The exploration of exchange bias (EB) on the nanoscale provides a novel approach to improving the anisotropic properties of magnetic nanoparticles for prospective applications in nanospintronics and nanomedicine. However, the physical origin of EB is not fully understood. Recent advances in chemical synthesis provide a unique opportunity to explore EB in a variety of iron oxide-based nanostructures ranging from core/shell to hollow and hybrid composite nanoparticles. Experimental and atomistic Monte Carlo studies have shed light on the roles of interface and surface spins in these nanosystems. This review paper aims to provide a thorough understanding of the EB and related phenomena in iron oxide-based nanoparticle systems, knowledge of which is essential to tune the anisotropic magnetic properties of exchange-coupled nanoparticle systems for potential applications.

We report a novel synthesis approach for the growth of core/shell FeO/Fe₃O₄ nanoparticles with controlled shape and size. FeO particles were partially oxidized to form core/shell FeO/Fe₃O₄ structures, as evidenced from transmission electron microscopy, X-ray diffraction, and magnetometry analysis. We find that the molar ratios and concentrations of surfactants are the key parameters in controlling the particle size. The particles can grow in either isotropic or anisotropic shapes, depending upon a chemical reaction scheme that is controlled kinetically or thermodynamically. The competitive growth rates of {111} and {100} facets can be used to tune the final shape of nanoparticles to spherical, cubic, octahedral, octopod, and cuboctahedral geometries. FeO particles can also be oxidized chemically or thermally to form Fe₃O₄ nanoparticles. By following the same synthesis technique, it is possible to synthesize rods and triangles of Fe₃O₄ by introducing twinnings and defects into the crystal structure of the seed. The thermally activated first-order Verwey transition at ∼120 K has been observed in all the synthesized FeO/Fe₃O₄ nanoparticles, indicating its independence from the particle shape. These core/shell nanoparticles exhibit a strong shift in field-cooled hysteresis loops accompanied by an increase in coercivity (the so-called exchange bias effect), but the low field-switching behavior appears to vary with the particle shape.

Disorder among surface spins is a dominant factor in the magnetic response of magnetic nanoparticle systems. In this work, we examine time-dependent magnetization in high-quality, monodisperse hollow maghemite nanoparticles (NPs) with a 14.8 ± 0.5 nm outer diameter and enhanced surface-to-volume ratio. The nanoparticle ensemble exhibits spin-glass-like signatures in dc magnetic aging and memory protocols and ac magnetic susceptibility. The dynamics of the system slow near 50 K and become frozen on experimental time scales below 20 K. Remanence curves indicate the development of magnetic irreversibility concurrent with the freezing of the spin dynamics. A strong exchange-bias effect and its training behavior point to highly frustrated surface spins that rearrange much more slowly than interior spins. Monte Carlo simulations of a hollow particle corroborate strongly disordered surface layers with complex energy landscapes that underlie both glass-like dynamics and magnetic irreversibility. Calculated hysteresis loops reveal that magnetic behavior is not identical at the inner and outer surfaces, with spins at the outer surface layer of the 15 nm hollow particles exhibiting a higher degree of frustration. Our combined experimental and simulated results shed light on the origin of spin-glass-like phenomena and the important role played by the surface spins in magnetic hollow nanostructures.

We report upon the excellent magnetocaloric properties of Gd53Al24Co20Zr3 amorphous microwires. In addition to obtaining the large magnetic entropy change (−ΔSM ∼ 10.3 J/kg K at TC ∼ 95 K), an extremely large value of refrigerant capacity (RC ∼ 733.4 J/kg) has been achieved for a field change of 5 T in an array of forty microwires arranged in parallel. This value of RC is about 79% and 103% larger than those of Gd (∼410 J/kg) and Gd5Si2Ge1.9Fe0.1 (∼360 J/kg) regardless of their magnetic ordering temperatures. The design and fabrication of a magnetic bed made of these parallel-arranged microwires would thus be a very promising approach for active magnetic refrigeration for nitrogen liquefaction. Since these microwires can easily be assembled as laminate structures, they have potential applications as a cooling device for micro electro mechanical systems and nano electro mechanical systems.