Magneto-optical Kerr Rotation Research Articles

Direct observation of magnetic exchange-spring development in epitaxial DyFe2/YFe2 superlattices by magneto-optical Kerr effect

The magnetic reversal properties of molecular beam epitaxy (MBE) grown Laves phase single crystal YFe 2-dominated DYFe 2/ YFe 2 superlattices were investigated using magneto-optical Kerr effect (MOKE) measurements. Single crystal DYFe 2/ YFe 2 superlattices with a thickness ratio of 1:4 were MBE-grown with a (110) growth direction. In the epitaxial superlattices, the development of exchange-spring structure during magnetic reversal is revealed by both magneto-optical reflectivity and Kerr rotation angle loops. The bending fields of the samples are found to be close due to nonideal anisotropy in both hard and soft layers. For the sample with thin soft/hard layers, large magneto-optical reflectivity change up to 0.25% was observed. This can be related with rotation of Fe spins in YFe 2 soft layers under a low pinning field, in consistence with the small switching field of thin DYFe 2 hard layers measured experimentally.

Strain dependent electron spin dynamics in bulk cubic GaN

The electron spin dynamics under variable uniaxial strain is investigated in bulk cubic GaN by time-resolved magneto-optical Kerr-rotation spectroscopy. Spin relaxation is found to be approximately independent of the applied strain, in complete agreement with estimates for Dyakonov-Perel spin relaxation. Our findings clearly exclude strain-induced relaxation as an effective mechanism for spin relaxation in cubic GaN.

Temperature dependent magnetic anisotropy of epitaxial Co2FeAl films grown on GaAs

Co2FeAl films with different thickness were prepared at different temperature by molecular beam epitaxy. Their magnetic anisotropy was studied by rotating magneto-optical Kerr rotation measurements under different temperature. It is found that the cubic anisotropy depends only on the temperature-dependent fourth order magneto-elastic coefficients. However, the results of growth and measurement temperature-dependent uniaxial anisotropy suggest that the uniaxial anisotropy of Co2FeAl films may be attributed to contributions from both shear strain and anisotropic interfacial bonding. Our experimental findings proposed a new point of view to understand the origin of magnetic anisotropy in ferromagnet/GaAs(001) heterostructures.

Photon spin splitting in magneto-optic Kerr effect

Photonic spin Hall effect (PSHE) of light, similar to the spin Hall effect of an electronic system, is the interaction between photon spin-orbit of spin splitting phenomenon in refractive index gradient field. PSHE provides a new way to manipulate photons and accurately measure relevant physical effects. This paper studies the photon spin splitting in the magneto-optic Kerr effect, and establishes the quantitative relationship between the magneto-optical Kerr rotation and the PSHE. We have observed the magneto-optic Kerr effect of the action of an iron film in the magnetic field. After finishing experiments, we firstly analyze the amplified shift of the spin changing with the magnetic intensity in the case of horizontal polarization and vertical polarization incidence. Then, the Kerr rotations changing with the incident angle for different magnetizations are measured separately. Finally, comparing the above two results, we obtain that the accurate magneto-optic Kerr rotation angle is 6.7×10-5o/μm. If the position sensor based on phase-locked amplifier (20 nm) is used to measure the magneto-optic Kerr rotation angle, its precision can be improved by one or two orders of magnitude. These results prove that the PSHE not only can be used to accurately measure the magneto-optic Kerr effect, but also have great significance in magnetic film research. Especially, in terms of magnetic-optic devices, PSHE can be used to achieve the superior performance of endurance cycling and data retention.

Tuning the magneto-optical response of TbPc2 single molecule magnets by the choice of the substrate

Magneto-optical Kerr rotation of thin films of TbPc2 single molecule magnets can be tuned at room temperature within almost two orders of magnitude by the choice of the substrate.

Suppressed decay of a laterally confined persistent spin helix

We experimentally investigate the dynamics of a persistent spin helix in etched GaAs wire structures of 2 to 80 um width. Using magneto-optical Kerr rotation with high spatial resolution, we determine the lifetime of the spin helix. A few nanoseconds after locally injecting spin polarization into the wire, the polarization is strongly enhanced as compared to the two-dimensional case. This is mostly attributed to a transition to one-dimensional diffusion, strongly suppressing diffusive dilution of spin polarization. The intrinsic lifetime of the helical mode is only weakly increased, which indicates that the channel confinement can only partially suppress the cubic Dresselhaus spin-orbit interaction.

