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Transition from itinerant electron to local moment system in Y(Mn1−xAlx)2
1073
Iournal of Magnetism and Magnetic Materials 54-57 (1986) 1073-1074 TRANSITION
FROM
ITINERANT
ELECTRON
TO LOCAL MOMENT
SYSTEM
IN
Y(Mnt-,,AI~), M. SHIGA,
H. WADA,
K. Y O S H I M U R A
a n d Y. N A K A M U R A
Department of Metal Science and Technology, Kyoto University, Kyoto 606, Japan
The magnetic susceptibility and thermal expansion have been measured for the pseudobinary Y(Mn 1_,AI~ )2 system. The thermal expansion anomaly, which was observed for YMn 2, rapidly decreases with increasing AI content and disappears around x = 0.10. Furthermore, the susceptibility, being almost temperature independent for YMn 2, becomes of the Curie-Weiss type as the AI content increases. These results strongly suggest that Y(Mn t ,AI~) 2 is a typical system which exhibits a transition from an itinerant electron to a local moment character.
1. Introduction ~'* The Laves phase c o m p o u n d Y M n 2 is an antiferrom a g n e t with /~Mn = 2.7 / ~ and T N = 110 K [1]. This c o m p o u n d shows a first-order transition at T N with a large thermal hysteresis of a b o u t 20 K, a c c o m p a n i e d with a large volume shrinkage of a b o u t 5% [2]. A b o v e T N, the magnetic susceptibility increases with increasing temperature, indicating an itinerant electron character. Therefore, the large volume change can be attributed to the collapse of M n m o m e n t s above T N. The magnetic data of R M n 2 c o m p o u n d s (R:rareearths) have suggested that the M n m a g n e t i s m in R M n 2 is sensitive to the atomic distance [3]. In this paper, we present the results of the magnetic susceptibility a n d the t h e r m a l expansion of p s e u d o b i n a r y Y ( M n ] _ ~ A I ~ ) 2 c o m p o u n d s in which AI substitution expands the lattice [4].
Y ( M n l _ x A[x) 2
25 "*
x=o.50
A
E
~20
~=0.~0
O~
r.
E
•
°
~
' /'N'
..
.
~ °o
o.
"x=O.18k °°
x
fo.°
IO
°°*"
% °°o°
%°o
°° .....
/x:O.10 "°*.°*°°, *°°°°O°.o
....-,,, x =0.05
°'°°"o..,°
......::: ........ :..:........... :,"
"*'o°°......................................... °°°°°o~°°°°°°°~oo°°°o°°°oo°°°
°°°O~oooo ° 0 o ~
5- x=0.02
2. Experiments The samples were p r e p a r e d by argon arc-melting, followed by annealing at 800°C for seven days. No foreign phase o t h e r t h a n C15 has been detected by X-ray analysis. The thermal expansion was measured by a differential transformer-type dilatometer between 4.2 K a n d room temperature. T h e susceptibility vs. temperature curve was measured by a magnetic torsion balance.
3. Results and discussion The t e m p e r a t u r e d e p e n d e n c e of the magnetic susceptibility, X, of the Y ( M n ] _ x A l x ) 2 c o m p o u n d s with 0.02 ~< x ~< 0.50 are shown in fig. 1. The C u r i e - W e i s s b e h a v i o r appears in the x - T curves with increasing x, as previously reported by Marei et al. [5]. Furthermore, a sharp m a x i m u m appears at low temperatures for higher Al concentration. In order to o b t a i n i n f o r m a t i o n o n the M n m o m e n t in this system, we have studied 5SMn N M R of these c o m p o u n d s with 0 ~< x-<.<0.28 at 1.3 K. T h e o b s e r v e d r e s o n a n c e frequencies of 0304-8853/86/$03.50
'~
,
...................... !
