Phase diagram of dipolar-coupled XY moments on disordered square lattices

Abstract

The effects of dilution disorder and random-displacement disorder are analyzed for dipolar-coupled magnetic moments confined in a plane, which were originally placed on the square lattice. In order to distinguish the different phases, new order parameters are derived and parallel tempering Monte Carlo simulations are performed for a truncated dipolar Hamiltonian to obtain the phase diagrams for both types of disorder. We find that both dilution disorder and random-displacement disorder give similar phase diagrams, namely disorder at small enough temperatures favors a socalled microvortex phase. This can be understood in terms of the flux closure present in dipolarcoupled systems.arXiv:1804.07498v2 [cond-mat.stat-mech]

Spatial and Temporal Correlations of XY Macro Spins

“…13,23 Structural disorder in imperfect square lattice may similarly lead to spin frustration and the nucleation of vortices. 26 The original motivation to study spin frustrated XY dipolar systems traces back to FeCl 3graphite intercalated compounds with strong dipolar and weak exchange interaction between layered Fe 3+ ions arranged in a honeycomb lattice. 13 Despite successful correlation between experimental and numerical integral properties, a direct visualization of the predicted magnetization configurations in XY dipolar systems, providing an unambiguous proof, is still lacking three decades later.…”

Advances in artificial spin ice

“…For more exotic lattices, it was found that the low-energy states adopted by an extended system could not be predicted by only considering the relative energies of the building blocks consisting of a few nanomagnets 18,105 . Going beyond Ising-like systems, dipolar XY systems with arrays of coupled discs exhibit complex ordering phenomena 19,20,22 , and the phase diagrams of such XY systems can be modified by structural disorder to give new phases that favour local flux closure 20 . Both the lattice and the moment degrees of freedom define the ordering behaviour.…”

“…In addition to using elongated magnets for Ising-like systems, it is possible use circular nanomagnets to create so-called dipolar-coupled XY systems with stripeordered low-energy states 19,22 . Introducing positional disorder in such artificial spin systems modifies the phase diagram to give a microvortex phase 20 . Introducing random interactions and/or removing crystalline order can be a route to obtain spin-glass behaviour 140 , which has been investigated in dipolar XY systems with positional disorder 20 and systems of randomly placed elongated magnets 141 .…”

“…Introducing positional disorder in such artificial spin systems modifies the phase diagram to give a microvortex phase 20 . Introducing random interactions and/or removing crystalline order can be a route to obtain spin-glass behaviour 140 , which has been investigated in dipolar XY systems with positional disorder 20 and systems of randomly placed elongated magnets 141 . In addition to using shape anisotropy to constrain the magnetic moments, it is possible to use single-crystal materials to define the in-plane magnetization direction, restricting the nanomagnet moments to point along the crystallographic axes.…”

“…This means that any arbitrary design can be realized, including more exotic arrays, not only based on extensions of the square and kagome geometries 7,[14][15][16] but also going beyond periodic systems to artificial quasicrystals 17,18 . In addition to systems with separated elongated nanomagnets, there are also noteworthy systems with circular [19][20][21][22][23] or square magnets 24 , or connected systems in which domain walls can travel through the network. The first efforts to create and characterize 3D artificial spin systems are also underway.…”