Magneto-optical Kerr Effect Resonance Research Articles

Element-selective magnetization states in a Gd23Fe67Co10 alloy, probed by soft X-ray resonant magneto-optical Kerr effect

The core-level resonant magneto-optical Kerr effect of a ferrimagnetic metal alloy, Gd23Fe67Co10, was measured at the Fe M-shell and Gd N-shell absorption edges using rotating analyzer ellipsometry. A large Kerr rotation angle of several degrees was detected at room temperature. The signal was found to be strong enough for element-selective magneto-optical experiments to trace various magnetic events, such as all-optical magnetization switching.

Probing anisotropy in epitaxial Fe/Pt bilayers by spin–orbit torque ferromagnetic resonance

We report the generation and detection of spin–orbit torque ferromagnetic resonance (STFMR) in micropatterned epitaxial Fe/Pt bilayers grown by molecular beam epitaxy. The magnetic field dependent measurements at an in-plane magnetic field angle of 45° with respect to the microwave-current direction reveal the presence of two distinct voltage peaks indicative of a strong magnetic anisotropy. We show that STFMR can be employed to probe the underlying magnetic properties, including the anisotropies in the Fe layer. We compare our STFMR results with broadband ferromagnetic resonance spectroscopy and magneto-optical Kerr effect measurements of the unpatterned bilayer thin films. The experimental STFMR measurements are interpreted using an analytical formalism and further confirmed using micromagnetic modeling which sheds light on the field-dependent magnetization alignment in the microstructures responsible for the STFMR rectification. Our results demonstrate a simple and efficient method for determining magnetic anisotropies in microstructures by means of rf spectroscopy.

Scanning magneto-optical Kerr effect (MOKE) measurement with element-selectivity by using a soft x-ray free-electron laser and an ellipsoidal mirror

The scanning magneto-optical Kerr effect (MOKE) method was developed by using a soft x-ray free-electron laser to visualize magnetic microstructures with element-selectivity. For focusing optics of soft x-rays, an ellipsoidal mirror with an achromatic property was utilized to obtain resonant MOKE spectra for various elements with a simple experimental setup. We performed a scanning MOKE experiment at Fe M-edge with a spatial resolution of 7 μm. This system is readily applicable to ultrafast MOKE experiments combined with a synchronized optical laser for spintronics research.

Magnetic field sensor based on magnetoplasmonic crystal

Here we report on designing a magnetic field sensor based on magnetoplasmonic crystal made of noble and ferromagnetic metals deposited on one-dimensional subwavelength grating. The experimental data demonstrate resonant transverse magneto-optical Kerr effect (TMOKE) at a narrow spectral region of 50 nm corresponding to the surface plasmon-polaritons excitation and maximum modulation of the reflected light intensity of 4.5% in a modulating magnetic field with the magnitude of 16 Oe. Dependences of TMOKE on external alternating current (AC) and direct current (DC) magnetic field demonstrate that it is a possibility to use the magnetoplasmonic crystal as a high-sensitive sensing probe. The achieved sensitivity to DC magnetic field is up to 10−6 Oe at local area of 1 mm2.

Element-selectively tracking ultrafast demagnetization process in Co/Pt multilayer thin films by the resonant magneto-optical Kerr effect

We examined the photo-induced dynamics of ferromagnetic Co/Pt thin films exhibiting perpendicular magnetic anisotropy by means of the resonant polar magneto-optical Kerr effect with element specificity. The investigation was conducted at Pt N6,7 and Co M2,3 edges using an x-ray free electron laser. The obtained results showed a clear element dependence of photo-induced demagnetization time scales: τdemag.Co=80±60 fs and τdemag.Pt=640±140 fs. This dependence is explained by the induced moment of the Pt atom by current flow from the Co layer through the interfaces. The observed magnetization dynamics can be attributed to the characteristics of photo-induced Co/Pt thin film phenomena including all-optical switching.

An apparatus and methodology for high-power SQUID-detected ferromagnetic resonance measurements

Historically, ferromagnetic resonance has been dominated by inductive techniques, for the best part of the last 80 years. It has been only in the last 20 years that non-inductive techniques, such as Ferromagnetic Resonance Force Microscopy (FMRFM) and Magneto-optical Kerr Effect (MOKE), have been used to study, for example, the spatial distribution of resonance modes. Neither of these techniques is absolute - i.e. provides information on the amplitude of excitation as a function of absorbed microwave power. Here we extend on the recent demonstration of SQUID-detected FMR [J. M. O’Reilly and P. Stamenov, Rev. Sci. Instrum. 89, 044701 (2018)], of absolute scalar resonance measurements in single-crystalline and poly-crystalline YIG, at various fields and temperatures, by introducing a new set-up, where the microwave power, instead of being sunk in a matched load at the cryogenic end of the measurement probe is brought back to the ambient environment and is both metered and sunk in high dissipation power (>50 W @ 50 Ω) matching load. The here suggested methodology allows for the absolute excitation amplitude of modes excited during high-power operation of critical microwave devices, such as filters and Y-junction stripline circulators, to be predicted based on direct measurements of the same material in a known geometry.

