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Magnetism resurgence in the Y-Ni alloys studied on the YNi3 compound
M A G N E T I S M R E S U R G E N C E IN T H E Y - N i ALLOYS S T U D I E D O N T H E YNi 3 C O M P O U N D
D. GIGNOUX, R. LEMAIRE, P. M O L H O Laboratoire Louis Neel, CNRS, 166X, 38042 Grenoble-c~dex, Franee
and F. TASSET Institut Laue-Langevin, 156X, 38042 Grenoble-cedex, France
Whereas YNi 5 is a Pauli paramagnet a magnetism resurgence occurs for larger Y amounts, as observed in Y2Ni7 and YNi 3. YNi 3 is a very weak itinerant ferromagnet which behaves as ZrZn 2. Polarized neutron experiments show that the magnetism resurgence arises essentially from the 3d Ni electrons.
In the Y - N i compounds, the 3d moment decreases rapidly as the Y content increases. Especially YNi 5 exhibits only a strong enhanced Pauli paramagnetism [1]. However, for larger Y amount, a magnetism resurgence is observed in the Y2Ni7 and YNi 3 compounds [2]. Does this resurgence originate from Y or Ni atoms? We have studied it on YNi 3 (R3m, PuNi3-type structure) by means of measurements of magnetization, susceptibility, specific heat and resistivity, and polarized neutron diffraction experiments on a single crystal. Below 30 K, YNi 3 is ferromagnetic. The bulk magnetic properties are similar to those of ZrZn 2 [3] and are well described in the model of the very weak itinerant ferromagnetism [4]. Magnetization measurements at 4.2 K on a single crystal show that the anisotropy is weak. However, the weight of the crystal being too weak the magnetization variation was studied on a heavier polycrystalline sample. In order to neglect the anisotropy effects, magnetization curves were analysed only above 15 kOe. Magnetization is always weak and strongly field dependent. Arrott plots (fig. 1) have a linear variation in a large temperature range. At Tc = 30 K, the Arrott straight line passes through the origin. The variation of M2(0, T) as a function of T 2 is linear; the extrapolated value of M(0, 0) is then 0.04/.tB/Ni. Above 30 K, YNi 3 is paramagnetic. However, the thermal variation of the reciprocal susceptibility is not linear. The thermal variation of the specific heat does not exhibit any anomaly around T¢. Experimental points can be analysed as resulting from an electronic and a lattice contribution only. This is consistent with the model of the very weak itinerant ferromagnetism which foresees a gap in the specific heat at T c of ACm - - 0 . 3 J m o l - l K - I , value smaller than the experimental uncertainty. The thermal variation of
I
i
42K~15K
"Z/ 0K.230K K •
3
o
0~5 K
YN~3
50 K 2
K
,-,o 801
0
2000
HIM
4000
(Koe/PB/Ni)
/1
Fig. 1. Arrott plots (M2(H, T) as a function of HIM(H, T)) in YNi 3.
the resistivity presents a weak discontinuity in the slope at T c. Below T¢ this variation can be written as p(T) - p(0) = B T 2, where B = 1.3 × 10 -8 ~2cm K -z. This law at low temperature, a general feature for Fermi liquids, is also observed in transition metals. However, B is usually of the order of m a g n i t u d e 10 - I 1 f~cm K -2. In YNi3, as in ZrZn 2 (B = 4.7 x 10 - s f~cm K -2) [5], it is three order of magnitude larger. Polarized neutron experiments were performed at 4.2 K with a field of 13.2 kOe applied vertically and parallel to the a-axis (hexagonal cell) which is the easy magnetization direction. A Fourier projection of the magnetization density along this axis was obtained from the measured structure factors. This density may show that magnetization is localized on the three Ni sites (FNi(3b)= 0.057/~B; ~Ni(6C) ---- 0.078/.tB; //,Ni(8h) = 0.065~a ). Between these sites one observes slight oscillations which seem associated with a nonuniformity of the dif-
Journal of Magnetism and Magnetic Materials 15-18 (1980) 289-290 ©North Holland
289
290
D. Gignoux et al./ Magnetism resurgence in Y N i 3
fuse p o l a r i z a t i o n . T h u s the m a g n e t i s m in Y N i 3 essentially arises from the 3d N i electrons. T h e m a g n e t i s m resurgence o b s e r v e d in the Y - N i system c a n be u n d e r s t o o d using the results of b a n d c a l c u l a t i o n s in Y N i 2 [6]. T h e alloy is f o r m e d b y the a s s o c i a t i o n of a n a r r o w 3d b a n d a n d a wider 4d b a n d . T h e e l e c t r o n e g a t i v i t y difference of the c o n s t i t u e n t s leads to a transfer of the 4d electrons ( 0 . 