Spin Canting in Nano Particles of Magnetite

5.5.2. Spin Canting in Nano Particles of Magnetite

There are some interesting Mössbauer results, when we apply a magnetic field (H ext ) parallel to the γ-ray direction. The Mössbauer spectra for 650 sample at 300°K, 4°K and 4°K + 50 KG are shown in Figure 5.14, and those for the 900 sample at 300°K, 4°K, 4°K + 10 KG and 4°K + 50 KG are shown in Figure 5.15. It is seen from the last figure that at 4°K and H ext = 0, the 2nd and the 5th spectral line

(i.e. ∆m I = 0 lines) are ‘visible’. When H ext = 10 KG, the 2nd and 5th lines are still visible with some intensities, and when H ext = 30 KG (not shown here) still show some finite intensities. But, when Hext = 50 KG, these lines are almost extinguished in the ‘longitudinal configuration’, when the γ-ray direc- tion is parallel to the direction of the applied magnetic moment of the Fe ions. It should be noted that from 650 to 900 sample, the line-width becomes smaller, which could be ascribed to a decrease of disorder of the magnetite lattice.

Figure 5.14: The Mössbauer spectra at 300°K, 4°K and 4°K + 50 KG for the basalt glass heat-treated at 650°C for 8 hours.

NANO MATERIALS

For the 650 sample, as shown in Figure 5.14, the 2 nd and 5 th lines do not vanish at all even at

H ext = 50 KG, showing a significant intensity, as marked by two arrows at the bottom of the figure. Accordin to Neel’s model of ferrimagnetism (as discussed in the section 1.6.1), antiparallel moments have unequal magnitude giving rise to a net resultant moment. When H ext is applied parallel to the γ-ray direction in the Mössbauer experiment, the effective field on each sub-lattices A and B is given by the following relation :

H eff (A, B) = H hf (A, B) ± H ext

(5.10) When H ext is applied parallel to the γ-ray propagation, the ∆m I = 0 lines, i.e. the 2nd and 5th lines

in the high field Mössbauer spectra, would vanish and the resultant spectrum consists of the superposition of the 4-line spectra with H ext given by equation (5.10) [34]. While almost similar behaviour is observed in the 900 sample, the Neel theory is not satisfied at all for the 650 and 700 samples. Yafet anf Kittel extended the Neel theory by allowing the “Canted Spin” arrangements as [35] :

(5.11) where, θ A,B is the angles between the ‘spin’ on the sub-lattices (A, B) and H ext .

H 2 2 eff 1/2 (A, B) = H ext +H hf (A, B) – 2 H ext H hf (A, B) (cos θ A,B )

Figure 5.15 : The Mössbauer spectra at 300°K, 4°K, 40K + 10 KG and 4°K + 50 KG for the basalt glass heat-treated at 900°C for 8 hours.

MECHANICAL PROPERTIES

201 The appearance of ∆m I = 0 lines with H ext ≠ 0 has also been explained by Haneda and Morrish

by assuming a “non-collinear” spin configuration of the surface Fe ions in small particles of γ-Fe 2 O 3 following equation (5.11), i.e. spin canting. This is perhaps the ‘first’ experimental confirmation of ‘spin canting’ of the surface spins in small nano particles of magnetite within a glassy matrix. This also clearly shows that unless we go down to the nano level, we cannot understand the sensitivity of the arrangements of the surface spins changing the pattern of alignment from the ‘canted mode’ to more or less completely organised spin configuration within 5.5 nm to 7.0 nm of particle size.

The above situation can be described in terms of a model shown in Figure - 5.16. For the 900 sample, some surface spins may be canted even for H ext = 50 KG, because the actual saturation of spin arrangements might occur at H ext > 50 KG. This model is shown for the 700 sample with a particle size of around 5.5 nm, although it is very much possible for the 650 sample, where more spins are present on the surface of the nano particles of magnetite, since the particle size is still smaller, i.e. 4.5 nm. As said earlier, in the 900 sample, these surface ions are more or less aligned, i.e. less canted, which is clearly evident from Figure - 5.15 as a function of magnetic field. It is noted from this figure that at H ext = 10 KG, there is still some significant intensity of 2nd and 5th lines, whereas with H ext = 50 KG, these lines are almost extinguished, indicating almost no ‘spin canting’ [9].

