A new way of controlling magnetic order in solids

A new way of controlling magnetic order in solids

 

An international team of researchers have studied structural and electronic phase transition in the transition metal oxide Ca3Ru2O7 using ARPES at beamline Bloch. The results show that the electronic structure is a driver in spin-reorientation transition. This is a step in the direction of new spintronic technologies.

Researchers are fascinated by transition metal oxides (TMO) because of their unique magnetic and electronic characteristics. Many of the properties arise as so-called correlated phenomena, meaning they originate from the interactions between electrons in the material. One of the future applications of TMOs is thought to be spin electronics or spintronics, where the operation is not only governed by the charge but also the spin property of the electrons. Spintronic applications require the ordering of the electrons magnetic moments.

In a recently published study, an international research team used beamline Bloch to study the temperature-dependent properties of the TMO Ca3Ru2O7 and particularly the phase transitions happening at two particular temperatures.

“From our study, we were able to find a new way in which magnetic order can be controlled in solids, based on the electronic structure of the material,” says Prof Phil King from University of St Andrews in the UK. “We termed this a ‘magneto-electronic anisotropy’ and showed how this ultimately explains a coupled spin-reorientation and structural phase transition in the correlated oxide Ca3Ru2O7.”

The image illustrates the changes across the Ts = 48 K phase transition, as indicated by the arrow on the far left. For both above and below Ts, we show a schematic of the orientation of the magnetic moments and the local inversion-breaking field on the left, which dictates whether the relevant spin-orbit term is allowed for hybridisation, and a measured ARPES dispersion in the respective phase on the right, showing the opening of the gap below Ts.

The Fermi surface, a concept in the science of solid materials, is of interest to investigate as it is linked to the crystal structure of the material and the energy levels of the electrons.

“What surprised us the most was the creation of an additional, large Fermi surface as the material is warmed through its phase transition,” adds King. “This needed a complete rethink on the temperature-dependent changes of electronic structure in Ca3Ru2O7, which we now believe to in fact be the driver of its phase transition.”

At beamline Bloch, the research team used a method called Angle-Resolved Photoemission Spectroscopy (ARPES). The method gives information about the electronic structure of the material, the energy landscape for the electrons. In ARPES, not only the energy but also the angle of the X-ray induced photoelectrons is measured.

“The temperature-dependent electronic dispersions we measured at Bloch clearly demonstrated the unexpected qualitative change of the electronic structure across the phase transition, giving us the key component for the understanding of the transition mechanism,” says Dr Igor Marković, from University of St Andrews.

The team are preparing for more experiments.

“We already have further experiments planned at the Bloch beamline to be carried out this year, if travel and other restrictions permit,” concludes Marković. “In these, we will investigate other unusual aspects of the electronic structure and properties of this interesting material.”

 

Publication

Igor Marković, Matthew D. Watson, Oliver J. Clark, Federico Mazzola, Edgar Abarca MoralesChris A. HooleyHelge RosnerCraig M. PolleyThiagarajan BalasubramanianSaumya MukherjeeNaoki KikugawaDmitry A. SokolovAndrew P. Mackenzie, Phil D. C. King. (23 June 2020) Electronically driven spin-reorientation transition of the correlated polar metal Ca3Ru2O7. PNAS 15524–15529. DOI: 10.1073/pnas.2003671117