Mossbauer spectroscopy and x-ray diffraction study of (Cd, Zn) substituted mixed ferrites.

Systematic decrease in the hyperfine fields indicating weakening of the magnetic coupling with an increase in x is observed. The spinel structure of the compounds studied in this work is confirmed by X-ray diffraction (XRD). However, an anomalous behavior of the lattice parameter is observed in Zn-based samples.

List of Tables

Introduction to ferrites

Crystal structure
Magnetic ordering in magnetic materials
Magnetic interactions in ferrites
Motivation for current work

The strength of the magnetic interaction in ferrites depends on the magnetic neighbors present in the Fe environment and is proportional to the Curie temperature [2]. It is in this perspective that a systematic study of the compositional dependence of the magnetic properties of substituted mixed ferrites was carried out in order to investigate the site occupation of Zn and Cd ions. It is interesting to see the effect of size differences on the magnetic properties and structure of compounds.

Figure 1.1: Schematic diagram of one eighth of the unit cell of the spinel structure [4].

Principles of Mossbauer Spectroscopy

The Mossbauer effect
Hyperfine interactions

Isomer shift
Electric quadrupole interactions
Magnetic hyperfine interactions
Combined magnetic and quadrupole interaction

Sample preparation
Mossbauer spectroscopy

The conservation of energy and momentum in the emission process gives the nuclear recoil energy. 2.1.4) where is the component of the mean-square vibrational amplitude of the atom that emits in the direction of the beam,. Equation (2.1.6) shows that f is large when OD is large (a strong lattice), the recoil energy and the temperature T are low. 2.1.7) Favorable conditions for resonant absorption impose limits on nuclei suitable for Mossbauer spectroscopy.

The energies of the nuclear states are weakly affected by the chemical environment of the sample. In a Mössbauer experiment, the isomer shift is measured independently of the nature of the source. The isomer shift is clearly a direct function of the electron density at the nuclear site of the absorber.

It is therefore sensitive to changes in orbital occupation in the valence shell of the absorber atom. The magnetic hyperfine division of the energy levels in the 57Fe nucleus is illustrated in Figure 2.2 (b). In the following chapter, a discussion of the experimental techniques and procedures used in this work is presented.

For the current investigation, shorter measurement times are possible by using 57Fe-enriched iron oxide in the production of the samples. The electromechanical drive system consists of the Mossbauer drive unit (MDU) model MR-350, function generator (FG) model FG-351 and a multi-channel scale PCA3 card (supplied by Oxford Instruments) mounted in a computer. The drive coil in the transducer produces a voltage signal that is proportional to the velocity of the transducer.

Figure 2.2: (a) Isomer shift, (b) Magnetic splitting and (c) Splitting due to combined magnetic and quadrupole splitting of the 3/2 - - 7 1/2 transition in 57Fe.

SkV peA3 bias

SV /REJ

The Mossbauer spectrometer was started and when it was running smoothly, the F1 key on the keyboard was pressed to start the MCS recording on the PCA3. If all cables are properly connected and everything is set up and powered on, you should be able to see counts in the pass block and the start of the baseline rising from the bottom of the screen. The analysis of spectra was performed using Mosswinn 3.Oi [29] and Recoil [30] Mossbauer software packages.

This makes the use of X-rays ideally suited for studying the structure of materials. Each particle in the powder acts as a crystal mounted randomly with respect to the beam. Equation (3.3.1) indicates that if an X-ray beam of known wavelength A is used, the distance between the planes of atoms (d) can be calculated.

Since in ferrites the spinel structure is cubic, d is related to the size of the unit cell lattice parameter a by [27]. 3.3.3) Equation (3.3.3) shows that to determine the grating constant one only needs to assign the correct Miller indices to each diffraction line. Measurements were carried out at the School of Geological and Computer Sciences at the University of Natal using a Phillips diffractometer type PW1710.

The nature, size of the unit cell and quality of the samples were determined from the diffraction patterns.

Introduction

X-ray diffraction results

Lattice parameters
X-ray and bulk densities
Porosity of samples
Grain sizes of the compounds

The values of the lattice parameter a of the Zn and Cd based spinel oxides are shown in Tables 4.1 and 4.2 respectively and plotted against x in Figure 4.3. The increase and decrease in a indicate an expansion and a contraction of the unit cell, respectively. The change in size of a unit cell with x can be explained based on the changes in the distances between the A or B position cations and the oxygen anions due to the difference in size of the cations involved.

The increase in RA in Cd-based compounds is higher than in Zn-based compounds, consistent with larger size of the Cd atom compared to Zn. This may be due to some of the Zn or Cd ions also going to B sites. This indicates a modification of the cation distribution in expression (4.2.1) to accommodate some of the diamagnetic ions that also go to B-site at high concentrations.

This is theoretically expected because both porosity and density of the samples are related to the heat treatment. The percentage porosity P of the samples was derived from the x-ray density Px and mass density Pb data using the formula [23]. This was calculated from the broadening of the (311) x-ray diffraction linewidth using the Sherrer formula [27J. g = - - -K).

