When exciting a magnetic material with a femtosecond laser pulse, the amplitude of magnetization is no longer constant and can decrease within a time scale comparable to the duration of the optical ...excitation. This ultrafast demagnetization can even trigger an ultrafast, out of equilibrium, phase transition to a paramagnetic state. The reciprocal effect, namely an ultrafast remagnetization from the zero magnetization state, is a necessary ingredient to achieve a complete ultrafast reversal. However, the speed of remagnetization is limited by the universal critical slowing down which appears close to a phase transition. Here we demonstrate that magnetization can be reversed in a few hundreds of femtoseconds by overcoming the critical slowing down thanks to ultrafast spin cooling and spin heating mechanisms. We foresee that these results outline the potential of ultrafast spintronics for future ultrafast and energy efficient magnetic memory and storage devices. Furthermore, this should motivate further theoretical works in the field of femtosecond magnetization reversal.
Since 2011, the Pays d’Épinal has promoted the “Hêtre des Vosges” brand, later renamed “Terres de Hêtre”. The Pays d’Épinal territory includes 196 municipalities. Forests cover 42% of its surface ...area (100,000 hectares). New outlets are being sought for the deciduous tree component (62% of the forested area), in particular for beech that covers 40,000 hectares of this territory. Support for the forestry and forest-based industry has therefore become a priority for maintaining stable economic activity in this rural area. This dynamic has given rise to multiple projects and achievements. The final goal is to develop the beech sector as a whole, from the tree to the manufacture of end products.
Le Pays d’Épinal porte depuis 2011 la marque « Hêtre des Vosges », devenue par la suite « Terres de Hêtre ». Le territoire du Pays d’Épinal, qui comporte 196 communes, a une couverture forestière de 42 % (100 000 hectares). La ressource feuillue (62% de la surface forestière) doit trouver des débouchés, notamment le Hêtre qui représente 40 000 hectares sur le territoire. Le soutien à la filière forêt-bois est donc devenu une priorité pour le maintien d’une activité économique stable en zone rurale. De multiples projets et réalisations naissent de cette dynamique. L’objectif ultime est de développer une filière Hêtre dans son ensemble, de l’arbre jusqu’à la production de produits finis.
Abstract An antiferromagnet emits spin currents when time-reversal symmetry is broken. This is typically achieved by applying an external magnetic field below and above the spin-flop transition or by ...optical pumping. In this work we apply optical pump-THz emission spectroscopy to study picosecond spin pumping from metallic FeRh as a function of temperature. Intriguingly we find that in the low-temperature antiferromagnetic phase the laser pulse induces a large and coherent spin pumping, while not crossing into the ferromagnetic phase. With temperature and magnetic field dependent measurements combined with atomistic spin dynamics simulations we show that the antiferromagnetic spin-lattice is destabilised by the combined action of optical pumping and picosecond spin-biasing by the conduction electron population, which results in spin accumulation. We propose that the amplitude of the effect is inherent to the nature of FeRh, particularly the Rh atoms and their high spin susceptibility. We believe that the principles shown here could be used to produce more effective spin current emitters. Our results also corroborate the work of others showing that the magnetic phase transition begins on a very fast picosecond timescale, but this timescale is often hidden by measurements which are confounded by the slower domain dynamics.
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.
We review the phenomenology of light-induced ultrafast magnetization dynamics, with emphasis on metallic compounds. The interaction of femtosecond laser pulses with magnetic thin films induces a wide ...range of out of equilibrium phenomena, leading to an ultrafast demagnetization, and a subsequent magnetization dynamics. The latter can lead to a deterministic reversal of the magnetization or allow for the controlled motion of domain walls. Those light-induced magnetization manipulations have potential application for ultrafast and energy efficient spintronic. The review summarizes experimental results and theoretical approaches on demagnetization and the all-optical helicity-dependent switching of the magnetization, whose underlying mechanism(s) are still heavily debated, even after 25 years of intensive research.
•The magnetic excitations responsible for the ultrafast demagnetization are debated.•A broad overview of the mechanisms through which they could be generated is provided.•Criteria allowing for the helicity-dependent all-optical switching are reviewed.•The origin of the helicity-dependence is examined.
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 Ni1–x /Pt spin-valve 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.
