All-optical Magnetization Switching Research Articles

Optomagnetism with a plasmonic skyrmion.

Research at the frontier between optics and magnetism is revealing a wealth of innovative phenomena and avenues of exploration. Optical waves are demonstrating the capacity to induce ultrafast magnetism, while optical analogs of magnetic states, such as magnetic skyrmions, offer the prospect of novel, to the best of our knowledge, spin-optical states. In this Letter, we strengthen the synergy between light and magnetism by exploring the ability of plasmonic Neel skyrmions to create an optomagnetic field, i.e., an opto-induced stationary magnetic field, within a thin gold film. We show that, when generated using a focused radially polarized vortex beam (RPVB), a plasmonic Neel skyrmion emerges as an optimum for inducing optomagnetism in a thin gold film. Optical skyrmions offer new degrees of freedom for enhancing and controlling optomagnetism in plasmonic nanostructures, with direct application in all-optical magnetization switching, magnetic recording, and the excitation of spin waves.

Tracking nuclear motion in single-molecule magnets using femtosecond X-ray absorption spectroscopy

The development of new data storage solutions is crucial for emerging digital technologies. Recently, all-optical magnetic switching has been achieved in dielectrics, proving to be faster than traditional methods. Despite this, single-molecule magnets (SMMs), which are an important class of magnetic materials due to their nanometre size, remain underexplored for ultrafast photomagnetic switching. Herein, we report femtosecond time-resolved K-edge X-ray absorption spectroscopy (TR-XAS) on a Mn(III)-based trinuclear SMM. Exploiting the elemental specificity of XAS, we directly track nuclear dynamics around the metal ions and show that the ultrafast dynamics upon excitation of a crystal-field transition are dominated by a magnetically active Jahn-Teller mode. Our results, supported by simulations, reveal minute bond length changes from 0.01 to 0.05 Å demonstrating the sensitivity of the method. These geometrical changes are discussed in terms of magneto-structural relationships and consequently our results illustrate the importance of TR-XAS for the emerging area of ultrafast molecular magnetism.

4f electron temperature driven ultrafast electron localization

Valence transitions in strongly correlated electron systems are caused by orbital hybridization and Coulomb interactions between localized and delocalized electrons. The transition can be triggered by changes in the electronic structure and is sensitive to temperature variations, applications of magnetic fields, and physical or chemical pressure. Launching the transition by photoelectric fields can directly excite the electronic states and thus provides an ideal platform to study the correlation among electrons on ultrafast timescales. The EuNi2(Si0.21Ge0.79)2 mixed-valence metal is an ideal material to investigate the valence transition of the Eu ions via the amplified orbital hybridization by the photoelectric field on subpicosecond timescales. A direct view on the 4f electron occupancy of the Eu ions is required to understand the microscopic origin of the transition. Here we probe the 4f electron states of EuNi2(Si0.21Ge0.79)2 at the sub-ps timescale after photoexcitation by x-ray absorption spectroscopy across the Eu M5-absorption edge. The observed spectral changes due to the excitation indicate a population change of total angular momentum multiplet states J = 0, 1, 2, and 3 of Eu3+ and the Eu2+ J = 7/2 multiplet state caused by an increase in 4f electron temperature that results in a 4f localization process. This electronic temperature increases combined with fluence-dependent screening accounts for the strongly nonlinear effective valence change. The data allow us to extract a time-dependent determination of an effective temperature of the 4f shell, which is also of great relevance in the understanding of metallic systems' properties, such as the ultrafast demagnetization of ferromagnetic rare-earth intermetallic and their all-optical magnetization switching. In addition, our results elucidate the energetics of charge fluctuations in valence-mixed electronic systems, which provide enriched knowledge regarding the role of valence transitions and orbital hybridization for quantum critical phenomena. Published by the American Physical Society 2024

Role of photonic angular momentum in all-optical magnetic switching

Role of photonic angular momentum in all-optical magnetic switching

Extending the scope and understanding of all-optical magnetization switching in Gd-based alloys by controlling the underlying temperature transients

Extending the scope and understanding of all-optical magnetization switching in Gd-based alloys by controlling the underlying temperature transients

Single-shot all-optical switching of magnetization in TbFe

Thermally induced magnetization switching (TIMS) relying solely on a single laser without any applied magnetic field is a key research direction of current spintronics. Most studies on TbFe so far have focused on helicity-dependent all-optical switching (HD-AOS). In this work, we observe the TIMS on TbFe alloys excited by atomic spin dynamics simulations combined with a two-temperature model. The results show that the magnetization switching of TbFe can be found under certain damping conditions. In addition, we further investigated the reasons why energy density leads to the opposite switching time behavior of Tb and Fe, and our research results also found that changes in damping can affect the concentration and energy density range of the switching, as well as the maximum pulse duration. The dynamic behavior indicates that TbFe switching in 2 ps or less. Our findings widen the basis for fast optical switching of magnetization and break new ground for engineered materials that can be used for nonvolatile ultrafast switching using ultrashort pulses of light.

