List of Figures

  1. Recoil in a free nucleus during gamma ray emission.
  2. Gamma ray energy distributions for emission and absorption in free atoms. The overlap is shown shaded and not to scale as it is extremely small.
  3. Example Mössbauer spectrum showing the simplest case of emitter and absorber nuclei in the same environment. The uncertainty in the energy of the excited state, $ \Gamma $, is shown exaggerated.
  4. The effect on the nuclear energy levels for a $ \nicefrac {3}{2}\to {}\nicefrac {1}{2}$ transition, such as in $ ^{57}$Fe or $ ^{119}$Sn, for an asymmetric charge distribution. The magnitude of quadrupole splitting, $ \Delta $ is shown.
  5. The effect of magnetic splitting on nuclear energy levels in the absence of quadrupole splitting. The magnitude of splitting is proportional to the total magnetic field at the nucleus.
  6. The effect of a first-order quadrupole perturbation on a magnetic hyperfine spectrum for a $ \nicefrac {3}{2} \rightarrow \nicefrac {1}{2}$ transition. Lines 2,3,4,5 are shifted relative to lines 1,6.
  7. Decay scheme of $ ^{57}$Fe following excitation of the $ 14.41\ensuremath{\unskip\,\mathrm{keV}}$ state.
  8. Probability of a 7.3keV K-conversion electron reaching the absorber surface in metallic iron.
  9. Effect of moment alignment on magnetisation: (a) Single magnetic moment, $ m$, (b) two identical moments aligned parallel and (c) antiparallel to each other.
  10. Typical effect on the magnetisation, $ M$, of an applied magnetic field, $ H$, on (a) a paramagnetic system and (b) a diamagnetic system.
  11. Schematic of a magnetisation hysteresis loop in a ferromagnetic material showing the saturation magnetisation, $ M_{s}$, coercive field, $ H_{c}$, and remanent magnetisation, $ M_{r}$. Virgin curves are shown dashed for nucleation (1) and pinning (2) type magnets.
  12. The process of magnetisation in a demagnetised ferromagnet.
  13. Shape of hysteresis loop as a function of $ \theta _{H}$, the angle between anisotropy axis and applied field $ H$, for: (a) $ \theta _{H} = 0^{\circ }$, (b) $ 45^{\circ }$ and (c) $ 90^{\circ }$.
  14. Rotation of sublattice magnetisation under an applied field, $ H$, perpendicular to the spin axis.
  15. Variation of reciprocal susceptibility with temperature for: (a) antiferromagnetic, (b) paramagnetic and (c) diamagnetic ordering.
  16. Variation of saturation magnetisation below, and reciprocal susceptibility above $ T_{c}$ for: (a) ferromagnetic and (b) ferrimagnetic ordering.
  17. Superconductor enclosing a non-superconducting region.
  18. Superconducting quantum interference device (SQUID) as a simple magnetometer.
  19. Critical measuring current, $ I_{c}$, as a function of applied magnetic field.
  20. Decay scheme for a $ ^{57}$Co source leading to gamma-ray emission. Internal conversion accounts for the remaining $ 91\%$ of $ 14.41\ensuremath{\unskip\,\mathrm{keV}}$ events.
  21. Mössbauer spectrometer schematic.
  22. CEMS detector used at Liverpool University.
  23. Illustration of an RSO measurement with a small amplitude. (a) shows the ideal SQUID response for a dipole and (b) shows the movement of the sample within the SQUID pickup coils.
  24. Variation of the indirect exchange coupling constant, $ j$, of a free electron gas in the neighbourhood of a point magnetic moment at the origin $ r=0$.
  25. The atomic positions in the C15 MgCu$ _{2}$ Cubic Laves Phase unit cell.
  26. Schematic of the Laves Phase sample construction.
  27. Comparison of spectra of DyFe$ _{2}$, YFe$ _{2}$ and HoFe$ _{2}$ thin films at room temperature under zero applied field.
  28. 750Å DyFe$ _{2}$ thin film under $ 0\ensuremath{\unskip\,\mathrm{kOe}}$ and $ 2.5\ensuremath{\unskip\,\mathrm{kOe}}$ in plane applied magnetic fields.
