Mossbauer spectra o f the unsintered samples showed that there are differences
between the metallic samples and the non-metallic samples. The metallic samples
seem to consist o f mostly o f magnetite (75%) with the remainder mostly being
wiistite and the unknown phase (Figure 2.30). There is also some starting material
such as Fe (0.5-1.8% ) and Fe2 0 3 (0.8-3.3% ). However it is not possible to
establish if the FeiOs detected is from starting materials or from oxidation o f Fe.
This suggests that the metallic parts do not go to completion in the initial SHS
step.
Figure 2.30 A plot of composition of metallic component against magnetic field used in synthesis.
100% a> O) 2 c 8 œ 6 15 2 0 S yn th es is F ie ld /T e s la ■ Unknow n □ W üstite □ Hematite □ Magnetite □ Alpha-Fe
The non-metallic samples seem to consist mostly {ca. 60 %) o f wüstite and hematite. There is also some magnetite (22.1-42.7 %) and Fe (5-8.2 %). It can
also be seen that the relative percentages o f magnetite increase (Figure 2.31) with
increasing field (this is the opposite o f what is happening in the barium ferrite
reactions). This suggests a slightly different mechanistic pathway for the
formation o f strontium ferrite. From the Mossbauer fits it can also be seen that the
non-metallic part is less fully combusted than the metallic part (there are higher
relative percentages o f Fe in the non-metallic parts). However the Mossbauer
does show for the 6 Tesla non-metallic part that there is some oxidation o f Fe to
value o f 33%. Therefore it is possible to say that for the non-metaUic parts synthesized in an applied field oxidation o f Fe to FeaOs is took place.
What is interesting to note is that in the non-metallic part synthesised in a 20 Tesla magnetic field maghemite is also formed. This indicates that the magnetic field is inducing areas o f space rich in oxygen, thus inducing the formation o f oxidised Fe304 (y-FeiOs). The formation o f y-Fe203 is easily accomplished by oxidising magnetite^^.
The unknown phase found in the unsintered materials is a reflection o f what is happening in barium ferrite. It is thought to be proto-strontium ferrite. That is strontium ferrite in its early stages o f formation. There is also the possibility that this unknown phase might be a precursor which then leads to the formation of strontium ferrite.
It was also noticed that the SrFe204 or SrFe0 3-x observed in the X-ray diffraction patterns for the metallic and non-metallic samples was not detected in the Mossbauer spectra. A doublet^^^ would be expected to be present in the Mossbauer spectra if SrFe0 3-x was present. A sextet^^^ would also be expected in the Mossbauer spectra if SrFc204 was present. All doublets were accounted for, in the spectra. However there was an unknown sextet that has been mentioned before. The parameter o f this sextet did not match the expected parameters for SrFe204. However it is possible that the unknown might be cation or oxygen deficient SrFe204 and hence does not show the expected Mossbauer parameters.
Figure 2.31 A plot of composition of non-metallic component against magnetic field used in synthesis.
100% 6 15 S y n th esis F ie ld /T e s la ■ Gamma-Fe203 ■ unknown □ VWslAe □ Hematite ■ Magnetite D Alpha-Fe 2 . S . 2 . 2 S i n t e r e d s a m p l e s .
The sintered samples showed that strontium ferrite was synthesised with the expeeted r a t i o s ' o f 4 : 6 : 12 for the sites, 4fvi, 4fiv + 2a, 12k. However the site occupancy for all sintered samples for the 2b site was lower than expected. This mirrors what has been observed before for reduced ferrites where the site occupancy of the 12k and 4fvi sites decrease with the appearance of paramagnetic Fe and a-Fe components in the M ossbauer'spectrum . This suggests that the lower lattice parameters observed in all SHS sintered samples might be due to reduced Fe in the 2b site. Table B.l in the appendix also shows that the fits needed a singlet ca. 2% to achieve a low chi squared value (a measure of goodness of fit). This singlet was not matched to any parameters in the literature, but it is thought to be paramagnetic Fe. This resembles what was observed for reduced ferrites. The parameters were on average 0.27 mms ' for the isomer shift.
