RAPID COMMUNICATION Nanocomposite (Nd,Dy)(Fe,Co,Nb,B)5.5/α-Fe multilayer magnets with high performance

The structural and magnetic properties of rare-earth nanocomposite magnets of a (Nd,Dy)(Fe,Co,Nb,B)5.5 single layer and a (Nd,Dy)(Fe,Co,Nb,B)5.5/α-Fe multilayer prepared by sputtering and heat treatment have been investigated. Incomplete exchange coupling behaviour is observed in the Ti-buffered NdDyFeCoNbB + 40 wt%Fe single layer magnet. After annealing, the laminated nanostructure consisting of mutually dispersed soft and hard phases results in remanence enhancement. The optimum magnetic properties of Jr = 1.11 T, μ0iHc = 0.88 T and (BH)max = 192 kJ m−3 are achieved in the multilayer magnet annealed at 550 ̊C for 30 min. The designed multilayer film with thinner soft magnetic layers may favour the formation of more ideal nanostructures for exchange coupling between softand hard-magnetic phases. Experimental researches on nanostructured exchange-coupled magnets have been performed since 1988 [1]. Taking advantage of high coercivity and high magnetization contributed by hardand soft-magnetic components, respectively, high remanence and a large maximum energy product would be obtained if a full exchange coupling existing between the grains of the two phases in nanocomposite magnets, as predicted based on micromagnetic calculation [2–4]. However, up to now, the energy products of the rare-earth nanocomposite magnets prepared by means of rapid quenching and mechanical alloying have been much lower than the theoretical expectation, due to difficulties in controlling the nanostructures [5–8]. 5 Also at: International Centre for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China. 6 Author to whom any correspondence should be addressed. Recently, some studies on exchange coupling were carried out for nanostructured CoSm/FeCo and PrCo/Co multilayers prepared by sputtering and subsequent heat treatment [9, 10]. The magnetic properties of exchange-coupled α-Fe/Nd–Fe–B multilayer magnets were investigated by Shindo [11] and observations for Nd–Fe–B/Fe/Nd–Fe–B trilayers were reported by Parhofer and Yang et al [12–14]. In our previous study [15], we investigated the magnetic properties of nanocomposite multilayer magnets of NdDyFeCoNbB/M (M = Co, Fe65Co35) on Ti-buffered Si substrates prepared by sputtering and subsequent heat treatment. In comparison with the single layer, the remanence of the multilayer magnets was found to increase noticeably. In this letter, we report the structural and magnetic properties of nanocomposite (Nd,Dy)(Fe,Co,Nb,B)5.5/Fe multilayer magnets synthesized 0022-3727/03/170067+04$30.00 © 2003 IOP Publishing Ltd Printed in the UK L63