Co_0.6Fe_0.4O–Co_1.4Fe_1.6O₄ core–shell nanoparticles with colossal exchange bias

The exchange bias (EB) effect is widely utilized in spintronics with 2D materials like thin films. Exploring the EB effect in nanoparticles opens up tremendous opportunities, such as miniaturization of devices, enhanced efficiency, and tunable properties, all of which are size-dependent. Due to the increased surface area to volume ratio, magnetic nanoparticles display unique characteristics, allowing for the manipulation of their magnetic properties, such as the EB effect commonly observed between antiferromagnetic (AFM) and ferro-/ferrimagnetic (FM/FiM) materials. This work employs a simple and highly reproducible one-step thermal decomposition method to fabricate colloidally stable Co_0.6Fe_0.4O-Co_1.4Fe_1.6O₄ core-shell (CS) nanoparticles with a lattice-matched interface and strong exchange coupling. We investigate their temperature and field-dependent magnetic properties using time-of-flight neutron diffraction and magnetometry. These nanoparticles exhibit the highest reported EB values among core-shell nanoparticles, reaching a maximum of 10.34 kOe. Additionally, the core exhibits antiferromagnetism above room temperature, with a Néel temperature of approximately 397 K, making it more suitable for high-temperature applications. This study paves the way for designing core-shell biphasic nanoparticles to enhance the EB effect and tune the effective magnetic anisotropy, offering potential future applications in nanospintronics and nanomedicine.