Researchers explore ultrafast control of magnetism across interfaces: A new study discovers how the sudden excitation of lattice vibrations in a crystal can trigger a change of the magnetic properties of an atomically-thin layer that lies on its surface.
A research team, led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter at CFEL in Hamburg, the University of Oxford, and the Université de Genève, used extremely short X-ray pulses from a free-electron laser to discover that melting of magnetic order in the thin layer is initiated at its interface with the substrate and progressively moves into the interior of the film on an ultra-short time scale. This novel type of ultrafast light control in materials engineered at the atomic scale may lead to new prospects in magnetic storage technologies. The results are reported online in the journal Nature Materials.
This image depicts the magnetic ordering, i.e. the antiparallel spin alignment, present in the NdNiO3 thin film that is grown on a LaAlO3 crystal substrate. A short mid-infrared laser pulse triggers vibrations in the substrate, indicated by the smeared oxygen atoms in red. This atomic motion initiates melting of the magnetic order in the functional film, starting locally at the interface between the two materials and progressively moving into the interior of the film.
Transition metal oxides, such as manganites, cuprates or nickelates, have attracted enormous interest among researchers since their electric and magnetic properties can be altered by even subtle changes of external parameters, such as temperature, and electric or magnetic fields. “But there is also a strong correlation between the atomic arrangement of the crystal lattice and these properties, so that controlled structural modifications allow for tuning the electronic and magnetic state of these materials, too,” said Michael Först, scientist at the Max Planck Institute and one of the lead authors of this work.
More recently, researchers started studying heterostructures composed of different oxide materials. An atomically-thin oxide film on a substrate can have properties very different from its bulk form due to a variety of interface effects, including mechanical strain produced at the interface between the substrate and the film. This makes complex oxide heterostructures a versatile tool for engineering materials and devices properties. “In the present work, we studied the possibility to dynamically control the properties of a thin functional film by modifying the atomic structure of the substrate with light,” said Andrea Caviglia, now at the Kavli Institute of Nanoscience at Delft University of Technology.
At cryogenic temperatures, neodymium nickelate (NdNiO3) is an antiferromagnetic insulator, i.e. the spins of the valence electrons align in an alternating pattern with no resulting net magnetization. Above 200 K, this material becomes a metal and the antiferromagnetic spin ordering disappears concomitantly. When an epitaxial NdNiO3 thin film is grown on a lanthanum aluminate (LaAlO3) substrate, the slightly different lattice constants of the two materials induce static strain in the film leading to a reduction of the insulator–metal-transition temperature from 200 K of the bulk material to about 130 K.
Interestingly, the electrical properties of the NdNiO3 film can be changed on ultrafast time scales by selectively exciting lattice vibrations in the LaAlO3 substrate. This was shown in an earlier publication of the main authors in Physical Review Letters. “In that experiment, a mid-infrared laser pulse at 15 µm wavelength triggered a vibrational mode in the substrate and a drastic change of electrical conductivity in the nickelate film was observed by measuring the change of the sample reflectivity with a terahertz, that is far-infrared, pulse,” said Caviglia.
In the current work, the group studied the effect this substrate excitation has on the magnetic properties of the nickelate film. To measure these changes with high spatial and temporal resolution, the team used a technique called time-resolved resonant soft X-ray diffraction at the Linac Coherent Light Source (LCLS) free-electron laser at SLAC, California. The LCLS femtosecond X-ray pulses scatter off the film, carrying time-stamped signatures of the material’s spin ordering that the physicists then used to reconstruct the spatiotemporal magnetic dynamics.
First, the researchers found that the magnetic ordering melts on a timescale of few picoseconds, i.e. at similar time scales as the insulator–metal transition observed earlier, thus suggesting that both processes are connected.
“Even more interestingly, the diffraction experiment showed that the magnetic melting in the nickelate starts locally at the interface to the substrate and propagates, comparable to a wave, from there into the NdNiO3 film,” said Först. “The high speed at which this wave front propagates, indicates that these dynamics are driven by local changes of the electronic structure at the interface”, he added.
