Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Light-driven atomic rotations excite magnetic waves

24.10.2016

Terahertz excitation of selected crystal vibrations leads to an effective magnetic field that drives coherent spin motion

Controlling functional properties by light is one of the grand goals in modern condensed matter physics and materials science. A new study now demonstrates how the ultrafast light-induced modulation of the atomic positions in a material can control its magnetization. An international research team led by Andrea Cavalleri from the Max Planck Institute for the Structure and Dynamics of Matter at CFEL in Hamburg used terahertz light pulses to excite pairs of lattice vibrations in a magnetic crystal.


Light-driven atomic rotations (spirals) induce coherent motion of the electronic spins (blue arrows).

Image: J.M. Harms/MPI for the Structure and Dynamics of Matter

These short bursts of light caused the lattice ions to rotate around their equilibrium positions, acting as an ultrafast effective magnetic field on the electronic spins to coherently drive a magnetic wave. These findings represent an important hallmark on how light interacts with matter and establish a novel approach in the control of magnetization at terahertz speed, making the research potentially relevant for magnetic storage technologies. The results are presented in the journal Nature Physics today.

Ultrafast vibrational control of materials

Understanding microscopic interactions and engineering collective responses to tailor material functionalities has been a driving force for the fields of condensed matter physics and materials science, both from a fundamental and technological point of view. Conventional static approaches to modify and control material properties include chemical substitution, i.e. the replacement of particular atoms in the crystal lattice by atoms of another chemical element, or the application of external perturbations like pressure and magnetic fields.

A conceptually different path consists in the ultrafast dynamical modulation of material parameters. In particular, the direct excitation of lattice vibrations in solid-state systems (collective excitations of the ions called phonons) by ultra-short and ultra-intense terahertz light pulses has proven to be an extremely efficient route to material control. The Hamburg group has been playing a pioneering role in this technique, called nonlinear phononics. Recent successful examples include the control of insulator–metal transitions, melting of magnetic order and enhancement of superconductivity.

The origin of this powerful tool lies in the nonlinear nature of the crystal lattice. Indeed, a phonon driven by laser light to large amplitude can transfer energy to other lower-frequency vibrational modes. This nonlinear phonon–phonon coupling results in a transient, directional and selective distortion of the crystal structure, i.e. specific atoms of the lattice move temporarily to different positions. This effect is of extreme relevance for complex materials, in which macroscopic electronic properties are strongly tied to the atomic arrangement.

Spin control by ionic loops

In an effort to generalize these principles and show that the coherent driving of phonons can not only affect the crystal structure but also directly control other properties like magnetization, an international team of scientists from Germany, the Netherlands and the USA studied terahertz stimulation of the magnetic material erbium orthoferrite (ErFeO3).

First, the researchers excited a single phonon and observed the typical signature of nonlinear phononics, i.e. the energy transfer to lower-frequency lattice vibrations. The key idea to go beyond this “conventional” observation was to combine the action of two different orthogonal phonons at the same time. Due to the modes’ slightly different frequencies, the atoms of the crystal lattice started rotating about their equilibrium positions resulting in a circularly polarized phononic field. This movement dynamically modulated the electric field felt by the electrons, perturbing their orbital motion. As a consequence, a high-frequency magnetic wave – a collective excitation of the electron spins – was excited. These results can be viewed as the generation of an ultrafast magnetic field pulse by the circular phonons.

“This is the first time that a direct and coherent control of spins by lattice vibrations has been observed,” says Tobia Nova, Ph.D. student at the MPSD in Hamburg and first author of the paper. The experiment successfully demonstrates that it is possible to transfer energy between driven phonons and magnetic excitations while “distorting” the spin ordering of a material, thus leading to an ultrafast control of its magnetization.

Further routes for research and potential applications

The magnetic wave amplitude scales quadratically with the terahertz electric field strength, implying that a moderate increase in the field strength may lead to immense phonon-driven magnetic dynamics and possibly toward magnetic switching. As the effect operates at terahertz frequencies, it might become applicable in new devices that operate at such high speeds.

Furthermore, possible applications of circularly polarized phonons might extend well beyond magnetization control. A time-dependent perturbation in the form of circularly polarized light has been shown to manipulate surface properties in a new class of materials, so-called topological insulators, and further has been predicted to induce analogous topological states in graphene. In a similar fashion, phonon-driven Floquet physics might be induced by lattice rotations.

This work was made possible by the ERC Synergy Grant “Frontiers in Quantum Materials’ Control” (Q-MAC) and The Hamburg Centre for Ultrafast Imaging (CUI). The Center for Free-Electron Laser Science (CFEL) is a joint enterprise of DESY, the Max Planck Society and the University of Hamburg. Further institutions involved in this collaboration were the Radboud University in Nijmegen and the University of Michigan.

