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

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 $(\hat{I}´,0,1)$ with $\hat{I}´\simeq 0.025$ and an ordered moment of about 1 $\hat{I}{¼}_{\mathrm{B}}$, with the eccentricity of the cycloid evolving with temperature. In an applied magnetic field applied parallel to the $b$-axis, the intensity of the $(\hat{I}´,0,1)$ satellite peaks decreases continuously up to about $\hat{I}{¼}_{0}H\simeq 5$ 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.

12 pages, 11 figures