F i g u r e 2 . 3 2 M o s s b a u e r f î t o f t h e s i n t e r e d n o n - m e t a l l i c s a m p l e s y n t h e s i s e d a t 2 0 T e s l a . c o 2- o (/> < m 0 10 - I D V (mm/s)
In the metallic parts a decrease in the ratios of populations of the 4fvi on the 20 Tesla sample from zero field suggests that the maximum lattice parameter shrinkage observed for the 20 Tesla sample is due to a reduction of the 4fvi (oxygen is desorbed from this site). However an accompanying increase of the relative populations of the singlet is not seen (table B.l in the appendices).
All of the 12k sites in the metallic samples are also slightly below the expected ratios, thus suggesting that some of the 12k sites have been reduced. Reduction of specific sites in ferrites has been described in the literature
The metallic parts of samples synthesised at 0 T and 20 T were also fitted with hematite ca. 15 %. This corroborates the X-ray data where hematite was observed for the same samples. The familiar unknown phase that was observed for the unsintered samples was also seen in metallic sintered samples synthesised in a 20 Tesla magnetic field.
In the non-metallic parts it was observed that the relative populations of the 4fvi sites were slightly less in the 6 Tesla sample (the sample with the highest lattice shrinkage). This might suggest that a similar process to that of barium ferrite synthesis where oxygen is thought to be coming off from the 4fvi site in the crystal lattice is occurring also for the strontium ferrite. This was also corroborated by an
increase in the population o f the singlet observed, thought to be paramagnetic Fe. In the 6 Tesla sample the highest populations for the singlet (3.9 %) were observed.
All o f the 12k sites are also slightly below the expected ratios as seen in the metallic samples and this also substantiates some reduction o f the 12 k sites.
2.5.3 Vibrating sample magnetometry (VSM) of sintered samples.
Coercivity, saturation magnetisation, and remanence values were obtained from the hysteresis loops and are shown in Table 2.8. The data demonstrates a clear-cut difference between samples synthesised in zero and samples synthesised in applied fields.
Table 2.8 Magnetic data for sintered strontium ferrite samples.
Sample Synthesis Field / Tesla Saturation / emu g‘* ± 0.1 emu g ' Coercivity / Oe ± 5 0 e Remanence / emu g'^ ±0.1 emu g ’ Metallic 0 39.3 1755 25.3 6 48.3 1395 30.7 15 47.6 1550 29.5 20 45.3 850 23.3 Non-metallic 0 46.3 1850 29.3 6 46.3 2140 31.0 15 47.1 2225 31.1 20 44.1 2320 29.2
From Table 2.8 it can be seen that coercivities o f the metallic samples synthesised in an applied field are lower than those made in zero field. This suggests that synthesising ferrites in a magnetic field is a good alternative to doping in order to manipulate coercivity. However, the opposite occurs for the non-metallic samples where the coercivities for materials synthesised in zero field are lower than those synthesised in an applied field (Figure 2.34). Overall coercivities o f non-metallic samples are higher than those o f metallic samples (Figure 2.33 and 2.34).
F i g u r e 2 . 3 3 H y s t e r e s i s l o o p s f o r m e t a l l i c a n d n o n - m e t a l l i c p a r t s o f s t r o n t i u m f e r r i t e s s y n t h e s i s e d i n a 2 0 T e s l a m a g n e t i c f i e l d . T h e b l u e l i n e r e p r e s e n t s t h e m e t a l l i c p a r t . T h e r e d l i n e r e p r e s e n t s t h e n o n - m e t a l l i c p a r t . — Non-metallic - - -6( -4C -2( 40 X) 6030 66^-- H / O e
Coercivities compared well to the values required for magnetic recording applications and were in general lower compared to traditional synthesis where values"^ above 2.66 kOe are common.