Contact persons:
Mr. Tobia Nova
Max Planck Institute for the Structure and Dynamics of Matter
Center for Free-Electron Laser Science
Luruper Chaussee 149
22761 Hamburg
Germany
+49 (0)40 8998-6574
tobia.nova@mpsd.mpg.de

Prof. Dr. Andrea Cavalleri
Max Planck Institute for the Structure and Dynamics of Matter
Center for Free-Electron Laser Science
Luruper Chaussee 149
22761 Hamburg
Germany
+49 (0)40 8998-5354
andrea.cavalleri@mpsd.mpg.de

Original publication:
T. F. Nova, A. Cartella, A. Cantaluppi, M. Först, D. Bossini, R. V. Mikhaylovskiy, A. V. Kimel, R. Merlin and A. Cavalleri, “An effective magnetic field from optically driven phonons,” Nature Physics, Advance Online Publication (October 24, 2016), DOI: 10.1038/nphys3925

Weitere Informationen:

http://dx.doi.org/10.1038/nphys3925 Original publication
http://qcmd.mpsd.mpg.de/ Research group of Prof. Dr. Andrea Cavalleri
http://www.mpsd.mpg.de/en Max Planck Institute for the Structure and Dynamics of Matter

Dr. Michael Grefe | Max-Planck-Institut für Struktur und Dynamik der Materie

More articles from Physics and Astronomy:

nachricht Hope to discover sure signs of life on Mars? New research says look for the element vanadium
22.09.2017 | University of Kansas

nachricht Calculating quietness
22.09.2017 | Forschungszentrum MATHEON ECMath

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: The pyrenoid is a carbon-fixing liquid droplet

Plants and algae use the enzyme Rubisco to fix carbon dioxide, removing it from the atmosphere and converting it into biomass. Algae have figured out a way to increase the efficiency of carbon fixation. They gather most of their Rubisco into a ball-shaped microcompartment called the pyrenoid, which they flood with a high local concentration of carbon dioxide. A team of scientists at Princeton University, the Carnegie Institution for Science, Stanford University and the Max Plank Institute of Biochemistry have unravelled the mysteries of how the pyrenoid is assembled. These insights can help to engineer crops that remove more carbon dioxide from the atmosphere while producing more food.

A warming planet

Im Focus: Highly precise wiring in the Cerebral Cortex

Our brains house extremely complex neuronal circuits, whose detailed structures are still largely unknown. This is especially true for the so-called cerebral cortex of mammals, where among other things vision, thoughts or spatial orientation are being computed. Here the rules by which nerve cells are connected to each other are only partly understood. A team of scientists around Moritz Helmstaedter at the Frankfiurt Max Planck Institute for Brain Research and Helene Schmidt (Humboldt University in Berlin) have now discovered a surprisingly precise nerve cell connectivity pattern in the part of the cerebral cortex that is responsible for orienting the individual animal or human in space.

The researchers report online in Nature (Schmidt et al., 2017. Axonal synapse sorting in medial entorhinal cortex, DOI: 10.1038/nature24005) that synapses in...

Im Focus: Tiny lasers from a gallery of whispers

New technique promises tunable laser devices

Whispering gallery mode (WGM) resonators are used to make tiny micro-lasers, sensors, switches, routers and other devices. These tiny structures rely on a...

Im Focus: Ultrafast snapshots of relaxing electrons in solids

Using ultrafast flashes of laser and x-ray radiation, scientists at the Max Planck Institute of Quantum Optics (Garching, Germany) took snapshots of the briefest electron motion inside a solid material to date. The electron motion lasted only 750 billionths of the billionth of a second before it fainted, setting a new record of human capability to capture ultrafast processes inside solids!

When x-rays shine onto solid materials or large molecules, an electron is pushed away from its original place near the nucleus of the atom, leaving a hole...

Im Focus: Quantum Sensors Decipher Magnetic Ordering in a New Semiconducting Material

For the first time, physicists have successfully imaged spiral magnetic ordering in a multiferroic material. These materials are considered highly promising candidates for future data storage media. The researchers were able to prove their findings using unique quantum sensors that were developed at Basel University and that can analyze electromagnetic fields on the nanometer scale. The results – obtained by scientists from the University of Basel’s Department of Physics, the Swiss Nanoscience Institute, the University of Montpellier and several laboratories from University Paris-Saclay – were recently published in the journal Nature.

Multiferroics are materials that simultaneously react to electric and magnetic fields. These two properties are rarely found together, and their combined...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

“Lasers in Composites Symposium” in Aachen – from Science to Application

19.09.2017 | Event News

I-ESA 2018 – Call for Papers

12.09.2017 | Event News

EMBO at Basel Life, a new conference on current and emerging life science research

06.09.2017 | Event News

 
Latest News

Rainbow colors reveal cell history: Uncovering β-cell heterogeneity

22.09.2017 | Life Sciences

Penn first in world to treat patient with new radiation technology

22.09.2017 | Medical Engineering

Calculating quietness

22.09.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>