Quadratic Magneto-optical Kerr Effect Research Articles

Magneto-optical properties of cobalt nanoparticles in porous silicon

We report on the magnetooptical response of cobalt nanoparticles deposited on porous silicon surface. The use of such unconventional surface leads to a random feature for crystal relaxation considerably different from those of the smoothed one. Interestingly, heat treatment of the silicon substrate shows a significant effect on the morphology of the cobalt nanoparticles. Annealing exhaustively changes the shape of the cobalt nanoparticles. The study of the magnetic properties of the elaborated samples shows a peculiar behavior. Noticeably, an asymmetric hysteresis loop of the deposited cobalt is obtained. This result is attributed to a quadratic magneto-optical Kerr effect (QMOKE) contribution which certainly finds its origins in the interaction between neighboring clusters.

Giant quadratic magneto-optical Kerr effect in (Eu,Gd)O films for magnetic field sensing

A quadratic magneto-optical Kerr effect (QMOKE) of exceptionally high value, strong polar and longitudinal linear Kerr effects have been demonstrated in thin films of ferromagnetic (Eu,Gd)O and the parent compound EuO epitaxially grown on yttria-stabilized zirconia. At a photon energy of 1.85 eV, the effect reaches 1 deg in Eu0.97Gd0.03O exceeding the reported values of giant QMOKE by an order of magnitude. It is demonstrated that the QMOKE in (Eu,Gd)O enables detailed optical studies of the in-plane magnetic anisotropy of the film. The observed giant QMOKE may be employed for the development of vectorial sensors of weak magnetic fields.

Linear and Nonlinear Magneto-optical Phenomena in Epitaxial Films of Europium Chalcogenides EuX (X = O, Se, Te)

The studies of magneto-optical effects in epitaxial films of magnetic semiconductors—europium chalcogenides EuX (X = O, Se, Te) are presented: linear and quadratic magneto-optical Kerr effects, photo-induced Faraday effect, magnetically-induced optical second and third harmonics generation, the inverse Faraday effect, and the phenomenon of optical orientation. A large quadratic magneto-optical Kerr effect was found in EuO and (Eu, Gd)O films. Photo-induced spin polarons with a huge value of the magnetic moment were studied in europium chalcogenides EuTe and EuSe using the optical pump-probe technique. In the same materials, a new type of nonlinear magneto-optical susceptibility, which is responsible for the optical second harmonic generation and associated with the magnetic ordering of the spins of Eu2+ ions, is revealed.

Линейные и нелинейные магнитооптические явления в эпитаксиальных плeнках халькогенидов европия EuX (X=O, Se, Te)

AbstractThe studies of magneto-optical effects in epitaxial films of magnetic semiconductors—europium chalcogenides EuX (X = O, Se, Te) are presented: linear and quadratic magneto-optical Kerr effects, photo-induced Faraday effect, magnetically-induced optical second and third harmonics generation, the inverse Faraday effect, and the phenomenon of optical orientation. A large quadratic magneto-optical Kerr effect was found in EuO and (Eu, Gd)O films. Photo-induced spin polarons with a huge value of the magnetic moment were studied in europium chalcogenides EuTe and EuSe using the optical pump-probe technique. In the same materials, a new type of nonlinear magneto-optical susceptibility, which is responsible for the optical second harmonic generation and associated with the magnetic ordering of the spins of Eu^2+ ions, is revealed.

Linear and quadratic magneto-optical Kerr effect investigation of Co2Mn1.30Si0.84 epitaxially grown on MgO

Abstract We investigated the magneto-optical properties of a L21 ordered nonstoichiometric Co2Mn1.30Si0.84 film epitaxially grown on a MgO-buffered MgO (001) single-crystal substrate. Longitudinal magneto-optical Kerr effects (LMOKE) and rotating magneto-optical Kerr effect (ROT-MOKE) measurements suggest that the film exhibits a cubic magnetic anisotropy with the extracted cubic anisotropy constant of K C = 6.7 × 10 4 e r g / c m 3 . Orientation-dependent ROT-MOKE suggest that the quadratic magneto-optical Kerr effects (QMOKE) components also show fourfold symmetry with a modest amplitude of 3 mdeg, which is in accordance with complex Kerr angle expression for cubic symmetry systems. Our results suggest that ROT-MOKE is not only an efficient method to determine magnetic anisotropy parameters but also a good method to extract QMOKE components.

Quantitative study of the quadratic magneto-optical Kerr effects in Fe films.