i
i
oooo°°°
°~°°o~°oo°Oo~°°~°°°°°°
I
i
i
0
I OO 200 300 TEMPERATURE ( K ) Fig. 1. Temperature dependence of the magnetic susceptibility of the Y(Mn~ _xAIx) 2 compounds. Y ( M n 1 xAlx)2 c o m p o u n d s are almost the same as that of Y M n 2 of 120 MHz, indicating an almost c o n s t a n t value of the M n m o m e n t of a b o u t 2.7/~ B in this system. We can estimate the p a r a m a g n e t i c effective moment, /%ff, from the C u r i e - W e i s s susceptibility. The estimated limiting value of #~ff of a b o u t 3 . 2 / , R / M n by extrapolating /~eff to the Mn-dilute region is in good correspondence to 2.7/1 B of #Mo below TN, which also implies a localized m o m e n t character in the AI substituted alloys. These results strongly suggest that the substitution of M n by AI stabilizes the M n m o m e n t not only in the g r o u n d state but also above T N. In other words, the system shows a transition from an itinerant electron to a localized m o m e n t type. This interpretation is consistent with the variation of thermal expansion curves of this system.
© E l s e v i e r S c i e n c e P u b l i s h e r s B.V.
1074
M. Shiga et al. / Transition in Y(Mn/
V (Mnl.xAIx) 2
I
:lxlO
,.AI~) 2
3
.(c)
7.75 o< v r~
<3 .< e,Q_ w u
7.70 ~ T
I
I
i
i
0
100 260 300 "fEI~PERATURE( K ) Fig. 2. Thermal expansion curves of the Y(Mn l_.~Alx) 2 compounds. The right hand scale indicates the corresponding lattice parameter of each compound, which is calculated from the dilatation curve and the lattice parameter at 290 K.
T h e thermal expansion curves of the Y ( M n l _ x A l x ) 2 c o m p o u n d s with 0.02 ~< x ~< 0.10 are s h o w n in fig. 2. T h e results i n d i c a t e t h a t the thermal expansion is strongly affected b y a small a m o u n t of substituted A1. T h e large volume change rapidly decreases with increasing A1 c o n t e n t a n d disappears for the c o m p o u n d s with x = 0.10. N o t h e r m a l hysteresis was observed for x >1 0.03. T h e t h e r m a l e x p a n s i o n coefficient immediately a b o v e TN, which is r e m a r k a b l y large (50 × 1 0 - 6 / K ) for Y M n 2, becomes smaller with increasing x. These behaviors c a n b e explained in terms of the spin fluctuation theory as follows. T h e variation of spin fluctuation a m p l i t u d e in this system is schematically s h o w n in fig. 3. T h e curve (a) corresponds to Y M n 2, in which ( S 2) drops almost zero at T N a n d then gradually increases with increasing t e m p e r a t u r e by thermal fluctuations. Since the magnetic volume change, ¢0, is
Fig. 3. Schematic representation of the temperature variation of the square average of longitudinal spin fluctuation amplitudes, ($2). The curve (a) corresponds to YMn 2 and the curve (c) shows a almost local moment case. The curve (b) is the intermediate case between (a) and (c).
p r o p o r t i o n a l to ( $ 2 ) , we observe a large s p o n t a n e o u s volume magnetostriction, ~0s, and an e n h a n c e d thermal e x p a n s i o n coefficient, c~, above T N. By substituting M n b y A1, the ( S [ ) vs. T curve changes from (a) to (b) and finally to (c), indicating a local m o m e n t system. The decrease of o~s a n d c~ with increasing A1 content can easily be u n d e r s t o o d by this picture. The lattice expansion due to A1 substitution may b e responsible for the stabilization of the M n m o m e n t . T h e authors are i n d e b t e d to Mr. H. N a k a m u r a for his assistance in the thermal expansion measurements. [1] Y. Nakamura, M. Shiga and S. Kawano, Physica 120B (1983) 212. [2] M. Shiga, H. Wada and Y. Nakamura, J. Magn. Magn. Mat. 31-34 (1983) 119. [3] Y. Nakamura, Proc. Intern. Conf. Magn. of Rare-Earths and Actinides, Bucharest (1983) vol. 2, p. 47. [4] K. Yoshimura, M. Takigawa, H. Yasuoka, M. Shiga and Y. Nakamura, J. Magn. Magn. Mat. 54-57 (1986). [5] S.A. Marei, R.S. Craig, W.E. Wallace and T. Tsuchida, J. Less-Common Metals 13 (1967) 391.
