5
14
M. Zaghrioui et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 509–514
where Z is the number of nearest neighbours (here Z¼6). Using
Eqs. (4) and (5), we can determine t value from the experimental
results assuming that the spin value of the system is a mean spin per
metallic ion. From that, t value was found to increase from 0.27 to 0.36
for 0:02rxr0:08 as observed in Fig. 7c. Above x¼0.08, t remains
practically constant. Note that this result fairly supports the evolution
of c parameter and then well correlates the presence of Fe2 site. In
addition, according to Capriotti etal. [24,26], the critical reduced
temperature evolution could be due to the increase of the spin of
the system and/or to the increase of magnetic anisotropy. In our case, as
evocated above, the resulting magnetization is due to the average
moment of metallic ions while the higher Fe substitution (x¼0.20)
gives rise to an increase of the moment of 5%. This is not sufficient
to explain the increase of t, especially the large variation observed
for the low rate substitution. Then, the increase of the magnetic
anisotropy originates the increase of t, i.e.iron substitution has a
consequence to favour the spin alignment in ab plane. Note that
of an additional weak ferromagnetic component below T
addition, Curie–Weiss temperature CW and effective magnetic
moment meff were found to decrease as Fe content increases,
N
. In
y
3
+
attributed to the random distribution of Fe
in the system as
5
7
evidenced by Fe M o¨ ssbauer spectrometry.
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7
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5
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increases.
wðTÞ measured in ZFC and FC modes evidenced a presence