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Mossbauer spectroscopy is a useful technique for the investigation o f the detailed micro structure o f as-deposited PLD samples. Obviously, however, it cannot easily be apphed in the case o f the above Co-Ag GMR materials, but given the similarity in alloying properties of iron and cobalt, a study of Fe-Ag and Fe-Co-Ag films would prove valuable. Given the thin film nature o f the sangles (with thicknesses below 2000 Â), the data were collected using a backscatter conversion electron detector, as described in chapter 3, and so the spectra appear in emission rather than absorption format.

The results are shown in figure 6.24. The first sample deposited was FesoCoso at a laser fluence of 3 J/cm^. There is no evidence o f an a-iron sextet, and the hyperfine field o f the main magnetic feature, at 35.0 T, corresponds closely to that o f the FesoCoso ball milled alloy ( Bhf = 34.7 T). This indicates that the iron and cobalt have alloyed during the deposition process, as would be expected for miscible elements. However, a significant fi*action (approximately 60 %) of iron atoms appear to be in a highly disordered or amorphous state, which could not be fitted with any unique hyperfine parameters. Additionally, there is a large asymmetry in lines three and four of the magnetic sextet, which has been fitted with an extra singlet, o f isomer shift + 0.86 mm/s. Given the large isomer shift compared with that of, for example, superparamagnetic a-iron (Ô = 0.1 mm/s), it is not certain whether this is a real feature, although it is possible that it is due to fine particles o f an Fe-Co alloy.

Looking now at the results for FesoAgyo deposited at 1.5 J/cm^ and 3 J/cw? it is clear that these Mossbauer spectra are rather different fi*om those seen for the

IRON-COBALT-SILVER ₁₆₃ 1.0 0.5 0.0 0.4 0.2 0.0

I

Velocity (mm/s)

Figure 6.24 Conversion electron Mossbauer spectra for selected iron-based thin jBlms grown by pulsed laser deposition at a range of laser fluences. The solid lines are least-squares fits to the data, as

discussed in the text.

equivalent ball milled material, which were composed mainly of a standard a-Fe magnetic sextet, indicating the presence of unalloyed iron granules. The major component in both the PLD samples is paramagnetic and has been fitted as a doublet with isomer shift 0.4 mm/s and quadrupole spfitting o f 0.9 mm/s. This result is similar to those seen for fee Fe-Ag metastable sohd solutions produced by sputtering and ion implantation [80, 113, 114], and also resembles that of fee Fe-Cu as described in chapter 5. Therefore it can be deduced that the PLD process produces a single phase metastable sohd solution under these deposition conditions. The broad emission lines in

1 6 4 Ch a p t e r 6

comparison with those of the sputtered sample also shown indicate a high level o f disorder in the lattice, as there was with iron-cobalt.

There is a small degree o f asymmetry in the two lines, but this is seen in the similar sputtered samples and may be the result o f preferred grain orientation. Alternatively, it could be explained in the following way. The sohd solution contains iron and silver atoms randomly distributed on an fee lattice. Most iron atoms will have a mix o f both iron and silver nearest neighbours, which has the effect o f destroying the local cubic symmetry at a given iron site, thus generating an electric field gradient and a quadrupole doublet in the Mossbauer spectrum. There will, however, be a small number o f iron atoms which are in a symmetric fee environment, surrounded entirely by either silver or iron. The Mossbauer signal fi*om these would be a singlet, with isomer shift of approximately -0.1 mm/s corresponding to fcc-Fe. These two components added together produce the observed asymmetric doublet.

The original reason for studying these materials using Mossbauer spectroscopy was to look for evidence of phase segregation m the as-deposited films, analogous to that found by XRD for Co-Ag. As mentioned in previous sections, iron and silver peaks cannot be separated by XRD, although the scans do at least provide information on particle sizes fi*om the line broadening. Given that it is shown that these particles are as large or larger than those found in the mechanically alloyed samples, the Mossbauer signal fi*om any phase segregated iron granules would be ferromagnetic rather than superparamagnetic in form, and thus a sextet should be visible in the spectrum.

No evidence is seen o f a magnetic sextet in the film deposited at 1.5 J/cm^ and therefore there appears to be no phase segregation in this sample. In the sample deposited at 3 J/cm^, however, a broad poorly resolved feature is seen in addition to the paramagnetic doublet. Given the very low signal strength it is not possible to fit this feature with any accuracy, but it is possible to approximate it with a sextet o f similar hyperfine parameters to a-Fe, and very broad lines. This degree of disorder is not surprising given the earlier results for FesoCoso, but makes the signal more difficult to detect. The fit itself is not good, but is an improvement over ignoring the magnetic component entirely. A similar but more convincing broad feature is seen in the spectrum of FeisCoisAgyo, implying a degree o f disorder but possible partial phase segregation in this sample also.

