13 research outputs found
A deeper look into natural sciences with physics-based and data-driven measures
No full textWith the development of machine learning in recent years, it is possible to glean much more information from an experimental data set to study matter. In this perspective, we discuss some state-of-the-art data-driven tools to analyze latent effects in data and explain their applicability in natural science, focusing on two recently introduced, physics-motivated computationally cheap tools—latent entropy and latent dimension. We exemplify their capabilities by applying them on several examples in the natural sciences and show that they reveal so far unobserved features such as, for example, a gradient in a magnetic measurement and a latent network of glymphatic channels from the mouse brain microscopy data. What sets these techniques apart is the relaxation of restrictive assumptions typical of many machine learning models and instead incorporating aspects that best fit the dynamical systems at hand
Current-induced H-shaped-skyrmion creation and their dynamics in the helical phase
No full textInevitable for the basic principles of skyrmion racetrack-like applications is not only their confined motion along one-dimensional channels but also their controlled creation and annihilation. Helical magnets have been suggested to naturally confine the motion of skyrmions along the tracks formed by the helices, which also allow for high-speed skyrmion motion. We propose a protocol to create topological magnetic structures in a helical background. We furthermore analyse the stability and current-driven motion of the skyrmions in a helical background with in-plane uniaxial anisotropy fixing the orientation of the helices
Resonant excitation of vortex gyrotropic mode via surface acoustic waves
No full textFinding new energy-efficient methods for exciting magnetization dynamics is one of the key challenges in magnonics. In this work, we present an approach to excite the gyrotropic dynamics of magnetic vortices through the phenomenon of inverse magnetostriction, also known as the Villari effect. We develop an analytical model based on the Thiele formalism that describes the gyrotropic motion of the vortex core including the energy contributions due to inverse magnetostriction. Based on this model, we predict excitations of the vortex core resonances by surface acoustic waves whose frequency is resonant with the frequency of the vortex core. We verify the model's prediction using micromagnetic simulations and show the dependence of the vortex core's oscillation radius on the surface acoustic wave amplitude and the static bias field. Our study contributes to the advancement of energy-efficient magnetic excitations by relying on voltage-induced driven dynamics, which is an alternative to conventional current-induced excitations
Spin-Wave Driven Bidirectional Domain Wall Motion in Kagome Antiferromagnets
No full textWe predict a mechanism to controllably manipulate domain walls in kagome antiferromagnets via a single linearly polarized spin-wave source. We show by means of atomistic spin dynamics simulations of antiferromagnets with kagome structure that the speed and direction of the domain wall motion can be regulated by only tuning the frequency of the applied spin wave. Starting from microscopics, we establish an effective action and derive the corresponding equations of motion for the spin-wave-driven domain wall. Our analytical calculations reveal that the coupling of two spin-wave modes inside the domain wall explains the frequency-dependent velocity of the spin texture. Such a highly tunable spin-wave-induced domain wall motion provides a key component toward next-generation fast, energy-efficient, and Joule-heating-free antiferromagnetic insulator devices
Spin-transfer torque driven motion, deformation, and instabilities of magnetic skyrmions at high currents
No full textIn chiral magnets, localized topological magnetic whirls, magnetic skyrmions, can be moved by spin polarized electric currents. Upon increasing the current strength, with prospects for high-speed skyrmion motion for spintronics applications in mind, isolated skyrmions deform away from their typical circular shape. We analyze the influence of spin-transfer torques on the shape of a single skyrmion, including its stability upon adiabatically increasing the strength of the applied electric current. For rather compact skyrmions at uniaxial anisotropies well above the critical anisotropy for domain wall formation, we find for high current densities that the skyrmion assumes a noncircular shape with a tail, reminiscent of a shooting star. For larger and hence softer skyrmions close to the critical anisotropy, in turn, we observe a critical current density above which skyrmions become unstable. We show that above a second critical current density the shooting star solution can be recovered also for these skyrmions
Nonlinear Dynamics of Topological Ferromagnetic Textures for Frequency Multiplication
No full textWe propose that the nonlinear radio-frequency dynamics and nanoscale size of topological magnetic structures associated with their well-defined internal modes advocate 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 bound modes with a higher amplitude. For example, we excite the eigenmodes of isolated ferromagnetic skyrmions by applying half, a third, and a quarter of the corresponding eigenfrequency. We predict that frequency multiplication via magnetic structures is a general phenomenon that is independent of the particular properties of the magnetic texture and works for magnetic vortices, droplets, and other topological textures
Characterizing breathing dynamics of magnetic skyrmions and antiskyrmions within the Hamiltonian formalism
