Search

International audience; Terahertz (THz) emission from magnetic films has recently become an important characterization tool of spintronic properties, particularly since no patterning is required. One such property of interest is the spin-current attenuation length. When separating a magnetic film from a spin-to-charge converter with a light metal, the emitted intensity reduces almost exponentially with the thickness of the spacer. However, the extracted characteristic length is more than an order of magnitude smaller than the spin diffusion length measured in equilibrium. In this work, we experimentally and theoretically demonstrate that most of the observed decay in the THz emission is of optical (THz) origin. We are able to estimate a spin current attenuation length for Cu of ∼50 nm in much closer agreement with spin diffusion length measurements. We conclude that THz emission remains a powerful characterization technique, but due to the high number of intricate conversion mechanisms, and most importantly, due to the high sensitivity to changes in the optical properties, extracting absolute numbers for spintronic phenomena remains extremely challenging.

The development of approaches that can efficiently control the magnetization of magnetic materials is central to the creation of fast and low-power spintronic devices. Spin transfer torque can be used to electrically manipulate magnetic order in devices, but is typically limited to nanosecond timescales. Alternatively, spin–orbit torque can be employed, and switching with current pulses down to ~200 ps has been demonstrated. However, the upper limit to magnetization switching speed remains unestablished. Here, we show that photoconductive switches can be used to apply 6-ps-wide electrical pulses and deterministically switch the out-of-plane magnetization of a common thin cobalt film via spin–orbit torque. We probe the ultrafast magnetization dynamics due to spin–orbit torques with sub-picosecond resolution using the time-resolved magneto-optical Kerr effect (MOKE). We also estimate that the magnetization switching consumes less than 50 pJ in micrometre-sized devices. The magnetization of a cobalt thin film can be reversed by spin–orbit torques using picosecond electrical pulses that are generated by photoconductive switches.

International audience; 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.

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., 30 pages, 5 figures, 7 supplemental figures

International audience; Many questions are still open regarding the physical mechanisms behind the magnetic switching in Gd-Fe-Co 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 Gd-Fe-Co 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 femtosecond 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 Gd-Fe-Co 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 ps 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 Gd-Fe-Co 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.

International audience; Since it was recently demonstrated in a spin-valve structure, magnetization reversal of a ferromagnetic layer using a single ultrashort optical pulse has attracted attention for future ultrafast and energy-efficient magnetic storage or memory devices. However, the mechanism and the role of the magnetic properties of the ferromagnet as well as the time scale of the magnetization switching are not understood. Here, we investigate single-shot all-optical magnetization switching in a GdFeCo/Cu/[Co x Ni 1−x /Pt] spinvalve structure. We demonstrate that the threshold fluence for switching both the GdFeCo and the ferromagnetic layer depends on the laser pulse duration and the thickness and the Curie temperature of the ferromagnetic layer. We are able to explain most of the experimental results using a phenomenological model. This work provides a way to engineer ferromagnetic materials for energy efficient single-shot all-optical magnetization switching.

New methods to induce magnetization switching in a thin ferromagnetic material using femtosecond laser pulses without the assistance of an applied external magnetic field have recently attracted a lot of interest. It has been shown that by optically triggering the reversal of the magnetization in a GdFeCo layer, the magnetization of a nearby ferromagnetic thin film can also be reversed via spin currents originating in the GdFeCo layer. Here, using a similar structure, it is shown that the magnetization reversal of the GdFeCo is not required in order to reverse the magnetization of the ferromagnetic thin film. This switching is attributed to the ultrafast spin current and can be generated by the GdFeCo demagnetization. A larger energy efficiency of the ferromagnetic layer single pulse switching is obtained for a GdFeCo with a larger Gd concentration. Those ultrafast and energy efficient switchings observed in such spintronic devices open a new path toward ultrafast and energy efficient magnetic memories., In GdFeCo/Cu/[Co/Pt] spin valve, the ultrafast demagnetization of the ferrimagnetic GdFeCo alloy, generated by a single femtosecond laser pulse, is shown to generate enough spin current to switch the magnetization of the ferromagnetic Co/Pt multilayer. By increasing the Gd concentration, the Co/Pt switching is found to be more energy efficient.