Frequency-independent optical spin injection in Weyl semimetals

We demonstrate that in Weyl semimetals, the momentum-space helical spin texture can couple to the chirality of the Weyl node to generate a frequency-independent optical spin injection. This frequency-independence is rooted in the topology of the Weyl node. Since the helicity and the chirality are always locked for Weyl nodes, the injected spin from a pair of Weyl nodes always add up, implying no symmetry requirements for Weyl semimetals. Finally, we show that such frequency-independent spin injection is robust against multiband corrections and lattice-regularization effect and capable of realizing all-optical magnetization switching in the THz regime.

Transient Non-Collinear Magnetic State for All-Optical Magnetization Switching.

Resonant absorption of a photon by bound electrons in a solid can promote an electron to another orbital state or transfer it to a neighboring atomic site. Such a transition in a magnetically ordered material could affect the magnetic order. While this process is an obvious road map for optical control of magnetization, experimental demonstration of such a process remains challenging. Exciting a significant fraction of magnetic ions requires a very intense incoming light beam, as orbital resonances are often weak compared to above-band-gap excitations. In the latter case, a sizeable reduction of the magnetization occurs as the absorbed energy increases the spin temperature, masking the non-thermal optical effects. Here, using ultrafast X-ray spectroscopy, this work is able to resolve changes in the magnetization state induced by resonant absorption of infrared photons in Co-doped yttrium iron garnet, with negligible thermal effects. This work finds that the optical excitation of the Co ions affects the two distinct magnetic Fe sublattices differently, resulting in a transient non-collinear magnetic state. The present results indicate that the all-optical magnetization switching (AOS) most likely occurs due to the creation of a transient, non-collinear magnetic state followed by coherent spin rotations of the Fe moments.

Dynamics of all-optical single-shot switching of magnetization in Tb/Co multilayers

Dynamics of all-optical single-shot switching of magnetization in Tb/Co multilayers

Analysis of ultrafast magnetization switching dynamics in exchange-coupled ferromagnet–ferrimagnet heterostructures

Magnetization switching in ferromagnets has so far been limited to the current-induced spin–orbit–torque effects. Recent observation of helicity-independent all-optical magnetization switching (HI-AOS) in an exchange-coupled ferromagnet–ferrimagnet (FM-FEM) heterostructures expanded the range and applicability of such ultrafast heat-driven magnetization switching. Here we report the element-resolved HI-AOS dynamics of such an exchange-coupled system, using a modified microscopic three-temperature model. We have studied the effect of (i) the Curie temperature of the FM, (ii) FEM composition, (iii) the long-range Ruderman–Kittel–Kasuya–Yosida (RKKY) exchange-coupling strength, and (iv) the absorbed optical energy on the element-specific time-resolved magnetization dynamics. The phase-space of magnetization illustrates how the RKKY coupling strength and the absorbed optical energy influence the switching time. Our analysis demonstrates that the threshold switching energy depends on the composition of the FEM and the switching time depends on the Curie temperature of the FM as well as RKKY coupling strength. This simulation anticipates new insights into developing faster and more energy-efficient spintronics devices.

Laser-induced magnetization switching in synthetic-ferrimagnetic bilayer

We present a theoretical study of all-optical magnetization switching in rare-earth/transition-metal bilayer, $\mathrm{R}{\mathrm{E}}_{n}/\mathrm{T}{\mathrm{M}}_{m}$, consisting of $n$ RE monolayers and $m$ TM monolayers. Using the Landau-Lifshitz-Bloch equation, we numerically calculate the spin dynamics, including spin transport in the bilayer, upon incidence of a single laser pulse. It is shown that the spin transfer between the RE and TM layers with $n>1$ or $m>1$ is necessary for the switching. The calculation shows that the spin transfer makes conditions for the switching similar to those for the ferrimagnetic alloys. When the spin transfer between the monolayers is absent the intensity of the exchange scattering decreases and the switching is possible only in the $\mathrm{R}{\mathrm{E}}_{1}/\mathrm{T}{\mathrm{M}}_{1}$ bilayer. A relationship between the spin relaxation and temperature dependence of the switching is discussed.

