Highlights

Exchange Bias in Fe/Fe₃O₄ Core-shell Magnetic Nanoparticles Mediated by Frozen Interfacial Spins

Scientific Achievement

The exchange bias (EB) effect is often manifest by the shifting and broadening of a magnetic hysteresis loop of a sample cooled under an applied field.  Even though the macroscopic model of EB has now been widely accepted, there have been ongoing debates regarding the microscopic origin of the EB.  One important issue pertains to the role of interfacial spins.  There is also a renewed interest to investigate EB effects in nanoparticle system, largely driven by the desire to overcome the superparamagnetic limit in nanoparticles for magnetic storage applications.  We have carried out a detailed magnetic properties study on Fe/Fe₃O₄ core-shell nanoparticles and Fe₃O₄ hollow-shell nanoparticles.  Magnetic hysteresis measurements of core-shell particles at 5 K after field-cooling exhibits a large loop shift associated with unidirectional anisotropy, whereas Fe₃O₄ hollow-shell nanoparticles support much smaller shifts.  Both core-shell and hollow-shell particles exhibit sharp demagnetization jumps at low field associated with sudden switching of shell moments, but the intensity of the former under the field-cooled conditions accounts for the majority of the shell moments, and is strongly affected by the cooling field.  Temperature-dependent magnetization measurements of core-shell particles at high field show deviation between field-cooled and zero-field-cooled curves below 30K, suggesting the presence of frozen spins at the interface of ferri- and ferromagnetic domains at low temperatures.  These frozen interfacial spins play an important role in mediating the exchange coupling between the ferromagnetic core and ferrimagnetic shell.

Significance

Exchange bias (EB) has been one of most widely studied phenomena in magnetic systems due to its applications in spin-valve and magnetic tunneling junction devices.  Although EB was originally proposed in partially oxidized cobalt nanoparticles, most of the subsequent studies haven focused on thin film systems.  Colloidal magnetic nanoparticles offers opportunities to study EB effects in nanoscale systems, but particle polydispersity and aggregation can often obscure the origin of these effects and hinder quantitative measurements.  Fortunately, recent synthetic developments can now provide routes to highly uniform core-shell and hollow-shell magnetic nanoparticles with well-defined oxide layers.  We have determined that the orientation of interfacial frozen spins at low temperature plays a key role in mediating the exchange coupling between core and shell moments.  We have also identified a novel demagnezation mechanism at low fields that is related to a sudden switch in shell moments, especially under field-cooled conditions for core-shell particles.  These findings should deepen our understanding of EB in nanoscale systems, with possible insights for device applications based on this phenomenon  This work has been submitted to Phys. Rev. Lett.. 

Performers

Q. K. Ong, A. Wei (Purdue U.); X.-M. Lin (Argonne-CNM)