Domain walls in magnetic thin films display a complex dynamical response when subject to an external drive. It is claimed that different dynamic regimes are correlated with the domain-wall roughness, i.e., with the fluctuations of domain-wall position due to the inherent disorder in the system. Therefore, key to understanding the dynamics of domain walls is to have a statistically meaningful measure of the domain-wall roughness. Here we present a thorough study of the roughness parameters, i.e., roughness exponent and roughness amplitude, for domain walls in a ferrimagnetic GdFeCo thin film in the creep regime. Histograms of roughness parameters are constructed with more than 40 independent realizations under the same experimental conditions, and the average values and standard deviations are compared in different conditions. We found that the most prominent feature of the obtained distributions is their large standard deviations, which is a signature of large fluctuations. We show that even if the roughness parameters for a particular domain wall are well known, these parameters are not necessarily representative of the underlying physics of the system. In the low field limit, within the creep regime of domain-wall motion, we found the average roughness exponent and roughness amplitude to be around 0.75 and 0.45 μm2, respectively. When an in-plane magnetic field is applied we observed that, even though the distributions are wide, changes in the mean values of roughness parameters can be identified; the roughness exponent decreasing to values around 0.72 while the roughness amplitude increases to 0.65 μm2. Our results call for a careful consideration of statistical averaging over different domains walls when reporting roughness exponents. Fil: Jordán Ringgold, Daniel. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro | Universidad Nacional de Cuyo. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro; Argentina Fil: Albornoz, Lucas Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; Argentina Fil: Gorchon, Jon. Lawrence Berkeley National Laboratory; Estados Unidos Fil: Lambert, Charles Henri. University of California at Berkeley; Estados Unidos Fil: Salahuddin, Sayeef. University of California at Berkeley; Estados Unidos Fil: Bokor, Jeffrey. University of California at Berkeley; Estados Unidos. Lawrence Berkeley National Laboratory; Estados Unidos Fil: Curiale, Carlos Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; Argentina Fil: Bustingorry, Sebastian. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; Argentina

International audience; The discovery of ultrafast demagnetization by Bigot and Beaurepaire in 1996 launched the field of ultrafast magnetism – the study of sub-picosecond magnetization dynamics in response to femtosecond laser excitations. In the same year, the discovery of spin-transfer torque switching of magnetic nanostructures [2,3] launched the field of spintronics – the control of magnetic order with electrical currents and voltage, germane to integrated electronic systems. Ultrafast magnetism may be particularly useful to future spintronic memory and logic devices by enabling magnetization switching at much faster time scales than in any existing spintronic technology. However, a number of obstacles stand in the way of integrating ultrafast magnetic phenomena into spintronic devices. To be useful for devices, ultrafast magnetization dynamics must be driven by electrical currents, not femtosecond lasers. The electrical currents need to be sourced by semiconductor transistors, which currently have a minimum gate delay on the order of picoseconds. Readout of the magnetic state must also be electrical, which will likely require a large magnetoresistance from a ferromagnetic tunnel junction. Finally, the switching energy must be minimized, which requires nanoscale device dimensions.This review discusses our most recent advances in addressing these challenges and bridging together the two fields of spintronics and ultrafast magnetism to enable the integration of ultrafast spintronic devices. Our research shows that, not only sub-picosecond optical pulses, but also picosecond excitations in the form of optical and electrical (heat current) pulses can result in the switching of the magnetization direction between two opposite states in ferrimagnetic GdFeCo. We also demonstrate ultrafast single shot switching of ferromagnetic Co/Pt exchange coupled to a GdFeCo layer. Furthermore, we observe ultrafast switching in nanoscale patterned GdCo dots. These achievements pave the way towards the construction of nanoscale ferromagnetic tunnel junctions capable of picosecond magnetization write times and electrical readout.