Low-power continuous-wave all-optical magnetic switching in ferromagnetic nanoarrays

All-optical magnetic switching promises ultrafast magnetization control without a magnetic field. Existing schemes typically require power-hungry femtosecond-pulsed lasers and complex magnetic materials. Here, we demonstrate deterministic, all-optical magnetic switching in simple ferromagnetic nanomagnets (Ni81Fe19, Ni50Fe50) with sub-diffraction limit dimensions using a focused low-power, linearly polarized continuous-wave laser. Isolated nanomagnets are switched across a range of dimensions, laser wavelengths, and powers. All square-geometry artificial spin ice vertex configurations are written at low powers (2.74 mW). Usually, switching with linearly polarized light is symmetry forbidden; here, the laser spot has a similar size to the nanomagnets, producing an absorption distribution that depends on the nanoisland-spot displacement. We attribute the deterministic switching to the transient dynamics of this asymmetric absorption. No switching is observed in Co or Ni nanostructures, suggesting the multi-species nature of NiFe plays a role. These results usher in inexpensive, low-power, optically controlled devices with impact across data storage, neuromorphic computation, and reconfigurable magnonics.

Probe and control of photo-excited magnetization precession in Co/Pd multilayer films at low laser fluence regime

We present femtosecond-pulse-induced precession of magnetization at low laser fluence (<5μJ/cm2) regime as a function of magnetic field and laser fluence in three Co/Pd multilayer (ML) systems. These systems belong to three different regimes of magnetic anisotropy that vary with Co thickness (tCo): in-plane (sample 1, tCo = 0.74 nm), weakly out-of-plane (sample 2, tCo = 0.6 nm), and out-of-plane (sample 3, tCo = 0.40 nm). Interestingly, we observed that the precession amplitudes increase significantly with decreasing the Co layer thickness. In this study, the influence of various spin dynamics and static magneto-optical parameters on precession amplitude is examined critically and compared with a previously proposed analytical expression that connects those quantities. It is found that the enhancement of structural-dependent energy transfer efficiency between charge and spin subsystems is indeed responsible for the observed variations in precession amplitudes. On the basis of this fact, we discuss that the spin–orbit interaction that yields perpendicularly spin-polarized electrons in the MLs through the Co/Pd interface is responsible for the observed increase in precession amplitudes of locally excited magnetization. Our approach of employing low-fluence laser excitation of magnetization precession could be practical for developing a non-thermal, all-optical magnetic switching toward photonic memory applications.

Element-selective magnetization states in a Gd23Fe67Co10 alloy, probed by soft X-ray resonant magneto-optical Kerr effect

The core-level resonant magneto-optical Kerr effect of a ferrimagnetic metal alloy, Gd23Fe67Co10, was measured at the Fe M-shell and Gd N-shell absorption edges using rotating analyzer ellipsometry. A large Kerr rotation angle of several degrees was detected at room temperature. The signal was found to be strong enough for element-selective magneto-optical experiments to trace various magnetic events, such as all-optical magnetization switching.

Energy Efficient Single Pulse Switching of [Co/Gd/Pt]N Nanodisks Using Surface Lattice Resonances.

The impact of plasmonic surface lattice resonances on the magneto-optical properties and energy absorption efficiency has been studied in arrays of [Co/Gd/Pt]N multilayer nanodisks. Varying the light wavelength, the disk diameter, and the period of the array, it is demonstrated that surface lattice resonances allow all-optical single pulse switching of [Co/Gd/Pt]N nanodisk arrays with an energy 400% smaller than the energy needed to switch a continuous [Co/Gd/Pt]N film. Moreover, the magneto-optical Faraday effect is enhanced at the resonance condition by up to 5,000%. The influence of the disk diameter and array period on the amplitude, width and position of the surface lattice resonances is in qualitative agreement with theoretical calculations and opens the way to designing magnetic metasurfaces for all-optical magnetization switching applications.

