Crystal distortion and magnetic structure of γ-Mn(Au) alloys

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Journal of Magnetism and Magnetic Materials 196-197 (1999) 663-664

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Crystal distortion and magnetic structure of 7-Mn(Au) alloys T. Hori a'*, Y. Tsuchiya b, S. Funahashi b'~, Y. Shimojo b, H. Shiraishi a, K. Hojou b, Y. N a k a g a w a c aShibaura Institute of Technology, Oomiya, Saitama 330-8570, Japan bJapan Atomic Energy Research Institute, TokaL Ibaraki 319-1106, Japan ~Tohoku Institute of Technology, Yagiyama, Sendal 982-8577, Japan

Abstract y-Mn alloys containing Au have been examined by magnetic measurements and X-ray and neutron diffraction experiments. The 9 at% Au alloy has a face centered orthorhombic lattice with a = 3.853, b = 3.794 and c = 3.731/~ at 11 K, and shows a non-collinear antiferromagnetic structure with 4 sublattices; the three-axis components of the magnetic moments are as follows:/~a = 0, #b = 1.10 and #c = 1.91 ~tB.A revised phase diagram for y-Mn(Au) alloy system with 8-16 at% Au is proposed. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Antiferromagnetism; Crystal distortion; Neutron diffraction

As is well known, most Mn-rich 7-Mn alloys undergo a distortion from the face centered cubic structure to a face centered tetragonal (FCT) structure with c/a < 1 below the N6el temperature [1]. In some cases a distortion to a FCT with c/a > 1 is also observed [2]. In the 7-Mn(Ni) alloys, there is a face centered orthorhombic (FCO) region in addition to the FCT regions with c/a < 1 and c/a > 1 [3]. Recently, the present authors [4] have found that these three regions exist in the 7-Mn(Ga) alloys, and determined a non-collinear magnetic structure with 4 sublattices for the FCO phase. The a-, b- and c-axes components of the magnetic moments, Pa, #b and Pc are as follows: #2 > / ~ > #2 = 0 in the FCO structure with a > b > c. The FCO phase of the y-Mn(Au) alloys was first reported by Cowlam et al. [5]. They made neutron diffraction experiments and assumed a simple antiferromagnetic structure, disregarding a possibility of the non-collinear structure. The phase diagram of the 7Mn(Au) alloys was determined by Harders et al. [6] on

* Corresponding author. Tel.: + 81-48-687-5162;fax: + 8148-687-5163; e-mail: [email protected]. 1present adress: Department of Technology Development ATOX Co. Ltd., 1408Takada, Kashiwa, Chiba 277-0861,Japan.

the basis of X-ray diffraction and Young's modulus measurements. In the present paper, more detailed X-ray and neutron diffraction experiments and magnetic susceptibility measurements on the 7-Mn(Au) alloys are reported. Methods of the sample preparation and magnetization measurements were similar to those described in our earlier paper [4]. The powder sample for X-ray and neutron diffraction was prepared by filing the ingot. In order to eliminate residual stress after filing, the powder sample was annealed for 2 h at 1000°C and quenched in water. Neutron diffraction experiments were made by using the HRPD and TAS II diffractometers in the JRR3M reactor at JAERI. The HRPD has a higher resolution than the TAS II. Fig. 1 shows a neutron diffraction pattern for the 9 at% Au alloy at 11 K obtained by the HRPD diffractometer (wavelength 2 = 1.823/~). The pattern can be indexed by assuming the FCO structure with a = 3.853, b = 3.794 and c = 3.731 ~ and non-collinear antiferromagnetic structure similar to that of the y-Mn(Ga) alloy [4]. The components of the magnetic moments, #a = 0, #b = 1.10 and #c = 1.91 IxB/Mn atom, were deduced from the magnetic diffraction lines 110 and 101 shown in the inset of Fig. 1. We also measured the temperature dependence of these lines using the TAS II

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T. Hori et al. / Journal of Magnetism and Magnetic Materials 196-197 (1999) 663-664 6000

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Fig. 1. Neutron diffraction pattern for y-Mn(Au) alloy with 9 at% Au at 10 K (), = 1.823 A).