Engineering of magnetic materials for developing better spintronic applications relies on the control of two key parameters: the spin polarization and the Gilbert damping, responsible for the spin angular momentum dissipation. Both of them are expected to affect the ultrafast magnetization dynamics occurring on the femtosecond timescale. Here, engineered Co

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., Includes article + supplementary information. Latest version uses full name of the first author. Nature Electronics (2020)

All-optical helicity-dependent switching in ferromagnetic Co/Pt multilayers is investigated using magneto-optical microscopy and anomalous Hall effect measurements. A state diagram is built by studying the effect of pulse duration, fluence, and spot size. We use numerical solutions of the three-temperature model to explain that the all-optical helicity-dependent switching mechanism relies on the spin bath reaching temperatures close to the Curie point. Further insights into the reversal process are provided by the experimental demonstration of significant helicity-dependent reversal after a single laser pulse that reveals the involvement of direct angular momentum transfer. Moreover, based on the observation that longer pulse durations and larger spot sizes lead to enhanced reversal efficiency, we identify experimental conditions that lead to saturated magnetization reversal after just a few tens of laser pulses.

By conducting helicity-dependent ultrafast magnetization dynamics in a CoTb ferrimagnetic alloy, we are able to quantitatively determine the magnetic circular dichroism (MCD) and resolve its role in the helicity-dependent all-optical switching (AOS). Unequivocal interpretation of the sign of the dichroism is provided by performing AOS and femtosecond laser-induced domain wall motion experiments. We demonstrate that AOS occurs when the magnetization is initially in the most absorbent state, according to the light helicity. Moreover, we evidence that the MCD creates a thermal gradient that drives a domain wall toward hotter regions. Our experimental results are in agreement with the purely thermal models of AOS.

Engineering of magnetic materials for developing better spintronic applications relies on the control of two key parameters: the spin polarization and the Gilbert damping, responsible for the spin angular momentum dissipation. Both of them are expected to affect the ultrafast magnetization dynamics occurring on the femtosecond timescale. Here, engineered Co2 MnAlx Si1- x Heusler compounds are used to adjust the degree of spin polarization at the Fermi energy, P, from 60% to 100% and to investigate how they correlate with the damping. It is experimentally demonstrated that the damping decreases when increasing the spin polarization from 1.1 × 10-3 for Co2 MnAl with 63% spin polarization to an ultralow value of 4.6 × 10-4 for the half-metallic ferromagnet Co2 MnSi. This allows the investigation of the relation between these two parameters and the ultrafast demagnetization time characterizing the loss of magnetization occurring after femtosecond laser pulse excitation. The demagnetization time is observed to be inversely proportional to 1 - P and, as a consequence, to the magnetic damping, which can be attributed to the similarity of the spin angular momentum dissipation processes responsible for these two effects. Altogether, the high-quality Heusler compounds allow control over the band structure and therefore the channel for spin angular momentum dissipation.

All-optical ultrafast magnetization switching in magnetic material thin film without the assistance of an applied external magnetic field is explored for future ultrafast and energy-efficient magnetic storage and memories. It is shown that femtosecond (fs) light pulses induce magnetization reversal in a large variety of magnetic materials. However, so far, only GdFeCo-based ferrimagnetic thin films exhibit magnetization switching via a single optical pulse. Here, the single-pulse switching of Co/Pt multilayers within a magnetic spin-valve structure ([Co/Pt]/Cu/GdFeCo) is demonstrated and four possible magnetic configurations of the spin valve can be accessed using a sequence of single fs light pulses. The experimental study reveals that the magnetization final state of the ferromagnetic [Co/Pt] layer is determined by spin-polarized currents generated by the light pulse interactions with the GdFeCo layer. This work provides an approach to deterministically switch ferromagnetic layers and a pathway to engineering materials for opto-magnetic multi-bit recording.

Here, we first demonstrate helicity-independent all-optical switching in GdCo, a material chosen for stronger perpendicular magnetic anisotropy (PMA) than GdFeCo but with similar ferrimagnetic properties; furthermore, we achieve reliable AOS down to 200 nm diameters. The greater challenge to scaling was maintaining the perpendicular magnetic anisotropy for smaller dot dimensions, as was found to be a challenge in. While ion milling is a common method for patterning MTJ pillars for MRAM, it was found to destroy the integrity of the PMA. Instead, a lift-off process with electron -beam lithography was used to pattern the nanodots, ranging in size from 15 pm down to 50 nm, into arrays. Each dot size of diameter d was arrayed with a pitch of 3d in a25 pm x 25 pm square region. The pitch was chosen to be large enough to prevent magnetistatic coupling between the dots, while simultaneously allowing a high areal density of the dots for maximum magnetic signal during subsequent optical measurements.

