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Nuclear spin order of Sc metal
PHYSICA
Physica B 194-196 (1994) 249-250 North-Holland
Nuclear spin order of Sc m e t a l Haruhiko Suzuki, Yoshihiro Koike, Yoshitomo Karaki*, Minoru Kubota* and Hidehiko Ishimoto* Department of Physics, Faculty of Science, Kanazawa University, Kanazawa 920-11, Japan *Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan
Ferromagnetic nuclear spin ordering of Sc metal was observed by a magnetic susceptibility measurement following a demagnetization cooling from the initial magnetic field of about 2.5T and the initial temperature of 0.277mK. Spin glass-like phenomena due to 3 ppm Fe impurities was also observed at slightly higher temperature than the nuclear spin ordering temperature.
1.INTRODUCTON
The clear evidence of the nuclear spin ordering has been reported only in Cu and Ag. In LT-19, we also reported the result of our experiments on nuclear spin ordering in Sc metal [1]. In this report we describe our new results of nuclear spin ordering experiments in Sc metal. Scandium metal has the following features compared with Cu and Ag. Sc metal is known as the highly exchange enhanced Pauli paramagnetic metal. The natural abundance of 45Sc(I=7/2, ~t=4.563PLN) is 1 0 0 % . S i n c e the crystal structure is hcp, the electric quadrupole interaction of nuclear spin exists. Very recently the sign of the electric field gradient was determined by Pollack et al[2]. From their result it is found that the ground state is ±7/2 in Sc metal. They also obtained the Korringa constant of the spin lattice r e l a x a t i o n t i m e a s 90±9 m s e e K at l o w temperatures. The ordering forces of the nuclear spins in Sc metal are expected to be dominated by the direct dipole-dipole coupling and by the RKKY indirect exchange force. From the calculation of the dipolar interaction, an antiferromagnetic state is stable below TN=130 nK. The RKKY interaction is v e r y hard to be calculated. Since Sc metal contains only one isotope, it is also h a r d to determine the strength of the RKKY interaction by experiments. If the very short relaxation time of Sc measured by Pollack et al comes from the RKKY interaction and not from paramagnetic impurities, we can expect the ordering temperature of about several p.K, comparing with the Korringa constants of other metals such as Cu, Ag. The thermal conductivity of Sc metal is rather poor compared to Cu and Ag.
2. E X P E R I M E N T A L
PROCEDURE.
A two stage nuclear demagnetizing cryostat was constructed. The first stage is c o m p o s e d of 50 effective mol copper which is cut from the bulk OFHC Cu, and the second stage is our specimen, Sc metal which was set in the experimental space of the first stage. A single crystal of Sc, grown at Ames Laboratory, had a dimension of 25x3x3 mm 3 with the long direction parallel to the c-axis. The home made second stage magnet, set also in the experimental space, produced about 4 T parallel to the c-axis. As usual in this kind of experiment, we did not use a heat switch between the first stage and the Sc. The main magnetic impurity in our crystal is 3 at. ppm Fe. The residual resistance ratio of our crystal was 37. The thermal connection between the Sc crystal and the silver thermal link plates which is conducted to the first nuclear stage was tightly fitted by screws. The magnetic susceptibility w a s m e a s u r e d b y a S Q U I D magnetometer. To produce zero magnetic field a ~tm e t a l s h i e l d s u r r o u n d e d the s a m p l e . T h e temperature of the c o p p e r n u c l e a r s t a g e was measured by a Pt NMR thermometer. 3. R E S U L T S A N D D I S C U S S I O N S By d e m a g n e t i z i n g f r o m 7 T , t h e l o w e s t temperature of the first stage was just below 100 ~tK. A t first, we m e a s u r e d the t e m p e r a t u r e dependence of the magnetic susceptibility o f S c metal in zero magnetic field from about 10 mK to 100 #K as shown in the inset of Fig.1. The
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0921-4526(93)E0704-K
250
