The field of ultrafast magnetism explores magnetization dynamics triggered by stimuli significantly faster than the time needed for thermodynamic equilibrium. A seminal discovery was femtosecond-laser-induced demagnetization of ferromagnetic Ni, much faster than previously understood spin interactions. This spurred numerous counter-intuitive findings, challenging existing theories and prompting investigation into fundamental limits on time and energy dissipation for writing magnetic bits. Ultrafast all-optical toggle switching, observed in metallic ferrimagnetic GdFeCo, represented a major advancement, inspiring current-induced switching studies in various metallic ferrimagnets. However, these methods rely on ultrafast heating, increasing dissipation. This research aimed to achieve all-optical toggle switching without heat, a mechanism seemingly contradicting Curie's principle (symmetry of cause and effect). The challenge was to generate an effective magnetic field on a timescale shorter than the magnetization's relaxation to equilibrium. This study proposes using photo-induced magnetic anisotropy in YIG:Co to achieve this "cold" toggle switching.

Existing research on ultrafast magnetism has predominantly focused on metallic magnets where laser-matter interaction involves free electrons, leading to inevitable energy transfer, entropy increase, and dissipation. The discovery of ultrafast all-optical toggle switching in GdFeCo was significant, yet it relied on ultrafast heating to create a strongly non-equilibrium state leading to magnetization reversal. Previous studies using photo-induced magnetic anisotropy in iron garnets to switch magnetization required a miscut substrate resulting in lower symmetry. This research builds on these prior works by addressing the limitations of heat-based methods and exploring the possibility of toggle switching in higher symmetry systems.

The study used thin films of Co-doped YIG grown via liquid phase epitaxy on a Gd3Ga5O12 (001) substrate without miscut, ensuring cubic symmetry. Static measurements used a polarization magneto-optical microscope with an LED light source. Toggle switching involved 50 fs pump pulses (1300 nm wavelength, linearly polarized along the [100] axis) resonantly pumping the d-d transition in Co ions. Time-resolved measurements used a laser system generating 35 fs pulses, employing a pump-probe setup with a modulated pump beam (1300 nm) and a probe beam (800 nm). Magnetization precession was measured at lower fluences to determine precession frequency. The switching time was determined by fitting an exponential increase to time-resolved images. Energy dissipation was estimated based on threshold fluence measurements as a function of fluence and temperature. The precession frequency was extracted from time-resolved Faraday rotation measurements using a damped sine function fit.

The researchers successfully demonstrated all-optical toggle switching in YIG:Co without relying on heat. Each laser pulse toggled the magnetization state, confirmed by differential magneto-optical images. The switching occurred over a remarkably broad temperature range (200-450 K), unlike heat-induced switching which is limited to specific temperatures. Time-resolved experiments revealed a switching time of 10-50 ps, strongly temperature-dependent and correlating with the magnetization precession frequency. The switching mechanism involves magnetization precession, with the switching time approximately a quarter of the precession period. Energy dissipation was significantly lower than in heat-induced switching (15-3 J/cm³ vs. 1500 J/cm³), although switching time and heat load were inversely related. The lack of miscut in the sample allowed for the toggle switching by ensuring the equivalence of switching processes.

The "cold" toggle switching demonstrated in this work contradicts Curie's principle, as toggling implies alternating symmetry of the effect despite identical excitation events. This is achieved by exploiting photo-induced magnetic anisotropy to generate an effective magnetic field on a timescale shorter than the magnetization's equilibration time. The broad temperature range and low energy dissipation represent significant advantages over heat-based methods. The observed relationship between switching time and energy dissipation highlights the trade-off between speed and efficiency. These results open avenues for high-frequency magnetization toggling with minimal heat generation.

This study successfully demonstrated ultrafast all-optical "cold" toggle switching of magnetization in YIG:Co, a mechanism that does not rely on heat and offers significantly reduced energy dissipation. The switching operates across a broad temperature range, showing a strong correlation between switching time and precession frequency. Future research could explore optimizing photo-sensitive ferrimagnets and antiferromagnets to further enhance speed and reduce dissipation, potentially enabling high-frequency (50 GHz) magnetization toggling for applications in magneto-photonics and magnonics.

The study primarily focuses on a specific material (YIG:Co) and excitation conditions. The generalizability of the "cold" switching mechanism to other materials needs further investigation. The current experiments use a single laser pulse; exploring the use of pulse trains to achieve even higher switching frequencies could be a valuable avenue for future research. While the energy dissipation is significantly lower than heat-induced methods, optimizing the trade-off between switching speed and energy consumption remains an area for future work.