Jahn-Teller Effect Induces Exchange Bias in Ultrasmall Magnetite.

Exchange bias, a quantum phenomenon confined to macroscopic interfacial ferromagnetic-antiferromagnetic heterostructures, has remained unobserved in superparamagnetic single-phase 0-dimensional (0D) nanosystems, where magnetic interfaces are absent. Here we demonstrate intrinsic exchange bias in freestanding magnetite (Fe3O4) nanoparticles (4.0 nm), mediated by symmetry-breaking Jahn-Teller-type lattice distortions. Ångström-scale symmetry-lowering lattice displacements in octahedral Fe2+ (3d6) modulate the local crystal field environment, induing a spin-state reconfiguration from a low-spin (S = 0) toward a high-spin state (S = 1), accompanied by the breaking of magnetic space-group and time-reversal (𝒯) symmetries. The high-spin state couples to 𝒯-symmetric antiparallel Fe3+ (3d5) spins via double exchange, bypassing conventional interfacial interactions. This work establishes Jahn-Teller driven symmetry breaking as a new mechanism for exchange bias in quantum-confined systems, opening pathways for engineering emergent spin correlations in single-phase 0D spintronic architectures with three-dimensional nanoconfinement (nm × nm × nm scale).