ARTICLE IN PRESS
J.M.A. Almeida et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e304–e307
e305
gelatin and 1.225 g of MnCl ꢀ 4H O (MERCK 99%),
thermal analysis DTA were performed and the results are
displayed in Fig. 1. The TG data indicate a pronounced
mass loss close to 100 1C, corresponding to the elimination
of water, and another one near 310 1C, attributed to the
break out of the polimeric chains of the organic substances
related to the gelatin addition. In this process, occurring up
to 600 1C, the mass loss can be mainly associated with CO2
and CO and the beginning of the NiMn O formation.
2
2
which were dissolved in 10 mL of water. Both solutions
were mixed and maintained under stirring at temperature
of 40 1C. The final solution, with pH nearly 4, was cooled
slowly and a gel was formed. The gel was dried at 80 1C
during 5 days and the resulted material was a xerogel
(
biopolymer). This xerogel was heated up to different
sintering temperatures 600, 700, 800, 900, and 1000 1C in a
rate of 10 1C/min. The annealing time interval was typically
of 6 h. The X-ray powder diffraction (XRPD) analysis was
performed in a Rigaku X-ray diffractometer using Brag-
g–Brentano geometry in the continuous mode with speed
of 0.51/min. Cu Ka radiation was used and the tube
operated at 40 kV and 25 mA. The X-ray diffraction
patterns were taken in the range of 17–911 in order to
cover the most intense peaks of the NiMn O phase. The
2
4
3.2. X-ray diffraction
Fig. 2 displays the calculated intensity (Rietveld refine-
ment) and the experimental XRPD patterns for all
samples. The results indicate that samples have the mixed
spinel structure and peaks belonging to NiO, which
decrease in intensity with increasing sintering temperature
2
4
phase identification analysis has been made with the help of
the powder diffraction database of International Centre for
Diffraction Data (ICDD). Rietveld refinement [10] proce-
dures were made by using the DBWS 9807 code [11]. The
pseudo-Voigt function has been used to fit the peak profiles
of the identified crystalline phases. The full-width at half-
maximum (FWHM) of the peaks has been used to estimate
the crystallites size. Asymmetry coefficients, scale factors,
and lattice parameters for each phase and the background
polynomial parameters, were simultaneously refined. The
calculation of the crystallites size for all powders has been
performed by using the Scherrer’s equation [12].
(
see Table 1). The presence of NiO in samples heat-treated
below 850 1C has been also observed elsewhere [5]. This
can be related to a small nonstoichiometry proportion of
nickel chloride or due to a large ionic distribution of Mn
mainly in the site 8a, as observed previously [14] in
(
NiFe O ) +(NiMn O ) for 0.5pxp1.0. The inset
2
4 1ꢁx
2
4 x
in Fig. 2 shows a systematic shift of the {3 3 3} plane to
lower 2y values with increasing sintering temperature, an
indicative of the expansion of the unit cell volume of
ꢃ0.9%.
The average size of the crystallites was calculated by
using the FWHM of the crystallographic families {0 2 2},
The FWHM parameter has been corrected by the
instrumental broadening using LaB powder standard, as
{
1 1 3}, {2 2 4}, and {3 3 3} which were extracted from the
6
Rietveld refinements. Fig. 3 exhibits a pronounced increase
of the average crystallites size and microstrain of the
crystallographic family {1 1 1} with increasing sintering
temperature, verified also for others planes families. Such
an increase in the microstrain is certainly related to the
large surface effect, an effect much more pronounced when
the crystallites size is largely reduced. Williamsom–
Hall [13] plots, used here to calculate the microstrain,
showed a positive angular coefficient, indicating that all
samples have a positive expansion in their structure along
described by Maia et al. [8]. The average microstrain (e), a
parameter extracted from the refinement, has been
determined for all samples. This microstrain and crystal-
lites size are convoluted in the integral breadth of the peak
profile and both parameters can be analytically separated
[
13] and calculated by using the relationship:
b cos y
1
4 ꢂ ꢀ ꢂ sin y .
¼
þ
(1)
l
D
l
The microstrain is given by the angular coefficient and the
crystallite size by the linear coefficient in a plot of b cos y/l
versus sin y.
Thermogravimetric analysis was performed in a Shi-
madzu Differential Thermal Analyzer. The measurements
were performed in airflow, with a flow rate of 50 mL/min,
and in the temperature range 25–1000 1C with a heating
rate of 10 1C/min. Measurements of magnetization as
function of the temperature were performed in SQUID
magnetometer for two samples of NiMn O which were
2
4
synthesized at 1000 and 600 1C.
3
. Results and discussions
3
.1. Thermogravimetric analysis
In order to gain further information regarding the
precursor material, thermogravimetric TG and differential
Fig. 1. TG and DTA analyses for the solution of the precursor material
2 4
for the synthesis of the NiMn O nanoparticles.
Almeida
