Fig. 8 The plot of MS versus 1/(particle size) for the La0.7Sr0.3MnO3
nanoparticles. The inset on the left is a plot of the HC versus 1/(particle
size)3/2, and the inset on the right is a plot of the MS versus particle
size
paramagnetic phase of LaMnO3 presented in the samples
prepared above 900◦C. In the case of the samples prepared
below 900◦C, there is still no saturation even after subtraction
of the M–H curve of LaMnO3. This may be due to the
presence of the other paramagnetic phases such as La(OH)3
(detected by TEM) in the samples. These results indicate the
presence of incomplete substitutions of Sr for La site in perovskite
structure.
Figure 8 shows the plot of saturated magnetization MS
versus the inverse of particle size of the La0.7Sr0.3MnO3
nanoparticles. It is clearly seen in the inset of Fig. 8 (right)
that the MS decreases with the increase in the particle size.
In a granular perovskite system, a grain of the perovskite
can be assumed to be a two-phase system, a body and a surface.
The body phase would have the same properties as the
bulk compound such as magnetic and transport properties,
but the surface phase would have low transition temperature
and magnetization [17]. A decrease in MS with decreasing
crystallize size in Fig. 8 may be attributed to the existence
of a nonmagnetic surface layer or to a non-collinear spin
arrangement at the surface of the crystal which is a consequence
of a so-called magnetically dead layer. The increase
of magnetically disordered states at the surface of a crystal
can be due to the termination of the crystal structure, breaking
of Mn–O–Mn paths, deviation of stoichiometric composition,
oxygen vacancies and dislocation [17]. Therefore,
the linear dependence of MS with the inverse of the mean
crystallite size observed in our La0.7Sr0.3MnO3 nanoparticles
is a clear indication that the reduction in the MS is only
a function of the crystallite size, and this suggests that the
magnetically dead layer is approximately the same for all
the samples [18].
It is known that ferromagnetic particles with diameter
smaller than a critical size have single-magnetic domain