IRON-COBALT-SILVER 165

Given restrictions in time and sample availability, it was not possible to carry out annealing experiments on the above films, but at a later date these experiments would be useful in order to clarify the phase segregation and grain growth processes. It would be particularly valuable to carry out heat treatment on the FesoAgyo 3 J/cm^ sample, since the magnetic sextet should grow in intensity at the expense o f the doublet as further phase segregation occurs. The lines should also sharpen, as annealing removes defects and structural disorder in the lattice and encourages the growth o f more regular crystallites. In addition, magnetoresistance measurements would provide a further source o f information on the microstructure o f these samples. Nevertheless, the existing Mossbauer spectra o f these as-deposited iron based samples have provided important structural and magnetic data to complement that obtained for the Co-Ag system.

6.4 Conclusions

In this chapter the structural and magnetoresistive properties of the iron-cobalt- silver system were investigated, with samples produced by mechanical alloying and pulsed laser deposition.

Using mechanical alloying a full study o f the ternary phase diagram was undertaken, fi'om which it was shown that it is possible, starting from separate elemental powders, to produce a granular magnetoresistive system comprising iron- cobalt alloy particles embedded in a silver matrix. When milling pure iron in Syalon it was found that the final particle size was sfightly larger, and the strain shghtly smaller, than when the same powder was milled in steel, implying a reduction in the relative impact energy. Milling o f cobalt favoured the formation o f a material with a dominant fee lattice as opposed to the mixed hcp/fcc structure of the original sample.

In the binary iron-cobalt system milling was carried out using both steel and Syalon containers, with the lattice structure and Mossbauer hyperfine parameters measured as a function of composition. The properties o f the samples milled in Syalon corresponded closely to those o f equüibrium formed alloys, with the structure being bcc when the iron concentration is greater than 25 at.%, fee when it is less than 20 at.% and mixed when it is between these values. In the steel milled material, however, the dual phase region

166 C h a p t e r 6 disappeared to be replaced with an extended bcc zone. This, together with the fact that milling times were reduced compared with the Syalon based samples, confirms that the impact energy available in steel containers is higher.

In the ternary material. X-ray diffraction and Mossbauer spectroscopy results confirmed the existence o f separate magnetic iron-cobalt and non-magnetic silver grains. In particular, a comparison o f the Mossbauer hyperfine field values in the (FexCoi-x)i-yAgy system with those in the equivalent Fe^Coi-x sangles showed that similar Fe-Co particles were present in each case. Although the magnetic and non­ magnetic grains are largely non-interacting in the ternary system, it was seen fi'om the secondary components in the Mossbauer spectra that there is a limited degree of alloying between them, particularly in those samples with cobalt rich magnetic particles.

Given the miscibility o f iron and cobalt, no phase separation was seen on thermal treatment o f the binary alloys at temperatures o f up to 600 °C, although lattice recovery and recrystaUisation processes were visible. In the case o f the ternary system, however, the limited alloying of the magnetic grains with the silver was removed during heating.

The magnetoresistance o f the binary cobalt-silver and ternary iron-cobalt-silver systems was investigated as a function o f composition, with the largest MR, o f 8.2 % in 9 T at 10 K, being found in (Feo.i5Coo.85)3oAg7o. Magnetoresistance values would have

been higher were it not for the large residual resistivities o f the sample pellets, caused by the high defect concentration and hrperfect compaction o f the ball milled powder.

A much greater MR was obtained in similar granular samples produced by pulsed laser deposition rather than mechanical alloying. Magnetoresistance values o f up to 41 %, at 4.2 K and 4.7 T, were obtained in CosoAgyo thin films deposited with a laser fluence o f 3 J/cm^ and annealed at 150 °C.

The structure o f the as-deposited PLD thin films was found to be a mixture of phase segregated and alloyed particles. The presence o f separate cobalt and silver granules could be seen in the x-ray patterns o f those samples deposited at laser fluences of 3 J/cm^ and above, although Mossbauer spectra o f similar iron-silver samples indicated that much of the material existed in the form of a single phase fee alloy.

In conclusion, high energy ball milling has been shown to be an effective method of producing complex GMR materials in which there is a high degree o f phase segregation between the magnetic and non-magnetic clusters. The resulting structure o f

IRON-COBALT-SILVER 167

the material can be predicted by considering the relative miscibihties of the constituent elements. The structure of comparable as-deposited PLD thin films is less well defined, with the possible existence o f both segregated clusters and a sohd solution alloy. Using the correct deposition and annealing conditions, however, it is possible to produce a

Chapter 7 : Conclusions

This research has been concerned with investigating the properties o f a range of metallic alloys, particularly those which may exhibit giant magnetoresistive effects. The materials were synthesised mainly in bulk powder form by mechanical alloying, although a set of equivalent samples was deposited as thin films using PLD.

