Emergent tri-criticality in magnetic metamaterials

TL;DR

This paper investigates magnetic metamaterials composed of nanostructured discs, revealing a complex phase transition that shifts from Kosterlitz-Thouless to first order via an emergent tri-critical point, with implications for experimental studies.

Contribution

It introduces a mesospin model with O(2) symmetry to describe internal magnetic textures and uncovers a novel tri-critical point in the phase transition of magnetic metamaterials.

Findings

Identification of a tri-critical point in the phase transition.

Evolution of critical exponents from 2D-XY to below half at the tri-critical point.

Numerical evidence of a transition from Kosterlitz-Thouless to first order.

Abstract

Metallic discs engineered on the 100 nm scale have an internal magnetic texture which varies from a fully magnetized state to a vortex state with zero moment. The interplay between this internal structure and the inter-disc interactions is studied in magnetic metamaterials made of square arrays of the magnetic discs. The texture is modeled by a mesospin of varying length with O(2) symmetry and the inter-disc interaction by a nearest neighbour coupling between mesospins. The thermodynamic properties of the model are studied numerically and an ordering transition is found which varies from Kosterlitz-Thouless to first order via an apparent tri-critical point. The effective critical exponent characterising the finite size magnetization evolves from the value for the 2D-XY model to less than half this value at the tri-critical point. The consequences for future experiments both in and out of equilibrium are discussed.