Magneto-optical Kerr Rotation Research Articles

Exciton spin Hall effect in arc-shaped strained WSe2

Generating a pure spin current using electrons, which have degrees of freedom beyond spin, such as electric charge and valley index, presents challenges. In response, we propose a mechanism based on intervalley exciton dynamics in strained transition metal dichalcogenides (TMDs) to achieve the in an electrically insulating regime, without the need for an external electric field. The interplay between strain gradients and strain-induced pseudomagnetic fields results in a net Lorentz force on long-lived intervalley excitons in WSe2, carrying nonzero spin angular momentum. This process generates an exciton-mediated pure spin Hall current, resulting in opposite-sign spin accumulations and local magnetization on the two sides of the single-layer arc-shaped TMD. We demonstrate that the magnetic field induced by spin accumulation, at approximately ∼mT, can be detected using techniques such as superconducting quantum interference magnetometry or spatially resolved magneto-optical Faraday and Kerr rotations. Published by the American Physical Society 2024

Fabrication of single-crystalline YFeO3 films with large antiferromagnetic domains

The antiferromagnetic orthoferrite YFeO3 possesses fascinating magnetic properties for spintronics, such as terahertz spin dynamics, ultrafast domain wall motion, and long magnon decay length. YFeO3 belongs to a special family of antiferromagnets that show an unusually strong non-trivial Kerr response due to its weak ferromagnetism. The highly stable antiferromagnetic domains without any spontaneous spin rotation transitions below the 645 K Néel temperature may be useful for nanoscale device applications. We report the successful fabrication of high-quality twinning-free (110)-oriented YFeO3 films by pulsed laser deposition. Detailed structural and magnetic characterization revealed that the crystal structure and magnetic properties of the YFeO3 films are comparable to bulk single crystals. We show that the spin rotation under high magnetic fields follows the two-sublattice approximation model. The film surface is atomically flat with step-terrace surface morphology. A longitudinal magneto-optic Kerr (MOKE) rotation of 10 mdeg was observed at room temperature, which is consistent with earlier reports on bulk single crystals. The in-plane anisotropy of the Kerr response corresponds to the obtained magnetic anisotropy from the SQUID measurement. The large MOKE signal enables the imaging of antiferromagnetic domains and their reversal. The domain size was found to be larger than 100 μm. These high-quality YFeO3 thin films facilitate the fabrication of antiferromagnetic spintronic devices and provide a convenient platform for studying various spin-related phenomena in thin films and at interfaces.

Excellent magnetic properties in multiferroic BiFeO3 based thin films for magnetic devices application

The operation of magnetic devices with a magnetic field or spin current causes high energy consumption. Very low energy consumption can be achieved by using multiferroic thin films, where we can control the magnetization direction by applying an electric field. Good magnetic properties in multiferroic thin films are needed for these magnetic device applications. However, multiferroic thin films generally do not have such good magnetic characteristics, like high saturation magnetization (Ms), perpendicular magnetic anisotropy (PMA, the ratio of coercivity between the alignment of film plane perpendicular and parallel to the applied field (Hc⊥/Hc//)), magneto-optical Kerr rotation angle (Θk) and Curie temperature (Tc). In this research, we have looked into improving magnetic characteristics such as Ms, Hc⊥/Hc//, Θk and Tc in (Bi,L)(Fe,Co)O3(L = Lanthanides) thin films by substituting different lanthanides with cobalt. The highest Ms value of 140 emu/cm3 and large Hc⊥/Hc// ratio of 2.6 were found in (Bi,Nd)(Fe,Co)O3 thin film, and maximum Θk of 0.67° and relatively high Hc⊥/Hc// of 1.6 were found for the (Bi,La)(Fe,Co)O3 thin film. These properties are suitable for magnetic recording devices and optical devices, respectively. A very small coercivity of 0.8 kOe, along with a high Ms value of 110 emu/cm3 and Tc of 450 °C, were observed in the (Bi,Eu)(Fe,Co)O3 thin film. So, the (Bi,Eu)(Fe,Co)O3 thin film is suitable for magnetic sensor applications because of its high sensitivity to magnetic fields. Moreover, the domain size of the above-mentioned thin films was observed in the small nano range, which is suitable for magnetic nano device applications.