Magnetic field sensing using evanescent waves in the Kretschmann configuration

To verify the feasibility of magnetic field sensing based on the interaction between evanescent waves and magnetic thin films, we have investigated the polar magneto-optical Kerr effect of Co thin films with various thicknesses in the Kretschmann attenuated total reflection configuration. Because of the influence of the glass magneto-optical Faraday rotation under a polar magnetic field, a multi-step experimental method has been applied to measure the intrinsic magneto-optical Kerr rotations of the thin Co films as a function of the angle of incidence. The significant effects of the angle of incidence, the incident light polarization state and the film thickness on the polar Kerr rotations of magnetic thin films in the Kretschmann configuration have been clearly shown in our experiments. The analysis of the experimental results shows that the interaction between the evanescent wave and the magnetic thin film is the main source of the observed magneto-optical activity. On the basis of these results, we discuss the practicalities of the development of a magnetic field sensor based on an evanescent wave penetrating into the magnetic thin film in the Kretschmann configuration, which is a simple structure that is compatible with both optical fibers and integrated optical waveguides.

Polar magneto-optical effects in magnetoplasmonic thin films illuminated by attenuated total reflection

We investigate the polar magneto-optical Kerr effect as a function of the angle of incidence in Co/Au thin films under attenuated total reflection conditions. An experimental method is proposed to measure the intrinsic polar magneto-optical Kerr rotation of the magnetoplasmonic films while considering the influence of the glass Faraday magneto-optical effect. The experimental results demonstrate that the Kerr rotations of these films are greatly affected by a number of factors including the angle of incidence, the linear polarization of the incident light, and the film thickness. The results suggest that the polar magneto-optical activity is affected by the evanescent fields decaying inside the films; these fields arise not only from attenuated total reflection at the glass/metal interface but also from the surface plasmon polaritons at the metal/air interface. A close relationship between the magneto-optical activity and the electromagnetic field distribution within the magnetoplasmonic films is clearly revealed.

Mirror-symmetric magneto-optical Kerr rotation using visible light in [(GeTe)2(Sb2Te3)1]n topological superlattices.

Interfacial phase change memory (iPCM), that has a structure of a superlattice made of alternating atomically thin GeTe and Sb2Te3 layers, has recently attracted attention not only due to its superior performance compared to the alloy of the same average composition in terms of energy consumption but also due to its strong response to an external magnetic field (giant magnetoresistance) that has been speculated to arise from switching between topological insulator (RESET) and normal insulator (SET) phases. Here we report magneto-optical Kerr rotation loops in the visible range, that have mirror symmetric resonances with respect to the magnetic field polarity at temperatures above 380 K when the material is in the SET phase that has Kramers-pairs in spin-split bands. We further found that this threshold temperature may be controlled if the sample was cooled in a magnetic field. The observed results open new possibilities for use of iPCM beyond phase-change memory applications.

Irreversible modification of magnetic properties of Pt/Co/Pt ultrathin films by femtosecond laser pulses

Annealing ultrathin Pt/Co/Pt films with single femtosecond laser pulses leads to irreversible spin-reorientation transitions and an amplification of the magneto-optical Kerr rotation. The effect was studied as a function of the Co thickness and the pulse fluence, revealing two-dimensional diagrams of magnetic properties. While increasing the fluence, the creation of two branches of the out-of-plane magnetization state was found.

Magneto-Plasmonic Effect in Cobalt Thin Film Incorporating Core–Shell Ag@Au Nanoparticles

We report enhanced magneto-optical Kerr rotation in the magneto-plasmonic structure of a cobalt thin film incorporating silver/gold core–shell nanoparticles. Metallic nanoparticles with core–shell structure of silver/gold were fabricated by laser ablation in liquid method and the magneto-optical medium was prepared by electron beam deposition technique. Excitation of localized surface plasmon resonance was demonstrated for various incidence angles. The experimental results show direct evidence for localized surface plasmon resonance enhancement effect on the polar magneto-optical Kerr effect of the cobalt thin film.

Application of longitudinal generalized magneto-optical ellipsometry in magnetic ultrathin films

The longitudinal generalized magneto-optical ellipsometry (GME) method is extended to the measurement of three-layer ultrathin magnetic films. In this work, the theory of the reflection matrix is introduced into the GME measurement to obtain the reflective matrix parameters of ultrathin multilayer magnetic films with different thicknesses. After that, a spectroscopic ellipsometry is used to determine the optical parameter and the thickness of every layer of these samples, then the magneto-optical coupling constant of the multilayer magnetic ultrathin film can be obtained. After measurements of a series of ultrathin Fe films, the results show that the magneto-optical coupling constant Q is independent of the thickness of the magnetic film. The magneto-optical Kerr rotations and ellipticity are measured to confirm the validity of this experiment. Combined with the optical constants and the Q constant, the Kerr rotations and ellipticity are calculated in theory. The results show that the theoretical curve fits very well with the experimental data.

The subwavelength tuned magneto-optical Kerr effect in L10-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.

Picosecond inverse magnetostriction in galfenol thin films

Coherent high-amplitude precession of the magnetization and spin waves with frequencies up to 40 GHz are generated by injecting picosecond compressive and shear acoustic pulses into nanometer-sized galfenol (Fe81Ga19) films. The magnetization modulation is due to the picosecond inverse magnetostrictive effect. The oscillations of the magnetization measured by magneto-optical Kerr rotation last for several nanoseconds, and the maximum modulation of the in-plane effective magnetic field is as high as 40 mT. These results in combination with a comprehensive theoretical analysis show that galfenol films possess excellent properties for ultrafast magnetization control based on the picosecond inverse magnetostrictive effect.

