Magneto-optical Polar Kerr Rotation Research Articles

Tunable polar magneto-optical Kerr rotation of light from a one-dimensional photonic crystal containing a Graphene/InAs slab at THz range

Magneto-optical effects of graphene and InAs layers generated by gyrotropic behaviors under an external magnetic field, make them desirable for magneto-optical applications. In this paper, the polar magneto-optical Kerr effect of reflected light from a one-dimensional photonic crystal containing a magneto-optical slab (a graphene sheet on an InAs layer) has been investigated in the THz frequency region. The intensity reflection coefficient and Kerr rotation angle spectra of the reflected light from the structure have been calculated using the developed 4 × 4 transfer matrix method. By adjusting the thickness of the InAs layer in the structure, a narrow deep mode inside the photonic bandgap appears. The results revealed that the corresponding enhanced Kerr rotation angle occurred at the deep mode frequency. The performance of the structure in the presence and absence of the graphene sheet is investigated considering the FOM behaviors. The tunability and enhancement of the Kerr rotation angle are investigated by varying the external magnetic field strength and the charge career density of graphene. Finally, the polarization state of reflected light at the deep mode frequency is visualized. The results of this work may have potential applications in designing magnetic field sensors and tunable Kerr-rotation-based devices.

Polar magneto-optical Kerr effect in antiferromagnetic M2As (M=Cr,Mn,Fe) under an external magnetic field

Antiferromagnetic (AFM) metals have attracted tremendous interest for memory applications due to their expected fast response dynamics in the terahertz frequency regime. Spin dynamics such as AFM resonance and the magnon relaxation rate have been measured using the linear magneto-optical Kerr effect (MOKE) with coherent spin precession induced by laser pumping. Polarized electromagnetic radiation is a promising alternative for probing the response in canted AFM systems. Hence, in this paper, we use first-principles simulations to study the magneto-optical response of AFM ${M}_{2}\mathrm{As} (M=\mathrm{Cr},\mathrm{Mn},\text{and} \mathrm{Fe})$ under external magnetic fields. We devise a computational scheme to compute the magnetic susceptibility from total-energy changes using constraints on magnetic-moment tilting. Our predictions of the spectral dependence of polar magneto-optical Kerr rotation and ellipticity allow us to attribute these effects to breaking of the magnetic symmetry. We show that tilting of magnetic moments affects the exchange interaction, while the spin-orbit interaction remains unaffected. In this paper, we provide an understanding of the polar MOKE on a band structure level and underscore the importance of the magnetic susceptibility when searching for materials with large magneto-optical response.

Engineered polar magneto-optical Kerr rotation through Wood–Rayleigh anomalies and magnetoplamon resonance coupling

Polar magneto-optical Kerr rotation (MOKR) is an important phenomenon in magneto-optical materials. Both intensive and narrow Kerr rotation spectra are needed to ensure the high performance of magneto-optical devices. In this work, by using a nanosphere template and Ag film deposition at different angles, glass/Cr/Ag/Fe/PSS/Ag samples with different Ag nanostructures between polystyrene spheres were fabricated, and different optical modes and their coupling were manipulated. Polar MOKR peaks were demonstrated and analyzed. Narrow and intensive polar MOKR peaks were presented with the Ag film deposited at a glancing angle θ = 30°. The physical mechanism was studied by simulating the electromagnetic-field distribution. It was found that, in the glass/Cr/Ag/Fe/PSS/Ag system with the Ag film deposited at different angles, different optical modes and their interaction are introduced. Polar MOKR peaks due to the coupling between Fabry–Perot and Bragg plasmons was broad. However, polar MOKR spectra governed by the coupling of WRA and magnetoplasmons demonstrated intensive and narrow peaks for samples with the Ag film deposited at θ = 30°, which would improve the performance of magneto-optical devices with high sensitivity, compact volume, high-density data storage devices, and so on.

Interface effects in nanometer-thick yttrium iron garnet films studied by magneto-optical spectroscopy

The properties of nanometer-thick yttrium iron garnet (YIG) films are strongly influenced by interfaces. This work employs spectral ellipsometry (SE) and magneto-optic polar Kerr rotation (PKR) to characterize YIG films with thickness, t, from 6 nm to 30 nm grown on Gd3Ga5O12 (GGG) substrates oriented parallel to (111) plane. The films display a surface roughness of 0.35 nm or lower. The analysis of the SE data at the photon energies of 1 eV < E < 6.5 eV provided the t and permittivity values. The PKR at 1.3 eV < E < 4.5 eV is reasonably explained with the optical model for the YIG film/GGG substrate system. Even better agreement is achieved by assuming a 1.07-nm-thick layer sandwiched between YIG and GGG that has Fe3+ sublattice magnetization opposite to that in the YIG volume. This suggests the existence of antiferromagnetic coupling between the Gd3+ and tetrahedral Fe3+.

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.

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.

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.

