Formations of narrow stripes and vortex-antivortex pairs in a quasi-two-dimensional ferromagnet K2CuF4

Togawa, Y., Akashi, T., Kasai, H., Paterson, G. W. , McVitie, S. , Kousaka, Y., Shinada, H., Kishine, J.-i. and Akimitsu, J. (2021) Formations of narrow stripes and vortex-antivortex pairs in a quasi-two-dimensional ferromagnet K2CuF4. Journal of the Physical Society of Japan(90), 014702. (doi: 10.7566/JPSJ.90.014702)

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Using cryogenic Lorentz transmission electron microscopy, we investigate the temperature evolution of magnetic structures in single crystal samples of K2CuF4 with a magnetic phase transition at 6.1 K. This material is known as one of the candidates for a two-dimensional (2D) XY magnet that may exhibit a Berezinskii–Kosterlitz–Thouless (BKT) phase transition. A fine magnetic stripe pattern was found with a period of about 120 nm in a direction along the c axis below 7.3 K in a thin sample with the c axis in the plane. Magnetic columns of vortices and antivortices with a separation of about three micrometers were observed below 6 K in a c-plane sample approximately 150 nm in thickness. The formation of two different types of magnetic structures at different threshold temperatures is likely to be consistent with a picture of two-step dimensional crossover in spin and real spaces previously derived from magnetization and neutron experiments in K2CuF4. These results indicate how the 2D XY character of K2CuF4 is incorporated in three-dimensional magnetic structures. Based on the experimental observations, we discuss the lengthscale of film thickness appropriate for expanding the 2D XY regime and generating the robust BKT excitations. We expect our study to be an important step in realizing the BKT phase transition in a real magnetic system.

Item Type:Articles
Additional Information:We acknowledge support from Grants-in-Aid for Scientific Research (Nos. 17H02767, 17H02923). This work includes the results obtained by using the research equipment shared in the MEXT Project for promoting public utilization of advanced research infrastructure (Program for supporting introduction of the new sharing system: Grant Number JPMXS0410500020) and the United Kingdom research council EPSRC (Grant Number EP=M024423=1).
Glasgow Author(s) Enlighten ID:Togawa, Dr Yoshihiko and McVitie, Professor Stephen and Paterson, Dr Gary
Authors: Togawa, Y., Akashi, T., Kasai, H., Paterson, G. W., McVitie, S., Kousaka, Y., Shinada, H., Kishine, J.-i., and Akimitsu, J.
College/School:College of Science and Engineering > School of Physics and Astronomy
Journal Name:Journal of the Physical Society of Japan
Publisher:Institute of Pure and Applied Physics
ISSN (Online):1347-4073
Published Online:07 December 2020
Copyright Holders:Copyright © 2020 The Physical Society of Japan
First Published:First published in Journal of the Physical Society of Japan 90:014702
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher
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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
190883Consortium for advanced materials based on spin chiralityStephen McVitieEngineering and Physical Sciences Research Council (EPSRC)EP/M024423/1P&S - Physics & Astronomy