magnetic susceptibility shows a dip at about 0.3 mK. The same kind of the dip was also observed in the time depencence of the magnetic susceptibility a n d t h e m a g n e t i z a t i o n f o l l o w i n g the demagnetization of the second stage shown later. Below this temperature it shows the Curie's law like behaviour. We cooled down the Sc crystal in the m a g n e t i c field o f 2.38T keeping the first nuclear stage at 0.277 mK for more than 10 days. We demagnetized the Sc metal from 2.38T to zero field w i t h i n 30 minutes. The demagnetizatized e x p e r i m e n t s in d i f f e r e n t condition were also performed. The magnetic susceptibility parallel to the c - a x i s r e c o r d e d d u r i n g the w a r m i n g up f o l l o w i n g the demagnetization cooling from Ti=0.589 mK and Hi=2.38 T is shown in figure 1. This result is very similar to the result for the demagnetization from Ti=0.277 mK. In the figure the static magnetization mesured simultaneously is also shown. After the quick demagnetization of the second stage, the first stage copper warmed up to 0.7 mK, probably due to the eddy current heating of the silver thermal link. Since the resistivity of Sc metal is rather poor, the eddy current heating of Sc c r y s t a l is so small. After the second stage demagnetizastion, the warming up time depends on the temperature of the first stage. As the specific heat o f the c o n d u c t i o n e l e c t r o n in m e t a l is negligibly small, the temperature of the electrons during the warming up should be determined by the s p i n - l a t t i c e r e l a x a t i o n t i m e and the thermal conductivity of Sc metal. If the Korringa constant of Sc metal is 90 msecK, the temperature of the e l e c t r o n s s h o u l d m o v e so q u i c k l y to t h e temperature of the nuclear spins during or after the demagnetization. Then, the coupled nuclear spins and electrons warm up to the temperature of the first stage due to the thermal conduction. As seen in the f i g u r e , We o b s e r v e d two peaks of the magnetic susceptibility. Considering our previous result [1], the peak at higher temperature should correspond to the freezing temperature of spin glass of Fe impurity. From the magnetic susceptibility at higher temperatures shown in Fig.l, we can estimate the temperature of the peak to be about 70 ~tK. Our previous crystal which contained 50 ppm Fe impurity showed the maximum in the magnetic susceptibility at about 0.8 inK. This r e s u l t is consistent with the concentration depemdence of the spin glass of Fe impurity in Pd[3]. Considering with the time dependence of the magnetization, the
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Figure 1. T i m e d e p e n d e n c e s of the magnetic susceptibility (open circle) and the magnetization (closs marck) following the demagnetization. The inset shows the magnetic susceptibility in our control experiment. Both of the magnetic susceptibilities are represented in the same unit. lower maximum of the magnetic susceptibility should correspond to the ferromagnetic ordering and the formation of the magnetic domains. Though the magnetic susceptibility peak at higher temperature is larger than the lower one, the magnetization at lower temperatures is much larger than at higher temperatures. This result suggests that the origins of the two peaks of the magnetic sucept~bility should be different. When the ground state of the nuclear spin is ±7/2, the quadrapole splitting does n o t p r o d u c e any m a x i m u m of the m a g n e t i c s u s c e p t i b i l i t y w i t h o u t any m a g n e t i c ordering Finally, we demagnetized the specimen at about 10 m K t o m a k e s u r e t h a t t h e p e a k s in t h e susceptibility are not artifact. We could detect no change in the susceptibility as a function of time within the width of the recording trace line.
REFERENCES 1.H.Suzuki, T.Sakon and N.Mizutani, Prc. of LT-19, Physica B, 165&166 (1990) 795. 2.L.Pollack, E.N.Smith and R.C.Richardson, J. Low Temp. Phys., 89 (1992) 169. 3.R.P.Peters,Ch.Buchal, M.Kubota, R.M.Muller and F.Pobell, Phys. Rev. Letters, 53 (1984) 1108.
