Magnetic non-contact friction from domain wall dynamics actuated by oscillatory mechanical motion

Magnetic friction is a form of non-contact friction arising from the dissipation of energy in a magnet due to spin reorientation in a magnetic field. In this paper we study magnetic friction in the context of micromagnetics, using our recent implementation of smooth spring-driven motion [Phys. Rev. E. 97, 053301 (2018)] to simulate ring-down measurements in two setups where domain wall dynamics is induced by mechanical motion. These include a single thin film with a domain wall in an external field and a setup mimicking a magnetic cantilever tip and substrate, in which the two magnets interact through dipolar interactions. We investigate how various micromagnetic parameters influence the domain wall dynamics actuated by the oscillatory spring-driven mechanical motion and the resulting damping coefficient. Our simulations show that the magnitude of magnetic friction can be comparable to other forms of non-contact friction. For oscillation frequencies lower than those inducing excitations of the internal structure of the domain walls, the damping coefficient is found to be independent of frequency. Hence, our results obtained in the frequency range from 8 to 112 MHz are expected to be relevant also for typical experimental setups operating in the 100 kHz range.Comment: 19 pages, 8 figure

Domain walls within domain walls in wide ferromagnetic strips

We carry out large-scale micromagnetic simulations which demonstrate that due to topological constraints, internal domain walls (Bloch lines) within extended domain walls are more robust than domain walls in nanowires. Thus, the possibility of spintronics applications based on their motion channeled along domain walls emerges. Internal domain walls are nucleated within domain walls in perpendicularly magnetized media concurrent with a Walker breakdown-like abrupt reduction of the domain wall velocity above a threshold driving force, and may also be generated within pinned, localized domain walls. We observe fast field and current driven internal domain wall dynamics without a Walker breakdown along pinned domain walls, originating from topological protection of the internal domain wall structure due to the surrounding out-of-plane domains.Comment: 5 pages, 6 figure

Dynamic hysteresis in cyclic deformation of crystalline solids

The hysteresis or internal friction in the deformation of crystalline solids stressed cyclically is studied from the viewpoint of collective dislocation dynamics. Stress-controlled simulations of a dislocation dynamics model at various loading frequencies and amplitudes are performed to study the stress - strain rate hysteresis. The hysteresis loop areas exhibit a maximum at a characteristic frequency and a power law frequency dependence in the low frequency limit, with the power law exponent exhibiting two regimes, corresponding to the jammed and the yielding/moving phases of the system, respectively. The first of these phases exhibits non-trivial critical-like viscoelastic dynamics, crossing over to intermittent viscoplastic deformation for higher stress amplitudes.Comment: 5 pages, 4 figures, to appear in Physical Review Letter

Universality classes and crossover scaling of Barkhausen noise in thin films

We study the dynamics of head-to-head domain walls separating in-plane domains in a disordered ferromagnetic thin film. The competition between the domain wall surface tension and dipolar interactions induces a crossover between a rough domain wall phase at short length-scales and a large-scale phase where the walls display a zigzag morphology. The two phases are characterized by different critical exponents for Barkhausen avalanche dynamics that are in quantitative agreement with experimental measurements on MnAs thin films.Comment: 5 pages, 5 figure

Multistep Bloch-line-mediated Walker breakdown in ferromagnetic strips

A well-known feature of magnetic field driven dynamics of domain walls in ferromagnets is the existence of a threshold driving force at which the internal magnetization of the domain wall starts to precess -- a phenomenon known as the Walker breakdown -- resulting in an abrupt drop of the domain wall propagation velocity. Here, we report on micromagnetic simulations of magnetic field driven domain wall dynamics in thin ferromagnetic strips with perpendicular magnetic anisotropy which demonstrate that in wide enough strips Walker breakdown is a multistep process: It consists of several distinct velocity drops separated by short linear parts of the velocity vs field curve. These features originate from the repeated nucleation, propagation and annihilation of an increasing number of Bloch lines within the domain wall as the driving field magnitude is increased. This mechanism arises due to magnetostatic effects breaking the symmetry between the two ends of the domain wall.Comment: 6 pages, 4 figures, to appear in Phys. Rev.

