Browsing by Author "Karna, Sunil K."

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    Annihilation and Control of Chiral Domain Walls with Magnetic Fields
    (ACS, 2021-02-10) Karna, Sunil K.; Marshall, Madalynn; Xie, Weiwei; DeBeer-Schmitt, Lisa; Young, David P.; Vekhter, Ilya; Shelton, William A.; Kovács, Andras; Charilaou, Michalis; DiTusa, John F.; Physics, School of Science
    The control of domain walls is central to nearly all magnetic technologies, particularly for information storage and spintronics. Creative attempts to increase storage density need to overcome volatility due to thermal fluctuations of nanoscopic domains and heating limitations. Topological defects, such as solitons, skyrmions, and merons, may be much less susceptible to fluctuations, owing to topological constraints, while also being controllable with low current densities. Here, we present the first evidence for soliton/soliton and soliton/antisoliton domain walls in the hexagonal chiral magnet Mn1/3NbS2 that respond asymmetrically to magnetic fields and exhibit pair-annihilation. This is important because it suggests the possibility of controlling the occurrence of soliton pairs and the use of small fields or small currents to control nanoscopic magnetic domains. Specifically, our data suggest that either soliton/soliton or soliton/antisoliton pairs can be stabilized by tuning the balance between intrinsic exchange interactions and long-range magnetostatics in restricted geometries.
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    Helical magnetic order and Fermi surface nesting in non-centrosymmetric ScFeGe
    (American Physical Society, 2021) Karna, Sunil K.; Tristant, D.; Hebert, J. K.; Cao, G.; Chapai, R.; Phelan, W. A.; Zhang, Q.; Wu, Y.; Dhital, C.; Li, Y.; Cao, H. B.; Tian, W.; Dela Cruz, C. R.; Aczel, A. A.; Zaharko, O.; Khasanov, A.; McGuire, M. A.; Roy, A.; Xie, W.; Browne, D. A.; Vekhter, I.; Meunier, V.; Shelton, W. A.; Adams, P. W.; Sprunger, P. T.; Young, D. P.; Jin, R.; DiTusa, J. F.; Physics, School of Science
    An investigation of the structural, magnetic, thermodynamic, and charge transport properties of noncentrosymmetric hexagonal ScFeGe reveals it to be an anisotropic metal with a transition to a weak itinerant incommensurate helimagnetic state below 𝑇𝑁=36 K. Neutron diffraction measurements discovered a temperature and field independent helical wave vector 𝒌 = (0 0 0.193) with magnetic moments of 0.53 𝜇𝐵 per Fe confined to the 𝑎⁢𝑏 plane. Density functional theory calculations are consistent with these measurements and find several bands that cross the Fermi level along the 𝑐 axis with a nearly degenerate set of flat bands just above the Fermi energy. The anisotropy found in the electrical transport is reflected in the calculated Fermi surface, which consists of several warped flat sheets along the 𝑐 axis with two regions of significant nesting, one of which has a wave vector that closely matches that found in the neutron diffraction. The electronic structure calculations, along with a strong anomaly in the 𝑐 -axis conductivity at 𝑇𝑁, signal a Fermi surface driven magnetic transition, similar to that found in spin density wave materials. Magnetic fields applied in the 𝑎⁢𝑏 plane result in a metamagnetic transition with a threshold field of ≈6.7 T along with a sharp, strongly temperature dependent discontinuity and a change in sign of the magnetoresistance for in-plane currents. Thus, ScFeGe is an ideal system to investigate the effect of in-plane magnetic fields on a helimagnet with a 𝑐 -axis propagation vector, where the relative strength of the magnetic interactions and anisotropies determine the topology and magnetic structure.