International audience; Composite multiferroic systems, consisting of a piezoelectric substrate coupled with a ferromagnetic thin film, are of great interest from a technological point of view because they offer a path toward the development of ultralow power magnetoelectric devices. The key aspect of those systems is the possibility to control magnetization via an electric field, relying on the magneto-elastic coupling at the interface between the piezoelectric and the ferromagnetic components. Accordingly, a direct measurement of both the electrically induced magnetic behavior and of the piezo-strain driving such behavior is crucial for better understanding and further developing these materials systems. In this work, we measure and characterize the micron-scale strain and magnetic response, as a function of an applied electric field, in a composite multiferroic system composed of 1 and 2 μm squares of Ni fabricated on a prepoled [Pb(Mg 1/3 Nb 2/3)O 3 ] 0.69 −[PbTiO 3 ] 0.31 (PMN−PT) single crystal substrate by X-ray microdiffraction and X-ray photoemission electron microscopy, respectively. These two complementary measurements of the same area on the sample indicate the presence of a nonuniform strain which strongly influences the reorientation of the magnetic state within identical Ni microstructures along the surface of the sample. Micromagnetic simulations confirm these experimental observations. This study emphasizes the critical importance of surface and interface engineering on the micron-scale in composite multiferroic structures and introduces a robust method to characterize future devices on these length scales.

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., Comment: 9 pages, 5 figures

© 2017 IEEE. Up until now, magnetic nanodots used for magnetic random access memory have required spin-polarized currents to transfer the angular momentum needed to switch the magnetization and thereby switch the magnetic memory bit. This particular switching process, however, is limited to nanosecond or greater timescales-too slow for use as low-level cache in energy efficient electronics systems. On the other hand, this work aims to achieve ultrafast femtosecond switching of nanomagnetic dots without the use of spin-polarized currents. Using just linearly polarized light, several research groups have demonstrated all-optical magnetization switching in large GdFeCo magnetic dots, ranging from several microns [1-4] down to 400 nm [5]; this work characterizes the switching behavior as these dots are scaled down further in size, with the aim of minimizing the energy required for switching the magnetic memory bit. The fabrication process, magnetization behavior and optical switching behavior are additionally characterized to better understand how size affects the functionality of these optically-switchable ferrimagnets. Knowledge of this behavior will allow future developments of simultaneously ultrasmall and ultrafast magnetic memory systems, thereby enabling increased data storage in future electronics.

When electrons in a magnetic metal are driven far from equilibrium via ultrafast heating of the electrons, the magnetic order undergoes radical changes within tens of femtoseconds due to massive flows of energy and angular momentum between electrons, spins, and phonons. In ferrimagnetic metals such as GdFeCo, ultrafast optical heating can deterministically reverse the magnetization in less than a picosecond. In this talk, I describe our experimental work to gain a better understanding of how energy is exchanged between electrons, phonon, and spins in a magnetic metal following ultrafast heating. We use time-resolved measurements of the magneto-optic Kerr effect to record the response of ferro- and ferri-magnetic metals to heating via ultrafast optical or electrical pulses. Picosecond electrical pulses are generated with photoconductive Auston switches. By comparing the magnetic dynamics that result from electrical vs. optical heating, we identify differences in the rate of energy transfer to phonons from thermal vs. nonthermal electrons. We also find that both optical and electrical heating are effective for ultrafast switching of ferrimagnetic metals. We observe deterministic, repeatable ultrafast reversal of the magnetization of a GdFeCo thin film with a single sub-10 ps electrical pulse. The magnetization reverses in ~10 ps, which is more than one order of magnitude faster than other electrically controlled magnetic switching mechanisms.

