Magneto-optical Kerr Effect Technique Research Articles

Phase shift of coherent magnetization dynamics after ultrafast demagnetization in strongly quenched nickel thin films.

The remagnetization process after ultrafast demagnetization can be described by relaxation mechanisms between the spin, electron, and lattice reservoirs. Thereby, collective spin excitations in form of spin waves and their angular momentum transfer play an important role on the longer timescales. In this work, we address the question whether the magnitude of demagnetization - the so-called quenching - affects the coherency and the phase of the excited spin waves. We present a study of coherent magnetization dynamics in thin nickel films after ultrafast demagnetization using the all-optical, time-resolved magneto-optical Kerr-effect (tr-MOKE) technique. The largest coherent precession amplitude was observed for strongly quenched systems, indicating a well-defined precession phase for all pump pulses at a demagnetization of up to 90% in this system. Moreover, the phase of the excited spin-waves in Ni increases with the pump fluence, indicating a delayed start of the precession during the remagnetization. We compare these findings to recent studies in Ni80Fe20 (permalloy), to evaluate the influence of the magneto-elastic coupling and non-linear spin wave dynamics on the magnetization dynamics.

Effect of growth conditions on magnetization reversal and magnetic anisotropy in Co2Fe0.5Ti0.5Si quaternary Heusler alloy thin films

Ferromagnetic resonance and magneto-optic Kerr effect (MOKE) techniques are employed to unravel the nature of ‘in-plane’ (IP) magnetic anisotropy and magnetization reversal (MR) processes in magnetron-sputtered 100 nm Co2Fe0.5Ti0.5Si (CFTS) thin films, deposited (and subsequently annealed) at different substrate temperatures (Ts) ranging from 200 °C to 550 °C. By varying TS, the CFTS films are produced with different amounts of anti-site (AS) atomic disorder. Irrespective of the degree of AS disorder, the IP uniaxial magnetic anisotropy (UMA) is prevalent in all the CFTS films. The TS450 and TS500 films, deposited at T S = 450 °C and 500 °C, stand out as they have (i) the least AS disorder, (ii) lowest value (α = 0.0055) of the Gilbert damping constant, (iii) high saturation magnetization ( ≅770 G ) at 300 K, (iv) UMA energy density as high as ≅1.6×104erg/cc at 120 K dropping to ≅1.0×104erg/cc at 300 K, (v) spin-wave stiffness ( D ) at T = 0 K, D0≅175 meVÅ2 and, as in other CFTS films, the electron–magnon interaction is primarily responsible for the thermal renormalization of D . Furthermore, in the TS450 and TS500 films, MOKE hysteresis loops at various angles ( ψH ) that the field makes with the easy axis in the film plane, reveals two mutually exclusive UMAs, UMA1≫UMA2 , with easy axes perpendicular to each other. By contrast, only a single UMA is observed in all the remaining films. When ψH=0∘ , MOKE domain images reveal that, consistent with the uniaxial nature of magnetic anisotropy, an abrupt reversal of magnetization is accompanied by the formation of 180° domains. When ψH = 90°, due to the presence of two UMAs in TS450 and TS500 films, following the field reversal, first the reverse domains nucleate and then grow at the expense of the fundamental domains through field-induced domain wall motion.

Enhanced magnetic anisotropy and its thermal stability in obliquely deposited Co-Film on the nanopatterned substrate

The deliberate manipulation of magnetic anisotropy through controlled adjustments to surface and interface morphology has become important due to its potential applications in spintronics and magnetic memory devices. In the present work, oblique angle deposition (OAD) at 65° to the surface normal on a rippled substrate, keeping nanopatterns on the SiO2 substrate perpendicular to the OAD projection, has been used to induce in-plane uniaxial magnetic anisotropy (UMA) and its thermal stability in cobalt (Co) thin film. Prior to film preparation, the patterned substrate of wavelength 68 ± 4 nm and amplitude 3.4 ± 0.2 nm was prepared separately using a low-energy ion beam erosion (IBE) process. Grazing incidence small-angle X-ray scattering (GISAXS) and magneto-optical Kerr effect (MOKE) techniques have been used for film characterization. While GISAXS provided information about film structure and morphology, MOKE provided information about magnetic properties, thus making it possible to correlate the temperature-dependent evolution of morphology with that of UMA in the film. A clear anisotropy in the growth of Co film is found to result in strong UMA. Unlike previous studies, which typically observe a reduction in UMA strength with thermal annealing, the present work demonstrates a 31 % increase in UMA strength after annealing up to 200 °C, underscoring the importance of oblique angle deposition on nano-rippled substrates for achieving high UMA and ensuring its thermal stability up to moderate temperatures. The appearance of large UMA and its unusual temperature dependence is understood in terms of the shadowing effects and column coalescence, resulting in more robust shape anisotropy. This work provides insights into morphological anisotropy, elucidating the interplay of shadowing effects, shape anisotropy, and dipolar interactions within interacting column and ripple structures. This method gives rise to morphology-induced anisotropy, which can also be applied to other ferromagnetic films to enhance the magnetic anisotropy and ensure thermal stability.

