Ultrafast all-optical toggle writing of magnetic bits without relying on heat

The study addresses whether deterministic, ultrafast all-optical toggle switching of magnetization can be achieved via a mechanism that does not rely on heat, thereby minimizing dissipation during magnetic bit writing. Prior work in ultrafast magnetism has largely focused on metallic systems where laser excitation leads to substantial, irreversible heating and entropy increase. Heat-driven toggle switching in metallic ferrimagnets (e.g., GdFeCo) requires ultrafast heating that drives the system into a highly nonequilibrium state from which magnetization reversal is the preferred relaxation pathway. In contrast, the authors aim to demonstrate a “cold” toggle switching mechanism in dielectric ferrimagnets where photoexcitation changes magnetic anisotropy on ultrafast timescales, producing an effective field that drives precessional switching within approximately a half precession period, circumventing Curie’s thermodynamic symmetry arguments. They target Co-substituted iron garnets (YIG:Co) without substrate miscut to ensure high crystal symmetry and degenerate bit states, enabling pulse-to-pulse toggling with identical linearly polarized light.

Key prior findings include ultrafast laser-induced demagnetization in Ni, and heat-driven all-optical toggle switching in metallic ferrimagnets like GdFeCo and other alloys, often near compensation points and relying on ultrafast heating of coupled sublattices (e.g., Ostler et al., 2012; Mangin et al., 2014; Banerjee et al., 2020). Ultrafast precessional switching by short magnetic field pulses demonstrates that when excitation occurs faster than thermal equilibration, Curie’s principle constraints can be bypassed (Back et al., 1999; Gerrits et al., 2002). In iron garnets, resonant excitation of Co d–d transitions can induce large transient changes in magnetic anisotropy (Stupakiewicz et al., 2017, 2019; Atoneche et al., 2010), enabling nonthermal optical control of magnetization. Earlier reports of polarization-dependent switching in garnets required low symmetry (substrate miscut), which breaks equivalence of bit states. This work builds on those foundations to achieve true toggle switching between equivalent (degenerate) states in high-symmetry garnet films.

Materials: Co-doped monocrystalline yttrium iron garnet films with composition Y₂CaFe₃.₉Co₀.₁GeO₁₂, 8 µm thick, grown by liquid phase epitaxy on Gd₃Ga₅O₁₂ (001) substrates with no miscut (precision 0.1°). Room-temperature parameters: 4πM_s = 75 G; Néel temperature T_N = 445 K; cubic anisotropy K_c = −5.5 × 10³ erg/cm³; uniaxial anisotropy K_u = 0.6 × 10³ erg/cm³. Four easy axes along ⟨111⟩ persist from 200–450 K; no compensation point reported for this composition. The non-miscut substrate yields regular domain structures and equivalent bit states. Static imaging and toggle tests: Magneto-optical (Faraday) microscopy with LED illumination to visualize domain patterns. For static switching, single 50 fs pump pulses, linearly polarized along [100], central wavelength 1300 nm (resonant with Co d–d transitions to maximize anisotropy modulation), fluences typically around 50 mJ/cm² were used. Domains before/after each pulse were imaged; differential images (after n-th minus after (n−1)-th pulse) highlight pulse-to-pulse toggling. Experiments conducted without external magnetic field. Time-resolved measurements: A Ti:sapphire oscillator-amplifier system (35 fs pulses, 1 kHz) with two OPAs generated pump (1300 nm) and probe beams. Pump polarization aligned along ⟨100⟩ to maximize response; pump modulated at 500 Hz for lock-in detection. Probe for Faraday rotation: 800 nm at normal incidence with balanced detection (half-wave plate + Wollaston prism to a balanced photodiode bridge) and boxcar integrator. For wide-field imaging, the probe wavelength was 650 nm and deliberately defocused to cover a large area; the pump was focused to ~120 µm diameter. Time delay Δt controlled via a motorized translation stage. Data acquisition and analysis: For time-resolved imaging, background (probe-only) and pump+probe images were acquired and subtracted to obtain differential images tracking ΔM along [001]. The development of switching contrast was fit to 1 − exp(−Δt/τ) to extract the characteristic switching time τ. For subswitching dynamics at lower fluence (e.g., 10 mJ/cm²), time-resolved Faraday rotation traces were fit with a damped sine Δθ_F(Δt) = A sin(2π f Δt + φ) exp(−Δt/τ_damp) to obtain the ferromagnetic resonance frequency f and damping behavior as a function of temperature (200–450 K). The effective anisotropy field H_A was estimated from Kittel’s formula for cubic (001): f = (γ/2π) √[H_A (H_A + 4πM_s)]. Heat load estimation: The absorbed energy/heat density during switching was estimated from the threshold fluence I_c and the film absorption at 1300 nm (absorption coefficient α_x ≈ 0.12) and thickness d = 8 µm, yielding q in J/cm³. Switched area as a function of fluence (1–90 mJ/cm²) and temperature provided I_c(T) by extrapolating normalized switched area to zero. Domain wall motion post-switching was noted to proceed on microsecond timescales, leaving ring-like features at the switched spot edge in static images.

