Jahn–Teller Effect Induces Exchange Bias in Ultrasmall Magnetite

ABSTRACT 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 (Fe 3 O 4 ) nanoparticles (4.0 nm), mediated by symmetry‐breaking Jahn–Teller‐type lattice distortions. Ångström‐scale symmetry‐lowering lattice displacements in octahedral Fe 2+ (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 Fe 3+ (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).