Scientists couple terahertz radiation with spin waves

An international research team led by Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has developed a new method to efficiently couple terahertz waves with much shorter wavelengths, so-called spin waves.As experts report in the journal natural physics, their experiments, combined with theoretical models, reveal the underlying mechanisms of this process previously thought impossible. This result is an important step for the development of new energy-saving spin-based techniques for data processing.

“We were able to efficiently excite high-energy spin waves Terahertz Light is generated in a sandwich-like material system consisting of two metal films a few nanometers thick. ferromagnetic layer Electrons have an effective spin that behaves like a top.

And like a gyroscope, external perturbations can tilt the axis of rotation of the spin. A gyroscope movement called precession follows. In ferromagnets, there is a very strong interaction between electron spins, resulting in locally initiated precession spin wave The entire layer of ferromagnetic material.

This is interesting because spin waves, like other waves, can be used as information carriers. In a considered ferromagnet, while each electron spin is in motion, it remains in its position in the atomic lattice, so no current flow is involved. Therefore, unlike current computer chips, spin-based devices do not lose heat through electrical current.

Conveniently, the characteristic frequency of high-energy spin waves is Terahertz rangeThis is exactly the target range for new ultra-high speed technologies for data transmission and processing. Therefore, combining optical terahertz technology with spin-based devices may enable entirely new efficient concepts in IT technology.

The Problem: Communication Between Different Types of Waves

Similar to light, we can also describe it in terms of individual particles called photons, but the energy of spin waves is quantized and the quanta of spin waves are called magnons. Magnons and terahertz photons have the same energy, so they should be easily convertible into each other. But there are problems along the way. The difference is that the two wave phenomena have completely different velocities.

terahertz wave electromagnetic radiation At the speed of light, spin waves are coupled to the presence of interacting spins. Its propagation speed is several hundred times faster than that of light. The wavelength of terahertz waves is less than 1 millimeter, while the wavelength of spin waves is about several nanometers. As a result, terahertz waves do not have the opportunity to specifically and directly transfer their energy to slower spin waves.

To solve this problem, the researchers devised a combination of very thin metallic layers of tantalum and platinum, with a thin layer of ferromagnetic nickel-iron alloy inserted in between. This combination of materials is precisely tuned to “convert” the signal from the optical world to the spin world.

Many steps from light to spin

They developed and manufactured functional layer materials at the HZDR Institute of Ion Beam Physics and Materials Research. To do so, they gradually deposited a metal film on a thin glass substrate. “In our experiments, we irradiated the sample with an intense terahertz pulse and measured the rapidly time-varying magnetization with an optical laser pulse. Absolutely,” Kovalev explains.

“We varied many factors, including the external magnetic field and different material compositions of the layers, until we could confidently show that these were indeed the spin waves we were looking for. I let him,” says teammate Dr. Ruslan Salikhov. magnetic material.

For this conversion of electromagnetic waves into spin waves, the team harnessed a whole range of different quantum effects. Metaphorically speaking, these effects ensure that terahertz and spin waves understand each other. beginning, terahertz radiation It accelerates the free electrons of heavy metals and enables the formation of microscopic currents.

These currents are converted into spin currents by the so-called spin Hall effect. That is, a stream of electrons that have only a very specific spin orientation and can carry the resulting angular momentum into local space. At the interface between a heavy metal and a ferromagnet, this angular momentum exerts a torque on the spin of the ferromagnet. This torque precisely provides the perturbation that leads to the formation of spin waves.

By comparing different samples, scientists were able to show that the terahertz field itself cannot directly generate spin waves. Only detours lead to success. In this way, they were able to confirm theoretical predictions about the efficiency of spin-orbit torque on the picosecond timescale.

The new sample system thus acts in principle as a terahertz-driven spin wave source that can be easily integrated into circuits. This work is an important step towards the use of terahertz technology in new electronic components. At the same time, the demonstrated method opens new possibilities for non-contact characterization of spin-based devices.