### Browsing by Author "Durin, Gianfranco"

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Item Creep turns linear in narrow ferromagnetic nanostrips(2016-02-04) Leliaert, Jonathan; Van de Wiele, Ben; Vansteenkiste, Arne; Laurson, Lasse; Durin, Gianfranco; Dupre, Luc; Van Waeyenberge, Bartel; Ghent University; Department of Applied Physics; ISI FoundationThe motion of domain walls in magnetic materials is a typical example of a creep process, usually characterised by a stretched exponential velocity-force relation. By performing large-scale micromagnetic simulations, and analyzing an extended 1D model which takes the effects of finite temperatures and material defects into account, we show that this creep scaling law breaks down in sufficiently narrow ferromagnetic strips. Our analysis of current-driven transverse domain wall motion in disordered Permalloy nanostrips reveals instead a creep regime with a linear dependence of the domain wall velocity on the applied field or current density. This originates from the essentially point-like nature of domain walls moving in narrow, line-like disordered nanostrips. An analogous linear relation is found also by analyzing existing experimental data on field-driven domain wall motion in perpendicularly magnetised media.Item Ground-state optimization and hysteretic demagnetization: The random-field Ising model(American Physical Society (APS), 2005) Alava, Mikko J.; Basso, Vittorio; Colaiori, Francesca; Dante, Lorenzo; Durin, Gianfranco; Magni, Alessandro; Zapperi, Stefano; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe compare the ground state of the random-field Ising model with Gaussian distributed random fields, with its nonequilibrium hysteretic counterpart, the demagnetized state. This is a low-energy state obtained by a sequence of slow magnetic-field oscillations with decreasing amplitude. The main concern is how optimized the demagnetized state is with respect to the best-possible ground state. Exact results for the energy in d=1 show that in a paramagnet, with finite spin-spin correlations, there is a significant difference in the energies if the disorder is not so strong that the states are trivially almost alike. We use numerical simulations to better characterize the difference between the ground state and the demagnetized state. For d⩾3, the random-field Ising model displays a disorder induced phase transition between a paramagnetic and a ferromagnetic state. The locations of the critical points R(DS)c and R(GS)c differ for the demagnetized state and ground state. We argue based on the numerics that in d=3 the scaling at the transition is the same in both states. This claim is corroborated by the exact solution of the model on the Bethe lattice, where the critical points are also different.Item Phase Transitions in a Disordered System in and out of Equilibrium(American Physical Society (APS), 2004) Colaiori, Francesca; Alava, Mikko J.; Durin, Gianfranco; Magni, Alessandro; Zapperi, Stefano; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceThe equilibrium and nonequilibrium disorder-induced phase transitions are compared in the random-field Ising model. We identify in the demagnetized state the correct nonequilibrium hysteretic counterpart of the T=0 ground state, and present evidence of universality. Numerical simulations in d=3 indicate that exponents and scaling functions coincide, while the location of the critical point differs, as corroborated by exact results for the Bethe lattice. These results are of relevance for optimization, and for the generic question of universality in the presence of disorder.