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We have investigated the effect of nanometric grain size on magneticproperties 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 offield cooled (FC) and zero-field cooled (ZFC) dc magnetization (DCM), themagnetic properties could be distinguished into two regimes: a relatively hightemperature 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 sharpmaximum (at T = TS) is related to the freezing of surface (shell) spins. Theunusual shape of M (H) loop at T = 1.5 K, temperature dependent feature ofcoercive field and remanent magnetization give a strong support of surface spinfreezing that are occurring at lower temperature regime (T < 40 K) in this LCMOnanoparticles. Additionally, waiting time (tw) dependence of ZFC relaxationmeasurements at T = 50 K show weak dependence of relaxation rate [S(t)] on twand dM/dln(t) following a logarithmic variation on time. Both of these featuresstrongly support the high temperature regime to be associated with the blockingof core moments. At T = 20 K, ZFC relaxation measurements indicates theexistence of two different types of relaxation processes in the sample withS(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-dependenteffect convincingly establish the surface spin freezing of our LCMOnanoparticles associated with a background of superparamagnetic (SPM) phase ofcore moments.

journal of applied physics

Enhanced grain surface effect on magnetic properties of nanometric  La0.7Ca0.3MnO3 manganite : Evidence of surface spin freezing of manganite  nanoparticles