# Local optical control of ferromagnetism and chemical potential in a topological insulator

## TL;DR

This paper demonstrates reconfigurable optical techniques to locally control magnetization and chemical potential in topological insulator films, enabling patterning and imaging at micron scales for potential quantum applications.

## Contribution

It introduces a novel optical method to independently manipulate magnetization and chemical potential in topological insulators at micron resolution.

## Key findings

- Optical modulation of coercivity enables writing and erasing magnetic patterns.
- Optical manipulation of the substrate's charge layer controls local chemical potential.
- Both effects are persistent and independently patternable.

## Abstract

Many proposed experiments involving topological insulators (TIs) require spatial control over time-reversal symmetry and chemical potential. We demonstrate reconfigurable micron-scale optical control of both magnetization (which breaks time-reversal symmetry) and chemical potential in ferromagnetic thin films of Cr-(Bi,Sb)$_2$Te$_3$ grown on SrTiO$_3$. By optically modulating the coercivity of the films, we write and erase arbitrary patterns in their remanent magnetization, which we then image with Kerr microscopy. Additionally, by optically manipulating a space charge layer in the underlying SrTiO$_3$ substrates, we control the local chemical potential of the films. This optical gating effect allows us to write and erase p-n junctions in the films, which we study with photocurrent microscopy. Both effects are persistent and may be patterned and imaged independently on a few-micron scale. Dynamic optical control over both magnetization and chemical potential of a TI may be useful in efforts to understand and control the edge states predicted at magnetic domain walls in quantum anomalous Hall insulators.