Physica B 194-196 (1994) 249-250 North-Holland
Nuclear spin order of Sc m e t a l Haruhiko Suzuki, Yoshihiro Koike, Yoshitomo Karaki*, Minoru Kubota* and Hidehiko Ishimoto* Department of Physics, Faculty of Science, Kanazawa University, Kanazawa 920-11, Japan *Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan
Ferromagnetic nuclear spin ordering of Sc metal was observed by a magnetic susceptibility measurement following a demagnetization cooling from the initial magnetic field of about 2.5T and the initial temperature of 0.277mK. Spin glass-like phenomena due to 3 ppm Fe impurities was also observed at slightly higher temperature than the nuclear spin ordering temperature.
1.INTRODUCTON
The clear evidence of the nuclear spin ordering has been reported only in Cu and Ag. In LT-19, we also reported the result of our experiments on nuclear spin ordering in Sc metal [1]. In this report we describe our new results of nuclear spin ordering experiments in Sc metal. Scandium metal has the following features compared with Cu and Ag. Sc metal is known as the highly exchange enhanced Pauli paramagnetic metal. The natural abundance of 45Sc(I=7/2, ~t=4.563PLN) is 1 0 0 % . S i n c e the crystal structure is hcp, the electric quadrupole interaction of nuclear spin exists. Very recently the sign of the electric field gradient was determined by Pollack et al[2]. From their result it is found that the ground state is ±7/2 in Sc metal. They also obtained the Korringa constant of the spin lattice r e l a x a t i o n t i m e a s 90±9 m s e e K at l o w temperatures. The ordering forces of the nuclear spins in Sc metal are expected to be dominated by the direct dipole-dipole coupling and by the RKKY indirect exchange force. From the calculation of the dipolar interaction, an antiferromagnetic state is stable below TN=130 nK. The RKKY interaction is v e r y hard to be calculated. Since Sc metal contains only one isotope, it is also h a r d to determine the strength of the RKKY interaction by experiments. If the very short relaxation time of Sc measured by Pollack et al comes from the RKKY interaction and not from paramagnetic impurities, we can expect the ordering temperature of about several p.K, comparing with the Korringa constants of other metals such as Cu, Ag. The thermal conductivity of Sc metal is rather poor compared to Cu and Ag.
2. E X P E R I M E N T A L
PROCEDURE.
A two stage nuclear demagnetizing cryostat was constructed. The first stage is c o m p o s e d of 50 effective mol copper which is cut from the bulk OFHC Cu, and the second stage is our specimen, Sc metal which was set in the experimental space of the first stage. A single crystal of Sc, grown at Ames Laboratory, had a dimension of 25x3x3 mm 3 with the long direction parallel to the c-axis. The home made second stage magnet, set also in the experimental space, produced about 4 T parallel to the c-axis. As usual in this kind of experiment, we did not use a heat switch between the first stage and the Sc. The main magnetic impurity in our crystal is 3 at. ppm Fe. The residual resistance ratio of our crystal was 37. The thermal connection between the Sc crystal and the silver thermal link plates which is conducted to the first nuclear stage was tightly fitted by screws. The magnetic susceptibility w a s m e a s u r e d b y a S Q U I D magnetometer. To produce zero magnetic field a ~tm e t a l s h i e l d s u r r o u n d e d the s a m p l e . T h e temperature of the c o p p e r n u c l e a r s t a g e was measured by a Pt NMR thermometer. 3. R E S U L T S A N D D I S C U S S I O N S By d e m a g n e t i z i n g f r o m 7 T , t h e l o w e s t temperature of the first stage was just below 100 ~tK. A t first, we m e a s u r e d the t e m p e r a t u r e dependence of the magnetic susceptibility o f S c metal in zero magnetic field from about 10 mK to 100 #K as shown in the inset of Fig.1. The
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0921-4526(93)E0704-K
250
