Surfaces and Interfaces 46 (2024) 104153 Available online 5 March 2024 2468-0230/© 2024 Elsevier B.V. All rights reserved. Influence and mechanism of surface defects on coercivity of M-type ferrite particles M. Haseeb a , Y.Q. Li a, * , H.G. Zhang a , W.Q. Liu a , P.J. Zhang b , M. Yue a, * a College of Materials Science & Engineering, Key Laboratory of Advanced Functional Materials, Ministry of Education of China, Beijing University of Technology, Beijing 100124, China b BGRIMM Magnetic Materials and Technology Co., Ltd. Beijing 102600, China A R T I C L E INFO Keywords: Hexagonal ferrites Coercivity Micromagnetic simulations Magnetization reversal mechanism Surface defects ABSTRACT M-type ferrite exhibits good magnetic characteristics and resistance to oxidation, enabling the production of stable nanoparticles for applications such as medical delivery, hyperthermia therapy, and magnetic recording. Additionally, it serves as a valuable tool for studying the theory of coercivity. Here, the impact of surface and size on coercivity and associated mechanisms in M-type hexagonal ferrites has been thoroughly analyzed by a combination of micromagnetic models and experimental investigations. Based on a cubic model and without considering surface defects, the coercivity still cannot reach the theoretical value. It decreases with an increase in particle size, and the quasi-coherent and quasi-flower reversal modes appear continuously. Besides, surface defects do not affect the reversal mode, but they do decrease the coercivity and magnetic hardening effect, hence further amplifying the difference between coercivity and its theoretical value. The simulation results are strongly supported by experimental data. This study extensively examines the coercivity of M-type ferrite particles pro- duced by various preparation methods, taking into account experimental observations and simulation results. Our findings can serve as guidance for the development of preparation technology for permanent magnet nanoparticles and contribute to a deeper understanding of the contradictions in coercivity and the mechanisms of magnetization reversal. 1. Introduction The magnetoplumbite-type hexagonal ferrites, also known as M-type ferrites, are widely regarded as permanent magnetic materials because of their elevated Curie temperature, high coercivity, and cost- effectiveness. The M-type ferrites also possess the advantage of being non-toxic and resistant to corrosion. Therefore, these materials are extensively utilized in magneto-dielectric devices, electronics, micro- waves, and magnetic data recording [1–7], with a yearly production of one million tons [8]. The newly developed M-type ferrite nanoparticles play a prominent role in biomedicine, contributing to applications such as magnetic resonance (MR) imaging, medicinal delivery, and hyper- thermia therapy. It is crucial to emphasize that these nanoparticles are vital across various applications, with particular significance as a mag- netic recording medium [9–18]. The applications of M-type ferrites heavily rely on their permanent magnet performance. Therefore, studying and understanding these characteristics is of great significance for further improving magnetic properties and expanding application fields. The M-type ferrites exhibit remarkable intrinsic properties, such as a Curie temperature above 740 K, a saturation magnetization (Ms) of 0.38 MA/m and a theoretical limit value of intrinsic coercivity (magneto- crystalline anisotropy field, Ha) of 1.7–1.8 T [19]. Nevertheless, the experimentally achieved coercivity of M-type ferrite nanoparticles is much lower at 627 kA/m (0.79 T) [20]. When examining coercivity dependencies thoroughly, a significant association with particle (grain) size is observed, revealing a peak at a critical single-domain size of approximately 600 nm. Coercivity increases with decreasing particle size until it approaches the single domain threshold. The difference between intrinsic and experimental coercivity values in M-type ferrite magnets is also worth studying. Shinde et al. found significant hysteresis in nanocrystalline ferromagnetic materials without regular magnetic domains [21]. Turilli et al. found that particle size and inherent mag- netic properties significantly reduce the coercivity of sintered barium ferrite powder around the single-multidomain transition area [22]. A strontium ferrite (SrM) powder milling investigation found that lengthy * Corresponding authors. E-mail addresses: yqli@bjut.edu.cn (Y.Q. Li), yueming@bjut.edu.cn (M. Yue). Contents lists available at ScienceDirect Surfaces and Interfaces journal homepage: www.sciencedirect.com/journal/surfaces-and-interfaces https://doi.org/10.1016/j.surfin.2024.104153 Received 10 January 2024; Received in revised form 20 February 2024; Accepted 4 March 2024