Room-temperature ferromagnetism of sol-gel-synthesized ${\mathrm{Sn}}_{1-x}{}^{57}\mathrm{Fe}_x{\mathrm{O}}_{2-\delta }$ powders

We report unusual observations in the magnetic behavior of ${\mathrm{Sn}}_{1-x}{}^{57}\mathrm{Fe}_x{\mathrm{O}}_{2-\delta }$ powders prepared by a sol-gel method. Mössbauer spectra showed three different sites for those irons in the $\mathrm{Sn}{\mathrm{O}}_2$ lattice. The samples seem to exhibit many sources of ferromagnetism, and the dominance of one of them greatly depends upon the synthesis conditions. In one sample, prepared using citric acid solution of metal Fe and annealed at $500°\mathrm{C}$ for two hours, we observed magnetization, but its room temperature Mössbauer spectrum did not show any magnetic component, suggesting that the ferromagnetism could not have originated from the iron ions but from magnetic defects. In another sample prepared from chloride acid solution of ${\mathrm{Fe}}_2{\mathrm{O}}_3$ and annealed at $600°\mathrm{C}$ for six hours, a large magnetization and a sextet with broad lines were observed and the ferromagnetism was ascribed to iron impurities. In another sample prepared from citric acid solution of metal Fe but annealed at $650°\mathrm{C}$ for two hours, we observed an intensive magnetic sextet with sharp lines but small magnetization, suggesting the presence of hematite doped with tin.

Figure 1

Average grain sizes from SEM versus iron content for all samples. Square symbols are data for samples coming from citric acid solutions of metal ${}^{57}\mathrm{Fe}$ and annealed at $500°\mathrm{C}$ for $2\mathrm{h}$. Up triangle symbols are data for samples prepared from HCl solutions of ${}^{57}\mathrm{Fe}_2{\mathrm{O}}_3$ and annealed at $600°\mathrm{C}$ for $6\mathrm{h}$. Diamonds represent for samples prepared using citric acid solutions of metal ${}^{57}\mathrm{Fe}$ and annealed at $650°\mathrm{C}$ for $2\mathrm{h}$.

Figure 2

XRD patterns of ${\mathrm{Sn}}_{1-x}{\mathrm{Fe}}_x{\mathrm{O}}_{2-\delta }$ samples prepared from citric acid solutions of metal ${}^{57}\mathrm{Fe}$ and annealed at $500°\mathrm{C}$ for $2\mathrm{h}$.

Figure 3

XRD pattern plotted in linear scale of sample $x=0.15$ prepared using citric acid solution of metal ${}^{57}\mathrm{Fe}$ and annealed at $650°\mathrm{C}$ for $2\mathrm{h}$. Solid spheres represent the experimental data, whereas the solid lines represent the fit using Rietveld method. The insets show some selected expanded regions plotted in log scale. The arrows point to hematite peaks.

Figure 4

Changes in the unit cell parameters with increasing iron nominal content for samples prepared from citric acid solutions of metal ${}^{57}\mathrm{Fe}$ and annealed at $650°\mathrm{C}$ for $2\mathrm{h}$ (upper part), HCl solutions of ${}^{57}\mathrm{Fe}_2{\mathrm{O}}_3$ and annealed at $600°\mathrm{C}$ for $6\mathrm{h}$ (middle part), and citric acid solutions of metal ${}^{57}\mathrm{Fe}$ and annealed at $500°\mathrm{C}$ for $2\mathrm{h}$ (lower part).

Figure 5

Lattice volumes versus iron content at different annealing temperatures. Dotted, solid, and dashed lines are linear fits to the data for each group of samples. Open squares represent data for samples coming from citric acid solutions of metal ${}^{57}\mathrm{Fe}$ and annealed at $500°\mathrm{C}$ for $2\mathrm{h}$. Open circles are data for samples prepared from HCl solutions of ${}^{57}\mathrm{Fe}_2{\mathrm{O}}_3$ and annealed at $600°\mathrm{C}$ for $6\mathrm{h}$. Filled up triangles are for samples prepared using citric acid solutions of metal ${}^{57}\mathrm{Fe}$ and annealed at $650°\mathrm{C}$ for $2\mathrm{h}$.

Figure 6

Room temperature hysteresis loops for ${\mathrm{Sn}}_{0.9}{\mathrm{Fe}}_{0.1}{\mathrm{O}}_{2-\delta }$ samples prepared from (a) HCl solutions of ${}^{57}\mathrm{Fe}_2{\mathrm{O}}_3$ and annealed at $600°\mathrm{C}$ for $6\mathrm{h}$, (b) citric acid solutions of metal ${}^{57}\mathrm{Fe}$ and annealed at $500°\mathrm{C}$ for $2\mathrm{h}$, and (c) citric acid solutions of metal ${}^{57}\mathrm{Fe}$ and annealed at $650°\mathrm{C}$ for $2\mathrm{h}$.

Figure 7

$M_s$ versus iron nominal content and annealing temperature for some selected samples. Samples annealed at $500°\mathrm{C}$ and at $650°\mathrm{C}$, both for $2\mathrm{h}$, come from citric acid solutions of metal ${}^{57}\mathrm{Fe}$, whereas sample annealed at $600°\mathrm{C}$ for $6\mathrm{h}$ was prepared from HCl solutions of ${}^{57}\mathrm{Fe}_2{\mathrm{O}}_3$.

Figure 8

Room temperature Mössbauer spectra of ${\mathrm{Sn}}_{0.9}{\mathrm{Fe}}_{0.1}{\mathrm{O}}_{2-\delta }$ samples prepared from (a) citric acid solutions of metal ${}^{57}\mathrm{Fe}$ and annealed at $650°\mathrm{C}$ for $2\mathrm{h}$, (b) HCl solutions of ${}^{57}\mathrm{Fe}_2{\mathrm{O}}_3$ and annealed at $600°\mathrm{C}$ for $6\mathrm{h}$, and (c) citric acid solutions of metal ${}^{57}\mathrm{Fe}$ and annealed at $500°\mathrm{C}$ for $2\mathrm{h}$.

Figure 9

Hysteresis loops for ${\mathrm{Sn}}_{0.9}{\mathrm{Fe}}_{0.1}{\mathrm{O}}_{2-\delta }$ samples prepared from citric solutions of metal ${}^{57}\mathrm{Fe}$ and annealed at $500°\mathrm{C}$ for $2\mathrm{h}$ and $6\mathrm{h}$ in air. The insets show their respective room temperature Mössbauer spectra.

Figure 10

SEM micrographs of some selected ${\mathrm{Sn}}_{1-x}{\mathrm{Fe}}_x{\mathrm{O}}_{2-\delta }$ samples prepared from HCl solutions of ${}^{57}\mathrm{Fe}_2{\mathrm{O}}_3$ and annealed at $600°\mathrm{C}$ for $6\mathrm{h}$.

Figure 11

Low temperature Mössbauer spectra of ${\mathrm{Sn}}_{0.9}{\mathrm{Fe}}_{0.1}{\mathrm{O}}_{2-\delta }$ sample prepared from HCl solution of ${}^{57}\mathrm{Fe}_2{\mathrm{O}}_3$ and annealed at $600°\mathrm{C}$ for $6\mathrm{h}$.