- Demonstration of cold all-optical toggle switching: In YIG:Co films grown on (001) GGG without miscut, each identical linearly polarized 1300 nm femtosecond pulse toggles magnetization between two degenerate bit states under zero external field. Differential magneto-optical images show pulse-to-pulse reversals while preserving domain wall positions. - Minimal heating: Estimated single-pulse temperature rise ~1 K; heat load required for switching is 3–15 J/cm³, orders of magnitude lower than heat-induced toggle switching in metallic ferrimagnets (~1500 J/cm³). - Broad temperature range: Toggle switching observed from 200 to 450 K, not confined to compensation temperatures. - Switching dynamics: Switching contrast develops over ~10–50 ps. The characteristic switching time τ scales with temperature similarly to the ferromagnetic resonance frequency f(T), indicating a precessional mechanism. τ is approximately a quarter of the precession period (τ ≈ T/4). - Precession and anisotropy: Subthreshold excitation induces coherent magnetization precession; f decreases with increasing temperature (measured via time-resolved Faraday rotation). Effective anisotropy field H_A(T) extracted through Kittel’s relation captures the temperature dependence. - Energy–time trade-off: Faster switching (shorter τ) correlates with higher dissipated energy density; choosing temperature or anisotropy allows tuning between speed and dissipation. The time/energy ratio increases with temperature with a linear slope of about 0.12 ± 0.01 ps·cm³/J·K (from Fig. 5). - High-frequency potential: With a pulse train, toggle frequencies up to ~50 GHz are anticipated with minimal cumulative heating (~0.6 K temperature rise).

The findings establish that transient photo-induced magnetic anisotropy in Co-doped iron garnets can generate effective fields on ultrafast timescales sufficient to drive precessional toggling between degenerate magnetization states with identical linear polarization pulses. By operating far from thermal equilibrium and within a time window shorter than equilibration, the switching mechanism circumvents Curie’s symmetry constraints, analogous to precessional switching by short magnetic field pulses. Removing substrate miscut restores high crystal symmetry (4mm point group) and equivalence of bit states, enabling true toggle operation with no helicity dependence. The strong correlation between τ and f(T) confirms a precessional pathway where switching occurs in roughly a quarter-period while the photo-anisotropy persists. Practically, the approach reduces dissipation by two to three orders of magnitude compared to heat-driven mechanisms and extends operational temperature ranges far beyond narrow compensation points. The observed energy–time competition implies device-level tunability: stronger (or longer-lived) photo-anisotropy fields and lower temperatures favor faster switching at the cost of higher energy input, while minimal dissipation requires accepting longer switching times. The mechanism’s compatibility with dielectric media opens opportunities in magneto-photonics, magnonics, and integration with low-loss or superconducting spintronic platforms.

This work demonstrates an all-optical, nonthermal (“cold”) toggle switching mechanism in dielectric ferrimagnets, specifically YIG:Co films on non-miscut GGG (001). Linearly polarized femtosecond pulses at 1300 nm modify magnetic anisotropy to drive precessional switching between degenerate bit states at zero field. Switching occurs over 10–50 ps across 200–450 K with heat loads of only 3–15 J/cm³, dramatically lower than heat-induced toggle schemes. The switching time scales as a quarter of the ferromagnetic resonance period, and an intrinsic trade-off between speed and dissipation is quantified, with the time/energy ratio increasing with temperature. The results point to high-frequency (>10 GHz, up to ~50 GHz) low-dissipation magnetic writing with modest temperature rises and suggest broad design avenues via tailoring anisotropy (e.g., cubic vs. growth-induced uniaxial components), crystal symmetry, and resonant optical coupling to anisotropy-active ions. Future work should optimize materials to enhance photo-anisotropy strength and lifetime, explore antiferromagnets and other ferrimagnets, and integrate device-scale architectures for high-repetition-rate, energy-efficient magnetic memory and photonic-magnonic systems.

- The mechanism requires high crystal symmetry (no substrate miscut) to ensure degenerate bit states; films with miscut exhibit asymmetric switching pathways. - While termed “cold,” a finite heat load remains (3–15 J/cm³; ~1 K per pulse), and performance depends on temperature and anisotropy parameters. - Domain wall motion contributes on microsecond timescales after switching, potentially affecting final domain configurations in device contexts. - Demonstrated in a specific material system (YIG:Co, 8 µm thick) under resonant excitation at 1300 nm with polarization along [100]; generalization to other materials and thicknesses requires validation. - A speed–dissipation trade-off persists; achieving the fastest switching necessitates higher energy input or specific anisotropy conditions. - Precession-based switching relies on sufficient lifetime and magnitude of photo-induced anisotropy; materials with weaker or shorter-lived anisotropy may not support robust toggling.