We report on element-specific measurements of ultrafast demagnetization and magnetization precession damping in permalloy thin films. Magnetization dynamics induced by an optical pump at 1.5 eV is probed simultaneously at the M2,3 edges of Ni and Fe with high-order harmonics for moderate demagnetization rates (less than 50%). The role of the intersublattice exchange interaction in both longitudinal and transverse dynamics is analyzed with a Landau-Lifshitz-Bloch description of ferromagnetically coupled Fe and Ni sublattices. It is shown that the intersublattice exchange interaction governs the dissipation during demagnetization as well as precession damping of the magnetization vector.

We report on an element- and time-resolved investigation of femtosecond laser induced ultrafast dynamics of 3d and 4f spins in a ferrimagnetic Co80Dy20 alloy as a function of temperature. We observe an increase of the Co3d characteristic demagnetization time and a decrease of the Dy4f demagnetization time when the temperature is approaching the Curie temperature. It suggests that the critical slowing down regime, which affects the laser induced ultrafast dynamics in pure 3d transition metals and 4f rare-earth ferromagnetic layers, vanishes for the Dy sublattice in the CoDy alloy, in line with the theoretical predictions of the Landau-Lifshitz-Bloch model.

Exchange coupled core-shell nanoparticles present high potential to tune adequately the magnetic properties for specific applications such as nanomedicine or spintronics. Here, we report on the design of core-shell nanoparticles by performing the successive thermal decomposition of Fe and Co complexes. Depending on the thermal stability and the concentration of the Co precursor, we were able to control the formation of a hard ferrimagnetic (FiM) Co-ferrite shell or an antiferromagnetic (AFM) CoO shell at the surface of a soft FiM Fe3-δO4 core. The formation of the Co-ferrite shell was also found to occur through two different mechanisms: the diffusion of Co or the growth at the iron oxide surface. The structural properties of core-shell nanoparticles were investigated by a wide panel of techniques such as HAADF, STEM and XRD. The distribution of Fe and Co elements in the crystal structure was described accurately by XAS and XMCD. The operating conditions influenced significantly the oxidation rate of Fe2+ in the core as well as the occupancy of Oh sites by Fe2+ and Co2+ cations. The structural properties of nanoparticles were correlated with their magnetic properties which were investigated by SQUID magnetometry. Each core-shell nanoparticle displayed enhanced effective magnetic anisotropy energy (Eeff) in comparison with pristine Fe3-δO4 nanoparticles because of magnetic coupling at the core-shell interface. The Co-ferrite FiM shells resulted in better enhancement of Eeff than a CoO AFM shell. In addition, the magnetic properties were also influenced by the core size. The coercive field (HC) was increased by core reduction while the blocking temperature (TB) was increased by a larger core. Element-specific XMCD measurements showed the fine coupling of Fe and Co cations which agree with Co-ferrite in each sample, e.g. the formation of a Co-doped interfacial layer in the Fe3-δO4@CoO nanoparticles.

We present the first comparative study of ultrafast demagnetization between native magnetite (Fe3O4) nanoparticles, and maghemite (gamma-Fe2O3) nanoparticles issued from the same batch of nanoparticles and obtained after thermal annealing of the native ones. We demonstrate that the demagnetizing process is accelerated and becomes simultaneous to the electron thermalization time as the degree of oxidation is increased. Our interpretation is that thermal annealing, inducing a phase transition between the mixed valence state Fe3O4 and its oxidized phase gamma-Fe2O3, reinforces antiferromagnetic superexchange interactions which govern their magnetic properties. Consequently, this speeds up the ultrafast magnetization in the very short time scale. We show the role played by the exchange interaction in the ultrafast demagnetization following a femtosecond laser pulse excitation.

Ultrashort laser pulses are used to induce changes of the magnetization in ferrimagnetic CoxTb1-x alloys. Ultrafast magnetization dynamics has been probed by tr-XMCD at the CoL3 and TbM5 edges. We demonstrated that demagnetization of the 4f magnetic moment is much faster than in pure Tb when the excited-state-temperature is below the compensation temperature.

In this work, we have studied the ultrafast dynamics of charges and spins in assemblies of magnetite (Fe3O4) and maghemite (γ-Fe2O3) nanoparticles. We demonstrate that using time-resolved magneto-optics one is able to disentangle those very similar iron oxide structures. The Fe3O4 nanoparticles are elaborated by hydrothermal decomposition and deposited by drop on a glass substrate. γ-Fe2O3 nanoparticles assemblies have been obtained by annealing the Fe3O4 nanoparticles. Comparing time resolved transmission and Faraday rotation, our measurements show that in case of Fe3O4 the demagnetization occurs after the thermalization of the charges, as expected from previous works on ultrafast quenching of magnetization in ferromagnetic nanostructures. On the contrary, in the case of maghemite nanoparticles, an acceleration of the demagnetizing occurs, leading to a simultaneous charges and spins dynamics. We attribute this behavior to the rearrangement of vacancies and annealing of crystal defects in maghemite.

