Tasa Interna de Retorno y Valor Actual Neto

The pSR experiments performed in this project were zero-field relaxation and longitudinal (with respect to the muon polarisation) field relaxation experiments. In zero-field experiments the muons will precess about any local

fields and will thus be very sensitive to spin-fluctuations, like superparamagnetic or spin glass relaxation. In the initial plan, the primary role of pSR in this study of magnetic relaxation effects is that o f a measurement technique of intermediate timescale, bridging the gap between magnetisation measurements and Mossbauer Spectroscopy. However, for our samples, transition points seem to be less easily visible in pSR than in many other techniques. The particularities of pSR in this system and the analysis o f pSR data will be covered in more detail in chapter 6.

v.A C S -T „ = l f f ‘ - l ( r ‘ s

AC Susceptibility measurements have been made at the Universidad de Cantabria, in Santander, Spain. This technique uses a small oscillating field and measures the

differential susceptibility y and the imaginary part o f the susceptibility x" though

pick-up coils. Since the AC field has a tuneable frequency, a variety o f measurement times can be explored under otherwise identical conditions. This makes it very useful for examination of superparamagnetic effects, in which one is interested in examining the variation o f the blocking temperature against measurement frequency.

3. Structural measurement techniques

A feature o f this project is that systems made from mechanical alloying are not made in as controlled a fashion as in, for example, sputtering or epitaxy. The samples obtained thus needed to be characterised structurally, for which a variety o f experimental techniques were used. The most important one of these was undoubtedly X-ray diffraction, which allowed, through detailed Rietveld peak profile analysis, an estimate of the mean crystallite sizes to be obtained. Combined with results already obtained from the hysteresis loop data, this gave a good idea o f the nanoscale make-up o f the systems. Other techniques used included Energy Dispersive Analysis of X-rays (EDAX) and Differential Scanning Calorimetry (DSC), to ascertain the stoichiometries and degrees of alloying respectively.

i. X-ray Diffraction

X-ray diffraction has been used for purposes of examining the structural properties o f the samples in this project. With Rietveld refinement o f the diffraction patterns, and using the well-known Scherrer formula, the crystallite sizes can be calculated. The X-ray diffraction patterns have been obtained either on the lab-based Philips Diffractometers, or at the CLRC Daresbury synchrotron facility.

d sin 0

Figure 3.8. Bragg reflection. The Bragg law can be deduced from this diagram

X-ray diffraction relies on the well-known Bragg law, 2d sin 9 = nX

(figure 3.8). Bragg reflection can thus only occur for wavelength X < 2d. Given

that d for most crystals is o f the order o f an Angstrom, visible light cannot be

used. While all planes reflect about 10’^ - 10'^ of the incident beam intensity, only some angles yield a powerful Bragg peak from the added effect o f 10^ - 10^ planes. These peak angles depend on the crystal structure o f the material examined. In this case, diffraction experiments were performed on powdered samples, in order to decrease the effect o f preferred orientations.

a. SEM and EDAX

The Scanning Electron Microscope (SEM) consists o f a finely focused beam of electrons, which is scanned over a rectangular raster over a surface. The electrons interact with the atoms, and can be reflected as back-scattered electrons or cause the ejection o f lower-energy electrons, called secondary electrons. A picture can

be formed from either group of electrons, showing the surface topography. The maximum resolution is about 30 Â for a SEM with a field emission source.

Through EDAX, which stands for Energy Dispersive Analysis o f X-rays, it was possible to verify the stoichiometric make-up o f the systems, as well as, through a compositional scan, investigate the homogeneity o f the systems on a micron scale. It was important to see that the systems in question were in fact nanoscale single-domain magnetic granular alloys and not simply multidomain systems that would, for instance, give superficially similar coercivity results in a hysteresis loop as a superparamagnetic system.

The SEM and EDAX experiments featured in this thesis were performed at the Institute o f Archaeology at University College London using either a Hitachi S-4000 or a Jeol EMA. The maximum resolution o f these machines was o f the order o f microns. Samples were studied after sealing the powder into an epoxy resin disc, which was later polished with increasingly fine standard abrasive paper.

Hi. Differential Scanning Calorimetry

Differential Scanning Calorimetry, usually abbreviated as DSC, consists of measuring the heat flow in or out o f a sample as the temperature is varied. The usual procedure is to heat a sample at a controlled rate in a nitrogen atmosphere, and registering the heat flow in and out of the sample. In the samples studied in this thesis, DSC gives a measure of the amount o f metastable alloying (which will break down upon heating). Also the defects induced by the ball milling process will be removed as the sample is heated, resulting in a release of stored energy.

Samples o f mass somewhere between 20 and 100 mg are placed in platinum pans, and heated under a constant flow o f nitrogen o f about 30 ml/min. The DSC thermograms are usually presented as a graph o f energy in mW/mg against temperature, though if the area under the mW/mg versus time graph is found, it represents the amount o f energy stored (see chapter 4).

The DSC experiments featured in this thesis were performed at the Thermal Methods laboratory at Birkbeck College, University of London, under the ULIRS scheme.

4. Conclusions

The chapter has described the techniques and the procedures used to obtain the data presented in this thesis. This leads directly to the following chapters, in which the data are presented, divided into structural, magnetic and muon data.