71 research outputs found
Twisted domain walls and skyrmions in perpendicularly magnetized multilayers
We present an analytical theory to describe three-dimensional magnetic
textures in perpendicularly magnetized magnetic multilayers that arise in the
presence of magnetostatic interactions and the Dzyaloshinskii-Moriya
interaction (DMI). We demonstrate that domain walls in multilayers develop a
complex twisted structure, which persists even for films with strong DMI. The
origin of this twist is surface-volume stray field interactions that manifest
as a depth-dependent effective field whose form mimics the DMI effective field.
We find that the wall twist has a minor impact on the equilibrium skyrmion or
domain size, but can significantly affect current-driven dynamics. Our
conclusions are based on the derived analytical expressions for the
magnetostatic energy and confirmed by micromagnetic simulations.Comment: 30 pages, 8 figure
Full phase diagram of isolated skyrmions in a ferromagnet
Magnetic skyrmions are topological quasi particles of great interest for data
storage applications because of their small size, high stability, and ease of
manipulation via electric current. Theoretically, however, skyrmions are poorly
understood since existing theories are not applicable to small skyrmion sizes
and finite material thicknesses. Here, we present a complete theoretical
framework to determine the energy of any skyrmion in any material, assuming
only a circular symmetric 360 domain wall profile and a homogeneous
magnetization profile in the out-of-plane direction. Our model precisely agrees
with existing experimental data and micromagnetic simulations. Surprisingly, we
can prove that there is no topological protection of skyrmions. We discover and
confirm new phases, such as bi-stability, a phenomenon unknown in magnetism so
far. The outstanding computational performance and precision of our model allow
us to obtain the complete phase diagram of static skyrmions and to tackle the
inverse problem of finding materials corresponding to given skyrmion
properties, a milestone of skyrmion engineering
Determination of the Spin-Hall-Effect-Induced and the Wedged-Structure-Induced Spin Torque Efficiencies in Heterostructures with Perpendicular Magnetic Anisotropy
We report that by measuring current-induced hysteresis loop shift versus
in-plane bias magnetic field, the spin Hall effect (SHE) contribution of the
current-induced effective field per current density, , can be
estimated for Pt and Ta-based magnetic heterostructures with perpendicular
magnetic anisotropy (PMA). We apply this technique to a Pt-based sample with
its ferromagnetic (FM) layer being wedged-deposited and discover an extra
effective field contribution, , due to the asymmetric nature of
the deposited FM layer. We confirm the correlation between and
the asymmetric depinning process in FM layer during magnetization switching by
magneto-optical Kerr (MOKE) microscopy. These results indicate the possibility
of engineering deterministic spin-orbit torque (SOT) switching by controlling
the symmetry of domain expansion through the materials growth process
Current-driven dynamics of chiral ferromagnetic domain walls
In most ferromagnets the magnetization rotates from one domain to the next
with no preferred handedness. However, broken inversion symmetry can lift the
chiral degeneracy, leading to topologically-rich spin textures such as
spin-spirals and skyrmions via the Dzyaloshinskii-Moriya interaction (DMI).
Here we show that in ultrathin metallic ferromagnets sandwiched between a heavy
metal and an oxide, the DMI stabilizes chiral domain walls (DWs) whose spin
texture enables extremely efficient current-driven motion. We show that spin
torque from the spin Hall effect drives DWs in opposite directions in
Pt/CoFe/MgO and Ta/CoFe/MgO, which can be explained only if the DWs assume a
N\'eel configuration with left-handed chirality. We directly confirm the DW
chirality and rigidity by examining current-driven DW dynamics with magnetic
fields applied perpendicular and parallel to the spin spiral. This work
resolves the origin of controversial experimental results and highlights a new
path towards interfacial design of spintronic devices
Large voltage-induced modification of spin-orbit torques in Pt/Co/GdOx
We report on large modifications of current-induced spin-orbit torques in a
gated Pt/Co/Gd-oxide microstrip due to voltage-driven O migration. The
Slonczewski-like and field-like torques are quantified using a low-frequency
harmonic technique based on the polar magneto-optical Kerr effect.
Voltage-induced oxidation of Co enhances the Slonczewski-like torque by as much
as an order of magnitude, and simultaneously reduces the anisotropy energy
barrier by a factor of ~5. Such magneto-ionic tuning of interfacial spin-orbit
effects may significantly enhance the efficiency of magnetization switching and
provide additional degrees of freedom in spintronic devices
Origins of the unidirectional spin Hall magnetoresistance in metallic bilayers
Recent studies evidence the emergence of asymmetric electron transport in
layered conductors owing to the interplay between electrical conductivity,
magnetization, and the spin Hall or Rashba- Edelstein effects. Here, we
investigate the unidirectional magnetoresistance (UMR) caused by the
current-induced spin accumulation in Co/Pt and CoCr/Pt bilayers. We identify
three competing mechanisms underpinning the resistance asymmetry, namely
interface and bulk spin-dependent electron scattering and electron-magnon
scattering. Our measurements provide a consistent description of the current,
magnetic field, and temperature dependence of the UMR and show that both
positive and negative UMR can be obtained by tuning the interface and bulk
spin-dependent scattering terms relative to the magnon population
Generalized analysis of thermally activated domain-wall motion in Co/Pt multilayers
Thermally activated domain-wall (DW) motion driven by magnetic field and
electric current is investigated experimentally in out-of-plane magnetized
Pt(Co/Pt) multilayers. We directly extract the thermal activation energy
barrier for DW motion and observe the dynamic regimes of creep, depinning, and
viscous flow. Further analysis reveals that the activation energy must be
corrected with a factor dependent on the Curie temperature, and we derive a
generalized Arrhenius-like equation governing thermally activated motion. By
using this generalized equation, we quantify the efficiency of current-induced
spin torque in assisting DW motion. Current produces no effect aside from Joule
heating in the multilayer with 7-\AA\ thick Co layers, whereas it generates a
finite spin torque on DWs in the multilayer with atomically thin 3-\AA\ Co
layers. These findings suggest that conventional spin-transfer torques from
in-plane spin-polarized current do not drive DWs in ultrathin Co/Pt
multilayers
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