43 research outputs found
Resonant translational, breathing and twisting modes of pinned transverse magnetic domain walls
We study translational, breathing and twisting resonant modes of transverse
magnetic domain walls pinned at notches in ferromagnetic nanostrips. We
demonstrate that a mode's sensitivity to notches depends strongly on the
characteristics of that particular resonance. For example, the frequencies of
modes involving lateral motion of the wall are the ones which are most
sensitive to changes in the notch intrusion depth (especially at the narrower,
more strongly confined end of the domain wall). In contrast, the breathing
mode, whose dynamics are concentrated away from the notches is relatively
insensitive to changes in the notches' sizes. We also demonstrate a sharp drop
in the translational mode's frequency towards zero when approaching depinning
which is found, using a harmonic oscillator model, to be consistent with a
reduction in the local slope of the notch-induced confining potential at its
edge.Comment: 11 pages, 10 figures, additional data and analysi
Additive Laser Excitation of Giant Nonlinear Surface Acoustic Wave Pulses
The laser ultrasonics technique perfectly fits the needs for non-contact,
non-invasive, non-destructive mechanical probing of samples of mm to nm sizes.
This technique is however limited to the excitation of low-amplitude strains,
below the threshold for optical damage of the sample. In the context of strain
engineering of materials, alternative optical techniques enabling the
excitation of high amplitude strains in a non-destructive optical regime are
seeking. We introduce here a non-destructive method for laser-shock wave
generation based on additive superposition of multiple laser-excited strain
waves. This technique enables strain generation up to mechanical failure of a
sample at pump laser fluences below optical ablation or melting thresholds. We
demonstrate the ability to generate nonlinear surface acoustic waves (SAWs) in
Nb:SrTiO substrates, at typically 1 kHz repetition rate, with associated
strains in the percent range and pressures close to 100 kbars. This study paves
the way for the investigation of a host of high-strength SAW-induced phenomena,
including phase transitions in conventional and quantum materials, plasticity
and a myriad of material failure modes, chemistry and other effects in bulk
samples, thin layers, or two-dimensional materials
Generation of coherent spin-wave modes in Yttrium Iron Garnet microdiscs by spin-orbit torque
Spin-orbit effects [1-4] have the potential of radically changing the field
of spintronics by allowing transfer of spin angular momentum to a whole new
class of materials. In a seminal letter to Nature [5], Kajiwara et al. showed
that by depositing Platinum (Pt, a normal metal) on top of a 1.3 m thick
Yttrium Iron Garnet (YIG, a magnetic insulator), one could effectively transfer
spin angular momentum through the interface between these two different
materials. The outstanding feature was the detection of auto-oscillation of the
YIG when enough dc current was passed in the Pt. This finding has created a
great excitement in the community for two reasons: first, one could control
electronically the damping of insulators, which can offer improved properties
compared to metals, and here YIG has the lowest damping known in nature;
second, the damping compensation could be achieved on very large objects, a
particularly relevant point for the field of magnonics [6,7] whose aim is to
use spin-waves as carriers of information. However, the degree of coherence of
the observed auto-oscillations has not been addressed in ref. [5]. In this
work, we emphasize the key role of quasi-degenerate spin-wave modes, which
increase the threshold current. This requires to reduce both the thickness and
lateral size in order to reach full damping compensation [8] , and we show
clear evidence of coherent spin-orbit torque induced auto-oscillation in
micron-sized YIG discs of thickness 20 nm
Degenerate and non-degenerate parametric excitation in YIG nanostructures
We study experimentally the processes of parametric excitation in microscopic
magnetically saturated disks of nanometer-thick Yttrium Iron Garnet. We show
that, depending on the relative orientation between the parametric pumping
field and the static magnetization, excitation of either degenerate or
non-degenerate magnon pairs is possible. In the latter case, which is
particularly important for applications associated with the realization of
computation in the reciprocal space, a single-frequency pumping can generate
pairs of magnons whose frequencies correspond to different eigenmodes of the
disk. We show that, depending on the size of the disk and the modes involved,
the frequency difference in a pair can vary in the range 0.1-0.8 GHz. We
demonstrate that in this system, one can easily realize a practically important
situation where several magnon pairs share the same mode. We also observe the
simultaneous generation of up to six different modes using a fixed-frequency
monochromatic pumping. Our experimental findings are supported by numerical
calculations that allow us to unambiguously identify the excited modes. Our
results open new possibilities for the implementation of reciprocal-space
computing making use of low damping magnetic insulators.Comment: 18 pages, 4 figure
Graphene-passivated nickel as an oxidation-resistant electrode for spintronics.
We report on graphene-passivated ferromagnetic electrodes (GPFE) for spin devices. GPFE are shown to act as spin-polarized oxidation-resistant electrodes. The direct coating of nickel with few layer graphene through a readily scalable chemical vapor deposition (CVD) process allows the preservation of an unoxidized nickel surface upon air exposure. Fabrication and measurement of complete reference tunneling spin valve structures demonstrate that the GPFE is maintained as a spin polarizer and also that the presence of the graphene coating leads to a specific sign reversal of the magneto-resistance. Hence, this work highlights a novel oxidation-resistant spin source which further unlocks low cost wet chemistry processes for spintronics devices.R.S.W. acknowledges funding from EPSRC
(Doctoral training award). S.H. acknowledges funding from ERC
Grant InsituNANO (Project Reference 279342). P.S. acknowledges
the Institut Universitaire de France for junior fellowship
support. This research was partially supported by the EU FP7
work programme under Grant GRAFOL (Project Reference
285275).This is the accepted manuscript. The final version is available from ACS at http://pubs.acs.org/doi/abs/10.1021/nn304424x
Optical Frequency Up-Conversion of the Ferromagnetic Resonance in an Ultrathin Garnet Mediated by Magnetoelastic Coupling
We perform ultrafast pump-probe measurements on a nanometer-thick crystalline Bi-doped yttrium iron garnet film with perpendicular magnetic anisotropy. Tuning the photon energy of the pump laser pulses above and below the material’s band gap, we trigger ultrafast optical and spin dynamics via both one- and two-photon absorption. Contrary to the common scenario, the optically induced excitation induces an increase up to 20% of the ferromagnetic resonance frequency of the material. We explain this unexpected result in terms of a modification of the magnetic anisotropy caused by a long-lived photo-induced strain, which transiently and reversibly modifies the magnetoelastic coupling in the material. Our results disclose the possibility to optically increase the magnetic eigenfrequency in nanometer-thick magnets.publishe
Observation of Spin Seebeck Effect in nanometer thick YIG/Pt stripe
Trabajo presentado en la 61st Annual Conference on Magnetism and Magnetic Materials, celebrada en New Orleans (Lousiana, EE.UU), del 31 de octubre al 4 de noviembre de 2016Peer reviewe