Magnetic Multipoles in a Ruthenate Ca3Ru2O7

TL;DR

This paper investigates the magnetic multipole structure of Ca3Ru2O7 using neutron and x-ray diffraction, revealing evidence of Dirac dipoles and proposing experiments with circularly polarized x-rays to detect chiral multipole order.

Contribution

It provides the first evidence of Dirac dipoles in Ca3Ru2O7 and suggests new diffraction experiments to detect chiral multipole order using circularly polarized x-rays.

Findings

Neutron diffraction shows evidence of Dirac dipoles.

X-ray diffraction data is ambiguous regarding Dirac dipoles.

Proposed experiments can detect chiral order of Dirac multipoles.

Abstract

Compulsory Dirac multipoles in the bilayer perovskite Ca3Ru2O7 are absent in published analyses of experimental data. In a first step at correcting knowledge of the magnetic structure, we have analysed existing Bragg diffraction patterns gathered on samples held well below the N\'eel temperature at which A-type antiferromagnetic order of axial dipoles spontaneously develops. Patterns were gathered with neutrons, and linearly polarized x-rays tuned in energy to a ruthenium atomic resonance. Neutron diffraction data contains solid evidence of Dirac dipoles (anapoles or toroidal moments). No such conclusion is reached with existing x-ray diffraction data, which instead is ambiguous on the question. To address this shortcoming by future experiments, we calculated additional diffraction patterns. Chiral order of Dirac multipoles is allowed by magnetic space-group PCna21, and it can be exposed in Bragg diffraction using circularly polarized x-rays. Likewise, a similar experiment can expose a chiral order of axial dipoles. A magnetic field applied parallel to the b-axis creates a ferrimagnetic structure in which bulk magnetization arises from field-induced nonequivalent Ru sites (magnetic space-group Pm'c'21).