Abstract
Where dust and gas are uniformly mixed, atomic hydrogen can be traced through the detection of far-infrared (FIR) or UV emission of dust. We considered, for the origin of discrepancies ...observed between various direct and indirect tracers of gas outside the Galactic plane, possible corrections to the zero levels of the Planck High Frequency Instrument (HFI) detectors. We set the zero levels of the Planck-HFI skymaps as well as the 100
μ
m map from COBE/DIRBE and IRAS from the correlation between FIR emission and atomic hydrogen column density excluding regions of lowest gas column density. A modified blackbody model fit to those new zero-subtracted maps led to significantly different maps of the opacity spectral index
β
and temperature
T
and an overall increase in the optical depth at 353 GHz
τ
353
of 7.1 × 10
−7
compared to the data release 2 Planck map. When comparing
τ
353
and the H
i
column density, we observed a uniform spatial distribution of the opacity outside regions with dark neutral gas and CO except in various large-scale regions of low
N
H
i
that represent 25% of the sky. In those regions, we observed an average dust column density 45% higher than predictions based on
N
H
i
with a maximum of 250% toward the Lockman Hole region. From the average opacity
σ
e
353
= (8.9 ± 0.1) × 10
−27
cm
2
, we deduced a dust-to-gas mass ratio of 0.53 × 10
−2
. We did not see evidence of dust associated with a Reynolds layer of ionized hydrogen. We measured a far-ultraviolet isotropic intensity of 137 ± 15 photons s
−1
cm
−2
sr
−1
Å
−1
in agreement with extragalactic flux predictions and a near-ultraviolet isotropic intensity of 378 ± 45 photons s
−1
cm
−2
sr
−1
Å
−1
corresponding to twice the predicted flux.
The discovery of spin-transfer torque (STT) enabled the control of the magnetization direction in magnetic devices in nanoseconds using an electrical current. Ultrashort optical pulses have also been ...used to manipulate the magnetization of ferrimagnets at picosecond timescales by bringing the system out of equilibrium. So far, these methods of magnetization manipulation have mostly been developed independently within the fields of spintronics and ultrafast magnetism. Here we show optically induced ultrafast magnetization reversal taking place within less than a picosecond in rare-earth-free archetypal spin valves of Pt/Co/Cu/Co/Pt commonly used for current-induced STT switching. We find that the magnetization of the free layer can be switched from a parallel to an antiparallel alignment, as in STT, indicating the presence of an unexpected, intense and ultrafast source of opposite angular momentum in our structures. Our findings provide a route to ultrafast magnetization control by bridging concepts from spintronics and ultrafast magnetism.
Oak (Quercus petraea and Quercus robur), an emblematic species in French forests, has a strong patrimonial value and plays a leading role in the forestry and forest-based industry. However, the ...industry appears to be confronted with major problems (e.g. supply to markets). We tried to better understand and analyse those problems by studying the oak chain in north eastern France (Alsace, Franche-Comté, Lorraine) through interviews conducted with public forest managers and players involved in primary processing. In order to survey a broad range of points of view and have a basis for comparison, the study area also included two Landers in south western Germany (Baden-Wurttemberg, Rhineland-Palatinate). A qualitative analysis of 25 interviews conducted on both sides of the Franco-German border enabled us to identify contrasting views and points of consensus about the issues relating to forest resources, the perception of the sector by the players and the entrepreneurial dynamic. Possible future developments could also be distinguished, some of which are already discussed in the literature while others were particular to this survey.
Le Chêne (Quercus petraea et Quercus robur), essence emblématique de la forêt française, a une valeur patrimoniale forte et joue un rôle majeur dans la filière forêt-bois. Néanmoins, cette filière semble être confrontée à des problèmes importants (par exemple au niveau de l’approvisionnement et des marchés) que nous avons cherché à mieux comprendre et à analyser. Notre étude s’intéresse plus particulièrement à la filière Chêne dans le nord-est de la France (Alsace, Franche-Comté, Lorraine) et se base sur des entretiens réalisés auprès de gestionnaires forestiers publics et d’acteurs de la première transformation. Afin d’obtenir une gamme large de points de vue et de disposer d’éléments de comparaison, la zone d’étude intègre également deux Länder du sud-ouest de l’Allemagne (Bade-Wurtemberg, Rhénanie-Palatinat). L’analyse qualitative de 25 entretiens, réalisés de part et d’autre de la frontière franco-allemande, a permis de dégager des avis divergents et des points de consensus au sujet de thématiques relatives à la ressource forestière, à la perception de la filière par les acteurs et aux dynamiques d’entreprise. Des perspectives d’évolution ont aussi pu être distinguées, comprenant des réflexions déjà évoquées dans la littérature et d’autres plus personnelles à cette étude.
PDF
