Abstract
We investigate the changes in spin and orbital patterns induced by magnetic transition-metal ions without an orbital degree of freedom doped in a strongly correlated insulator with spin-orbital order. In this context, we study the ion substitution in transition-metal oxides in the case of doping at either or sites, which realizes orbital dilution in a Mott insulator. Although we concentrate on this doping case as it is known experimentally and more challenging than other oxides due to finite spin-orbit coupling, the conclusions are more general. We derive the effective (or ) 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 orbital. One finds that in some cases, due to the quench of the orbital degree of freedom at the impurity, the spin and orbital order within the host is drastically modified by doping. The impurity either acts as a spin defect accompanied by an orbital vacancy in the spin-orbital structure when the host-impurity coupling is weak or favors doubly occupied active orbitals (orbital polarons) along the bond leading to antiferromagnetic or ferromagnetic spin coupling. This competition between different magnetic couplings leads to quite different ground states. In particular, for the case of a finite and periodic atom substitution, it leads to striped patterns either with alternating ferromagnetic or antiferromagnetic domains or with islands of saturated ferromagnetic order. 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. Orbital quantum fluctuations modify quantitatively spin-orbital order imposed by superexchange. In contrast, 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 transition-metal oxides doped with ions. We suggest how the local or global changes of the spin-orbital order induced by such impurities could be detected experimentally.
- Received 7 August 2014
DOI:https://doi.org/10.1103/PhysRevX.5.011037
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Published by the American Physical Society
Popular Summary
Understanding the origin of the complex behavior of transition-metal oxides with orbital degrees of freedom is a central problem in strongly correlated electrons and quantum magnetism. The interplay of spin-charge degrees of freedom may be responsible for qualitatively new and unexpected behavior such as high-temperature superconductivity. Mobile charge carriers in manganites with spin-orbital-charge degrees of freedom trigger a phase transition from antiferromagnetic to ferromagnetic order and also cause drastic changes in resistivity at finite temperatures, known as colossal magnetoresistance. Applying magnetic impurities to systems with spin-orbital order is particularly challenging and is still unexplored. Immobile defects with no orbital degree of freedom in spin-orbital-ordered Mott insulators may disturb both magnetic and orbital order; such “substitutional doping” can generate novel types of order at high doping concentrations. We study local changes in spin-orbital order in an insulating transition-metal oxide when impurities ( and ions) with no orbital degrees of freedom are added, leading to orbital dilution.
We employ a microscopic model whose physical properties are controlled by a few parameters. Our analysis demonstrates that the impurities can behave either as spin defects surrounded by inactive orbitals or polarized orbitals around them that change spin interactions to ferromagnetic. We investigate a few representative doping concentrations and show that certain unexpected changes in global spin-orbital order are triggered by finite doping and that frustration of impurity spins, which emerges classically at the crossover between different types of magnetic order, is removed by quantum effects. We predict local and global changes of spin-orbital order induced by such impurities, and we suggest how the final spin-orbital order could be detected experimentally.
Our results are generic and pave the way toward a better understanding of heavy ( and ) transition-metal oxides with immobile magnetic ions. Our theory provides the framework to investigate the consequences of quantum fluctuations and of spin-orbit coupling in Mott insulators, which will likely lead to novel quantum behavior in spin-orbital systems.
