Surface vacancy mediated pinning in maghemite NP

Surface vacancy mediated pinning of the magnetization in γ−Fe2O3 nanoparticles: A micromagnetic simulation study

Bassel Alkadour, J. I. Mercer, J. P. Whitehead, J. van Lierop, and B. W. Southern

Phys. Rev. B 93, 140411(R) (2016)

The energy landscape for a nanoparticle selected at random from the K10 enemble. Each point on the surface of the sphere represents the energy associated with the alignment of the magnetic moment. The energy is calculated using a mean field approximation based on the distribution of surface vacancies and the average angular distribution of the energy per spins at T=0 shown in Fig. 4. The energy scale associated with the color map shown on the right is given in K.

SW excitation from pinned magnetic DWs

Tunable short-wavelength spin wave excitation from pinned magnetic domain walls.
Ben van de Viele, Sampo J. Hämäläinen, Pavel Baláž, Federico Montoncello & Sebastiaan van Dijken
Scientific Reports 6, 21330 (2016)

(a) Schematic of the ferroelectric-ferromagnetic bilayer structure. The polarization of the ferroelectric a1 and a2 domains is aligned along the elongated in-plane axis of the tetragonal BaTiO3 lattice and it rotates by 90 degrees at the domain boundaries. Via strain transfer and inverse magnetostriction, uniaxial anisotropy is induced in the ferromagnetic layer. The resulting ferromagnetic domain pattern fully correlates with the ferroelectric domain structure and magnetic domain walls are strongly pinned onto the ferroelectric domain boundaries by abrupt 90 degree rotations of magnetic anisotropy. (b) Micromagnetic simulation geometry for an extended film. The computational area is restricted to one period of the stripe domain structure, i.e. a single a1 and a2 domain, both having a width of 3.2 μm. Periodic boundary conditions in the x- and y-direction are used to account for the continuous film geometry. The simulated magnetization pattern near the magnetic domain wall is shown as inset.

Special on DW propagation in nanowires

Spin wave emission in field-driven domain wall motion.
X. S. Wang and X. R. Wang
Phys. Rev. B 90, 184415 (2014)

(a) Schematic diagram of a 1D head-to-head DW (top-left inset) and the snapshot of s components at t=200 for α=0,J=53.7,Dz=0.317,Dz/Dx=10, and H=0.5. Lower-left inset: The s profile of DW. Symbols are numerical results, the vertical dashed line indicates the DW center position at z=0.9, and solid lines are the Walker profile (4) with ϕ=Ht and Z=0.9. Right inset: The simulated motion evolution of the DW center (black curve), the time dependence of the DW center by the collective coordinate model (red curve), and their difference (blue curve). (b) The field dependence of average DW speed v for different Dz and Dx. Vertical dashed and solid lines correspond to critical fields Hc1 and Hc2, respectively. Inset: Simulated DW center for fields below and above Hc1 (indicated by arrows in the main figure).

Domain wall pinning in notched nanowires.
H. Y. Yuan and X. R. Wang

Phys. Rev. B 89, 054423 (2014)

Snapshots of the spin configuration around the notch as the field increases from zero up over the depinning field. (a) H=0 Oe, (b) H=100 Oe, (c) H=170 Oe, and (d) H=180 Oe. For clarity, each spin represents the average magnetization of four 4 × 4 × 4 nm cells. The nanowire dimensions are 1000 × 64 × 4 nm, and the notch dimensions are 40 × 32 × 4 nm. (e) The evolution of m¯zCD and m¯zEF with applied field.

Instability of Walker Propagating Domain Wall in Magnetic Nanowires

B. Hu and X. R. Wang

Phys. Rev. Lett. 111, 027205 (2013)

Illustration of transverse head-to-head DW of width Δ in a nanowire, with easy axis along ẑ and hard axis along x̂. In the absence of external magnetic field (upper), a static DW exists between two domains with m_z=±1. Under a field parallel to the easy axis, the Walker propagating DW moves towards the energy minimum state (m_z=-1) at a speed v while the DW profile is preserved.

Direct Imaging of Thermally Driven Domain Wall Motion in Magnetic Insulators.
Wanjun Jiang, Pramey Upadhyaya, Yabin Fan, Jing Zhao, Minsheng Wang, Li-Te Chang, Murong Lang, Kin L. Wong, Mark Lewis, Yen-Ting Lin, Jianshi Tang, Sergiy Cherepov, Xuezhi Zhou, Yaroslav Tserkovnyak, Robert N. Schwartz, and Kang L. Wang
Phys. Rev. Lett. 110, 177202 (2013)

Experimental demonstration of DW motion driven by a temperature gradient. (a) The schematic illustration of polar MOKE microscope for DW imaging. (b) and (c) M vs H hysteresis loops measured using a MOKE magnetometer for polar and longitudinal orientations, respectively. (d) Snapshots of the position of the DW as a function of time in the cold region, and hot region (e), respectively. It is noted that there is no time correlation between figures (d) and (e). The blue color corresponds to magnetization along +z direction, and white color associates with the magnetization along -z direction. Each fingerlike pattern thus contains two parallel DWs

Domain Wall Propagation through Spin Wave Emission.
X. S. Wang, P. Yan, Y. H. Shen, G. E. W. Bauer, and X. R. Wang
Phys. Rev. Lett. 109, 167209 (2012)

Schematic transverse head-to-head DW of width Δ in a magnetic nanowire. H⃗ is an external field along the wire axis defined as the z direction. DW breathing and other types of periodic DW deformations emit spin waves, denoted by the wavy lines with arrows.

DW pinning suppression

Suppression of the intrinsic stochastic pinning of domain walls in magnetic nanostripes.
Manuel Muñoz, and José L. Prieto
Nature Comms. 2, 562 (2011)

Enhanced pinning in the precessional mode and the pinning probability for different types of magnetic domain wall.

Spin transfer in transverse DWs

Enhanced spin transfer torque effect for transverse domain walls in cylindrical nanowires.
Matteo Franchin, Andreas Knittel, Maximilian Albert, Dmitri S. Chernyshenko, Thomas Fischbacher, Anil Prabhakar, and Hans Fangohr
Phys. Rev. B 84, 094409 (2011)

The relaxed magnetization configurations for θP = 0◦ (a) and θP = 90◦ (b). The sketches on the corners of the
two figures show that the DW moves into the barrier to allow the magnetization to align along the pinning direction in the barrier.