  29. Spectra for 1000Å YFe$ _{2}$ thin film under $ 0\ensuremath{\unskip\,\mathrm{kOe}}$ and $ 2.5\ensuremath{\unskip\,\mathrm{kOe}}$ in plane applied fields.
  30. Spectra for $ \left[ \mathrm{DyFe}_{2}(x\mathrm{\AA})/\mathrm{Dy}(y\mathrm{\AA})\right]_{z}$, multilayers.
  31. Exchange coupling between iron sublattices in DyFe$ _2$/YFe$ _2$ multilayers.
  32. Spectra for $ \left[ \mathrm{DyFe}_{2}(x\,\mathrm{\AA})/\mathrm{YFe}_{2}(y\,\mathrm{\AA})\right]_{z}$, multilayers.
  33. Normalised magnetisation vs temperature scans for Ce(20Å)/Fe($ x$Å) multilayers. Cerium layer thickness is constant.
  34. Normalised magnetisation vs temperature curves for Ce(27Å)/Fe($ x$Å) multilayers. Cerium layer thickness is constant.
  35. Normalised magnetisation vs temperature scans for Ce($ x$Å)/Fe(10Å) multilayers.
  36. Normalised magnetisation vs temperature scans for Ce($ x$Å)/Fe(15Å) multilayers.
  37. Normalised magnetisation vs temperature scans for Ce($ x$Å)/Fe(20Å) multilayers.
  38. Percentage change in resistance under an applied field for the $ \left[\mathrm{Ce}(20\,\mathrm{\AA{}})/\mathrm{Fe}(17\,\mathrm{\AA{}})\right]_{60}$ sample.
  39. Exploded view of a Ce/Fe multilayer showing the antiferromagnetic coupling between the iron layers.
  40. Variation of antiferromagnetic coupling constant, $ J(z)$, with cerium layer thickness, $ z$. The dashed line, representing a theoretical picture of oscillatory coupling, is added to guide the eye.
  41. Hysteresis loops taken at room temperature for all measured Ce/Fe samples. The magnetisation is normalised relative to the iron layers volume only. The hysteresis loop for the 20/10 sample is shown truncated in the main plot for clarity, but is compared to the other loops in the inset.
  42. CEMS spectra for U/Fe multilayers: $ \left[ \mathrm{U}(40)/\mathrm{Fe}(60) \right]_{10}$, $ \left[ \mathrm{U}(80)/\mathrm{Fe}(60) \right]_{20}$, $ \left[ \mathrm{U}(101)/\mathrm{Fe}(60) \right]_{20}$ and $ \left[ \mathrm{U}(42)/\mathrm{Fe}(113) \right]_{21}$. The spectra are unaffected by the uranium layer thicknesses.
  43. CEMS spectra for U/Fe multilayers: $ \left[ \mathrm{U}(28)/\mathrm{Fe}(30) \right]_{31}$, $ \left[ \mathrm{U}(28)/\mathrm{Fe}(43) \right]_{31}$ and $ \left[ \mathrm{U}(22)/\mathrm{Fe}(180) \right]_{5}$. Increasing crystallinity is observed within the iron layers as the thickness increases.
  44. DCEMS spectra recorded from the $ \left[ \mathrm{U}(28)/\mathrm{Fe}(43) \right]_{31}$ sample. Spectrum (a) is obtained from the top $ \sim 5$ bilayers in the sample, whilst spectrum (b) was recorded from the remaining layers underneath.
  45. X-ray reflectivity scans for a) $ \left[ \mathrm{U}(42)/\mathrm{Fe}(113) \right]_{21}$ and b) $ \left[ \mathrm{U}(22)/\mathrm{Fe}(180) \right]_{5}$ samples. Sharp peaks indicate well defined interfaces.
  46. Room temperature Mössbauer spectra for all toner samples.
  47. 77K Mössbauer spectra for all toner samples.
  48. Fe$ _3$O$ _4$ thin films on (a) platinum and (b) sapphire substrates. Sample (b) was grown using an oxygen plasma source.
  49. Fe$ _3$O$ _4$ thin film on a sapphire substrate, grown with a standard sputtering source. (a) shows the fitted spectrum and (b) shows the hyperfine field distribution.

Dr John Bland, 15/03/2003