Magnon
excitation. Courtesy: Daria Sokol/MIPT Press Office
Scientists
Excite Magnons in Nanostructures With Laser Pulses
Physicists
from MIPT and the Russian Quantum Center, joined by colleagues from Saratov
State University and Michigan Technological University, have demonstrated new
methods for controlling spin waves in nanostructured bismuth iron garnet films
via short laser pulses. Presented in Nano Letters, the solution has potential
for applications in energy-efficient information transfer and spin-based
quantum computing.
A
particle’s spin is its intrinsic angular momentum, which always has a
direction. In magnetized materials, the spins all point in one direction. A
local disruption of this magnetic order is accompanied by the propagation of
spin waves, whose quanta are known as magnons.
Unlike the
electrical current, spin wave propagation does not involve a transfer of
matter. As a result, using magnons rather than electrons to transmit
information leads to much smaller thermal losses. Data can be encoded in the
phase or amplitude of a spin wave and processed via wave interference or
nonlinear effects.
Simple
logical components based on magnons are already available as sample devices.
However, one of the challenges of implementing this new technology is the need
to control certain spin wave parameters. In many regards, exciting magnons
optically is more convenient than by other means, with one of the advantages
presented in the recent paper in Nano Letters.
The researchers
excited spin waves in a nanostructured bismuth iron garnet. Even without
nanopatterning, that material has unique optomagnetic properties. It is
characterized by low magnetic attenuation, allowing magnons to propagate over
large distances even at room temperature. It is also highly optically
transparent in the near infrared range and has a high Verdet constant.
The film
used in the study had an elaborate structure: a smooth lower layer with a
one-dimensional grating formed on top, with a 450-nanometer period (fig. 1).
This geometry enables the excitation of magnons with a very specific spin
distribution, which is not possible for an unmodified film.
To excite
magnetization precession, the team used linearly polarized pump laser pulses,
whose characteristics affected spin dynamics and the type of spin waves
generated. Importantly, wave excitation resulted from optomagnetic rather than
thermal effects.
The
researchers relied on 250-femtosecond probe pulses to track the state of the
sample and extract spin wave characteristics. A probe pulse can be directed to
any point on the sample with a desired delay relative to the pump pulse. This
yields information about the magnetization dynamics in a given point, which can
be processed to determine the spin wave’s spectral frequency, type, and other
parameters.
Unlike the
previously available methods, the new approach enables controlling the
generated wave by varying several parameters of the laser pulse that excites
it. In addition to that, the geometry of the nanostructured film allows the
excitation center to be localized in a spot about 10 nanometers in size. The
nanopattern also makes it possible to generate multiple distinct types of spin
waves. The angle of incidence, the wavelength and polarization of the laser
pulses enable the resonant excitation of the waveguide modes of the sample,
which are determined by the nanostructure characteristics, so the type of spin
waves excited can be controlled. It is possible for each of the characteristics
associated with optical excitation to be varied independently to produce the
desired effect.
“Nanophotonics
opens up new possibilities in the area of ultrafast magnetism, said the
study’s co-author, Alexander Chernov, who heads the Magnetic Heterostructures
and Spintronics Lab at MIPT. The creation of practical applications will
depend on being able to go beyond the submicrometer scale, increasing operation
speed and the capacity for multitasking. We have shown a way to overcome these
limitations by nanostructuring a magnetic material. We have successfully
localized light in a spot few tens of nanometers across and effectively excited
standing spin waves of various orders. This type of spin waves enables the
devices operating at high frequencies, up to the terahertz range.
The paper
experimentally demonstrates an improved launch efficiency and ability to
control spin dynamics under optical excitation by short laser pulses in a
specially designed nanopatterned film of bismuth iron garnet. It opens up new
prospects for magnetic data processing and quantum computing based on coherent
spin oscillations.