Novel Spin-Orbital Phases Induced by Orbital Dilution

We demonstrate that magnetic $3d$ impurities with $S=3\mathrm{/}2$ spins and no orbital degree of freedom induce changes of spin-orbital order in a $4d^4$ Mott insulator with $S=1$ spins. Impurities act either as spin defects which decouple from the surrounding ions, or trigger orbital polarons along $3d$-$4d$ bonds. The $4d$-$4d$ superexchange in the host $J_{\mathrm{h}\mathrm{o}\mathrm{s}\mathrm{t}}$ competes with $3d$-$4d$ superexchange $J_{\mathrm{i}\mathrm{m}\mathrm{p}}$ — it depends on which orbital is doubly occupied. The spin-orbital order within the host is totally modified at doping $x=1\mathrm{/}4$. Our findings provide new perspective for future theoretical and experimental studies of doped transition-metal oxides.