Fabrication of Exchange-Coupled α-Fe/Nd-Fe-B Multilayer Thin-Film Magnets

Abstract

Abstract---Multilayers having the form of TilFe/ [Nd-F e-BIF e ]x5ffi/glass were fabricated on glass substrates by means of rf sputtering and subsequent annealing. We have confirmed that these films keep clear multilayer structure and Nd2FelJ3 grains were randomly aligned. We have observed the spring-back behavior in the magnetization curves, which confirms the exchange coupling between Fe and Nd-Fe-B layers. Moreover, micromagnetic calculations applied for nanocomposite magnets have been performed, which indicated that good alignment of hard magnetic phase and crystal grain size less than 10 nm are extremely important factors to derive the potential for nanocomposite magnets. I . INTRODUCTIONNanocomposite magnets [l] are expected to have a potential for high performance magnetic powders for bonded magnets. The magnetic properties of nanocomposite magnets strongly depend on their nanostructures, so it is necessary to investigate the relation between magnetic properties and structures in detail. Nanocomposite magnets are obtained mainly by melt-spinning or rapid-quenching process[2], [3]. However, it is not easy to control the nanostructure by using such a method. As proposed by Skomski and Coey[4], a multilayered two phase magnet, consisting of alternating hard and soft magnetic layers has the advantage that individual layer thickness can be controlled precisely. AI-Omari and Sellmyer[5] found the exchange-coupled behavior in CoSmlFeCo bilayer films, while a remanence-enhancement behavior have been reported by Parhofer et al. [6] observed for Nd-FeBlFe/Nd-Fe-B trilayers. We have succeeded in fabricating exchange-coupled a-Fe/Nd-Fe-B multilayer films with Ti both for underlayers and overlayers, and reported the dependence of magnetic properties on layer thickness [7]. Moreover, we have estimated the coupling strength between a-Fe and Nd-Fe-B layers to be about 10% of the FelFe and Nd-Fe-B/Nd-Fe-B couplings by comparing the experiments with micromagnetic calculations [8]. In this paper, we will report the more advanced experimental and calculated results and discuss the directions of improvement for magnetic properties of nanocomposite magnets. All the films were annealed at 873 or 923 K for 30 minutes with the pressure below 4.0xlO-4 Pa to crvstallize the Nd2Fe14B phase. The magnetization curves were measured at room temperature by means of vibrating sample magnetometer with a maximum field of l.2 MAIm applied parallel to the film plane. X-ray diffractometer was used to study the structural properties of the films. The composition of the as-sputtered Nd-Fe-B films were determined by the EPMA. Transmission electron microscopy (TEM) images and Auger profiles were used to confirm the multilayer structure of annealed samples. Ill. EXPERIMENT AL RESULTS AND DISCUSSIONSAccording to the EPMA results, the composition ofthe as-sputtered Nd-Fe-B films were found to be Nd13-1SFebalB7-11, so the each Nd-Fe-B layer in a-Fe/NdFe-B muItilayer films is expected to be almost of single phase containing NdzFel4B grains. We have co...