Decoupling of the Antiferromagnetic and Insulating States in Tb doped Sr2IrO4

TL;DR

This study demonstrates that minimal Tb doping in Sr2IrO4 decouples its antiferromagnetic order from its insulating state, revealing a complex interplay of spin-orbit coupling, crystal field effects, and Hund's rule coupling that challenges conventional understanding.

Contribution

It provides the first detailed investigation showing how Tb doping suppresses magnetic order without metallization, highlighting an unconventional decoupling of magnetic and insulating states in iridates.

Findings

Suppression of AFM transition at 3% Tb doping

Persistence of insulating state despite magnetic suppression

Incommensurate magnetic order with spiral or spin density wave characteristics

Abstract

Sr2IrO4 is a spin-orbit coupled insulator with an antiferromagnetic (AFM) transition at TN=240 K. We report results of a comprehensive study of single-crystal Sr2Ir1-xTbxO4. This study found that mere 3% (x=0.03) tetravalent Tb4+(4f7) substituting for Ir4+ (rather than Sr2+) completely suppresses the long-range collinear AFM transition but retains the insulating state, leading to a phase diagram featuring a decoupling of magnetic interactions and charge gap. The insulating state at x=0.03 is characterized by an unusually large specific heat at low temperatures and an incommensurate magnetic state having magnetic peaks at (0.95, 0, 0) and (0, 0.95, 0) in the neutron diffraction, suggesting a spiral or spin density wave order. It is apparent that Tb doping effectively changes the relative strength of the SOI and the tetragonal CEF and enhances the Hund's rule coupling that competes with the SOI, and destabilizes the AFM state. However, the disappearance of the AFM accompanies no metallic state chiefly because an energy level mismatch for the Ir and Tb sites weakens charge carrier hopping and renders a persistent insulating state. This work highlights an unconventional correlation between the AFM and insulating states in which the magnetic transition plays no critical role in the formation of the charge gap in the iridate.