Manipulation with light can lead to a change of the magnetic properties of oxide thin films on ultrafast timescales. This novel type of ultrafast light control may lead to new prospects in magnetic storage technologies. A research team, led by scientists of the Max Planck Institute in Hamburg and TU Delft, has published their results online in the journal Nature Materials.

Transition metal oxides, such as 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. But there is also a strong correlation between these properties and the atomic arrangement of the crystal lattice, so that controlled structural modifications allow for tuning the electronic and magnetic state of these materials, too.  

Recently, researchers started studying heterostructures composed of different oxide materials. An oxide thin film on a substrate can have very different properties than the bulk constituent due to strain produced at the interface between the substrate and the film. This makes complex oxide heterostructures a versatile tool for engineering the device 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', says Andrea Caviglia from the TU Delft. 

At cryogenic temperatures, NdNiO₃ is an antiferromagnetic insulator. Above 200 K, this material becomes a metal and the antiferromagnetic spin ordering disappears. When a 30-nanometer NdNiO₃ thin film is grown on a 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 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. In an earlier experiment, a laser pulse (15 µm) triggered a vibrational mode in the substrate and a drastic change of electrical conductivity in the nickelate film. 

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 the team used a technique called time-resolved resonant soft X-ray diffraction at SLAC, California.  

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 from there into the NdNiO₃ film. 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.

Visit Nature.com for the publication: Spatially resolved ultrafast magnetic dynamics initiated at a complex oxide heterointerface