Electrical detection and current control of all-optical magnetization switching in GdFeCo ferrimagnetic alloy thin film

We experimentally demonstrated electrical detection of all-optical magnetization switching (AOS) induced by a single femtosecond laser pulse irradiation by measuring alternate rapid changes in anomalous Hall voltage and magneto-optic image pulse by pulse in a Hall-cross shape ferrimagnetic GdFeCo alloy thin film. We also demonstrated that the amplitude of the change in anomalous Hall voltage depended on the position of the AOS-created magnetic domain on the Hall cross. Furthermore, the AOS-created magnetic domains were stable against subsequent current applications in the Hall cross circuit, whereas reversed magnetic domains were not created when the laser pulse was irradiated with a high current. We found that the cooperative effect among magnetism, light, and electric current was assumed to have effects on the absence of the AOS. Combining the AOS phenomenon and electrical measurement/control techniques can realize ultrafast, deterministic, and distinguishable applications.

Plasmonic dichroism and all-optical magnetization switching in nanophotonic structures with GdFeCo.

We report on a phenomenon of plasmonic dichroism observed in magnetic materials with transverse magnetization under excitation of surface plasmon polariton waves. The effect originates from the interplay of the two magnetization-dependent contributions to the material absorption, both of which are enhanced under plasmon excitation. Plasmonic dichroism is similar to circular magnetic dichroism, which is at the base of all-optical helicity-dependent switching (AO-HDS) but observed for linearly polarized light, and the dichroism acts upon in-plane magnetized films, where AO-HDS does not take place. We show by electromagnetic modeling that laser pulses exciting counter-propagating plasmons can be used to write +M or -M states in a deterministic way independent of the initial magnetization state. The presented approach applies to various ferrimagnetic materials with in-plane magnetization, exhibiting the phenomenon of all-optical switching of a thermal nature and broadens the horizons of their applications in data storage devices.

Different helicity dependency of all-optical magnetization switching in GdFeCo films with optical interference layer

We observed helicity-dependent all-optical magnetization switching (HD-AOS) phenomena in GdFeCo films, which have equivalent magnetic properties but different optical properties due to optical interference layer thickness. Consequently, we found that these films have different properties of HD-AOS with the duality relation between magnetization direction and light helicity in light absorption, which generally means magnetic circular dichroism (MCD). Therefore, we also evaluated the effective absorption of the same GdFeCo samples which have similar intrinsic MCD by CW laser and intense femtosecond laser pulses. We confirmed that the contribution of this duality relation is much smaller in the absolute value of absorption. Furthermore, the magnitude and the sign of the duality relation change by the interference layer’s thickness. As a result, we concluded that the duality relation in HD-AOS is derived from the duality relation in absorption. Furthermore, its duality relation can be changed effectively using multiple interference effects.

Enhancing all-optical switching of magnetization by He ion irradiation

All-optical switching (AOS) of magnetization by a single femtosecond laser pulse in Co/Gd based synthetic ferrimagnets is the fastest magnetization switching process. On the other hand, He ion irradiation has become a promising tool for interface engineering of spintronic material platforms, giving rise to significant modification of magnetic properties. In this paper, we explore the use of He ion irradiation to enhance single pulse AOS of Co/Gd bilayer-based synthetic ferrimagnets. The intermixing of the constituent magnetic layers by He ion irradiation was both numerically simulated and experimentally verified. We theoretically modeled the effects of intermixing on AOS using the layered microscopic 3-temperature model and found that AOS is enhanced significantly by breaking the pristine Co/Gd interface through intermixing. Following this notion, we studied the threshold fluence of AOS as a function of He ion irradiation fluence. We found that the AOS threshold fluence can be reduced by almost 30%. Our study reveals the control of AOS by He ion irradiation, which opens up an industrially compatible approach for local AOS engineering.

Probing laser-induced spin-current generation in synthetic ferrimagnets using spin waves

Several rare-earth transition-metal ferrimagnetic systems exhibit all-optical magnetization switching upon excitation with a femtosecond laser pulse. Although this phenomenon is very promising for future opto-magnetic data storage applications, the role of non-local spin transport in these systems is scarcely understood. Using Co/Gd and Co/Tb bilayers we isolate the contribution of the rare-earth materials to the generated spin currents by using the precessional dynamics they excite in an adjacent ferromagnetic layer as a probe. By measuring THz standing spin-waves as well as GHz homogeneous precessional modes, we probe both the high- and low-frequency components of these spin currents. The low-frequency homogeneous mode indicates a significant contribution of Gd to the spin current, but not from Tb, consistent with the difficulty in achieving all-optical switching in Tb-containing systems. Measurements on the THz frequency spin waves reveal the inability of the rare-earth generated spin currents to excite dynamics at the sub-ps timescale. We present modelling efforts using the $s$-$d$ model, which effectively reproduce our results and allow us to explain the behavior in terms of the temporal profile of the spin current.