F i g u r e 2 . 3 4 C o e r c i v i t y o f m e t a l l i c a n d n o n - m e t a l l i c p a r t s v e r s u s s y n t h e s i s f i e l d . 2500 2000 1500 -- Metalic Non-metallic U 1000 500
The non-metallic samples showed similar values for saturation magnetisation irrespective o f the magnetic field used in the initial synthesis. However for the metallic samples there seems to be a higher saturation magnetisation for the samples produced in the applied field compared to the samples synthesised in zero field. Saturation magnetisation values were lower than in conventionally prepared materials, which have a value‘s® o f 71 em ug'\ However absolute saturation magnetisation was probably not achieved since only a field o f 5 kOe was used. Literature reports* for saturation magnetisation are usually for fields o f 7.5 kOe and above. Overall remanence values compared well to those reported in the literature*^ (36 emu g *).
2 . 5 . 4 S c a n n i n g e l e c t r o n m i c r o s c o p y ( S E M ) .
It is interesting to note that at zero field and at low fields (6 T) the particle sizes for metallic and non-metallic pre sintered materials are almost identical (ranging fi*om 0.3-6 pm), see Figure 2.35. However, at high fields (20 T) there is a clear difference in particle sizes between the metallic and non-metallic component. In these cases smaller particle sizes (0.6-3pm) are observed for the non-metallic samples while larger particle sizes are observed for the metallic samples (1-6 pm), see Figure 2.35. It is worth noting that the difference in particle size between metallic and non-metallic samples is carried through even after sintering. In this case smaller particle sizes (0.5-2.4 pm) are observed for the non-metallic while even larger average particle sizes are observed for the metallic (7-15 pm) samples synthesised in a 20 Tesla magnetic field. This compares well with the VSM data where the metallic samples which contain the larger particles are the ones with the lowest coercivity. Literature has shown that the higher the average particle sizes the lower the coercivity^^.
F i g u r e 2 . 3 5 S c a n n i n g e l e c t r o n m i c r o g r a p h s o f s t r o n t i u m f e r r i t e s . O n t h e l e f t t h e S E M o f t h e n o n - m e t a l l i c s a m p l e o f p o s t - S H S s t r o n t i u m f e r r i t e s y n t h e s i s e d i n a 6 T e s l a m a g n e t i c f i e l d . O n t h e r i g h t t h e S E M o f t h e n o n - m e t a l l i c s i n t e r e d s t r o n t i u m f e r r i t e s a m p l e s y n t h e s i s e d i n a 2 0 T e s l a m a g n e t i c f i e l d . 4 1 9 0 2 0 25KV X3 . 5 0 K 8 . 6 u i 2 . 5 . 5 E n e r g y d i s p e r s i v e a n a l y s i s o f X - r a y s ( E D A X ) a n d E l e c t r o n p r o b e r e s u l t s .
Acicular particulates were observed in the electron-probe maps o f the post-SHS
samples. These acicular particles were only present in the metallic samples
synthesised in both zero and applied magnetic field. The acicular particles were
very abundant, ca. 50 percent o f the metallic parts. However they were less abundant in the zero field ca. 5%. The acicular particles measure between 5-10 pm in width to 100-300 pm in length (Figure 2.36). The acicular particles seem to
have a ratio o f iron : strontium o f 12:1 for both zero field and applied magnetic
field. This implies the formation o f SrFeizOig. The areas surrounding the acicular
particles showed differences between zero field and applied magnetic field
samples. The zero field samples showed ratios o f 2.8:1 to 4:1 o f Fe : Sr. This
could be due to a mixture o f all o f the materials detected by X-ray diffraction such
as FeO, Fe2 0 3, Fe3 0 4, SrFe:0 4, Fe and SrFei20i9. The applied field samples in the