Coexistence of Ferroelectricity and Ferromagnetism in Atomically Thin Two-Dimensional Cr2S3/WS2 Vertical Heterostructures.

Two-dimensional (2D) heterostructures with ferromagnetism and ferroelectricity provide a promising avenue to miniaturize the device size, increase computational power, and reduce energy consumption. However, the direct synthesis of such eye-catching heterostructures has yet to be realized up to now. Here, we design a two-step chemical vapor deposition strategy to growth of Cr2S3/WS2 vertical heterostructures with atomically sharp and clean interfaces on sapphire. The interlayer charge transfer and periodic moiré superlattice result in the emergence of room-temperature ferroelectricity in atomically thin Cr2S3/WS2 vertical heterostructures. In parallel, long-range ferromagnetic order is discovered in 2D Cr2S3 via the magneto-optical Kerr effect technique with the Curie temperature approaching 170 K. The charge distribution variation induced by the moiré superlattice changes the ferromagnetic coupling strength and enhances the Curie temperature. The coexistence of ferroelectricity and ferromagnetism in 2D Cr2S3/WS2 vertical heterostructures provides a cornerstone for the further design of logic-in-memory devices to build new computing architectures.

Structural and magnetic properties of YIG thin films deposited by pulsed laser deposition and RF magnetron sputtering technique

Yttrium iron garnet (YIG) has been extensively explored for its potential avenues in spintronic applications. A majority of these studies employ thin films grown by PLD at high substrate temperature, which generally leads to an interfacial dead layer with cations interdiffusion hindering their technological implications. In this communication, we report the growth of YIG thin films at room temperature by PLD and RF sputtering techniques. Detailed structural investigation confirms the thin films’ single-phase growth and epitaxial nature. We have a further detailed investigation of magnetic properties by dc magnetization, magneto-optical Kerr effect and FMR techniques. Although our thin films exhibit a comparatively lower magnetic performance in terms of saturation magnetization and damping constant, we have obtained a significantly lower interfacial dead layer thickness of ∼1 nm, which is quite promising for spin transport applications. The present study, therefore, calls for future studies for simultaneous optimization of magnetic performance and interfacial dead layer with room temperature grown YIG thin films by both PLD and RF sputtering methods.

Enhancing spin-transfer torque in magnetic tunnel junction devices: Exploring the influence of capping layer materials and thickness on device characteristics

We have developed and optimized two categories of spin-ransfer torque magnetic tunnel junctions (STT-MTJs) that exhibit a high tunnel magnetoresistance ratio, low critical current, high outputpower in the micro-watt range, and auto-oscillation behavior. These characteristics demonstrate the potential of STT-MTJs for low-power, high-speed, and reliable spintronic applications, including magnetic memory, logic, and signal processing. The only distinguishing factor between the two categories, denoted as A-MTJs and B-MTJs, is the composition of their free layers, two CoFeB/0.21 Ta/6 CoFeSiB for A-MTJs and two CoFeB/0.21 Ta/7 NiFe for B-MTJs. Our study reveals that B-MTJs exhibit lower critical currents for auto-oscillation than A-MTJs. We found that both stacks have comparable saturation magnetization and anisotropy field, suggesting that the difference in auto-oscillation behavior is due to the higher damping of A-MTJs compared to B-MTJs. To verify this hypothesis, we employed the all-optical time-resolved magneto-optical Kerr effect technique, which confirmed that STT-MTJs with lower damping exhibited auto-oscillation at lower critical current values. Additionally, our study aimed to optimize the STT-MTJ performance by investigating the impact of the capping layer on the device’s response to electronic and optical stimuli.

