Magnetic phase transitions in DyAg2Si2

Magnetic phase transitions in DyAg2Si2

PHYSICA ELSEVIER Physica B 213&214 (1995) 312 314 Magnetic phase transitions in DyAg2Si2 M. OhashP'*, K. Koizuka a' ~, H. Onodera a, H. Yamauchi a, ...

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PHYSICA ELSEVIER

Physica B 213&214 (1995) 312 314

Magnetic phase transitions in DyAg2Si2 M. OhashP'*, K. Koizuka a' ~, H. Onodera a, H. Yamauchi a, Y. YamaguchP, T. Kaneko a, S. Funahashi b "lnstituteJbr Materials Research. Tohoku University. Katahira, Sendai 980-77, Japan bDepartment of Materials Science and Engineering, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-11, Japan

Abstract DyAg2Si 2 has been studied by powder neutron diffraction. The magnetic phase transition T, = 4.3 K and the N6el temperature T N = 10.3 K have been determined by the temperature dependences of the intensities of the (x2 012)and (0 la 1) magnetic peaks. Between T, and TN, the magnetic structure is a collinear antiferromagnet with k I = (0z0), in which two kinds of Dy moment aligned along the a-axis,/1 ~ = 6.7,uB and #z = 3.4#B (at 7 K) are coupled antiferromagnetieally with the sequence T,xT~s along the b-axis. Below T,, the two propagation vectors k2 = (½0 ½) and k~ give a non-collinear antiferromagnet with two kinds of Dy moment of lq -- 8.9#B and tz2 = 7.5#B (at 1.7 K).

DyAg2Si2 has the body-centered tetragonal crystal structure of ThCr2Si2-type with the space group I4/mmm [1], in which the Dy, Ag and Si atoms occupy the 2(a),4(d), and 4(c) sites, respectively. The c-planes containing the same atoms are stacked in the sequence DySi-Ag-Si-Dy-. Sakurada [1] has suggested from magnetometric and electrical-resistivity measurements that this compound is an incommensurate antiferromagnet with Neel temperature Tr~ = 10 K, and becomes commensurate below T, = 4 K The resistivity versus temperature curve clearly shows anomalies characteristic of the order-order transition T, and the Neel temperature Tr~ as shown in Fig. 1, although the transition at TN is obscure in the susceptibility versus temperature curve. On the other hand, Onodera et al. [2] reported from the 161Dy M6ssbauer spectrum of DyAg2Si2 that there are magnetically two kinds of Dy atoms, with hyperfine fields of 808 and 712MHz. In the present study, neutron-diffraction measurements have been carried out on a pow* Corresponding author. 1Also at: Fujitu Laboratories, Atsugi, Morinosato-Wakamiya, Atsugi, Kanagawa 293-01, Japan.

der sample of DyAg2Si2, and the magnetic structures in both magnetic states and the magnetic phase transitions between them determined. The Dy and Ag metals in the sample preparation were of purity 99.9% and the Si sample was 99.999% pure. The compounds were synthesized by conventional argon arc-melting. The product was turned over and remelted 5 times, followed by annealing at 850°C for a week, for improved homogeneity. X-ray diffraction measurement using Fe Kct radiation showed that the diffraction lines can be indexed in the ThCr2Si2-type structure. The aand c-lattice parameters were obtained to be 4.11 .A and 10.75 ~,, respectively, in good accordance with those of previous work. Fig. 2 shows the temperature dependence of the magnetic susceptibility of powder DyAg2Si2, measured using a vibrating-sample magnetometer. The transition at TN observed in the p - T curve is also not so clear in this experiment. The cusp at T, = 4.3 K is clearly observed and corresponds to the anomaly in the electrical-resistivity experiments. The inverse susceptibility satisfies the Curie Weiss law above 100K and gives 0p = - l l . 7 K and /~erf= ll.7#B/Dy. Neutron-diffraction experiments

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M. Ohashi et aL /Physica B 213&214 (1995) 312 314