Mimicking complex dislocation dynamics by interaction networks

Two-dimensional discrete dislocation models exhibit complex dynamics in relaxation and under external loading. This is manifested both in the time-dependent velocities of individual dislocations and in the ensemble response, the strain rate. Here we study how well this complexity may be reproduced using so-called Interaction Networks, an Artificial Intelligence method for learning the dynamics of complex interacting systems. We test how to learn such networks using creep data, and show results on reproducing individual and collective dislocation velocities. The quality of reproducing the interaction kernel is discussed

The effect of disorder on transverse domain wall dynamics in magnetic nanostrips

We study the effect of disorder on the dynamics of a transverse domain wall in ferromagnetic nanostrips, driven either by magnetic fields or spin-polarized currents, by performing a large ensemble of GPU-accelerated micromagnetic simulations. Disorder is modeled by including small, randomly distributed non-magnetic voids in the system. Studying the domain wall velocity as a function of the applied field and current density reveals fundamental differences in the domain wall dynamics induced by these two modes of driving: For the field-driven case, we identify two different domain wall pinning mechanisms, operating below and above the Walker breakdown, respectively, whereas for the current-driven case pinning is absent above the Walker breakdown. Increasing the disorder strength induces a larger Walker breakdown field and current, and leads to decreased and increased domain wall velocities at the breakdown field and current, respectively. Furthermore, for adiabatic spin transfer torque, the intrinsic pinning mechanism is found to be suppressed by disorder. We explain these findings within the one-dimensional model in terms of an effective damping parameter $\alpha^*$ increasing with the disorder strength.Comment: 5 pages, 3 figure

Avalanches and clusters in planar crack front propagation

We study avalanches in a model for a planar crack propagating in a disordered medium. Due to long-range interactions, avalanches are formed by a set of spatially disconnected local clusters, the sizes of which are distributed according to a power law with an exponent $\tau_{a}=1.5$. We derive a scaling relation $\tau_a=2\tau-1$ between the local cluster exponent $\tau_a$ and the global avalanche exponent $\tau$. For length scales longer than a cross-over length proportional to the Larkin length, the aspect ratio of the local clusters scales with the roughness exponent of the line model. Our analysis provides an explanation for experimental results on planar crack avalanches in Plexiglas plates, but the results are applicable also to other systems with long-range interactions.Comment: 7 pages, 6 figures, accepted for publication in Physical Review

Verification and Simplification of DMN Decision Tables

Decision Model and Notation (DMN) on standardne notatsioon, mida kasutatakse ärirakendustes otsuste loogika kirjeldamiseks. Otsustabelid on DMNi üks peamisi osi. DMNi otsustabelite suurenev kasutatavus igapäevaste äriotsuste ülesmärkimiseks ja automatiseerimiseks on tõstatanud vajadust analüüsida otsustabeleid. See lõputöö annab ülevaate DMN otsustabelist ja kirjeldab kolme skaleeruvat algoritmi, mis on mõeldud leidmaks kattuvaid reegleid ja puuduvaid reegleid ning lihtsustada otsustabeleid kasutades reeglite ühendamist. Kõik välja pakutud algoritmid on implementeeritud avatud lähtekoodiga DMN redaktorisse ja katsetatud suurte otsustabelite peal, mis pärinevad krediidiandmise andmebaasist.The Decision Model and Notation (DMN) is a standard notation to specify decision logic in business applications. A central construct in DMN is a decision table. The rising use of DMN decision tables to capture and to automate everyday business decisions raises the need to support analysis tasks on decision tables. This thesis provides scalable algorithms to tackle three analysis tasks: detection of overlapping rules, detection of missing rules and simplification of decision tables via rule merging. All proposed algorithms have been implemented in an open-source DMN editor and are tested on large decision tables derived from a credit lending data-set