Iournal of Magnetism and Magnetic Materials 54-57 (1986) 1073-1074 TRANSITION
FROM
ITINERANT
ELECTRON
TO LOCAL MOMENT
SYSTEM
IN
Y(Mnt-,,AI~), M. SHIGA,
H. WADA,
K. Y O S H I M U R A
a n d Y. N A K A M U R A
Department of Metal Science and Technology, Kyoto University, Kyoto 606, Japan
The magnetic susceptibility and thermal expansion have been measured for the pseudobinary Y(Mn 1_,AI~ )2 system. The thermal expansion anomaly, which was observed for YMn 2, rapidly decreases with increasing AI content and disappears around x = 0.10. Furthermore, the susceptibility, being almost temperature independent for YMn 2, becomes of the Curie-Weiss type as the AI content increases. These results strongly suggest that Y(Mn t ,AI~) 2 is a typical system which exhibits a transition from an itinerant electron to a local moment character.
1. Introduction ~'* The Laves phase c o m p o u n d Y M n 2 is an antiferrom a g n e t with /~Mn = 2.7 / ~ and T N = 110 K [1]. This c o m p o u n d shows a first-order transition at T N with a large thermal hysteresis of a b o u t 20 K, a c c o m p a n i e d with a large volume shrinkage of a b o u t 5% [2]. A b o v e T N, the magnetic susceptibility increases with increasing temperature, indicating an itinerant electron character. Therefore, the large volume change can be attributed to the collapse of M n m o m e n t s above T N. The magnetic data of R M n 2 c o m p o u n d s (R:rareearths) have suggested that the M n m a g n e t i s m in R M n 2 is sensitive to the atomic distance [3]. In this paper, we present the results of the magnetic susceptibility a n d the t h e r m a l expansion of p s e u d o b i n a r y Y ( M n ] _ ~ A I ~ ) 2 c o m p o u n d s in which AI substitution expands the lattice [4].
Y ( M n l _ x A[x) 2
25 "*
x=o.50
A
E
~20
~=0.~0
O~
r.
E
•
°
~
' /'N'
..
.
~ °o
o.
"x=O.18k °°
x
fo.°
IO
°°*"
% °°o°
%°o
°° .....
/x:O.10 "°*.°*°°, *°°°°O°.o
....-,,, x =0.05
°'°°"o..,°
......::: ........ :..:........... :,"
"*'o°°......................................... °°°°°o~°°°°°°°~oo°°°o°°°oo°°°
°°°O~oooo ° 0 o ~
5- x=0.02
2. Experiments The samples were p r e p a r e d by argon arc-melting, followed by annealing at 800°C for seven days. No foreign phase o t h e r t h a n C15 has been detected by X-ray analysis. The thermal expansion was measured by a differential transformer-type dilatometer between 4.2 K a n d room temperature. T h e susceptibility vs. temperature curve was measured by a magnetic torsion balance.
3. Results and discussion The t e m p e r a t u r e d e p e n d e n c e of the magnetic susceptibility, X, of the Y ( M n ] _ x A l x ) 2 c o m p o u n d s with 0.02 ~< x ~< 0.50 are shown in fig. 1. The C u r i e - W e i s s b e h a v i o r appears in the x - T curves with increasing x, as previously reported by Marei et al. [5]. Furthermore, a sharp m a x i m u m appears at low temperatures for higher Al concentration. In order to o b t a i n i n f o r m a t i o n o n the M n m o m e n t in this system, we have studied 5SMn N M R of these c o m p o u n d s with 0 ~< x-<.<0.28 at 1.3 K. T h e o b s e r v e d r e s o n a n c e frequencies of 0304-8853/86/$03.50
'~
,
...................... !
