An innovative technique has been created by an international research team under the direction of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) for the effective coupling of terahertz waves with much shorter wavelength spin waves. Their investigations, along with theoretical models, help to clarify the basic workings of this process, which were previously thought to be impossible.
The outcomes mark a critical milestone in creating innovative, energy-efficient spin-based data processing technologies.
“We were able to efficiently excite high-energy spin waves using terahertz light in a sandwich-like material system consisting of two metal films a few nanometers thick, with a ferromagnetic layer sandwiched in between,”
said Dr. Sergey Kovalev of the Institute of Radiation Physics at HZDR, where the experiments were conducted.
Spin Wave Benefits
Schematic of the experiment. The single-cycle terahertz pulse is incident on the Ta/Py/Pt sample, which excites standing SWR modes in the Py layer. The z-axis dynamic component of the Py magnetization is detected using a 100 fs laser-probe pulse via the Faraday rotation effect.
Credit: Nat. Phys. (2023) DOI: 10.1038/s41567-022-01908-1
The effective spin of an electron is similar to the behaviour of a spinning top. Similarly to a gyroscope, an outside disturbance can cause the spin’s axis of rotation to tilt: precession, a gyroscopic motion, follows suit.
Because of the intense interaction between electron spins in ferromagnetic materials, a precession that began locally continues as a spin wave throughout the entire layer of ferromagnetic material. This is intriguing because a spin wave can serve as an information carrier just like any other wave.
While each electron spin is in motion, it remains in its position in the atomic lattice in ferromagnets, so no current flow occurs. In spin-based devices, unlike today’s computer chips, there are no heat losses due to currents.
The characteristic frequencies of the high-energy spin waves are conveniently in the terahertz range. This is precisely the target range for novel ultrafast data transmission and processing technologies. Combining optical terahertz technology with spin-based devices could thus enable completely new and efficient IT technology concepts.
Issues With Magnons And Terahertz Photons
A terahertz light wave (from left) is converted into a spin wave (right) in a sample of thin metallic layers.
In a heavy metal layer (left), electrical currents are first excited by the terahertz field. Within an ultrashort time, the spin Hall effect leads to the accumulation of spins with a certain orientation at the interface with a ferromagnetic layer (right).
This directed spin current then triggers a coherent, nanomater-wavelength spin wave in the ferromagnetic material.
Credit: HZDR/Juniks
Similar to light, which can also be described in terms of discrete particles known as photons, the energies of spin waves are quantized, and their quanta are known as magnons. Magnons and terahertz photons possess identical energies and should therefore be easily interchangeable.
But there is a problem along the way: the two wave phenomena have completely different speeds.
Terahertz waves are electromagnetic radiation that travel at the speed of light, whereas spin waves are associated with the interaction of spins. Their speed of propagation is hundreds of times slower than light’s.
And, whereas terahertz waves have a wavelength of slightly less than a millimeter, spin waves have a wavelength of only a few nanometers. As a result, the terahertz wave has no chance of specifically and directly transferring its energy to a much slower spin wave.
To address this issue, the researchers devised a combination of extremely thin tantalum and platinum metallic layers, in the middle of which they inserted a thin layer of a ferromagnetic nickel-iron alloy. This particular material combination has been precisely tuned to “translate” signals from the world of light into the world of spins.
Magnetization Oscillations
They created and manufactured their functional layer material at the HZDR Institute of Ion Beam Physics and Materials Research. They did this by gradually vapor depositing metal films onto a thin glass substrate.
“In the experiment, we then bombarded the samples with intense terahertz pulses and measured their rapidly time-varying magnetization with optical laser pulses. We found characteristic oscillations of the magnetization, even for times when the exciting terahertz pulse was no longer interacting with the sample at all,”
Kovalev explained.
“We varied many factors, such as external magnetic fields and different material compositions of the layers, until we could confidently show that these were indeed the spin waves we were looking for,”
said Dr. Ruslan Salikhov, a team member working on new functional magnetic materials. For this transformation of an electromagnetic wave into a spin wave, the team used a variety of quantum effects.
These effects ensure that the terahertz wave and spin wave can communicate with one another. Terahertz radiation first accelerates free electrons in the heavy metal, allowing the formation of microscopic currents.
Detour To Success
The so-called spin Hall effect converts these currents into spin currents, i.e., electron currents with a very specific spin orientation that can transport the resulting angular momentum in local space.
This angular momentum then exerts a torque on the spins in the ferromagnet at the interfaces between the heavy metal and ferromagnet. This torque produces the exact disturbance that results in the formation of spin waves.
By comparing various samples, scientists have demonstrated that the terahertz field itself is incapable of producing spin waves directly. Only a detour will result in success.
Thus, they were able to validate theoretical predictions regarding the efficiency of spin-orbit torques on picosecond timescales. Therefore, the new sample system functions as a terahertz-driven source of spin waves that, in theory, could be easily integrated into circuits.
This research represents a significant step toward the implementation of terahertz technology in novel electronic components. Moreover, the demonstrated method offers new opportunities for non-contact spin-based device characterization.
Reference: Salikhov, R., Ilyakov, I., Körber, L. et al. Coupling of terahertz light with nanometre-wavelength magnon modes via spin–orbit torque. Nat. Phys. (2023) DOI: 10.1038/s41567-022-01908-1
