arXiv subjects

Jon Gorchon

Publications and source records attributed to Jon Gorchon.

At least 19 recordsLinked to original sources

arXiv2026

Type-II-like ultrafast demagnetization behavior in NiCo2O4 thin films

Rare-earth-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 time-resolved magneto-optical Faraday effect using two independent pump-probe configurations: 1030/515 nm 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 Mult sublattice spin dynamics on ultrafast timescales.

cond-mat.mtrl-sci↗

arXiv2025

Spin Accumulation based deep MOKE Microscopy

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 (SA-MOKE) microscopy technique that enables imaging of a magnetic thin-films 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 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.

cond-mat.mes-hall↗

arXiv2025

Single-shot laser-pulse-induced magnetization reversal in CoFeB/MgO-based magnetic tunnel junctions

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 (STT-MRAM). By tuning the Ru capping layer thickness, we modify the laser energy absorption profile and observe magnetization reversal from the parallel (P) to antiparallel (AP) state, with switching observed for $t_\text{Ru} \geq 2.0\,$ nm. Furthermore, we detect magnetization reversal in a micro-scale MTJ device via the tunnel magnetoresistance (TMR) 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.

cond-mat.mes-hall↗

arXiv2025

Exploring Co, Fe, and Ni Reference Layers for Single-Pulse All-Optical Reversal in Ferromagnetic Spin Valves

We investigate the magnetization reversal process induced by a single femtosecond laser pulse in ferromagnetic spin valves by systematically comparing reference layers composed of pure Co, Ni, and Fe. To circumvent the loss of perpendicular magnetic anisotropy associated with changes in reference layer material and thickness, we design spin valves with in plane magnetizations. While antiparallel to parallel switching is observed for all three elements, parallel to antiparallel switching occurs only with a Co reference layer and is absent with Ni and Fe. This difference is attributed to the distinct ultrafast magnetization dynamics of the reference materials. Our results support the hypothesis that parallel to antiparallel switching requires a rapid remagnetization of the reference layer, which generates a substantial negative spin current polarized opposite to the free layer magnetization an essential condition for triggering its reversal.

cond-mat.mes-hall↗

arXiv2025

Ultrafast Spin Accumulations Drive Magnetization Reversal in Multilayers

Engineering and controlling heat and spin transport on the femtosecond time-scale 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, non-equilibrium spin dynamics. In this study, 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.

cond-mat.mes-hall↗

arXiv2025

Single-Shot Magnetization Reversal in Ferromagnetic Spin Valves Enabled via Heat Control

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 ($t_{\mathrm{Cu}}$), 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.

cond-mat.mtrl-sci↗

arXiv2025

Large spin accumulation signals in ultrafast magneto-optical experiments

Magneto-optical techniques have become essential tools in spintronics, enabling the investigation of spin dynamics in the ultrafast regime. A key challenge in this field has been to accurately isolate the contributions to magneto-optical signals of spin transport phenomena from the local magnetization dynamics. The contribution of transported and accumulated spins was long believed to be orders of magnitude smaller than that of the magnetization and thus previous approaches to disentangle these signals have relied on specific experimental designs, usually including thick metal layers. Here, we present experimental evidence demonstrating that the magneto-optical signal from ultrafast spin accumulations can, under certain conditions, be comparable to or even exceed that of the magnetic layer in a standard ultrafast demagnetization experiment. Our findings provide a new framework for accessing and isolating these spin accumulations, allowing for time and depth dependent probing of transported spin and/or orbital angular momentum.

cond-mat.mes-hall↗

arXiv2024

Energy-efficient picosecond spin-orbit torque magnetization switching in ferro- and ferrimagnetic films

