312 research outputs found
Laser induced THz emission from femtosecond photocurrents in Co/ZnO/Pt and Co/Cu/Pt multilayers
The ultrashort laser excitation of Co/Pt magnetic heterostructures can
effectively generate spin and charge currents at the interfaces between
magnetic and nonmagnetic layers. The direction of these photocurrents can be
controlled by the helicity of the circularly polarized laser light and an
external magnetic field. Here, we employ THz time-domain spectroscopy to
investigate further the role of interfaces in these photo-galvanic phenomena.
In particular, the effects of either Cu or ZnO interlayers on the photocurrents
in Co/X/Pt (X = Cu, ZnO) have been studied by varying the thickness of the
interlayers up to 5 nm. The results are discussed in terms of spin-diffusion
phenomena and interfacial spin-orbit torque.Comment: 15 pages, 6 figures, 2 table
H-T Phase Diagram of Rare-Earth -- Transition Metal Alloy in the Vicinity of the Compensation Point
Anomalous hysteresis loops of ferrimagnetic amorphous alloys in high magnetic
field and in the vicinity of the compensation temperature have so far been
explained by sample inhomogeneities. We obtain H-T magnetic phase diagram for
ferrimagnetic GdFeCo alloy using a two-sublattice model in the paramagnetic
rare-earth ion approximation and taking into account rare-earth (Gd) magnetic
anisotropy. It is shown that if the magnetic anisotropy of the -sublattice
is larger than that of the -sublattice, the tricritical point can be at
higher temperature than the compensation point. The obtained phase diagram
explains the observed anomalous hysteresis loops as a result of high-field
magnetic phase transition, the order of which changes with temperature. It also
implies that in the vicinity of the magnetic compensation point the shape of
magnetic hysteresis loop is strongly temperature dependent.Comment: 8 pages, 3 figure
Supervised learning of an opto-magnetic neural network with ultrashort laser pulses
The explosive growth of data and its related energy consumption is pushing
the need to develop energy-efficient brain-inspired schemes and materials for
data processing and storage. Here, we demonstrate experimentally that Co/Pt
films can be used as artificial synapses by manipulating their magnetization
state using circularly-polarized ultrashort optical pulses at room temperature.
We also show an efficient implementation of supervised perceptron learning on
an opto-magnetic neural network, built from such magnetic synapses.
Importantly, we demonstrate that the optimization of synaptic weights can be
achieved using a global feedback mechanism, such that the learning does not
rely on external storage or additional optimization schemes. These results
suggest there is high potential for realizing artificial neural networks using
optically-controlled magnetization in technologically relevant materials, that
can learn not only fast but also energy-efficient.Comment: 9 pages, 4 figure
Selection Rules for All-Optical Magnetic Recording in Iron Garnet
Finding an electronic transition a subtle excitation of which can launch
dramatic changes of electric, optical or magnetic properties of media is one of
the long-standing dreams in the field of photo-induced phase transitions [1-5].
Therefore the discovery of the magnetization switching only by a femtosecond
laser pulse [6-10] triggered intense discussions about mechanisms responsible
for these laser-induced changes. Here we report the experimentally revealed
selection rules on polarization and wavelengths of ultrafast photo-magnetic
recording in Co-doped garnet film and identify the workspace of the parameters
(magnetic damping, wavelength and polarization of light) allowing this effect.
The all-optical magnetic switching under both single pulse and multiple-pulse
sequences can be achieved at room temperature, in narrow spectral ranges with
light polarized either along or crystallographic axes of the
garnet. The revealed selection rules indicate that the excitations responsible
for the coupling of light to spins are d-electron transitions in octahedral and
tetrahedral Co-sublattices, respectively
Laser-driven quantum magnonics and THz dynamics of the order parameter in antiferromagnets
The impulsive generation of two-magnon modes in antiferromagnets by
femtosecond optical pulses, so-called femto-nanomagnons, leads to coherent
longitudinal oscillations of the antiferromagnetic order parameter that cannot
be described by a thermodynamic Landau-Lifshitz approach. We argue that this
dynamics is triggered as a result of a laser-induced modification of the
exchange interaction. In order to describe the oscillations we have formulated
a quantum mechanical description in terms of magnon pair operators and coherent
states. Such an approach allowed us to} derive an effective macroscopic
equation of motion for the temporal evolution of the antiferromagnetic order
parameter. An implication of the latter is that the photo-induced spin dynamics
represents a macroscopic entanglement of pairs of magnons with femtosecond
period and nanometer wavelength. By performing magneto-optical pump-probe
experiments with 10 femtosecond resolution in the cubic KNiF and the
uniaxial KNiF collinear Heisenberg antiferromagnets, we observed
coherent oscillations at the frequency of 22 THz and 16 THz, respectively. The
detected frequencies as a function of the temperature ideally fit the
two-magnon excitation up to the N\'eel point. The experimental signals are
described as dynamics of magnetic linear dichroism due to longitudinal
oscillations of the antiferromagnetic vector.Comment: 25 pages, 10 figure
High Field Anomalies of Equilibrium and Ultrafast Magnetism in Rare-Earth-Transition Metal Ferrimagnets
Magneto-optical spectroscopy in fields up to 30 Tesla reveals anomalies in
the equilibrium and ultrafast magnetic properties of the ferrimagnetic
rare-earth-transition metal alloy TbFeCo. In particular, in the vicinity of the
magnetization compensation temperature, each of the magnetizations of the
antiferromagnetically coupled Tb and FeCo sublattices show triple hysteresis
loops. Contrary to state-of-the-art theory, which explains such loops by sample
inhomogeneities, here we show that they are an intrinsic property of the
rare-earth ferrimagnets. Assuming that the rare-earth ions are paramagnetic and
have a non-zero orbital momentum in the ground state and, therefore, a large
magnetic anisotropy, we are able to reproduce the experimentally observed
behavior in equilibrium. The same theory is also able to describe the
experimentally observed critical slowdown of the spin dynamics in the vicinity
of the magnetization compensation temperature, emphasizing the role played by
the orbital momentum in static and ultrafast magnetism of ferrimagnets
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