  Antiferromagnetic (AF) nanostructures from Co3O4, CoO and Cr2O3 were prepared
by the nanocasting method and were characterized magnetometrically. The field
and temperature dependent magnetization data suggests that the nanostructures
consist of a core-shell structure. The core behaves as a regular
antiferromagnet and the shell as a two-dimensional diluted antiferromagnet in a
field (2d DAFF) as previously shown on Co3O4 nanowires [Benitez et al., Phys.
Rev. Lett. 101, 097206 (2008)]. Here we present a more general picture on three
different material systems, i.e. Co3O4, CoO and Cr2O3. In particular we
consider the thermoremanent (TRM) and the isothermoremanent (IRM) magnetization
curves as "fingerprints" in order to identify the irreversible magnetization
contribution originating from the shells. The TRM/IRM fingerprints are compared
to those of superparamagnetic systems, superspin glasses and 3d DAFFs. We
demonstrate that TRM/IRM vs. H plots are generally useful fingerprints to
identify irreversible magnetization contributions encountered in particular in
nanomagnets.
