Spin and charge frozen by strain

In the development of next-generation microelectronics, a great deal of attention has been given to the use of epitaxy (the deposition of a crystalline overlayer on a crystalline substrate) to tailor the properties of materials to suit particular applications. Correlated electron systems provide an excellent platform for the development of new microelectronic devices due to the presence of multiple competing ground states of similar energy. In some cases, strain can drive these systems between two or more such states, resulting in phase transitions and dramatic changes in the properties of the material. Often, the specific mechanism by which strain accomplishes such a feat is unknown. This was precisely the case in lanthanum cobaltite, LaCoO3, which undergoes a strain-induced transition from paramagnet to ferromagnet, until a recent study carried out at the U.S. Department of Energy’s Advanced Photon Source (APS) revealed the intriguing microscopic phenomena at work in this system. These phenomena may play a role in spin-state and magnetic-phase transitions, regardless of stimulus, in many other correlated systems.

Lanthanum cobaltite is a perovskite, which means the structure can be thought of as made up of distorted cubes with cobalt at the cube centers, oxygen at the cube faces, and lanthanum at the cube corners. The cobalt ions have a nominal 3+ valence, meaning they lose three electrons to the neighboring oxygen ions. Bulk LaCoO3 is paramagnetic (that is, having a net magnetization only in the presence of an externally applied magnetic field) above 110 Kelvin, and non-magnetic below that temperature. In its ground state, all the electrons on a given cobalt ion are paired, meaning their magnetic spins cancel each other out. These are so-called low-spin (LS) Co3+ ions, and when all of the cobalt ions are in this form, LaCoO3 is non-magnetic.

>Read more on the Advanced Photon Source website

Image: Upper left: Resonant x-ray scattering at the cobalt K-edge. Inversion of the spectra at the reflections shown indicates the presence of charge order. Upper right: X-ray diffraction reciprocal space maps at the (002) and (003) reflection indicating the high epitaxial quality of the films. The satellite peaks result from lattice modulations associated with the reduced symmetry in the film. Lower left: Schematic crystal structure of epitaxial LaCoO3 showing the arrangement of cobalt sites with different charge and spin. The circulated charge transfer from oxygen to the different cobalt sites is also shown. Lower right: Calculated total energy as a function of the difference between the in-plane Co-O bond lengths of HS and LS cobalt ions (∆rCo-O).

“X-ray streaking” allows ultrafast processes to be followed using a single pulse of light

Grazing light for rapid events

An international team of scientists has developed a new experimental method at the FLASH X-ray laser which allows the sequence of events involved in a process to be observed using a single, ultrashort pulse of light from FLASH. Their method is called “X-ray streaking” and enables researchers to observe ultrafast processes continuously, instead of being confined to taking snapshots at discrete intervals using separate X-ray pulses. Apart from the extreme brightness of the FLASH beam, the scientists also made use of an X-ray lens which they introduced into the beamline in a particular configuration, so as to capture a chronological sequence of events using a single X-ray pulse. To demonstrate the functionality of X-ray streaking, they observed the ultrafast demagnetisation of cobalt.

The invention of X-ray lasers has considerably boosted the study of the dynamics of matter. Pump-probe experiments allow artificially induced (“pumped”) processes and reactions to be photographed (“probed”) using an extremely short X-ray pulse at predetermined intervals. Ideally, these photographs, taken with different time delays, can then be assembled to create a film showing the sequence of events during an ultrafast process with a temporal resolution of the order of femtoseconds. One limitation of this otherwise promising experimental technique is, however, that the experiment has to be conducted all over again for each time delay. This means that before each observation, the process of interest must be triggered using the same starting conditions and it must run through the same sequence of events – both of which rule out extreme experimental conditions.

>Read more on the FLASH website

Image Caption: (a,b) Raw images from the reflection and reference detectors respectively. Both the images for the pumped and the un-pumped event are acquired using a single x-ray pulse. (c) Transient reflectivity image (as defined in the text) calculated from the images shown in (a,b). (d) Reshaped transient reflectivity image after calibration of the time window. Article published in Scientific Reports.