The magnetic structure and field dependence of the cycloid phase mediating the spin reorientation transition in Ca$_3$Ru$_2$O$_7$

TL;DR

This study elucidates the magnetic cycloid phase in Ca$_3$Ru$_2$O$_7$, detailing its temperature and field dependence, phase transitions, and underlying interactions, using neutron diffraction, magnetometry, and resistivity measurements.

Contribution

It provides the first comprehensive experimental phase diagram of the cycloid phase in Ca$_3$Ru$_2$O$_7$, highlighting the field-induced expansion of this phase and the role of Dzyaloshinskii–Moriya interactions.

Findings

Cycloid phase exists between 46.7 K and 49.0 K in zero field.

Applied magnetic field increases the cycloid wavevector and eccentricity.

The phase diagram shows the cycloid phase expanding with magnetic field up to 5 T.

Abstract

We report a comprehensive experimental investigation of the magnetic structure of the cycloidal phase in Ca$_3$Ru$_2$O$_7$, which mediates the spin reorientation transition, and establishes its magnetic phase diagram. In zero applied field, single-crystal neutron diffraction data confirms the scenario deduced from an earlier resonant x-ray scattering study: between $46.7$~K $< T < 49.0$~K the magnetic moments form a cycloid in the $a-b$ plane with a propagation wavevector of $(\delta,0,1)$ with $\delta \simeq 0.025$ and an ordered moment of about 1 $\mu_{\rm{B}}$, with the eccentricity of the cycloid evolving with temperature. In an applied magnetic field applied parallel to the $b$-axis, the intensity of the $(\delta,0,1)$ satellite peaks decreases continuously up to about $\mu_0 H \simeq 5$ T, above which field the system becomes field polarised. Both the eccentricity of the cycloid and the wavevector increase with field, the latter suggesting an enhancement of the anti$-$symmetric Dzyaloshinskii$-$Moriya interaction via magnetostriction effects. Transitions between the various low-temperature magnetic phases have been carefully mapped out using magnetometry and resistivity. The resulting phase diagram reveals that the cycloid phase exists in a temperature window that expands rapidly with increasing field, before transitioning to a polarised paramagnetic state at 5 T. High-field magnetoresistance measurements show that below $T\simeq 70$ K the resistivity increases continuously with decreasing temperature, indicating the inherent insulating nature at low temperatures of our high-quality, untwinned, single-crystals. We discuss our results with reference to previous reports of the magnetic phase diagram of Ca$_3$Ru$_2$O$_7$ that utilised samples which were more metallic and/or poly-domain.