Preparation of ferromagnetic (In,Mn)As with a high Curie temperature of 90K

The authors found high Curie temperatures of up to 90K in annealed (In,Mn)As epilayers grown by molecular beam epitaxy using a relatively high V/III beam flux ratio. Magnetization data suggest that increasing the As beam flux results in an increase in the effective Mn content, and the carrier transport data indicate an increase in the hole concentration from a 1019to1020cm−3 range with low-temperature annealing after growth. The highest Curie temperature was obtained in lattice matched (In,Mn)As∕Al(As,Sb) samples. Both Hall effect and magneto-optical polar Kerr rotation measurements confirm that the high Curie temperature is due to hole-mediated ferromagnetism.

Cyclotron frequency coupled enhancement of Kerr rotation in low refractive index-dielectric/magneto-optic bilayer thin-film structures

We investigate the enhancement of magneto-optic polar Kerr rotation over a broad range of optical frequencies. The Drude model for the dielectric tensor is considered. Resonance-like peaks in the Kerr rotation and ellipticity caused in the vicinity of a plasma edge is extended throughout the visible spectrum by deriving an expression for the cyclotron frequency such that Re(εxx)∼1 at any given incident photon energy and plasma frequency of the material. The Kerr rotation obtained by use of this expression was studied for the case of InSb and further enhancement of Kerr rotation was achieved in the case of LiF/InSb bilayer thin-film structure, grown on a glass substrate. The numerical analysis was carried out using a 2×2 characteristic matrix, which takes into account multiple reflections and interface effects within the medium. In addition the role of various magneto-optic material parameters and layer thicknesses is investigated in determining the optical frequency at which the maximum Kerr rotation occurs, under the present cyclotron frequency condition. Enhanced Kerr rotation greater than 1.5° over a broad range of optical frequencies was obtained. Tables showing optimum figure of merit for repeated LiF/InSb bilayers at 3.1 eV for different plasma frequencies are also provided.

SHG anisotropy in Au/Co/Au/Cu/vicinal Si(1 1 1)

The second harmonic generation (SHG) reflectivity on magnetic multilayers is a very sensitive technique to reveal the crystallography of buried interfaces. We have used the azimuthal anisotropy of SHG to demonstrate that the vicinal character of Si(1 1 1) substrate is duplicated in the metallic multilayer Au/Co/Au/Cu. The magnetic properties of these multilayers as anisotropy and magneto-optic polar Kerr rotation were studied by linear magneto-optic effects in correlation with SHG experiments, by varying the Co and Au buffer thicknesses as well as the Cu buffer deposition condition.

Temperature Dependence of Magneto-Optical Spectra in EuO Epitaxial Films.

Ferromagnetic EuO films were epitaxially grown on MgO substrate. The Curie temperature Tc of the obtained film was 69K. Spectra of magneto-optical polar Kerr rotation and Kerr ellipticity were measured from the substrate side of the MgO/EuO/Ag structure using the photoelastic modulator technique for photon energies between 1.2 and 4 eV under the magnetic field of 1T. Huge magneto-optical effect was obtained. A few spectral structures were observed for photon energies between 1.2 and 2.6eV, which undergo strong temperature dependence.

Dielectric tensor for interfaces and individual layers in magnetic multilayer structures

The magneto-optical Kerr response of metallic magnetic multilayers has been studied by determining the dielectric tensors (dielectric functions) for individual layers, including the magnetic and nonmagnetic interfacial layers. The diagonal components of these tensors were determined using in situ ellipsometric analysis, where the ellipsometric data were taken in real time during multilayer deposition. The off-diagonal components were determined by regression fitting magneto-optic polar Kerr rotation and ellipticity data to models supported by electromagnetic theory. The Voigt parameters (ratio between off-diagonal and diagonal components of dielectric tensors) were determined from these model fits. Higher magnitudes for the Voigt parameters were found at interfaces, corresponding with stronger Kerr responses observed in those materials. Five different magnetic multilayer systems were studied, including Pt/Co, Pd/Co, Au/Co, Cu/Co, and Pt/Fe multilayer structures. The Voigt parameters for the magnetic layers and magnetic–nonmagnetic interfaces in all five structures were determined, and in turn the dielectric tensors for the respective layers were also determined.

Giant magneto-optical Kerr effect enhancement in layered CoMn/Si structures

For sputtered Co57Mn43/Si film samples with different ferromagnetic thickness, the spectra of the complex magneto-optical polar Kerr rotation and optical constants were ex-situ measured. For 5 nm-thick Co57Mn43 film, the Kerr rotation around 4.3 eV was enhanced by a factor of about 10 times with lower loss of the signal intensity as compared to the thick film. Numerical calculations show that the enhancement effect is attributed to an interplay between optical properties of Co57Mn43 and substrate Si that has a strong interband transition (E2) near 4.3 eV.

Magneto-optical studies in NiO/Fe2O3 multilayers

[ Fe 2 O 3 /NiO ] N multilayer films prepared by pulsed laser deposition display oscillation observed in the magnetization M as a function of N or the number of interfaces i=2N−1 indicating that they are not symmetric. In this work magneto-optic (MO) polar Kerr rotation spectra of some samples in the photon energy range 1.2–4.5 eV are reported. Faraday rotation (FR) was studied at 632.6 and 550 nm laser wavelengths. It shows the oscillations with N in accordance with M. Polar Kerr rotation spectra show a series of peaks which could be assigned to nickel ferrite NiFe2O4 formed at the interfaces between NiO and αFe2O3. The amplitudes change strongly with N as expected from N dependence of FR and M. A simple analytical expression is proposed to explain the complex phase relations among the contributions from individual ferrimagnetic interfaces to the total MO Kerr rotation in the multilayers.

