Abstract
The magnetic properties of Mg
0.95Mn
0.05Fe
2O
4 ferrite samples with an average particle size of ∼6.0±0.6 nm have been studied using X-ray diffraction, Mössbauer spectroscopy, dc magnetization and frequency dependent real
χ
′
(
T
)
and imaginary
χ
″
(
T
)
parts of ac susceptibility measurements. A magnetic transition to an ordered state is observed at about 195 K from Mössbauer measurements. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization have been recorded at low field and show the typical behavior of a small particle system. The ZFC curve displays a broad maximum at
T
mean
=
195
±
5
K
, a temperature which depends upon the distribution of particle volumes in the sample. The FC curve was nearly flat below
T
mean
, as compared with monotonically increasing characteristics of non-interacting superparamagnetic systems indicating the existence of strong interactions among the nanoparticles. A frequency-dependent peak observed in
χ
′
(
T
)
is well described by Vogel–Fulcher law, yielding a relaxation time
τ
0
=
5.8
×
1
0
−
12
s
and an interaction parameter
T
0
=
195
±
3
K
. Such values show the strong interactions and rule out the possibility of spin-glass (SG) features among the nanoparticle system. On the other hand fitting with the Néel–Brown model and the power law yields an unphysical large value of
τ
0
(∼6×10
−69 and 1.2×10
−22 s respectively).