La feminización de la pobreza: la mujer en el sistema económico globalizado

6.2.1 Introduction

The expected outcome o f milling this ternary material can be predicted by first considering the properties o f each o f the three binary systems. As described in the last chapter, iron and silver are almost entirely immiscible, and remain so even after prolonged ball milling. Given the results o f previous research in this area [13, 95, 96], the Co-Ag system is expected to behave similarly. Iron and cobalt, on the other hand, are closely matched in atomic size and so alloy under equilibrium conditions, as described in chapter 4. They should therefore be easily miscible in the conditions provided by a high energy null, although the final structure o f the material, particularly in the two phase region, needs investigation. The facts for the three binary combinations imply that, for the ternary system, simultaneous milling o f the three elemental powders would result in a material consisting o f particles o f silver mixed with separate particles of an Fe-Co alloy.

Mossbauer spectroscopy is a particularly useful tool for analysing the resulting Fe-Ag and Fe-Co-Ag materials. Since the signal is a superposition of absorption profiles fi'om individual iron atoms, it allows each distinct structural environment within a sample to be separately identified, and thus enables the degree of alloying to be quantified. This analysis is not possible for pure Co-Ag sanç)les, which are known to be the most promising o f the granular GMR materials. However, given the high miscibihty between iron and cobalt it is possible to dope small quantities o f iron into the cobalt granules during milling. One particular aim o f the current investigation is thus to analyse the structural, magnetic and electrical properties o f these ‘high cobalt’ (FexCoi-x)i-yAgy

systems, and fiirther to show how they change with increasing iron content.

Previous work has been done on the properties of granular (Fe%Coi_x)i.yAgy thin films which were deposited using Fe-Co and Ag targets [97, 98], but not on the equivalent mechanically alloyed system. An analysis o f the ball-milled samples in terms o f structure, particle size, magnetic state and magnetoresistance is therefore valuable. Work has though been previously carried out on the mechanically alloyed binary Fe-Co system [99-101]. This current investigation, however, enables comparisons to be made between the properties o f the pure ball-milled Fe-Co with that o f the Fe-Co clusters in

IRON-COBALT-SILVER ₁₂₇ silver. Differences in the respective Mossbauer spectra allow the degree o f interaction between the Fe-Co and Ag to be investigated.

The investigaton has been conducted in a systematic manner, with full details o f the preparation and characterisation o f the three binary systems being given, before moving on to look at the ternary structure. Particular attention is paid to Fe-Co, given that there is a structural phase change as the composition is varied. Additionally, the earher work on Fe-Ag was repeated, given that there has been a switch in milling containers from steel to Syalon, and consequently the alloying properties, particle sizes and impurity levels may have changed.

6.2.2 Experim ental details

Thirty-eight different samples were prepared, with compositions chosen to systematically cover the majority o f the ternary phase diagram, as shown in Figure 6.1. Attention was focused on two particular regions, namely the binary Fe-Co system where structural phase transitions occur with varying composition, and the region of high silver concentration within which the most significant giant magnetoresistive effects are expected.

1 2 8 Ch a p t e r 6 The ternary alloys can be written in the form (FexCoi-x)i-yAgy. Several sample series were prepared, with either x or y fixed, as marked in the figure. A fixed value o f x allowed the effects o f varying the silver concentration to be independently investigated, whilst a fixed value for y enabled the effects of changing the Fe:Co ratio on both the alloy structure and the magnetoresistance to be studied.

The milling parameters were similar to those used in the previous chapter, except that all experiments were carried out in an argon atmosphere, and the milling containers were constructed from Syalon (silicon nitride - SigN#) rather than stainless steel. Syalon containers were chosen in preference to stainless steel, as this eliminated the possibility of iron-based impurities from the steel affecting the final sample compositions, or showing up independently as contaminants in the Mossbauer spectra. A further difference in the two materials is the density (3.2 x 10^ kg/m^ for Syalon compared with 7.9 X 10^ kg/m^ for steel). Apart from the resulting reduction in ball to powder mass ratio to 5:1 from 12:1, this change in density would be expected to influence the impact energy during milling and thus the structure of the final product. To investigate this possibility, equivalent Fe-Co samples were milled both in Syalon and steel containers, with all other factors remaining constant at the values quoted in table 4.1.

During milling o f the Fe-Co, an aliquot o f sample was removed for analysis at regular intervals, which enabled the progress o f alloying with time to be studied for different sample compositions. This type o f investigation is particularly valuable as a test of the miscibility o f the separate elements. Iron and cobalt, for example, would be expected to alloy in a much shorter time than did the iron and copper. However, less obviously, for the Fe-Co system it is also possible that sample compositions close to the fcc/bcc structural transition region may take longer to alloy than those more distant from it.

Structural and magnetic characterisation of the samples was carried out using XRD, Mossbauer spectroscopy and DSC as described earlier. Additionally, magnetic measurements were made using a VSM in fields up to 0.75 T and temperatures down to 77 K. Magnetoresistance measurements were mainly carried out in Spain by Dr. Luis Fernandez, using a four point probe resistometer in fields up to 9 T and in the temperature range 10 - 300 K. Impurity levels in the samples were measured using EDAX techniques.

IRO N-CO BA LT-SILVER ₁₂₉ 6.2.3 M illing of pure Fe, Co and Ag

As mentioned in chapter 4, cobalt can be found in both hep and fee forms at room temperature, and XRD scans show that these two allotropes are present, in the ratio of approximately 2:1, in the as purchased powder (Aldrich Chemicals). However, prolonged milling o f the elemental powder favours the formation of the fee structure at the expense o f the hep material. Under equihbrium conditions the fee phase is the more stable o f the two only at temperatures over 450 °C, and thus its formation here probably results from the raised temperature at the impact sites. The XRD scans in figure 6.2 additionally show that the although the hep structure is largely transformed, it is not eliminated entirely even after 70 hours milling. This fact makes it difficult to obtain accurate FWHM values and thus calculate crystaUite size and strain parameters, since the lower intensity fee and hep peaks merge. Fortunately it is possible to fit the main f c c ( lll) peak at 20.0°, and using the Scherrer formula estimate a crystallite size of 60 Â. This result will be shghtly too small, as the broadening was assumed to be due solely to particle size effects, rather than a combination of size and strain. Nevertheless it is o f the same order as that of milled iron, which is to be expected given the similarity in melting points o f the two elements.

Cobalt