arXiv subjects

Morteza Mohseni

Publications and source records attributed to Morteza Mohseni.

At least 19 recordsLinked to original sources

arXiv2026

Spinning test particles in the spacetime of a global monopole

We investigate the motion of spinning test particles in the spacetime of a global monopole in the framework of the Mathisson-Papapetrou-Dixon equations. By making use of the symmetries of the spacetime, we obtain a general exact solution to the equations of motion. We show that the particle's trajectories, momenta, and spin can be expressed in terms of three specific functions of the polar and azimuthal angles. We also show that the system is completely integrable. We obtain the general non-geodesic trajectory of the particle, and also examine the particular cases of radial and planar motion. We compare the non-geodesic trajectories of a spinning particle with a non-spinning particle.

gr-qc↗

arXiv2025

Interaction of exact gravitational waves with matter

We consider interactions of exact (i.e., solutions of full nonlinear field equations) gravitational waves with matter by using the Einstein-Boltzmann equation. For a gravitational wave interacting with a system of massless particles, we compute the perturbed energy-momentum tensor and obtain explicit form of a set of Einstein-Boltzmann equations. We find solution to this system of equations to obtain the gravitational wave profile. The interaction superposes a static term on the gravitational wave profile which depends on the difference between square of the temperatures of the system in the absence and in the presence of the wave. We compute this perturbed term when the states of the system obey Bose-Einstein, Fermi-Dirac, and Maxwell-Boltzmann distributions, respectively. The relative strength of this term is roughly half for the Fermi-Dirac, and one-third for the Maxwell-Boltzmann distributions compared with that of the Bose-Einstein distribution. We consider both Minkowski and flat Friedmann-Robertson-Walker backgrounds.

gr-qc↗

arXiv2024

On optical appearance of Einstein-Maxwell-Æther black holes surrounded by various accretions

In this paper, we investigate the effects of the æther field and the electric charge on the observed shadow of two types of charged black holes in the Einstein-Maxwell-Æther theory. By considering that the Einstein-Maxwell-Æther black holes surrounded by the static/infalling spherical accretion flows, as well as an optically and geometrically thin disk accretion flow, we study the shadow luminosities and the observed specific intensity of the image for these various profiles of accretion flows. We find that in the thin disk accretion model the location and the emitted model of the accretion gas affect on the optical appearance of charged Einstein-Æther black holes in contrast to the spherical accretion flows. For a thin disk profile, we show that the observer will receive more intensity for an emitted model as Gaussian function when the innermost radiation radius lies in the innermost stable circular orbit. Finally, comparing the results of the charged Einstein-Æther black holes with neutral Einstein-Æther black holes, we show that the charged Einstein-Æther black holes have smaller dark area, whereas wider lensed ring and photon ring. Comparing the charged Einstein-Æther black holes with Reissner-Nordstrom black hole gives a reverse effect.

gr-qc↗

arXiv2023

Deep learning in bifurcations of particle trajectories

We show that deep learning algorithms can be deployed to study bifurcations of particle trajectories. We demonstrate this for two physical systems, the unperturbed Duffing equation and charged particles in magnetic reversal by using the AI Poincaré algorithm. We solve the equations of motion by using a fourth-order Runge-Kutta method to generate a dataset for each system. We use a deep neural network to train the data. A noise characterized by a noise scale L is added to data during the training. By using a principal component analysis, we compute the explained variance ratios for these systems which depend on the noise scale. By plotting explained ratios against the noise scale, we show that they change at bifurcations. For different values of the Duffing equation parameters, these changes are of the form of different patterns of growth-decline of the explained ratios. For the magnetic reversal, the changes are of the form of a change in the number of principal components. We comment on the use of this technique for other dynamical systems with bifurcations.

physics.comp-ph↗

arXiv2022

Parametric Excitation and Instabilities of Spin Waves driven by Surface Acoustic Waves

The parametric excitation of spin waves by coherent surface acoustic waves is demonstrated experimentally in metallic magnetic thin film structures. The involved magnon modes are analyzed with micro-focused Brillouin light scattering spectroscopy and complementary micromagnetic simulations combined with analytical modelling are used to determine the origin of the spin-wave instabilities. Depending on the experimental conditions, we observe spin-wave instabilities originating from different phonon-magnon and magnon-magnon scattering processes. Our results demonstrate that an efficient excitation of high amplitude, strongly nonlinear magnons in metallic ferromagnets is possible by surface acoustic waves, which opens novel ways to create micro-scaled nonlinear magnonic systems for logic and data processing that can profit from the high excitation efficiency of phonons using piezoelectricity.

