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Applied Optics Group demonstrates Strong Coupling between Magnons and Phonons on a Single Nanomagnet

Professor Holger Schmidt in the Applied Optics Group lab
Professor Holger Schmidt in the Applied Optics Group lab
Graduate Assistant Cassidy Berk
Graduate Assistant Cassidy Berk
Laboratory images of the nanomagnet sample and results
Laboratory images of the nanomagnet sample and results
Monday, July 8, 2019
James McGirk

UC Santa Cruz researchers discover interplay between magnetic and vibrational resonances within single nanomagnets

SANTA CRUZ, CA—UC Santa Cruz Electrical Engineer Holger Schmidt’s Applied Optics Group uses nanofabrication, laser pulses, and an observation technique called magneto-optic Kerr effect to unlock the fundamental properties of miniature magnets.

Schmidt and Cassidy Berk, then an electrical engineering Ph.D. student working in the Applied Optics Group, discovered a phenomenon called strong coupling between the magnetic and vibrational dynamics within a single nanomagnet.

“No one has ever observed strong coupling between a magnetic and vibrational system before,” Associate Dean of Research and Professor of Electrical and Computer Engineering Holger Schmidt said. “We now have another knob we can use to control how we affect magnetic states in a device.”

The work is of particular interest to the data storage industry for its potential to reduce power consumption and increase efficiency in high-density magnetic storage and memory.

Conventional hard drives work by magnetizing granules to encode information. There are limits to how densely information can be encoded this way. Researchers like Berk and Schmidt seek ways of precisely arranging and tooling the magnetized particles to cram data into ever smaller spaces.

“One of the things being considered in next generation storage is the use of individual but perfectly patterned nanomagnets,” Schmidt said. By carefully shaping and placing these nano-scale magnets, researchers have been able to observe and manipulate physical phenomena.

“If you hit a magnet with a light pulse, this will kick the array out of its equilibrium state, and the magnetization start to spin around,” Schmidt said. “Metal magnets also absorb optical energy and get hot. When you hit them with a very short pulse of light, they heat up and expand and then eventually go back to their original state and this will cause vibration; like pulling on a spring and letting go.”

By making a single metal nanomagnet vibrate at resonant frequencies, Schmidt’s applied optics team looked for a strong coupling effect between the mechanical vibrations and the magnetization oscillations. Strong coupling refers to a type of interdependency between two connected systems.

“Let’s assume you have two ball-and-spring systems,” Schmidt said. “If there’s no connection between them and you pull them out and let go and they will bounce at a particular frequency, but if you put another spring between them and change the mass of the second spring, you’re effectively coupling the two systems, and it turns out they cannot run at the frequency they individually want to be at, and you get two new frequencies instead.”

They looked for areas where the two systems observed (magnetism and vibration) could not reach their resonant frequencies, evidence of strong coupling.

Measurements for the experiment were made using an ultra fast femtosecond pulse laser aimed at a single 330nm x 330nm x 30nm nickel nanomagnet. “We take the pulse that comes out of the laser and split it,” Schmidt explained. “The first pulse hits the sample and starts the oscillations; we delay the second pulse so it hits the sample slightly later, and we use that measurement to figure out what the magnetization is doing.”

It was the first time anyone had ever observed strong coupling in a magnetic-vibrational system. Berk created a mathematical model explaining how the phenomenon worked from their findings.

“[What this means is that] energy can be transferred back and forth between the magnetic system and the mechanical system,” Schmidt said. “This is fundamental work but could have relevance for industry in the sense that for some of these devices you might be able to lower the amount of energy being used to store information with the help of vibrational energy. Essentially it’s another tool for us to use at this level and Cassidy [Berk] made a really wonderful model to explain this behavior.”

Their findings were published in Nature Communications as “Strongly Coupled Magnon-Phonon Dynamics in a Single Nanomagnet.” Listed as co-authors are UC Santa Cruz engineers M. Jaris, W. Yang, t and S. Cabrini and S. Dhuey from the Molecular Foundry at the University of California, Berkeley.

The work was supported by the W.M. Keck Center for Nanoscale Optofluidics. The samples used in the experiment were made at the Molecular Foundry at UC Berkeley. The work was supported by the National Science Foundation and the U.S. Department of Energy.

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