X-ray magnetic circular dichroism in quasicrystals

Materials Science and Engineering 294–296 (2000) 617–620

X-ray magnetic circular dichroism in quasicrystals Yasuhiro Watanabe a,∗ , Junpei Tamura a , Hironobu Shoji a , Susumu Nanao a , Tetsuya Nakamura b , Yoshihiko Yokoyama c a

Institute of Industrial Science, University of Tokyo, Minato-ku, Roppongi 7-22-1, Tokyo 106-8558, Japan b The Institute of Physical Chemical Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan c Faculty of Engineering, Himeji Institute of Technology, Shosha, Himeji, Hyogo 671-22, Japan Received 21 September 1999; accepted 10 April 2000

Abstract The spectra of X-ray magnetic circular dichroism (XMCD) and extended X-ray absorption fine structure (EXAFS) were measured in the ferromagnetic quasicrystals (a stable decagonal phase of Al40 Mn25 Fe15 Ge20 , a unstrained icosahedral phase of Al50 Pd15 Mn20 B15 , a strained icosahedral phase of Al42.5 Pd7.5 Mn30 B20 ) and the orthorhombic approximant crystalline phase of Al30 Mn40 B30 . In Al40 Mn25 Fe15 Ge20 , the intensity of XMCD at Mn K-edge and Fe K-edge were −7×10−5 and −5×10−5 , respectively, which are normalized by the edge jump of the normal absorption spectra. The values suggest that Mn and Fe in this alloy contribute to the magnetism to the same degree. From the result of EXAFS, the nearest neighbors of Mn atoms and Fe atoms were analyzed to be at the same distance. The environment around Mn and Fe were found not so similar except for the nearest neighbor distance. In Al50 Pd15 Mn20 B15 , Al42.5 Pd7.5 Mn30 B20 and Al30 Mn40 B30 alloys, the intensity of XMCD around the Mn K-edge were about −3×10−4 . The magnetic moments of Mn atoms in these alloys are roughly estimated to be 0.8␮B . The difference of a magnetic structure was detected between the quasicrystals and the approximant. The result of EXAFS analysis indicates that the distances from Mn to its first nearest neighbor and its second nearest neighbor are almost equal. The existence of phason strain field have no appreciable influence on the moment of Mn. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Magnetic circular dichroism; Decagonal quasicrystal; Icosahedral quasicrystal; EXAFS

1. Introduction An Al–Mn–Fe–Ge decagonal phase has been known to have the largest magnetization in all the stable quasicrystals. A decagonal Al40 Mn15 Fe15 Ge20 has an Al4 Mn-type structure and exhibits the magnetization of 6.3 emu/g in an applied field of 10 kOe at 40 K [1,12]. It have been reported in aluminum–manganese base alloys that some metastable icosahedral quasicrystals prepared by rapid solidification have a, rather, large coercivity (Al65 Mn20 Ge15 [2], Al40 Cu10 Mn25 Ge25 [3]) or exhibit ferromagnetic properties (Al–Mn–Si [4]). These alloys, however, have not large magnetization. In an Al–Pd–Mn–B system, there are metastable icosahedral phases and stable approximant crystalline phases. An icosahedral Al50 Pd15 Mn20 B15 alloy and an orthorhombic Al30 Mn40 B30 alloy were found to be unstrained and an icosahedral Al42.5 Pd7.5 Mn30 B20 alloy strained [5,6,11], by X-ray powder diffraction and electronic diffraction. In these ∗ Corresponding author. Tel.: +81-3-3402-3950; fax: +81-3-3402-6350. E-mail address: [email protected] (Y. Watanabe).

alloys, the magnetization increased with increase of Mn and B concentration. X-ray magnetic circular dichroism (XMCD) is being used to investigate the magnetic properties of the ferromagnetic or ferrimagnetic materials. XMCD is defined experimentally as the relative difference of the absorption for right and left polarized X-rays in magnetized materials around an absorption edge. It reflects the differences of spin densities of the empty state [7]. The magnetic moment on 4p band is created by the spin-dependent hybridization with the 3d band close to the Fermi level. Therefore, the intensity of XMCD has a strong positive correlation with the contribution of the relevant element to the magnetization of material. The objective of this study is to apply the XMCD method to study the magnetism of the ferromagnetic quasicrystals. It is a main target to clarify the difference between the contribution of Mn and Fe atom to the magnetism in an Al40 Mn25 Fe15 Ge20 alloy, and the effects of phason strain on the magnetism of Mn in Al–Pd–Mn–B alloys. Local structures around Mn and Fe atoms were also investigated by extended X-ray absorption fine structure (EXAFS).

0921-5093/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 1 0 3 - 5

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Y. Watanabe et al. / Materials Science and Engineering 294–296 (2000) 617–620

Table 1 Samples designation and their characteristics Designation Composition

Structure

Stability

Magnetization (emu/g)

A B C D

Decagonal Icosahedral Icosahedral Orthorhombic

Stable Metastable Metastable Stable

6.3 (40 K) 22 (r.t.) 40 (r.t.) 80 (r.t.)