magnetic susceptibility shows a dip at about 0.3 mK. The same kind of the dip was also observed in the time depencence of the magnetic susceptibility a n d t h e m a g n e t i z a t i o n f o l l o w i n g the demagnetization of the second stage shown later. Below this temperature it shows the Curie's law like behaviour. We cooled down the Sc crystal in the m a g n e t i c field o f 2.38T keeping the first nuclear stage at 0.277 mK for more than 10 days. We demagnetized the Sc metal from 2.38T to zero field w i t h i n 30 minutes. The demagnetizatized e x p e r i m e n t s in d i f f e r e n t condition were also performed. The magnetic susceptibility parallel to the c - a x i s r e c o r d e d d u r i n g the w a r m i n g up f o l l o w i n g the demagnetization cooling from Ti=0.589 mK and Hi=2.38 T is shown in figure 1. This result is very similar to the result for the demagnetization from Ti=0.277 mK. In the figure the static magnetization mesured simultaneously is also shown. After the quick demagnetization of the second stage, the first stage copper warmed up to 0.7 mK, probably due to the eddy current heating of the silver thermal link. Since the resistivity of Sc metal is rather poor, the eddy current heating of Sc c r y s t a l is so small. After the second stage demagnetizastion, the warming up time depends on the temperature of the first stage. As the specific heat o f the c o n d u c t i o n e l e c t r o n in m e t a l is negligibly small, the temperature of the electrons during the warming up should be determined by the s p i n - l a t t i c e r e l a x a t i o n t i m e and the thermal conductivity of Sc metal. If the Korringa constant of Sc metal is 90 msecK, the temperature of the e l e c t r o n s s h o u l d m o v e so q u i c k l y to t h e temperature of the nuclear spins during or after the demagnetization. Then, the coupled nuclear spins and electrons warm up to the temperature of the first stage due to the thermal conduction. As seen in the f i g u r e , We o b s e r v e d two peaks of the magnetic susceptibility. Considering our previous result [1], the peak at higher temperature should correspond to the freezing temperature of spin glass of Fe impurity. From the magnetic susceptibility at higher temperatures shown in Fig.l, we can estimate the temperature of the peak to be about 70 ~tK. Our previous crystal which contained 50 ppm Fe impurity showed the maximum in the magnetic susceptibility at about 0.8 inK. This r e s u l t is consistent with the concentration depemdence of the spin glass of Fe impurity in Pd[3]. Considering with the time dependence of the magnetization, the
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3
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ea 1.5
.t~
.......
4-
o o,I
"~,"
l
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T[mX]
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+ +
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+ +
.++++
•. : .. : ' .°. . ~ o . . .. . . °. . . . .°
. . . . ... .
-4.0
0.5 + + + + -¢on, ""e+p + +
4-
+
+
+
i.5.o
0
100
200
300
400
500
Time[mini
Figure 1. T i m e d e p e n d e n c e s of the magnetic susceptibility (open circle) and the magnetization (closs marck) following the demagnetization. The inset shows the magnetic susceptibility in our control experiment. Both of the magnetic susceptibilities are represented in the same unit. lower maximum of the magnetic susceptibility should correspond to the ferromagnetic ordering and the formation of the magnetic domains. Though the magnetic susceptibility peak at higher temperature is larger than the lower one, the magnetization at lower temperatures is much larger than at higher temperatures. This result suggests that the origins of the two peaks of the magnetic sucept~bility should be different. When the ground state of the nuclear spin is ±7/2, the quadrapole splitting does n o t p r o d u c e any m a x i m u m of the m a g n e t i c s u s c e p t i b i l i t y w i t h o u t any m a g n e t i c ordering Finally, we demagnetized the specimen at about 10 m K t o m a k e s u r e t h a t t h e p e a k s in t h e susceptibility are not artifact. We could detect no change in the susceptibility as a function of time within the width of the recording trace line.
REFERENCES 1.H.Suzuki, T.Sakon and N.Mizutani, Prc. of LT-19, Physica B, 165&166 (1990) 795. 2.L.Pollack, E.N.Smith and R.C.Richardson, J. Low Temp. Phys., 89 (1992) 169. 3.R.P.Peters,Ch.Buchal, M.Kubota, R.M.Muller and F.Pobell, Phys. Rev. Letters, 53 (1984) 1108.