the enhancement mechanism is presented in which giant TMOKE values can be used for sensing and biosensing.

Enhanced magneto-optical Kerr effect and index sensitivity in Au/Fe_xCo_1-x magnetoplasmonic transducers

Magnetoplasmonic sensors are attractive candidates for ultrasensitive chemical and biomedical sensor applications. A variety of ferromagnetic metal thin films have been used for magnetoplasmonic device applications, yet the dependence of sensor performance on the optical and magneto-optical properties of ferromagnetic metal materials has been rarely studied. In this work, we report the study of enhanced magneto-optical Kerr effect (MOKE) and sensing performance in Au/FexCo1−x bilayer magneto-optical surface plasmon resonance (MOSPR) transducers. The optical constants of FexCo1−x (x=0, 0.29, 0.47, 0.65, and 1) in a sputter-deposited Au/FexCo1−x device are characterized by the attenuated total internal reflection (ATR) method. FexCo1−x thin films show different MOKEs as a function of the chemical concentration, with the highest transverse MOKE signal observed in Fe0.7Co0.3. Index sensing performance is closely related to the material’s optical and magneto-optical constants. By studying the sensing performance in the parameter space of the Au/FexCo1−x bilayer thicknesses, the highest sensitivity is found to be 0.385 (theoretical) and 0.306 RIU−1 (experimental) in the Au/Fe0.7Co0.3 MOSPR devices. Our research highlights the influence of the optical properties of ferromagnetic material to device sensitivity in MOSPR transducers. The high sensitivity in Au/FexCo1−x MOSPR devices make these structures attractive candidates for chemical and biomedical sensing applications.

Spin Hall effect of reflected light in dielectric magneto-optical thin film with a double-negative metamaterial substrate

We study spin the Hall effect (SHE) of reflected light in a dielectric magneto-optical thin film of Ce1Y2Fe5O12 (Ce:YIG) with a double-negative (DNG) metamaterial substrate. The spin-dependent splitting expressions of left- and the right-handed circularly polarized (LHCP and RHCP) components in longitudinal, polar and transverse magneto-optical Kerr effect (MOKE) configurations are obtained. Meanwhile we first obtain the analytical expressions of the SHE shift of reflected light for three MOKE configurations by proper approximation. Owing to the enhancement of the MOKE by DNG metamaterial, the external magnetic field shows a large enhancement and modulation to spin-dependent splitting of reflected light. Based on simulation results, the influences of magnetic field direction and substrate material on the transverse centroid shifts of the reflected left- and right-handed circularly polarized light perpendicular to incident plane are analyzed. We find the maximum spin-dependent splitting between LHCP and RHCP components achieves about 9.2 μm and the maximum value of the magneto-optical spin Hall effect (MOSHE) shift reaches 9 μm in polar MOKE configuration. In order to make our results convincing we use a realizable DNG metamaterial with silver nanostructures as substrate to verify our conclusion. The DNG metamaterial provides a flexible method to manipulate and enhance SHE of light.

“Synthesis of Co1−xGdxFe2O4+δ (0 ≥ x ≤ 0.1) and study of their magnetic, electric, and magneto-optical properties”

In this investigation, pure and gadolinium-doped cobalt ferrites (Co1−xGdxFe2O4+δ, x= 0, 0.02, 0.04, 0.06, 0.08 y 0.1), were synthesized through a modified route of the coprecipitation method. X-ray diffraction technique shows a cubic phase of the spinel-type structure typically associated with these compounds. According to Scherrer's equation, the crystallite size can be tuned through the Gd3+ content (26 nm to x= 0 and 9 nm to x= 0.1) or through the temperature of the thermal treatment (21 nm to 400 °C and 28 nm to 700 °C). The obtained powders show a ferrimagnetic behavior, at room temperature, with magnetization (reached at the maximum applied field) and coercivity values showing a dependence on the substitution grade of cobalt (II) by gadolinium (III) ions. At x= 0.02 the magnetization shows an increase of 7% (even when the particle size decreases a 40%) and the coercivity shows a decrease of 45%. DC conductivity of sintered ferrites is enhanced up to 3 orders of magnitude in the x= 0.1 substituted materials; while, keeping a constant transverse magneto-optical Kerr effect around 0.35%.

Ultrasensitive transverse magneto-optical Kerr effect measurements by means of effective polarization change detection

We perform a detailed comparative study of conventional transverse magneto-optical Kerr effect (T-MOKE) measurements and a methodology that utilizes an effective polarization detection scheme for mixed s- and p-polarized incoming light. To test the ultimate sensitivity of both methods, we also design a series of specialized samples in which the T-MOKE signal of a Co-film is artificially reduced by means of a Ag overcoat of varying thickness. We find that the effective polarization detection scheme leads to a more than 30-fold increase of the T-MOKE signal and signal-to-noise ratio, even under general operation conditions which were not individually optimized. This allowed for the observation of T-MOKE hysteresis loops of Co-films that were buried under 80 nm of Ag, for which the MOKE signal was only 1/600 of that for an uncoated Co-film. In comparison, conventional T-MOKE measurements did not succeed for Ag overcoats thicker than 40 nm.