64 research outputs found
Indirect Exchange Coupling in Sputtered Magnetic Multilayers
This thesis describes the design, construction, and use of a sputter deposition facility at the University of Leeds for the growth of magnetic multilayer samples. Now completed, the machine is a UHV sputtering facility, capable of growing up to 15 samples of arbitrary complexity under computer control in a single vacuum cycle. The system currently has five sputter targets.
The system was used to grow Co/Cu multilayers. It was found that the level of residual gas in the chamber is of primary importance in determining the final quality of the samples. A good vacuum is required to achieve good antiferromagnetic coupling between adjacent Co layers in the multilayer stack. Consequentially the giant magnetoresistance of such samples is very high. A poor vacuum leads to poor coupling between Co layers, and the magnetoresistance of such samples is minimal.
When grown under clean conditions the giant magnetoresistance of the Co/Cu multilayers was very high - as much as 75% at room temperature, rising to 130% at 4.2K. Coupling oscillations as the Cu spacer thickness was varied were strong, whilst none were detected as a function of Co thickness.
It was found that the >< Cu spacer layer was the part of the multilayer where the effects of damage by residual gases were most severe, and that damage to this area could cause biquadratic coupling. This form of the coupling arranges spins in adjacent layers at 90° to one another, rather than antiparallel, reducing the giant magnetoresistance response of the sample. The saturation magnetisation, bilinear and biquadratic coupling energies of 180° and 90° coupled samples were measured. These three quantities were found to scale as a function of temperature
Dynamics of bistable N\'eel domain walls under spin-orbit torque
N\'eel magnetic domain walls that are stabilized by achiral energy terms
instead of the usual Dzyaloshinskii-Moriya interaction will be bistable, with
the two possible chiral forms being degenerate. Here we focus on the
theoretical study of the spin-orbit torque driven dynamics of such bistable
N\'eel domain walls. We find that, for a given domain wall, two propagation
directions along a nanowire are possible, depending on its initial state. These
dynamics also exhibit complex dependence on the spin-orbit torque magnitude,
leading to important transient regimes. Finally, a few ways are proposed for
controlled or random reversal of the domain wall propagation direction. A
robust analytical model which handles all the observed behaviors of such domain
walls is developed and validated by comparing with numerical simulations. The
obtained new dynamics open the way for new uses of domain walls in information
storage and processing devices
Magnetic microscopy of topologically protected homochiral domain walls in an ultrathin perpendicularly magnetized Co film
Next-generation concepts for solid-state memory devices are based on
current-driven domain wall propagation, where the wall velocity governs the
device performance. It has been shown that the domain wall velocity and the
direction of travel is controlled by the nature of the wall and its chirality.
This chirality is attributed to effects emerging from the lack of inversion
symmetry at the interface between a ferromagnet and a heavy metal, leading to
an interfacial Dzyaloshinskii-Moriya interaction that can control the shape and
chirality of the magnetic domain wall. Here we present direct imaging of domain
walls in Pt/Co/AlO films using Lorentz transmission electron microscopy,
demonstrating the presence of homochiral, and thus topologically protected,
N\'{e}el walls. Such domain walls are good candidates for dense data storage,
bringing the bit size down close to the limit of the domain wall width
Skyrmions in thin films with easy-plane magnetocrystalline anisotropy
We demonstrate that chiral skyrmionic magnetization configurations can be
found as the minimum energy state in B20 thin film materials with easy-plane
magnetocrystalline anisotropy with an applied magnetic field perpendicular to
the film plane. Our observations contradict results from prior analytical work,
but are compatible with recent experimental investigations. The size of the
observed skyrmions increases with the easy-plane magnetocrystalline anisotropy.
We use a full micromagnetic model including demagnetization and a
three-dimensional geometry to find local energy minimum (metastable)
magnetization configurations using numerical damped time integration. We
explore the phase space of the system and start simulations from a variety of
initial magnetization configurations to present a systematic overview of
anisotropy and magnetic field parameters for which skyrmions are metastable and
global energy minimum (stable) states.Comment: 5 pages, 3 figure
Magnetism and magnetotransport in sputtered Co-doped FeSi films
FeSi is a non-magnetic narrow-gap semiconductor that can be doped n-type by Co, which also gives rise to magnetic order. Here we report on the growth of sputtered thin films of Fe 0.8 Co 0.2 Si, which are predominantlyphase (B20 lattice structure), and possess that phase's characteristic magnetotransport properties. The ordinary Hall coefficient shows that each Co atom donates roughly one electron, whilst the magnetometry suggests that each gives rise to close to one Bohr magneton of moment. These results indicate that a highly spinpolarised electron gas persists despite the inevitable disorder in these thin films, suitable for spintronic devices
Differential Phase Contrast Imaging of the Magnetostructural Transition and Phase Boundary Motion in Uniform and Gradient-doped FeRh-based Thin Films
No abstract available
Quantitative Differential Phase Contrast Imaging of the Magnetostructural Transition and Current-driven Motion of Domain Walls in FeRh Thin Films
No abstract available
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