Talapatra, A and Mohanty, Jyoti Ranjan
(2018)
Creation and Modification of Perpendicular
Magnetic Anisotropy in Thin Films and Multilayer
for Possible Applications.
PhD thesis, Indian institute of technology Hyderabad.
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Abstract
The thesis deals with the creation and control of perpendicular magnetic anisotropy (PMA) in
technologically important material systems e.g. Co/Pt, Co/Pd multilayer (ML), Rare-earth (RE)
transition-metal (TM) alloy films, heterostructure and magnetic bit patterned media (BPM) for
high density (1T b/inch2
) data storage application.
Co/Pt ML (Si(substrate)/P t(1.3)/[Co(1.2)/P t(0.7)]×50/T a(2)nm) has been experienced where
the PMA originates from the strong hybridization between Co (3d element) and Pt (5d elements)
at the interface which results in strong spin-orbit coupling. The ML system shows labyrinth-like
domain pattern of dimension 110(±2) nm. Local modification of the magnetic domains has been
observed employing ultrashort laser pulse, where the tuning of magnetism is limited within the
length scale of few µm. The area where the laser is focused shows the formation of disordered
sub-domains that minimizes the dipolar energy. The demagnetized random domains appear at the
centre of the laser spot and a rim is observed at the edge of the demagnetized area which signifies
a deterministic switching. Rotation of domains at the central area with the application of small
transverse field (100 mT) proves the region to be magnetically weaker. Micromagnetic simulation
replicates the phenomenon by considering a spatial variation of magnetic anisotropy.
The optimization of PMA has been studied in Gd19F e81 thin film, a member of RE-TM alloy
family. The development of perpendicular anisotropy was observed in terms of stronger contrast of
magnetic domains having a characteristic size of 138(±3) nm at the thickness of 100 nm. Global
modification of magnetic domains and magnetic properties in amorphous Gd-Fe films has been
studied with systematic rapid thermal processing (RTP) at various temperatures (250 oC to 550 oC),
and with different time intervals viz., 2, 5, 10 and 20 minutes. The films processed at 250 oC for
various time intervals show a successive reduction in magnetic phase contrast and domain size.
The domain pattern completely disappeared, and topography dominated mixed magnetic phase
has been obtained for the films processed at 450 oC for time intervals greater than 2 minutes.
Finally, no stripe pattern was observed when the RTP temperature reached 550 oC and particulate
growth is observed over the film surface. The magnetization measurements clearly indicate a reorientation
of the magnetization direction from perpendicular to the plane of the surface. Depthresolved
structural studies revealed the crystallization of Fe, predominantly nucleating at the film
surface and the film-substrate reaction was evidenced at the interface at higher temperature due to
the diffusion of Si into the film. As the magnetic moment of free Fe is more than that of Gd-Fe, thus
saturation magnetization increases after RTP and hence PMA decreases. The experimental results
were complemented by extensive micromagnetic simulations.
After the spin re-orientation studies in continuous thin films, the experimental intricacies of
patterning nanostructures are highlighted. The effect of experiment induced challenges have been
modeled to study their impact on the performance of magnetic BPM. Micromagnetic study of variation
of switching field distribution (SFD) in a high-density patterned media is presented as a function
of magnetic anisotropy of the system. The study includes manifold effect of magnetic anisotropy in
terms of its magnitude, tilt in anisotropy axis and random arrangements of magnetic islands with
random anisotropy values. Calculation shows that reduction in anisotropy causes a linear decrease
in coercivity because the anisotropy energy tries to align the spins along a preferred crystallographic
direction. Tilt in anisotropy axis results in a decrease in squareness of the hysteresis loop and hence
facilitates switching. Finally, the experimental challenges like lithographic distribution of magnetic
viii
islands, their orientation, the creation of defects, etc. demanded the distribution of anisotropy to
be random along with random repetitions. It is explained that the range of anisotropy values and
the number of bits with different anisotropy play the key role for determining the SFD, whereas the
position of the bits and their repetitions do not show a considerable contribution. The final part
of the thesis deals with magnetic skyrmion, a promising candidate for future generation ultra-high
density data storage with an energy efficient data transfer. We have discussed the possible ways
of scaling of skyrmion diameter inside magnetic nanostructure, possessing high PMA. A detailed
study has been shown on the dependence of geometry and magnetic properties of the nanostructure
over the phase transition from a single domain magnetic state to a multi-domain state via skyrmion
configuration. Special emphasis has been given towards the understanding of the role of the defect,
its threshold dimension and magnetic anisotropy to tune the skyrmion size. The transformation to
an isolated skyrmion and multiple skyrmion lattice from a multi-domain state has been shown as a
function of externally applied out-of-plane magnetic field and diameter of the nanostructure. Hence,
the thesis provides a broader aspect towards the understanding of creation and control of PMA for
the applications in high-density magnetic data storage.
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