Ordering and defects in artificial magnetic square ice: thermodynamic and field-driven processes

This thesis addresses the behaviour of artificial magnetic square spin ice patterns in response to applied magnetic fields and thermal activation. Two main points of focus are the access of well-defined statistical states and the properties of fractionalised “monopole” charge defects.
Experimental investigations are conducted using magnetic force microscopy of athermal remanent states.

Magnetic reversal of a square ice subject to magnetic fields applied slightly off a diagonal symmetry axis is found to mediate via incremental sublattice-independent processes. Dipolar correlations manifest as charge defect propagation and ordering, along and between adjacent
flipped moment chains respectively. Quenched disorder, while allowing for bulk defect nucleation, is responsible for strongly suppressing correlations.

A constant magnitude rotating field protocol is investigated. The acquisition of strong ground state ordering via highly correlated edgenucleated reversal processes, as recently predicted, is not found, attributable
to the enhancement of bulk processes by quenched disorder.
An optimally tuned field allows for efficient demagnetisation towards the ground state.

A study of as-fabricated states is presented, magnetically self-ordered following formation of magnetic elements via evaporative vacuum deposition. Compelling evidence is presented, in a case study of a sample closely approaching the ground state, for superparamagnetism during early deposition stages which is subsequently arrested. Large ground state domains are observed sustaining Boltzmann factor weighted monopole excitations. Evidence is present for monopole interactions playing a role in thermal ordering, as excitations explore a dipolar energy band structure. Further, order is found to increase with interaction strength, and preliminary results suggest that quenched disorder acts to suppress it. Statistical mechanical calculations show that states achieved correspond closely to arrested thermal equilibrium.
Consideration of an effective thermodynamic model presented for ac field demagnetised states is given. Comparison allows for discussion of the general role of quenched disorder and the effective temperature of a “frozen” state.