Quantum theory of helimagnetic thin films

H. T. Diep
Phys. Rev. B 91, 014436 – Published 29 January 2015

Abstract

We study properties of a helimagnetic thin film with a 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.

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  • Received 19 November 2014
  • Revised 9 January 2015

DOI:https://doi.org/10.1103/PhysRevB.91.014436

©2015 American Physical Society

Authors & Affiliations

H. T. Diep*

  • Laboratoire de Physique Théorique et Modélisation, Université de Cergy-Pontoise, Centre National de la Recherche Scientifique, Unités Mixtes de Recherche, 8089 2 Avenue Adolphe Chauvin, 95302 Cergy-Pontoise Cedex, France

  • *diep@u-cergy.fr

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Vol. 91, Iss. 1 — 1 January 2015

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