Magneto-optical Kerr Effect Spectroscopy Research Articles

Polar magneto-optical Kerr effect spectroscopy with a microscope arrangement for studies on 2D materials.

We describe a setup for magneto-optical Kerr effect (MOKE) spectroscopy suitable for Kerr rotation (ϕ) and ellipticity (η) measurement on microscopic samples, such as flakes of two-dimensional materials. A spatial resolution of ∼25μm, limited by the demagnified monochromator exit slit image, was achieved. The use of mirrors allows for measurement in polar MOKE geometry with a conventional electro-magnet, without requiring holes in the magnet pole pieces. The microscope-like optics also has a 90° twisted periscope arrangement of two mirrors that helps transport light without change in its circular polarization state. A Jones matrix analysis of the setup brings out the influence of the beam-splitter on the measured signals. Its correction requires the ellipsometry parameters of the beam-splitter in transmission mode, which were measured separately. The working of the setup is tested by measuring the ϕ and η spectra of 2H-WS2 flakes at low temperature, verifying them using Kramers-Kronig analysis and extracting the Landé g-factor of the ground state exciton from them.

Relationship between magnetic asymmetry and magnetization in ultrafast transverse magneto-optical Kerr effect spectroscopy in the extreme ultraviolet spectral range

Relationship between magnetic asymmetry and magnetization in ultrafast transverse magneto-optical Kerr effect spectroscopy in the extreme ultraviolet spectral range

Effect of granule sizes on magneto-optical spectra of nanocomposites

The magneto-optical transverse Kerr effect (TKE) spectroscopy allows non-contact investigation of nanostructures, giving valuable information about the microstructure of samples. In this paper, within the framework of effective medium approach we analyze the effect of granule size on the magneto-optical spectra of magnetic nanocomposites. We consider the influence of various microscopic parameters such as plasma frequency, relaxation time, the coefficient of the anomalous Hall effect inside the granule and on its surface on the amplitude and profile of the TKE spectrum. Three main mechanisms are considered: the quasi-classical size effect, spin–orbit interaction enhancement at the granule surface and change of the percolation threshold.

Boron-induced magneto-optical Kerr spectra and dielectric tensors in ferrimagnetic (Mn4N)B antiperovskite thin films

Boron-induced electronic states were investigated via a combination of polar magneto-optical Kerr effect (p-MOKE) spectroscopy and spectroscopic ellipsometry for one of the antiperovskite nitrides, Mn4N. The boron content in the Mn4N film varied from 0 to 4.3 at.%, for which the crystal structure was maintained. The amplitude of p-MOKE spectra and the diagonal and off-diagonal dielectric tensors decreased with increasing boron content, which is in agreement with the magnetic properties such as magnetic anisotropy and saturation magnetization. These results were related to the lattice expansion and displacement of the charge density in the Mn4N by boron doping. However, the peak energy of the Lorentz oscillator in the diagonal elements of dielectric tensors suggests that a dominant inter-band transition was independent of boron content.

Spin dynamics in the axion insulator candidate EuIn2As2

We report the investigation of spin dynamics in the antiferromagnetic metal ${\mathrm{EuIn}}_{2}{\mathrm{As}}_{2}$ with the Zintl phase by using time-resolved magneto-optical Kerr effect (TR-MOKE) and transient reflectivity spectroscopy. In TR-MOKE measurement, we observe a spin precession mode with a frequency of about 18 GHz below 1.4 T and a new emerging coherent acoustic phonon mode near 35 GHz at a higher field where the latter shows a field-independent behavior. With temperature increasing, the spin precession disappears above N\'eel temperature ${T}_{N}$, whereas the acoustic phonon persists up to 25 K due to existence of magnetic-field-induced polarized paramagnetism. In contrast, the quasiparticle dynamics of ${\mathrm{EuIn}}_{2}{\mathrm{As}}_{2}$ does not show dramatic change in the vicinity of ${T}_{N}$. By comparing the spin and quasiparticle dynamics, we attribute the appearance of an acoustic phonon in TR-MOKE to photoinduced transient perturbation of magneto-optical coefficient. Our paper provides an in-depth investigation on the dynamical magnetic properties and broadens the understanding of the ${\mathrm{EuIn}}_{2}{\mathrm{As}}_{2}$ system.

