Effective field theories and spin-wave excitations in helical magnets

TL;DR

This paper derives effective field theories for two types of helical magnets, explaining their spin-wave excitations and connecting theoretical predictions with experimental observations in cuprates and iron oxides.

Contribution

It introduces specific effective field theories for helical magnets near antiferromagnetic and ferromagnetic orderings, linking them to experimental spin-wave spectra.

Findings

Hourglass dispersion linked to antiferromagnetic spin spiral

Quantum fluctuations affect ferromagnetic spin-wave features

Theories applicable to cuprates and Fe-based oxides

Abstract

We consider two classes of helical magnets. The first one has magnetic ordering close to antiferromagnet and the second one has magnetic ordering close to ferromagnet. The first case is relevant to cuprate superconductors and the second case is realized in FeSrO$_3$ and FeCaO$_3$. We derive the effective field theories for these cases and calculate corresponding excitation spectra. We demonstrate that the "hourglass" spin-wave dispersion observed experimentally in cuprates is a fingerprint of the "antiferromagnetic spin spiral state". We also show that quantum fluctuations are important for the "ferromagnetic spin spiral", they influence qualitative features of the spin-wave dispersion.