6 6 / Y ) t o w a r d s the 3d b a n d of lower energy; the two b a n d s are b r o u g h t nearer. F u r t h e r m o r e a n h y b r i d i z a t i o n of 3 d - 4 d states occurs which gives rise to a strong positive curvature of the total density of states n(e) as illustrated on fig. 2. ] ' h e F e r m i level of the N i rich Y - N i c o m p o u n d s lies in this region very sensitive to the Y a m o u n t variations. As the Y c o n t e n t goes up the increase in the n u m b e r of h y b r i d i z e d states in this region c a n l e a d to a n increase of n(eF) in spite of the a d d i t i o n of transferred electrons. T h e l o c a l i z a t i o n of this m a g n e t i s m is d e d u c e d f r o m the local b a n d structures. O n the N i sites the b a n d is a l m o s t filled, the states have an a n t i b o n d i n g c h a r a c t e r . A m a g n e t i s m with a localization a n a l o g o u s to that of N i m e t a l m u s t be o b s e r v e d on the N i sites. O n the c o n t r a r y on the Y sites the b a n d is a l m o s t e m p t y a n d the states have a b o n d i n g character. T h e local d e n s i t y of states at the F e r m i level is very weak. T h e m a g n e t i c c o n t r i b u t i o n of the e l e c t r o n s of this local
rl c
-
EF E EF E EF E Fig. 2. Schematic representation of the total and local in Ni and Y densities of states in YNi3. nc is the critical value of n(E) above which the compound is ferromagnetic. b a n d is thus w e a k a n d s t r o n g l y delocalized. It c a n give a c c o u n t for the oscillations in the m e a s u r e d diffused polarization. F o r all the Y - N i alloys the local d e n s i t y of states on the Y sites will have the s a m e c h a r a c t e r ; thus a m a g n e t i s m localized on the Y sites is not realistic.
References [1] D. Gignoux, D. Givord and A. del Moral, Solid State Commun. 19 (1976) 891. [2] B. Barbara, D. Gignoux, D. Givord, F. Givord and R. Lemaire, Intern. J. Magn. 4 (1973) 77. [3] S. Ogawa and N. Sakamoto, J. Phys. Soc. Japan 22 (1967) 1214. [4] D. M. Edwards and E. P. Wohlfarth, Proc. Roy. Soc. A303 (1968) 127. [5] S. Ogawa, Physica 91B (1977) 82. [6] M. Cyrot and M. Lavagna, J. de Phys. 40 (1979).
D. GIGNOUX, R. LEMAIRE, P. M O L H O Laboratoire Louis Neel, CNRS, 166X, 38042 Grenoble-c~dex, Franee
and F. TASSET Institut Laue-Langevin, 156X, 38042 Grenoble-cedex, France
Whereas YNi 5 is a Pauli paramagnet a magnetism resurgence occurs for larger Y amounts, as observed in Y2Ni7 and YNi 3. YNi 3 is a very weak itinerant ferromagnet which behaves as ZrZn 2. Polarized neutron experiments show that the magnetism resurgence arises essentially from the 3d Ni electrons.
In the Y - N i compounds, the 3d moment decreases rapidly as the Y content increases. Especially YNi 5 exhibits only a strong enhanced Pauli paramagnetism [1]. However, for larger Y amount, a magnetism resurgence is observed in the Y2Ni7 and YNi 3 compounds [2]. Does this resurgence originate from Y or Ni atoms? We have studied it on YNi 3 (R3m, PuNi3-type structure) by means of measurements of magnetization, susceptibility, specific heat and resistivity, and polarized neutron diffraction experiments on a single crystal. Below 30 K, YNi 3 is ferromagnetic. The bulk magnetic properties are similar to those of ZrZn 2 [3] and are well described in the model of the very weak itinerant ferromagnetism [4]. Magnetization measurements at 4.2 K on a single crystal show that the anisotropy is weak. However, the weight of the crystal being too weak the magnetization variation was studied on a heavier polycrystalline sample. In order to neglect the anisotropy effects, magnetization curves were analysed only above 15 kOe. Magnetization is always weak and strongly field dependent. Arrott plots (fig. 1) have a linear variation in a large temperature range. At Tc = 30 K, the Arrott straight line passes through the origin. The variation of M2(0, T) as a function of T 2 is linear; the extrapolated value of M(0, 0) is then 0.04/.tB/Ni. Above 30 K, YNi 3 is paramagnetic. However, the thermal variation of the reciprocal susceptibility is not linear. The thermal variation of the specific heat does not exhibit any anomaly around T¢. Experimental points can be analysed as resulting from an electronic and a lattice contribution only. This is consistent with the model of the very weak itinerant ferromagnetism which foresees a gap in the specific heat at T c of ACm - - 0 . 3 J m o l - l K - I , value smaller than the experimental uncertainty. The thermal variation of
I
i
42K~15K
"Z/ 0K.230K K •
3
o
0~5 K
YN~3
50 K 2
K
,-,o 801
0
2000
HIM
4000
(Koe/PB/Ni)
/1
Fig. 1. Arrott plots (M2(H, T) as a function of HIM(H, T)) in YNi 3.