Peculiar temperature dependence of magneto-optic Kerr rotation associated with antiferromagnetic–paramagnetic transition

We investigate the magneto-optic Kerr effect in perpendicularly magnetized Pt/Co/Ir/Cr2O3/Pt thin films, associated with the antiferromagnetic–paramagnetic transition of the Cr2O3 layer. The magneto-optic Kerr rotation angle (θ K) shows oscillatory behavior as a function of the photon energy of incident light owing to interference in the Cr2O3 layer. The temperature dependence of θ K at 2.67 eV (λ = 465 nm), at which the largest θ K is obtained, shows a sharp dip at 287.0 K. The dip temperature is similar to the reported Néel temperature for Cr2O3 thin films. Although the θ K spectra measured at several temperatures are generally explained by the classical interference model, θ K is enhanced at 2.36–2.79 eV (λ = 525–445 nm) close to the dip temperature. This peculiar enhancement in θ K is discussed on the basis of the anomaly in the optical parameters of the Cr2O3 layer associated with the antiferromagnetic–paramagnetic transition.

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.

Spatiotemporal spin dynamics of two-dimensional electron gas with ballistic motion in persistent spin helix state

The spatiotemporal spin dynamics of two-dimensional electron spins with ballistic motion in a GaAs quantum well is investigated using time- and spatially resolved magneto-optical Kerr rotation measurements. The ballistic movement of photoexcited spins, which maintains the width of the excitation spot size, results in a wave-packet-like motion of the spin distribution. The time evolution of the spin distribution measured at different excitation intensities reveals that the spin diffusion slows down and the wave-packet-like motion disappears at higher photoexcited spin densities, at which the spin distribution converges to the helical spin mode observed in the diffusive regime. The variation in the spatiotemporal spin distribution can be controlled by spin diffusion, while the spin precession length is determined by the strength of the spin-orbit interaction.

Temperature lag with the onset of exchange bias, superparamagnetic blocking, and antiferromagnetic ordering in ultrathin ferromagnet/antiferromagnet thin film

The magnetization of a nanosized magnet, such as an ultrathin film, thermally fluctuates and can become superparamagnetic. In ferromagnetic/antiferromagnetic thin films, superparamagnetism can be suppressed in accordance with antiferromagnetic ordering. The exchange bias can also be induced at the ferromagnetic/antiferromagnetic interface, and it is nontrivial whether the superparamagnetic blocking temperature (TB_SPM) can match either the onset temperature of the exchange bias (TB_EB) or the Néel temperature (TN). In this study, we investigated the temperature dependence of parameters such as coercivity, exchange bias field, magneto-optic Kerr rotation (θK), and AC magnetization (MAC) to elucidate the matching of TB_EB, TB_SPM, and TN in a Pt/Co/Au/Cr2O3/Pt thin film. Based on the temperature dependences of MAC, TB_SPM was yielded as about 283 K. TB_EB and TN, which were determined based on the temperature dependence of θK, were 278 and 282 K, respectively. TB_SPM was almost equal to TN but TB_EB was smaller. This temperature lag was caused by the difference in the magnetic anisotropy energy required to induce the exchange bias and suppress superparamagnetism.

Independent control of the longitudinal magneto-optical Kerr effect and its refractive index sensing in Ag-Co composite nanogratings

Conventionally, tuning magneto-optical Kerr rotation via the excitation of surface plasmons (SPs) usually results in extremely low light energy at the Kerr null point position in the optical spectrum. Realizing independent control of the magneto-optical effect is considered as an effective solution. The decoupling between the Kerr effect and optical response is achieved in our constructed Ag/Co composite nanogratings, and the Kerr rotation used as the refractive index sensing signal can be obtained without SP excitation. The refractive index sensing performance is improved by reducing electron collision frequency in Ag/Co nanogratings and the figure of merit reaches 96/RIU, far exceeding that of Co nanogratings.

Study of a Partly Spin-Polarized Two-Dimensional Electron System, According to Time-Resolved Magneto-Optical Kerr Rotation

A spin-depolarized electron system with filling factor ν = 1 in a GaAs quantum well is investigated according to time-resolved Kerr rotation. A transition from a new spin-correlated state to one with the low spin stiffness characteristic of a single-particle electron system is observed at temperatures above 5 K. A nonlinear contribution to the Larmor oscillations decay is distinguished at low temperatures, where spin–spin correlations determine the ground state of a two-dimensional electron system. The parameters of the fluctuating magnetic field acting on individual electron spins are determined.

Control of spin relaxation anisotropy by spin-orbit-coupled diffusive spin motion

Spatiotemporal spin dynamics under spin-orbit interaction is investigated in a (001) GaAs two-dimensional electron gas using magneto-optical Kerr rotation microscopy. Spin polarized electrons are diffused away from the excited position, resulting in spin precession because of the diffusion-induced spin-orbit field. Near the cancellation between spin-orbit field and external magnetic field, the induced spin precession frequency depends nonlinearly on the diffusion velocity, which is unexpected from the conventional linear relation between the spin-orbit field and the electron velocity.This behavior originates from an enhancement of the spin relaxation anisotropy by the electron velocity perpendicular to the diffused direction. We demonstrate that the spin relaxation anisotropy, which has been regarded as a material constant, can be controlled via diffusive electron motion.

