Phase Transition in Frustrated Magnetic Thin Film—Physics at Phase Boundaries

Abstract

In this review, I outline some principal theoretical knowledge on the properties of frustrated systems and thin films. The two points I would like to emphasize: i) the physics in low dimensions where exact solutions can be obtained, ii) the physics at phase boundaries where spectacular phenomena can occur due to competing interactions of the two phases around the boundary. This competition causes a frustration. I will concentrate my attention to thin films and phenomena occurring near the boundary of two phases of different symmetries. The case of two-dimensional (2D) systems is in fact the limiting case of thin films with a monolayer. Naturally, I will treat this case at the beginning. After a short introduction on frustrated spin systems, I show several 2D frustrated Ising spin systems which can be exactly solved by using vertex models. These systems contain most of the spectacular effects due to the frustration: high ground-state degeneracy, existence of several phases in the ground-state phase diagram, multiple phase transitions with increasing temperature, reentrance, disorder lines, partial disorder at equilibrium. Evidences of such effects in non solvable models are also shown and discussed. Thin films are next presented with different aspects: surface elementary excitations (surface spin-waves), surface phase transition and criticality. Several examples are shown and discussed. New results on skyrmions in thin films and superlattices are also displayed.

Static and dynamic properties of bimerons in a frustrated ferromagnetic monolayer

“…However, the dynamics of magnetic bimerons driven by different external forces as well as the static properties of different forms of bimerons still remain elusive, especially for the bimerons in frustrated magnetic systems. Some most recent studies have focused on the existence and manipulation of skyrmions in frustrated magnetic systems [48,[64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82][83]. Therefore, it is also imperative to study bimerons in frustrated magnetic systems, of which the physical properties are essential for designing future bimeron-based device applications.…”

Skyrmions in a frustrated model of multiferroic superlattices

“…In nanofilms of superlattices the magnetoelectric interaction is crucial for the creation of non-collinear long-range spin order. DM interaction has been identified as a key ingredient in the creation, stabilization of skyrmions and chiral domain walls [18,19,16,22]. In the case when the magnetic film has a frustration and a thickness, the angle between NN spins in each magnetic layer is different from that of the neighboring layer.…”

“…3). In the case of a single magnetic layer skyrmions are uniform on a plane (2D sheet) (see [22]). And also as shown our calculations in a case where the frustration is sufficiently strong (see Fig.…”

Dynamic transformation between a skyrmion string and a bimeron string in a layered frustrated system

“…To be specific, we consider a 3D skyrmion string forming by 11 aligned stacks of 2D pancake skyrmions in a frustrated spin system. Each 2D FM layer has 25 × 25 spins and is described by a J 1 -J 2 -J 3 classical Heisenberg model on a simple square lattice [18,21,24,33,[39][40][41]50], of which the Hamiltonian H n reads…”