Magnetic properties of the hydrides of YFe3-related compounds

Journal of Magnetism and Magnetic Materials 104-107 (1992) 729-730 North-Holland

Magnetic properties of the hydrides of YFe3-related compounds T. F u t a k a t a ~, M. Y a m a g u c h i ~, I. Y a m a m o t o and S. M o r i m o t o c

a

M.I. B a r t a s h e v i c h

a,1 T.

Goto

b

A. Ito c

Faculty of Engineering, Yokohama National Unit,ersity, Tokiwadai, Hodogaya-ka, Yokohama 240, Japan h Institute for Solid State Physics, The UniL'ersity of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan ¢ Department of Physics, Faculty of Science, Ochanomizu Uniuersity, Otsuka, Bunkyo-ku, Tokyo 112, Japan Magnetization and M6ssbauer spectra were obtained at 4.2 K for Y(Fe x yCoy)3H x (y = 0, 0.2, 0.4; x = 0, 1.2, 1.8, 3.2, 3.8). As a result of hydrogen absorption, the Fe moment is enhanced for y = 0, Fe and Co moments are, respectively, increased and decreased for y = 0.2 and both moments are scarcely influenced at y = 0.4. These effects are discussed in relation to the band structure and the 3d band occupancy. T h e Y T 3 (T = Fe, Co, Ni) c o m p o u n d s are typical itinerant f e r r o m a g n e t s and, chemically, excellent hydrogen-absorbers. H y d r o g e n absorption strongly affects t h e i r m a g n e t i c properties. A recent high-field m a g n e t i z a t i o n m e a s u r e m e n t [1,2] indicates t h a t the magnetic states of the Y T 3 p s e u d o b i n a r y c o m p o u n d s with n3d _> 7.4 as well as t h e i r hydrides are i n t e r p r e t e d by the density-of-states ( D O S ) curve of YCo 3 as a first approximation, w h e r e n3d stands for 3d electrons p e r 3d atom a n d is assumed to be 7, 8 a n d 9 for Fe, Co and Ni, respectively. In this p a p e r we investigate the hydrogen effects on the m a g n e t i c p r o p e r t i e s of the Fe-rich c o m p o u n d s with n3d < 7.4, t h a t is, Y F e 3 , Y(Fe0.sCo0.2) 3 a n d Y(Fe0.4Co0.6) 3. T h e c o m p o u n d s were p r e p a r e d by the same m e t h o d as previously r e p o r t e d [1]. F o r each comp o u n d , four kinds of hydrides (x = 1.2 a n d 1.8 in the 13-phase and x = 3.2 a n d 3.8 (3.6) in the ~-phase) were used for m a g n e t i z a t i o n a n d M 6 s s b a u e r m e a s u r e m e n t s . Powder X-ray diffraction lines of these hydrides can be indexed by the PuNi 3 structure. M a g n e t i z a t i o n was m e a s u r e d at 4.2 K in pulsed fields up to 36 T and M 6 s s b a u e r spectra were o b t a i n e d at 4.2 K in zero field. In the M 6 s s b a u e r experiment, we used YFe2.gH x instead of Y F e 3 H x since Y F e 3 showed extra absorption lines caused by metallic iron. Fig. 1 p r e s e n t s the m e a s u r e d m a g n e t i z a t i o n curves: N o n e of t h e m show a m e t a m a g n e t i c transition in high fields. T h e s e m a g n e t i z a t i o n curves give the spontan e o u s m o m e n t M~ (the m o m e n t e x t r a p o l a t e d from high fields to zero field) a n d the high field m o m e n t Mhf vs. x shown in fig. 2. It is notable t h a t the hydrogen u p t a k e from x = 0 to 1.8 leads to a considerable increase in the m o m e n t for Y F e 3 (M~ is 1.55/x B at x = 0 a n d 1.88#B at x = 1 . 8 ) while it leads to a decrease in the m o m e n t for Y(Feo.sCo0.2) 3 in spite of an