Concomitant modulation of interlayer exchange coupling and Gilbert damping in Fe/CoO with spin conductor Ag layer at interface

We report on the modulation of interlayer exchange coupling (IEC) between the ferromagnet (FM) Fe and insulating antiferromagnet (AFM) CoO and its impact on the Gilbert damping by a time-resolved magneto-optical Kerr effect technique. By inserting a wedge spin conductor Ag layer at the interface of Fe/CoO, it is revealed that both uniform spin precession frequency and Gilbert damping constant of Fe film decrease with increasing Ag thickness up to ∼2 nm, and above that with eliminated IEC, these two parameters reach the intrinsic values of the Fe film. The precession frequency and damping also show similar variation tendency with temperature for attenuated IEC. These results prove that the exchange coupling at the FM–AFM interface plays an essential role for the spin angular momentum transfer to the insulating AFM layer, and the spin pumping is insignificant even for the very thin spin conductor layer. Our findings may have general insights into the role of the FM/AFM interface in the magnetization dynamics and spin angular momentum transfer for future spintronic applications.

Optical Creation of Skyrmions by Spin Reorientation Transition in Ferrimagnetic CoHo Alloys.

Manipulating magnetic skyrmions by means of a femtosecond (fs) laser pulse has attracted great interest due to their promising applications in efficient information-storage devices with ultralow energy consumption. However, the mechanism underlying the creation of skyrmions induced by an fs laser is still lacking. As a result, a key challenge is to reveal the pathway for the massive reorientation of magnetization from trivial to nontrivial topological states. Here, we studied a series of ferrimagnetic CoHo alloys and investigated the effect of a single laser pulse on the magnetic states. Thanks to the time-resolved magneto-optical Kerr effect and imaging techniques, we demonstrate that the laser-induced phase transitions from single domains into a topological skyrmion phase are mediated by the transient in-plane magnetization state, in real time and space domains, respectively. Combining experiments and micromagnetic simulations, we propose a two-step process for creating skyrmions through laser pulse irradiation: (i) the electron temperature enhancement induces a spin reorientation transition on a picosecond (ps) timescale due to the suppression of perpendicular magnetic anisotropy (PMA) and (ii) the PMA slowly restores, accompanied by out-of-plane magnetization recovery, leading to the generation of skyrmions with the help of spin fluctuations. This work provides a route to control skyrmion patterns using an fs laser, thereby establishing the foundation for further exploration of topological magnetism at ultrafast timescales.

Nonlocal Manipulation of Magnetism in an Itinerant Two-Dimensional Ferromagnet.

Two-dimensional (2D) magnets are crucial in the construction of 2D magnetic and spintronic devices. Many devices, including spin valves and multiple tunneling junctions, have been developed by vertically stacking 2D magnets with other functional blocks. However, owing to limited local interactions at the interfaces, the device structures are typically extremely complex. To solve this problem, the nonlocal manipulation of magnetism may be a good solution. In this study, we use the magneto-optical Kerr effect technique to demonstrate the nonlocal manipulation of magnetism in an itinerant 2D ferromagnet, Fe3GeTe2 (FGT), whose magnetism can be manipulated via an antiferromagnet/ferromagnet interface or a current-induced spin-orbital torque placed distant from the local site. It is discovered that the coupling of a small piece of MnPS3 (∼40 μm2) with FGT can significantly enhance the coercive field and emergence of exchange bias in the entire FGT flake (∼2000 μm2). Moreover, FGT flakes with different thicknesses have the same coercive field at low temperatures if they are coupled together. Our study provides an understanding of the basic magnetism of 2D itinerant ferromagnets as well as opportunities for engineering magnetism with an additional degree of freedom.

Quantifying spin Hall topological Hall effect in ultrathin Tm3Fe5O12/Pt bilayers

Recent reports have shown that thulium iron garnet (TmIG) based bilayers are promising material platforms for realizing small, room-temperature skyrmions. For potential applications, it is imperative to accurately evaluate electrical readout signals of skyrmions. In this context, the topological Hall effect has been considered as a characteristic signature of skyrmion formation. Unlike previous studies that have modeled the anomalous Hall effect in ultrathin TmIG/Pt bilayers, we isolate its contribution to the electrical readout signal by directly measuring the magnetic hysteresis loops using a sensitive Sagnac magneto-optical Kerr effect technique. Our combined optical and electrical measurements reveal that the spin Hall topological Hall resistivity is considerably larger than previously estimated values. Our finding further indicates that skyrmions can exist at room-temperature and near-zero applied magnetic fields.

