期刊
JOURNAL OF APPLIED PHYSICS
卷 104, 期 10, 页码 -出版社
AMER INST PHYSICS
DOI: 10.1063/1.3020524
关键词
calcium compounds; grain size; high-temperature effects; lanthanum compounds; magnetic moments; magnetic relaxation; magnetic transitions; magnetisation; nanoparticles; particle size; superparamagnetism; surface magnetism
资金
- CSIR India
- DST-NSTI project India [SR/S5/NM-04/2005]
- BRNS-DAE, India [2006/37/52/BRNS/2376]
We have investigated the effect of nanometric grain size on magnetic properties of La0.7Ca0.3MnO3 nanoparticles having average particle size (Phi) of similar to 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 (approximate to 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 T-S 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/d ln(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.
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