Magnetism in grain-boundary phase of a NdFeB sintered magnet studied by spin-polarized scanning electron microscopy Appl. Phys. Lett.Analysis of the demagnetization process of Nd-Fe-B sintered magnets at elevated temperatures by magnetic domain observation using a Kerr microscope Sintered ͓Nd 0.45 ͑Y 3 Dy 1 ͒ 0.25 ‫ء‬ 0.55 ͔ 2.8 Fe 14 B magnets were prepared for the first time. Magnetic properties and microstructures of the magnets were investigated by magnetic measurements and electron microprobe analysis. The microstructure consists of a MRE 2 Fe 14 B ͑2-14-1͒ phase matrix having a grain size of ϳ10 m and a RE-rich grain boundary phase. However, sintering resulted in segregation of Y to the inner and Nd to the outside of the 2-14-1 grains. The magnet has a room temperature ͑BH͒ max of 25.4 MGOe, which is two times higher than that of the isotropic melt spun ribbons with similar compositions. The temperature coefficients of Br ͑␣͒ and Hcj ͑␤͒ for the magnet are Ϫ0.150 and −0.632% /°C from 27 to 127°C, respectively. These temperature coefficients, especially for ␤, are also much higher than those of melt spun ribbons. The composition segregation in the 2-14-1 grains is believed to be responsible for the higher temperature coefficients.

Control of microstructure and texture is of critical importance in forming rare earth (RE)-iron-boron particulate suitable for anisotropic polymer-bonded permanent magnets and anisotropic sintered permanent magnets. In this study, the selected approach to controlling grain size, while maintaining texture, is through stabilization and refinement of directional growth in melt-spun ribbons. Varying concentrations of Ag were added to melt-spun ribbons of composition (Y0.55Nd0.45)2.2Fe14B1.1. Effects on microstructure and texture were observed through scanning electron microscopy (SEM) and x-ray diffraction (XRD). It was determined that Ag stabilized columnar growth (compared to alloys with no Ag added) with additions as small as 0.3 at. %, but the Ag also produced a unique texture in the ribbons. In RE-Fe-B ribbons without Ag, strong <00l> texture is observed at the free surface and a mechanism has been established. In all Ag-containing ribbons, the observed texture is canted to both the c- and a-axes, but the mechanism remains unclear.

The devitrification behavior of nanocrystalline MRE2(Fe,Co)14B+ZrC (MRE=Nd+Y+Dy) was studied using differential scanning calorimetry (DSC), synchrotron high temperature x-ray diffraction, and analytical transmission electron microscopy (TEM) techniques. Alloy ribbons were melt spun at 25 m/s to obtain an amorphous structure. Optimum hard magnetic properties (Br=7.2 kG, Hc=12.7 kOe and (BH)max=10.8 MG Oe) were obtained in ribbons annealed at 750 °C for 15 min. A reduced annealing temperature of 638 °C and holding time from 0 to 11 min were chosen based on DSC analysis. Large changes in both microstructure and hard magnetic properties were found in a narrow window of annealing time, 4.5–6 min, resulting in a dramatic increase in energy product, remanence and coercivity: 0.96 MG Oe, 5.2 kG, 2.7 kOe to 5.7 MG Oe, 7.2 kG, 8.5 kOe for (BH)max, Br and Hc, respectively. Energy dispersive x-ray spectroscopy and energy filtered TEM analyses indicate that Zr- and C-rich particles (∼5 nm) and thin grain boundary layers (1–2 nm thick) are formed surrounding 2-14-1 hard phase grains when the annealing time is over 6 min. Further annealing resulted in a more distinct hard phase surrounded by a nonmagnetic grain boundary phase ∼1 nm in thickness. The thin grain boundary layer phase starts to disappear with annealing time over 11 min. The partitioning behavior of various elements at different annealing conditions appears to be associated with significant changes in magnetic properties, leading to an improved optimum microstructure.

The grain pinning behavior of TiC particles in a rapidly solidified MRE-Fe-B (MRE = Nd + Y + Dy) nanocrystalline hard magnet was studied using electron tomography (ET). The 3D reconstruction overcomes the inherent 2D nature of conventional transmission electronmicroscopy (TEM) to reveal how this grain boundary phase controls the nanoscale structure in the rapidly solidified alloy. The 3D reconstruction was performed on the optimally annealed alloy (750 °C/15 min) with hard magnetic properties of Mr = 8.1 kGs, Hc = 6.2 kOe, (BH)max = 11.2 MGOe measured at 300 k. The sampled volume, 425 × 425 × 92.5 nm3, contains more than 20 grains of the RE2-14-1 phase and more than 70 TiC nanoparticles. The TiC grains’ shapes depend on their sizes and locations along the grain boundary. Most of the TiC particles are oval or short rod like shapes and range from 5 nm to 10 nm. TiC particles less than 10 nm formed between adjacent 2-14-1 grains, while the largest ones formed at triple junctions. There are ∼1.7 × 108 TiC particles within a 1 mm3 volume in the alloy. This accounts for the strong grain boundary pinning effect, which limits grain growth during annealing.