Browsing by Author "Herranen, Touko"

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  • Barkhausen Noise from Precessional Domain Wall Motion
    (2019-03-21) Herranen, Touko; Laurson, Lasse
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The jerky dynamics of domain walls driven by applied magnetic fields in disordered ferromagnets - the Barkhausen effect - is a paradigmatic example of crackling noise. We study Barkhausen noise in disordered Pt/Co/Pt thin films due to precessional motion of domain walls using full micromagnetic simulations, allowing for a detailed description of the domain wall internal structure. In this regime the domain walls contain topological defects known as Bloch lines which repeatedly nucleate, propagate, and annihilate within the domain wall during the Barkhausen jumps. In addition to bursts of domain wall propagation, the in-plane Bloch line dynamics within the domain wall exhibits crackling noise and constitutes the majority of the overall spin rotation activity.
  • Bloch line dynamics within magnetic domain walls
    (2018) Herranen, Touko
     School of Science | Doctoral dissertation (article-based)
    Magnetic domains are uniformly magnetized regions within a ferromagnet separated by magnetic domain walls. The internal degrees of freedom of a domain wall can be excited by applying a magnetic field greater than so called Walker field. As a result the domain wall velocity experiences the Walker breakdown, an abrupt drop of the average velocity, and the magnetization of the domain wall starts a cyclic rotation. If this mechanism is triggered in a domain wall with a dimension greater than a material characteristic Bloch line width, the excitations become non-uniform, which results in nucleation of domain walls within the domain wall called Bloch lines. The dynamics of domain walls in disordered media have been studied extensively using various computational methods as well as experimentally. In this doctoral dissertation we use a micromagnetics software to simulate the Bloch line dynamics and the effects of Bloch lines on domain wall dynamics in samples with perpendicular magnetic anisotropy. In Publication I we study the domain wall dynamics in CoPtCr thin films with different widths. We observe nucleation of Bloch lines within domain walls in disordered and perfect samples when driven with a magnetic field higher than the Walker field. We construct a geometry to confine the domain wall between two notches, and use it to study Bloch line dynamics under an in-plane field. Finally we demonstrate the performance potential of an electrical current operated Bloch line memory. In Publication II we study the effects of boundary conditions and thickness effects on the domain wall dynamics in a magnetic garnet, and we find they determine the internal dynamics allowed for the magnetization of the domain wall. The sample thickness limits the maximum achievable stable velocity before the breakdown. The velocity limit is also found to be related to the spatial width of Bloch lines. In Publication III we use a micromagnetics approach to study the Barkhausen effect and avalanche statistics in a thin Pt/Co/Pt multilayer. The domain wall is driven using a quasistatic constant velocity. The novel approach enables us to determine magnetization of the domain wall segment where avalanches are triggered. Internal magnetization dynamics show that during avalanches the activity of in-plane magnetization, i.e. the Bloch line motion, is higher than the activity related to the domain wall motion. Avalanche size and duration distributions obtained from the activity signals follow power law scaling, and the corresponding features extracted from the domain wall velocity show no significant difference. The analysis also shows that the results obtained using micromagnetic simulations are close to the values expected from a simpler model describing a short-range elastic string in a random medium.
  • Bloch-line dynamics within moving domain walls in 3D ferromagnets
    (2017-10-18) Herranen, Touko; Laurson, Lasse
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We study field-driven magnetic domain wall dynamics in garnet strips by large-scale three-dimensional micromagnetic simulations. The domain wall propagation velocity as a function of the applied field exhibits a low-field linear part terminated by a sudden velocity drop at a threshold field magnitude, related to the onset of excitations of internal degrees of freedom of the domain wall magnetization. By considering a wide range of strip thicknesses from 30 nm to 1.89μm, we find a nonmonotonic thickness dependence of the threshold field for the onset of this instability, proceeding via nucleation and propagation of Bloch lines within the domain wall. We identify a critical strip thickness above which the velocity drop is due to nucleation of horizontal Bloch lines, while for thinner strips and depending on the boundary conditions employed, either generation of vertical Bloch lines, or close-to-uniform precession of the domain wall internal magnetization takes place. For strips of intermediate thicknesses, the vertical Bloch lines assume a deformed structure due to demagnetizing fields at the strip surfaces, breaking the symmetry between the top and bottom faces of the strip, and resulting in circulating Bloch line dynamics along the perimeter of the domain wall.
  • Multistep Bloch-line-mediated Walker breakdown in ferromagnetic strips
    (2019-05-24) Hütner, Johanna; Herranen, Touko; Laurson, Lasse
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    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.