Magnetic reorientation transition in a three orbital model for $\rm Ca_2 Ru O_4$ -- Interplay of spin-orbit coupling, tetragonal distortion, and Coulomb interactions

TL;DR

This paper investigates the complex magnetic reorientation transition in a three-orbital model of $ m Ca_2 Ru O_4$, revealing how spin-orbit coupling, tetragonal distortion, and Coulomb interactions interplay to produce novel magnetic phases and anisotropies.

Contribution

It introduces a comprehensive three-orbital model including orbital off-diagonal condensates, uncovering new features like anisotropic SOC renormalization and magnetic reorientation transitions.

Findings

Planar antiferromagnetic order with easy $a-b$ plane at moderate tetragonal distortion.

Transition to $c$ axis ferromagnetic order with decreasing tetragonal distortion.

Strong coupling between orbital magnetic moments and orbital fields.

Abstract

Including the orbital off-diagonal spin and charge condensates in the self consistent determination of magnetic order within a realistic three-orbital model for the $4d^4$ compound $\rm Ca_2 Ru O_4$, reveals a host of novel features including strong and anisotropic spin-orbit coupling (SOC) renormalization, coupling of strong orbital magnetic moments to orbital fields, and a magnetic reorientation transition. Highlighting the rich interplay between orbital geometry and overlap, spin-orbit coupling, Coulomb interactions, tetragonal distortion, and staggered octahedral tilting and rotation, our investigation yields a planar antiferromagnetic (AFM) order for moderate tetragonal distortion, with easy $a-b$ plane and easy $b$ axis anisotropies, along with small canting of the dominantly $yz,xz$ orbital moments. With decreasing tetragonal distortion, we find a magnetic reorientation transition from the dominantly planar AFM order to a dominantly $c$ axis ferromagnetic (FM) order with significant $xy$ orbital moment.