Fig. 1. Observed XAFS spectra at the Sm K-edge (46.836 keV) for Sm 2 Fe 17 N x (x= 0.0, ≤ 0.1, 0.5, 2.0 and 3.0). The origin of each spectrum is shifted [6]. 48 HIGH ENERGY XAFS STUDIES OF THE Sm: K -EDGE IN Sm 2 Fe 17 N x COMPOUNDS Sm 2 Fe 17 N x compounds (x=0 ∼ 3) are relatively new hard magnetic materials. Upon introduction of three nitrogen atoms into Sm 2 Fe 17 , the Curie temperature increases dramatically from 398 K to 752 K [1]. The saturation magnetization of Sm 2 Fe 17 N 3 is comparable to that of Nd 2 Fe 14 B, while its uniaxial magnetic anisotropy is three times larger. Several studies have been performed on the change of the crystal structure of Sm 2 Fe 17 N x compounds due to the absorption of nitrogen atoms [ 2 ] . T h e c r y s t a l s t r u c t u r e o f S m 2 F e 1 7 i s rhombohedral Th 2 Zn 17 -type (R 3m ) and the lattice expands more than 6% to accommodate three nitrogen atoms at the interstitial site. Two possible sites – 9 e and 18g sites – are inferred from the results of the powder neutron diffraction studies of Pr 2 Fe 17 N x , Nd 2 Fe 17 N x and Y 2 Fe 17 N x . Several XAFS studies [3-5] were carried out at room temperature on the L 3 -edge of samarium atom and on the K -edge of iron atom in Sm 2 Fe 17 and Sm 2 Fe 17 N x (x = 2.8 ± 1.0). The XANES spectra of the Sm L 3 -edge indicate that the Sm valence was not changed upon nitrogenation. Furthermore, the XANES spectra of the Fe K -edge of Sm 2 Fe 17 , Sm 2 Fe 17 N x and α -Fe indicate that the environment of iron atoms in the Sm 2 Fe 17 N x was intermediate between its anionic state in Sm 2 Fe 17 and its covalent state in α -Fe. Also, a small edge shift occurred, implying that some charge transfer had occurred between the nitrogen and iron atom s. On the other hand, the magnetically-oriented powder EXAFS spectra of the Sm L 3 -edge suggested that the distance between samarium and iron atoms is expanded by nitrogenation, and the interstitial site of the nitrogen atoms in Sm 2 Fe 17 N x is the 9 e site. The study of the structural change in the intermediate nitrogen concentration (0 < x < 3) in Sm 2 Fe 17 N x compounds is important in order to clarify the mechanism of the change in magnetic properties by nitrogenation. These studies have not yet been performed, except on the lattice parameters and the unit cell volume. We carried out XAFS studies on the Sm K -edge in Sm 2 Fe 17 N x (x= 0.0, ≤ 0.1, 0.5, 2.0 and 3.0) at room temperature [6] in order to obtain more precise structural data. In the case of Sm L 3 -edge, the results were limited by the presence of the Fe K -edge at k=10.2 Å -1 . A Sm 2 Fe 17 alloy was prepared by the high f r e q u e n c y i n d u c t i o n - m e l t m e t h o d . F i r s t , nitrogenation was carried out under N 2 gas at 1 atm pressure and ∼ 400 ° C for several hours. After that, we exchanged the gas from N 2 to Ar, and annealing was performed continuously under Ar gas at 1 atm pressure and ∼ 400 ° C for a few days. From the results of the preliminary X-ray powder diffraction measurements, a few % α -Fe was mixed in Sm 2 Fe 17 N x (x ≤ 0.1, 0.5 and 2.0). XAFS investigation was carried out at beamline BL01B1 . Measurements were made on the Sm K - edge (46.834 keV) using the transmission mode with an Si(511) adjustable inclined double-crystal monochromator. The incident and transmitted X- ray intensities were monitored with ionization chambers filled with Kr gas. The estimated energy resolution was about 4 eV. Energy (keV) μ d 45.5 46.0 46.5 48.0 47.0 47.5 48.5 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 x = 3.0 x = 2.0 x = 0.5 x c 0.1 x = 0.0 Fig. 2. Enlargement of the edge region of the spectra. The structure indicated by arrow changes by nitrogenation [6]. Fig. 3. XAFS oscillations χ (k) in Sm 2 Fe 17 N x [6]. 49 References [1] J. M. D. Coey and H. Sun, J. Magn. Magn. Mater. 87 (1990) L251. [2] H. Fujii and H. Sun, Handbook of Magnetic Materials 9 (1995) 303; K. Kobayashi, 13th Int. Workshop on RE Magnets & their Applications p.717. [3] J. M. D. Coey et al. , J. Appl. Phys. 69 (1991) 3007. [4] T. W. Capehart et al. , Appl. Phys. Lett. 58 (1991) 1395. [5] T. W. Capehart et al. , J. Appl. Phys. 75 (1994) 7018. [6] H. Kasatani, M. Ohmura, K. Yokoyama, K. Kobayashi, Y. Nishihata, K. Yagi and H. Terauchi, Proc. Int. Conf. SRMS-2, Jpn. J. Appl. Phys. 38 (1999) 433. Sm K -edge XAFS spectra of Sm 2 Fe 17 N x (x= 0.0, ≤ 0.1, 0.5, 2.0 and 3.0) are shown in Fig. 1. The origin of each XAFS spectrum is shifted. Good XAFS signals were observed, despite the existence of blunt edge jumps ( ∼ 50 eV) and reduction of the EXAFS signal at higher energy. The structure above the absorption edge changed with nitrogen content as shown in Fig. 2. Results of Sm L -edge X A N E S a n d F e K - e d g e X A N E S s t u d i e s b y Capehart et al. [5] imply that the structure indicated by an arrow in Sm 2 Fe 17 is due to the 1 s → 6 p transition, because samarium is trivalent in both Sm 2 Fe 17 and Sm 2 Fe 17 N 2.8 and the 6 p band is unoccupied. Although a clear peak is observed in x ≤ 0.1 compound, it becomes obscure with the increase of nitrogen content. Since the 1 s → 6 p transition is forbidden, this structure cannot be observed in L 3 -edge XANES . It is inferred that the interstitial nitrogen atom directly changes the electrical state of the samarium atom. The XAFS oscillation ( χ ( k ) ) and the radial distribution function ( φ ( r ) ) indicate that the local structures around the samarium atom were changed upon nitrogenation. Figure 3 shows the XAFS oscillation ( χ ( k ) ) for Sm 2 Fe 17 N x (x=0.0, 2.0 and 3.0). The period of each XAFS oscillation becomes shorter with increasing numbers of nitrogen atoms. The Sm-Fe distance expanded monotonously upon nitrogenation and most of the expansion in lattice parameters and unit cell volume did not occur as previously indicated by X- ray diffraction studies. In order to clarify the precise nitrogen-induced local structural changes around the samarium atom in Sm 2 Fe 17 N x compounds by nitrogenation, further experiments will be conducted. Hirofumi Kasatani Shizuoka Institute of Science and Technology Email: kasatani@ms.sist.ac.jp Energy (keV) μ d 46.8 46.9 47.0 47.1 47.2 -1.08 -1.06 -1.04 -1.02 -1.00 x = 3.0 x = 2.0 x = 0.5 x c 0.1 x = 0.0 k (Å -1 ) 4 6 7 10 12 -0.02 0.00 0.02 x = 3.0 x = 2.0 x = 0.0 χ (k)
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