Electrical current pulses can be used to manipulate magnetization efficiently via spin-orbit torques (SOTs). Pulse durations as short as a few picoseconds have been used to switch the magnetization of ferromagnetic films, reaching the THz regime. However, little is known about the reversal mechanisms and energy requirements in the ultrafast switching regime. In this work, we quantify the energy cost for magnetization reversal over 7 orders of magnitude in pulse duration, in both ferromagnetic and ferrimagnetic samples, bridging quasi-static spintronics and femtomagnetism. To this end, we develop a method to stretch picosecond pulses generated by a photoconductive switch by an order of magnitude. Thereby, we can create current pulses from picoseconds to durations approaching pulse width available with commercial instruments. We show that the energy cost for SOT switching decreases by more than an order of magnitude in all samples when the pulse duration enters the picosecond range. We project an energy cost of 9 fJ for a 100 x 100 nm 2 ferrimagnetic device. Micromagnetic and macrospin simulations unveil a transition from a non-coherent to a coherent magnetization reversal with a strong modification of the magnetization dynamical trajectories as pulse duration is reduced. Our results cement the potential for high-speed magnetic spin-orbit torque memories and highlights alternative magnetization reversal pathways at fast time scales.

cond-mat.mtrl-sci↗

arXiv2023

Influence of interlayer exchange coupling on ultrafast laser-induced magnetization reversal in ferromagnetic spin valves

In this study, we explore the influence of interlayer exchange coupling on magnetization reversal triggered by femtosecond laser pulses in ferromagnetic spin valves. Our experiments, focused on femtosecond laser-induced magnetization reversal, methodically vary the thickness of the copper (Cu) spacer layer. We identify a critical Cu thickness threshold at 2.32 nm. Above this threshold, a stable reversed magnetic domain is consistently generated upon exposure to a single laser pulse. Conversely, with a Cu spacer thinner than 2.32 nm, the observed magnetization reversal from parallel (P) to anti-parallel (AP) states occurs only under continuous laser irradiation. Once the laser is stopped, the magnetic configuration relaxes back to its initial P state, influenced by ferromagnetic exchange coupling. This research enhances our understanding of the mechanisms that drive optically induced ultrafast magnetization reversal in ferromagnetic spin valves.

cond-mat.mtrl-sci↗

arXiv2023

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

We use the thickness of Cu layers to control all-optical switching of magnetization in adjacent Gd$_{24}$(Fe$_9$0Co$_{10}$)$_{76}$ films. While increasing the Cu thickness from 5 to 900nm has no effect on the switching threshold, it significantly enlarges the fluence and pulse duration at which multiple domains emerge. Having shown that thermally activated multi-domain formation limits the maximum fluence and pulse duration for controlled switching, we demonstrate that continuous magnetization reversal precedes multi-domain formation in Gd$_{18}$Dy$_4$Co$_{78}$ films excited with fluences slightly larger than the multi-domain threshold.

cond-mat.mtrl-sci↗

arXiv2023

Single laser pulse induced magnetization switching in in-plane magnetized GdCo alloys

The discovery of all-optical ultra-fast deterministic magnetization switching has opened up new possibilities for manipulating magnetization in devices using femtosecond laser pulses. Previous studies on single pulse all-optical helicity-independent switching (AO-HIS) have mainly focused on perpendicularly magnetized thin films. This work presents a comprehensive study on AO-HIS for in-plane magnetized GdxCo100-x thin films. Deterministic single femtosecond laser pulse toggle magnetization switching is demonstrated in a wider concentration range (x=10% to 25%) compared to the perpendicularly magnetized counterparts with GdCo thicknesses up to 30 nm. The switching time strongly depends on the GdxCo100-x concentration, with lower Gd concentration exhibiting shorter switching times (less than 500 fs). Our findings in this geometry provide insights into the underlying mechanisms governing single pulse AO-HIS, which challenge existing theoretical predictions. Moreover, in-plane magnetized GdxCo100-x thin films offer extended potential for opto-spintronic applications compared to their perpendicular magnetized counterparts.

cond-mat.mes-hall↗

arXiv2022

Picosecond Spin-Orbit Torque Induced Coherent Magnetization Switching in a Ferromagnet

