1,087 research outputs found
A high-temperature expansion method for calculating paramagnetic exchange interactions
The method for calculating the isotropic exchange interactions in the
paramagnetic phase is proposed. It is based on the mapping of the
high-temperature expansion of the spin-spin correlation function calculated for
the Heisenberg model onto Hubbard Hamiltonian one. The resulting expression for
the exchange interaction has a compact and transparent formulation. The quality
of the calculated exchange interactions is estimated by comparing the
eigenvalue spectra of the Heisenberg model and low-energy magnetic part of the
Hubbard model. By the example of quantum rings with different hopping setups we
analyze the contributions from the different part of the Hubbard model spectrum
to the resulting exchange interaction.Comment: 8 pages, 8 figure
Neural network agent playing spin Hamiltonian games on a quantum computer
Quantum computing is expected to provide new promising approaches for solving
the most challenging problems in material science, communication, search,
machine learning and other domains. However, due to the decoherence and gate
imperfection errors modern quantum computer systems are characterized by a very
complex, dynamical, uncertain and fluctuating computational environment. We
develop an autonomous agent effectively interacting with such an environment to
solve magnetism problems. By using the reinforcement learning the agent is
trained to find the best-possible approximation of a spin Hamiltonian ground
state from self-play on quantum devices. We show that the agent can learn the
entanglement to imitate the ground state of the quantum spin dimer. The
experiments were conducted on quantum computers provided by IBM. To compensate
the decoherence we use local spin correction procedure derived from a general
sum rule for spin-spin correlation functions of a quantum system with even
number of antiferromagnetically-coupled spins in the ground state. Our study
paves a way to create a new family of the neural network eigensolvers for
quantum computers.Comment: Local spin correction procedure was used to compensate real device
errors; comparison with variational approach was adde
Monte Carlo study of magnetic nanoparticles adsorbed on halloysite nanotubes
We study properties of magnetic nanoparticles adsorbed on the halloysite
surface. For that a distinct magnetic Hamiltonian with random distribution of
spins on a cylindrical surface was solved by using a nonequilibrium Monte Carlo
method. The parameters for our simulations: anisotropy constant, nanoparticle
size distribution, saturated magnetization and geometrical parameters of the
halloysite template were taken from recent experiments. We calculate the
hysteresis loops and temperature dependence of the zero field cooling (ZFC)
susceptibility, which maximum determines the blocking temperature. It is shown
that the dipole-dipole interaction between nanoparticles moderately increases
the blocking temperature and weakly increases the coercive force. The obtained
hysteresis loops (e.g., the value of the coercive force) for Ni nanoparticles
are in reasonable agreement with the experimental data. We also discuss the
sensitivity of the hysteresis loops and ZFC susceptibilities to the change of
anisotropy and dipole-dipole interaction, as well as the 3d-shell occupation of
the metallic nanoparticles; in particular we predict larger coercive force for
Fe, than for Ni nanoparticles.Comment: 10 pages, 12 figure
Bimeron nanoconfined design
We report on the stabilization of the topological bimeron excitations in
confined geometries. The Monte Carlo simulations for a ferromagnet with a
strong Dzyaloshinskii-Moriya interaction revealed the formation of a mixed
skyrmion-bimeron phase. The vacancy grid created in the spin lattice
drastically changes the picture of the topological excitations and allows one
to choose between the formation of a pure bimeron and skyrmion lattice. We
found that the rhombic plaquette provides a natural environment for
stabilization of the bimeron excitations. Such a rhombic geometry can protect
the topological state even in the absence of the magnetic field.Comment: 5 pages, 7 figure
Profile approach for recognition of three-dimensional magnetic structures
We propose an approach for low-dimensional visualisation and classification
of complex topological magnetic structures formed in magnetic materials. Within
the approach one converts a three-dimensional magnetic configuration to a
vector containing the only components of the spins that are parallel to the z
axis. The next crucial step is to sort the vector elements in ascending or
descending order. Having visualized profiles of the sorted spin vectors one can
distinguish configurations belonging to different phases even with the same
total magnetization. For instance, spin spiral and paramagnetic states with
zero total magnetic moment can be easily identified. Being combined with a
simplest neural network our profile approach provides a very accurate phase
classification for three-dimensional magnets characterized by complex
multispiral states even in the critical areas close to phases transitions. By
the example of the skyrmionic configurations we show that profile approach can
be used to separate the states belonging to the same phase
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