Emergent chirality in magnetic and electronic textures

Program Description

Chirality determines many important properties in nature, ranging from theelectroweak interaction in particle physics to the reactions of enzymes that arefundamental for life. In condensed matter physics, chiral structures determine thefunctional behavior of magnetic and other correlated-electron materials. Underlyingthese functionalities are a wealth of intriguing nanoscale ordering phenomena suchas charge, orbital, and spin superstructures, as well as periodic arrays of domainsand domains walls or vortices.

This talk focuses on the chiral nanostructures in two distinct material systems:Chiral magnetic domain walls (DWs) in Co/Pt/Cu multilayers; and the emergentchirality of electric polarization vortices in PbTiO3/SrTiO3 superlattices.

Our experimental measurements of DW chirality in ultrathin films are importantfor both the fundamental understanding of DWs and in research on topologicallyprotected nanomagnetic structures [1]. Additionally, they provide criticalinformation for the development of DW-based spintronic devices, where DWchirality has been suggested to greatly suppress the critical current density fordriving DW motion.

Recently we have discovered chiral topologies of electric polarization that arereminiscent of rotational spin topologies. These nanometer-scale arrays of counterrotatingvortex pairs can be created by employing the competition between charge,orbital, and lattice degrees of freedom in superlattices of alternating PbTiO3 andSrTiO3 layers [2]. These observations have implications for the creation of newstates of matter (such as dipolar skyrmions, hedgehog states) and associatedphenomena in ferroic materials, such as electrically controllable chirality.

For both studies we employ the unique capabilities of resonant soft x-raydiffraction (RSXD) as a tool for investigating electronic and magneticnanostructures. RSXD uses x-ray wavelengths of ~1‒3 nm that are well matched tothe periodicity of the nanostructures. By selecting wavelengths that correspond toresonant electronic transitions we gain sensitivity to probe the magnetic or electricpolarization orientations. In particular, x-ray circular dichroism (XCD)—thedifference in diffraction intensity when circularly polarized x-rays of oppositehelicity are used—is central to identifying chiral polarization textures that emergein magnetic domain walls and in vortex superlattices composed of non-chiralconstituents.[1] G. Chen et al., Nature Communications 4, 2671 (2013).[2] A. K. Yadav et al., Nature 530, 198 (2016).

Biography

Padraic Shafer is a Research Scientist at the Advanced Light Source, an x-raysynchrotron facility at Lawrence Berkeley National Laboratory. He received a Ph.D.in Materials Science and Engineering from the University of California, Berkeley forwork on nanoscale ferroelectric domains and vibrational energy scavenging. As aPostoctoral Research Fellow at the ALS, Padraic designed and commissioned abeamline endstation for resonant x-ray scattering studies of strongly correlatedelectronic materials. He joined the Magnetic Spectroscopy and Scattering Group asan ALS scientist in 2012. Previously he was a research and development engineer atthe Microelectronics Division of IBM, after receiving a B.S. from the University ofIllinois at Urbana-Champaign.

Padraic studies electronic order at domain walls and thin film interfaces in complexoxides and magnetic metals using a combination of resonant soft x-ray absorptionand scattering. He also investigates spin-pumped dynamics in magnetic multilayersusing x-ray based ferromagnetic resonance.