  We have investigated the effect of nanometric grain size on magnetic
properties of single phase, nanocrystalline, granular La0.7Ca0.3MnO3 (LCMO)
sample. We have considered core-shell structure of our LCMO nanoparticles,
which can explain its magnetic properties. From the temperature dependence of
field cooled (FC) and zero-field cooled (ZFC) dc magnetization (DCM), the
magnetic properties could be distinguished into two regimes: a relatively high
temperature regime T > 40 K where the broad maximum of ZFC curve (at T = Tmax)
is associated with the blocking of core particle moments, whereas the sharp
maximum (at T = TS) is related to the freezing of surface (shell) spins. The
unusual shape of M (H) loop at T = 1.5 K, temperature dependent feature of
coercive field and remanent magnetization give a strong support of surface spin
freezing that are occurring at lower temperature regime (T < 40 K) in this LCMO
nanoparticles. Additionally, waiting time (tw) dependence of ZFC relaxation
measurements at T = 50 K show weak dependence of relaxation rate [S(t)] on tw
and dM/dln(t) following a logarithmic variation on time. Both of these features
strongly support the high temperature regime to be associated with the blocking
of core moments. At T = 20 K, ZFC relaxation measurements indicates the
existence of two different types of relaxation processes in the sample with
S(t) attaining a maximum at the elapsed time very close to the wait time tw =
1000 sec, which is an unequivocal sign of glassy behavior. This age-dependent
effect convincingly establish the surface spin freezing of our LCMO
nanoparticles associated with a background of superparamagnetic (SPM) phase of
core moments.
