Recent advances in SmFe12-based permanent magnets.

To realize a sustainable society, 'green technology' with low (or even zero) CO2 emissions is required. A key material in such technology is a permanent magnet because it is utilized for electric-power conversion in several applications including electric vehicles (EVs), hybrid EVs (HEVs), and turbines for wind power generation. To realize highly efficient electric-power conversion, a stronger permanent magnet than Nd-Fe-B is necessary. One potential candidate is a Fe-rich SmFe12-based compound with a ThMn12 structure. In this paper, the phase stability, structure, and intrinsic and extrinsic magnetic properties in both film and bulk forms are reviewed. Based on these results, a possible way to realize a strong SmFe12-based permanent magnet in bulk form is discussed.

Magnetic refrigeration material operating at a full temperature range required for hydrogen liquefaction.

Magnetic refrigeration (MR) is a key technique for hydrogen liquefaction. Although the MR has ideally higher performance than the conventional gas compression technique around the hydrogen liquefaction temperature, the lack of MR materials with high magnetic entropy change in a wide temperature range required for the hydrogen liquefaction is a bottle-neck for practical applications of MR cooling systems. Here, we show a series of materials with a giant magnetocaloric effect (MCE) in magnetic entropy change (-∆Sm > 0.2 J cm-3K-1) in the Er(Ho)Co2-based compounds, suitable for operation in the full temperature range required for hydrogen liquefaction (20-77 K). We also demonstrate that the giant MCE becomes reversible, enabling sustainable use of the MR materials, by eliminating the magneto-structural phase transition that leads to deterioration of the MCE. This discovery can lead to the application of Er(Ho)Co2-based alloys for the hydrogen liquefaction using MR cooling technology for the future green fuel society.

Multi-scale characterization by FIB-SEM/TEM/3DAP.

In order to improve properties of functional materials, it is important to understand the relation between the structure and the properties since the structure has large effect to the properties. This can be done by using multi-scale microstructure analysis from macro-scale to nano and atomic scale. Scanning electron microscope (SEM) equipped with focused ion beam (FIB), transmission electron microscope (TEM) and 3D atom probe (3DAP) are complementary analysis tools making it possible to know the structure and the chemistry from micron to atomic resolution. SEM gives us overall microstructural and chemical information by various kinds of detectors such as secondary electron, backscattered electron, EDS and EBSD detectors. Also, it is possible to analyze 3D structure and chemistry via FIB serial sectioning. In addition, using TEM we can focus on desired region to get more complementary information from HRTEM/STEM/Lorentz images, SAED/NBD patterns and EDS/EELS to see the detail micro or nano-structure and chemistry. Especially, combination of probe Cs corrector and split EDS detectors with large detector size enable us to analyze the atomic scale elemental distribution. Furthermore, if the specimen has a complicated 3D nanostructure, or we need to analyze light elements such as hydrogen, lithium or boron, 3DAP can be used as the only technique which can visualize and analyze distribution of all constituent atoms of our materials within a few hundreds nm area. Hence, site-specific sample preparation using FIB/SEM is necessary to get desired information from region of interest. Therefore, this complementary analysis combination works very well to understand the detail of materials.In this presentation, we will show the analysis results obtained from some of functional materials by Carl Zeiss CrossBeam 1540EsB FIB/SEM, FEI Tecnai G(2) F30, Titan G2 80-200 TEMs and locally build laser assisted 3DAP. As the one of the example, result of multi-scale characterization for ultra-fine grain Nd-Fe-B permanent magnet will be shown [1]. In order to improve the magnetic properties, especially to increase the coercivity (resistance against magnetization reversal) of the magnet, decreasing the grain size and isolating each grain by non-ferromagnetic grain boundary phase are quite important since the nucleation of magnetic reversal from grain boundary phase can be suppressed and pinning force of magnetic domain wall at the grain boundary phase can be strengthened. Therefore, micro and nano structure and chemistry analysis can shed a light do grain boundary engineering.Figure 1(a,b) shows SEM BSE images of ultrafine grain Nd-Fe-B sintered magnet and the reconstructed 3D tomography of Nd-rich phases obtained by FIB/SEM serial sectioning. This data can provide us information about the distribution of Nd-rich phase and its volume fraction. Moreover, the HRTEM image from the grain boundary phase, the 3DAP maps and the concentration depth profiles are shown in Fig. 1(c,d,e). This magnet shows high coercivity (1517kA/m), and by comparing these results with the microstructures of low coercivity specimen, importance of grain boundary formation was confirmed and it gives us hint to improve the coercivity further. We will show the detail and results from other materials.jmicro;63/suppl_1/i6/DFU046F1F1DFU046F1Fig. 1.(a) SEM BSE images of ultrafine grain Nd-Fe-B sintered magnet. (b) 3D FIB/SEM tomography of Nd-rich phases. (c) HRTEM image from the grain boundary phase. (d) 3DAP maps of Nd, Cu and Al. (e) Concentration depth profiles for Fe, Nd+Pr, B, Co, Cu and Al, determined from the selected box in (d)[1].

Photoemission electron microscopy study of sub-200 nm self-assembled La0.7SrLa0.3MnO3 epitaxial islands.

The chemical composition and the magnetic structure of individual La0.7Sr0.3MnO3 (LSMO) ferromagnetic manganite epitaxial nanostructures less than 200 nm in width are explored using Photoemission Electron Microscopy (PEEM). X-ray absorption spectra (XAS) provide separate information on the surface and the bulk composition of the nanoislands and give evidence of Mn(2+) present on the surface of otherwise stoichiometric nanostructures. Ferromagnetic domains less than 70 nm are resolved using X-ray magnetic circular dichroism (XMCD), which allows for the detection of magnetic vortex states in both (001)LSMO square and (111)LSMO triangular manganite nanoislands. The evolution of single nanostructures under an in-plane magnetic field is seen to depend on the specific nanoisland size and geometry. In particular, PEEM XMCD imaging allows detecting opposite chiralities as well as a variety of magnetization behaviors for different nanoislands.

Quantitative laser atom probe analyses of hydrogenation-disproportionated Nd-Fe-B powders.

We report a successful atom probe tomography of hydrides in hydrogenation-disproportionated Nd-Fe-B powder using a green femtosecond laser. The atom probe specimens were prepared from one particle of powder using the focused ion beam lift-out method. The atom probe tomography taken from an α-Fe/NdH(2) structure suggested that B and Ga (trace added element) were partitioned in the NdH(2) phase. The hydrogen concentration of 64 at% determined from the atom probe analysis was in excellent agreement with the stoichiometry of the NdH(2) phase.

Broadening the applications of the atom probe technique by ultraviolet femtosecond laser.

Laser assisted field evaporation using ultraviolet (UV) wavelength gives rise to better mass resolution and signal-to-noise ratio in atom probe mass spectra of metals, semiconductors and insulators compared to infrared and green lasers. Combined with the site specific specimen preparation techniques using the lift-out and annular Ga ion milling in a focused ion beam machine, a wide variety of materials including insulating oxides can be quantitatively analyzed by the three-dimensional atom probe using UV laser assisted field evaporation. After discussing laser irradiation conditions for optimized atom probe analyses, recent atom probe tomography results on oxides, semiconductor devices and grain boundaries of sintered magnets are presented.