All-optical Magnetic Recording Research Articles

Magnetization properties of radially polarized Bessel-Gaussian vortex beams with radial phase modulation in a 4π focusing system.

This paper explored the optically induced magnetization properties of radially polarized Bessel-Gaussian vortex beams with radial phase modulation in a 4π high numerical aperture (NA) focusing system, which is based on the vector diffraction theory and the inverse Faraday effect. The results show that in the case of radial modulation parameter L=0, one longitudinal magnetization chain with adjustable length can be obtained by modulating the truncation parameter β. When the radial modulation parameter L=1.3, two magnetization chains can be obtained by modulating the truncation parameter β. By modulating the radial modulation parameter L, two magnetization chains along the optical axis can be generated, each with four dark magnetic traps; meanwhile, the spacing between two magnetization chains can be adjusted. These results may be helpful in high-density all-optical magnetic recording, atom capture, and magnetic resonance microscopy.

All-optical magnetic recording in garnets using a single laser pulse at L-band telecom wavelengths

Ultrafast photo-magnetism in dielectric garnets reveals a possibility to perform magnetization switching with a single laser pulse. We demonstrate ultrafast photo-magnetic switching in Co-doped iron garnet films at wavelengths relevant to the more-enabled optical L-band window. We studied both photo-induced magnetization precession and switching of magnetization within the near-infrared spectral range using magneto-optical tools. Single laser pulse switching of magnetization around the 1590 nm wavelength has been found in a narrow resonance band using magneto-optical imaging. The high efficiency of this switching, along with a very low-photon absorption in the L-band, makes this phenomenon interesting for both all-optical magnetic recording and data transfer using telecom systems.

Generation of pure longitudinal magnetization focal spot with a triplex metalens.

A pure longitudinal magnetization focal spot is greatly desired by all-optical magnetic recording. In this Letter, a triplex metalens is proposed and demonstrated to possess triple functions of an azimuthal polarization converter, a helical phase plate, and a focusing lens. The three-in-one combination enables conversion of linearly polarized incident light to the first-order azimuthally polarized vortex beam and focusing into a diffraction-limited spot. The state of polarization of the focal spot is measured to be pure circular polarization implying that a pure longitudinal magnetization field can be induced by the inverse Faraday effect. The lateral full width at half-maximum of the focal spot is 1.9 µm, and the experimental conversion efficiency of the metasurface is about 19.3%.

Dual-shot dynamics and ultimate frequency of all-optical magnetic recording on GdFeCo

Although photonics presents the fastest and most energy-efficient method of data transfer, magnetism still offers the cheapest and most natural way to store data. The ultrafast and energy-efficient optical control of magnetism is presently a missing technological link that prevents us from reaching the next evolution in information processing. The discovery of all-optical magnetization reversal in GdFeCo with the help of 100 fs laser pulses has further aroused intense interest in this compelling problem. Although the applicability of this approach to high-speed data processing depends vitally on the maximum repetition rate of the switching, the latter remains virtually unknown. Here we experimentally unveil the ultimate frequency of repetitive all-optical magnetization reversal through time-resolved studies of the dual-shot magnetization dynamics in Gd27Fe63.87Co9.13. Varying the intensities of the shots and the shot-to-shot separation, we reveal the conditions for ultrafast writing and the fastest possible restoration of magnetic bits. It is shown that although magnetic writing launched by the first shot is completed after 100 ps, a reliable rewriting of the bit by the second shot requires separating the shots by at least 300 ps. Using two shots partially overlapping in space and minimally separated by 300 ps, we demonstrate an approach for GHz magnetic writing that can be scaled down to sizes below the diffraction limit.

Interlayer exchange coupling modulated all-optical magnetic switching in synthetic ferrimagnetic heterostructures

All-optical magnetic switching in ferromagnetic thin films reveals an unprecedented route to manipulate magnetic configurations for future ultrafast and energy-efficient magnetic storage. In this study, the relationship between the all-optical magnetic switching and interlayer exchange coupling was investigated in a [Co/Pt]n/Ru/GdFeCo synthetic ferrimagnetic heterostructure. While a thermal demagnetization is dominant in a single GdFeCo alloy film after laser sweep, the magneto-optical interaction transits to a clear all-optical magnetic switching after GdFeCo film being exchange coupled with a [Co/Pt]n multilayer. Meanwhile, the established interlayer exchange coupling can also significantly increase the optical-induced magnetization switching ratio in [Co/Pt]n multilayer after single-pulse laser illumination. A threshold interlayer exchange coupling strength of ∼0.31 erg cm−2 was confirmed for achieving a deterministic (∼90%) optical-induced magnetization switching ratio after multi-pulse laser illumination. This study demonstrates a clear correlation between interlayer exchange coupling and all-optical magnetic switching, offering a promising engineering pathway for high density all-optical magnetic recording.