Ultrafast element- and depth-resolved magnetization dynamics probed by transverse magneto-optical Kerr effect spectroscopy in the soft x-ray range

We report on time- and angle-resolved transverse magneto-optical Kerr effect spectroscopy in the soft x-ray range that, by analysis via polarization-dependent magnetic scattering simulations, allows us to determine the spatio-temporal and element-specific evolution of femtosecond laser-induced spin dynamics in nanostructured magnetic materials. In a ferrimagnetic GdFe thin film system, we correlate a reshaping spectrum of the magneto-optical Kerr signal to depth-dependent magnetization dynamics and disentangle contributions due to non-equilibrium electron transport and nanoscale heat diffusion on their intrinsic time scales. Our work provides a quantitative insight into light-driven spin dynamics occurring at buried interfaces of complex magnetic heterostructures, which can be tailored and functionalized for future opto-spintronic devices.

Field orientation dependent magnetization reversal and dynamics in sub-100 nm wide permalloy nanowires

We have investigated the magnetization reversal mechanism and dynamic behaviour of 90 nm wide permalloy nanowires patterned over a large area as a function of thickness (t) in the range from 20 nm to 70 nm and applied field orientation. The reversal processes and dynamic properties are probed using the magneto-optical Kerr effect and broadband ferromagnetic resonance (FMR) spectroscopy respectively. We have observed that both the static and dynamic properties are markedly sensitive to the film thickness. For t= 20 nm, the magnetization reversal process is dominated by coherent rotation mode due to the strong shape anisotropy and the onset of curling mode is observed. The FMR spectra display the existence of two well-separated modes, unlike the single mode which existed in the large width nanowires of the same thickness. The two modes at high and low frequencies correspond to the excitations at the centre and edges of the wire respectively. For t ⩾ 50 nm, the magnetization reversal process is dominated by curling mode of reversal. The centre mode frequency is found to be sensitive to the film thickness and shows two fold anisotropy. The experimental results are substantiated with micromagnetic simulations. These results have potential implications in designing reconfigurable magnonic crystals for microwave devices.

Ultrafast optical excitation of magnetic dynamics in van der Waals magnets: Coherent magnons and BKT dynamics in NiPS3

Optical pump-probe experiments carried out in the time domain reveal both the intrinsic low-energy dynamics and its connections to higher-energy excitations in correlated electron systems. In this work, we propose two microscopic mechanisms for the optical generation of coherent magnetic modes in van der Waals magnets, and derive the corresponding effective light-spin interactions: either through pumping atomic orbital excitations resonantly or via a light-induced Floquet spin Hamiltonian, the ground state of the system is driven out of equilibrium. The subsequent long-time relaxational dynamics can then be probed using, e.g., the magneto-optical Kerr effect or transient grating spectroscopy. As an example, we apply our framework to ${\mathrm{NiPS}}_{3}$, which is magnetically ordered in the bulk, and is conjectured to realize the XY model in the monolayer limit. Our theory makes explicit how the material's low-energy response depends sensitively on the microscopic details of the light-spin coupling as well as pump fluence, frequency, and polarization. For the case of bulk ${\mathrm{NiPS}}_{3}$, we find quantitative agreement with recent experiments [D. Afanasiev et al., Sci. Adv. 7, eabf3096 (2021)]. We further propose pump-probe experiments for monolayer ${\mathrm{NiPS}}_{3}$ and detail how anomalous relaxational behavior may reveal excitations of a (proximate) Berezinskii-Kosterlitz-Thouless phase in a proposed effective XY model.

Optical N-invariant of graphene\u2019s topological viscous Hall fluid

Over the past three decades, graphene has become the prototypical platform for discovering topological phases of matter. Both the Chern Cin {mathbb{Z}} and quantum spin Hall upsilon in {{mathbb{Z}}}_{2} insulators were first predicted in graphene, which led to a veritable explosion of research in topological materials. We introduce a new topological classification of two-dimensional matter – the optical N-phases Nin {mathbb{Z}}. This topological quantum number is connected to polarization transport and captured solely by the spatiotemporal dispersion of the susceptibility tensor χ. We verify N ≠ 0 in graphene with the underlying physical mechanism being repulsive Hall viscosity. An experimental probe, evanescent magneto-optic Kerr effect (e-MOKE) spectroscopy, is proposed to explore the N-invariant. We also develop topological circulators by exploiting gapless edge plasmons that are immune to back-scattering and navigate sharp defects with impunity. Our work indicates that graphene with repulsive Hall viscosity is the first candidate material for a topological electromagnetic phase of matter.

In situ electromagnet with active cooling for real-time magneto-optic Kerr effect spectroscopy.

