Enhanced grain surface effect on magnetic properties of nanometric La_0.7Ca_0.3MnO₃ manganite: evidence of surface spin freezing of manganite nanoparticles

Abstract

We have investigated the effect of nanometric grain size on magnetic properties of La_0.7Ca_0.3MnO₃ nanoparticles having average particle size (Φ) of ∼17 nm. Temperature dependence of field-cooled (FC) and zero-FC (ZFC) dc magnetization indicate the existence of two different types of relaxation processes: a relatively high temperature regime where there is a broad maximum of the ZFC curve at T = T_max (>40 K) and another is a relatively low temperature regime that is characterized by a sharp maximum at T = T_S (≈40 K). We believe that the broad maximum at T_max is associated with the blocking of core particle moments, whereas the sharp maximum at TS is related to the freezing of surface spins. Waiting time (t_w) dependence of ZFC relaxation measurements at T = 50 K show weak dependence of relaxation rate [S(t)] on t_w and dM/dln(t) following a logarithmic variation in time. These features strongly support superparamagnetic (SPM) blocking of core particle moments at T_max. At T = 20 K, S(t) attains a maximum at t_w = 1000 s that establishes freezing phenomenon occurring at T_S. The combination of topological disorder and magnetic phase separation may result in a magnetically disordered state at the grain surface. This, in turn, results in magnetically uncoupled assembly of nanoparticles that eventually exhibits SPM blocking. The glassy behavior at T_S has been attributed to the competing magnetic interactions stabilizing a spin-glass-like frozen state at the surface region of the nanometric grains.