We present a rotating field method to separate the linear and quadratic magneto-optical Kerr effects (LMOKE and QMOKE) in Fe/GaAs(001) films. The LMOKE is isotropic in crystal orientation, while the QMOKE has both isotropic and anisotropic contributions. The experimental observation is well explained by Yeh's 4×4 matrix formalism. We also report the incident angle and the thickness dependences of the LMOKE and QMOKE, and extract the material's index of refraction n and the magneto-optical coupling constant K and G. The study gives a full description of the Kerr effect in Fe films, and the proposed method can be applied to other magneto-optical coupling systems.

Co2FeAl thin films grown on MgO substrates: Correlation between static, dynamic, and structural properties

Co2FeAl (CFA) thin films with thickness varying from 10 nm to 115 nm have been deposited on MgO(001) substrates by magnetron sputtering and then capped by Ta or Cr layer. X-rays diffraction (XRD) revealed that the cubic $[001]$ CFA axis is normal to the substrate and that all the CFA films exhibit full epitaxial growth. The chemical order varies from the $B2$ phase to the $A2$ phase when decreasing the thickness. Magneto-optical Kerr effect (MOKE) and vibrating sample magnetometer measurements show that, depending on the field orientation, one or two-step switchings occur. Moreover, the films present a quadratic MOKE signal increasing with the CFA thickness, due to the increasing chemical order. Ferromagnetic resonance, MOKE transverse bias initial inverse susceptibility and torque (TBIIST) measurements reveal that the in-plane anisotropy results from the superposition of a uniaxial and of a fourfold symmetry term. The fourfold anisotropy is in accord with the crystal structure of the samples and is correlated to the biaxial strain and to the chemical order present in the films. In addition, a large negative perpendicular uniaxial anisotropy is observed. Frequency and angular dependences of the FMR linewidth show two magnon scattering and mosaicity contributions, which depend on the CFA thickness. A Gilbert damping coefficient as low as 0.0011 is found.

Quadratic magneto-optical Kerr effect in Co2MnSi

Quadratic magneto-optical Kerr effects (QMOKE) are investigated in epitaxial Co2MnSi thin films as a function of the post-deposition annealing temperature. We determine the amplitude of the QMOKE signal for different annealing temperatures, which provide various degrees of L21 crystal ordering, as manifested by X-ray diffraction. We observe that QMOKE is significantly present only when the L21 ordering phase is also present, and that the QMOKE signal increases with the higher degree of L21 ordering. Additionally, we notice that the linear magneto-optical Kerr effect (LMOKE) decreases with higher annealing temperature. A comparison of the linear and quadratic MOKE contributions shows the amplitudes are of similar magnitude. From these results, we conclude that the presence of QMOKE is linked to the higher degree of L21 ordering in Co2MnSi.

Note: Probing quadratic magneto-optical Kerr effects with a dual-beam system

In this Note, we present a dual-beam magneto-optical Kerr effect (MOKE) magnetometer for the study of quadratic MOKE in magnetic thin films. The two beams simultaneously probe the sample, located in the middle of a quadrupole magnet, at two angles of incidence (0 degrees and 45 degrees). This combination of two systems allows one to automatically and routinely perform measurements that are sensitive to the combined longitudinal and quadratic MOKE signals (45 degrees), or the quadratic effect alone (0 degrees). Orientation-dependent and automated quantitative analyses of the quadratic effect's amplitude are also implemented. We present representative measurements on Heusler compound thin films to illustrate the newly combined capabilities of this instrument.

Tandem magneto-optical Kerr effect magnetometer for the study of quadratic effects

It has recently been observed that strong quadratic magneto-optical Kerr effects (QMOKE) are present in cobalt-based Heusler compounds thin films. In this contribution we present a magnetometer dedicated to the study of QMOKE in magnetic thin films. The system consists of two MOKE magnetometers working in tandem, and is equipped with a quadrupole magnet which affords any in-plane field. The first magnetometer operates at an angle of incidence of 45°, and is sensitive to the combined longitudinal MOKE and QMOKE signals. In the second magnetometer, the light impinges on the sample at perpendicular incidence, and this magnetometer is thus only sensitive to the QMOKE component. Representative measurements illustrating the capabilities of this instrument are presented.