The time resolved reflectivity ΔR(t) and magneto-optical Kerr response Δθ(t) of assemblies of Co–Pt nanoparticles are studied with femtosecond laser pulses. In ordered arrangements of such superparamagnetic nanoparticles ΔR(t) displays a low frequency mode (145 ps) characteristic of collective vibrations, while Δθ(t) varies monotonously. In contrast, after the nanoparticles are thermally annealed, Δθ(t) displays a motion of precession and a monotonous variation of ΔR(t) both characteristic of a ferromagnetic phase and disordered arrangement.

Femtosecond laser pulses can be used to induce ultrafast changes of the magnetization in magnetic materials. However, one of the unsolved questions is that of conservation of the total angular momentum during the ultrafast demagnetization. Here we report the ultrafast transfer of angular momentum during the first hundred femtoseconds in ferrimagnetic Co0.8Gd0.2 and Co0.74Tb0.26 films. Using time-resolved X-ray magnetic circular dichroism allowed for time-resolved determination of spin and orbital momenta for each element. We report an ultrafast quenching of the magnetocrystalline anisotropy and show that at early times the demagnetization in ferrimagnetic alloys is driven by the local transfer of angular momenta between the two exchange-coupled sublattices while the total angular momentum stays constant. In Co0.74Tb0.26 we have observed a transfer of the total angular momentum to an external bath, which is delayed by ~150 fs. Femtosecond laser pulses can induce ultrafast changes to the magnetization in magnetic materials. Here, the authors show that the ultrafast demagnetization in ferrimagnets is driven by the transfer of angular momenta between two coupled sublattices whilst the total angular momentum remains constant.

Spherical coreshell CoxFe1xO@CoyFe3-yO4 nanoparticles (NPs) as well as spherical and cubic shaped CoFe2O4 NPs were synthesized through a thermal decomposition method by adjusting parameters such as the nature of precursors and ligands. The use of metal (iron and/or cobalt) oleates and stearates as precursors in the presence of oleic acid as ligand leads to coreshell NPs, due to the reducing environment provided by oleate groups from the oleic acid and precursors. By contrast, the use of oleylamine as ligand favored the decomposition of precursors and less reducing medium, which allows obtaining NPs with homogeneous composition. In addition, cobalt ferrite cubic-shaped NPs were synthesized using mixed oleate formed in situ from metal iron chloride and cobalt chloride in the presence of sodium oleate. The as-synthesized NPs were carefully characterized by combining several techniques including TEM, XRD, Fe-57 Mossbauer spectrometry, STEM-EELS, and XMCD. The correlation between the crystalline structure and the magnetic properties was investigated by carrying out magnetic measurements as a function of an applied field and of temperature. The CoFe2O4 NPs were found to display high coercivity due to their homogeneous composition, while the coreshell NPs show higher blocking temperature and exchange bias properties originating from the interaction between the antiferromagnetic (AFM) core and the ferrimagnetic (FIM) layer at the surface.

Spherical coreshell CoxFe1xO@CoyFe3-yO4 nanoparticles (NPs) as well as spherical and cubic shaped CoFe2O4 NPs were synthesized through a thermal decomposition method by adjusting parameters such as the nature of precursors and ligands. The use of metal (iron and/or cobalt) oleates and stearates as precursors in the presence of oleic acid as ligand leads to coreshell NPs, due to the reducing environment provided by oleate groups from the oleic acid and precursors. By contrast, the use of oleylamine as ligand favored the decomposition of precursors and less reducing medium, which allows obtaining NPs with homogeneous composition. In addition, cobalt ferrite cubic-shaped NPs were synthesized using mixed oleate formed in situ from metal iron chloride and cobalt chloride in the presence of sodium oleate. The as-synthesized NPs were carefully characterized by combining several techniques including TEM, XRD, Fe-57 Mossbauer spectrometry, STEM-EELS, and XMCD. The correlation between the crystalline structure and the magnetic properties was investigated by carrying out magnetic measurements as a function of an applied field and of temperature. The CoFe2O4 NPs were found to display high coercivity due to their homogeneous composition, while the coreshell NPs show higher blocking temperature and exchange bias properties originating from the interaction between the antiferromagnetic (AFM) core and the ferrimagnetic (FIM) layer at the surface.

