A research team has achieved the first direct observation of magnon spin currents, a critical development for the field of spintronics.
“Controlling spin currents, that is, the flow of spin angular momentum, in small magnetic devices, is the principal objective of spin electronics, a main contender for future energy-efficient information technologies,” said the team in a new study.
Using an advanced X-ray technique, the team was able to measure the flow of angular momentum through a material without an electrical charge. This process has previously only been studied indirectly.
Spintronics is an emerging field that utilizes the spin of electrons to create devices that can store and transmit information more efficiently and at higher densities.
A key step for this technology is controlling spin currents, which are notoriously difficult to measure. Until now, scientists typically converted a spin current into an electrical signal to detect it, a process that obscures direct spin information.
“A pure spin current has never been measured directly because the associated electric stray fields and/or shifts in the non-equilibrium spin-dependent distribution functions are too small for conventional experimental detection methods optimized for charge transport,” added the study.
Using the RIXS technique
Led by scientists at the National Synchrotron Light Source II (NSLS-II) at Brookhaven National Laboratory, the team used a technique called resonant inelastic X-ray scattering (RIXS).
By applying a temperature gradient, they created a device that generates a magnon spin current within a magnetic insulator, yttrium iron garnet (YIG). Magnons are quantized excitations that carry angular momentum in a material’smagnetic structure.
“Our goal was to reveal the magnons involved in spin currents,” said Yanhong Gu, former postdoctoral fellow in Bisogni’s group.
“These are not moving spins, but moving angular momenta forming a spin wave, while the electron charges remain still.”
The RIXS technique proved sensitive enough to detect small imbalances in the magnon intensity, which reflected changes in the magnon distribution as they moved. This provided a microscopic picture of which specific excitations carried the spin current and at what momentum.
An artist’s representation of the spin current generated in yttrium iron by the spin Seebeck effect. Image credit: Valerie A. Lentz/Brookhaven National Laboratory
Measuring the momentum distribution
This approach contrasts with previous methods that lacked the ability to measure the momentum distribution of excitations within the spin current.
Using a mathematical model, the team was able to calculate how long the magnons lasted and how they moved. These details are essential for developing future magnon-based spintronic devices.
“We can now detect changes in the excitation spectral weight as we drive a material out of equilibrium,” said Valentina Bisogni, a lead author of the study and beamline scientist.
“That opens many other research directions, including other forms of non-conventional, charge-less transport, like phonons, orbitals, or plasmons, which promise to be faster and resilient to magnetic fields.”
The immediate next steps for the research include replicating the results in thin films to compare them with bulk crystals. The team also plans to use RIXS to study other unconventional forms of transport in materials.
“They hope to find applications in systems like graphene and magnetic van der Waals materials,” concluded the press release.