All-Optical Manipulation of Magnetization in Ferromagnetic Thin Films Enhanced by Plasmonic Resonances.

In this paper, we report all-optical manipulation of magnetization in ferromagnetic Co/Pt thin films enhanced by plasmonic resonances. By annealing a thin Au layer, we fabricate large-area Au nanoislands on top of the Co/Pt magnetic thin films, which show plasmonic resonances around the wavelength of 606 nm. Using a customized magneto-optical Kerr effect setup, we experimentally observe an 18.5% decrease in the minimum laser power required to manipulate the magnetization, comparing the on- and off-resonance conditions. The results are in very good agreement with numerical simulations. Our research findings demonstrate the possibility to achieve an all-optical magnetic recording with low energy consumption, low cost, and high areal density by integrating plasmonic nanostructures with magnetic media.

Hysteretic optomagnetic bistability based on the plasmon-induced inverse Faraday effect in magnetoplasmonic disk resonators

We investigate the inverse Faraday effect (IFE) by surface plasmon traveling modes in an ultracompact magnetoplasmonic disk resonator side-coupled to a plasmonic waveguide. The magnetic field by the IFE reacts to the traveling modes and leads to a nonlinear shift of the resonance in the disk resonator. Describing those nonlinear phenomena analytically by using the temporal coupled mode theory and the Lorentz reciprocity theorem, we propose a conception of hysteretic optomagnetic bistability that the system has two stable states of optically induced magnetic field to switch on and off, depending upon the history of the incident optical power. The numerical simulations agree well with the analytical prediction. The proposed conception may find potential applications in all-optical magnetic recording and switching with nanometer resolution.

Investigation on magnetization induced by tightly focused azimuthally polarized fractional vortex beam

All-optical magnetic recording based on the inverse Faraday effect has become a research hotspot in recent years due to its ultra-high storage density and ultra-fast magnetization reversal rate. Existing studies have shown that by optimizing the phase modulation or performing 4π tight focusing of azimuthally polarized vortex beams, high-resolution longitudinal magnetization fields with different axisymmetric intensity patterns can be generated. In order to meet the requirements of more complex all-optical magnetic recording and asymmetric magnetic particle capture and manipulation, it is particularly important to generate an asymmetric light-induced magnetization field with adjustable center position. Studies have shown that the fractional vortex phase could lead to the asymmetric focal field distribution generation under tight focusing conditions, which means that the tightly-focused azimuthally polarized light carrying the fractional vortex phase can produce a novel asymmetric light-induced magnetization field. As a new degree of freedom for the regulation of the magnetization field, the fractional topological charge will bring more new phenomena, new effects and new applications in the field of interaction between light and matter. In this work, for the first time to our knowledge, the magnetization induced by tightly focused azimuthally polarized fractional vortex beam is studied based on the Richard-Wolf vector diffraction theory and the inverse Faraday effect. The equivalent approximation of the magnetization induced by azimuthally polarized fractional vortex beam regarded as a weighted superposition of magnetization induced and crossly induced by a finite number of azimuthally polarized adjacent integer-order vortex beams, where the number of the equivalent terms is chosen by using the histogram intersection method of the intensity distribution image of the magnetization field. The magnetization field distribution under different values of <i>α</i> are also numerically simulated. Studies have shown that magnetization induced by the azimuthally polarized fractional vortex beam is asymmetrically distributed. When the fractional vortex topological charge α belongs to [0.5,1.5], as the vortex topological load increases, the splitting phenomenon of the transverse distribution of magnetization field appears with the magnetization spot position shift in the direction perpendicular to the optical axis. When <i>α</i> equals 0.5 or 1.5, the maximum offset of the center of the magnetization spot is 0.24<i>λ</i>. When the fractional vortex topological charge α belongs to [2,3], the transverse distribution of magnetization field splits two hot intensity spots with gradually growing outer ring diameter. When the fractional vortex topological charge <i>α</i> tends to be an integer 3, the transverse distribution of magnetization field also gets round and symmetrical. In particular, when the fractional vortex topological charge α is a half-integer, especially larger than 3. The number of hot spots (<inline-formula><tex-math id="M8">\begin{document}$\alpha - 0.5 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20200269_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20200269_M8.png"/></alternatives></inline-formula>) of the intensity of the magnetization field and the number of dark spots (<inline-formula><tex-math id="M9">\begin{document}$ \alpha - 1.5 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20200269_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20200269_M9.png"/></alternatives></inline-formula>) surrounded by them all have a positive correlation with the number of vortex order. The research in this paper is expected to have new applications in the fields of all-optical magnetic recording and the capture and manipulation of magnetic particles.

