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BEGIN:VEVENT
DTSTART:20230620T180000Z
UID:173637@events.unl.edu
DTSTAMP:20230614T133820Z
ORGANIZER;CN=Physics Department:MAILTO:paoffice2@unl.edu
SUMMARY:Topological Hall Effect in Particulate Magnetic Nanostructure
STATUS:CONFIRMED
DESCRIPTION:Ahsan Ullah will present his Thesis Defense topic\, "Topologica
l Hall Effect in Particulate Magnetic Nanostructure" in-person and via Zoo
m. \n\nJoin Zoom Meeting\nhttps\://unl.zoom.us/j/97355081955\nMeeting ID\:
973 5508 1955\n\nAbstract\: Conduction electrons change their spin direct
ion due to the exchange interaction with the lattice spins. Ideally\, the
spins of the conduction electrons follow the atomic spin environment adiab
atically\, so that spins like S1\, S2\, and S3 can be interpreted as time-
ordered sequences t1 < t2 < t3. Such spin sequences yield a quantum-mechan
ical phase factor in the wave function\, ???ei??\, where ? is known as the
Berry phase. The corresponding spin rotation translates into a Berry curv
ature and an emergent magnetic field and subsequently\, a Hall-effect cont
ribution known as the topological Hall-effect. In my dissertation defense\
, I will talk about the phenomena of the topological Hall effect in magnet
ic materials in confined geometries\, where noncollinear spin textures are
stabilized as a consequence of competition between different magnetic int
eractions. The topologically non-trivial spin textures in these nanostruct
ures are flower states\, curling states\, vortex\, and magnetic bubbles\,
which give rise to the topological Hall effect and have finite skyrmion nu
mbers. The topological Hall effect is investigated in noninteracting nanop
articles\, exchanges coupled centrosymmetric nanoparticles\, exchanges cou
pled noncentrosymmetric nanoparticles which possess Dzyaloshinskii-Moriya
interaction (DMI)\, and exchanged coupled Hard and soft magnetic films. Mi
cromagnetic simulations\, experimental methods\, and analytical calculatio
ns are used to determine the topological Hall effect. In noninteracting na
noparticles\, the reverse magnetic fields enhance the skyrmion number due
to the flower state until the reversal occurs\, whereas\, for particles wi
th a radius greater than the coherence radius\, the skyrmion number jumps
to a larger value at the nucleation field representing the transition from
the flower state to the curling state. The comparisons of magnetization p
atterns between experimental and computed magnetic force microscopy (MFM)
measurements show the presence of spin chirality. Magnetic and Hall-effect
measurements identify the topological Hall effect in the exchange-coupled
Co and CoSi nanoparticle films. The origin of the topological Hall effect
namely\, the chiral domains with domain-wall chirality quantified by an i
nteger skyrmion number in Co film and chiral spins with partial skyrmion n
umber in CoSi film. These spin structures are different from the skyrmions
due to DMI in B-20 crystals and multilayered thin films with Cnv symmetry
.
LOCATION:Jorgensen Hall Room 207
URL://events.unl.edu/cas/2023/06/20/173637/
DTEND:20230620T200000Z
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