Ultrafast optically induced spin transfer in ferromagnetic alloys

The vision of using light to manipulate electronic and spin excitations in materials on their fundamental time and length scales requires new approaches in experiment and theory to observe and understand these excitations. The ultimate speed limit for all-optical manipulation requires control schemes for which the electronic or magnetic subsystems of the materials are coherently manipulated on the time scale of the laser excitation pulse. In our work, we provide experimental evidence of such a direct, ultrafast, and coherent spin transfer between two magnetic subsystems of an alloy of Fe and Ni. Our experimental findings are fully supported by time-dependent density functional theory simulations and, hence, suggest the possibility of coherently controlling spin dynamics on subfemtosecond time scales, i.e., the birth of the research area of attomagnetism

Ultrafast optically induced spin transfer in ferromagnetic alloys

The vision of using light to manipulate electronic and spin excitations in materials on their fundamental time and length scales requires new approaches in experiment and theory to observe and understand these excitations. The ultimate speed limit for all-optical manipulation requires control schemes for which the electronic or magnetic subsystems of the materials are coherently manipulated on the time scale of the laser excitation pulse. In our work, we provide experimental evidence of such a direct, ultrafast, and coherent spin transfer between two magnetic subsystems of an alloy of Fe and Ni. Our experimental findings are fully supported by time-dependent density functional theory simulations and, hence, suggest the possibility of coherently controlling spin dynamics on subfemtosecond time scales, i.e., the birth of the research area of attomagnetism

Deterministic Switching of the Magnetization States in Cobalt Nanorings

We studied the magnetoresistance of symmetric cobalt nanorings with nanowires attached at diametrically opposite positions. Deterministic switching, induced by an external magnetic field between the stable magnetization states, is demonstrated using histograms of the resistance changes as a function of an external magnetic field. The changes of the switching fields were studied as the lateral dimensions of these elements were varied systematically. The field required to switch between the vortex state and the onion state of the rings decreases as the line widths of the elements were increased. The vortex state is unstable at the larger line widths. Our experimental observations are explained using micromagnetic simulations

Control of stable magnetization states in permalloy nanorings using magnetic nanowires

We study the evolution of the magnetic field-induced switching between the stable domain configurations of permalloy nanoring structures, when magnetic nano-wires are attached to them. Magnetoresistance measurements were performed on such devices for two configurations of the attached nano-wires: (i) when they are at diametrically opposite ends of the nanoring, and (ii) when the nanowires are at an obtuse angle with respect to each other. During the measurements, the direction of application of the in-plane magnetic field is varied to understand the switching properties of the devices. Micromagnetic simulations were carried out in order to understand the domain configuration and reversal mechanism. We show that due to the nature of domain walls created by the presence of the nanowires in the obtuse configuration, a vortex state can be stabilized in the nano-ring. We extended our studies to various nanoring devices with different widths while keeping a constant thickness. (C) 2017 Elsevier B.V. All rights reserved

Control of vortex state in cobalt nanorings with domain wall pinning centers

Magnetic rings at the mesoscopic scale exhibit new spin configuration states and switching behavior, which can be controlled via geometrical structure, material composition and applied field. Vortex states in magnetic nanorings ensure flux closure, which is necessary for low stray fields in high packing density in memory devices. We performed magnetoresistance measurements on cobalt nanoring devices and show that by attaching nanowires to the ring, the vortex state can be stabilized. When a square pad is attached to the free end of the wire, the domain wall nucleation field in the nanowire is reduced. In addition, the vortex state persists over a larger range of magnetic fields, and exists at all in-plane orientations of the magnetic field. These experimental findings are well supported by our micromagnetic simulations. (C) 2017 Author(s)

Enhancement of uniaxial magnetic anisotropy in Fe thin films grown on GaAs(001) with an MgO underlayer

The understanding and control of anisotropy in Fe films grown on cubic systems such as GaAs and MgO has been of interest from the point of view of applications in devices. We report magnetic anisotropy studies on Fe/GaAs(001) and Fe/MgO/GaAs(001) prepared by pulsed laser deposition. In Fe/GaAs(001), magneto optical Kerr effect (MOKE) measurements revealed a dominant uniaxial anisotropy for Fe thickness less than 20 monolayers (ML) and this was confirmed by ferromagnetic resonance (FMR) studies. Multiple steps in the hysteresis loops were observed for Fe films of thickness 20 and 25 ML. Whereas, in Fe/MgO/GaAs(001), even at 25 ML of Fe, the uniaxial anisotropy remained dominant. The anisotropy constants obtained from FMR spectra have shown that the relative strength of uniaxial anisotropy is higher as compared to the cubic anisotropy constant in the case of Fe/MgO/GaAs(001). (C) 2011 American Institute of Physics. doi:10.1063/1.3556941

Investigation on two magnon scattering processes in pulsed laser deposited epitaxial nickel zinc ferrite thin film

Ferromagnetic resonance (FMR) measurements are employed to evaluate the presence of the two magnon scattering contribution in the magnetic relaxation processes of the epitaxial nickel zinc ferrite thin films deposited using pulsed laser deposition (PLD) on the (0 0 1) MgAl2O4 substrate. Furthermore, the reciprocal space mapping reveals the presence of microstructural defects which acts as an origin for the two magnon scattering process in this thin film. The relevance of this scattering process is further discussed for understanding the higher FMR linewidth in the in-plane configuration compared to the out-of-plane configuration. FMR measurements also reveal the presence of competing uniaxial and cubic anisotropy in the studied films

High photon flux 70 eV HHG source for ultrafast dynamics

We present a high harmonic generation source driven by a nonlinearly compressed fiber laser system resulting in a record high photon flux of 1011 photons/s in single harmonics from 50-70 eV. This unique property of the HHG source is underlined by static T-MOKE experiments with permalloy samples

Nuclear inelastic scattering studies of a 1D- polynuclear spin crossover complex of Fe(II) urea-triazoles

The partial density of vibrational states (pDOS) of the low-spin isomer of the tosylate salt of [Fe{(N-Propyl)-N′-(1,2,4-triazole-4-yl-urea)}$_3$]$^{2+}$ was determined by nuclear inelastic scattering experiments performed at 8 K. The pDOS features a rich band structure from 320 to 500 cm$^{−1}$, characteristic of the low spin state of the complex. Density functional theory calculations (B3LYP/CEP-31G) were used to assign molecular modes to the experimentally observed peak in the pDOS

Optical pump - nuclear resonance probe experiments on spin crossover complexes

A novel sample environment enabling optical pump – nuclear resonance probe experiments has been installed at the beamline P01, Petra III, DESY Hamburg. This set-up has been used to investigate optically induced spin state changes of spin crossover (SCO) complexes by nuclear resonant scattering immediately after excitation by an optical laser pulse. Here, we report the technical details as well as first results of the experiments performed at 290 K and 80 K on the SCO complexes [Fe (NH$_2$trz)$_3$]Cl$_2$ and [Fe(PM-BiA)$_2$(NCS)$_2$], respectively. The $^{57}$Fe-enriched SCO complexes were excited by a 531 nm laser with a pulse length < 100 ps. Evaluation of the nuclear forward scattering data clearly indicate the presence of high spin (HS) states when the complexes are excited by laser pulses and a pure low spin (LS) state in the absence of any laser pulse. Furthermore, the dependence of the optically excited HS-fraction has been determined as a function of the average optical power