Octahedral tilting induced isospin reorientation transition in iridate heterostructures

Iridate heterostructures are gaining interest as their magnetic properties are much more sensitive to structural distortion compared to pure spin systems due to spin-orbital entanglement. While bulk monolayer and bilayer iridates show $ab$-plane canted and $c$-axis antiferromagnetic (AFM) order, recent experiments on layered iridate superlattices (SLs) have revealed striking properties, especially in the bilayer SLs. A spin model is presented to illustrate the proclivity towards $ab$-plane canted AFM order. A realistic Hubbard model including spin-dependent hopping terms is constructed for the bilayer SL, and magnetic excitations are investigated in the self-consistently determined magnetic state. The Hubbard model analysis confirms the spin model results and shows strongly reduced magnon energy gap and an isospin reorientation transition from $c$-axis to $ab$-plane canted AFM order with increasing tilting.

Figure 1

The $x,y$ (a) and $z$ components (b) of the ${\mathbf{t}}_{\mathbf{ij}}^{\prime }$ terms for intra- and interlayer sites ($\parallel$ and $\perp$). Here $A$ and $B$ denote the two magnetic sublattices. Due to the staggered octahderal rotation and tilting, ${\mathbf{t}}_{\mathbf{ji}}^{\prime }=-{\mathbf{t}}_{\mathbf{ij}}^{\prime }$. (c) shows $xy\text{−}xz$ orbital mixing ($\pi$ overlap) due to both tiltings (i) and (ii) for intralayer neighbors, but due to only (i) for interlayer neighbors.

Figure 2

Calculated electronic band structure in the self-consistent AFM state of the bilayer Hubbard model with hopping parameter values as given in Table 1.

Figure 3

Calculated magnon dispersions for the bilayer Hubbard model (a) with and (b) without the hopping terms $t_x,t_y$, showing strong magnon gap reduction (from 75 to 30 meV) induced by the octahedral tilting in the bilayer SL. Here, the bilayer reference case (b) corresponds to reduced octahedral rotation effect but no tilting.

Figure 4

Strong magnon gap variation with the octahedral tilting induced hopping terms $t_{x,y}^{\parallel }$, showing the AFM I-II reorientation transition. Inset shows only the planar part of the magnetic order. Phases I and II are dominantly $c$-axis and $ab$-plane AFM orders.