i
i
oooo°°°
°~°°o~°oo°Oo~°°~°°°°°°
I
i
i
0
I OO 200 300 TEMPERATURE ( K ) Fig. 1. Temperature dependence of the magnetic susceptibility of the Y(Mn~ _xAIx) 2 compounds. Y ( M n 1 xAlx)2 c o m p o u n d s are almost the same as that of Y M n 2 of 120 MHz, indicating an almost c o n s t a n t value of the M n m o m e n t of a b o u t 2.7/~ B in this system. We can estimate the p a r a m a g n e t i c effective moment, /%ff, from the C u r i e - W e i s s susceptibility. The estimated limiting value of #~ff of a b o u t 3 . 2 / , R / M n by extrapolating /~eff to the Mn-dilute region is in good correspondence to 2.7/1 B of #Mo below TN, which also implies a localized m o m e n t character in the AI substituted alloys. These results strongly suggest that the substitution of M n by AI stabilizes the M n m o m e n t not only in the g r o u n d state but also above T N. In other words, the system shows a transition from an itinerant electron to a localized m o m e n t type. This interpretation is consistent with the variation of thermal expansion curves of this system.
© E l s e v i e r S c i e n c e P u b l i s h e r s B.V.
1074
M. Shiga et al. / Transition in Y(Mn/
V (Mnl.xAIx) 2
I
:lxlO
,.AI~) 2
3
.(c)
7.75 o< v r~
<3 .< e,Q_ w u
7.70 ~ T
I
I
i
i
0
100 260 300 "fEI~PERATURE( K ) Fig. 2. Thermal expansion curves of the Y(Mn l_.~Alx) 2 compounds. The right hand scale indicates the corresponding lattice parameter of each compound, which is calculated from the dilatation curve and the lattice parameter at 290 K.
T h e thermal expansion curves of the Y ( M n l _ x A l x ) 2 c o m p o u n d s with 0.02 ~< x ~< 0.10 are s h o w n in fig. 2. T h e results i n d i c a t e t h a t the thermal expansion is strongly affected b y a small a m o u n t of substituted A1. T h e large volume change rapidly decreases with increasing A1 c o n t e n t a n d disappears for the c o m p o u n d s with x = 0.10. N o t h e r m a l hysteresis was observed for x >1 0.03. T h e t h e r m a l e x p a n s i o n coefficient immediately a b o v e TN, which is r e m a r k a b l y large (50 × 1 0 - 6 / K ) for Y M n 2, becomes smaller with increasing x. These behaviors c a n b e explained in terms of the spin fluctuation theory as follows. T h e variation of spin fluctuation a m p l i t u d e in this system is schematically s h o w n in fig. 3. T h e curve (a) corresponds to Y M n 2, in which ( S 2) drops almost zero at T N a n d then gradually increases with increasing t e m p e r a t u r e by thermal fluctuations. Since the magnetic volume change, ¢0, is
Fig. 3. Schematic representation of the temperature variation of the square average of longitudinal spin fluctuation amplitudes, ($2). The curve (a) corresponds to YMn 2 and the curve (c) shows a almost local moment case. The curve (b) is the intermediate case between (a) and (c).
p r o p o r t i o n a l to ( $ 2 ) , we observe a large s p o n t a n e o u s volume magnetostriction, ~0s, and an e n h a n c e d thermal e x p a n s i o n coefficient, c~, above T N. By substituting M n b y A1, the ( S [ ) vs. T curve changes from (a) to (b) and finally to (c), indicating a local m o m e n t system. The decrease of o~s a n d c~ with increasing A1 content can easily be u n d e r s t o o d by this picture. The lattice expansion due to A1 substitution may b e responsible for the stabilization of the M n m o m e n t . T h e authors are i n d e b t e d to Mr. H. N a k a m u r a for his assistance in the thermal expansion measurements. [1] Y. Nakamura, M. Shiga and S. Kawano, Physica 120B (1983) 212. [2] M. Shiga, H. Wada and Y. Nakamura, J. Magn. Magn. Mat. 31-34 (1983) 119. [3] Y. Nakamura, Proc. Intern. Conf. Magn. of Rare-Earths and Actinides, Bucharest (1983) vol. 2, p. 47. [4] K. Yoshimura, M. Takigawa, H. Yasuoka, M. Shiga and Y. Nakamura, J. Magn. Magn. Mat. 54-57 (1986). [5] S.A. Marei, R.S. Craig, W.E. Wallace and T. Tsuchida, J. Less-Common Metals 13 (1967) 391.