Magnetic properties of Gd-substituted Y2Co7 hydrides

0038-1098/87 $3.00 + .00 © 1987 Pergamon Journals Ltd.

Solid State Communications, Vol. 63, No. 4, pp. 285-287, 1987. Printed in Great Britain.

MAGNETIC PROPERTIES OF Gd-SUBSTITUTED Y2Co7 HYDRIDES M. Yamaguchi and T. Ohta Faculty of Engineering, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240, Japan T. Goto and T. Sakakibara The Institute for Solid State Physics, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan and T. Katayama Electrotechnical Laboratory, Sakuramura, Ibaraki 305, Japan

(Received 18 February 1987 by J. Kanamori) The magnetization of (Yl_yGdy)2Co7Hx(y = 0.1, 0.2, 0.4 and 1; 0 ~< x ~< 3) has been measured at 4.2K up to 30T. The exchange field introduced by the Gd substitution enhances the Co moment in the hydrides. The origin of the disappearance of ferromagnetism in the flL hydride (y = 0; x = 1.7) has been discussed. INTRODUCTION THE MAGNETIC PROPERTIES OF Y2Co7 drastically vary upon hydrogen absorption. The hydrogen-free compound is ferromagnetic, but the flL hydride, Y2 Co7 Hi. 7, loses this property. However, the tim hydride, Y2Co7H3.0, recovers ferromagnetism. In addition, the YCo3Hx system exhibits similar features with increasing x [1, 2]. We believe that the origin of the hydrogen-induced variation in the magnetic properties is common to the two systems [3]. Previous magnetization measurements of Nisubstituted Y2Co7 hydrides have revealed that hydrogen absorption leads to the increase in the densityof-states (DOS) at the Fermi level and that the DOS in the flH hydride becomes large enough to induce ferromagnetism [4, 5]. In this paper, we observe the influence of the substitution of Gd for Y on the Co moment to study the origin of the disappearance of ferromagnetism in the flL hydride. It is expected that this substitution causes the increase in the exchange field acting upon Co atoms. EXPERIMENTAL The compounds of (Yl_yGdy)2Co7(y = 0.1, 0.2, 0.4 and 1) were made by arc-melting, followed by annealing in argon. X-ray powder diffraction showed that they were single phase with the Gd2Co7-type structure. For each compound, the formation of hydrides was confirmed by measuring a pressurecomposition isotherm (PCT) at 348K up to 5MPa. Here, the hydrogen composition is denoted by x in the

formula (Yl_yGdy)2Co7Hx. The PCT's for the compounds at y = 0.1, 0.2 and 0.4 indicate that they, as well as Y2Co7Hx, form the fl phase between x = 1.7 and 3.0. In the PCT for GdECOTHx, however, the fl phase exists only between x -- 2.0 and 2.6. Hydrogenated samples were prepared in the composition range less than x = 3 for the magnetization measurement. The magnetization was measured at 4.2K in pulsed fields up to 30T by the induction method des" cribed elsewhere [6]. RESULTS AND DISCUSSION Figure 1 gives the magnetization curves at 4.2K for the hydrogen-free compounds and their hydrides. The salient features of them are as follows: The hydrides of the compounds at y = 0.1 and 0.2 show the reduction in the spontaneous magnetization and the occurrence of metamagnetic transition in the same manner as the Y2Co 7 hydride. In contrast to this, the spontaneous magnetization of the compound at y = 0.4 is independent of the hydrogen composition, indicating no metamagnetic transition. In the compound at y = 1, Gd2Co7, the spontaneous magnetization of the hydrides at x = 2.0 and 2.4 are almost the same as that of the hydrogen-free compound although their magnetization curves tend to increase in higher fields. The sample at x = 2.9 has slightly larger magnetization. Probably, it contains the ~, phase (x -~ 6). In addition to these results, we refer to the earlier magnetic data on the Y2CoTHx system reported in [1, 2].

