Single-shot helicity-independent all-optical switching of magnetization in ferrimagnets represents the fastest known approach for deterministic data recording. Recently, it was shown that 15-ps-long optical pulses could suffice in triggering the magnetic switching in certain Gd-Fe-Co alloys, generating enormous controversy about the underlying mechanism. Here, we demonstrate how the exact composition of the ferrimagnet affects the kinetics of the reversal process and facilitates the use of thermal pulses with a duration spanning all relevant timescales within the nonadiabatic limit. By modelling a generic ferrimagnet as two coupled macrospins, we show that the magnetization reversal can occur via distinctly different pathways, depending on the duration of the heater. We experimentally reveal that pulses with a duration below and above a critical pulse width respectively enable and disable the capability of all-optical magnetization switching in Gd-Fe-Co alloys, and that modest change of the alloy composition leads to drastic variation of the critical pulse width, by almost 2 orders of magnitude. Our interpretation and results resolve an urgent and outstanding technologically relevant controversy, and provide crucial but previously overlooked guidelines for how to engineer deterministic all-optical switching of magnetization in suitable ferrimagnets.

Ever since the first observation of all-optical switching of magnetization in the ferrimagnetic alloy GdFeCo using femtosecond laser pulses, there has been significant interest in exploiting this process for data-recording applications. in particular, the ultrafast speed of the magnetic reversal can enable the writing speeds associated with magnetic memory devices to be potentially pushed towards tHz frequencies. this work reports the development of perpendicular magnetic tunnel junctions incorporating a stack of tb/co nanolayers whose magnetization can be all-optically controlled via helicity-independent single-shot switching. toggling of the magnetization of the tb/co electrode was achieved using either 60 femtosecond-long or 5 picosecond-long laser pulses, with incident fluences down to 3.5 mJ/cm 2 , for co-rich compositions of the stack either in isolation or coupled to a cofeBelectrode/Mgo-barrier tunnel-junction stack. Successful switching of the cofeB-[tb/co] electrodes was obtained even after annealing at 250 °c. After integration of the [tb/co]-based electrodes within perpendicular magnetic tunnel junctions yielded a maximum tunneling magnetoresistance signal of 41% and RxA value of 150 Ωμm 2 with current-in-plane measurements and ratios between 28% and 38% in nanopatterned pillars. these results represent a breakthrough for the development of perpendicular magnetic tunnel junctions controllable using single laser pulses, and offer a technologically-viable path towards the realization of hybrid spintronic-photonic systems featuring tHz switching speeds. Ferrimagnetic systems based on rare earth (RE)-transition metal (TM) alloys and multilayers have been extensively studied in recent decades, largely due to their potential application in the field of magneto-optical recording 1. The strong perpendicular magnetocrystalline anisotropy inherent to amorphous RE-TM systems have allowed these alloys to play a key role in the historical transition from longitudinal to perpendicular magnetic recording structures 2 , and made them ideal for handling magnetic bit instabilities arising from superparamagnetic effects 3. Binary and ternary RE-TM systems such as GdFeCo, GdCo, TbCo or GdFe are still driving forward new developments pertaining to spintronic devices, including spin valves for magnetic read heads 4 , perpendicular magnetic tunnel junctions (p-MTJs) 5,6 or spin-orbit-torque phenomena 7. Recent works in this field have also revealed that RE-TM-based films (amorphous or multilayered) represent ideal materials for the observation and study of the phenomena of all-optical switching (AOS) 8-10. In these systems, it is possible to switch the magnetization using suitable laser pulses without the application of any external magnetic field. Depending on whether the laser-pulses need to be circularly-polarized, AOS of magnetization can be classed as either helicity-dependent (HD-AOS) or helicity-independent (HI-AOS). Furthermore, subject to the material in question, the switching process can be achieved with either a...

Revealing the ultimate speed limit at which magnetic order can be controlled, is a fundamental challenge of modern magnetism having far reaching implications for magnetic recording industry. Exchange interaction is the strongest force in magnetism, being responsible for ferromagnetic or antiferromagnetic spin order. How do spins react after being optically perturbed on an ultrashort timescales pertinent to the characteristic time of the exchange interaction? Here we demonstrate that femtosecond measurements of X-ray magnetic circular dichroism provide revolutionary new insights into the problem of ultrafast magnetism. In particular, we show that upon femtosecond optical excitation the ultrafast spin reversal of Gd(FeCo) -a material with antiferromagnetic coupling of spins -occurs via a transient ferromagnetic state. The latter one emerges due to different dynamics of Gd and Fe magnetic moments: Gd switches within 1.5 ps while it takes only 300 fs for Fe. Thus, by using a single fs laser pulse one can force the spin system to evolve via an energetically unfavorable way and temporary switch from an antiferromagnetic to ferromagnetic type of ordering. These observations supported by atomistic simulations, present a novel concept of manipulating magnetic order on different classes of magnetic materials on timescales of the exchange interaction.

