J–T Instability in Oxygen-Deficient Orthoferrite CaFeO _3-δ Nanoparticles: Implications for Magnetic Exchange Bias and Magnetocaloric Effect-Based Applications

In this manuscript, we aim to investigate the impact of Jahn–Teller (J–T) instability on the various magnetic properties of orthoferrite CFO (CaFeO 3 ) nanoparticles containing a high-spin d 4 (t3 2g e 1g ) configuration. The average particle size calculated from XRD data using Debye–Scherrer formula was 15–20 nm which was further confirmed by high-resolution transmission electron microscopy. The presence of d 4 electron configuration in transition-metal (TM) perovskites is highly unstable according to J–T theory, and we observe an unusual peak in M–T data around ∼150 K, which is sensitive on the external applied magnetic field. This may be due to electron localization process during charge ordering which CFO undergoes to remove degeneracy in the d orbital to minimize Coulomb repulsion and J–T instability. Experiments show a huge exchange bias with a peak of ∼1700 Oe in zero field-cooled hysteresis data measured at 150 K, inverse magnetocaloric effect (IMCE), and kinetically arrested magnetic glass state between re-entrant phases for CFO nanoparticles which can be accounted on the basis of charge ordering accompanied by structural transition and metal-to-insulator transition. Magnetocaloric effect (MCE) measurement shows IMCE with entropy change up to 0.04 J/kg.K in this sample which is exotic. The cause lies on specific approaches to remove d orbital degeneracy to minimize the Coulomb repulsion and J–T instability. CFO removes the degeneracy by charge ordering like 2Fe 4+ = Fe 5+ + Fe 3+ . Other parameters like metal–oxygen bond strength, electronegativity of the TM ions, ionic size of cations, and geometry of the crystal structure play an important role. Emergence of exchange bias without applying any external magnetic field can find potential applications in magneto-resistive devices, spintronics, and magneto-refrigeration combined with the IMCE.