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Topological Hall Effect in Particulate Magnetic Nanostructure

Thesis Defense

1:00 pm–3:00 pm
Jorgensen Hall Room: 207
Target Audiences:
Physics Department, (402) 472-2770,
Ahsan Ullah will present his Thesis Defense topic, “Topological Hall Effect in Particulate Magnetic Nanostructure” in-person and via Zoom.

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Meeting ID: 973 5508 1955

Abstract: Conduction electrons change their spin direction due to the exchange interaction with the lattice spins. Ideally, the spins of the conduction electrons follow the atomic spin environment adiabatically, so that spins like S1, S2, and S3 can be interpreted as time-ordered sequences t1 < t2 < t3. Such spin sequences yield a quantum-mechanical phase factor in the wave function, ???ei??, where ? is known as the Berry phase. The corresponding spin rotation translates into a Berry curvature and an emergent magnetic field and subsequently, a Hall-effect contribution known as the topological Hall-effect. In my dissertation defense, I will talk about the phenomena of the topological Hall effect in magnetic materials in confined geometries, where noncollinear spin textures are stabilized as a consequence of competition between different magnetic interactions. The topologically non-trivial spin textures in these nanostructures are flower states, curling states, vortex, and magnetic bubbles, which give rise to the topological Hall effect and have finite skyrmion numbers. The topological Hall effect is investigated in noninteracting nanoparticles, exchanges coupled centrosymmetric nanoparticles, exchanges coupled noncentrosymmetric nanoparticles which possess Dzyaloshinskii-Moriya interaction (DMI), and exchanged coupled Hard and soft magnetic films. Micromagnetic simulations, experimental methods, and analytical calculations are used to determine the topological Hall effect. In noninteracting nanoparticles, the reverse magnetic fields enhance the skyrmion number due to the flower state until the reversal occurs, whereas, for particles with 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 patterns 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 integer skyrmion number in Co film and chiral spins with partial skyrmion number 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.

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