Magnetic and Magneto-Optical Properties of Fe-Pt Alloy Thin Films

A study of magnetic and magneto-optical properties of Fe100-xPtx (0≦x≦60) alloy thin films as functions of composition and substrate deposition temperature in conjunction with structure is carried out. It is found that the coercivities for both perpendicular and in-plane magnetization curves become very large for the mixed phase of fcc and fct. For Fe50Pt50 thin films, the perpendicular magnetic anisotropy constant Ku is found to increase with increasing deposition temperature (Ts) and becomes nearly constant (5×107erg/cc) at Ts beyond 250°C. Also, the Ku of Fe50Pt50 films increases with the intensity ratio of I(001)/I(002), being closely related with the tetragonality (c/a). These results indicate that the evolution of Ku is due to the occurrence of the fct phase. The magneto-optical polar Kerr rotation and Kerr ellipticity are found to exhibit different behaviors with tetragonality. In particular, the Kerr rotation and Kerr ellipticity at photon energies from 1.5∼2.5eV are found to be sensitive to the tetragonality (c/a).

The Magnetic Coupling, Optical and Magneto-Optical Properties of Oscillation Observed in Sputtered Co–Al/Cu Multilayers

We have studied the optical and magneto-Optical properties, and the behavior of interlayer coupling and magnetoresistance in sputtered [Co90Al10(1.6 nm)/Cu]30 multilayers. It is found that for Co–Al/Cu multilayers, the magnetoresistance ratio ΔR/R, the in-plane saturation field Hs and coercive force Hc oscillate with a period of about 0.9 nm, as a function of the Cu layer thickness. It is significant to find that the magneto-Optical polar Kerr rotation θK, and particularly the effective dielectric constant ε oscillate in a period of about 0.9 nm. This result can be attributed to the oscillations of the optical constants in Cu spacer layers, induced by the oscillatory interlayer coupling or quantum well effect. Numerical calculations show that the oscillations of the effective optical constants play an important role in the magneto-Optical Kerr effect oscillations.

Magneto-optical properties of U 5f electrons inU3P4

To investigate the nature of the 5f electrons in the noncollinear ferromagnet ${\mathrm{U}}_{3}$${\mathrm{P}}_{4}$ the photon-frequency dependence of the normal-incidence reflectivity and the polar magneto-optical Kerr rotation are studied theoretically and compared with available experimental data. The calculations are carried out treating the U 5f electrons as itinerant and the results are found to be in good agreement with experimental data. We show that the U 5f states contribute substantially to the optical properties. We discuss the role played by the noncollinearity of the magnetic structure in determining the optical properties. We conclude that the magnetism of this compound can be described in terms of well-defined moments of the U atoms that are formed by itinerant U 5f electrons.

Novel giant magneto-optical Kerr effect in cerium compounds (invited)

The recently discovered novel giant polar magneto-optical Kerr rotation of 90° in CeSb is compared to previous record Kerr signals of the order of 10°, and to another newly discovered giant Kerr signal of 22° in CeS. An analysis of the temperature and field dependence of the off-diagonal conductivity spectra of CeSb is presented which allows an empirical assignment of several structures to electronic transitions. In particular, a folding of bands due to magnetic superstructures in a ferrimagnetic phase is shown to account for three additional peaks.

Optical and magnetooptical characterization of FeNi/Cu multilayers

A series of [FeNi(1.7 nm)/Cu(tCu)]30/Fe(10 nm) multilayers with the range of tCu from 0.36nm to 4.32nm deposited on glass substrates was studied by measuring the effective optical constants n and k, saturation magnetooptical polar Kerr rotation θk and ellipticity ηk and evaluating the effective complex off-diagonal elements of the dielectric tensor exy′ and exy″. It was found that the four primary quantities exy′, exy″, n and k vary concurrently with respect to tCu, while the dependences of θk and ηk on tCu differ drastically. To explain this difference the contributions of both the real and imaginary parts of the off- diagonal dielectric tensor elements and the effect of complex optical constants have to be considered. No obvious correlation was found between the variation of n, e~xy and the oscillations of GMR of the multilayers.

Magneto-Optical Spectra of MnAs on GaAs Grown by HWE

Ferromagnetic MnAs thin films were grown on GaAs(100) and (111)B substrates by the atomic-hydrogen-assisted hot-wall epitaxy technique, using polycrystalline powder of MnAs as evaporation source. The source and substrate temperatures were 700°C and 400°C, respectively. The surface orientations of the MnAs films grown on (100) GaAs substrates (1011) and (1012), (111)B GaAs was (0001). The polar magneto-optical Kerr rotation and ellipticity spectra and reflectivity spectrum were measured in the films. The dielectric permeability spectra were calculated from experimental data, from which the magneto-optical spectra were analyzed.