The controls of magnetization dynamics and magneto-optic properties in single-crystalline yttrium iron garnet capped by rare-earth dysprosium nano-films

The dynamic magnetization of magnetic materials is of fundamental interest and is essential for various applications in magnetic storage, spin wave and wireless communication areas. Traditionally, rare-earth-doped ferromagnets show an enhancement of the Gilbert damping parameter due to the slow relaxation of the 4f-electron spins of rare-earth atoms. The exchange coupling between a ferromagnetic garnet thin film and a nanometer rare-earth thin film has been rarely explored. In this work, we fabricated and systematically studied microstructural, static and dynamic magnetic properties of yttrium iron garnet (YIG)/dysprosium (Dy) heterojunctions. Large modulations of the out-of-plane magnetic anisotropy field, Gilbert damping parameter and magneto-optic Kerr rotation angle have been realized in single-crystalline YIG thin films capped by rare-earth Dy nano-films. Both the effective Gilbert damping parameter, αeff, and the Kerr rotation angle, increase by 300% for the YIG/Dy(60 nm) heterostruture compared to the bare YIG ferromagnet. The enhanced αeff strongly depends on the out-of-plane magnetic anisotropy field, which arises from the interfacial exchange coupling between YIG and Dy nano-films. Our findings provide a robust route to control both magnetization dynamics and magneto-optic properties using rare-earth nano-films.

Electrodeposited Heusler alloy films with enhanced magneto-optical property

Growth of Heusler alloy films through techniques other than conventional physical vapor deposition presents a challenge for spintronic materials research. In this study an electrochemical deposition approach for these alloys is demonstrated in which sub-micron thickness control can be achieved. By using magneto optic Kerr effect and Kerr microscopy measurements a systematic study of surface magnetization reversal behavior of these films is presented. The results demonstrate that the growth of sub-micron films of Co2FeSn leads to significant enhancement in magneto optic Kerr rotation. A large Kerr rotation value up to 300 mdeg was observed for thin films of thickness ∼200 nm. The modified scheme reported here opens up a path for exploration of electrochemical method as a facile route for fabricating Heusler alloys with improved surface property.

Measurement of the magnetic properties of thin films based on the spin Hall effect of light.

Using the spin Hall effect of light, this work proposes a measurement technique of the magnetic properties of thin films. The beam shift of the spin Hall effect of light is used to replace the magneto-optical Kerr rotation angle as a parameter to characterize the magnetism of thin films. The technique can easily achieve an accuracy of 10-6 rad of the magneto-optical Kerr rotation angle which can, in theory, be further improved to 10-8 rad. We also proposed two methods to solve the problem of the exceeding linear response region of the measurement under high magnetic field intensity, making it more conducive to practical application. This technique has great potential for application in the magnetic measurement of ultra-thin films with particular emphasis on thicknesses within several atomic layers.

Tunable optical and magneto-optical Faraday and Kerr rotations in a dielectric slab doped with double-V type atoms

We theoretically investigate the optical and magneto-optical Faraday and Kerr rotations of a probe field that propagates through a nonmagnetic dielectric slab doped with double-V type atoms. Both rotations and corresponding ellipticities, as well as the intensities of transmitted and reflected beams, are modified by quantum coherence induced in the atomic system. We show that applying a control laser field makes the system optically active and simultaneously, transparent to one component of the probe field. We demonstrate that the response of the slab can be modified both electrically and magnetically. Applying the second control laser field with different Rabi frequencies improves the optical properties of the slab due to the induced coherent effects. We present analytical expressions for facilitating the detailed study of the system behaviors. Magneto-optical Faraday rotation 45° with transmission close to 100 % and large Kerr rotation with high reflection are significant results from the influence of both the control and magnetic fields on such a small structure. By prevailing over the tradeoff between reflection and rotation, the proposed model could be considered as a special candidate for rotating the polarization plane of the transmitted and reflected beams, simultaneously.

