HR-TEM, STEM and EELS
Grain and phase boundaries are known to be of critical importance for the coercivity in Nd-Fe-B and other hard magnetic materials. In Nd-Fe-B sintered magnets, a 1-3 nm thick layer of the Nd-rich phase between the grains of Nd2Fe14B is thought to be the optimum configuration. The boundaries between larger Nd-rich grains and the Nd2Fe14B are also increasingly being thought of as important for the coercivity of these materials. The study of the structure and chemical composition of such features is extremely challenging and characterisation techniques with atomic scale resolution are needed. Aberration-corrected scanning transmission electron microscopy can be used to obtain images with < 1 Å spatial resolution. An example of HR-STEM is shown in the High Angle Annular Dark Field (HAADF) image in fig. 1, where the crystal structure of the Nd2Fe14B phase can be seen in real space with so-called "Z contrast" i.e. heavier atoms appear brighter. The rings composed of Fe atoms can be clearly seen between the planes containing the Nd and B atoms.
Figure 1: Aberration-corrected HR-STEM HAADF image of a grain of Nd2Fe14B with the <100> axis parallel to the electron beam (i.e. out of the plane of the image). An atomistic model of the crystal structure has been overlaid on the image in order to show the direct visualisation of the crystal structure by this imaging technique. Fe atoms are shown in red, Nd in blue and B in green.
Electron energy loss spectroscopy (EELS) can in combination with HR-STEM yield analysis of the spatial distribution of the elements on a sub-nm length scale. An EELS map showing the distribution of Fe and Nd in a region containing two Nd2Fe14B grains with an 1 nmm thick intergranular phase between them is shown in fig. 2. The distributions of Fe and Nd in the lattice planes in the 2:14:1 grain at the top of the image can be clearly seen, as can the enrichment of Nd in the intergranular phase.
Fig. 2 Aberration-corrected HR-STEM EELS map showing the spatial distribution of Fe and Nd at a boundary between two Nd2Fe14B grains.
These techniques have recently been used to investigate the local chemical composition and structure of phase boundaries in hot deformed Nd-Fe-B magnets in which the coercivity had been enhanced by infiltrating the grain boundaries of the magnet with a Nd-Cu eutectic liquid [TGW Acta].
In order to aid interpretation of the experimental results, various modelling techniques can provide useful information. In order to make the models close to reality, as many parameters determined from experimental results as possible should be used as input. An HR-TEM image of a phase boundary between Nd2Fe14B and a large grain of Nd2O3 can be seen in fig. 3. The lattice fringes of both grains can be clearly seen. This enables the orientation of the grains to be determined. This information was then used as input for atomistic simulations (right image in fig. 3). The simulations predict the presence of distorted layers of the crystals on either side of the boundary [GH APL]. These distorted regions could act as weak points in the microstructure, leading to a reduction in coercivity. For further details on this combination of cutting edge experimental work with detailed atomistic simulations please refer to [GH APL].
Fig. 3 HR-TEM image of a phase boundary in a Nd-Fe-B sintered magnet (left) and an atomistic simulation (right) of the same boundary showing distorted regions of the crystal structures near the boundary [GH APL].
[TGW Acta] T.G. Woodcock, Q.M. Ramasse, G. Hrkac, T. Shoji, M. Yano, A. Kato and O. Gutfleisch, Atomic-Scale Features of Phase Boundaries in Hot Deformed Nd-Fe-Co-B-Ga Magnets Infiltrated with a Nd-Cu Eutectic Liquid, Acta Materialia 77 (2014), 111-124
[GH APL] G. Hrkac, T.G. Woodcock, C. Freeman, A. Goncharov, J. Dean, T. Schrefl and O. Gutfleisch, The Role of Local Anisotropy Profiles at Grain Boundaries on the Coercivity of Nd2Fe14B Magnets, Applied Physics Letters, 2010, 97 (23), 232511