Accelerated Ultrafast Magnetization Dynamics at Graphene/CoGd Interfaces.

Atomically thin graphene layers can act as a spin-sink material when adjacent to a nanoscale magnetic surface. The enhancement in the extrinsic spin-orbit coupling (SOC) strength of graphene plays an important role in absorbing the spin angular momentum injected from the magnetic surface after perturbation with an external stimulus. As a result, the dynamics of the excited spin system is modified within the magnetic layer. In this paper, we demonstrate the modulation of ultrafast magnetization dynamics at graphene/ferrimagnet interfaces using the time-resolved magneto-optical Kerr effect (TRMOKE) technique. Magnetically modified interfaces with a systematic increase in the number of graphene layers coupled with the 10 nm-thick Co74Gd26 layer are studied. We find that the variation in the dynamical parameters, i.e., ultrafast demagnetization time, remagnetization times, decay time, effective damping, precessional frequency, etc., observed at different time scales is interconnected. The demagnetization time and decay time for the ferrimagnet become approximately two times faster than the corresponding intrinsic values. We found a possible correlation between the demagnetization time and damping. The effect is more pronounced for the interfaces with monolayer graphene and graphite. The spin-mixing conductance is found to be approximately 0.8 × 1015 cm-2. The effect of SOC, pure spin current, the appearance of structural defects, and thermal properties at the graphene/ferrimagnet interface are responsible for the modifications of several dynamical parameters. This work demonstrates some important properties of the graphene/ferrimagnet interface which may unravel the possibilities of designing spintronic devices with elevated performance in the future.

Ultra-long spin relaxation in two-dimensional ferromagnet Cr2Ge2Te6 flake

By utilizing time-resolved magneto-optical Kerr effect technique, we investigated both demagnetization and remagnetization processes of a typical two-dimensional (2D) van der Waals (vdW) ferromagnetic (FM) semiconductor Cr2Ge2Te6 (CGT) and two comparative Te-based crystals, i.e. three-dimensional (3D) FM metal Cr3Te4 (CT) and 2D vdW FM metal Fe3GeTe2 (FGT). Different from 3D CT showing one-step laser-induced demagnetization, two-steps demagnetization behavior is dominated in 2D vdW FGT and CGT, which indicates that the 2D vdW magnets have relatively weaker electron-spin coupling. Of particular interest, we discovered an ultra-long remagnetization behavior in the 2D vdW CGT flake. The laser pumping induced spin demagnetization would not recover even in the time scale of 3500 ps (experimental setup limitation). To the best of our knowledge, this is the longest remagnetization process observed so far and it might be one of the particular characterizations of 2D vdW ferromagnet semiconductors. Based on the modified three temperature model, the key roles played by the crystal dimensionality and thermal diffusion anisotropy were revealed in the spin dynamics of 2D vdW magnets. Our findings would not only open the door to the thoroughly understanding of the origin of the magnetism in 2D FM materials but also undoubtedly find suitable applications in spintronics and magnonics devices.

Giant anisotropic Gilbert damping versus isotropic ultrafast demagnetization in monocrystalline Co50Fe50 films

The magnetization orientation dependence of Gilbert damping and ultrafast demagnetization were investigated in single-crystalline ${\mathrm{Co}}_{50}{\mathrm{Fe}}_{50}$ films grown on a MgO(100) substrate by the time-resolved magneto-optical Kerr effect technique. The intrinsic Gilbert damping coefficient extracted from the time evolution of magnetization precessions shows a remarkably large anisotropy with a ratio of more than 300% for magnetization orientations along the $\ensuremath{\langle}100\ensuremath{\rangle}$ and $\ensuremath{\langle}110\ensuremath{\rangle}$ axes. Such a large anisotropy of Gilbert damping persists to high frequencies up to $50\phantom{\rule{4pt}{0ex}}\mathrm{GHz}$, where the effect of two-magnon scattering is suppressed. In contrast to the anisotropic Gilbert damping, the ultrafast demagnetization time and ratio for different in-plane magnetization orientations are nearly isotropic with the magnetization orientation. Although anisotropic Gilbert damping can be explained by the variation of spin-orbit coupling with the magnetization orientations, our results demonstrate that the role of spin-orbit coupling in the high nonequilibrium state after ultrafast laser excitation is isotropic in driving ultrafast spin-flip processes.

