Correlated Paramagnetism and Interplay of Magnetic and Phononic Degrees of Freedom in 3d-5d Coupled La2CuIrO6

TL;DR

This study reveals that La2CuIrO6 exhibits correlated paramagnetism well above its magnetic transition temperature, driven by strong spin-phonon interactions and broken rotational symmetry, as evidenced by temperature-dependent Raman spectroscopy.

Contribution

It provides the first comprehensive Raman spectroscopic evidence of correlated paramagnetism and strong phonon-magnetic coupling in La2CuIrO6, highlighting complex spin-orbit and lattice interactions.

Findings

Correlated paramagnetic state exists above TN.

Persistent phonon-magnetic interactions up to ~3.5TN.

Anomalous phonon self-energy changes below TN.

Abstract

Conventional Paramagnetism - a state with finite magnetic moment per ion sans long range magnetic ordering, but with lowering temperature the moment on each ion picks up a particular direction, breaking rotational symmetry, and results into long range magnetic ordering. However, in systems with competing multiple degrees of freedom this conventional notion may easily breaks and results into short range correlation much above the global magnetic transition temperature. La2CuIrO6 with complex interplay of spins (s =1/2) on Cu site and pseudo-spin (j =1/2) on Ir site owing to strong spin-orbit coupling provides fertile ground to observe such correlated phenomena. By a comprehensive temperature dependent Raman study, we have shown the presence of such a correlated paramagnetic state in La2CuIrO6 much above the long range magnetic ordering temperature (TN ). Our observation of strong interactions of phonons, associated with Cu/Ir octahedra, with underlying magnetic degrees of freedom mirrored in the observed Fano asymmetry, which remarkably persists as high as ~ 3.5TN clearly signals the existence of correlated paramagnetism hence broken rotational symmetry. Our detailed analysis also reveals anomalous changes in the self-energy parameters of the phonon modes, i.e. mode frequencies and linewidth, below TN, providing a useful gauge for monitoring the strong coupling between phonons and magnetic degrees of freedom.