In a 5d² double perovskite, strong spin-orbit coupling produces octupolar magnetic order — a form of order described not by conventional dipole moments but by higher-order multipoles of the electron density. The undoped material is a ferromagnetic octupolar insulator.
Doping with sodium introduces small polarons: localized charge carriers dressed by a surrounding lattice distortion. These polarons do something unexpected — they reverse the dominant magnetic interaction from ferromagnetic to antiferromagnetic.
The mechanism is not a simple dilution effect (removing magnetic sites) or a conventional double-exchange (adding mobile carriers that mediate ferromagnetism). The polaron locally modifies the superexchange pathways by changing the orbital occupancy and distorting the oxygen cage around the magnetic ion. This distortion alters the balance between competing exchange channels, flipping the sign of the dominant interaction.
The result is that doping progressively weakens the ferromagnetic ordering: the transition temperature drops, and the system moves toward antiferromagnetic correlations. A few percent of dopant-induced polarons can reverse the ground state.
The finding demonstrates that in materials with strong spin-orbit coupling and multipolar order, conventional intuitions about doping break down. A polaron is not just a localized hole — it is a local structural and electronic perturbation that rewires the magnetic interaction network. Control over polaron formation is, in this system, control over the sign of the magnetic exchange. The carrier is the switch.