Mechanical alloying was found to be an effective technique for producing a range o f materials, including sohd solution alloys o f normally immiscible elements. The effect o f parameters such as milling time and energy were discussed and the evolution of structure with time investigated. Alloy formation was seen to be gradual, with the ultimate time required depending on the energy o f the milling process and the miscibihty o f the constituent elements. For example, as described in chapter 6, milling times were lower when steel containers rather than Syalon containers were used, due to the higher density and therefore higher impact energy of steel. Alloying was also more rapid in the miscible Fe-Co system than the immiscible Fe-Cu system

Careful consideration was given to the issue of sample contamination by atmospheric and milling tool impurities. Systematic studies showed that gaseous contamination could be mmhnised by conducting the experiments in an inert argon atmosphere, but that impurities arising from the wear of the milling containers was, to some extent, inevitable. The magnitude and nature o f this contamination, however, depended on the container material used. Comparative experiments using stainless steel and Syalon milling tools demonstrated that the absolute impurity level was higher for samples milled in Syalon, but that the impurities remained immiscible with the sample material. By contrast, when stainless steel was used, the iron and chromium worn from the tools tended to alloy with the sample, affecting to a degree its structure and properties, as seen through a comparison o f the relevant Mossbauer spectra.

The main investigations were centred on the iron-copper-silver and iron-cobalt- silver systems. The pure elements and binary systems were examined first in order to provide a basis for the study o f the more complex ternary combiuations. A series of samples were prepared in each case and the variation in structural, magnetic and electrical properties of the materials analysed as the composition was adjusted.

170 CHAPTER 7 In the Fe-Cu-Ag mechanically alloyed system discussed in chapter 5, samples were prepared to systematically cover the complete phase diagram and so allow trends in the various properties to be mapped. Alloying occurred across a wide composition region, despite the equihbrium immiscibiÜty o f the constituent elements. The binary Cu-Ag and ternary copper and silver rich alloys had single phase fee structures. The structure o f the binary Fe-Cu system, however, was dependent on the relative proportions of the two elements, as a result o f the different initial elemental structures. A single phase bcc lattice was formed where the iron concentration was above 80 at.% and an fee lattice was formed where it was below 60 at.%. Mossbauer spectroscopy, however, showed that those alloys containing above 30 at.% iron were ferromagnetic, irrespective of structure. In the binary Fe-Ag system, and in the iron rich area of the ternary system, the stored energy imparted during the milling process is insufficient for alloying to occur. The result was a microcrystalline mixture o f elemental particles, with a limited amount o f intermetallic alloying.

Towards the centre o f the ternary phase diagram, as the relative proportion of the three elements became more equal, the grain size o f the resulting material decreased and the structure became less crystalline. The ternary samples which contained approximately equal proportions o f the starting elements were found to be highly disordered or amorphous as measured by Mossbauer spectroscopy. However, x-ray diffraction, magnetoresistance and differential scanning calorimetry data cast some doubt on there being a truly amorphous state at the atomic level, implying that the samples retained some degree of crystalhnity.

All alloys formed in this system were metastable and were found to decompose and phase segregate on heating. More detailed study o f the thermal treatment showed there was a gradual decomposition o f the sample into its constituent elements at temperatures below 400 °C, followed by recrystallisation and grain growth at higher temperatures. The binary Cu-Ag alloys were seen to be less thermally stable than the equivalent Fe-Cu alloys, as a result of the greater mismatch in atomic sizes, undergoing phase separation at temperatures below 200 °C. The absence of equivalent low temperature exotherms in the DSC scans on Fe-Ag confirmed the granular structure and lack o f alloying in this system. Additionally, the different quantities of heat released from each o f the pure milled elements showed that iron has a greater propensity to sustain lattice distortion

C o n c l u s i o n s 171

than either copper or silver. The final crystallite size obtained from x-ray measurements is also significantly smaller.

The structural phase diagram of the Fe-Co-Ag system, studied in chapter 6, was found to differ markedly from that o f Fe-Cu-Ag, as a result o f the differing miscibihties of the elements. During the mechanical alloying process, the iron and cobalt, which are miscible under equihbrium conditions, aUoyed fuUy, but there was only limited ahoying with the süver. The structure of the Fe-Co grains, for samples mihed in Syalon, corresponded to that found in the equihbrium phase diagram, with a bcc lattice being formed when the iron concentration was greater than 25 at.%, an fee lattice formed when it was less than 20 at.% and a mixed structure created at intermediate values. In the steel containers, with a higher impact energy, the mixed phase region was replaced with an extended bcc zone.

The absolute degree of ahoying between the magnetic Fe-Co grains and the silver particles could be seen through a comparison o f the Mossbauer spectra o f the ternary (FexCoi-x)i-yAgy samples with their binary FexCoi.% counterparts. The main magnetic component in each case, with characteristic hyperfine field, was due to the Fe-Co ahoy, but additional components could be identified in the spectra o f the ternary material, and these varied in relative area with composition.

The resulting structure was nevertheless largely granular in form, and therefore useful for GMR apphcations, with the magnetic Fe-Co particles being embedded in the Ag matrix. Thus it has been demonstrated that mechanical ahoying could produce these granular GMR materials with a range o f compositions, starting with the binary Fe-Ag samples discussed earher, through the ternary Fe-Co-Ag, to the binary Co-Ag. Giant magnetoresistance may also be found in other systems such as Fe-Cu, but in this case the mihed material would require annealing in order to encourage phase segregation.

Magnetoresistance measurements were made for the Fe-Co-Ag samples as a function of composition, with the largest MR, o f 8.2 % in 9 T at 10 K, being found in (Feo.i5Coo.85)3oAg7o. Magnetoresistance values would have been higher were it not for