Al40 Mn15 Fe15 Ge20 Al50 Pd15 Mn20 B15 Al42.5 Pd7.5 Mn30 B20 Al30 Mn40 B30

2. Experimental procedure Samples preparation in detail should be referred in the previous reports [1,5,6,12]. The characteristics of the samples are shown in Table 1. An annealed ingot of the Al40 Mn25 Fe15 Ge20 alloy and the rapid quenched Al–Pd–Mn–B ribbons were powdered and were fixed on pieces of Scotch tape for the measurement of X-ray transmission. The measurements of XMCD were carried out at PF-28B in KEK (Institute of Material Structure Science, High Energy Accelerator Research Organization in Japan). The circular polarized beam was generated by an ellipsoid multi-pole wigller and monochromatized by a Si (1 1 1) double-crystal monochromator with an energy resolution of 1 eV. The absorption spectra were measured with a transmission geometry in a magnetic field of 6 kOe, which direction was, alternatively switched from +45 to –45◦ to the direction of the incident X-ray beam. The XMCD spectra were measured for the Al40 Mn25 Fe15 Ge20 alloy at the Mn K-edge and Fe K-edge and at 20 K, and they were measured for all the Al–Pd–Mn–B alloys at the Mn K-edge at room temperature. EXAFS spectra were also measured for all the samples at the same absorption edges and were analyzed using the program of EXAFSH [8]. XMCD is given by the difference of the absorption coefficients of X-rays 1µ given by Eq. (1). 1µ(E) =

ln(I (↑↑)) − ln(I (↑↓)) η

(1)

where (I (↑↑)) and (I (↑↓)) are the intensities of transmitted X-rays with k vector parallel and antiparallel to the magnetization, respectively, and η is a jump of the absorption coefficient at the absorption edge.

3. Results and discussion 3.1. XMCD and EXAFS in Al40 Mn25 Fe15 Ge20 The XMCD spectra in Al40 Mn25 Fe15 Ge20 are shown in Fig. 1. The intensity of the peak at the Mn K-edge is −7.0×10−5 and that at the Fe K-edge is −5.1×10−5 , respectively. The 4p bands of the two metals are polarized to the same direction because the sign of the main peak is negative for the both spectra. On the assumption that the hybridization aligns the p moment antiparallel to the 3d

Fig. 1. XMCD spectra in a decagonal Al40 Mn25 Fe15 Ge20 alloy; (a) measured at the Mn K-edge; (b) Fe K-edge and (c) the XMCD spectrum in pure iron at the Fe K-edge.

moment, the moments due to Mn 3d band should have the same orientation as that on Fe. The contribution of each element to the magnetization is to be almost the same order of magnitude, considering the result that the intensities of these peaks are approximately equal. On the other hand, the intensity of the peaks were one order of magnitude lower than that of pure iron (−3.4×10−4 ), which spectrum was measured at room temperature in the present experiment as a standard. This is reasonable because the magnetization of this alloy is one order of magnitude smaller than that of iron. The shape of a XMCD spectrum is clearly different between the spectrum of Fe in this sample and that of the pure iron. This result confirms that ␣-iron impurity, which was not detected by X-ray powder diffraction, is not a origin of the relatively large magnetization of this alloy. EXAFS spectra χ (k) for the Mn and Fe K-edges are shown Fig. 2(a). The peak positions in the Mn spectrum and Fe one coincide within 0.02 Å−1 in the k range up to 7 Å−1 . In the both spectra, the first peak locates at 2.51 Å−1 , and the second one at 4.3 Å−1 . A small hump indicated by a arrow in Fig. 2(a) is observed at 3.20 Å−1 in the Mn spectrum, while no clear one is observed in the Fe spectrum. The absolute value functions of the Fourier transformation (FT) of k3 χ(k) are shown in Fig. 2(b). The position of the main peak is nearly the same for the Mn spectrum as for Fe. In the Mn spectrum, there is observed a clear shoulder indicated by an arrow p in Fig. 2(b), while a very small shoulder indicated by an arrow q appears at the corresponding position in the Fe spectrum. In the range of R above these shoulder, both spectra are quite different. The imaginary part of the FT (Fig. 2(c)) indicates that the main peak positions of the Mn and Fe spectrums are 2.00 and 1.96 Å,

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Fig. 3. XMCD spectra at the Mn K-edge: B, C, D and MnB indicates an unstrained icosahedral Al50 Pd15 Mn20 B15 alloy, an strained icosahedral Al42.5 Pd7.5 Mn30 B20 alloy, an orthorhombic Al30 Mn40 B30 alloy and a MnB compound, respectively.

