TEM / STEM study of rapidly quenched hard magnetic Nd‐Fe‐B ribbons

Abstract

The increase of coercivity and simultaneously reducing the amount of heavy rare earth elements of Nd‐Fe‐B magnets is of great economic and scientific interest. Both, the grain size of the hard magnetic Nd 2 Fe 14 B phase and the presence of a grain boundary (GB) phase and its chemical composition have a crucial influence on the coercivity of sintered Nd‐Fe‐B magnets [1], [2]. Besides the sinter processing also the production route of rapid solidification, also called melt‐spinning, for Nd‐Fe‐B magnets satisfies the demand of the industry for magnets with high coercivity and energy product [3]. The variation of the process parameters has a significant influence on the microstructure, like grain size and occurring phases. Two isotropic Nd‐Fe‐B melt‐spun ribbons, ms‐A and ms‐B , with an average grain size of about 19 nm (STDV = 6 nm) ( ms‐B ) and 60 nm (STDV = 22 nm) ( ms‐A ) and a coercive field ranging from µ 0 H c = 0.6 T ( ms‐A ) to 1.0 T ( ms‐B ) were investigated in a nanoanalytical TEM/STEM study carried out on an analytical field emission transmission electron microscope (TEM) (FEI Tecnai F20) at 200 kV, which is equipped with a high angle annular dark field detector (HAADF), a silicon drift energy dispersive X‐ray detector (EDX) from EDAX and a Gatan Tridem GIF electron energy loss spectrometer (EELS). Conventional TEM sample preparation (cutting, thinning, ion milling) was conducted, in order to investigate large sample areas. For detailed nanoanalytical investigations Focused Ion Beam (FIB) samples were prepared in an FEI Quanta 200 3D DualBeam‐FIB using the lift‐out technique. A TEM bright field (BF) image shows the large grains of sample ms‐A (Fig.1a). Besides selected area electron diffraction (SAED) analysis (Fig1.b), the occurring phases were identified with Fast Fourier Transformation (FFT) (Fig.1c) of High Resolution (HR) TEM images (Fig.1d). The [100], [110] and [111] lattice fringes of the single crystalline (Pr,Nd) 2 (Fe,Co) 14 B phase (φ) are visible in Fig.1d. Beside the 2‐14‐1 phase two further Fe dominating phases Fe‐1 and Fe‐2 were found. Two single crystalline bcc‐Fe (α) grains of phase Fe‐1, which contains mostly Fe and small amounts of Co and O, are displayed in Fig.1d. This phase shows characteristic dotted morphology, were else the Fe‐2 phase has a homogeneous contrast like the Nd‐Fe‐B phase. Besides Fe also significant amounts of Nb and O were found in this phase. Large area EDX analysis have indicated about 40 % of the grains to be one of these two Fe phases and only 60 % are Nd‐Fe‐B grains and the ratio of the two Fe phases is approximately 50/50. The smaller grain size of sample ms‐B is observed in the TEM‐BF image (Fig.2a). The [001] lattice plains of an Nd‐Fe‐B grain was indexed with an FFT (Fig.2c) of the TEM Dark Field (DF) image (Fig.2d). An EELS line scan (ls) over an interface of two Nd‐Fe‐B grains shows no change in the chemical composition, implying that a grain boundary phase is not present in this material (Fig.3b,d). On the basis of the information on the microstructure obtained by this TEM study a numerical micromagnetic finite element model was created to simulate the influence of microstructural features like grain size and other occurring phases (Fig.3c). The micromagnetic simulation of the demagnetization curve of randomly oriented grains with direct intergranular coupling shows a decrease of the coercive field with increasing grain size (Fig.3a), which is in good agreement with the measured coercive field of the two samples.