the resistivity presents a weak discontinuity in the slope at T c. Below T¢ this variation can be written as p(T) - p(0) = B T 2, where B = 1.3 × 10 -8 ~2cm K -z. This law at low temperature, a general feature for Fermi liquids, is also observed in transition metals. However, B is usually of the order of m a g n i t u d e 10 - I 1 f~cm K -2. In YNi3, as in ZrZn 2 (B = 4.7 x 10 - s f~cm K -2) [5], it is three order of magnitude larger. Polarized neutron experiments were performed at 4.2 K with a field of 13.2 kOe applied vertically and parallel to the a-axis (hexagonal cell) which is the easy magnetization direction. A Fourier projection of the magnetization density along this axis was obtained from the measured structure factors. This density may show that magnetization is localized on the three Ni sites (FNi(3b)= 0.057/~B; ~Ni(6C) ---- 0.078/.tB; //,Ni(8h) = 0.065~a ). Between these sites one observes slight oscillations which seem associated with a nonuniformity of the dif-
Journal of Magnetism and Magnetic Materials 15-18 (1980) 289-290 ©North Holland
289
290
D. Gignoux et al./ Magnetism resurgence in Y N i 3
fuse p o l a r i z a t i o n . T h u s the m a g n e t i s m in Y N i 3 essentially arises from the 3d N i electrons. T h e m a g n e t i s m resurgence o b s e r v e d in the Y - N i system c a n be u n d e r s t o o d using the results of b a n d c a l c u l a t i o n s in Y N i 2 [6]. T h e alloy is f o r m e d b y the a s s o c i a t i o n of a n a r r o w 3d b a n d a n d a wider 4d b a n d . T h e e l e c t r o n e g a t i v i t y difference of the c o n s t i t u e n t s leads to a transfer of the 4d electrons ( 0 . 6 6 / Y ) t o w a r d s the 3d b a n d of lower energy; the two b a n d s are b r o u g h t nearer. F u r t h e r m o r e a n h y b r i d i z a t i o n of 3 d - 4 d states occurs which gives rise to a strong positive curvature of the total density of states n(e) as illustrated on fig. 2. ] ' h e F e r m i level of the N i rich Y - N i c o m p o u n d s lies in this region very sensitive to the Y a m o u n t variations. As the Y c o n t e n t goes up the increase in the n u m b e r of h y b r i d i z e d states in this region c a n l e a d to a n increase of n(eF) in spite of the a d d i t i o n of transferred electrons. T h e l o c a l i z a t i o n of this m a g n e t i s m is d e d u c e d f r o m the local b a n d structures. O n the N i sites the b a n d is a l m o s t filled, the states have an a n t i b o n d i n g c h a r a c t e r . A m a g n e t i s m with a localization a n a l o g o u s to that of N i m e t a l m u s t be o b s e r v e d on the N i sites. O n the c o n t r a r y on the Y sites the b a n d is a l m o s t e m p t y a n d the states have a b o n d i n g character. T h e local d e n s i t y of states at the F e r m i level is very weak. T h e m a g n e t i c c o n t r i b u t i o n of the e l e c t r o n s of this local
rl c
-
EF E EF E EF E Fig. 2. Schematic representation of the total and local in Ni and Y densities of states in YNi3. nc is the critical value of n(E) above which the compound is ferromagnetic. b a n d is thus w e a k a n d s t r o n g l y delocalized. It c a n give a c c o u n t for the oscillations in the m e a s u r e d diffused polarization. F o r all the Y - N i alloys the local d e n s i t y of states on the Y sites will have the s a m e c h a r a c t e r ; thus a m a g n e t i s m localized on the Y sites is not realistic.
References [1] D. Gignoux, D. Givord and A. del Moral, Solid State Commun. 19 (1976) 891. [2] B. Barbara, D. Gignoux, D. Givord, F. Givord and R. Lemaire, Intern. J. Magn. 4 (1973) 77. [3] S. Ogawa and N. Sakamoto, J. Phys. Soc. Japan 22 (1967) 1214. [4] D. M. Edwards and E. P. Wohlfarth, Proc. Roy. Soc. A303 (1968) 127. [5] S. Ogawa, Physica 91B (1977) 82. [6] M. Cyrot and M. Lavagna, J. de Phys. 40 (1979).