Heavy ion irradiation induced structural and magnetic modifications in Co/Alq3 bilayer

This article reports on the Ar+ ion irradiation induced modifications of Co/Alq3/Si-substrate bilayer thin film. Irradiation was carried out with 10 keV Ar+ ions at a fluence of 1 × 1016 ions/cm2. To study structural and magnetic modifications, x-ray reflectivity (XRR) and magneto-optical Kerr effect (MOKE) techniques, respectively have been utilized. XRR analysis reveals a significant reduction of electron density of the Co layer and enhancement of intermixing of Co and Alq3 at the Co/Alq3 interface after ion irradiation. MOKE measurements affirm that saturation magnetization reduces by a factor of about 3 due to irradiation without any detectable changes in the coercivity. XRR analysis and Monte Carlo simulation demonstrate the ion irradiation induced defects formation due to Ar implantation and enhancement of interface mixing between Co and Alq3 is the main cause of the reduction in saturation magnetization.

Structural Phase-Dependent Giant Interfacial Spin Transparency in W/CoFeB Thin-Film Heterostructures.

Pure spin current has transformed the research field of conventional spintronics due to its various advantages, including energy efficiency. An efficient mechanism for generation of pure spin current is spin pumping, and high effective spin-mixing conductance (Geff) and interfacial spin transparency (T) are essential for its higher efficiency. By employing the time-resolved magneto-optical Kerr effect technique, we report here a giant value of T in substrate/W (t)/Co20Fe60B20 (d)/SiO2 (2 nm) thin-film heterostructures in the beta-tungsten (β-W) phase. We extract the spin diffusion length of W and spin-mixing conductance of the W/CoFeB interface from the variation of damping as a function of W and CoFeB thickness. This leads to a value of T = 0.81 ± 0.03 for the β-W/CoFeB interface. A stark variation of Geff and T with the thickness of the W layer is obtained in accordance with the structural phase transition and resistivity variation of W with its thickness. Effects such as spin memory loss and two-magnon scattering are found to have minor contributions to damping modulation in comparison to the spin pumping effect which is reconfirmed from the unchanged damping constant with the variation of Cu spacer layer thickness inserted between W and CoFeB. The giant interfacial spin transparency and its strong dependence on crystal structures of W will be important for future spin-orbitronic devices based on pure spin current.

Spin reorientation transition induced by surface reconstruction in epitaxial Fe/Co bilayers

Surface reconstruction is a common physical phenomenon, driving the atoms near the surface to form a different spacing or symmetry from the bulk atoms to minimize the free enthalpy of the system. The formation of an asymmetric (7 × 2) reconstruction was observed on (112)-oriented Fe surface, which may be related to the anisotropic strain distribution induced by lattice mismatch. The influence of reconstruction on in-plane magnetic anisotropy has been studied in epitaxial Fe/Co bilayers grown by molecular beam epitaxy. The evolutions of Co layer thickness-dependent magnetic anisotropy for reconstructed and unreconstructed configurations were investigated by magneto-optic Kerr effect technique. The most significant difference caused by reconstruction took place when tCo = 3 nm, showing a reconstruction-dependent in-plane spin reorientation transition with a rotation of 90°, which can be attributed to the dramatically enhanced magnetic interface anisotropy of Fe/Co bilayers.