Synthesis of yttrium iron garnet/bismuth quantum dot heterostructures with localized plasmon enhanced magneto-optical performance

Interactions between light and magnetic matter attracted great attention lately due to their potential applications in nanophotonics, spintronics, and high-accuracy sensing. Here, we grew bismuth quantum dots (Bi–QDs) with strong spin–orbit coupling on a magnetic insulator yttrium iron garnet (YIG) via molecular beam epitaxy. The YIG/Bi–QDs material shows an enhanced magneto-optical Kerr rotation up to 130% compared with that of a bare YIG film. The Bi–QDs were also introduced onto a lutetium–bismuth co-doped YIG film to form a hybrid system with remarkably enhanced Kerr rotation (from 1626 to 2341 mdeg). Ferromagnetic resonance measurements showed an increased effective magnetization as well as interfacial spin–orbit field in the YIG/Bi–QD heterostructures. Localized plasmons were mapped using electron energy loss spectroscopy with high spatial resolution, revealing enhanced plasmon intensity at both the Bi–QD surface and YIG/Bi–QD interface. Introducing Bi-QDs onto the YIG film enhanced Kerr rotation owing to the attenuated optical reflection and increased effective magnetization. The Bi–QD-enhanced magneto-optical effect enables development of efficient nanoscale light switching, spintronics, and even plasmonic nano-antennas.

SOS: symmetry-operational similarity

Symmetry often governs condensed matter physics. The act of breaking symmetry spontaneously leads to phase transitions, and various observables or observable physical phenomena can be directly associated with broken symmetries. Examples include ferroelectric polarization, ferromagnetic magnetization, optical activities (including Faraday and magneto-optic Kerr rotations), second harmonic generation, photogalvanic effects, nonreciprocity, various Hall-effect-type transport properties, and multiferroicity. Herein, we propose that observable physical phenomena can occur when specimen constituents (i.e., lattice distortions or spin arrangements, in external fields or other environments) and measuring probes/quantities (i.e., propagating light, electrons, or other particles in various polarization states, including vortex beams of light and electrons, bulk polarization, or magnetization) share symmetry-operational similarity (SOS) in relation to broken symmetries. In addition, quasi-equilibrium electronic transport processes such as diode-type transport effects, linear or circular photogalvanic effects, Hall-effect-type transport properties ((planar) Hall, Ettingshausen, Nernst, thermal Hall, spin Hall, and spin Nernst effects) can be understood in terms of symmetry-operational systematics. The power of the SOS approach lies in providing simple and physically transparent views of otherwise unintuitive phenomena in complex materials. In turn, this approach can be leveraged to identify new materials that exhibit potentially desired properties as well as new phenomena in known materials.

Optical excitation density dependence of spin dynamics in bulk cubic GaN

The influence of the optical excitation density on the electron spin dynamics is experimentally investigated in bulk cubic GaN by time-resolved magneto-optical Kerr-rotation spectroscopy. The nanosecond spin relaxation times in moderately n-doped β-GaN decrease with increasing excitation density, though the effective lifetimes of the optically excited carriers are almost two orders of magnitude shorter than the spin relaxation times. This counterintuitive finding is explained by the heating of the electron system due to the excitation process. The spin relaxation times in degenerately n-doped β-GaN are found to be independent of excitation density as the very high electron Fermi temperature completely dominates over carrier heating processes in this case.

Effect of structural disordering on magnetic and magneto-optical properties of Fe3Si

The electronic, magnetic, transport, and magneto-optical properties of the $D{0}_{3}$ and amorphous $\mathrm{F}{\mathrm{e}}_{3}\mathrm{Si}$ are investigated by using the first-principles calculations. A strong correlation between local magnetization and atomic arrangement is established. The amorphization significantly alters the spin polarization and the magneto-optical Kerr rotation spectrum but has a little influence on anomalous Hall conductivity. Analyses in band structures and interband matrix elements provide clear insights for the understanding of these results.

Tuning the shape of magneto-optical Kerr spectrum by changing the strip width in one-dimensional Ag-Co-Ag magnetoplasmonic nanogratings

Magnetoplasmonic nanostructures have received significant attention as they have the ability to manipulate the magneto-optical response more precisely and efficiently. Here, we report a method to manipulate the shape of the longitudinal magneto-optical Kerr rotation spectrum in one-dimensional magnetoplasmonic nanogratings. It is found that changing the strip width of the nanograting can alter the sign of the maximal Kerr rotation of the two Kerr spectra. Experimentally, we obtained the maximal Kerr rotation angles of 0.30° and −0.18° at the wavelength of 530 nm for strip widths of 200 nm and 160 nm, respectively, in the Ag/Co/Ag grating sample. It has been revealed that the sign change of the two maximal Kerr angles is attributed to the relative contributions of the dipole-dipole interactions. These findings are of potential values for practical applications of magneto-optical devices.