Monte Carlo simulations on rare-earth Holmium ultra-thin films

TL;DR

This study uses Monte Carlo simulations to explore magnetic phases and critical behaviors in ultra-thin Holmium films, revealing size-dependent phase transitions and surface effects that align with experimental observations.

Contribution

It provides a comprehensive simulation analysis of Holmium thin films, identifying new intermediate phases and surface effects not previously characterized.

Findings

For n>16, bulk magnetic properties are recovered.

Intermediate-temperature block phase with coexisting ordered and disordered layers.

Inner layers exhibit Kosterlitz-Thouless transition signatures.

Abstract

Motivated by recent experimental results in ultra-thin helimagnetic Holmium films, we have performed an extensive classical Monte Carlo simulation of films of different thickness, assuming a Hamiltonian with six inter-layer exchange constants. Both magnetic structure and critical properties have been analyzed. For n>16 (n being the number of spin layers in the film) a correct bulk limit is reached, while for lower n the film properties are clearly affected by the strong competition among the helical pitch and the surface effects, which involve the majority of the spin layers: In the thickness range n=9-16 three different magnetic phases emerge, with the high-temperature, disordered, paramagnetic phase and the low-temperature, long-range ordered one separated by an intriguing intermediate-temperature block phase, where outer ordered layers coexist with some inner, disordered ones. The phase transition of these inner layers displays the signatures of a Kosterlitz-Thouless one. Finally, for n<~7 the film collapse once and for all to a quasi-collinear order. A comparison of our Monte Carlo simulation outcomes with available experimental data is also proposed, and further experimental investigations are suggested.