Type of presentation: Poster

MS-12-P-1643 TEM investigation of grain boundaries from Nd2Fe14B hard magnets

Zickler G. A.1, Wallisch W.1, Stöger-Pollach M.2, Bernardi J.2, Üstüner K.3, Fidler J.1
1Institute of Solid State Physics, Vienna University of Technology, Vienna, Austria, 2USTEM, Vienna University of Technology, Vienna, Austria, 3Vacuumschmelze GmbH, Hanau, Germany
zickler@ifp.tuwien.ac.at

Nd2Fe14B exhibits a high magnetocristalline anisotropy and is suitable for the usage as a hard magnetic material. [1,2,3] The aim of our study is to determine the discrepancy between the theoretical and the experimental coercivity. We have investigated the role of the sintering temperature on the formation of the grain boundary phases of various sintered magnets with a standard TEM/STEM instrument (FEI TECNAI G20) and a field-emission analytical TEM/STEM (FEI TECNAI F20).
Standard methods for TEM preparation (cutting, thinning, ion milling) were used to prepare a sample parallel (p) and normal (n), respectively, to the preferable direction of the magnetisation (equal to [001] of the grains).
In Fig. 1 a TEM bright field image of two Nd2Fe14B grains from a magnet with Br =1.27 T and Hc=1630 kA/m are shown. The specimen normal and the grain boundary (GB) with 10 nm thickness are perpendicular to [001] of the right grain. This GB-phase is a characteristic attribute for a type "x" GB. In Fig.2 a TEM bright field image of a specimen with two Nd2Fe14B grains and a Nd2O3 (Mn2O3 structure) grain boundary junction is shown. The [001] direction is parallel to the specimen normal. Therefore the grain boundary is parallel to [001] and forming a type "y" GB. The nature of the observed Nd2O3 (Mn2O3) grain boundary junction phase differs from the one of the Nd-rich type “x” and “y” grain boundary phases. The corresponding lattice plains of the hard magnetic Nd2Fe14B grains are indexed in the FFT images of Fig.1 and 2. The coercive field of the investigated magnets are directly related to the morphology of the grain boundary and grain boundary junction phases.
Detailed investigations including Energy Dispersive X-Ray Spectroscopy (EDXS), Electron Energy-Loss Spectrometry (EELS) and High Annular Dark-Field STEM (HAADF-STEM) imaging will be presented.

 

References:

[1] K. Khlopkov, O. Gutfleisch, D. Eckert, D. Hinz, B. Wall,W. Rodewald, K.-H. Müller. L. Schultz, “Local texture in Nd–Fe–B sintered magnets with maximised energy density” J. Alloys and Compounds 365 (2004) 259–265.
[2] S.C. Wang, Y. Li. “In situ TEM study of Nd-rich phase in NdFeB magnet” J. Magn. Magn. Mater. 285 (2005) 177–182.
[3] G. Hrkac, T.G. Woodcock, K.T. Butler, L. Saharan, M.T. Bryan, T. Schrefl, O. Gutfleisch “Impact of different Nd-rich crystal-phases on the coercivityof Nd–Fe–B grain ensembles” Scripta Materialia 70 (2014) 35–38.

The funding from the European Community´s Seventh Framework Programme (FP7-NMP) under the grant agreement no. 309729 (ROMEO) is acknowledged.

Fig. 1: Bright field image showing a grain boundary of type "x" between two Nd2Fe14B grains. Specimen normal is perpendicular to [001].

Fig. 2: Bright Field image showing two Nd2Fe14B grains and a grain boundary junction. Specimen normal is parallel to [001]. A grain boundary of type "y" is marked.