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Fig. 2. Temperature dependence o~ the 110 and 101 intensities of the neutron diffraction for 7-Mn(Au) alloy with 9 at% Au. diffractometer (2 = 2.376,~). Although the 110 and 101 reflections were not resolved, we obtained the intensities, l l l o and 1101, by curve-fitting analysis using the double Gaussian functions. The results are shown in Fig. 2. The temperature dependence of the lattice parameters a, b, and c was determined by X-ray diffraction experiments using the 200, 202 and 002 reflections. Fig. 3 shows the results for the 9 at% Au alloy, which is orthorhombic below To ( = 230 K), and tetragonal with c/a < 1 between To and Tt ( = 400 K). The neutron diffraction intensities shown in Fig. 2 are consistent with the crystal distortion; I~o~ and Ii10 vanish at To and T , respectively. The N6el temperature TN coincides with the tetragonal distortion temperature T,. The 1 2 a t % Au alloy has a cubic lattice at room temperature and distorts to a tetragonal lattice with c/a > 1 below Tt ( = 230 K). The neutron diffraction pattern at 10 K exhibits the intense 101 or 011 line without the 110 line, suggesting a simple antiferromagnetism with the magnetic m o m e n t lying on the c plane. The value of the m o m e n t determined from the 101 line intensity is 2.20 pB/Mn atom. The 101 line intensity monotonically decreases with increasing temperature up to the N6el temperature TN ( = 300 K) which is higher than the crystal distortion temperature Tt ( = 2 3 0 K ) . (There is no distinction between 101 and 110 above T,) The alloys containing more than 14 a t % Au have the cubic lattice down to 15 K. In the neutron diffraction pattern for the 15 a t % Au alloy, the 110 line with a diffuse magnetic scattering was observed up to TN ( = 150 K). A strong diffuse magnetic scattering was also observed around the 100 reflection. The magnetic mo-

Fig. 4. Phase diagram of the y-Mn(Au) alloy system. C (cubic). tl {tetragonal with c/a < lh t2 (tetragonal with c/a > 1). TN. Nbel temperature.

ment is estimated to be 0.7 laB/Mn atom at 10 K. The 110 line was not observed for the 18 at% Au alloy even at 10 K. We have determined the N6el temperature also by the magnetic susceptibility measurements. The phase boundaries of the lattice distortion have been determined mainly by the X-ray diffraction. A phase diagram for the y-Mn(Au) alloys thus obtained is shown in Fig. 4. This phase diagram is not contradictory to the results of Cowlam et al. [5], but evidently different from the phase diagram of Harders et al. [6]. Jo et al. [7] have proposed a theoretical phase diagram and magnetic structures for various y-Mn alloys on the basis of Landau expansion of the free energy. For example, the components of the magnetic moments along the three crystallographic axes/~,, lib and #c in the F C O structure with a > b > c are as follows; ~2 > #~ > #~. The results of the present experiments are consistent with this theory.

References [1] G. Bacon, L.W. Dunmur, J.H. Smith, R. Street, Proc. Soc. R. London A 241 (1957) 223. [2] H. Uchishiba, J. Phys. Soc. Jpn. 31 (1971) 436. [3] N. Honda, Y. Tanji, Y. Nakagawa, J. Phys. Soc. Jpn. 41 (1976) 1931. [4] T. Hori, Y. Morii, S. Funahashi, H. Niida, M. Akimitsu, Y. Nakagawm Physica B 213 & 214 (1995) 354. [5] N. Cowlam, G.Y.M. Al-Shahery, Physica B 86-88 (1977) 267. [6] T.M. Harders, J.H. Smith, E.R. Vance, J. Magn. Magn. Mater. 15-18 (1980) 1177. [7] T. Jo, K. Hirai, J. Phys. Soc. Jpn. 55 (1986) 1614.