Femtosecond resonant magneto-optical Kerr effect measurement on an ultrathin magnetic film in a soft X-ray free electron laser

Time-resolved magneto-optical Kerr effect (MOKE) measurement was demonstrated on a sample of the Au/Fe/Au heterostructure with the Fe layer of 0.35 nm thickness under Fe M-edge resonance condition. An ultrabrilliant free electron laser (FEL) in the soft X-ray range was facilitated for the detection of transient signals of resonant MOKE from the ultrathin Fe film. A variation in the Kerr rotation angle was successfully observed on the femtosecond timescale. This technique enables us to reveal the transient magnetization dynamics of such a-few-monolayer magnetic films, which promote the development of spintronic devices.

Influence of Nanovoids Depth on the Surface Magneto-Optical Kerr Effect Modulations in Magnetic Photonic Crystal Slab

Magnetic photonic crystal slab with nanovoids buried in Ni films were fabricated using nanosphere lithography combined with electrochemical deposition. One featured structure was dominated the spectra of polar surface magneto-optical Kerr effect (MOKE) of samples with different voids depth. For the shallow ones, surface lattice resonance (SLR) was revealed through angle resolved spectra analysis, where the location of degeneracy points corresponded to that of the featured structure in MOKE signal. For the deeper ones, localized plasmons and degeneracy points were all demonstrated in angle resolved reflection spectra, where localized plasmons dominated the resonant MOKE signal, and the degeneracy points blue-shifted compared with that of shallow ones. Changing the lattice of samples, similar phenomena were presented. The physical mechanism of SLR could be attributed to the diffraction of surface plasmons by the reciprocity vector, and the localized plasmons were attributed to the Mie scattering. The featured structure in polar MOKE spectra demonstrated high sensitivity, which might pave the way to on-chip MO devices.

L-edge resonant magneto-optical Kerr effect of a buried Fe nanofilm

The Fe $L$-edge resonant magneto-optical Kerr effect of a buried Fe nanofilm was investigated by rotating-analyzer ellipsometry and the results were compared with those from three theoretical simulations. The reversal of the Kerr rotation angle ${\ensuremath{\theta}}_{\mathrm{K}}$ between the ${L}_{3}$ and ${L}_{2}$ edges, observed in the experiment, was consistent with classical electromagnetic simulation using empirical optical constants. The spectral ${\ensuremath{\theta}}_{\mathrm{K}}$ feature was reproduced by the first-principles calculation of the KKR-Green's function method on the itinerant electronic system. The demonstration indicates that spectra of the $L$-edge resonant MOKE can be understood in terms of both the macroscopic and microscopic pictures.

Resonant magneto–optical Kerr effect induced by hybrid plasma modes in ferromagnetic nanovoids**Project supported by the Natural Science Foundation of Shandong Province, China (Grant No. ZR2015AM024) and the Doctoral Research Started Funding of Qufu Normal University, China (Grant No. BSQD20130152).

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.

Resonant magneto-optical Kerr effect measurement system with polarization analysis using a high harmonic generation laser

We developed a resonant magneto-optical Kerr effect (RMOKE) measurement system that includes polarization analysis by using a high harmonic generation laser. Energy components around the Fe 3p absorption edge were separated by using multilayer mirrors. By using rotating-analyzer ellipsometry, we demonstrated longitudinal RMOKE measurements from the Fe(001)-p(1×1) terminated with oxygen.

Measurement of the Resonant Magneto-Optical Kerr Effect Using a Free Electron Laser

We present a new experimental magneto-optical system that uses soft X-rays and describe its extension to time-resolved measurements using a free electron laser (FEL). In measurements of the magneto-optical Kerr effect (MOKE), we tune the photon energy to the material absorption edge and thus induce the resonance effect required for the resonant MOKE (RMOKE). The method has the characteristics of element specificity, large Kerr rotation angle values when compared with the conventional MOKE using visible light, feasibility for M-edge, as well as L-edge measurements for 3d transition metals, the use of the linearly-polarized light and the capability for tracing magnetization dynamics in the subpicosecond timescale by the use of the FEL. The time-resolved (TR)-RMOKE with polarization analysis using FEL is compared with various experimental techniques for tracing magnetization dynamics. The method described here is promising for use in femtomagnetism research and for the development of ultrafast spintronics.

Gilbert damping in CoFeB/GaAs(001) film with enhanced in-plane uniaxial magnetic anisotropy

A 3.5 nm amorphous CoFeB film was sputtered on GaAs (001) wafer substrate without applying magnetic field during deposition, and a significant in-plane uniaxial magnetic anisotropy (UMA) field (Hu) of about 300 Oe could be achieved. To precisely determine the intrinsic Gilbert damping constant (α) of this film, both ferromagnetic resonance (FMR) and time-resolved magneto-optical Kerr effect (TRMOKE) techniques were utilized. With good fitting of the dynamic spectra of FMR and TRMOKE, α is calculated to be 0.010 and 0.013, respectively. Obviously, the latter is 30% larger than the former, which is due to the transient heating effect during the TRMOKE measurement. In comparison with ordinary amorphous CoFeB films with negligible magnetic anisotropies, α is enhanced significantly in the CoFeB/GaAs(001) film, which may be mainly resulted from the enhanced spin-orbit coupling induced by the CoFeB/GaAs interface. However, the significant in-plane UMA plays minor role in the enhancement of α.

