Magnetic switching in granular FePt layers promoted by near-field laser enhancement

Light-matter interaction at the nanoscale in magnetic materials is a topic of intense research in view of potential applications in next-generation high-density magnetic recording. Laser-assisted switching provides a pathway for overcoming the material constraints of high-anisotropy and high-packing density media, though much about the dynamics of the switching process remains unexplored. We use ultrafast small-angle x-ray scattering at an x-ray free-electron laser to probe the magnetic switching dynamics of FePt nanoparticles embedded in a carbon matrix following excitation by an optical femtosecond laser pulse. We observe that the combination of laser excitation and applied static magnetic field, one order of magnitude smaller than the coercive field, can overcome the magnetic anisotropy barrier between "up" and "down" magnetization, enabling magnetization switching. This magnetic switching is found to be inhomogeneous throughout the material, with some individual FePt nanoparticles neither switching nor demagnetizing. The origin of this behavior is identified as the near-field modification of the incident laser radiation around FePt nanoparticles. The fraction of not-switching nanoparticles is influenced by the heat flow between FePt and a heat-sink layer

Ultrafast manipulation of the NiO antiferromagnetic order via sub gap optical excitation

Wide band gap insulators such as NiO offer the exciting prospect of coherently manipulating electronic correlations with strong optical fields. Contrary to metals where rapid dephasing of optical excitation via electronic processes occurs, the sub gap excitation in charge transfer insulators has been shown to couple to low energy bosonic excitations. However, it is currently unknown if the bosonic dressing field is composed of phonons or magnons. Here we use the prototypical charge transfer insulator NiO to demonstrate that 1.5 eV sub gap optical excitation leads to a renormalised NiO band gap in combination with a significant reduction of the antiferromagnetic order. We employ element specific X ray reflectivity at the FLASH free electron laser to demonstrate the reduction of the upper band edge at the O 1s 2p core valence resonance K edge whereas the antiferromagnetic order is probed via X ray magnetic linear dichroism XMLD at the Ni 2p 3d resonance L2 edge . Comparing the transient XMLD spectral line shape to ground state measurements allows us to extract a spin temperature rise of 65 5 K for time delays longer than 400 fs while at earlier times a non equilibrium spin state is formed. We identify transient mid gap states being formed during the first 200 fs accompanied by a band gap reduction lasting at least up to the maximum measured time delay of 2.4 ps. Electronic structure calculations indicate that magnon excitations significantly contribute to the reduction of the NiO band ga

Ultrafast modification of the electronic structure of a correlated insulator

A nontrivial balance between Coulomb repulsion and kinematic effects determines the electronic structure of correlated electron materials. The use of electromagnetic fields strong enough to rival these native microscopic interactions allows us to study the electronic response as well as the time scales and energies involved in using quantum effects for possible applications. We use element specific transient x ray absorption spectroscopy and high harmonic generation to measure the response to ultrashort off resonant optical fields in the prototypical correlated electron insulator NiO. Surprisingly, fields of up to 0.22 V lead to no detectable changes in the correlated Ni 3d orbitals contrary to previous predictions. A transient directional charge transfer is uncovered, a behavior that is captured by first principles theory. Our results highlight the importance of retardation effects in electronic screening and pinpoints a key challenge in functionalizing correlated materials for ultrafast device operatio

Magnetic circular dichroism in Co 2p photoemission of Co/Cu(1 1 13): Separation of the fundamental spectra

PACS. 75.30.Pd Surface magnetism –, 75.25.+z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.) –, 71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect –, 79.60.-i Photoemission and photoelectron spectra,

Mn 3d electronic configurations in Ga xMnx As ferromagnetic semiconductors and their influence on magnetic ordering

We applied x ray absorption spectroscopy and x ray magnetic circular dichroism XMCD at the Mn 2p 3d resonances to study the Mn 3d electronic configuration and the coupling of Mn 3d magnetic moments in various Ga1 amp; 8722;xMnxAs films. The homogeneity of the Mn depth profile throughout the Ga1 amp; 8722;xMnxAs film was tested by additional structure sensitive x ray resonant reflectivity measurements. In all investigated Ga1 amp; 8722;xMnxAs films the electronic and magnetic configuration of the Mn impurities varies throughout the Mn doped layer. This inhomogeneity is caused by the surface segregation of nonferromagnetic Mn in a d5 configuration. X ray resonant reflectivity data show that the accumulation of nonferromagnetic Mn near the surface is strongly enhanced by low temperature annealing. By XMCD we identified the Mn species responsible for the long range ferromagnetic coupling. It is characterized by an Mn 3d5 3d6 mixed valence acceptor state that is unchanged at all investigated Mn concentrations, ranging from 1 to 6 . Additional nonferromagnetic Mn occurs in the bulk of high concentration samples. We discuss a model in which the latter is due to antiferromagnetic Mn Mn nearest neighbor pair