Fig. 2. EXAFS spectra in a decagonal Al40 Mn25 Fe15 Ge20 alloy; (a) χ (k) spectra; (b) absolute values of Fourier transformation of k3 χ (k) and (c) imaginary parts of Fourier transformation.

respectively. From the result of EXAFS, the nearest neighbors of Mn and Fe atoms were analyzed to be at the same distance. For father atomic shells, the distances are not in good agreement. 3.2. XMCD and EXAFS in Al–Pd–Mn–B alloys The XMCD spectra for Al–Pd–Mn–B systems are shown in Fig. 3. The spectrum in MnB [9] is also shown in Fig. 3. A MnB compound have a B27 structure and a ferromagnetism, in which the magnetic moment of Mn is 1.92␮B [10]. The ratio of XMCD to the total absorption are about −3×10−4 in the samples B, C and D. Assuming the amplitude of XMCD profile is proportional to the magnetic moment, it is roughly estimated that the magnetic moments of the samples B, C and D are about 0.8␮B . The XMCD profiles of the sample B (an unstrained quasicrystal), C (a strained quasicrystal) and D (an orthorhombic approximant) are in accordance within the experimental errors in the energy range up to 15 eV. Because in this energy region a XMCD spectrum reflects directly the dif-

ferences of spin densities of the empty state, the electronic structure in these alloys are indicated almost the same. A slight difference is observed near at neighborhood of 18 eV where there are no peaks in the crystalline samples (D and MnB) in spite that there appeared small positive peaks in the quasicrystalline samples (B and C). Because the magnetic structure is also reflected in this energy region, there should be small difference of magnetic structure between quasicrystals and crystals. The ratio of XMCD to the total absorption are about −3×10−4 in the samples B, C and D. Assuming the amplitude of XMCD profile is proportional to the magnetic moment, it is roughly estimated that the magnetic moments of the samples B, C and D are about 0.8␮B . EXAFS spectra χ (k) of Mn are quite similar below 5 Å−1 in the three samples, as shown in Fig. 4(a), which indicates that a Mn atom is in the similar environment. The peak positions of all the samples coincide within the difference of 0.08 Å−1 in the k range up to 7 Å−1 . The intensities of main peaks in the k range from 2 to 3 Å−1 are slightly different each other. Some intensity difference is observed between the quasicrystals and the approximant at the peaks of 5.3 and 4.8 Å−1 , which positions are indicated by arrows in Fig. 4(a). In the (FT) functions, which are shown in Fig. 4(b), the coordination number of the first shell accord within 1% for the both quasicrystals. The integrated intensity ratio of the peak q to the peak p of the samples B, C, and D are 0.32,0.28 and 0.18, respectively. It means that the coordination number of the second nearest neighbor in the approximant is relatively smaller than those of the quasicrystals. The imaginary parts of FTs in Fig. 4(c) indicate that the distances to the first and the second nearest neighbors accord within 0.6 Å.

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Mn atom. In the father region, the environment around Mn and Fe are more different. In Al50 Pd15 Mn20 B15 , Al42.5 Pd7.5 Mn30 B20 and Al30 Mn40 B30 alloys, the intensity of XMCD around Mn the K-edge were about −3×10−4 . The Magnetic moments of Mn atoms in these alloys were estimated to be 0.8␮B . The difference of an empty state density between the major and minor bands is almost the same up to 15 eV from EF . The difference was detected between the magnetic structures of the quasicrystals and the crystals (the approximant and the MnB compound). From the result of the EXAFS analysis, the distances to the first nearest neighbor and the second nearest neighbor found to be the same for these samples within the experimental error. The density of the second neighbor shell of the approximant is, however, considerably smaller than that of the quasicrystals. The existence of phason strain field have no appreciable influence on the magnetic moment and the local environment of Mn.

Acknowledgements The authors are grateful to T. Iwazumi for his technical support of all the synchrotron experiments in KEK. XMCD and EXAFS was measured under the approval of the Photon Facility Program Advisory Committee (Proposal No. 97G282). This work is financially supported in part by a Grant-in-Aid from the Ministry of Education, Science, Sports and Culture, Japan. It is also supported in part by a Grant-in-aid from the Light Metal Educational Foundation. They also thank Dr. Mizumaki for providing the XMCD data in MnB. References

Fig. 4. EXAFS spectra in Al–Pd–Mn–B system: (a) χ(k) spectra; (b) absolute values of Fourier transformation of k3 χ(k) and (c) imaginary parts of Fourier transformation, B, C, D indicates an unstrained icosahedral Al50 Pd15 Mn20 B15 alloy, a strained icosahedral Al42.5 Pd7.5 Mn30 B20 alloy and an orthorhombic Al30 Mn40 B30 alloy, respectively.

4. Summary In Al40 Mn25 Fe15 Ge20 , the intensities of XMCD at Mn K-edge and Fe K-edge were −7×10−5 and −5×10−5 , respectively. It means that Mn and Fe in this alloy contribute to the magnetism to the same degree. From the result of EXAFS, the nearest neighbors of Mn and Fe atoms were analyzed to be at the same distance. The environment around Mn and Fe were found not so similar, except for the nearest neighbor distance. The density around an Fe atom in the second nearest neighbor is relatively smaller than that around a

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