Thickness dependence of linear and quadratic magneto-optical Kerr effects in ultrathin Fe(001) films

Magneto-optical (MO) Kerr effect (MOKE) magnetometry is one of the most widely employed techniques for the characterization of ferromagnetic thin-film samples. Some information, such as the magnitude of coercive fields or anisotropy strengths, can be readily obtained without any knowledge of the optical and MO properties of the material. On the other hand, a quantitative analysis, which requires precise knowledge of the material's index of refraction $n$ and the MO-coupling constants $K$ and $G$, is often desirable, for instance, for the comparison of samples which are different with respect to ferromagnetic layer thicknesses, substrates, or capping layers. While the values of the parameters $n$ and the linear MO-coupling parameter $K$ reported by different authors usually vary considerably, the relevant quadratic MO-coupling parameters $G$ even for Fe are completely unknown. Here, we report on measurements of the thickness dependence (0--60 nm) of the linear and quadratic magneto-optical effects in epitaxial bcc-Fe(001) wedge-type samples performed at a commonly used laser wavelength of 670 nm. By fitting the thickness dependence we are able to extract a complete set of parameters $n$, $K$, $({G}_{11}\ensuremath{-}{G}_{12})$, and ${G}_{44}$ for the quantitative description of the MOKE response of bcc-Fe(001). We find the parameters $n$, $K$, and $G$ to significantly differ for films thinner than about 10 nm as compared to those for thicker films, which is indicative of a thickness dependence of the electronic properties or of surface contributions to the MOKE. The magnitude of the quadratic magneto-optical effect is found to be about one-third of the record values reported recently for ${\text{Co}}_{2}\text{FeSi}$.

Huge quadratic magneto-optical Kerr effect and magnetization reversal in the Co2FeSi Heusler compound

Co2FeSi(1 0 0) films with L21 structure deposited onto MgO(1 0 0) were studied using both the longitudinal and quadratic (QMOKE) magneto-optical Kerr effect. The films exhibit a huge QMOKE signal with a maximum contribution of up to 30 mdeg, which is the largest QMOKE signal in reflection that has been measured thus far. This large value is a fingerprint of an exceptionally large spin–orbit coupling of second or higher order. The Co2FeSi(1 0 0) films exhibit a rather large coercivity of 350 Oe and 70 Oe for film thicknesses of 22 nm and 98 nm, respectively. Despite the fact that the films are epitaxial, they do not provide an angular dependence of the anisotropy and the remanence in excess of 1% and 2%, respectively.

Ion beam induced modification of exchange interaction and spin–orbit coupling in the Co2FeSi Heusler compound

A Co2FeSi (CFS) film with L21 structure was irradiated with different fluences of 30 keV Ga+ ions. Structural modifications were subsequently studied using the longitudinal (LMOKE) and quadratic (QMOKE) magneto-optical Kerr effect. Both the coercivity and the LMOKE amplitude were found to show a similar behaviour upon irradiation: they are nearly constant up to ion fluences of ≈6 × 1015 ion cm−2, while they decrease with further increasing fluences and finally vanish at a fluence of ≈9 × 1016 ion cm−2, when the sample becomes paramagnetic. However, contrary to this behaviour, the QMOKE signal nearly vanishes even for the smallest applied fluence of 3 × 1014 ion cm−2. We attribute this reduction of the QMOKE signal to an irradiation-induced degeneration of second or higher order spin–orbit coupling, which already happens at small fluences of 30 keV Ga+ ions. On the other hand, the reduction of coercivity and LMOKE signal with high ion fluences is probably caused by a reduction of the exchange interaction within the film material.

Linear and quadratic magneto-optical Kerr effects in continuous and granular ultrathin monocrystalline Fe films

The effects of nanostructuration on the magneto-optic properties of ultrathin monocrystalline iron films grown on MgO (001) are investigated through the magneto-optic coefficients, which have a linear and quadratic dependence on the magnetization. The photon energy dependence of such magneto-optic coefficients is determined by measuring the relative variations of the reflectivity (ΔR p p /R p p ) for p-polarized incident and reflected light (p-p polarized) when the magnetization rotates in the plane of the sample. Thick Fe films present a magneto-optical anisotropy, which has a quadratic dependence on the magnetization. Such anisotropy is strongly reduced in nanostructured iron thin films formed by nanometric iron islands. The modifications induced by the nanostructuration are stronger for the magneto-optical coefficients, which have a quadratic dependence on the magnetization in contrast to the linear terms. A self-consistent effective-medium formalism is presented that explains the modifications induced in the magneto-optical coefficients by nanostructuration.