Size-dependent surface effects in maghemite nanoparticles and its impact on interparticle interactions in dense...

Area of Science:

• Materials Science
• Nanotechnology
• Magnetism

Background:

• Understanding interparticle magnetic interactions in nanoparticle assemblies is crucial for their applications.
• Bare maghemite nanoparticles (NPs) of varying diameters (6.2-11.5 nm) were investigated to determine dominant magnetic interactions.

Purpose of the Study:

• To identify the primary interparticle magnetic interaction in random closely packed maghemite nanoparticle assemblies.
• To investigate the influence of particle size and surface spin disorder on magnetic interactions and collective freezing behavior.

Main Methods:

• Single-particle magnetic property measurements (anisotropy, exchange bias field) in dilute systems.
• Analysis of closely packed NP assemblies using X-ray diffraction.
• Investigating collective (superspin) freezing behavior as a function of particle size.

Main Results:

• Surface spin disorder was observed in maghemite NPs with diameters ≥ 8 nm, attributed to crystallographic origins, but absent in smaller NPs (d = 6.2-6.3 nm).
• Collective superspin freezing in larger NPs (d ≥ 8 nm) is dominated by interparticle dipole interactions.
• Smaller NPs (d = 6.2-6.3 nm) showed higher freezing temperatures than predicted by dipole interactions alone, suggesting an alternative mechanism.

Conclusions:

• The dominant interparticle magnetic interaction in maghemite NP assemblies is size-dependent.
• For larger NPs (≥ 8 nm), dipole interactions govern superspin freezing.
• For smaller NPs (< 8 nm), the absence of surface spin disorder facilitates superexchange interactions between superspins, leading to enhanced freezing temperatures.