All-optical switching in granular ferromagnets caused by magnetic circular dichroism

Magnetic recording using circularly polarised femto-second laser pulses is an emerging technology that would allow write speeds much faster than existing field driven methods. However, the mechanism that drives the magnetisation switching in ferromagnets is unclear. Recent theories suggest that the interaction of the light with the magnetised media induces an opto-magnetic field within the media, known as the inverse Faraday effect. Here we show that an alternative mechanism, driven by thermal excitation over the anisotropy energy barrier and a difference in the energy absorption depending on polarisation, can create a net magnetisation over a series of laser pulses in an ensemble of single domain grains. Only a small difference in the absorption is required to reach magnetisation levels observed experimentally and the model does not preclude the role of the inverse Faraday effect but removes the necessity that the opto-magnetic field is 10 s of Tesla in strength

Magnetisation switching of FePt nanoparticle recording medium by femtosecond laser pulses

Manipulation of magnetisation with ultrashort laser pulses is promising for information storage device applications. The dynamics of the magnetisation response depends on the energy transfer from the photons to the spins during the initial laser excitation. A material of special interest for magnetic storage are FePt nanoparticles, for which switching of the magnetisation with optical angular momentum was demonstrated recently. The mechanism remained unclear. Here we investigate experimentally and theoretically the all-optical switching of FePt nanoparticles. We show that the magnetisation switching is a stochastic process. We develop a complete multiscale model which allows us to optimize the number of laser shots needed to switch the magnetisation of high anisotropy FePt nanoparticles in our experiments. We conclude that only angular momentum induced optically by the inverse Faraday effect will provide switching with one single femtosecond laser pulse.EC under Contract No. 281043, FemtoSpin. The work at Greifswald University was supported by the German research foundation (DFG), projects MU MU 1780/8-1, MU 1780/10-1. Research at Göttingen University was supported via SFB 1073, Projects A2 and B1. Research at Uppsala University was supported by the Swedish Research Council (VR), the Röntgen-Ångström Cluster, the Knut and Alice Wallenberg Foundation (Contract No. 2015.0060), and Swedish National Infrastructure for Computing (SNIC). Research at Kiel University was supported by the DFG, projects MC 9/9-2, MC 9/10-2. P.N. acknowledges support from EU Horizon 2020 Framework Programme for Research and Innovation (2014-2020) under Grant Agreement No. 686056, NOVAMAG. The work in Konstanz was supported via the Center for Applied Photonics

Beyond a phenomenological description of magnetostriction

We use ultrafast x-ray and electron diffraction to disentangle spin-lattice coupling of granular FePt in the time domain. The reduced dimensionality of single-crystalline FePt nanoparticles leads to strong coupling of magnetic order and a highly anisotropic three-dimensional lattice motion characterized by a- and b-axis expansion and c-axis contraction. The resulting increase of the FePt lattice tetragonality, the key quantity determining the energy barrier between opposite FePt magnetization orientations, persists for tens of picoseconds. These results suggest a novel approach to laser-assisted magnetic switching in future data storage applications.Comment: 12 pages, 4 figure

Chiral anisotropic magnetoresistance of ferromagnetic helices

ISSN:0003-6951ISSN:1077-311

Sol-gel route: An original strategy to chemically stabilize proton exchange membranes for fuel cell

International audienceWe present the elaboration, via a Sol-Gel (SG) route, of a new generation of hybrid membranes for PEMFC applications, in order to improve their performances and durability. The strategy was to create, within a commercial sPEEK membrane, a reactive SG phase able to reduce the oxidative species generated during FC operation and to improve the proton conductivity. The SG content is adjusted by tuning the SG precursors/sPEEK ratio used for the impregnation step. Raman analyses show that a uniform distribution of the SG phase in the membrane section is obtained when NMR analyses demonstrate its high extent of condensation, and reveal the structuring effect of the sPEEK membrane on the SG phase at the nanoscale. The membranes exhibit a better liquid water uptake and proton conductivity than pristine sPEEK membrane up to a SG uptake of 18%, with a gas permeability slightly higher than sPEEK but still lower than the benchmark Nafion. H2O2 accelerated aging tests evidenced the ability of the SG phase to prevent the oxidative degradation of sPEEK. Finally, the FC operability tests showed better and more durable performances for the hybrid membranes, without any increase of the gas permeability during operation

