STUDY OF ARTIFICIAL FRUSTRATED ARRAYS OF SINGLE-DOMAIN FERROMAGNETS

Open Access
Author:
Li, Jie
Graduate Program:
Physics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
April 20, 2011
Committee Members:
  • Peter E Schiffer, Dissertation Advisor
  • Peter E Schiffer, Committee Chair
  • Vincent Henry Crespi, Committee Member
  • Nitin Samarth, Committee Member
  • James Hansell Adair, Committee Member
  • Jayanth R Banavar, Committee Member
Keywords:
  • frustration
Abstract:
Geometrical frustration is an intriguing area where systems show exotic spin states that are not exhibited in any other systems, including spin fluctuation down to lowest temperature in spin liquid materials, finite entropy at absolute zero and magnetic monopoles as emergent particles in spin ice materials, as well as the connection to neuron networks (chapter 1). Despite the fertile physics in geometrical frustration, many microscopic phenomena remain unclear, for example how the spin frustration is accommodated and how the defects affect the local frustration? In our research, we adapt a different approach – we carefully design systems of artificial frustration where the constitutive elements are large enough to be directly imaged by advance techniques such as magnetic force microscopy or Lorentz-TEM, but still small enough to maintain single-domain magnetic moments (chapter 2 and 3). Such elements are precisely controlled to form different arrays that have frustrated neighboring interactions, i.e. not all the magnetostatic interaction between elements can be simultaneously satisfied. However, the strength of this interaction places it beyond the limit of thermal annealing – instead we design demagnetization protocols using external magnetic field, and first study the consequence of protocols with different patterns and step sizes in chapter 4. Having optimized the demagnetization protocol, we investigate the effects of the geometry on energetics and moment correlations by fabricating a series of lattice arrays sharing the same symmetry or topology in chapter 5. Finally the finite size effect is also studied by comparing isolated clusters with frustrated magnetostatic interaction to those without frustration in chapter 6 – the distinction between the two is attributed to kinetic barriers in the demagnetization process.