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., 11 pages, 3 figures, supplementary materials

We report the magnetic response of Au/GdFeCo bilayers to optical irradiation of the Au surface. For bilayers with Au thickness greater than 50 nm, the great majority of energy is absorbed by the Au electrons, creating an initial temperature differential of thousands of Kelvin between the Au and GdFeCo layers. The resulting electronic heat currents between the Au and GdFeCo layers last for several picoseconds with energy flux in excess of 2 TW m-2, and provide sufficient heating to the GdFeCo electrons to induce deterministic reversal of the magnetic moment., Comment: 17 pages, 5 figures

Magnetic switching is induced in 10 ps by electrical current pulses., The field of spintronics involves the study of both spin and charge transport in solid-state devices. Ultrafast magnetism involves the use of femtosecond laser pulses to manipulate magnetic order on subpicosecond time scales. We unite these phenomena by using picosecond charge current pulses to rapidly excite conduction electrons in magnetic metals. We observe deterministic, repeatable ultrafast reversal of the magnetization of a GdFeCo thin film with a single sub–10-ps electrical pulse. The magnetization reverses in ~10 ps, which is more than one order of magnitude faster than any other electrically controlled magnetic switching, and demonstrates a fundamentally new electrical switching mechanism that does not require spin-polarized currents or spin-transfer/orbit torques. The energy density required for switching is low, projecting to only 4 fJ needed to switch a (20 nm)3 cell. This discovery introduces a new field of research into ultrafast charge current–driven spintronic phenomena and devices.

© 2017 American Physical Society. We report the magnetic response of Co/Pt multilayers to picosecond electrical heating. Using photoconductive Auston switches, we generate electrical pulses with 5.5 ps duration and hundreds of pico-Joules to pass through Co/Pt multilayers. The electrical pulse heats the electrons in the Co/Pt multilayers and causes an ultrafast reduction in the magnetic moment. A comparison between optical and electrically induced demagnetization of the Co/Pt multilayers reveals significantly different dynamics for optical vs electrical heating. We attribute the disparate dynamics to the dependence of the electron-phonon interaction on the average energy and the total number of initially excited electrons.

All optical magnetic switching (AOS) is a recently observed rich and puzzling phenomenon that offers promis- ing technological applications. However, fundamental understanding of the underlying mechanisms remains elusive. Here we present a new model for multi-shot helicity-dependent AOS in ferromagnetic materials based on a purely heat-driven mechanism in the presence of Magnetic Circular Dichroism (MCD). We predict that AOS should be possible with as little as 0.5% of MCD, after a minimum number of laser shots. Finally, we re- produce previous AOS results by simulating the sweeping of a laser beam on an FePtC granular ferromagnetic film., Comment: 10 pages, 4 figures, supplementary materials

We report a comparative study of magnetic field driven domain wall motion in thin films made of different magnetic materials for a wide range of field and temperature. The full thermally activated creep motion, observed below the depinning threshold, is shown to be described by a unique universal energy barrier function. Our findings should be relevant for other systems whose dynamics can be modeled by elastic interfaces moving on disordered energy landscapes., Comment: 10 pages, 3 figures

Ultrafast optical heating of the electrons in ferrimagnetic metals can result in all-optical switching (AOS) of the magnetization. Here we report quantitative measurements of the temperature rise of GdFeCo thin films during helicity-independent AOS. Critical switching fluences are obtained as a function of the initial temperature of the sample and for laser pulse durations from 55 fs to 15 ps. We conclude that non-equilibrium phenomena are necessary for helicity-independent AOS, although the peak electron temperature does not play a critical role. Pump-probe time-resolved experiments show that the switching time increases as the pulse duration increases, with 10 ps pulses resulting in switching times of ~sim 13 ps. These results raise new questions about the fundamental mechanism of helicity-independent AOS., 18 pages, 6 figures and supplementary materials

The shape instability of magnetic domain walls under current is investigated in a ferromagnetic (Ga,Mn)(As,P) film with perpendicular anisotropy. Domain wall motion is driven by the spin transfer torque mechanism. A current density gradient is found either to stabilize domains with walls perpendicular to current lines or to produce finger-like patterns, depending on the domain wall motion direction. The instability mechanism is shown to result from the non-adiabatic contribution of the spin transfer torque mechanism., 5 pages, 3 figures + supplementary materials

Magnetic-field-driven domain wall motion in an ultrathin $\mathrm{Pt}/\mathrm{Co}(0.45\text{ }\mathrm{nm})/\mathrm{Pt}$ ferromagnetic film with perpendicular anisotropy is studied over a wide temperature range. Three different pinning dependent dynamical regimes are clearly identified: the creep, the thermally assisted flux flow, and the depinning, as well as their corresponding crossovers. The wall elastic energy and microscopic parameters characterizing the pinning are determined. Both the extracted thermal rounding exponent at the depinning transition, $\ensuremath{\psi}=0.15$, and the Larkin length crossover exponent, $\ensuremath{\phi}=0.24$, fit well with the numerical predictions.

