Spin-Orbital Order Modified by Orbital Dilution in Transition Metal Oxides: From Spin Defects to Frustrated Spins Polarizing Host Orbitals

TL;DR

This paper investigates how 3d ion doping in 4d transition metal oxides affects spin-orbital order, revealing impurity-induced modifications, frustration, and anisotropic patterns with implications for experimental detection.

Contribution

It derives the effective superexchange interactions in doped 4d oxides, highlighting the qualitative differences introduced by 3d impurities and their impact on spin-orbital structures.

Findings

Impurities can act as spin defects or orbital polarons.

Doping can drastically modify host spin and orbital order.

Spin-orbit coupling induces anisotropic spin and orbital patterns.

Abstract

We study the $3d$ substitution in $4d$ transition metal oxides in the cases of $3d^3$ doping at either $3d^2$ or $4d^4$ sites which realize orbital dilution. We derive the effective $3d-4d$ (or $3d-3d$) superexchange in a Mott insulator with different ionic valencies, underlining the emerging structure of the spin-orbital coupling between the impurity and the host sites and demonstrate that it is qualitatively different from that encountered in the host itself. This derivation shows that the interaction between the host and the impurity depends in a crucial way on the type of doubly occupied $t_{2g}$ orbital. One finds that in some cases, due to the quench of the orbital degree of freedom at the $3d$ impurity, the spin and orbital order within the host is drastically modified by doping. The impurity acts either as a spin defect accompanied by an orbital vacancy in the spin-orbital structure when the host-impurity coupling is weak, or it favors doubly occupied active orbitals (orbital polarons) along the $3d-4d$ bond leading to antiferromagnetic or ferromagnetic spin coupling. This competition between different magnetic couplings leads to quite different ground states. We find that magnetic frustration and spin degeneracy can be lifted by the quantum orbital flips of the host but they are robust in special regions of the incommensurate phase diagram. The spin-orbit coupling can lead to anisotropic spin and orbital patterns along the symmetry directions and cause a radical modification of the order imposed by the spin-orbital superexchange. Our findings are expected to be of importance for future theoretical understanding of experimental results for doped $4d$ transition metal oxides doped with $3d^3$ ions. We suggest how the local or global changes of the spin-orbital order induced by such impurities could be detected experimentally.