Physicists manipulate magnetism with light
Dr Adrian Po (Croucher Fellowship 2018) was a member of a research team at MIT which created an experimental setup for observing exotic physics and found a new way to manipulate magnetism with light. The work has potential applications in memory storage and was published in Nature Communications.
Adrian is now an assistant professor at the Hong Kong University of Science and Technology. Croucher caught up with him to discuss the findings.
What was your involvement or role in this research?
As a theorist, my main involvement in the research was to help identify the physical processes which led to the unusual results obtained by our experimental colleagues. Contrary to what one might imagine, interesting scientific discoveries often result from experiments which fail to confirm one’s expectations. In this project, our colleagues got hold of a new sample of a magnetic material, and they obtained some unusual results when they probed it with laser pulses. Their results could not be readily explained by the usual mechanisms relating magnetic responses to laser stimulus. They then consulted us, the theorists, on some candidate theories which might explain what happened inside this material. It was how we got involved and, long story short, we realised what happened inside this fascinating sample was vastly different from what we first imagined.
What is the significance, or potential application, of this discovery?
Our modern digital world is powered by very advanced electronics, which, as the name suggests, could be viewed as the art of controlling electrons. As a fundamental particle, electrons come with two key physical attributes: charge and spin. In conventional electronics, the electron charges, leading to electrical current and voltages, are utilised for fast information processing. In contrast, the electron spin, which leads to magnetism, is only used for long-term storage (as in conventional hard disks, which are beginning to be phased out). It is of great technological interest to perform fast information processing with the electron spin instead of its charge. Breakthroughs in this area, known as spintronics, could significantly reduce the power consumption in information processing. In this work, we discovered an interesting mechanism in which a magnetic insulator could act as a transducer converting energy from a laser pulse into excitations in both the electron charges and spins, all happening in a matter of picoseconds (trillionths of a second). This could open a new door towards realising ultrafast electronics and spintronics.
This discovery was made using a “playground” for observing exotic physics. Can you describe what that means?
Imagine a playground: there could be a slide, a roundabout, a swing, a seesaw etc. A common feature among all these facilities is how energy gets converted from one form into another, but through many different means. For instance, imagine a happy child climbing up a play castle, sliding down the slide, and then dashing to get on a swing. In this process, chemical energy inside the child’s body is first converted into gravitational potential energy at the top of the slide, which becomes kinetic energy as she slides down and dashes towards the swing, and then back into gravitational potential energy at the top of the arc of the swing. What we discovered was that a similar playground existed for the electrons in the magnetic insulator we studied. What is quite exotic about this material is the simultaneous existence of several “recreational equipment” for the electrons, all connected to each other. Using laser pulses as stimuli, we could coax the electrons to go through an unconventional routine and unleash the energy in new forms.
It was very much like playing a detective game, and it was very satisfying when we finally put all the pieces together to reveal the hidden story happening inside this material within trillionths of a second!
What was your reaction when you realised the discovery you had helped make?
We were quite puzzled when our experimental colleagues first showed us their data. The measurements were convincing, and we had every reason to believe the data were not the results of some unintentional artifacts from the experimental setup. Yet, we were not aware of any mechanism which could explain all the salient features presented in the data set. We then went through a month-long brainstorming process to explore and rule out the many possibilities which could be happening behind the scenes. Finally, with insights from our experimental colleagues, we arrived at a coherent picture which explains how the electron shuffles energy between these different channels. It was very much like playing a detective game, and it was very satisfying when we finally put all the pieces together to reveal the hidden story happening inside this material within trillionths of a second!