It is clear that fluctuations in grain sizes of the samples must be due to the grinding of samples by hand with a mortar and pestle and can therefore be reduced by using high energy ball milling.

Figure 4.1: X-ray diffraction patterns for ZnxCo o. g Fe1.7_xTi o .404.

Mossbauer spectroscopy results

The average isomer shift values found in the currently examined samples are shown in Table 4.6. As expected, both quadrupole splitting and isomer shift values at the A site are smaller than those at the B site. High quadrupole splitting at the B than A site is due to low symmetry at the B site as discussed in section (1.1).

A systematic decrease of the hyperfine fields acting on A and B sites in the 57Fe nuclei metx indicates the weakening of the magnetic coupling. The variation of the fraction of Fe ions on A or B sites with x is shown in Figure 4.11. The Fe fraction is proportional to the area of a sextet or a doublet in the Mossbauer spectra.

Therefore, these areas can provide information about the occupation of interstitial sites by Zn or Cd ions. A broad maximum in the Fe fraction was observed around x=0.7 at site A and a minimum at site B for Zn-based compounds. A maximum in the Fe fraction was observed around x=0.8 at site A and a minimum at site B for Cd-based compounds.

The occupancy of B sites by Zn ions is one of the models that has been used to explain the reduction of magnetization above x=0.5 in ZnxCol-xFe204 [6].

Figure 4.8: M6ssbauer spectra for ZnxCao.gFe1.7_xTi o .404.

Introduction

X-ray diffraction results

Lattice parameters
Bulk and X-ray densities of compounds
Porosity of samples
Grain sizes in the compounds

The values of the lattice parameter a found in the samples (see Tables 5.1 and 5.2) studied in this experiment compare very well with previously reported values for similar compounds. The tetrahedral radius RA and octahedral radius RB for Cd- or Zn-based samples can be calculated from the cation distribution given by equation (5.2.1). The increase in RA for Cd- and Zn-based samples is predicted by the positive slopes in equations (5.2.4) and (5.2.5).

Similarly, an increase in RA with x in Zn-based samples is due to larger Zn2+ ions replacing the smaller Fe3+ ions at an A site. The higher increase RA in Cd than in Zn-based samples is consistent with larger size of the Cd atom. In Cd-based compounds, the increase in RA is very large compared to the decrease in RB' This leads to an expansion of the unit cell and shows as an increase in the lattice parameter with x.

This leads to the contraction of the unit cell and shows up as a decrease in the lattice parameter with x. For x~O.l a mass density minimum was observed in Cd based samples and a mass density maximum in Zn based samples. There is a general increase and a general decrease with x in porosity for x2:0.1 in Zn-based and Cd-based compounds, respectively (see figure 5.6).

An anomalous decrease in grain size with x was observed in Zn-based composites for x2:0.4.

Table 5.1: Bulk Pb and X-ray Px densities, porosity P and lattice parameter a for ZnxCol-xFe2-xAlx04.

Mossbauer spectroscopy results

The values of isomer shifts, quadrupole splitting and hyperfine fields for Cd and Zn based samples are shown in Tables 5.3 and 5.4. No significant variation in the isomer shift with . In Cd-based compounds, the isomer shift decreases with an increase in inx at an A-position for x;:::0.5.

This means that the charge distribution of Fe3+ s-electrons at the A site is affected by a high concentration of Cd ions. However, Cd-based compounds generally have larger quadrupole splitting and isomer shift compared to Zn-based compounds. Gibbs [2] reported greater quadrupole splitting and isomer shift in CdFe204 than in ZnFe204 ferrites.

This compares well with the shorter bond length of Zn-based compared to Cd-based samples (see Figure 5.4) predicted by the general cation distribution [1, 8]. This is expected because the magnetic coupling depends on the magnetic neighbors present in the Fe environment. Similarly, the reduction of hyperfine fields at site A is due to the replacement of Fe3+ ions by diamagnetic Cd2+ ions.

A systematic reduction in the proportion of Fe atoms on A site to zero does not occur for Zn-based samples.

Figure 5.8: Mossbauer spectra for ZnxCol-xFe2-xAlx04.

Conclusions

This shows that the frustration effects and spin disorder occur faster when dilution occurs simultaneously on both A and B sites. Both isomer shifts and quadrupole splitting were found to be generally higher in Cd-based than in Zn-based compounds. The decrease in hyperfine fields with x is attributed to the weakening of the magnetic coupling as the magnetic ions are diluted with diamagnetic ions.

The magnetic properties investigated in this work are similar to those found by Thanki et al [8] in Zn-based compounds. There was no indication of an effect of sample preparation in oxygen flow as in air. The magnetic properties of the investigated Cd- and Zn-based samples appear to be similar as expected.

However, the present results show that diamagnetic Cd2+ and Zn2+ ions are replacing Fe ions in both A and B sites contrary to the commonly assumed distribution of cations given by equation (1.4.1). This systematic study of the compounds provides useful information on the evolution of the magnetic and structural properties of spinel oxides. It is believed that the results presented in this dissertation make a useful contribution to the study of mixed ferrites, worthy of publication [36, 37].

However, the study of the temperature dependence of the materials investigated in this work is not presented.

Bibliography