Our joint experimental and theoretical paper on Science Advances demonstrates the chirality of microwave evanescent fields that can unidirectionally pump spin waves with a focusing due to the anisotropic dispersion
Magnets, probably the most conventional materials that has attracted the attention of physicists for thousands of years, are important components in every computer for logic operations. The spins in the magnets are coupled via the exchange interaction, which form waves when they move, which can avoid Joule heating in insulators and can propagate very far. More attractively, the spins can only precess anticlockwise around the magnetic field, known as chirality, which promise lots of functionalities such as unidirectional spin transport, chiral spin Seebeck effect, non-contact spin pumping, magnonic non-Hermitian skin effect, spin blockage/trap with perfect energy transfer between magnets, and magnon/ phonon/microwave photon/electron diode effects, as revealed by a theoretical team in MPSD, Delft University of technology and Tohoku university in several theoretical papers on Physical Review Letters (Yu et al., Chiral Pumping of Spin Waves, Magnon Accumulation in Chirally Coupled Magnets, Noncontact Spin Pumping by Microwave Evanescent Fields, Unidirectional Pumping of Phonons by Magnetization Dynamics). However, directly imaging the spin waves and their chirality are ever challenging since they are inertial to the light and cannot be easily controlled by conventional electric method.
The theoretical team now collaborate with Van der Sar lab in Delft University of technology to image the chirality of spin waves. Van der Sar lab introduces a platform for probing coherent spin waves based on magnetic resonance imaging with electron spins in diamond. A significant technique advance allows them to image the spin-wave packet in time-domain. Just like watching water waves in the lake by eyes, scientists now can directly watch how the spin waves move. These results are the most direct demonstration of spin-wave chirality that deepen our understanding of spin waves dynamics, which could help to design pure spin-wave based circuit with unidirectional transport in the future. The team’s work has been published in Science Advances recently.
The theoreticians reveal or clarify a large class of devices that support chiral interaction, viz., a nanomagnet can only couple with the waves propagating along one direction, among various quasiparticles including magnon, electrons and phonons. A key prediction in the chiral coupling rely on the dipolar field from the spin waves. Dipolar field can be imagined as lots of force lines that connect the north pole of a magnet to its south pole. Since the spins of magnons are precessing, they generate dynamical dipolar field that are propagating with the spin waves. In a magnetic film, the right-propagating spin waves can only generate the dipolar field at one side of the film, say above the film, the left-propagating spin waves can only dwell below the film. This property, however, is difficult to be probed directly since the dipolar field is very tiny and decays fast in hundreds of nanometers away from the film.
Spins in nitrogen-vacancy (NV) center of diamond, which is ever a good candidate as qubit for quantum computing, is very sensitive to the tiny magnetic field and can be put very close to the magnetic film that turns out to be very powerful to detect the dipolar field of the spin waves. Iacopo Bertelli et al. in Van der Sar lab use the NV center of diamonds to directly image spin-wave interference with phase resolution, revealing unidirectional, autofocused spin-wave patterns with frequency-controlled numerical apertures. To see how the spin waves move, they introduce external microwaves as reference that interferes with the dipolar field of spin waves, also a kind of microwaves, allowing them to see the standing-wave patterns that evolve with time. Surprisingly, the ultrasensitive NV center images only spin wave propagating in one direction and in half space, a key characteristic of the spin-wave chirality. Also, very intriguingly, the excited spin waves are focused at one point in the film, at which point the amplitude of spin waves are much larger than the other places. We can expect that the unidirectional, self-focused spin waves can transport information in a very efficient way. These observations are explained by the theoreticians in MPSD and Tohoku University in terms of chiral spin-wave excitation and dipolar-field coupling to the sensor spins.
Please see press release at https://www.mpsd.mpg.de/480764/2020-11-mri-yu?c=273566