  We report exchange bias (EB) in single-phase CoO nanoparticles, where two
magnetic phases naturally emerge as the crystallite size decreases from 34.6 to
10.8 nm. The N\'eel temperature (TN) associated with antiferromagnetic ordering
decreases monotonically with the reduction in crystallite size, highlighting
the significant influence of size effects. The 34.6 nm nanoparticles exhibit
magnetization irreversibility between zero field cooled (ZFC) and field-cooled
(FC) states below TN. This irreversibility appears well above TN with further
reduction in size, resulting in the absence of true paramagnetic regime which
indicates the occurrence of an additional magnetic phase. The
frequency-dependent ac-susceptibility in 10.8 nm nanoparticles suggests slow
dynamics of disordered surface spins above TN, coinciding with the
establishment of long-range order in the core. The thermoremanent magnetization
(TRM) and isothermoremanent magnetization (IRM) curves suggest a core-shell
structure: the core is antiferromagnetic, and the shell consists of disordered
surface spins causing ferromagnetic interaction. Hence, the exchange bias in
these CoO nanoparticles results from the exchange coupling between an
antiferromagnetic core and a disordered shell that exhibits unconventional
surface spin characteristics.
