ESR Spectroscopy

5.6. ESR Spectroscopy

The theory of ESR is given in the section 1.6.2. The measurements of ESR was carried out at 300°K in a Brucker spectrometer operating in X-band frequency (9.2 GHz), the magnetic field modula- tion frequency was 18 KHz. All the low temperature measurement were carried out using Varian Asso- ciate spectrometer at X-band frequency and the modulation frequency was 100 KHz. All precautions were taken to reduce the noise level to a minimum with a proper amplification and tuning to get the best possible signals. The ESR spectra were recorded in a standard plotter. Since the total intensity of ESR is

203 dependent on the number of paramagnetic ions per unit volume, the weight of each sample was meas-

MECHANICAL PROPERTIES

ured before inserting through a special quartz tube in the microwave cavity. The typical ESR spectra, taken at 300°K, for some of the smples are shown in Figure 5.17. The

peak-to-peak intensity (I), line-width (∆H) and the integrated intensity (I × ∆H 2 ) for the g = 2.0 reso- nance are shown in Table 5.2, along with the respective weight of the samples. The spectra for the blank glass and 600 samples show both the resonances at g = 4.3 and g = 2.0 respectively, which are usually observed for paramagnetic materials containing Fe 3+ ions.

Table 5.2. The ESR Data for Nano Particles of Magnetite.

Heat-Treatment Intensity, I (AU)

I. ∆H 2 Sammple Weight Temperature

∆H (G)

(gm) As-annealed

Sample Blank at 300 K

Sample 650 at 300 K

500 G

500 G

Magnetic field

Magnetic field

NANO MATERIALS

g = 2.0

g = 2.0

Sample : 700 at 300 K

Sample 900 at 300 K

500 G

500 G

Magnetic field

Magnetic field

Figure 5.17 : The ESR spectra at 300°K for different samples.

The line-width for the g = 2.0 resonance is very high showing that there is a considerable ‘ex- change interaction’ between the Fe 3+ ions in these samples. For the blank glass, the magnetic ordering of the Fe 3+ ions, as revealed by the Mössbauer spectra at 4°K (see Figure 5.13), could also contribute to the observed line-width. Similar inference can be drawn for the 600 sample as well.

At 4°K, the ESR spectra of the blank glass showed a strong g = 4.3 resonance with a very weak

g = 2.0 resonance, but there was a strong absorption in the higher field region. From the analysis of the exchange Hamiltonian proposed by Owen [36], it can be suggested that the weakening of the g = 2.0 resonance at 4°K is due to ‘antiferromagnetic interaction’ between two Fe 3+ ions, which are incorpo- rated into small ‘nano’ clusters, as also revealed by the Mössbauer spectra at 4°K. If the interactions were between Fe 2+ and Fe 3+ ions, the g = 2.0 resonance should be stronger at 4°K than that at 300°K, which were not observed.

For the g = 4.3 resonance of the blank glass, it is found that the line-width increases from 108 G to 220 G between 4°K and 300°K. This is ascribed to the ‘spin-spin’ interaction rather than ‘spin- lattice’ interaction [37], which could cause an opposite temperature dependence of the line-width. Hence, even the Fe 3+ ions in the glass which give rise to crystal field resonances are subject to ‘exchange interaction’ [38]. At 300°K, the g = 4.3 resonance progressively disappears from the blank glass to the other samples. The barely discernible appearance of this resonance for the 650 sample shows that the isolated Fe 3+ ions have started migrating into the formation of the nano crystals of magnetite. It should

be noted that for the 700 sample, this resonance almost disappears for the 700 sample. The weight of the last four samples were 25 times lesser than those of the first two samples, but

the observed intensities of the g = 2.0 resonance are still very high. This shows the presence of nano crystalline phases with super-paramagnetic behaviour in these samples. The spectrum for the 650 sam- ple shows super-paramagnetic behaviour. This spectrum is very similar to that obtained for small nano particles of magnetite embedded in an inert carbonaceous matrix, giving the g = 4.3 resonance, but the line-width is very small [14].