We present a compact in situ electromagnet with an active cooling system for use in ultrahigh vacuum environments. The active cooling enhances the thermal stability and increases the electric current that can be applied through the coil, promoting the generation of homogeneous magnetic fields, required for applications in real-time deposition experiments. The electromagnet has been integrated into a reflectance difference magneto-optic Kerr effect (RD-MOKE) spectroscopy system that allows the synchronous measurement of the optical anisotropy and the magneto-optic response in polar MOKE geometry. Proof of principle studies have been performed in real time during the deposition of ultra-thin Ni films on Cu(110)-(2 × 1)O surfaces, corroborating the extremely sharp spin reorientation transition above a critical coverage of 9 monolayers and demonstrating the potential of the applied setup for real-time and in situ investigations of magnetic thin films and interfaces.

Thickness dependent magnetic properties of ferromagnetic films (Fe, Co) interfaced with Ta

In the present work, magnetic properties of ferromagnetic films viz. Co and Fe interfaced with Ta have been studied using magneto-optical Kerr effect (MOKE), X-ray reflectivity (XRR) and soft X-ray absorption spectroscopy (SXAS) on wedge shaped samples. Thickness of ferromagnetic film at different points of wedge sample as determined from XRR were employed to study the evolution of magnetic properties with increasing film thickness using MOKE measurements. Co film exhibits a uniaxial magnetic anisotropy on Ta which varies as a function of increasing film thickness, while no thickness dependent magnetic anisotropy has been observed for Fe film grown on Ta. The variation of coercivity with increasing film thickness is in agreement with the observed variation of magnetic anisotropy. SXAS measurements qualitatively support alloying / intermixing at interface. Thicknesses of interfacial magnetic dead layer as seen from variation of Kerr intensity with film thickness is relatively higher for Co as compared to Fe, while interfaced with Ta.

Magnetic resonance force spectroscopy of multilayer films Co/Pt with perpendicular magnetic anisotropy

The magnetic resonance force microscopy (MRFM) in combination with magneto-optical Kerr effect spectroscopy and magnetic force microscopy is used to study ferromagnetic resonance in multilayer thin-film structures [Co(0.5 nm)/Pt(1 nm)]5 with perpendicular magnetic anisotropy. Both the uniformly magnetized state and nonhomogeneous domain structures with different spatial scales are studied. It is shown that labyrinth domain structure manifests itself in the MRFM spectra as specific resonances that are split in an external magnetic field. In zero magnetic field the resonant frequencies depend on the size of oppositely magnetized domains due to internal demagnetizing fields and magnetostatic interaction through overlapping strain fields. For composite sample [Co(0.5 nm)/Pt(1 nm)]5Co(1.3 nm) the decrease in the spatial scale of domain structure and decrease in resonance frequencies are observed, which are due to the effect of interface exchange field. The obtained results demonstrate the ability of MRFM to study the resonance properties of highly inhomogeneous magnetic states of ferromagnetic structures, especially in zero magnetic fields.

Crystallization of optically thick films of CoxFe80−xB20 : Evolution of optical, magneto-optical, and structural properties

CoFeB alloys are highly relevant materials for spintronic applications. In this work, the crystallization of CoFeB alloys triggered by thermal annealing was investigated by x-ray diffraction techniques and scanning electron microscopy, as well as spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy for annealing temperatures ranging from 300 to ${600}^{\ensuremath{\circ}}\mathrm{C}$. The transformation of \ensuremath{\sim}100-nm-thick ${\mathrm{Co}}_{x}{\mathrm{Fe}}_{(80\ensuremath{-}x)}{\mathrm{B}}_{20}$ films from amorphous to polycrystalline was revealed by the sharpening of spectral features observed in optical and magneto-optical dielectric functions spectra. The influence of B on the dielectric function was assessed both experimentally and by optical modeling. By analyzing the Drude component of the optical dielectric function, a consistent trend between the charge-carrier scattering time/resistivity and the annealing temperature was observed, in agreement with the electrical investigations by means of the four-point-probe method.

Control of magneto-optical properties of cobalt-layers by adsorption of α-helical polyalanine self-assembled monolayers

Adsorption of α-helical polyaniline self-assembled monolayers onto magnetic substrates containing a thin Co layer determines the magneto-optical response.

Spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy study of thermally treated Co60Fe20B20 thin films

We report the optical and magneto-optical properties of amorphous and crystalline Co60Fe20B20 films with thicknesses in the range of 10 nm to 20 nm characterized using spectroscopy ellipsometry (SE) and magneto-optical Kerr effect (MOKE) spectroscopy. We derived the spectral dependence of the dielectric tensor from experimental data for samples prior and after annealing in vacuum. The features of the dielectric function can be directly related to the transitions between electronic states and the observed changes upon annealing can be ascribed to an increase of the crystalline ordering of CoFeB.