We show experimental evidence of magnetization switching in a single (Ga,Mn)(As,P) semiconducting ferromagnetic layer, attributed to a strong reduction of the magnetization and the anisotropy due to current injection. The nucleation of magnetization reversal is found to occur even in the absence of a magnetic field and to be both anisotropic and stochastic. Our findings highlight a new mechanism of magnetization manipulation based on spin accumulation in a semiconductor material. Fil: Gorchon, J.. Universite Paris Sud; Francia Fil: Curiale, Carlos Javier. Universite Paris Sud; Francia. Laboratoire de Photonique et de Nanostructures; Francia. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comision Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Resonancias Magnéticas; Argentina Fil: Lemaître, A.. Laboratoire de Photonique et de Nanostructures; Francia Fil: Moisan, N.. Universite Paris Sud; Francia Fil: Cubukcu, M.. Universite Pierre et Marie Curie; Francia Fil: Malinowski, G.. Universite Paris Sud; Francia Fil: Ulysse, C.. Laboratoire de Photonique et de Nanostructures; Francia Fil: Faini, G.. Laboratoire de Photonique et de Nanostructures; Francia Fil: Von Bardeleben, H.J.. Universite Pierre et Marie Curie; Francia Fil: Jeudy, V.. Universite Paris Sud; Francia. Université Cergy-Pontoise; Francia

The motion of elastic interfaces in disordered media is a broad topic relevant to many branches of physics. Field-driven magnetic domain wall motion in ultrathin ferromagnetic Pt/Co/Pt films can be well interpreted within the framework of theories developed to describe elastic interface dynamics in the presence of weak disorder. Indeed, the three theoretically predicted dynamic regimes of creep, depinning, and flow have all been directly evidenced in this model experimental system. We discuss these dynamic regimes and demonstrate how field-driven creep can be controlled not only by temperature and pinning, but also via interactions with magnetic entities located inside or outside the magnetic layer. Consequences of confinement effects in nano-devices are briefly reviewed, as some recent results on domain wall motion driven by an electric current or assisted by an electric field. Finally new theoretical developments and perspectives are discussed.

Strong spin-orbit coupling in non-magnetic heavy metals has been shown to lead to large spin currents flowing transverse to a charge current in such a metal wire. This in turn leads to the buildup of a net spin accumulation at the lateral surfaces of the wire. Spin-orbit torque effects enable the use of the accumulated spins to exert useful magnetic torques on adjacent magnetic layers in spintronic devices. We report the direct detection of spin accumulation at the free surface of nonmagnetic metal films using magnetization-induced optical surface second harmonic generation. The technique is applied to probe the current induced surface spin accumulation in various heavy metals such as Pt, β-Ta, and Au with high sensitivity. The sensitivity of the technique enables us to measure the time dynamics on a sub-ns time scale of the spin accumulation arising from a short current pulse. The ability of optical surface second harmonic generation to probe interfaces suggests that this technique will also be useful for studying the dynamics of spin accumulation and transport across interfaces between non-magnetic and ferromagnetic materials, where spin-orbit torque effects are of considerable interest.

