Observation and characterisation of magnetisation ripple using Lorentz transmission electron microscopy

  • Alison Cowan

Student thesis: Doctoral ThesisDoctor of Philosophy


Lorentz transmission electron microscopy (LTEM) has proved itself to be an invaluable technique when investigating in-situ micromagnetic behaviour of magnetic thin films. The nano-scale visualisation of the magnetic structure allows for not only the mapping of hysteresis behaviour, but quantitative characterisation of the materials micromagnetic properties. In this thesis, much of the previous work of experimental magnetisation ripple characterisation is reviewed. A newly developed methodology for characterisation of large sets of Fresnel images displaying magnetisation ripple properties is presented which is utilised throughout the rest of the body of work.

Ni45Fe55 has long been a high moment alternative for the more commonly used permalloy composition of Ni80Fe20 in hard disk (HDD) read-write head design. In this work a conventional TEM, and LTEM study was undertaken to investigate the effect of ultra-thin Ni79Fe21 seed layers on the physical and magnetic properties of Ni45Fe55. The dramatic effect of seed layer addition resulted in grain size and texture reduction, an increase in uniaxial anisotropy and a reduction in magnetisation ripple properties. This suggests that the film is a good candidate for controlling properties of Ni45Fe55 thin films, whilst maintaining a high magnetic moment density.

Micromagnetic modelling of magnetisation ripple using MuMax3 software has been presented using two different methods. Both models produced visually representative simulated Fresnel images, with work specifically focusing on magnetisation reversal processes and the quantitative analysis of magnetisation ripple properties. Models produced insights into the effect of parameters such as grain size, inter-granular exchange and magnetocrystalline anisotropy directionality.

Lastly, we can quantitatively assess the physical and magnetic variations between permalloy magnetostrictive samples with varying signs, without the need for external straining, to represent normal operating conditions in HDD devices. Analysis revealed near identical physical properties, with a subtle variation in bulk and magnetic ripple properties. However, it is not possible to determine if these variations are exclusively due to differences in magnetostrictive properties or if it is more likely due to compositional variations.
Date of AwardJul 2021
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SponsorsEngineering & Physical Sciences Research Council
SupervisorStephen McVitie (Supervisor) & Robert Bowman (Supervisor)


  • Lorentz microscopy
  • magnetism
  • magnetisation ripple
  • thin film magnetism
  • micromagnetic modelling
  • electron microscopy

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