  Exchange bias effects linked to surface spin freezing (SSF) are commonly
found in iron oxide nanoparticles, while signatures of SSF in low-field
temperature-dependent magnetization curves have been much less frequently
reported. Here, we present magnetic properties of dense assemblies of
similar-sized (~ 8 nm diameter) particles synthesized by a magnetite (sample
S1) and a maghemite (sample S2) method, and the influence of long-term (4-year)
sample aging under ambient conditions on these properties. The size of the
exchange bias field of the different sample (fresh or aged) states is found to
correlate with (a) whether a low-temperature hump feature signaling the SSF
transition is detected in out-of-phase ac susceptibility or zero-field-cooled
(ZFC) dc magnetization recorded at low field and with (b) the prominence of
irreversibility between FC and ZFC curves recorded at high field. Sample S1
displays a lower magnetization than S2, and it is in S1 where the largest SSF
effects are found. These effects are significantly weakened by aging but remain
larger than the SSF effects in S2, where the influence of aging is considerably
smaller. A non-saturating component due to spin disorder in S1 also weakens
with aging, accompanied by, we infer, an increase in the superspin and the
radius of the ordered nanoparticle cores. X-ray diffraction and M\"ossbauer
spectroscopy provide indication of maghemite-like stoichiometry in both aged
samples as well as thicker disordered particle shells in aged-S1 relative to
aged-S2 (crystallographically-disordered and spin-disordered according to
diffraction and M\"ossbauer, respectively). The pronounced diminution in SSF
effects with aging in S1 is attributed to a (long-term) transition, caused by
ambient oxidation, from magnetite-like to maghemite-like stoichiometry, and a
concomitant softening of the spin-disordered shell anisotropy...
