English abstract
In the last few decades, a race has been accelerated to achieve high-capacity data
storage to satisfy the growing demand for storage memory. Domain wall (DW)
memory devices can be an alternative scheme to achieve high capacity storage. In DW
memory, the manipulation of magnetic domain walls (DW)s in ferromagnetic
nanowires by a polarized current and a magnetic field has become the booming
research activity in theoretical and experimental exploration. It attracts much attention
recently due to its applications in logic and magnetic memory devices. This thesis
explores a stepped magnetic microstructure with perpendicular magnetic anisotropy
for multi-bit per cell memory. In the proposed wire, data is stored in a magnetic domain
wall that can be moved along the wire using a magnetic field or current. The wire
exhibits two magnetic domains, which are separated by a DW. Thus, for this nanowire
to exhibit high-density data storage, it is essential to stabilize the DW in many sites
along the wire using a current or magnetic field.
To study the structure and dynamics of magnetic domains in a material, a magneto optic Kerr effect (MOKE) microscopy instrument is required. It is a powerful imaging
technique that can be employed to study the magnetization reversal of thin films
and nanoscale structures. In this work, we built a MOKE microscope to observe the
structures of magnetic domains and plot the hysteresis loop of their magnetization
reversal. It is possible to image the magnetization evolution in a micron-sized magnetic
domain on a sub-ms time scale with a lateral spatial resolution down to 300 nm for thin
films and nanowires of magnetic multilayers (MLs).
After that, the study was directed to investigate thin films multilayers to select suitable
material for domain wall devices. Hence, the magnetic domain structures and magnetic
properties of multilayers (Co/Ni), (Co/Pt), and (Co/Pd) thin films multilayers have
been investigated. The domain size and nucleation field of (Co/Ni) multilayers were
found to be reduced as the number of bilayers increases. On the other hand, (Co/Pd)×10
multilayer has a much larger domain size, faster magnetization switching, and a higher
nucleation field than Co/Ni and Co/Pt. The magnetization reversal for Co/Pt
MLs occurs by enlarging the domain in all directions as dendritic domain wall
vi
propagation. The evolution started in many sites, and then the domains grow up with
no new nucleated sites when a fixed or increased magnetic field was applied for Co/Ni
and Co/Pt. The magnetic domain size for (Co/Ni)×12 and (Co/Pt)×12 were less than 200
nm. Hence they were selected for the lithography process to fabricate the nanowire
devices.
A dynamic micromagnetic simulation is carried out using the Object-Oriented
MicroMagnetic Framework (OOMMF) software to study the domain wall dynamics
in the stepped magnetic wires (Co/Ni)×12. A stepped nanowire with 200×40 nm multi bit per cell scheme. It was found that DW pinning strength depends on the length (d)
and width (λ) of the step. The DW speed was found to increase when nanowire width
(𝑊) decreases and thickness ( tz ) increases. A higher depinning current (Jde) could be
achieved with larger thickness and lower widths of the nanowire. As step length (d)
increases, the depinning current increases. Also, DW pinning strength at stepped
positions could be controlled by the materials properties such as magnetic anisotropy
Ku and saturation magnetization Ms. It was shown that the depinning current of DW
from the stepped position increase with increasing Ku and decreasing Ms, offering thus
more flexibility in adjusting the writing current for memory applications.
Experimentally, micro-devices of (Co/Pt)×12 and (Co/Ni)×12 stepped wires (in 50 µm
length and 1 µm width) were fabricated by electron-beam and laser lithography. Along
the wire, six nano-constrictions with off-set in x and y directions, represented by and
d, were created. We found from MOKE microscopy imagining; the (DW) could be
manipulated and pinned precisely through six positions using STT and magnetic field.
It was possible to stabilize the DW at each step by a magnetic field in the stepped wire
with d = 600 nm and = 0 nm. The DW could be moved to the first step by applied
65 mT for (Co/Pt)×12 and 35 mT for (Co/Ni)×12. Moreover, a current around 4 mA with
an assisted 6 mT magnetic field was required to stabilize the DW along (Co/Pt)×12
device. Finally, it was found that the DW will remain stable for more than five years
in the (Co/Pt)×12 device. Thus, the proposed staggering configuration could be
employed to fine-tune DW devices properties for memory applications with multiple bit-per-cell memory