A pure longitudinal magnetization induced by 4π tightly focusing of two linearly polarized vortex beams

In this paper, according to the inverse Faraday effect, a pure longitudinal magnetization field induced by 4π tightly focusing of two linearly polarized vortex beams (LPVBs) is investigated. We find that the orbital angular momentum (OAM) of these two LPVBs could be transformed into spin angular momentum in the tightly focusing condition, and a longitudinal magnetization with purity of 100% is generated, if the topological charges of the two counterpropagating LPVBs are two identical odd numbers. In addition, the distribution and strength of induced longitudinal magnetization could be regulated by turning the OAM of the two counterpropagating LPVBs and numerical aperture of the two focal lenses, respectively. Besides, the local polarization states of the light field and a three-dimensional magnetization array in the focal region is surveyed. We believe that this 4π setup of generating a pure longitudinal magnetization could potentially be used to the field of storage technology especially for the using of massively parallelized all-optical magnetic recording.

Creation of an ultralong non-diffracting magnetization light beam with multiple energy oscillations using the inverse Faraday effect.

Diffraction and scatter effects are two big challenges that make the magnetization induced by a light beam endure a finite propagation distance and vulnerable to the defect of magneto-optic film. Here we propose a method to overcome both challenges by creating an ultralong non-diffracting (UND) magnetization light beam with multiple energy oscillations. By simply increasing the number of energy oscillations using the optical pen, the magnetization light beam can propagate over an ultralong distance without significant divergence. Besides, as a kind of non-diffracting light beam, this magnetization light beam possesses the property of self-healing, which makes it robust to the scatter effect. This Letter demonstrates for the first time, to the best of our knowledge, the creation of an UND magnetization beam, which may open a new avenue for high-density all-optical magnetic recording and atomic trapping, as well as confocal and magnetic resonance microscopy.

Selection rules for all-optical magnetic recording in iron garnet

Rapid growth of the area of ultrafast magnetism has allowed to achieve a substantial progress in all-optical magnetic recording with femtosecond laser pulses and triggered intense discussions about microscopic mechanisms responsible for this phenomenon. The typically used metallic medium nevertheless considerably limits the applications because of the unavoidable heat dissipation. In contrast, the recently demonstrated photo-magnetic recording in transparent dielectric garnet for all practical purposes is dissipation-free. This discovery raised question about selection rules, i.e. the optimal wavelength and the polarization of light, for such a recording. Here we report the computationally and experimentally identified workspace of parameters allowing photo-magnetic recording in Co-doped iron garnet using femtosecond laser pulses. The revealed selection rules indicate that the excitations responsible for the coupling of light to spins are d-d electron transitions in octahedral and tetrahedral Co-sublattices, respectively.

All-optical generation of magnetization with arbitrary three-dimensional orientations.

In this Letter, all-optical generation of magnetization with arbitrary three-dimensional (3D) orientations is numerically demonstrated through the inverse Faraday effect (IFE) by using a reversing calculation method. The IFE-induced magnetization with an expected 3D orientation is initially conceived by coherently configuring two orthogonally arranged electric dipoles with a phase difference of π/2 in the focal region of a to-be-determined incident light field. Based on the dipole antenna theory, this required incident light field can be deduced analytically according to the orientations of the electric dipoles. By utilizing this field as illumination and reversing the field propagation, magnetization with the expected orientation can be obtained in the focal region through the IFE. Moreover, this method showcases a high magnetization orientation purity (greater than 93%) within the focal volume defined by the full width at half maximum when the numerical aperture of the focal lens is 0.95. This result demonstrates extended flexibility of magnetization manipulations in an all-optical fashion and possesses great potential in spintronics and all-optical magnetic recording.

Magnetization Switching of L10-FePt Nanoparticles by Combination Effect From Multishot Circularly Polarized Laser Pulses

All-optical magnetic switching observed on some materials provides a potential recording option due to its ultrashort recording time and low-power consumption. In most studies, this phenomenon is attributed to the inverse Faraday effect that a circularly polarized laser will induce an optomagnetic field or magnetic circular dichroism that one magnetization orientation will absorb more energy than another. L10-FePt is a promising candidate for high-density magnetic recording due to its high perpendicular anisotropy, whose magnetization is fixed out of plane either up or down. However, the mechanism about all-optical switching on FePt is still under debate. In this paper, we show that it benefits from both inverse Faraday effect and magnetic circular dichroism. We use an accumulative model to demonstrate the results of an ensemble of single-domain L10-FePt nanoparticles after multishot circularly polarized femtosecond laser pulses. The simulation results agree with the observation results of Lambert et al. and demonstrate the possibility of reaching deterministic all-optical magnetic recording by optimizing the laser fluence and the optomagnetic field strength.