Fe-rich c o m p o u n d ( M s is 1.58#B at x = 0 and 1.48/x B at x = 1.8). In Y F e 3 H ~, M s takes a m a x i m u m a r o u n d x = 1.8-3.2. Moreover, Ms tends to a smaller value above x = 3.2. This m o m e n t d e c r e a s e in hydrogendense hydrides is exhibited by the o t h e r two comp o u n d s and is c o m m o n to o t h e r F e - b a s e d hydrides [3]. Fig. 3 exemplifies the variation of the M 6 s s b a u e r spectra u p o n hydrogen uptake. T h e spectra were analyzed by four sextets due to the Fe sites in the Y F e 3 crystal, 12(h I + h2), 6h3, 6c a n d 3b [4]. Here, we assumed that in all cases the spins lay in the c-plane as is k n o w n to be the case in Y F e 3 H ~ [5] a n d Y(Fe I yCoy) 3 (y < 0 . 5 ) [6]. G e n e r a l satisfactory fits were o b t a i n e d a l t h o u g h r e a s o n a b l e fitting c a n n o t be X= j3,2

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40 ('r) Fig. 1. Magnetization curves at 4.2 K for the Y(Fe t _yCoy)3H x system. MAGNETIC

l On leave from Institute of Physics and Applied Mathematics, Ural State University, Sverdlovsk, USSR.

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T. Futakata et al.

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HYDROGEN COMPOSITION, x Fig. 2. Dependences of the spontaneous moment (open symbols) and the high-field moment (closed symbols) on hydrogen composition x for YFe3H ~ (0, o), Y(Fe0.sCo02)3H~ ( n , II) and Y(Feo.~Coo.4)3H ~ (~, A). made for some hydrogen-rich hydrides because of the complex nature of lines. Then, the average hyperfine fields Hhpf at the Fe nuclei were derived as illustrated in fig. 4. In YFe3Hx, Hhp f is approximately proportional to M s obtained from the magnetization measurement. In Y(Feo.6COo.4)3, Hhp f varies scarcely with x below x = 1.8: This tendency is consistent with that of M~. But Hhpf of Y(Feo.sCoo.2) 3 is increased upon hydrogenation in opposition to a gradual decrease in M s. Therefore, the Fe moment in Y(Feo.sCoo.2) 3 is enhanced by hydrogen absorption like YFe3; on the other hand, the Co moment is suppressed. Y(Feo.6Coo.4) 3 is believed to be a saturated ferromagnet (SF) where the Fermi level in the majority spin -8

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This work was partially supported by Grant-in-Aid for Scientific Research (No. 61550008) from the Ministry of Education, Science and Culture of Japan. References

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states, Ef+, is located at the top of the 3d band [1]. With increasing x for Y(Fe0.6Co0.4)3Hx, HhN keeps about 260 kOe and M s is scarcely decreased. This means that both Fe and Co moments maintain the SF state against hydrogen absorption. In addition, there are no charge transfer which change the 3d band occupancy on both Fe and Co sites. While the M6ssbauer spectra showed small and positive changes in the isomeric shift upon hydrogenation ( + 0 . 2 to 0.3 m m / s per H). However, this does not immediately means that the moment increase is caused by the decrease in 3d band occupancy [7]. It is hard to consider that the nature of charge transfer between hydrogen and the 3d state varies critically above and below n3a = 7.4. Thus, no charge transfer occur in Y F e 3 H x and Y(Fe0.sCo0.z)3Hx like Y(Fe0.6Co0a)3H~. We attribute the opposite hydrogen effects for the Fe and Co moments to different roles of the reduction in the 3 d - 3 d contact upon hydrogenation for both moments.

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i I I 1 2 3 4 HYDROGEN COMPOSITION, × Fig. 4. Dependence of the average hyperfine field at Fe nuclei on hydrogen composition x for YFe2.gH ~ (0), Y(F%.sCoo 2) H, (El) and Y(Feo.sCoo.2)3H ~ qa.).

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Fig. 3.57Fe M6ssbauer spectra of Y(Feo sCoo.2)3H x.

[1] T. Goto, M. Yamaguchi, T. Kobayashi and T. Yamamoto, Solid State Commun. 77 (1991) 867. [2] M. Yamaguchi, T. Kobayashi, I. Yamamoto and T. Goto, ibid. 77 (1991) 871. [3] P.J. Viccaro, G.K. Shenoy, B.D. Dunlap, D.G. Westlake and J.F. Miller, J. de Phys. 40 (1979) C2-198. [4] A.M. van Kraan, P.C.M. Gubbens and K.H.J. Buschow, Phys. Stat. Sol. (a) 31 (1975) 495. [5] M.I. Bartashevich et al., (in preparation). [6] S.K. Arif, D.St.P. Bunbury and G.J. Bowden, J. Phys. F 5 (1975) 1792. [7] K.H.J. Buschow, P.H. Smit and R.M. van Essen, J. Magn. Magn. Mater. 15-18 (1980) 1261.