cond-mat.other↗

arXiv2022

Parametric generation of spin waves in nano-scaled magnonic conduits

The research feld of magnonics proposes a low-energy wave-logic computation technology based on spin waves to complement the established CMOS technology and provide a basis for emerging unconventional computation architectures. However, magnetic damping is a limiting factor for all-magnonic logic circuits and multi-device networks, ultimately rendering mechanisms to effciently manipulate and amplify spin waves a necessity. In this regard, parallel pumping is a versatile tool since it allows to selectively generate and amplify spin waves. While extensively studied in microscopic systems, nano-scaled systems are lacking investigation to assess the feasibility and potential future use of parallel pumping in magnonics. Here, we investigate a longitudinally magnetized 100 nm-wide magnonic nano-conduit using space and time-resolved micro-focused Brillouin-light-scattering spectroscopy. Employing parallel pumping to generate spin waves, we observe that the non-resonant excitation of dipolar spin waves is favored over the resonant excitation of short wavelength exchange spin waves. In addition, we utilize this technique to access the effective spin-wave relaxation time of an individual nano-conduit, observing a large relaxation time up to (115.0 +- 7.6) ns. Despite the significant decrease of the pumping effciency in the investigated nano-conduit, a reasonably small threshold is found rendering parallel pumping feasible on the nano-scale.

cond-mat.mes-hall↗

arXiv2022

Fully Resonant Magneto-elastic Spin-wave Excitation by Surface Acoustic Waves under Conservation of Energy and Linear Momentum

We report on the resonant excitation of spin waves in micro-structured magnetic thin films by surface acoustic waves (SAWs). The spin waves as well as the acoustic waves are studied by micro-focused Brillouin light scattering spectroscopy. Besides the excitation of the ferromagnetic resonance, a process which does not fulfill momentum conservation, also the excitation of finite-wavelength spin waves can be observed at low magnetic fields. Using micromagnetic simulations, we verify that during this excitation both energy and linear momentum are conserved and fully transferred from the SAW to the spin wave.

cond-mat.other↗

arXiv2022

Classical analog of qubit logic based on a magnon Bose-Einstein condensate

We present a classical version of several quantum bit (qubit) functionalities using a two-component magnon Bose-Einstein condensate formed at opposite wavevectors in a room-temperature yttrium-iron-garnet ferrimagnetic film. The macroscopic wavefunctions of these two condensates serve as orthonormal basis states that form a system being a classical counterpart of a single qubit. Solving the Gross-Pitaevskii equation and employing micromagnetic numerical simulations, we first show how to initialize the system in one of the basis states: the application of wavevector-selective parallel parametric pumping allows us to form only a single condensate in one of the two lowest energy states of the magnon gas. Next, by translating the concept of Rabi-oscillations into the wavevector domain, we demonstrate how to manipulate the magnon-BEC system along the polar axis in the Bloch sphere representation. We also discuss the manipulation regarding the azimuthal angle.

cond-mat.mes-hall↗

arXiv2021

Stabilization of a nonlinear bullet coexisting with a Bose-Einstein condensate in a rapidly cooled magnonic system driven by a spin-orbit torque

We have recently shown that injection of magnons into a magnetic dielectric via the spin-orbit torque (SOT) effect in the adjacent layer of a heavy metal subjected to the action of short (0.1 $μ$s) current pulses allows for control of a magnon Bose-Einstein Condensate (BEC). Here, the BEC was formed in the process of rapid cooling (RC), when the electric current heating the sample is abruptly terminated. In the present study, we show that the application of a longer (1.0 $μ$s) electric current pulse triggers the formation of a nonlinear localized magnonic bullet below the linear magnon spectrum. After pulse termination, the magnon BEC, as before, is formed at the bottom of the linear spectrum, but the nonlinear bullet continues to exist, stabilized for additional 30 ns by the same process of RC-induced magnon condensation. Our results suggest that a stimulated condensation of excess magnons to all highly populated magnonic states occurs.

cond-mat.quant-gas↗

arXiv2021

Nonlinear dynamics of topological ferromagnetic textures for frequency multiplication

