Condensed matter seminar: Novel Magnetic Textures and the Topological Hall Effect
Jeffrey Lynn, NIST
Understanding the origin and properties of magnetoresistance in topological systems is a topic of high interest currently, and we discuss recent work on materials in this category. First we present work on the half-Heusler systems such as RPdBi and RPtBi (R = rare earth) [1,2]. Then we will talk about the singular angular magnetoresistance (SAMR) discovered in CeAlGe(Si), which orders ferrimagnetically below 5.6 K, with two inequivalent Ce sites with different moments that are oriented in the a-b tetragonal plane. For a magnetic field applied in this plane the magnetoresistance appears to diverge in an appropriate temperature and magnetic field range when the field is parallel to the [1,0,0], in a very narrow angular range . Neutron diffraction was employed to determine the magnetic structures, and this combined with theory led to understanding that the SAMR results from switching of domains coupled with Fermi surface mismatch near Weyl nodes. Finally, we will present work on YMn6Sn6, which orders antiferromagnetically at 345 K and exhibits quite a variety of magnetic structures as a function of temperature and magnetic field, mostly incommensurate, revealed by neutron scattering. Only one such structure—the transverse conical spiral—exhibits a topological Hall effect that develops at elevated temperatures. Detailed theoretical calculations explain not only all the observed phases and magnetic structures but also the topological Hall effect, which originates via a new mechanism of thermally induced chiral magnons.
 Topological RPdBi half-Heusler semimetals: a new family of non-centrosymmetric magnetic superconductors, Y. Nakajima, R. Hu, K. Kirshenbaum, A. Hughes, P. Syers, X. Wang, K. Wang, R. Wang, S. Saha, D. Pratt, J.W. Lynn, and J. Paglione, Science Advances 1, e1500242 (2015).
 Large Anomalous Hall Effect in a Half Heusler Antiferromagnet, T. Suzuki, R. Chisnell, A. Devarakonda, Y.-T. Liu, J. W. Lynn, and J. G. Checkelsky, Nature Physics 12, 1119 (2016)
 Competing Magnetic Phases and Fluctuation-driven Scalar Spin Chirality in the Kagome Metal YMn6Sn6, Nirmal J. Ghimire, Rebecca L. Dally, L. Poudel, D. C. Jones, D. Michel, N. Thapa Magar, M. Bleuel, Michael A. McGuire, J. S. Jiang, John F. Mitchell, Jeffrey W. Lynn, and I. I. Mazin, Science Advances 6, eabe2680 (2020); Chiral properties of the zero-field spiral state and field-induced magnetic phases of the itinerant kagome metal YMn6Sn6, Rebecca L. Dally, Jeffrey W. Lynn, Nirmal J. Ghimire, Dina Michel, Peter Siegfried, and Igor I. Mazin, Phys. Rev. B 103, 094413 (2021); Topological Phase Transition and Charge-spin Coupling in the Kagome Metal YMn6Sn6, Peter E. Siegfried, Hari Bhandari, David C. Jones, Madhav P. Ghimire, Rebecca L. Dally, Lekh Poudel, Markus Bleuel, Jeffrey W. Lynn, Igor I. Mazin, and Nirmal J. Ghimire, (submitted).