Thermal string excitations in artificial spin-ice square dipolar arrays

Abstract

We report on a theoretical investigation of artificial spin-ice dipolar arrays using a nanoisland shape adopted from recent experiments (Farhan et al 2013 Nature Phys. 9 375). The number of thermal magnetic string excitations in the square lattice is drastically increased by a vertical displacement of rows and columns. We find large increments especially for low temperatures and for string excitations with quasi-monopoles of charges ± 4. By kinetic Monte Carlo simulations we address the thermal stability of such excitations, thereby providing time scales for their experimental observation.

Pinning of thermal excitations at defects in artificial dipolar arrays: A theoretical investigation

“…In this study we focus on rounded rectangles (details in Appendix A). As pointed out elsewhere [46] the dipolar interaction [47,48] is relevant only for first-(1NN) and for second-nearest (2NN) neighbors [49], with energies E 1NN and E 2NN , respectively. For a discussion of the actual E 1NN and E 2NN we refer to Ref.…”

“…interaction energies and formation probabilities of magnetic charges, scale with the lattice constant, as demonstrated in Ref. [46]. The magnetic single-domain state of each elongated nanoisland is described by a magnetization vector M ± ('spin') aligned along the large island axis.…”

“…By displacing vertically the rows and columns of the square lattice by z δ , the degeneracies of the above configurations can be tuned to 6 for a critical z c δ [46], that is, to the same degree of degeneracy as in water ice [5], in pyrochlore lattices [50,51], and in kagome lattices [52]. z c δ has been obtained requiring…”

Broken vertex symmetry and finite zero-point entropy in the artificial square ice ground state

“…One type of artificial spin ice is the square ice, consisting of magnetic nanoislands patterned on a square lattice [3][4][5][6][7] designed to mimic the frustration found in rare earth titanate pyrochlore systems. While the pyrochlore-based spin ices have been shown to have a finite zero-point entropy 8,9 , this is not expected to be the case in artificial spin ices, due to their 'flat' geometry 10,11 . Here, we show that artificial square spin ice can equally exhibit a finite zero-point entropy in the ground state, and that this entropy can moreover be tailored through film thickness.…”

“…(c) Peak amplitude evolution of the two modes following pulses along the (11) (red curves) and (1-1) directions (blue curves) as a function of increasing number of introduced e-II vertices. (d) Ground state spectrum (grey) for a lattice with e-0 type symmetric vertices following a pulse along (11). The spectrum is identical if the pulse is along (1-1) (blue).…”