We propose that the non-linear radio-frequency dynamics and nanoscale size of topological magnetic structures associated to their well-defined internal modes advocate for their use as in-materio scalable frequency multipliers for spintronic systems. Frequency multipliers allow for frequency conversion between input and output frequencies, and thereby significantly increase the range of controllably accessible frequencies. In particular, we explore the excitation of eigenmodes of topological magnetic textures by fractions of the corresponding eigenfrequencies. We show via micromagnetic simulations that low-frequency perturbations to the system can efficiently excite bounded modes with a higher amplitude. For example, we excited the eigenmodes of isolated ferromagnetic skyrmions by applying half, a third and a quarter of the corresponding eigenfrequency. We predict that the frequency multiplication via magnetic structures is a general phenomenon which is independent of the particular properties of the magnetic texture, and works also for magnetic vortices, droplets and other topological textures.

cond-mat.mes-hall↗

arXiv2021

Tuning the dynamics of magnetic droplet solitons using dipolar interactions

Magnetic droplets are dissipative magnetodynamical solitons that can form under current driven nanocontacts in magnetic layers with large perpendicular magnetic anisotropy. Here, we extend the original droplet theory by studying the impact of the dipolar interactions on the dynamics of droplet solitons. By varying the thickness of the free layer of a spin torque nano-oscillator, we systematically tune the internal field of the free layer to investigate the dynamics of droplet solitons. Our numerical results show that increasing the free layer thickness increases the droplet threshold current, decreases the droplet frequency and diameter, enlarges the current hysteresis and also modifies the structure of the droplet. The Oersted field of the current breaks the phase coherency and deteriorates the stability of the droplet in free layers with larger thicknesses. Moreover, our findings show a simple relation to determine the impact of the free layer thickness on the droplet nucleation boundaries. Our study presents the missing brick on the physics behind magnetic droplet solitons, and further illustrates that magnetic droplets in thinner layers possess more promising characteristics for spintronic applications and enable devices with higher speed of operation.

physics.app-ph↗

arXiv2020

A nonlinear magnonic nano-ring resonator

The field of magnonics, which aims at using spin waves as carriers in data processing devices, has attracted increasing interest in recent years. We present and study micromagnetically a nonlinear nanoscale magnonic ring resonator device for enabling implementations of magnonic logic gates and neuromorphic magnonic circuits. In the linear regime, this device efficiently suppresses spin-wave transmission using the phenomenon of critical resonant coupling, thus exhibiting the behavior of a notch filter. By increasing the spin-wave input power, the resonance frequency is shifted leading to transmission curves, depending on the frequency, reminiscent of the activation functions of neurons or showing the characteristics of a power limiter. An analytical theory is developed to describe the transmission curve of magnonic ring resonators in the linear and nonlinear regimes and validated by a comprehensive micromagnetic study. The proposed magnonic ring resonator provides a multi-functional nonlinear building block for unconventional magnonic circuits.

physics.app-ph↗

arXiv2020

Bose-Einstein Condensation of Nonequilibrium Magnons in Confined Systems

We study the formation of a room temperature magnon Bose-Einstein condensate (BEC) in nanoscopic systems and demonstrate that its lifetime is influenced by the spatial confinement. We predict how dipolar interactions and nonlinear magnon scattering assist in the generation of a metastable magnon BEC in energy-quantized nanoscopic devices. We verify our prediction by a full numerical simulation of the Landau-Lifshitz-Gilbert equation and demonstrate the generation of magnon BEC in confined insulating magnets of yttrium iron garnet. We directly map out the nonlinear magnon scattering processes behind this phase transition to show how fast quantized thermalization channels allow the BEC formation in confined structures. Based on our results, we discuss a new mechanism to manipulate the BEC lifetime in nanoscaled systems. Our study greatly extends the freedom to study the dynamics of magnon BEC in realistic systems and to design integrated circuits for BEC-based applications at room temperature.

cond-mat.mes-hall↗

arXiv2020

Controlling the propagation of dipole-exchange spin waves using local inhomogeneity of the anisotropy

Spin waves are promising candidates to carry, transport, and process information. Controlling the propagation characteristics of spin waves in magnetic materials is an essential ingredient for designing spin-wave based computing architectures. Here, we study the influence of surface inhomogeneities on the spin-wave signals transmitted through thin films. We use micromagnetic simulations to study the spin-wave dynamics in an in-plane magnetized yttrium iron garnet thin film with a thickness in the nanometre range in the presence of surface defects in the form of locally introduced uniaxial anisotropies. These defects are used to demonstrate that the Backward Volume Magnetostatic Spin Waves (BVMSW) are more responsive to backscattering in comparison to Magnetostatic Surface Spin Waves (MSSWs). For this particular defect type, the reason for this behavior can be quantitatively related to the difference in the magnon band structures for the two types of spin waves. To demonstrate this, we develop a quasi-analytical theory for the scattering process. It shows an excellent agreement with the micromagnetic simulations, sheds light on the backscattering processes and provides a new way to analyze the spin-wave transmission rates in the presence of surface inhomogeneities in sufficiently thin films, for which the role of exchange energy in the spin-wave dynamics is significant. Our study paves the way to designing magnonic logic devices for data processing which rely on a designed control of the spin-wave transmission.

