Laser Pulses Enable Tunable Spin Wave Excitation

A team of researchers from the Moscow Institute of Physics and Technology (MIPT), the Russian Quantum Center, Saratov State University, and Michigan Technological University has demonstrated a method for controlling spin waves in nanostructured bismuth iron garnet films using short laser pulses. The technique has potential for applications in energy-efficient information transfer and spin-based quantum computing.

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-nm period. That geometry enabled the excitation of magnons with a very specific spin distribution, which is not possible for an unmodified film.

The laser pump pulse generates magnons by locally disrupting the ordering of spins — in orange — in bismuth iron garnet. A probe pulse — in blue — is then used to recover information about the excited magnons. Courtesy of Alexander Chernov et al./Nano Letters.

Spin refers to a particle’s intrinsic angular momentum, which always has a direction. In magnetized materials, the spins point in a uniform direction. A local disruption of this magnetic order is accompanied by the propagation of spin waves, the quanta of which are magnons.

Unlike electrical current, spin wave propagation does not involve a transfer of matter; using magnons instead of 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 already exist as sample devices, though one of the challenges in implementing the technology is the need to control certain spin wave parameters.

Even without nanopatterning, bismuth iron garnet film has distinct optomagnetic properties, such as low magnetic attenuation, which makes it possible for magnons to propagate over large distances, even at room temperature. It is also highly optically transparent in the near-infrared range, and it has a high Verdet constant.

To excite magnetization precession, the team used linearly polarized pump laser pulses. The pulses affected spin dynamics and the type of spin waves the system generated.

The researchers used 250 femtosecond probe pulses to track the state of the sample and extract spin wave characteristics. The team directed a probe pulse to any point on its sample, with a desired delay relative to the pump pulse, yielding information about the magnetization dynamics in a given point.

That information determined the spin wave’s spectral frequency, type, and additional parameters.

In contrast to previous methods, the approach enables the control of the generated wave by varying certain parameters of the laser pulse. The geometry of the nanostructured film also allowed the excitation center to be localized in a spot that was approximately 10 nm, and the nanopattern allowed the generation of multiple, distinct types of spin waves.

The angle of incidence, the wavelength, and polarization of the laser pulses enabled the resonant excitation of the sample’s waveguide modes, which are determined by the characteristics of the nanostructure so the spin waves being excited may 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, head of 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 a few tens of nanometers across and effectively excited standing spin waves of various orders. This type of spin wave enables the devices operating at high frequencies, up to the terahertz range.”

The research was published in Nano Letters (www.doi.org/10.1021/acs.nanolett.0c01528).