Transient quantum isolation and critical behavior in the magnetization dynamics of half-metallic manganites

We combine time resolved pump-probe Magneto-Optical Kerr Effect and Photoelectron Spectroscopy experiments supported by theoretical analysis to determine the relaxation dynamics of delocalized electrons in half-metallic ferromagnetic manganite $La_{1-x}Sr_{x}MnO_{3}$. We observe that the half-metallic character of $La_{1-x}Sr_{x}MnO_{3}$ determines the timescale of both the electronic phase transition and the quenching of magnetization, revealing a quantum isolation of the spin system in double exchange ferromagnets extending up to hundreds of picoseconds. We demonstrate the use of time-resolved hard X-ray photoelectron spectroscopy (TR-HAXPES) as a unique tool to single out the evolution of strongly correlated electronic states across a second-order phase transition in a complex material.

Electronic structure of La2/3Sr1/3MnO3 : Interplay of oxygen octahedra rotations and epitaxial strain

Influence of epitaxial strain and oxygen octahedra rotations on electronic structure of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_{3}$ ultrathin films was systematically studied. The investigated films were grown by pulsed laser deposition on four different substrates: cubic (001)-oriented ${\mathrm{LaAlO}}_{3}$, (001) ${({\mathrm{LaAlO}}_{3})}_{1/3}{({\mathrm{Sr}}_{2}{\mathrm{AlTaO}}_{6})}_{2/3}$, (001) ${\mathrm{SrTiO}}_{3}$, and orthorhombic (110) ${\mathrm{DyScO}}_{3}$, providing a broad range of induced epitaxial strains. Magnetic properties were found to deteriorate with increasing value of the epitaxial strain, as expected due to unit cell distortion increasingly deviating from the bulk and effect of the magnetically inert layer. A combination of spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy was used to determine spectra of the diagonal and off-diagonal elements of permittivity tensor. The off-diagonal elements at room temperature confirmed the presence of two previously reported electronic transitions in the spectra of all films. Moreover they revealed another electronic transition around 4.3 eV only in the spectra of films grown under compressive strain. We proposed classification of this transition as a crystal field paramagnetic Mn ${t}_{2g}\ensuremath{\rightarrow}{e}_{g}$ transition. Ab initio calculations were employed to distinguish between the potential influence of oxygen octahedra rotations and distortions. The ab initio calculations indicated a negligible influence of oxygen octahedra rotations on magneto-optical properties of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_{3}$. They further supported the proposed classification of the additional electronic transition, showing a key role of strain in controlling the electronic structure of ultrathin perovskite films.

Quadratic magnetooptic spectroscopy setup based on photoelastic light modulation

In most of the cases the magnetooptic Kerr effect (MOKE) techniques rely solely on the effects linear in magnetization (M). Nevertheless, a higher-order term being proportional to M2 and called quadratic MOKE (QMOKE) can additionally contribute to experimental data. Handling and understanding the underlying origin of QMOKE could be the key to utilize this effect for investigation of antiferromagnetic materials in the future due to their vanishing first order MOKE contribution. Also, better understanding of QMOKE and hence better understanding of magnetooptic (MO) effects in general is very valuable, as the MO effect is very much employed in research of ferro- and ferrimagnetic materials. Therefore, we present our QMOKE and longitudinal MOKE spectroscopy setup with a spectral range of 0.8–5.5 eV. The setup is based on light modulation through a photoelastic modulator and detection of second-harmonic intensity by a lock-in amplifier. To measure the Kerr ellipticity an achromatic compensator is used within the setup, whereas without it Kerr rotation is measured. The separation of QMOKE spectra directly from the measured data is based on measurements with multiple magnetization directions. So far the QMOKE separation algorithm is developed and tested for but not limited to cubic (001) oriented samples. The QMOKE spectra yielded by our setup arise from two quadratic MO parameters Gs and 2G44, being elements of quadratic MO tensor G, which describes perturbation of the permittivity tensor in the second order in M.

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.

All-optical measurement of interlayer exchange coupling in Fe/Pt/FePt thin films

Time Resolved Magneto Optic Kerr Effect spectroscopy was used to all-optically study the dynamics in exchange coupled Fe(10 nm)/Pt(x = 0–5 nm)/FePt (10 nm) thin films. As the Pt spacer decreases, the effective magnetization of the layers is seen to evolve towards the strong coupling limit where the two films can be described by a single effective magnetization. The coupling begins at x = 1.5 nm and reaches a maximum exchange coupling constant of 2.89 erg/cm2 at x = 0 nm. The films are ferromagnetically coupled at all Pt thicknesses in the exchange coupled regime (x ≤ 1.5 nm). A procedure for extracting the interlayer exchange constant by measuring the magnetic precession frequencies at multiple applied fields and angles is outlined. The dynamics are well reproduced using micromagnetic simulations.