Ultrafast magnetization dynamics induced by femtosecond laser pulses have been measured in ferrimagnetic Co0.8Gd0.2, Co0.74Tb0.26, and Co0.86Tb0.14 alloys. Using element sensitivity of x- ray magnetic circular dichroism at the Co L-3, Tb M-5, and Gd M-5 edges, we see that the demagnetization dynamics is element dependent. We show that a thermalization time as fast as 280 +/- 30 fs is observed for the rare earth in the alloy when the excited-state temperature is below the compensation temperature. It is limited to 500 +/- 100 fs when the excited-state temperature is below the Curie temperature (T-C). Therefore, for transition-metal rare-earth alloys, we propose that critical spin fluctuations in the vicinity of T-C reduce the demagnetization rates of the 4f electrons, whereas far from T-C the limited demagnetization rates should be avoided.

Comparing the magnetization dynamics in CoPd films using time resolved Magneto-Optics and X-rays Magnetic Circular Dichroism, we confirm that the demagnetization dynamics of ferromagnetic metals occurs during the thermalization time of the spins.

Optical Properties of Metal Clusters deals with the electronic structure of metal clusters determined optically. Clusters - as state intermediate between molecules and the extended solid - are importa

A precise control and understanding of the magnetization dynamics of nanostructures is an important topic in applied nanosciences. Herein, we perform such control by annealing crystalline (Co/core)-(Pt/shell) nanoparticles. Using electron tomography, temperature dependent electron microscopy and time-resolved magneto-optics, we establish a clear correlation between the magnetization dynamics and the crystalline structure of the nanoparticles. For a mild laser annealing (370 K) the Co-Pt nanoparticles keep their core-shell structure and remain superparamagnetic with a blocking temperature T(B) = 66 K. Their time-resolved reflectivity shows that they are locally organized into a supra-crystalline ordered layer in the region of the laser spot. In contrast, a thermal annealing at higher temperatures (up to 700 K) modifies the structure of the individual nanoparticles into a CoPt crystalline ferromagnetic phase, with T(B,anneal) = 347 K. Correspondingly, the magneto-crystalline anisotropy of the annealed CoPt nanoparticles increases and their magnetization dynamics displays a motion of precession, characteristic of ferromagnetic nanostructures and which is absent in the superparamagnetic Co-Pt core-shells.

The magnetization dynamics of CoPd films excited by circularly polarized ultrashort laser pulses is studied by time-resolved x-ray magnetic circular dichroism. In those films the ultrafast dynamics measured at the Co-L-3 edge is strongly sensitive to the orbitalmagnetic moment L-z. The amount of angular momentum transferred by the circularly polarized ultrashort laser pulses to the ferromagnetic films is evaluated to +/- 0.1 (h) over bar /atom, which is above the detection limit of the experiment. Despite this, no polarization-dependent difference on the magnetization dynamics could be evidenced. These results are explained by ultrafast electronic relaxation mechanisms of the transferred angular momentum, faster than similar to 100 fs. This experiment sets the methodology as well as an upper time limit for determination of angular momentum relaxation processes.

Understanding the induced demagnetization of magnetic metals interacting with femtosecond laser pulses necessitates taking into consideration the spin-orbit coupling. Here we explore the dynamics of this fundamental interaction in the presence of the laser field.

Spin dissection The magnetism produced by electrons in a solid can have two components — spin and orbital moments — that are interchangeable on femtosecond timescales. Christine Boeglin and colleagues have used ultrashort pulses of light to modify the orbital angular momentum of electrons in a magnetic material and to observe, with X-ray pulses, how rapidly this momentum is transferred to the spins. By disentangling the changes in these two components in this way, it is possible to obtain insights into the underlying dynamical processes that could be of value for ultrafast magnetic recording.

Using femtosecond laser pulses, we have observed in real time the coherent motion of nanocrystals self-assembled in a three-dimensional (3D) supra-crystal. The self-organisation of nanosystems is of great interest since it leads to the elaboration of mesoscopic materials which have specific physical properties differing both from those of the bulk and of the nano-material. The nanostructures that we have studied are 3D long-range ordered or amorphous assemblies of cobalt nanoparticles elaborated by soft chemistry. It consists in a micellar solution of cobalt nanoparticles passivated by aliphatic chains and evaporated on an HOPG substrate. Depending on the evaporation conditions, it results in amorphous assemblies or 3D supra-crystals with perfectly organised pavements of 10 mum2.

We study the trajectory of the magnetization in cobalt nanoparticles using femtosecond pulses. We show that the initial pathway of the magnetization vector is mostly determined by the magnetic anisotropy of the assembly of nanoparticles.

Using femtosecond optical spectroscopy, we study the ultrafast dynamics of the surface plasmon polaritons in gold arrays of subwavelength holes. A large time dependent spectral broadening and shift of the surface plasmon resonances are reported. The experimental results are modeled by the diffraction of a transverse electromagnetic field through the nanostructure, taking into account both the electron dynamics near the interband transitions and the Drude-like conductivity of the metal. Our analysis, using either a theoretical or an experimentally determined dielectric function of gold, suggests that the losses propagation in plasmonic devices is strongly influenced by intrinsic and extrinsic electron scattering mechanisms.