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were performed using the TAS-2 spectrometer, in doubleaxis mode, installed in the J R R - 3 M Guide Hall at JAERI. The incident neutron beam had a wavelength of 1.45 A, obtained using a PG(002) monochromator. The powder sample was sealed in a parallel-sided 0.5 mm thin container with He gas. The bottom diffraction pattern in Fig. 3, taken at 78 K, exhibits only the nuclear scattering based on the ThCrzSiz-type structure and gives the lattice constants and the Si atom position parameter a = 4.110A, c = 10.653 A, Zsi = 0.393 with R = 2%. The nuclear scattering lengths were taken from Ref. [3]. The diffraction pattern at 7K, as shown in the middle of Fig. 3, shows additional magnetic peaks, which satisfy the indices (hk/31) with h + k + l = e v e n and give k~ = (0z0). The magnetic unit cell requires triple the length of the chemical one along the b axis. The magnetic ordering can be explained as a stacking of two kinds of ferromagnetic layers with the sequence 1'~~T+~ " along the b-axis as shown in the upper part of Fig. 4. The two kinds of Dy magnetic moment have 6.7 + 0.41ts and 3.4 + 0.2gB (at 7K) aligned along the a-axis with

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M. Ohashi et al./ Physica B 213&214 (1995) 312-314

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Fig. 5. Temperature dependence of the peak intensity of the characteristic (½0½) and (0~ l) magnetic reflection. require another kind of magnetic structure. It is antiferromagnetic with Dy moments aligned along the b-axis, coupled ferromagnetically in the (101) plane and coupled antiferromagnetically between adjacent (1 0 1} planes. The intensities of the representative magnetic reflections of the (0½ 1) and (½0½) seem to be constant below 2.5 K, as seen in Fig. 5. Therefore, the low temperature phase is considered to be in a single magnetic phase. The synthesis of sub-magnetic structures with k~ and k2 results in the complicated non-collinear antiferromagnetic structure illustrated in Fig. 4. The Dy moments of 8.9 + 0.3yB and 7.5 _+ 0.2#B are arranged noncollinearly in the c-plane where the ratio of two kinds of moments is 1 : 2. In other words, the smaller magnetic moments run in pairs along the b-axis. This result agrees well with the fact that 161Dy M6ssbauer spectrum of this material is best interpreted by two distinct subspectra with hyperfine parameters of 808 and 712 MHz at 3 K and a ratio of 0.3 : 0.7 between the two kinds of moment. Fig. 5 shows the temperature dependence of the peak intensity of the (0 i3 1) and (½0 ½}magnetic reflections. The

collinear to non-collinear transition T~ = 4.3 K and the Neel temperature T N = 10.3 K are defined by the appearance of the (½0 ½) and (0 ½ 1) magnetic reflections, respectively. The non-collinear magnetic-moment arrangement seems to develop rapidly just below T, and to approach a balanced state with decreasing temperature. The electrical resistivity changes abruptly at T, as seen in Fig. 1. Then, this transition is considered to be first-order. According to the CEF parameter estimated from the ~55Gd Mrssbouer experiments of the GdAg2Si2 [5], the single ion anisotropy of the Dy atom may predict that the easy direction of the Dy moments of DyAgzSi2 is in the c-plane. It is presumed that the complicated non-collinear structure can be attributed to weak anisotropy in the c-plane of the tetragonal structure. The authors would like to thank Messrs. N. Minakawa, Y. Shimojo and K. Nemoto for their kind support in doing the neutron-diffraction experiments.

References

[1] S. Sakurada, Masters thesis, Tohoku University, 1990. [2] H. Onodera, A. Murata, M. Koizuka, M. Ohashi and Y. Yamaguchi, Science Report of the Research Institutes, Tohoku University, A40 (1994) 177. [3] V.F. Sears, in: Methods of Experimental Physics, eds. K. Skold and D.L. Price, Vol. 23, Part A (Academic Press, Orlando, 1986)p. 521. [4] A.J. Freeman and J.P. Desclaucx, J. Magn. Magn. Mater. 12 (1979) 11. [5] M.W. Dirken, R.C, Thiel and K.H.J. Buschow, J. LessCommon Met. 147 (1989) 97; G. Czjzek, V. Oestreich, H. Schmidt, K. Latka and K. Tomala, J. Magn. Magn. Mater, 79 (1989) 42.