title: Visualization of superparamagnetic dynamics in magnetic topological insulators.
abstract: Quantized Hall conductance is a generic feature of two-dimensional electronic systems with broken time reversal symmetry. In the quantum anomalous Hall state recently discovered in magnetic topological insulators, time reversal symmetry is believed to be broken by long-range ferromagnetic order, with quantized resistance observed even at zero external magnetic field. We use scanning nanoSQUID (nano-superconducting quantum interference device) magnetic imaging to provide a direct visualization of the dynamics of the quantum phase transition between the two anomalous Hall plateaus in a Cr-doped (Bi,Sb)2Te3 thin film. Contrary to naive expectations based on macroscopic magnetometry, our measurements reveal a superparamagnetic state formed by weakly interacting magnetic domains with a characteristic size of a few tens of nanometers. The magnetic phase transition occurs through random reversals of these local moments, which drive the electronic Hall plateau transition. Surprisingly, we find that the electronic system can, in turn, drive the dynamics of the magnetic system, revealing a subtle interplay between the two coupled quantum phase transitions.
authors.affiliation: Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
authors.forename: Ella O
authors.initials: EO
authors.lastname: Lachman
authors.affiliation: Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel. ; Department of Physics, University of California, Santa Barbara, Santa Barbara, CA 93106-9530, USA.
authors.forename: Andrea F
authors.initials: AF
authors.lastname: Young
authors.affiliation: Department of Physics, Pennsylvania State University, University Park, PA 16802, USA.
authors.forename: Anthony
authors.initials: A
authors.lastname: Richardella
authors.affiliation: Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
authors.forename: Jo
authors.initials: J
authors.lastname: Cuppens
authors.affiliation: Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
authors.forename: H R
authors.initials: HR
authors.lastname: Naren
authors.affiliation: Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
authors.forename: Yonathan
authors.initials: Y
authors.lastname: Anahory
authors.affiliation: Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
authors.forename: Alexander Y
authors.initials: AY
authors.lastname: Meltzer
authors.affiliation: Department of Physics, Pennsylvania State University, University Park, PA 16802, USA.
authors.forename: Abhinav
authors.initials: A
authors.lastname: Kandala
authors.affiliation: Department of Physics, Pennsylvania State University, University Park, PA 16802, USA.
authors.forename: Susan
authors.initials: S
authors.lastname: Kempinger
authors.affiliation: Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
authors.forename: Yuri
authors.initials: Y
authors.lastname: Myasoedov
authors.affiliation: Department of Physics, University of Colorado Denver, Denver, CO 80217, USA.
authors.forename: Martin E
authors.initials: ME
authors.lastname: Huber
authors.affiliation: Department of Physics, Pennsylvania State University, University Park, PA 16802, USA.
authors.forename: Nitin
authors.initials: N
authors.lastname: Samarth
authors.affiliation: Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
authors.forename: Eli
authors.initials: E
authors.lastname: Zeldov
keywords: Bi_2Te_3; Cr-Bi_2Te_3; QAHE; Qhantum Anomalous Hall Effect; SQUID; Topological Insulators; magnetism; nanoSQUID; superparamagnetic