Electrically controllable non-volatile magnetic memories show great potential for the replacement of semiconductor-based technologies. Recently there has been strong interest in spin-orbit torque (SOT) induced magnetization reversal due to the device's increased lifetime and speed of operation. However, recent SOT switching studies reveal an incubation delay in the ~ns range due to stochasticity in the nucleation of a magnetic domain during reversal. Here, we experimentally demonstrate ultrafast SOT-induced magnetization switching dynamics of a ferromagnet with no incubation delay by avoiding the nucleation process and driving the magnetization coherently. We employ an ultrafast photo-conducting switch and a co-planar strip line to generate and guide ~ps current pulses into the heavy metal/ferromagnet layer stack and induce ultrafast SOT. We use magneto-optical probing to investigate the magnetization switching dynamics with sub-picosecond time resolution. Depending on the relative current pulse and in-plane magnetic field polarities, we observe either an ultrafast demagnetization and subsequent recovery along with a SOT-induced precessional oscillation, or ultrafast SOT switching. The magnetization zero-crossing occurs in ~70 ps, which is approximately an order of magnitude faster than previous studies. Complete switching needs ~250 ps and is limited by the heat diffusion to the substrate. We use a macro-magnetic simulation coupled with an ultrafast heating model to analyze the effect of ultrafast thermal anisotropy torque and current-induced torque in the observed dynamics. Good agreement between our experimental results and the macro-spin model shows that the switching dynamics are coherent and present no noticeable incubation delay. Our work suggests a potential pathway toward dramatically increasing the writing speed of SOT magnetic random-access memory devices.

cond-mat.mes-hall↗

arXiv2021

Spin current transport in hybrid Pt / multifunctional magnetoelectric Ga0.6Fe1.4O3 bilayers

The low power manipulation of magnetization is currently a highly sought-after objective in spintronics. Non ferromagnetic large spin-orbit coupling heavy metal (NM) / ferromagnet (FM) heterostructures offer interesting elements of response to this issue, by granting the manipulation of the FM magnetization by the NM spin Hall effect (SHE) generated spin current. Additional functionalities, such as the electric field control of the spin current generation, can be offered using multifunctional ferromagnets. We have studied the spin current transfer processes between Pt and the multifunctional magnetoelectric Ga0.6Fe1.4O3 (GFO). In particular, via angular dependent magnetotransport measurements, we were able to differentiate between magnetic proximity effect (MPE)-induced anisotropic magnetoresistance (AMR) and spin Hall magnetoresistance (SMR). Our analysis shows that SMR is the dominant phenomenon at all temperatures and is the only one to be considered near room temperature, with a magnitude comparable to those observed in Pd/YIG or Pt/YIG heterostructures. These results indicate that magnetoelectric GFO thin films show promises for achieving an electric-field control of the spin current generation in NM/FM oxide-based heterostructures.

cond-mat.mtrl-sci↗

arXiv2021

Differentiating Contributions of Electrons and Phonons to the Thermoreflectance Spectra of Gold

To better understand the many effects of temperature on the optical properties of metals, we experimentally and theoretically quantify the electron vs. phonon contributions to the thermoreflectance spectra of gold. We perform a series of pump/probe measurements on nanoscale Pt/Au bilayers at wavelengths between 400 and 1000 nm. At all wavelengths, we find that changes in phonon temperature, not electron temperature, are the primary contributor to the thermoreflectance of Au. The thermoreflectance is most sensitive to the electron temperature at wavelength of ~480 nm due to interband transitions between d-states and the Fermi-level. In the near infrared, the electron temperature is responsible for only ~2% of the total thermoreflectance. We also compute the thermoreflectance spectra of Au from first principles. Our calculations further confirm that phonon temperature dominates thermoreflectance of Au. Most of Au's thermoreflectance is due to the effect of the phonon population on electron lifetime.

cond-mat.mtrl-sci↗

arXiv2020

Picosecond Spin Orbit Torque Switching

Reducing energy dissipation while increasing speed in computation and memory is a long-standing challenge for spintronics research. In the last 20 years, femtosecond lasers have emerged as a tool to control the magnetization in specific magnetic materials at the picosecond timescale. However, the use of ultrafast optics in integrated circuits and memories would require a major paradigm shift. An ultrafast electrical control of the magnetization is far preferable for integrated systems. Here we demonstrate reliable and deterministic control of the out-of-plane magnetization of a 1 nm-thick Co layer with single 6 ps-wide electrical pulses that induce spin-orbit torques on the magnetization. We can monitor the ultrafast magnetization dynamics due to the spin-orbit torques on sub-picosecond timescales, thus far accessible only by numerical simulations. Due to the short duration of our pulses, we enter a counter-intuitive regime of switching where heat dissipation assists the reversal. Moreover, we estimate a low energy cost to switch the magnetization, projecting to below 1fJ for a (20 nm)^3 cell. These experiments prove that spintronic phenomena can be exploited on picosecond time-scales for full magnetic control and should launch a new regime of ultrafast spin torque studies and applications.

