15, 16, 17 In particular, the greater surface-to-volume ratios of nanoparticles supports a higher number of spins at bilayer interfaces, which enables the use of magnetic characterization techniques that are typically applied toward bulk samples. However, recent advances in the chemical synthesis of highly monodisperse magnetic nanoparticles have created new opportunities to investigate EB phenomena in core–shell systems. Most EB studies are based on magnetic thin films produced by top-down deposition processes. 13 Despite their nearly negligible contribution toward overall magnetization, such frozen interfacial spins may be largely responsible for mediating most observable EB effects, including horizontal as well as vertical hysteresis loop shifts. used x-ray magnetic circular dichroism (XMCD) to show that unidirectional anisotropy can be induced at low temperatures by just a small fraction (<4%) of aligned interfacial spins. However, recent experimental studies point to spin frustration as a relevant factor in exchange anisotropy, which can be generated from frozen spins at an AFM interface 14 or within disordered spin glass layers. 13 These diverse arguments are difficult to resolve because of the challenges in measuring and characterizing the magnetic properties of the FM–AFM interface. 1, 2, 10 For example, the origins of exchange coupling between FM and AFM layers have been proposed to be derived from random fields generated by interface roughness and the formation of vertical AFM domains, 11 from AFM domains aligned parallel to the bilayer interface, 12 and from defect-induced domains within the AFM layer. 7Īlthough the macroscopic model of EB has been in existence since its discovery over fifty years ago, 8, 9 the microscopic mechanisms of EB are still being actively debated in the magnetic thin film community. 3, 4, 5, 6 These binary systems are often comprised of ferromagnetic (FM) and antiferromagnetic (AFM) layers, but the latter can be replaced with ferrimagnetic materials or even amorphous spin glasses that lack long-range magnetic order. The physical basis for the loop shift is attributed to a unidirectional anisotropy induced by the exchange coupling between two different layers of materials. 1, 2 A signature of the EB effect is a shift in the magnetic hysteresis loop in the direction of the applied field under field cooling (FC) conditions. Magnetic exchange bias (EB) has received considerable attention due to its broad impact on magnetoresistive materials and devices, nonvolatile random access memory, and spintronics. The exchange-bias field becomes negligible upon deliberate oxidation of 3O 4 nanoparticles into yolk–shell particles, with a nearly complete physical separation of core and shell. The increase in frozen spin population with age is responsible for the overall retention of exchange bias, despite void formation and other oxidation-dependent changes. Energy-filtered and high-resolution transmission electron microscopy both indicate further oxidation of the shell layer, but the Fe core is remarkably well preserved. These changes are accompanied by a structural evolution from well-defined core–shell structures to particles containing multiple voids, attributable to the Kirkendall effect. Aging of the core–shell nanoparticles under ambient conditions results in a gradual decrease in magnetization but overall retention of H C and H E, as well as a large increase in the population of frozen spins. The population of such frozen spins has a direct impact on both coercivity (H C) and the exchange-bias field (H E), which are modulated by external physical parameters such as the strength of the applied cooling field and the cycling history of magnetic field sweeps (training effect). These spins are frozen into magnetic alignment with field cooling, and are depinned in a temperature-dependent manner. Core–shell 3O 4 nanoparticles exhibit substantial exchange bias at low temperatures, mediated by unidirectionally aligned moments at the core–shell interface.
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