31 research outputs found
Spin relaxation signature of colossal magnetic anisotropy in platinum atomic chains
Recent experimental data demonstrate emerging magnetic order in platinum
atomically thin nanowires. Furthermore, an unusual form of magnetic anisotropy
-- colossal magnetic anisotropy (CMA) -- was earlier predicted to exist in
atomically thin platinum nanowires. Using spin dynamics simulations based on
first-principles calculations, we here explore the spin dynamics of atomically
thin platinum wires to reveal the spin relaxation signature of colossal
magnetic anisotropy, comparing it with other types of anisotropy such as
uniaxial magnetic anisotropy (UMA). We find that the CMA alters the spin
relaxation process distinctly and, most importantly, causes a large speed-up of
the magnetic relaxation compared to uniaxial magnetic anisotropy. The magnetic
behavior of the nanowire exhibiting CMA should be possible to identify
experimentally at the nanosecond time scale for temperatures below 5 K. This
time-scale is accessible in e.g., soft x-ray free electron laser experiments.Comment: 9 pages, 3 figure
The effect of confinement and defects on the thermal stability of skyrmions
The stability of magnetic skyrmions against thermal fluctuations and external
perturbations is investigated within the framework of harmonic transition state
theory for magnetic degrees of freedom. The influence of confined geometry and
atomic scale non-magnetic defects on the skyrmion lifetime is estimated. It is
shown that a skyrmion on a track has lower activation energy for annihilation
and higher energy for nucleation if the size of the skyrmion is comparable with
the width of the track. Two mechanisms of skyrmion annihilation are considered:
inside the track and escape through the boundary. For both mechanisms, the
dependence of activation energy on the track width is calculated. Non-magnetic
defects are found to localize skyrmions in their neighborhood and strongly
decrease the activation energy for creation and annihilation. This is in
agreement with experimental measurements that have found nucleation of
skyrmions in presence of spin-polarized current preferably occurring near
structural defects
Tailed skyrmions -- an obscure branch of magnetic solitons
We report tailed skyrmions -- a new class of stable soliton solutions of the
2D chiral magnet model. Tailed skyrmions have elongated shapes and emerge in a
narrow range of fields near the transition between the spin spirals and the
saturated state. We analyze the stability range of these solutions in terms of
external magnetic field and magnetocrystalline anisotropy. Minimum energy paths
and the homotopies (continuous transitions) between tailed skyrmions of the
same topological charge have been calculated using the geodesic nudged elastic
bands method. The discovery of tailed skyrmions extends the diversity of
already-known solutions illustrated by complex morphology solitons, such as
tailed skyrmion bags with and without chiral kinks.Comment: 7 pages, 6 figure
Reduction of energy cost of magnetization switching in a biaxial nanoparticle by use of internal dynamics
A solution to energy-efficient magnetization switching in a nanoparticle with
biaxial anisotropy is presented. Optimal control paths minimizing the energy
cost of magnetization reversal are calculated numerically as functions of the
switching time and materials properties, and used to derive energy-efficient
switching pulses of external magnetic field. Hard-axis anisotropy reduces the
minimum energy cost of magnetization switching due to the internal torque in
the desired switching direction. Analytical estimates quantifying this effect
are obtained based on the perturbation theory. The optimal switching time
providing a tradeoff between fast switching and energy efficiency is obtained.
The energy cost of switching and the energy barrier between the stable states
can be controlled independently in a biaxial nanomagnet. This provides a
solution to the dilemma between energy-efficient writability and good thermal
stability of magnetic memory elements.Comment: 13 pages, 10 figure
Energy surface and lifetime of magnetic skyrmions
The stability of skyrmions in various environments is estimated by analyzing
the multidimensional surface describing the energy of the system as a function
of the directions of the magnetic moments in the system. The energy is given by
a Heisenberg-like Hamiltonian that includes Dzyaloshinskii-Moriya interaction,
anisotropy and external magnetic field. Local minima on this surface correspond
to the ferromagnetic and skyrmion states. Minimum energy paths (MEP) between
the minima are calculated using the geodesic nudged elastic band method. The
maximum energy along an MEP corresponds to a first order saddle point on the
energy surface and gives an estimate of the activation energy for the magnetic
transition, such as creation and annihilation of a skyrmion. The
pre-exponential factor in the Arrhenius law for the rate, the so-called attempt
frequency, is estimated within harmonic transition state theory where the
eigenvalues of the Hessian at the saddle point and the local minima are used to
characterize the shape of the energy surface. For some degrees of freedom,
so-called 'zero modes', the energy of the system remains invariant. They need
to be treated separately and give rise to temperature dependence of the attempt
frequency. As an example application of this general theory, the lifetime of a
skyrmion in a track of finite width for a PdFe overlayer on a Ir(111) substrate
is calculated as a function of track width and external magnetic field. Also,
the effect of non-magnetic impurities is studied. Various MEPs for annihilation
inside a track, via the boundary of a track and at an impurity are presented.
The attempt frequency as well as the activation energy has been calculated for
each mechanism to estimate the transition rate as a function of temperature
Thermal generation of droplet soliton in chiral magnet
Controlled creation of localized magnetic textures beyond conventional π-skyrmions is an important problem in the field of magnetism. Here, by means of spin dynamics simulations, Monte Carlo simulations, and harmonic transition state theory we demonstrate that an elementary chiral magnetic soliton with zero topological charge—the chiral droplet—can be created by thermal fluctuations in the presence of the tilted magnetic field. The proposed protocol relies on an unusual kinetics combining the effects of the entropic stabilization and low-energy barrier for the nucleation of a topologically trivial state. Following this protocol by varying temperature and the tilt of the external magnetic field, one can selectively generate chiral droplets or π-skyrmions in a single system. The coexistence of two distinct magnetic solitons establishes a basis for a rich magnetization dynamics and opens up the possibility for the construction of more complex magnetic textures such as skyrmion bags and skyrmions with chiral kinks