For the samples heat-treated at lower temperatures with still smaller particles size, these surface spins (as shown in the model figure) are quite difficult to be aligned even with high magnetic field. Therefore, the delicate role of smaller nano particles of having the surface spins aligned is emphasized here in this model. This also highlights the effective role of particle anisotropy for the alignment of the surface spins in terms of the ‘fast relaxation’ of the nano particles.

Crystallites of Magnetite

Surface Iron Ions Isolated Iron Ions

Figure 5.16 : Magnetic model of the basalt glass heat-treated at 650 or 700°C, where the spins appear to be canted.

NANO MATERIALS

As pointed out earlier, when the particles are small, the ‘number of spins’ on the surface is naturally more. Since the particle size of the 650 and 700 samples is less than that of the 900 sample, the surface spins are ‘canted’ in the former samples with smaller nano particles. It is quite remarkable to note that within such a narrow range of nano particle size, such a possibility can occur between 4.5 nm/

i.e. for 600 or 700 sample showing spin canting, and 7.0 nm i.e. for 900 sample not showing any appreciable canting of spins. So, there might be a sort of “critical size” even within this nano range, wherein the spins start getting aligned, say between 5.5 nm and 7.0 nm. Then, let us say that this critical particle size is 6.0 nm, which is so sensitive in that this critical size determines whether or not a composite Mössbauer spectra of super-paramagnetic and ferrimagnetic contribution can be observed together, like in the case of 800 and 900 samples at 300°K. It also determines the extent of spin canting,

5.5 nm,

i.e. how the surface canted spins observed below this critical size are progressively getting aligned, which means a ‘saturation’. In the SANS data, some sort of criticality has also been observed in terms of re-dissolution of smaller nano particles when the larger nano particles grow, also within a narrow range (see the section 5.7).

Finally, it should be pointed out that the hyperfine field at saturation, say at H ext = 50 KG, is found to be 528 KG, which is not significantly higher than that found at H ext = 0. At H ext = 50 KG, the hyperfine field is found to be 516 KG at H ext = 10 KG. This again shows that the magnetic field neces- sary to align the surface canted spins arrangements in the 650 and 700 samples should be substantially higher than 50 KG, as also noted in several other magnetic materials [34].

In summary, it can be said that for the nano particles of magnetite within a basalt glass with an average partile size of 7.0 nm, the Mössbauer spetra at 300°K show both super-paramagneti and ferrimagneti contributions, while below a critial size of about 6.0 nm, only super-paramagnetic behav- iour is observed. Below the so-called blocking temperature, the Mössbauer spectra at 4°K show mag- netic ‘hyperfine splitting’ due to ferrimagnetic magnetite. This behaviour can be ascribed to the ‘relaxa- tion effect’ of the small nano particles of magnetite with respect to the time of the Mössbauer experi- ment, i.e. inverse of the Larmor frequency. The hyperfine field at 4°K for the samples seems to be independent of particle size of the nano particles confirming a ‘magnon’ model. From the ferimagnetic part of the spectra at 300°K, the hyperfine field is found to be lower than that of the bulk magnetite, and the anisotropy constant of the nano particles of magnetite seems to be very high above the critical size.

In the low temperature and high field Mössbauer data, for the smaller nano particles, the appear- ance of ∆m I = 0 lines with H ext ≠ 0 confirms the spin canting at the surface, while the disappearance of these lines with the progressive application of H ext above the critical size shows the ‘alignment’ of the surface spins. A model for the smaller nano particles of magnetite is also given to elucidate their interesting magnetic behaviour. The present results definitely show the usefulness of the Mössbauer technique in elaborating the magnetic behaviour of single domain small super-paramagnetic nano parti- cles, which were “uniquely” created within a glassy matrix at different heat-treatment temperatures, at different conditions of the measurement.