MECHANICAL PROPERTIES

205 This behaviour is ascribed to a strong ‘exchange interaction’ and also to a motional narrowing,

because the higher flipping rate of the Fe 3+ ions. The slight asymmetry in the shape of the spectrum results from the ‘non-equivalence of the positions’ of the Fe 3+ ions in the nano crystalline magnetite lattice. This is also caused by the ‘electron hopping’ in the octahedral site in magnetite [14]. This shows that there is some kind of ‘disorder’ in the nano particles of magnetite for the 650 sample. This ‘disor- der’ decreases in the 700 sample, because the observed spectrum is more symmetric. This is also re- flected in the evloution of the Mössbauer parameters (i.e. δ and ∆) showing high symmetry around 700°C of heat-treatment (see Figure 5.12).

For the 700 sample, the spectrum still shows a super-paramagnetic behaviour, and the spectrum gives quite symmetric resonance at g-value little higher than 2.0, but the line-width is very large. It seems that both the ‘exchange’ and ‘motional’ narrowing decrease with increasing particle size from 650 to 700 sample. The reason for this behaviour is very difficult to justify at this stage. However, the contribution of the ferrimagnetic particles, although not detected in Mössbauer spectra at 300°K due to faster relaxation, to the spectrum of the 700 sample cannot be completely ruled out. This was evident in the spectra of the 800 and 900 samples in that the g-value was shifted to a still lower value, and the both the line-width and intensity increased considerably (see Table 5.2). All these results are quite consistent with the room temperature Mössbauer data, as described in the section 5.5.

For the 700 sample, the spectra showed a continuous change from 300°K down to 130°K in that the line-width increased rapidly with the decrease of intensity. The spectrum at liquid nitrogen tempera- ture, i.e. at 77°K, shows a broad resonance at g-value much higher than 2.0, as shown in Figure 5.18. This is typical ferrimagnetic resonance [39, 40]. Although the magnetization measurements at 77°K show super-paramagnetic magnetite particles, but the Mössbauer spectra at 77°K show ferrimagnetic behaviour, the presence of ferrimagnetic particles gives such a strong resonance that the super-paramanetic part of the resonance observed at 300°K is washed out. This was evidently manifested in the evolution of the spectra with decreasing temperature.. This shows that the interpretation of the ESR spectra is much more complicated than those of the other measurements, as discussed in the previous sections, because of the contribution of different ‘magnetic interactions’ to the observed line-width of a single resonance.

Sample : 700 at 77 K

625 G

Figure 5.18 : The ESR spectra at 77 K for the 700 sample.

However, it seems more likely that the time required for the ESR measurement (τ obs ), which is almost comparable with that of Mössbauer measurement, is much less than the relaxation time (τ) of the nano particles at 77°K making the ferrimagnetic part of the resonance appear in the observed spectra. The importance of different measurements at different temperatures (300°K to 4°K) as highlighted be- fore is again brought to the focus here.

NANO MATERIALS

g = 4.3

Sample : 650 at 4 K 625 G

Magnetic field

Figure 5.19 : The ESR spectra at 4°K for the 650 sample.

A very similar behaviour for the g = 2.0 resonance was observed for the 650 sample down to 135°K, but the g = 4.3 resonance remains the same as that at 300°K. No measurement was carried out at 77°K. However, the ESR spectra at 4°K show a strong g = 4.3 resonance with a broad ferrimagnetic component at a lower g-value, as shown in Figure 5.19. The appearance of the g = 4.3 resonance at 4°K with a significant intensity could be thought to be due to the increase of Boltzman’s population between two energy levels, but it is not understood why this resonance remained stagnant down to 135°K. This behaviour could be mainly attributed to the ‘spin-spin’ relaxation for the 650 sample with a particle size of nano particles of magnetite of 4.5 nm. There are still some Fe 3+ ions, which are expected to remain outside the crystalline magnetite phase. The ESR spectra show that in addition to the magnetic interac- tions between the Fe 3+ ions inside and outside the nano-crystalline magnetite particles, there could be magnetic interactions between the isolated Fe 3+ ions and the Fe 3+ ions on the surface of the nano-grains.

In summary, it can be said that although the time for ESR measurement is comparable to that of Mössbauer experiment, the contribution of different magnetic interactions complicates the analysis of

the line-width. However, in general, the ESR data at 300°K support the Mössbauer data.