Phase Transition in Frustrated Magnetic Thin Film-Physics at Phase Boundaries.

Hung Diep

doi:10.3390/e21020175

Abstract

In this review, we outline some principal theoretical knowledge of the properties of frustrated spin systems and magnetic thin films. The two points we would like to emphasize: (i) the physics in low dimensions where exact solutions can be obtained; (ii) the physics at phase boundaries where interesting phenomena can occur due to competing interactions of the two phases around the boundary. This competition causes a frustration. We will concentrate our attention on magnetic thin films and phenomena occurring near the boundary of two phases of different symmetries. Two-dimensional (2D) systems are in fact the limiting case of thin films with a monolayer. Naturally, we will treat this case at the beginning. We begin by defining the frustration and giving examples of frustrated 2D Ising systems that we can exactly solve by transforming them into vertex models. We will show that these simple systems already contain most of the striking features of frustrated systems such as the high degeneracy of the ground state (GS), many phases in the GS phase diagram in the space of interaction parameters, the reentrance occurring near the boundaries of these phases, the disorder lines in the paramagnetic phase, and the partial disorder coexisting with the order at equilibrium. Thin films are then 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. By the examples presented in this review we show that the frustration when combined with the surface effect in low dimensions gives rise to striking phenomena observed in particular near the phase boundaries.

Highlights

Extensive investigations on materials have been carried out over the past three decades
We will show that these simple systems already contain most of the striking features of frustrated systems such as the high degeneracy of the ground state (GS), many phases in the GS phase diagram in the space of interaction parameters, the reentrance occurring near the boundaries of these phases, the disorder lines in the paramagnetic phase, and the partial disorder coexisting with the order at equilibrium
By the examples presented in this review we show that the frustration when combined with the surface effect in low dimensions gives rise to striking phenomena observed in particular near the phase boundaries

Summary

Introduction

Extensive investigations on materials have been carried out over the past three decades. This is due to an enormous number of industrial applications which drastically change our lifestyle. The progress in experimental techniques, the advance on theoretical understanding, and the development of high-precision simulation methods together with the rapid increase of computer power have made possible the rapid development in material science. The purpose of this review is to look back at early and recent results in the physics of frustrated spin systems at low dimensions: 2D systems and magnetic thin films. We would like to connect these results, published over a large period of time, on a line of thoughts: physics at phase boundaries

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Conclusion