Indeed, a theoretical model, assuming the creation of freely movable charge carriers at the hetero-interface by the substrate lattice vibrations, supports this picture. These charges are likely to scramble the antiferromagnetic order as they propagate into the film.
This work was made possible by the ERC Synergy Grant “Frontiers in Quantum Materials’ Control” (Q-MAC), which brings together scientists from the institutions listed above. Further institutions involved in the collaboration are the Diamond Light Source, the Brookhaven National Laboratory, the Lawrence Berkeley National Laboratory, the Stanford Linear Accelerator Center, and the National University of Singapore. CFEL is a cooperation of DESY, the Max Planck Society and the University of Hamburg.
Dr. Michael Först
Max Planck Institute for the Structure and Dynamics of Matter
Center for Free-Electron Laser Science
Luruper Chaussee 149
+49 (0)40 8998-5360
M. Först, A. D. Caviglia, R. Scherwitzl, R. Mankowsky, P. Zubko, V. Khanna, H. Bromberger, S. B.Wilkins, Y.-D. Chuang, W. S. Lee, W. F. Schlotter, J. J. Turner, G. L. Dakovski, M. P. Minitti, J. Robinson, S. R. Clark, D. Jaksch, J.-M. Triscone, J. P. Hill, S. S. Dhesi, and A. Cavalleri, "Spatially resolved ultrafast magnetic dynamics initiated at a complex oxide heterointerface", Nature Materials, 2015; DOI: 10.1038/nmat4341
http://dx.doi.org/10.1038/nmat4341 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
Igniting a solar flare in the corona with lower-atmosphere kindling
29.03.2017 | New Jersey Institute of Technology
NASA spacecraft investigate clues in radiation belts
28.03.2017 | NASA/Goddard Space Flight Center
The Institute of Semiconductor Technology and the Institute of Physical and Theoretical Chemistry, both members of the Laboratory for Emerging Nanometrology (LENA), at Technische Universität Braunschweig are partners in a new European research project entitled ChipScope, which aims to develop a completely new and extremely small optical microscope capable of observing the interior of living cells in real time. A consortium of 7 partners from 5 countries will tackle this issue with very ambitious objectives during a four-year research program.
To demonstrate the usefulness of this new scientific tool, at the end of the project the developed chip-sized microscope will be used to observe in real-time...
Astronomers from Bonn and Tautenburg in Thuringia (Germany) used the 100-m radio telescope at Effelsberg to observe several galaxy clusters. At the edges of these large accumulations of dark matter, stellar systems (galaxies), hot gas, and charged particles, they found magnetic fields that are exceptionally ordered over distances of many million light years. This makes them the most extended magnetic fields in the universe known so far.
The results will be published on March 22 in the journal „Astronomy & Astrophysics“.
Galaxy clusters are the largest gravitationally bound structures in the universe. With a typical extent of about 10 million light years, i.e. 100 times the...
Researchers at the Goethe University Frankfurt, together with partners from the University of Tübingen in Germany and Queen Mary University as well as Francis Crick Institute from London (UK) have developed a novel technology to decipher the secret ubiquitin code.
Ubiquitin is a small protein that can be linked to other cellular proteins, thereby controlling and modulating their functions. The attachment occurs in many...
In the eternal search for next generation high-efficiency solar cells and LEDs, scientists at Los Alamos National Laboratory and their partners are creating...
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are less stable. Now researchers at the Technical University of Munich (TUM) have, for the first time ever, produced a composite material combining silicon nanosheets and a polymer that is both UV-resistant and easy to process. This brings the scientists a significant step closer to industrial applications like flexible displays and photosensors.
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are...
20.03.2017 | Event News
14.03.2017 | Event News
07.03.2017 | Event News
29.03.2017 | Materials Sciences
29.03.2017 | Physics and Astronomy
29.03.2017 | Earth Sciences