Dimensionality Driven Spin-Flop Transition in Layered Iridates

TL;DR

This paper investigates how the number of IrO$_2$ layers influences the magnetic structure in layered iridates, revealing a spin-flop transition driven by competition among anisotropic exchange interactions due to strong spin-orbit coupling.

Contribution

It demonstrates the layer-dependent spin-flop transition in iridates and elucidates the microscopic origin of anisotropic exchange interactions in these materials.

Findings

Observation of c-axis collinear antiferromagnetic order in Sr$_3$Ir$_2$O$_7$

Identification of spin-flop transition related to layer number

Explanation of the transition via competition among pseudo-dipolar interactions

Abstract

Using resonant x-ray diffraction, we observe an easy c-axis collinear antiferromagnetic structure for the bilayer Sr$_3$Ir$_2$O$_7$, a significant contrast to the single layer Sr$_2$IrO$_4$ with in-plane canted moments. Based on a microscopic model Hamiltonian, we show that the observed spin-flop transition as a function of number of IrO$_2$ layers is due to strong competition among intra- and inter-layer bond-directional pseudo-dipolar interactions of the spin-orbit entangled $J_{eff}$=1/2 moments. With this we unravel the origin of anisotropic exchange interactions in a Mott insulator in the strong spin-orbit coupling regime, which holds the key to the various types of unconventional magnetism proposed in 5$d$ transition metal oxides.