(ESICMM-G8 Symposium on Next Generation Permanent Magnets, Tsukuba, 2015)
Electron holography studies on magnetization and lattice strain in Nd-Fe-B permanent magnets
Y. Murakami1,2, T. Sasaki3, T. Tanigaki4, S. Kaneko1, K. Niitsu2,
Z. Akase1,2, D. Shindo1,2, T. Ohkubo3, and K. Hono3
1 Tohoku University, Japan,
2 RIKEN, Japan,
3 NIMS, Japan,
4 Hitachi Ltd., Japan
Abstract:
Precise analysis of the magnetic and crystallographic microstructures is vitally important for understanding of the coercivity mechanism of permanent magnets. Transmission electron microscopy can be a powerful tool for the examinations of both magnetism and structure of sintered magnets. This paper reports on our recent electron holography studies, which aimed at determination of the magnetic flux density from a thin grain boundary phase and the lattice strain within Nd-Fe-B sintered magnets.
Following the three-dimensional atom probe study by Sepehri-Amin et al. [1], researchers attempted to understand the magnetism of ultrathin grain boundary phase produced in Nd-Fe-B sintered magnets [2-4]. To tackle this problem, we employed split-illumination electron holography [4] which significantly improved the precision in measuring the phase shift of electrons. The holography study determined the phase shift due to the thin grain boundary phase, which was sandwiched by Nd2Fe14B grains, to be 0.34 rad (±0.08 rad). Based on this phase shift measurement, the magnetic flux density of the grain boundary phase was evaluated at 1.0 ± 0.1 T. The observation indicates that the grain boundary phase is ferromagnetic, rather than nonferromagnetic as it was assumed in previous studies.
Fig. 1 Electron holography (EH) observations of
Nd-Fe-B magnet. (a) TEM image of a specimen
containing a spherical Nd2O3 precipitate.
(b) Magnetic flux lines revealed by conventional
EH. (c) Elongation/contraction of the c plane in
the Nd2Fe14B matrix, determined by dark-field
EH.
In addition to the functionality of magnetic flux mapping, such as shown in Figs. 1(a),(b), electron holography provides information about the lattice strain, which appears to be responsible for the coercivity mechanism in Nd-Fe-B sintered magnets [5,6]. To reveal the lattice strain near a Nd2O3 precipitate, electron holograms were generated by using Bragg reflections from the Nd2Fe14B matrix, which carried information about the elongation and contraction of lattice. Although this method (i.e., dark-field electron holography [7]) had only been applied to nonmagnetic semiconductors, we have established a technique that allows for mapping of both the strain and the electromagnetic field. In the neighborhood of the spherical Nd2O3 precipitate, the magnitude of elongation/contraction in the c plane of Nd2Fe14B was smaller than 1%: refer to Fig. 1(c). The observation was consistent with the strain map determined by Moiré patterns, which were observed in scanning transmission electron microscopy images.
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