GROWTH AND MAGNETIC PROPERTIES OF DC MAGNETRON SPUTTERED Co0.2Fe0.8 THIN FILM

We report the growth and characterization of the [Formula: see text][Formula: see text] thin film. The thin film was deposited on Si (100) substrate by using a direct-current magnetron sputtering deposition technique. The Grazing Incidence X-ray Diffraction (GIXRD) analysis suggests a bcc crystal structure with a lattice constant of 2.87[Formula: see text]. The X-ray reflectivity (XRR) data analysis shows the thickness of the [Formula: see text][Formula: see text] thin film is about 30 nm. The Energy Dispersive X-ray (EDAX) analysis shows the presence of 22.94% of Co and 77.06% of Fe in [Formula: see text][Formula: see text] thin film. The Magneto-Optic Kerr Effect (MOKE) technique was used to investigate the magnetic properties and results support that [Formula: see text][Formula: see text]thin film exhibits soft magnetic character. The analysis of the M–H loop suggests that the value of the coercive field decreases with the increase of angle with respect to the magnetic easy axis. The MOKE measurement suggests that [Formula: see text][Formula: see text] thin film alloy exhibits the least anisotropy with two-fold magnetic anisotropy. Magnetization reversibility was observed by using high-resolution Kerr microscopy. The domain images showed that reversibility occurs through nucleation and domain wall motion. Magnetic anisotropy dispersion was also observed which may be due to the misalignment of grains in the film due to which we got the least anisotropy. These properties of the prepared sample found potential application in the field of spintronics and magnetic sensors.

Magnetic anisotropy and magnetization reversal in cobalt-iron thin film

Cobalt-iron thin film was deposited on rotating silicon (100) substrate using the DC-magnetron sputtering deposition unit. The structure is examined by Grazing Incidence X-ray Diffraction and results reveal that the sample has a bcc crystal structure with lattice constant 2.65 Å. The X-ray Reflectivity data analysis confirms that the thickness of the thin film alloy is about 33 nm. The Energy Dispersive X-ray Analysis confirms the presence of 30.56% of cobalt and 69.44% of iron in cobalt-iron thin film. The study of magnetic properties by Magneto-Optic Kerr Effect technique suggests that sample exhibits soft magnetic behavior. The M–H loop study shows that the coercive field decreases with the increase of rotation of the substrate which may be due to reduced grain size or crystal orientation. The study of remanence magnetization (Mr/Ms) confirms that the sample has a twofold magnetic structure and cubic magnetic anisotropy. The occurrence of the magnetization reversal in the sample is due to the formation of the ripples and change in the contrast in the color of domain images along the easy axis and through coherent rotation of spins along the hard axis. This confirms that the sample has uniaxial nature and found very useful applications in the magnetic sensors, memory storage devices and spintronics.

Femtosecond laser-induced spin dynamics in single-layer graphene/CoFeB thin films.

Graphene/ferromagnet hybrid heterostructures are important building blocks of spintronics due to the unique ability of graphene to transport spin current over unprecedented distances and possible increase in its spin-orbit coupling due to proximity and hybridization. Here, we present magnetization dynamics over a femtosecond to nanosecond timescale by employing an all-optical time-resolved magneto-optical Kerr effect technique in single-layer graphene (SLG)/CoFeB thin films with varying CoFeB thickness and compared them with reference CoFeB thin films without an SLG underlayer. Gilbert damping variation with CoFeB thickness is modelled to extract spin-mixing conductance for the SLG/CoFeB interface and isolate the two-magnon scattering contribution from spin pumping. In SLG/CoFeB, we have established an inverse relationship between ultrafast demagnetization time (τm) and the Gilbert damping parameter (α) induced by interfacial spin accumulation and pure spin-current transport via a spin pumping mechanism. This systematic study of ultrafast demagnetization in SLG/CoFeB heterostructures and its connection with magnetic damping can help to design graphene-based ultrahigh-speed spintronic devices.

Laser melting and quenching effects on magnetic domains in quasi-two-dimensional ferromagnetic Cr5Te8 crystal

By utilizing magneto-optical microscopy combined with the polar magneto-optical Kerr effect (MOKE) techniques, the photo-induced melting and quenching effects on magnetic domains in a quasi-two-dimensional (quasi-2D) ferromagnetic Cr5Te8 crystal were investigated. With irradiation of a pulsed laser, the magnetic domain of the Cr5Te8 crystal can be melted at a critical laser power density (2.55 × 108 mW/cm2), which has been confirmed by both MOKE images and curves. Moreover, it is intriguing to find that after laser melting and quenching, the dendritic type domains of the Cr5Te8 crystal are broken and the regrown spherical type domains are isolated from the original dendrites. Simultaneously, the magnetization behavior as a function of external field was modulated. These findings suggest that laser melting and quenching is an effective technique to tailor the magnetic domains in quasi-2D magnets and it provides a viable route in the creation of optical spintronic devices.