Thèse confidentielle jusqu'au 26 octobre 2022.; The need for memory storage devices has skyrocketed over the last few decades especially after the development of the internet. This need has reached enormous heights in the past two years, soon after the pandemic due to COVID-19. Hard disk drives (HDDs) are known to have the potential to meet up with the high-density data storage demands. This thesis deals with one of the major challenges faced within the spintronic community to improve the speed and the energy consumption of memory devices.The speed of operation during the writing of a magnetic bit depends on the magnetization switching mechanism employed. The switching mechanism is itself dependent on the intrinsic magnetic properties of the sample and the externally induced excitation that drives the reversal of the magnetic bit 1. In this thesis, we will focus on the use of spin-orbit torque (SOT) excitations to drive the reversal, which is a relatively new but fast and energy-efficient approach in comparison with other state-of-the-art methods.The typical speed of magnetization reversal using SOTs is in the range of few nanoseconds, far slower than the picosecond-long switching that is possible with charge-based memory devices2. In fact, a record reversal speed with electrical pulses as short as ~200ps was reported by Garello et. al., 3 in 2011 using SOTs. This thesis reports further efforts to speed up the magnetization reversal by almost 2 orders of magnitude by exploiting such SOTs. To this aim, THz electrical pulses were generated via the use Auston photoconductive switches. We demonstrate that a single 6ps wide electrical pulse can induce a SOT to a 1nm thin Co ferromagnetic layer and result in a full magnetization reversal. A systematic study to understand SOTs in the picosecond time regime is also undertaken via using different magnetic nanostructures.In magnetic memory devices, a “read-head” is used to read the stored information in the device. Typically, in spintronic devices, giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR) based read heads are used for such operations. In this thesis, we also report on the attempts of developing a GMR sensor working in the THz regime.To undertake the aforementioned studies, a pump-probe optical and optoelectrical experimental setup has also been built and a detailed report of the same is also provided in the thesis.; Le besoin de dispositifs de stockage de mémoire a explosé au cours des dernières décennies, en particulier après le développement d'Internet. Ce besoin a atteint des sommets énormes au cours des deux dernières années, peu après la pandémie due au COVID-19. Les disques durs (HDD) sont connus pour avoir le potentiel de répondre aux demandes de stockage de données haute densité. Cette thèse traite de l'un des défis majeurs rencontrés au sein de la communauté spintronique pour améliorer la vitesse et la consommation d'énergie des dispositifs de mémoire. La vitesse de fonctionnement lors de l'écriture d'un bit magnétique dépend du mécanisme de commutation de magnétisation utilisé. Le mécanisme de commutation est lui-même dépendant des propriétés magnétiques intrinsèques de l'échantillon et de l'excitation induite de l'extérieur qui entraîne l'inversion du trépan magnétique 1. Dans cette thèse, nous nous concentrerons sur l'utilisation des excitations du couple spin-orbite (SOT) pour entraîner l'inversion, qui sont une approche relativement nouvelle mais rapide et économe en énergie par rapport à d'autres méthodes de pointe. La vitesse typique d'inversion de magnétisation à l'aide des SOT est de l'ordre de quelques nanosecondes, bien plus lente que la commutation longue de la picoseconde qui est possible avec les dispositifs de mémoire basés sur la charge 2. En fait, une vitesse d'inversion record avec des impulsions électriques aussi courtes que ~ 200 ps a été signalée par Garello et. al., 3 en 2011 en utilisant des SOT. Cette thèse rapporte des efforts supplémentaires pour accélérer l'inversion de l'aimantation de près de 2 ordres de grandeur en exploitant de tels SOT. Dans ce but, des impulsions électriques THz ont été générées via l'utilisation de commutateurs photoconducteurs Auston. Nous démontrons qu'une seule impulsion électrique de 6ps de large peut induire un SOT sur une couche ferromagnétique de Co d'une épaisseur de 1 nm et entraîner une inversion complète de l'aimantation. Une étude systématique pour comprendre les SOT dans le régime temporel picoseconde est également entreprise via l'utilisation de différentes nanostructures magnétiques. Dans les dispositifs à mémoire magnétique, une "tête de lecture" est utilisée pour lire les informations stockées dans le dispositif. Typiquement, dans les dispositifs spintroniques, des têtes de lecture à magnétorésistance géante (GMR) ou à magnétorésistance tunnel (TMR) sont utilisées pour de telles opérations. Dans cette thèse, nous rapportons également les tentatives de développement d'un capteur GMR fonctionnant en régime THz. Pour entreprendre les études susmentionnées, un montage expérimental optique et optoélectrique pompe-sonde a également été construit et un rapport détaillé de celui-ci est également fourni dans la thèse.