Gregory Malinowski

All-optical magnetization switching is attractive for ultrafast, energy-efficient magnetic information technologies, but most nanoscale demonstrations have relied on ferrimagnetic alloys, rather than ferromagnetic multilayers closer to spintronic device architectures. The authors investigate all-optical switching in patterned Pt/Gd/Co/Pt disks with just a little Gd in an otherwise ferromagnetic stack. Switching grows progressively less deterministic as disk diameter decreases, with an unexpected exponential size dependence arising from competition between weak switching bias and finite-size magnetization fluctuations---a fundamental constraint for future devices.

Rare-earth-free ferrimagnetic oxides are emerging as attractive platforms for investigating ultrafast spin dynamics. Here, we study the photoinduced magnetization dynamics of epitaxial NiCo2O4 (NCO) thin films by the time-resolved magneto-optical Faraday effect using two independent pump–probe configurations: 1030/515 and 800/400 nm. In both measurements, photoexcitation induces an immediate reduction of the magneto-optical signal within the experimental time resolution, followed by a reproducible slower demagnetization component with a characteristic timescale of approximately 5–6 ps and a subsequent recovery on the ∼100 ps timescale. Importantly, this picosecond demagnetization component is observed consistently across the two experimental configurations and excitation wavelengths, demonstrating that it is an intrinsic feature of the ultrafast magnetic response of NCO thin films. Because the earliest-time dip may contain a transient optical contribution, we describe the overall response as type-II-like, rather than assigning a definitive textbook type-II classification solely on the basis of the sub-resolution signal. These results establish a robust two-step ultrafast demagnetization behavior in NCO and highlight rare-earth-free oxide ferrimagnets as promising systems for exploring multisublattice spin dynamics on ultrafast timescales.

We demonstrate single-shot laser-pulse-induced magnetization reversal in rare-earth-free CoFeB/MgO magnetic tunnel junctions (MTJs), a material system widely adopted in spin-transfer torque magnetic random-access memory. By tuning the Ru capping layer thickness, we modify the laser energy absorption profile and observe magnetization reversal from the parallel to antiparallel state, with switching observed for tRu ≥ 2.0 nm. Furthermore, we detect magnetization reversal in a micro-scale MTJ device via the tunnel magnetoresistance effect. Our findings suggest that ultrafast spin transport, dipolar interactions, or a combination of both may contribute to the switching process, although the precise mechanism remains to be clarified. This work represents a significant step toward integrating ultrafast optical control with MTJ technology.

This work investigates the influence of bias voltage on the optical switching behavior of a Tb/Co-based Magnetic Tunnel Junction (MTJ) in an 80-nm diameter pillar device. The experimental results demonstrate enhanced switching performance at high bias voltage, with a maximum switching probability achieved at bias voltage values of 200 and 400 mV. At high bias, Joule heating effect reduces the effective magnetic anisotropy of the pillar device, thereby favoring in-plane magnetization precession, lowering the switching threshold, and expanding the toggle switching window. The optical switching exhibits a pronounced asymmetry, showing a favorable switching under negative voltage polarity. Under this polarity, tunneling electrons are injected toward the top electrode, causing the decrease of magnetic anisotropy in the Tb/Co-based free layer and, therefore, facilitating the magnetization reversal at a laser pulse. The thermal analysis of All-Optical Switching (AOS)-MTJ devices under opposite bias polarities shows a temperature difference of 5–10 K, depending on the pillar diameter. These findings demonstrate voltage-selective control for multiple AOS devices integrated on the same chips, enabling an application for CMOS-compatible magneto-photonic memory.

Magnetic imaging techniques are widespread critical tools used in fields such as magnetism, spintronics, or even superconductivity. Among them, one of the most versatile methods is the magneto-optical Kerr effect. However, as soon as light is blocked from interacting with the magnetic layer, such as in deeply buried layers, optical techniques become ineffective. In this work, we present a spin accumulation based magneto-optical Kerr effect microscopy technique that enables imaging of a magnetic thin-film covered by thick and opaque metallic layers. The technique is based on the generation and detection of transient spin accumulations that propagate through the thick metallic layer. These spin accumulation signals are directly triggered and detected optically on the same side, lifting any substrate transparency requirements. The spin accumulation signals detected on a Cu layer decay with a characteristic length of 60 nm, much longer than the 12 nm optical penetration depth, allowing for the detection of magnetic contrast with Cu capping layers up to hundreds of nm. This method should enable magnetic imaging in a wide range of experiments where the surface of interest is covered by electrodes.

Engineering and controlling heat and spin transport on the femtosecond timescale in spintronic devices opens up new ways to manipulate magnetization with unprecedented speed. Yet the underlying reversal mechanisms remain poorly understood due to the challenges of probing ultrafast, nonequilibrium spin dynamics. In this Letter, we demonstrate that typical magneto-optical experiments can be leveraged to access the time evolution of the spin accumulation generated within a magnetic multilayer following an ultrafast laser excitation. Furthermore, our analysis shows that the final magnetic state of the free layer in a spin valve is mainly dictated by the ultrafast dynamics of the reference-layer magnetization. Our results disentangle magnetization and spin transport dynamics within a multilayer stack and identify demagnetization and remagnetization-driven spin accumulation as the key mechanism for all-optical switching. These findings establish new design principles for ultrafast spintronic devices based on tailored spin current engineering.

We study laser-induced ultrafast magnetization reversal in a ferromagnetic spin valve by comparing the effects of direct laser excitation and ultrashort hot-electron pulses. A wedged Cu layer is deposited atop the spin valve to tune energy transmission to the magnetic stack for both optical and hot-electron excitation. We demonstrate single-shot magnetization reversal of the free layer using hot-electron pulses. Moreover, such reversal is achieved even with picosecond laser pulses. The influence of laser fluence, Cu thickness (t_{Cu}), and pulse duration is investigated in detail. Our results indicate that the key factor enabling magnetization reversal is full demagnetization of the free layer, driven by a rapid rise in its electronic temperature-achieved via either direct laser or hot-electron excitation. This work advances the understanding of ultrafast magnetization reversal via nonlocal heat and spin transport under strongly out-of-equilibrium conditions.

Light-induced magnetisation switching is one of the most intriguing and promising areas where an ultrafast phenomenon can be utilised in technological applications. So far, experiment and theory have considered the origin of all-optical helicity-independent magnetisation switching (AO-HIS) in individual magnetic films only as a microscopically local, thermally-driven process of angular momentum transfer between different subsystems. Here, we demonstrate that this local picture is insufficient and that AO-HIS must also be regarded as a spatially inhomogeneous process along the depth within a few-nanometre thin magnetic layer. Two regions of opposite magnetisation directions are observed, separated by a highly mobile boundary, which propagates along the depth of a 9.4 nm thin Gd₂₅Co₇₅ alloy. The dynamics of this transient boundary determines the final magnetisation state as well as the speed of AO-HIS throughout the entire magnetic layer. The ability to understand the influence of nanoscale and transient inhomogeneities on ultrafast switching phenomena and more generally on phase transitions will open new routes for material design and excitation scenarios in future devices for transferring and storing information.