Time-resolved magneto-optical imaging reveals that the dynamics of the helicity-dependent alloptical switching (HD-AOS) of Co/Pt ferromagnetic multilayers occurs on the timescales from nanoseconds to seconds. We find HD-AOS proceeds by two stages. First, for an optimized laser fluence, the ultrashort laser pulse demagnetizes the film to 25% of the initial magnetization. Subsequent laser pulses aids nucleation of small reversed domains. The observed nucleation is stochastic and independent of the helicity of laser light. At the second stage circularly polarized light breaks the degeneracy between the magnetic domains promoting a preferred direction of domain wall motion. One circular polarization results in a collapse of the reversed magnetic domains. The other polarization causes the growth of reversed magnetic domain from the nucleation sites, via deterministic displacement of the domain wall resulting in magnetization reversal. This mechanism is supported by further imaging studies of deterministic laser-induced displacement of the domain walls when excited by circular polarized optical pulses.

Sub-picosecond magnetisation manipulation via femtosecond optical pumping has attracted wide attention ever since its original discovery in 1996. However, the spatial evolution of the magnetisation is not yet well understood, in part due to the difficulty in experimentally probing such rapid dynamics. Here, we find evidence of a universal rapid magnetic order recovery in ferrimagnets with perpendicular magnetic anisotropy via nonlinear magnon processes. We identify magnon localisation and coalescence processes, whereby localised magnetic textures nucleate and subsequently interact and grow in accordance with a power law formalism. A hydrodynamic representation of the numerical simulations indicates that the appearance of noncollinear magnetisation via optical pumping establishes exchange-mediated spin currents with an equivalent 100% spin polarised charge current density of 10 7 A cm −2 . Such large spin currents precipitate rapid recovery of magnetic order after optical pumping. The magnon processes discussed here provide new insights for the stabilization of desired meta-stable states.

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.

Using spin-wave tomography (SWaT), we have investigated the excitation and the propagation dynamics of optically-excited magnetoelastic waves, i.e. hybridized modes of spin waves and elastic waves, in a garnet film. By using time-resolved SWaT, we reveal the excitation dynamics of magnetoelastic waves through coherent-energy transfer between optically-excited pure-elastic waves and spin waves via magnetoelastic coupling. This process realizes frequency and wavenumber selective excitation of spin waves at the crossing of the dispersion relations of spin waves and elastic waves. Finally, we demonstrate that the excitation mechanism of the optically-excited pure-elastic waves, which are the source of the observed magnetoelastic waves, is dissipative in nature.The development of spintronics is attracting a lot of attention due to the scaling limits of silicon based electronics. One of the concepts for future spintronic devices relies on the transfer of data via collective oscillations of spins [1-7], so-called spin waves or magnons. This approach is expected to provide novel functionalities such as multi-bit parallel processing [2], low-energy consumption [7], and quantum computation [1]. In this framework, femtosecond laser pulses have already demonstrated great potential, given their ability in the generation, manipulation and observation of the precessional motion of electron spins, even with femtosecond period and nanometer wavelength [8][9][10]. Moreover, an alloptical scheme allows real-time imaging of the photoinduced spatially propagating spin waves [11][12][13][14][15][16][17][18] and the reconstruction of spin wave dispersions [16].In magnetic media, spin waves and lattice vibrations (phonons or elastic waves) are hybridized due to the magnetoelastic coupling [19]. In particular, when an elastic wave and a spin wave have the same frequency and wavenumber, one can observe hybridization behavior, socalled magnetoelastic waves. The concept of a magnetoelastic wave was first suggested by C. Kittel [20] and then extensively investigated theoretically [21][22][23][24][25][26] and experimentally [23,[27][28][29][30][31][32][33][34][35].It has recently been discovered that magnetoelastic waves can be generated by femtosecond optical excitation via the magnetoelastic coupling [36]. The optical generation of magnetoelastic waves allowed the manipulation of spin textures, such as magnetic bubbles and domain walls [36]. Although in this study the excitation of the magnetoelastic waves was attributed to impulsive stimulated Raman scattering (ISRS), this interpretation is controversial since the reported excitation fluence dependence exhibited threshold behavior, which has never been observed in any previous ISRS experiment [8,37].In this study, we investigate the excitation mechanism of the optically-generated magnetoelastic waves in a ferrimagnetic garnet film by spin-wave tomography (SWaT). By using time-resolved SWaT, we found that the magnetoelastic waves are excited by a coherent energy transfer from the optically...

New possibilities for magnetic domain studies are demonstrated using a combination of nonlinear magneto-optical microscopy and a conventional linear polarizing microscope. The use of an optical response that is governed by a higher rank tensor offers sensitivity to additional combinations of magnetization directions and optical wave vector and polarization, which is demonstrated in magnetic garnet films of different crystallographic orientations. We observed a nontrivial modulated domain structure in a (210) film and a clear domain contrast for a (111) film, where the linear image only indicated simple up–down domains and no domain contrast for these two situations, respectively.