physics.app-ph↗

arXiv2020

Propagating magnetic droplet solitons as moveable nanoscale spin-wave sources with tunable direction of emission

Magnetic droplets are strongly nonlinear and localized spin-wave solitons that can be formed in current-driven nanocontacts. Here, we propose a simple way to launch droplets in an inhomogeneous nanoscopic waveguide. We use the drift motion of a droplet and show that in a system with broken translational symmetry, the droplet acquires a linear momentum and propagates. We find that the droplet velocity can be tuned via the strength of the break in symmetry and the size of the nanocontact. In addition, we demonstrate that the launched droplet can propagate up to several micrometers in a realistic system with reasonable damping. Finally, we demonstrate how an annihilating droplet delivers its momentum to a highly nonreciprocal spin-wave burst with a tunable wave vector with nanometer wavelengths. Such a propagating droplet can be used as a moveable spin-wave source in nanoscale magnonic networks. The presented method enables full control of the spin-wave emission direction, which can largely extend the freedom to design integrated magnonic circuits with a single spin-wave source.

cond-mat.mes-hall↗

arXiv2019

Slow-wave based magnonic diode

Spin waves, the collective excitations of the magnetic order parameter, and magnons, the associated quasiparticles, are envisioned as possible data carriers in future wave-based computing architectures. On the road towards spin-wave computing, the development of a diode-like device capable of transmitting spin waves in only one direction, thus allowing controlled signal routing, is an essential step. Here, we report on the design and experimental realization of a microstructured magnonic diode in a ferromagnetic bilayer system. Effective unidirectional propagation of spin waves is achieved by taking advantage of nonreciprocities produced by dynamic dipolar interactions in transversally magnetized media, which lack symmetry about their horizontal midplane. More specifically, dipolar-induced nonreciprocities are used to engineer the spin-wave dispersion relation of the bilayer system so that the group velocity is reduced to very low values for one direction of propagation, and not for the other, thus producing unidirectional slow spin waves. Brillouin light scattering and propagating spin-wave spectroscopy are used to demonstrate the diode-like behavior of the device, the composition of which was previously optimized through micromagnetic simulations. simulations.

physics.app-ph↗

arXiv2019

Nanoscale spin-wave wake-up receiver

We present the concept of a passive spin-wave device which is able to distinguish different radio-frequency pulse trains and validate its functionality using micromagnetic simulations. The information is coded in the phase of the individual pulses which are transformed into spin-wave packets. The device splits every incoming packet into two arms, one of which is coupled to a magnonic ring which introduces a well-defined time delay and phase shift. Since the time delay is matched to the pulse repetition rate, adjacent packets interfere in a combiner which makes it possible to distinguish simple pulse train patterns by the read-out of the time-integrated spin-wave intensity in the output. Due to its passive construction, this device may serve as an energy-efficient wake-up receiver used to activate the main receiver circuit in power critical IoT applications.

physics.app-ph↗

arXiv2018

Magnetic Droplet Soliton Nucleation in Oblique Fields

We study the auto-oscillating magnetodynamics in orthogonal spin torque nano-oscillators (STNOs) as a function of the out-of-plane (OOP) magnetic field angle. In perpendicular fields and at OOP field angles down to approximately 50 degrees we observe the nucleation of a droplet. However, for field angles below 50 degrees, experiments indicate that the droplet gives way to propagating spin waves, in agreement with our micromagnetic simulations. Theoretical calculations show that the physical mechanism behind these observations is the sign changing of spin-wave nonlinearity (SWN) by angle. In addition, we show that the presence of a strong perpendicular magnetic anisotropy (PMA) free layer in the system reverses the angular dependence of the SWN and dynamics in STNOs with respect to the known behavior determined for the in-plane magnetic anisotropy free layer. Our results are of fundamental interest in understanding the rich dynamics of nanoscale solitons and spin-wave dynamics in STNOs.

cond-mat.mes-hall↗