  Disorder among surface spins largely dominates the magnetic response of
ultrafine magnetic particle systems. In this work, we examine time-dependent
magnetization in high-quality, monodisperse hollow maghemite nanoparticles with
a 14.8 $\pm$ 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
slows near 50 K, and becomes 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 with bulk coordination. Monte
Carlo simulations of a hollow particle reproducing the experimental morphology
corroborated 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 study sheds light on
the origin of spin-glass-like phenomena and the role of surface spins in
magnetic hollow nanostructures.
