Scientists have revealed how lattice vibrations and spins talk to each other in a hybrid excitation known as an electromagnon. To achieve this, they used a unique combination of experiments at the X-ray free electron laser SwissFEL. Understanding this fundamental process at the atomic level opens the door to ultrafast control of magnetism with light.
Within the atomic lattice of a solid, particles and their various properties cooperate in wave like motions known as collective excitations. When atoms in a lattice jiggle together, the collective excitation is known as a phonon. Similarly, when the atomic spins – the magnetisation of the atoms – move together, it’s known as a magnon.
The situation gets more complex. Some of these collective excitations talk to each other in so-called hybrid excitations. One such hybrid excitation is an electromagnon. Electromagnons get their name because of the ability to excite the atomic spins using the electric field of light, in contrast to conventional magnons: an exciting prospect for numerous technical applications. Yet their secret life at an atomic level is not well understood.
It’s been suspected that during an electromagnon the atoms in the lattice wiggle and the spins wobble in an excitation that is essentially a combination of a phonon and a magnon. Yet since they were first proposed in 2006, only the spin motion has ever been measured. How the atoms within the lattice move – if they move at all – has remained a mystery. So too has an understanding of how the two components talk to each other.
Now, in a sophisticated series of experiments at the Swiss X-ray free-electron laser SwissFEL, researchers at PSI have added these missing pieces to the jigsaw. “With a better understanding of how these hybrid excitations work, we can now start to look into opportunities to manipulate magnetism on an ultrafast timescale,” explains Urs Staub, head of the Microscopy and Magnetism Group at PSI, who led the study.
First the atoms, then the spins
In their experiments at SwissFEL, the researchers used a terahertz laser pulse to induce an electromagnon in a crystal of multiferroic hexaferrite. Using time-resolved X-ray diffraction experiments they then took ultrafast snapshots of how the atoms and spins moved in response to the excitation. With this, they proved both that the atoms within the lattice really do move in an electromagnon and also revealed how energy is transferred between lattice and spin.
A striking outcome of their study was that the atoms move first, with the spins moving fractionally later. When the terahertz pulse strikes the crystal, the electric field pushes the atoms into motion, initiating the phononic part of the electromagnon. This motion creates an effective magnetic field that subsequently moves the spins.
“Our experiments revealed that the excitation does not move the spins directly. It was previously unclear whether this would be the case,” explains Hiroki Ueda, beamline scientist at SwissFEL and the first author of the publication.
Going further, the team could also quantify how much energy the phononic component acquires from the terahertz pulse and how much energy the magnonic component acquires through the lattice. “This is an important piece of information for future applications in which one seeks to drive the magnetic system,” adds Ueda.
Read more on PSI website
Image: Hiroki Ueda, first author of the paper, working at the new Furka experimental at SwissFEL Here, using soft X-rays, Ueda and colleagues could reveal the motion of the spins during an electromagnon, complementing hard X-ray measurements of lattice vibrations made at the Bernina experimental station.
Credit: Paul Scherrer Institute/Markus Fischer