physics.app-ph↗

arXiv2020

Unifying femtosecond and picosecond single-pulse magnetic switching in GdFeCo

Many questions are still open regarding the physical mechanisms behind the magnetic switching in GdFeCo alloys by single optical pulses. Phenomenological models suggest a femtosecond scale exchange relaxation between sublattice magnetization as the driving mechanism for switching. The recent observation of thermally induced switching in GdFeCo by using both several picosecond optical laser pulse as well as electric current pulses has questioned this previous understanding. This has raised the question of whether or not the same switching mechanics are acting at the femo- and picosecond scales. In this work, we aim at filling this gap in the understanding of the switching mechanisms behind thermal single-pulse switching. To that end, we have studied experimentally thermal single-pulse switching in GdFeCo alloys, for a wide range of system parameters, such as composition, laser power and pulse duration. We provide a quantitative description of the switching dynamics using atomistic spin dynamics methods with excellent agreement between the model and our experiments across a wide range of parameters and timescales, ranging from femtoseconds to picoseconds. Furthermore, we find distinct element-specific damping parameters as a key ingredient for switching with long picosecond pulses and argue, that switching with pulse durations as long as 15 picoseconds is possible due to a low damping constant of Gd. Our findings can be easily extended to speed up dynamics in other contexts where ferrimagnetic GdFeCo alloys have been already demonstrated to show fast and energy-efficient processes, e.g. domain-wall motion in a track and spin-orbit torque switching in spintronics devices.

cond-mat.mtrl-sci↗

arXiv2018

Single shot time-resolved magnetic x-ray absorption at a Free Electron Laser

Ultrafast dynamics are generally investigated using stroboscopic pump-probe measurements, which characterize the sample properties for a single, specific time delay. These measurements are then repeated for a series of discrete time delays to reconstruct the overall time trace of the process. As a consequence, this approach is limited to the investigation of fully reversible phenomena. We recently introduced an off-axis zone plate based X-ray streaking technique, which overcomes this limitation by sampling the relaxation dynamics with a single femtosecond X-ray pulse streaked over a picosecond long time window. In this article we show that the X-ray absorption cross section can be employed as the contrast mechanism in this novel technique. We show that changes of the absorption cross section on the percent level can be resolved with this method. To this end we measure the ultrafast magnetization dynamics in CoDy alloy films. Investigating different chemical compositions and infrared pump fluences, we demonstrate the routine applicability of this technique. Probing in transmission the average magnetization dynamics of the entire film, our experimental findings indicate that the demagnetization time is independent of the specific infrared laser pump fluence. These results pave the way for the investigation of irreversible phenomena in a wide variety of scientific areas.

cond-mat.mtrl-sci↗

arXiv2017

Single shot ultrafast all optical magnetization switching of ferromagnetic Co/Pt multilayers

In a number of recent experiments, it has been shown that femtosecond laser pulses can control magnetization on picosecond timescales, which is at least an order of magnitude faster compared to conventional magnetization dynamics. Among these demonstrations, one material system (GdFeCo ferromagnetic films) is particularly interesting, as deterministic toggle-switching of the magnetic order has been achieved without the need of any symmetry breaking magnetic field. This phenomenon is often referred to as all optical switching (AOS). However, so far, GdFeCo remains the only material system where such deterministic switching has been observed. When extended to ferromagnetic systems, which are of greater interest in many technological applications, only a partial effect can be achieved, which in turn requires repeated laser pulses for full switching. However, such repeated pulsing is not only energy hungry, it also negates the speed advantage of AOS. Motivated by this problem, we have developed a general method for single-shot, picosecond timescale, complete all optical switching of ferromagnetic materials. We demonstrate that in exchange-coupled layers of Co/Pt and GdFeCo, single shot, switching of the ferromagnetic Co/Pt layer is achieved within 7 picoseconds after irradiation by a femtosecond laser pulse. We believe that this approach will greatly expand the range of materials and applications for ultrafast magnetic switching.

cond-mat.mes-hall↗