Quantum Theory of Helimagnetic Thin Films

TL;DR

This paper investigates the quantum properties of helimagnetic thin films, analyzing surface spin configurations, spin-wave spectra, and magnetization behaviors, revealing surface-localized modes and quantum fluctuation effects that influence transition temperatures.

Contribution

It introduces a quantum Green's function approach to study surface effects and spin dynamics in helimagnetic thin films, including surface reconstruction and quantum fluctuation impacts.

Findings

Surface spin angles are strongly modified at the surface.

A surface-localized spin-wave mode exists.

Quantum fluctuations cause a crossover in magnetizations.

Abstract

We study properties of a helimagnetic thin film with quantum Heisenberg spin model by using the Green's function method. Surface spin configuration is calculated by minimizing the spin interaction energy. It is shown that the angles between spins near the surface are strongly modified with respect to the bulk configuration. Taking into account this surface spin reconstruction, we calculate self-consistently the spin-wave spectrum and the layer magnetizations as functions of temperature up to the disordered phase. The spin-wave spectrum shows the existence of a surface-localized branch which causes a low surface magnetization. We show that quantum fluctuations give rise to a crossover between the surface magnetization and interior-layer magnetizations at low temperatures. We calculate the transition temperature and show that it depends strongly on the helical angle. Results are in agreement with existing experimental observations on the stability of helical structure in thin films and on the insensitivity of the transition temperature with the film thickness. We also study effects of various parameters such as surface exchange and anisotropy interactions. Monte Carlo simulations for the classical spin model are also carried out for comparison with the quantum theoretical result.