Polarization dependence of resonant magneto-optical Kerr effect measured by two types of figure-8 undulators

The resonant magneto-optical Kerr effect of an Fe nanofilm at the L-edge was investigated by theoretically and experimentally using the polarization controlled undulator. Large values of the Kerr rotation angle (θK) were measured at the L2 and L3 absorption edges for both of s- and p-polarized incident lights. Furthermore, the sign changes of θK depending on the photon energy and the polarization of incident light were also observed.

Ultrafast spin-switching of a ferrimagnetic alloy at room temperature traced by resonant magneto-optical Kerr effect using a seeded free electron laser.

Ultrafast magnetization reversal of a ferrimagnetic metallic alloy GdFeCo was investigated by time-resolved resonant magneto-optical Kerr effect measurements using a seeded free electron laser. The GdFeCo alloy was pumped by a linearly polarized optical laser pulse, and the following temporal evolution of the magnetization of Fe in GdFeCo was element-selectively traced by a probe free electron laser pulse with a photon energy tuned to the Fe M-edge. The results have been measured using rotating analyzer ellipsometry method and confirmed magnetization switching caused by ultrafast heating.

Surface Plasmon Resonance Magneto-Optical Kerr Effect in Au/Co/Au Magneto-Plasmonic Multilayer

Surface plasmon resonance magneto-optical Kerr effect is studied in magneto-plasmonic multilayer as Au (11 nm)/Co (11 nm)/Au (11 nm). Our experimental setup is consists of spectral magneto-optical rotation in Kretschmann-based attenuated total reflection condition as surface plasmon resonance magneto-optical Kerr effect. Based on this new experimental setup, the sample exposed under external magnetic filed at surface plasmon resonance angle. Our results show sufficient surface plasmon resonance magneto-optical Kerr effect in visible region, thanks to the resonant excitation of surface plasmons which is very suitable for miniaturized and controllable magneto-optical imaging systems, memory, and also magneto-optical isolators.

Magnetization dynamics for L1 MnGa/Fe exchange coupled bilayers

The precessional magnetization dynamics of L10 MnGa/Fe bilayers with a large perpendicular magnetic anisotropy are investigated using vector network analyzer ferromagnetic resonance (VNA-FMR) and time-resolved magneto-optical Kerr effect (TRMOKE). The MnGa/Fe(1 nm) bilayer exhibited perpendicular magnetization and MnGa/Fe(2 and 5 nm) bilayers showed in-plane magnetizations. The VNA-FMR and TRMOKE data for these bilayers are well explained from calculations, based on the coupled Landau-Lifshitz equations, taking into account the interfacial exchange coupling of 2.4 erg/cm2.

Magnetic and Gilbert damping properties of L21-Co2FeAl film grown by molecular beam epitaxy

Co2FeAl film with L21 structure was prepared. Its magnetic and Gilbert damping properties were studied by ferromagnetic resonance (FMR) and time-resolved magneto-optical Kerr effect (TR-MOKE), respectively. It is observed that the apparent damping parameter decreases drastically with increasing magnetic field at low field regime and eventually becomes a constant value of 0.004 at high field regime by TR-MOKE measurements. A Gilbert damping parameter of 0.008 in the hard axis by FMR measurement has also been obtained, which is comparable with that extracted from TR-MOKE measurements at low external field, indicating the extrinsic damping processes involved in the low field regime.

Transversal magneto-optical Kerr effect in two-dimensional nickel magnetoplasmonic crystals

Resonant transversal magneto-optical Kerr effect (TKE) is experimentally studied in 2D nickel magnetoplasmonic crystal. Gradual transition between collinear and bi-directional types of surface plasmon-polariton phase matching and its influence on TKE resonances are observed by measuring and analyzing dependence of reflectance and TKE spectra on the azimuthal angle.

Coupled oscillations in noncollinear microscale rectangular magnets

The eigenmode spectrums of dense arrays of single-layer rectangular elements periodically arranged in a two-dimensional (2D) magnonic crystal with a complex unit cell have been characterized using broadband ferromagnetic resonance (FMR) spectroscopy and magneto-optical Kerr effect measurements. The crystal's unit cell consists of noncollinear orientations of constituting elongated rectangular elements, which makes it significantly different from a normal 2D magnonic crystal with similar elements. Microscopy and numerical simulations of the ground state reveal that the elements are statically dipole coupled and that the ground state is of collective nature. Two distinct collective FMR modes were observed in the frequency range from 1 to 20 GHz, the frequency positions of which depend on the angle of the applied field with the selected crystal axis. These modes were explained as predominantly localized in elements with different easy-axis orientations. Coherence of oscillations in the elements of the same type is ensured by mediators in the form of the elements of the other types. Angular FMR measurements confirmed uniaxial configurational anisotropy for the array with a not-so-well-resolved frequency gap between the two main modes.