36,108 research outputs found
Is perpendicular magnetic anisotropy essential to all-optical ultrafast spin reversal in ferromagnets?
All-optical spin reversal presents a new opportunity for spin manipulations,
free of a magnetic field. Most of all-optical-spin-reversal ferromagnets are
found to have a perpendicular magnetic anisotropy (PMA), but it has been
unknown whether PMA is necessary for the spin reversal. Here we theoretically
investigate magnetic thin films with either PMA or in-plane magnetic anisotropy
(IMA). Our results show that the spin reversal in IMA systems is possible, but
only with a longer laser pulse and within a narrow laser parameter region. The
spin reversal does not show a strong helicity dependence where the left- and
right-circularly polarized light lead to the identical results. By contrast,
the spin reversal in PMA systems is robust, provided both the spin angular
momentum and laser field are strong enough while the magnetic anisotropy itself
is not too strong. This explains why experimentally the majority of all-optical
spin-reversal samples are found to have strong PMA and why spins in Fe
nanoparticles only cant out of plane. It is the laser-induced spin-orbit torque
that plays a key role in the spin reversal. Surprisingly, the same spin-orbit
torque results in laser-induced spin rectification in spin-mixed configuration,
a prediction that can be tested experimentally. Our results clearly point out
that PMA is essential to the spin reversal, though there is an opportunity for
in-plane spin reversal.Comment: 20 pages, 4 figures and one tabl
Switching ferromagnetic spins by an ultrafast laser pulse: Emergence of giant optical spin-orbit torque
Faster magnetic recording technology is indispensable to massive data storage
and big data sciences. {All-optical spin switching offers a possible solution},
but at present it is limited to a handful of expensive and complex rare-earth
ferrimagnets. The spin switching in more abundant ferromagnets may
significantly expand the scope of all-optical spin switching. Here by studying
40,000 ferromagnetic spins, we show that it is the optical spin-orbit torque
that determines the course of spin switching in both ferromagnets and
ferrimagnets. Spin switching occurs only if the effective spin angular momentum
of each constituent in an alloy exceeds a critical value. Because of the strong
exchange coupling, the spin switches much faster in ferromagnets than
weakly-coupled ferrimagnets. This establishes a paradigm for all-optical spin
switching. The resultant magnetic field (65 T) is so big that it will
significantly reduce high current in spintronics, thus representing the
beginning of photospintronics.Comment: 12 page2, 6 figures. Accepted to Europhysics Letters (2016). Extended
version with the supplementary information. Contribution from Indiana State
University,Europhysics Letters (2016
Laser-induced spin protection and switching in a specially designed magnetic dot: A theoretical investigation
Most laser-induced femtosecond magnetism investigations are done in magnetic
thin films. Nanostructured magnetic dots, with their reduced dimensionality,
present new opportunities for spin manipulation. Here we predict that if a
magnetic dot has a dipole-forbidden transition between the lowest occupied
molecular orbital (LUMO) and the highest unoccupied molecular orbital (HOMO),
but a dipole-allowed transition between LUMO+1 and HOMO, electromagnetically
inducedtransparency can be used to prevent ultrafast laser-induced spin
momentum reduction, or spin protection. This is realized through a strong dump
pulse to funnel the population into LUMO+1. If the time delay between the pump
and dump pulses is longer than 60 fs, a population inversion starts and spin
switching is achieved. Thesepredictions are detectable experimentally.Comment: 6 pages, three figur
Instability of nonminimally coupled scalar fields in the spacetime of slowly rotating compact objects
Nonminimally coupled free scalar fields may be unstable in the spacetime of
compact objects. Such instability can be triggered by classical seeds or, more
simply, by quantum fluctuations giving rise to the so-called {\em vacuum
awakening effect}. Here, we investigate how the parameter space which
characterizes the instability is affected when the object gains some rotation.
For this purpose, we focus on the stability analysis of nonminimally coupled
scalar fields in the spacetime of slowly spinning matter shells.Comment: 11 pages, 6 figure
Quantum versus classical instability of scalar fields in curved backgrounds
General-relativistic stable spacetimes can be made unstable under the
presence of certain nonminimally coupled free scalar fields. In this paper, we
analyze the evolution of linear scalar-field perturbations in spherically
symmetric spacetimes and compare the classical stability analysis with a
recently discussed quantum field one. In particular, it is shown that vacuum
fluctuations lead to natural seeds for the unstable phase, whereas in the
classical framework the presence of such seeds in the initial conditions must
be assumed.Comment: 5 pages, 1 figure; condensed and revised version matching published
on
Magnetic spin moment reduction in photoexcited ferromagnets through exchange interaction quenching: Beyond the rigid band approximation
The exchange interaction among electrons is one of the most fundamental
quantum mechanical interactions in nature and underlies any magnetic phenomena
from ferromagnetic ordering to magnetic storage. The current technology is
built upon a thermal or magnetic field, but a frontier is emerging to directly
control magnetism using ultrashort laser pulses. However, little is known about
the fate of the exchange interaction. Here we report unambiguously that
photoexcitation is capable of quenching the exchange interaction in all three
ferromagnetic metals. The entire process starts with a small number of
photoexcited electrons which build up a new and self-destructive potential that
collapses the system into a new state with a reduced exchange splitting. The
spin moment reduction follows a Bloch-like law as , where is
the absorbed photon energy and is a scaling exponent. A good agreement
is found between the experimental and our theoretical results. Our findings may
have a broader implication for dynamic electron correlation effects in
laser-excited iron-based superconductors, iron borate, rare-earth
orthoferrites, hematites and rare-earth transition metal alloys.Comment: 16 pages, 3 figures, one supplementary material fil
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