Towards massively parallelized all-optical magnetic recording

We demonstrate an approach to parallel all-optical writing of magnetic domains using spatial and temporal interference of two ultrashort light pulses. We explore how the fluence and grating periodicity of the optical transient grating influence the size and uniformity of the written bits. Using a total incident optical energy of 3.5 μJ, we demonstrate the capability of simultaneously writing 102 spatially separated bits, each featuring a relevant lateral width of ∼1 μm. We discuss viable routes to extend this technique to write individually addressable, sub-diffraction-limited magnetic domains in a wide range of materials.

Generation of uniformly oriented in-plane magnetization with near-unity purity in 4π microscopy.

In this Letter, we numerically demonstrate the all-optical generation of uniformly oriented in-plane magnetization with near-unity purity (more than 99%) under a 4π microscopic configuration. This is achieved through focusing two counter-propagating vector beams consisting of coherently configured linear and radial components. Based on the Debye diffraction theory, constructive and destructive interferences of the focal field components can be tailored under the 4π configuration to generate high-purity uniformly polarized transverse and longitudinal electric-field components in the center of the focal region. Consequently, near-unity purity in-plane magnetization with a uniform orientation within the focal volume defined by the full width at half-maximum can be created through the inverse Faraday effect. In addition, it reveals that the purity of the in-plane magnetization is robust against the numerical aperture of the focal lens. This result expands the flexibility of magnetization manipulations through light and holds great potential in all-optical magnetic recording and spintronics.

Generating sub wavelength pure longitudinal magnetization probe and chain using complex phase plate

Based on inverse Faraday Effect, the three dimensional magnetization field distribution induced by the azimuthally polarized annular multi Gaussian transmitted through a multi belt complex phase filter (MBCPF) is investigated numerically using vector diffraction theory. Numerical results shows that by properly adjusting the radii of different rings of MBCPF, one can achieve sub wavelength (0.286λ) pure longitudinal magnetic probe with ultra-long focal depth of (35λ). We also noticed that magnetization chain composed of eight, four and two magnetic spots of subwavelength scale is also achieved by properly modulating the radii of MBCPF. The authors expects such a pure ultra-long subwavelength magnetic probe and magnetic chain can be used to realize all optical magnetic recording (AOMR), multilayer magneto-optical data storage, ultra-compact optomagnetic devices, magnetic particle trapping and transportation, fabricating magnetic lattices for spin wave operation, as well as confocal and magnetic resonance microscopy.

Theoretical guideline for generation of an ultralong magnetization needle and a super-long conveyed spherical magnetization chain.

Considering an azimuthally polarized vortex beam with a Gaussian annulus as an incoming light, light induced magnetization fields for both a single high NA lens and a pair of high NA lenses are investigated theoretically. We deduce analytical formulas for the parameters of a magnetization needle and a magnetization chain when the angular width of the incident beam is far less than its central angular position. Through these analytical formulas, the properties of the magnetization needle and the magnetization chain are very clear and distinct. Compared with parameter optimizing to produce an ultralong magnetization needle with lateral sub-wavelength scale and a super-long spherical magnetization chain with three-dimensional super resolution, the analytical method is direct and has a theoretical guideline. The validity of these formulas is proved, compared to numerical solutions. The present work regarding these super-resolution magnetization patterns is of great value in high density all-optical magnetic recording, atomic trapping as well as confocal and magnetic resonance microscopy.

Spherical and sub-wavelength longitudinal magnetization generated by 4π tightly focusing radially polarized vortex beams.

Based on the vector diffraction theory and the inverse Faraday effect, we numerically study the light-induced magnetization near the focus of a 4π high numerical aperture focusing configuration under the illumination of two counter- propagating radially polarized hollow Gaussian vortex beams. The simulated results demonstrate that, by selecting higher-order vortex beam modes (e.g. n=4with n - the beam order) and proper truncation parameter (e.g. β=1.75 with β- the ratio of the pupil radius to the incident beam waist), spherical and sub-wavelength longitudinal magnetization can be generated in the vicinity of focus. Such special magnetization feature is attributed to not only the interaction between optical vortices and the radially polarized beams, but also the completely destructive interference of azimuthal components and the constructive interference of the longitudinal component of the two counter-propagating radially polarized vortex beams. This spherical and sub-wavelength longitudinal magnetization distribution may be of interest for applications in all-optical magnetic recording and confocal and magnetic resonance microscopy.

Novel Development of Ultra-Fast and Ultra-High Density Magnetic Recording

This paper reviews our recent progress of a novel magnetic recording method which is based on all-optical magnetic recording and plasmonic antennas with bit-patterned media. Using our technique, magnetization reversal occurs at the femtosecond time scale and recording density is over 2 Tbit/inch2. We will verify our recording method in terms of precise electromagnetic simulation for ultra-short time light pulse.

Circularly Polarized Light Generated by Plasmon Antenna for All-Optical Magnetic Recording

Circularly Polarized Light Generated by Plasmon Antenna for All-Optical Magnetic Recording