Impact of the SG phase morphology on the performances and durability of hybrid polymer membranes for fuel cell applications

International audienceProton-Exchange Membrane Fuel Cells (PEMFC) has emerged as a promising emission-free energy conversion device. However, the ionomer membrane at the heart of the device fails to deliver durable performance (to be achieved: 8000h for transportation, 50000h for stationary) at high temperature (100-150°C vs 80°C for std. Nafion) and low relative humidity (30%RH). The aim of our work is to improve existing membranes (better chemical and thermomechanical stabilities, better conductivities) by Sol-Gel (SG) hybridization. SG precursors are selected to diffuse through commercial membranes and introduce stabilizing organo-functional groups offering either a sacrificial stabilization (consumed over time) or a redox stabilization (regenerable) by degrading oxidizing agents produced during Fuel Cell operation. As the morphology (size, interaction/dispersion, connectivity) and localization (polar/apolar regions) of the SG phase inside the host matrix are parameters expected to be crucial for properties (H+ conductivity, water uptake), durability (H2O2-accelerated aging tests to assess the effectiveness of the reactive SG phase) and performances (FC operation) of the hybrid membranes, we explored their morphology at all relevant length scales. In this purpose, we use a combination of direct space (AFM/SEM/TEM) and reciprocal space (contrast variation SANS/SAXS) techniques (dimensional scale covered: from a hundred to a few nanometers) with regard to the chemistry of the SG Precursors (SGPs) (stabilization group, number of hydrolysable functions), yielding a variety of morphology (mass fractal structure vs. dispersed spherical aggregates vs. interconnected ones). H2O2-accelerated aging tests and preliminary fuel cell tests show promising operability of the hybrid membranes and the potential of the SG phase to inhibit the chemical ageing of sPEEK. With this work, we are confident to reach a predictive approach of the key parameters governing the final properties

Impact of the SG phase morphology on the performances and durability of hybrid polymer membranes for fuel cell applications

International audienceProton-Exchange Membrane Fuel Cells (PEMFC) has emerged as a promising emission-free energy conversion device. However, the ionomer membrane at the heart of the device fails to deliver durable performance (to be achieved: 8000h for transportation, 50000h for stationary) at high temperature (100-150°C vs 80°C for std. Nafion) and low relative humidity (30%RH). The aim of our work is to improve existing membranes (better chemical and thermomechanical stabilities, better conductivities) by Sol-Gel (SG) hybridization. SG precursors are selected to diffuse through commercial membranes and introduce stabilizing organo-functional groups offering either a sacrificial stabilization (consumed over time) or a redox stabilization (regenerable) by degrading oxidizing agents produced during Fuel Cell operation. As the morphology (size, interaction/dispersion, connectivity) and localization (polar/apolar regions) of the SG phase inside the host matrix are parameters expected to be crucial for properties (H+ conductivity, water uptake), durability (H2O2-accelerated aging tests to assess the effectiveness of the reactive SG phase) and performances (FC operation) of the hybrid membranes, we explored their morphology at all relevant length scales. In this purpose, we use a combination of direct space (AFM/SEM/TEM) and reciprocal space (contrast variation SANS/SAXS) techniques (dimensional scale covered: from a hundred to a few nanometers) with regard to the chemistry of the SG Precursors (SGPs) (stabilization group, number of hydrolysable functions), yielding a variety of morphology (mass fractal structure vs. dispersed spherical aggregates vs. interconnected ones). H2O2-accelerated aging tests and preliminary fuel cell tests show promising operability of the hybrid membranes and the potential of the SG phase to inhibit the chemical ageing of sPEEK. With this work, we are confident to reach a predictive approach of the key parameters governing the final properties