Tunable Magneto-Optical Kerr Effect in Fe Films Underneath a Two-Dimensional Array of Polystyrene Spheres Covered by Au Nanocaps

For Fe layers covered by two-dimensional arrays of hexagonal close-packed polystyrene spheres which are hemispherically covered by Au films on the top, fine structures are observed in reflection, magneto-optical Kerr rotation and ellipticity spectra. The Kerr rotation achieves peaks or exhibits a narrow resonance-like spectral lineshape near the reflection minima, depending on the mechanism of the reflection dips. The Kerr rotation peaks and the resonance-like lineshape both are shifted with the sphere diameters. These phenomena can be attributed to the excitation of surface plasmon resonance and Fabry—Pérot modes.

Electron spin dynamics study of bulk p-GaAs: The screening effect

In this work, the electron spin dynamics of bulk p-GaAs doped with Be grown by molecular beam epitaxy is investigated by using the time-resolved magneto-optical Kerr rotation technique. The spin relaxation/dephasing times T1 and T2* are systematically investigated as a function of hole doping density and photo-excitation density as well as temperature. The complex hole doping density dependence of spin relaxation times T1 and T2* is observed experimentally, which agrees well with predictions of the kinetic spin Bloch equation theory published previously [J. Jiang and M. Wu, Phys. Rev. B 79, 125206 (2009)]. D'yakonov-Perel's mechanism [M. Dyakonov and V. Perel, Sov. Phys. Solid State 13, 3023 (1972)] is discussed to dominate the electron spin relaxation process in p-GaAs, with the effect of hole screening proven to play an important role.

Surface modes enhanced magneto-optical Kerr effect in Fe films underneath two-dimensional array of polystyrene spheres

For bilayers consisting of 20 nm Fe film and top two-dimensional arrays of hexagonal close-packed polystyrene spheres, fine structures are observed in the reflection R and magneto-optical polar Kerr rotation θK spectra. The θK achieves narrow peaks or dips near R minima and are both shifted with the sphere diameter. All phenomena are attributed to the excited guided waves and surface plasmonic modes. The θK peaks and dips near the R minima can be mainly explained as a result of the pure optical constant effect.

Direct mapping of the formation of a persistent spin helix

The spin-orbit interaction (SOI) in zincblende semiconductor quantum wells can be set to a symmetry point, in which spin decay is strongly suppressed for a helical spin mode. Signatures of such a persistent spin helix (PSH) have been probed using the transient spin grating technique, but it has not yet been possible to observe the formation and the helical nature of a PSH. Here we directly map the diffusive evolution of a local spin excitation into a helical spin mode by a time- and spatially resolved magneto-optical Kerr rotation technique. Depending on its in-plane direction, an external magnetic field interacts differently with the spin mode and either highlights its helical nature or destroys the SU(2) symmetry of the SOI and thus decreases the spin lifetime. All relevant SOI parameters are experimentally determined and confirmed with a numerical simulation of spin diffusion in the presence of SOI.

Effect of Ga+ irradiation on the magneto-optic spectra of Pt/Co/Pt sandwiches

Interfacial changes in rf sputtered Pt/Co(2.6nm)/Pt sandwiches grown onto sapphire (Al2O3) substrates induced by irradiation of 30keV Ga+ ions at low dose (1014 ions/cm2) have been investigated by magneto-optic polar Kerr rotation (PKR) spectroscopy between 1 and 5eV. The irradiation resulted in an increase of PKR over the whole spectral range. The measured PKR spectra were compared with those computed from the transfer matrix formalism using known polar Kerr rotation and ellipticity spectra for Co and five CoxPt1−x alloys. The comparison between measured and computed PKR spectra provided an in-depth profile of Co and Pt ion distributions across the sandwich and confirmed that irradiation favors alloying in the vicinity of the two interfaces. These results are in a good agreement with the profile evaluated independently by TRIDYN simulations. Our results evidence an asymmetry in the irradiation effect due to an excess of Pt–Co alloying at the upper interface. Moreover, the observation of a negative PKR peak around 3.2eV states definitively the presence of a chemically ordered Co0.75Pt0.25 alloy phase inside the irradiated film structure.

Morphology, microstructure and magnetic properties of thermionic vacuum arc deposited NiFeCu ferromagnetic thin films

AbstractNiFeCo granular ferromagnetic thin films were deposited on glass and silicon wafer substrates in thermionic vacuum arc plasma with simultaneous ignition of plasma in Cu and NiFe vapors. The structural and morphological properties of the prepared films were investigated by TEM Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). Elemental composition of the films was revealed after X-ray energy dispersive spectroscopy analysis (EDAX). The magneto-optical longitudinal Kerr rotation spectra of the samples were also measured and compared.