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We study the $3d$ substitution in $4d$ transition metal oxides in the casesof $3d^3$ doping at either $3d^2$ or $4d^4$ sites which realize orbitaldilution. We derive the effective $3d-4d$ (or $3d-3d$) superexchange in a Mottinsulator with different ionic valencies, underlining the emerging structure ofthe spin-orbital coupling between the impurity and the host sites anddemonstrate that it is qualitatively different from that encountered in thehost itself. This derivation shows that the interaction between the host andthe 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 degreeof freedom at the $3d$ impurity, the spin and orbital order within the host isdrastically modified by doping. The impurity acts either as a spin defectaccompanied by an orbital vacancy in the spin-orbital structure when thehost-impurity coupling is weak, or it favors doubly occupied active orbitals(orbital polarons) along the $3d-4d$ bond leading to antiferromagnetic orferromagnetic spin coupling. This competition between different magneticcouplings leads to quite different ground states. We find that magneticfrustration and spin degeneracy can be lifted by the quantum orbital flips ofthe host but they are robust in special regions of the incommensurate phasediagram. The spin-orbit coupling can lead to anisotropic spin and orbitalpatterns along the symmetry directions and cause a radical modification of theorder imposed by the spin-orbital superexchange. Our findings are expected tobe of importance for future theoretical understanding of experimental resultsfor doped $4d$ transition metal oxides doped with $3d^3$ ions. We suggest howthe local or global changes of the spin-orbital order induced by suchimpurities could be detected experimentally.

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