Influence of Annealing on Magnesioferrite Nanoparticles Synthesized by a Sol-Gel/Combustion Method

X ray diffraction patterns were obtained using an XPert Pro Panalitical diffractometer with Cu Kα radiation (λ = 1.5418 Å). The average particle size was calculated from line broadening using the TOPAS application, academic edition. Mössbauer spectra were recorded at several temperatures between 25 K and room temperature in a homemade instrument using a source of 57Co(Rh) with an activity of about 50 mCi.

to 200 °C for 20 minutes so that auto-combustion would take place.Finally, the product was annealed in a furnace for 2 hours at different temperatures.

Measurements
X ray diffraction patterns were obtained using an XPert Pro Panalitical diffractometer with Cu Kα radiation (λ = 1.5418Å).The average particle size was calculated from line broadening using the TOPAS application 3 , academic edition.
Mössbauer spectra were recorded at several temperatures between 25 K and room temperature in a homemade instrument using a source of 57Co(Rh) with an activity of about 50 mCi.

Experimental Results and Analysis
Table 1 shows the average particle sizes, as calculated from the X ray spectra shown in Figure 1, for samples as prepared and after annealing at three different temperatures.
The Mössbauer spectra of sample 2 in Table 1 are shown in Figure 2 for several different measurement temperatures.While at room temperature there is a doublet due to superparamagnetic relaxation, at lower temperatures one sees a sextet which is characteristic of bulk magnesioferrite 4 .By taking the ratio of the area under the doublet to the area under the sextet, it is possible to estimate the volume fraction of unblocked particles for each measurement temperature.The

Introduction
Nanosized ferrites display physical and chemical properties which may be quite different from those of their bulk counterparts.Magnesioferrite (MgFe 2 O 4 ) is one of the most interesting, because, due to its small magnetocrystalline anisotropy, superparamagnetic properties are still present at relatively low temperatures and/or high magnetic fields.The purpose of this work was to investigate the influence of annealing on the average particle size and particle size distribution of MgFe 2 O 4 nanoparticles prepared using the sol-gel/ combustion method 1 , a fast and relatively inexpensive technique.The average particle size was determined by X ray diffraction, while the particle size distribution was obtained by measuring at several temperatures the relative intensity of the Mössbauer spectra due to superparamagnetic particles and to ferrimagnetic particles, a method that has already been applied successfully 2 to another nanosized ferrite, CoFe 2 O 4 .3, where the dots are the experimental points and the line is a fit to a cumulative log-normal function

Sample preparation
where T is the absolute temperature, erf(T) is the error function, C 1 and C 2 are constants and µ and δ are adjustable parameters.The best fit was obtained with C 1 = 0.64, C 2 = 0.61, µ = 5.70 and δ = 0.64.The distribution of unblocking temperatures of the system is given by 5,6

P T C T df T dT
where C is a normalization constant.The temperature dependence of Equation 2 may be converted to a dependence on particle diameter (thus yielding the particle size distribution) using the relation 5,6

D T D T T c
where D is the particle diameter, 〈D〉 is the average particle diameter, as estimated from the X ray results, and 〈T c 〉 is the average blocking temperature, given by

Conclusions
The sol-gel/combustion technique has been used to prepare MgFe 2 O 4 nanoparticles, which were annealed at different temperatures.X ray diffraction patterns show broad peaks in the positions corresponding to the crystal structure of magnesioferrite.Mössbauer spectra exhibit superparamagnetic behavior, confirming that the particles are in the nanometric range.Analysis of the broadening of the X ray lines shows that the average particle size increases with increasing annealing temperature.Analysis of the temperature dependence of the Mössbauer spectra of as prepared and annealed samples yields fairly narrow lognormal distributions of particle sizes.The results suggest that the sol-gel/ combustion technique may be used to synthesize nanosized MgFe 2 O 4 powders whose average size can be controlled by subsequent annealing without appreciably changing the distribution of particle sizes.
The result is shown in Figure 4 (curve 2), which is a log-normal distribution with average diameter 〈D〉 = 13.2 nm, most probable diameter Dm = 11.6 nm and full width at half maximum ∆D = 5.5 nm.Similar results for samples as prepared and annealed at different temperatures are shown in Table 1 and Figure 4.

Figure 1 .
Figure 1.X ray diffraction patterns of MgFe 2 O 4 samples as prepared and annealed at different temperatures.

Figure 3 .
Figure 3. Temperature dependence of the volume fraction of unblocked particles in sample 2 ofTable 1 (annealed at 400 °C), calculated from the Mössbauer spectra.The dots are experimental points; the line is a fit to a cumulative log-normal function (see text).

Influence of Annealing on Magnesioferrite Nanoparticles Synthesized by a Sol-Gel/Combustion Method Ronaldo Sergio de Biasi a *, Lúcia Helena Guimarães Cardoso a *, José Brant de Campos b *, Dalber Ruben Sanchez c *, João Batista Marimon da Cunha d *
Analytical grade Fe(NO 3 ) 3 .9H 2 O, Mg(NO 3 ) 2 .6H 2 O, and C 6 H 8 O 7 .H 2 O were dissolved in deionized water to obtain the starting solution.The concentrations of ferric nitrate, magnesium nitrate and citric acid were 0.5, 0.25 and 0.75 M. The solution was stirred for 4 hours at 70 °C, heated to 90 °C and kept at this temperature until the sol turned into a transparent gel.The gel was then heated result is shown in Figure

Table 1 .
Properties of MgFe 2 O 4 nanoparticles as prepared and annealed at three different temperatures.

Table 1 (
annealed at 400 °C), calculated from the Mössbauer spectra.The dots are experimental points; the line is a fit to a cumulative log-normal function (see text).