Preparation, Microstructure, and Magnetic Properties of Electrodeposited Nanocrystalline L1 0 FePt Films
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ORIGINAL PAPER
Preparation, Microstructure, and Magnetic Properties of Electrodeposited Nanocrystalline L10 FePt Films Yao Ying 1 & Huibin Wang 1 & Jingwu Zheng 1 & Jing Yu 1 & Wangchang Li 1 & Liang Qiao 1 & Wei Cai 1 & Shenglei Che 1 Received: 31 May 2020 / Accepted: 21 July 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract In this work, we prepared the nanocrystalline near-equiatomic FePt thin films by electrodeposition and explored the effects of annealing temperature and annealing time on morphology, structure, and magnetic properties of FePt film. The fcc-FePt phase starts to form when annealing at 400 °C under the 15% H2 gas mixture. With increasing annealing temperature from 400 to 800 °C, the FePt film experiences a gradual phase transformation from the soft magnetic fcc phase to the permanent magnetic fct L10 phase. The transition degree of the L10 phase is achieved to be 0.94 and coercivity reaches a maximum of 1.5 T for the sample annealed at 800 °C for 4 h. A shoulder behavior in the M(H) curves is found in the electrodeposited FePt films. Through fitting the M(H) curves by the model based on the transition degree of the L10 FePt phase, the shoulder behavior is quantitatively confirmed to originate from the coexistence of the soft magnetic fcc phase and the permanent magnetic L10 phase. The magnetic exchange coupling between the soft magnetic and permanent magnetic nanocrystalline phases is negligible in the electrodeposited FePt films due to that the substrate Ag diffusion into the FePt film separates the FePt nanocrystalline phases. Keywords L10 FePt . Electrodeposition . Phase transformation . Coercivity
1 Introduction In recent years, the chemically ordered face-centered tetragonal (fct or L10) FePt alloy was identified as a promising permanent magnetic material for potential applications in high-density magnetic storage and micro-electromechanical systems (MEMS) [1–3] because the ordered L10 FePt possesses a very high magnetocrystalline anisotropy and its uniaxial anisotropy constant Ku reaches as high as 6.6–10 × 107 erg/cm2 [4, 5]. Generally, FePt films have been mostly prepared by various deposition techniques such as sputtering [6, 7], vapor deposition [8], pulsed laser deposition [9], accumulative roll bonding [10],
and electrodeposition [3, 11–13]. In MEMS, thick and continuous films are required. Electrodeposition has the advantage to prepare thicker films more efficiently and less costly [3, 11, 12]. In the method of electrodeposition, the post-annealing treatment at 400~900 °C is needed to form the high-coercivity L10 phase [3]. Leistner et al. prepared L10 FePt thin films with a small grain size of about 13 nm by electrodeposition and found that the coercivity of the film increases with raising annealing temperature and reaches 1 T after annealing at 600 °C [12]. Fardi-Ilkhchy et al. successfully synthesized FePt nanowires with graded composition and heat treatment at 550 °C to obtain a maximum coercivity of about 1810 Oe and rectangularity (Mr/Ms) of
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