Research ArticleMATERIALS SCIENCE

Visualization of superparamagnetic dynamics in magnetic topological insulators

Science Advances  06 Nov 2015:
Vol. 1, no. 10, e1500740
DOI: 10.1126/sciadv.1500740

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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.

Keywords
  • Topological Insulators
  • superparamagnetic
  • Qhantum Anomalous Hall Effect
  • QAHE
  • nanoSQUID
  • Cr-Bi_2Te_3
  • Bi_2Te_3
  • magnetism
  • SQUID

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

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