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M A G N E T I C PROPERTIES OF Gd-SUBSTITUTED Y2Co7 H Y D R I D E S 80

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Fig. 1. Magnetization curves at 4.2K for the (Y~_yGd>.)=CovHx system at (a) y = 0.1, (b) y = 0.2, (c) y = 0.4 a n d ( d ) y = 1. For the hydrogen-free compounds, extrapolating the magnetization curves to zero field gives the spontaneous moment of 8.4, 6.3, 5.8, 3.0 and 2.7#n/F. U. at y = 0, 0.1, 0.2, 0.4 and 1 respectively. These values can be explained by the ferrimagnetic arrangement between the Co and the Gd spins. This consideration agrees with that reported by Burzo et al. [7]. Assuming that such a ferrimagnetic arrangement is still maintained in their hydrides and that the Gd atoms contributes 7/~B to the moment even after absorbing hydrogen, we obtain Fig. 2 summarizing the variation in the Co moment in zero field. It is obvious that the Gd substitution enhances the Co moment in the fl phase. Especially, the Co moment in the flL hydride is approximately proportional to the Gd content in the region less than y = 0.4. As the charge transfer of the Gd atom is thought to be similar to that of the Y atom, this enhancement is due to the exchange field from the Gd spin. The field assists the splitting between the majority and the minority bands of the 3d state. This fact implies that the disappearance of ferro-

magnetism in Y2Co7HI.7is caused by a weakening in the exchange interaction between the Co spins upon the absorption of hydrogen. 2 ---4J

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Vol. 63, No. 4 MAGNETIC PROPERTIES OF Gd-SUBSTITUTED Y2Co7 HYDRIDES The metamagnetic transition shown by the flL hydrides at y = 0.1 and 0.2 is considered to be the itinerant electron metamagnetism of the Co moment as observed in the Y2C07 and the YC03 hydrides [2]. Some paramagnetic Co atoms achieve a spin-splitting state above the critical field as a result of the sum of the applied field and the exchange field. The gradual increase in the magnetization observed in the Gd2CoTHx samples is interpreted by the field-induced decrease in the Co moment, provided that the ferrimagnetic spin structure is not broken in higher fields. The Co moment decreases by approximately 0.2 ~t~ between zero field and 30T. It is likely that the total field at the position of the Co atom decreases with increasing field because the applied field is antiparallel to the Co moment in Gd2CoTHx, where the moment of the Gd sublattice is larger than that of the Co sublattice. In conclusion, all results indicate that the Co moment depends strongly upon the field polarizing it. The exchange field produced by the Gd spin assists the increase in the Co moment. The disappearance of

287

ferromagnetism in the flL hydride (y = 0; x = 1.7) is caused by a weakening in the exchange interaction upon hydrogen absorption. The similar conclusion can be obtained in the case of the YCo3Hx system. REFERENCES 1. 2. 3. 4. 5. 6. 7.

M. Yamaguchi, H. Ikeda, T. Ohta, T. Katayama & T. Goto J. Less-Common Met. 106, 165 (1985). M. Yamaguchi, H. Ikeda, T. Ohta, T. Goto & T. Katayama, Solid State Commun., 53, 383 (1985). M. Yamaguchi, D.K. Ross, T. Goto & T. Ohta, Z. Phys. Chem. N.F. 145, S.101 (1985). T. Goto, T. Sakakibara & M. Yamaguchi, J. Mag. Magn. Mater. 54-57, 1085 (1986). M. Yamaguchi, T. Ohta, T. Goto, T. Sakakibara & T. Katayama, J. Less-Common Met., 130, 47 (1987). N. Yamada, S. Takeyama, T. Sakakibara, T. Goto & N. Miura, Phys. Rev. B34, 4121 (1986). E. Burzo, V. Pop & N. Plugaru, J. Less-Common Met. 111, 97 (1985).