No full textWe derive an effective Hamiltonian system describing the low-energy dynamics of circular magnetic skyrmions and antiskyrmions. Using scaling and symmetry arguments, we model (anti)skyrmion dynamics through a finite set of coupled, canonically conjugated, collective coordinates. The resulting theoretical description is independent of both micromagnetic details as well as any specificity in the ansatz of the skyrmion profile. Based on the Hamiltonian structure, we derive a general description for breathing dynamics of (anti)skyrmions in the limit of radius much larger than the domain wall width. The effective energy landscape reveals two qualitatively different types of breathing behavior. For small energy perturbations, we reproduce the well-known small breathing mode excitations, where the magnetic moments of the skyrmion oscillate around their equilibrium solution. At higher energies, we find a breathing behavior where the skyrmion phase continuously precesses, transforming Néel to Bloch skyrmions and vice versa. For a damped system, we observe the transition from the continuously rotating and breathing skyrmion into the oscillatory one. We analyze the characteristic frequencies of both breathing types, as well as their amplitudes and distinct energy dissipation rates. For rotational (oscillatory) breathing modes, we predict on average a linear (exponential) decay in energy. We argue that this stark difference in dissipative behavior should be observable in the frequency spectrum of excited (anti)skyrmions
Towards a New Generation of Nonvolatile Memory Devices: Creation and Manipulation of Topological Magnetic Structures by Electric Current
No full textIn this thesis we propose a novel method to study the dynamics of topological magnetic textures.
Based on the stability of these objects, scaling and symmetry arguments, we show that,
despite the complexity of the micromagnetic model, the electric and magnetic driven dynamics
can be described in terms of a few relevant dynamical parameters. This method reproduces well
known behaviors reported in the literature without the assistance of sophisticated micromagnetic
numerical calculations. Moreover, it allows for the study of new phenomena relevant for proposing
new memory devices based on topological textures.
Based on a specific configuration of a nanowire with a strong pinning point, we predict a periodic
injection of domain walls by all electrical means. Our analytical results reveal the existence of
a critical current. For currents below the critical current, the magnetic configuration is stable and
fully defined by a single parameter. For currents slightly above the critical current, this parameter
becomes dynamical and is associated to the periodic injection of domain walls into the nanowire.
The period is given by a universal exponent T (wavy line) (j – jvc)1/2. The process is very general and independent
of microscopic details. A major feature is that the process is independent of "twisting"
terms or applied external magnetic field.
We also propose a Hamiltonian dynamics formalism for the current and magnetic field driven
dynamics of ferromagnetic and antiferromagnetic domain walls in one-dimensional systems. We
obtain Hamiltonian equations for pairs of the dynamical parameters that describe the low energy
excitations of domain walls. This model independent formalism includes both the undamped and
damped dynamics. We use it to study current induced domain wall motion in ferromagnetic and
antiferromagnetic materials. In the second material, we include also the influence of magnetic
fields and predict an orientation switch mechanism for antiferromagnetic domain walls which can
be tested experimentally.
Moreover, we extend the formalism from nanowires to thin-films and study extended domain
walls as string objects. The description includes the dynamics of vortices and curvatures along
the domain wall as well as boundary effects. We provide an effective action that describes the
dynamics of domain walls with periodic boundary conditions. By considering closed domain walls,
we included the dynamics of smoothly deformed skyrmions in the large radius limit. Our theory
provides an analytical description of the excitation modes of magnetic skyrmions in a natural
way. The method developed along the thesis proves to be rich and powerful, being crucial for the
development of a new generation of memory devices based on magnetic topological textures
Ultra-sensitive voltage-controlled skyrmion-based spintronic diode
No full textWe have designed a passive spintronic diode based on a single skyrmion
stabilized in a magnetic tunnel junction and studied its dynamics induced by
voltage-controlled anisotropy (VCMA) and Dzyaloshinskii-Moriya interaction
(VDMI). We have demonstrated that the sensitivity (rectified voltage over input
microwave power) with realistic physical parameters and geometry can be larger
than 10 kV/W which is one order of magnitude better than diodes employing a
uniform ferromagnetic state. Our numerical and analytical results on the VCMA
and VDMI-driven resonant excitation of skyrmions beyond the linear regime
reveal a frequency dependence on the amplitude and no efficient parametric
resonance. Skyrmions with a smaller radius produced higher sensitivities,
demonstrating the efficient scalability of skyrmion-based spintronic diodes.
These results pave the way for designing passive ultra-sensitive and energy
efficient skyrmion-based microwave detectors.Comment: 11 pages, 3 figure
Additional information from Wild dogs at stake: deforestation threatens the only Amazon endemic canid, the short-eared dog (<i>Atelocynus microtis</i>)
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