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Anomalous magnetic torque in the heavy-fermion superconductor UBe13
PHYSICA
Physica B 194-196 (1994) 245-246 North-Holland
Anomalous
magnetic
torque
in t h e h e a v y - f e r m i o n
superconductor
UBela
G. M. S c h m i e d e s h o f f a, Z. Fisk b a n d J. L. S m i t h b a D e p a r t m e n t of Physics, Occidental College, Los Angeles, California 90041, USA bLos A l a m o s N a t i o n a l L a b o r a t o r y , Los Alamos, New Mexico 87545, USA M e a s u r e m e n t s of t h e m a g n e t i c t o r q u e a c t i n g u p o n a single c r y s t a l of t h e h e a v y - f e r m i o n s u p e r c o n d u c t o r U B e l 3 have b e e n m a d e at t e m p e r a t u r e s f r o m 0.5 K to 30.0 K a n d in m a g n e t i c fields to 23 T using a capacitive m a g n e t o m e t e r . We find t h a t a large, a n o m a l o u s c o n t r i b u t i o n to t h e m a g n e t i c t o r q u e a p p e a r s in at low t e m p e r a t u r e s a n d in h i g h fields. The a n o m a l o u s t o r q u e coexists w i t h t h e s u p e r c o n d u c t i n g s t a t e at low t e m p e r a t u r e . We propose t h a t t h e a n o m a l o u s t o r q u e reflects t h e existence of a f i e l d - i n d u c e d m a g n e t i c phase transition. T h e m a g n e t i c t o r q u e on a single crystal s a m p l e of U B e l 3 has b e e n m e a s u r e d w i t h a capacitive m a g n e t o m e t e r described elsewhere [1]. In high fields t h e sign of t h e m a g n e t i c t o r q u e changes w i t h falling temp e r a t u r e suggesting two c o m p e t i n g t o r q u e m e c h a n i s m s . We believe t h a t t o r q u e resulting f r o m s h a p e effects (as e x p e c t e d [1]) coexists, at low t e m p e r a t u r e s a n d high m a g n e t i c fields w i t h t o r q u e r e s u l t i n g f r o m the a p p e a r a n c e of a t r a n s v e r s e c o m p o n e n t of t h e m a g n e t i z a t i o n M z . T o r q u e resulting f r o m an M±is e x p e c t e d to be m u c h larger t h a n t h a t r e s u l t i n g f r o m shape effects [2]. T h e t e m p e r a t u r e a n d field dep e n d e n c e of M i i s shown in Fig. 1. T h e change in sign of M ± ( T ) w i t h falling temp e r a t u r e is a p p a r e n t in Fig. l a . T h e behavior of M ± ( H ) shown in Fig. l b suggests t h e p r e s e n c e of an onset field of 3-5 T above which M ± a p p e a r s (the oscillatory b e h a v i o r is discussed elsewhere). We can e s t i m a t e t h e m a g n i t u d e of M± by c a l i b r a t i n g t h e m a g n e t o m e t e r u n d e r the a s s u m p t i o n t h a t t h e high t e m p e r a t u r e d a t a results f r o m s h a p e effects only. Calib r a t i n g t h e m a g n e t o m e t e r f r o m t h e change in c a p a c i t a n c e o b s e r v e d at 20 K a n d 20 T, we find t h a t , at 1 K a n d 20 T (the maxi m u m value of M ± i n Fig. l b ) , M± ~ 0.23
e m u / g . M±is t h u s a b o u t 4% of MII at 20 T as m e a s u r e d by a v i b r a t i n g s a m p l e magn e t o m e t e r at t h e same t e m p e r a t u r e . We p r o p o s e t h a t a f i e l d - i n d u c e d magnetic p h a s e t r a n s i t i o n , p e r h a p s a m e t a m agnetic t r a n s i t i o n , is responsible for M±. A l t h o u g h we do n o t find a s h a r p f e a t u r e in a n y of t h e d a t a p r e s e n t e d in this p a p e r (a c h a r a c t e r i s t i c of m e t a m a g n e t i c transitions), t h e onset of this b e h a v i o r at low t e m p e r a t u r e s correlates quite well w i t h o t h e r i n d i c a t i o n s of p h a s e t r a n s i t i o n s at low t e m p e r a t u r e s a n d in high m a g n e t i c fields: T h e onset field is in r e a s o n a b l e agreement with a phase b o u n d a r y near 2 T observed up to a b o u t 0.5 K in field dependent specific h e a t m e a s u r e m e n t s [3]. T h e onset field is also in g o o d a g r e e m e n t w i t h a sharp kinks in H c 2 ( T ) o b s e r v e d in t h e same s a m p l e [4]. A p h a s e d i a g r a m for our p r o p o s e d fieldi n d u c e d m a g n e t i c t r a n s i t i o n is shown in Fig. 2, w h e r e T is p l o t t e d on a logarithmic scale. T h e small solid circles r e p r e s e n t m e a s u r e m e n t s of H c 2 ( T ) r e p o r t e d elsewhere [4]. T h e large o p e n circles denote t h e positions of sharp kinks in Hc2 (T) which are too small to be easily visible in Fig. 2. T h e large closed circles represent t h e onset of t h e a n o m a l o u s m a g n e t i c
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)E0702-I
246
i0
i2
UBeta alngla crystal
UBea
t0
H U [~.00]
sample #3
"'3.
@ ,..~
d
sample #3
£ 2 Ca)
-2
•
i
'
i
i
t0
20
T (K)
i
J
I
30
T
t.t 0.9
4
Cb)
0.7 D m
!
0.5
/
0.3
/ =
I
t "l
0
singla crystal
/
0.t -0.t o
UBe~a
0.1
s a ~ l e #3
i'o
=
|
t5
=
'
10
T (K)
1.OK
=
i
25
H (T)
F i g u r e 1. (a) T h e t r a n s v e r s e m a g n e t i z a t i o n (M_L, see t e x t ) p l o t t e d as a f u n c t i o n o f t e m p e r a t u r e a t f i x e d fields o f 10 T ( s q u a r e s ) , 15 T ( t r i a n g l e s ) , a n d 20 T (circles). T h e solid lines a r e g u i d e s t o t h e eye. (b) M ± p l o t t e d as a f u n c t i o n o f field a t 1.0 K.
t o r q u e d e t e r m i n e d in a m a n n e r d e s c r i b e d e l s e w h e r e [2]. T h e solid line is a g u i d e t o the eye. T h i s w o r k was s u p p o r t e d b y t h e National Science Foundation under DMR-9019661 and by a Cottrell College Science Grant from the Research Corporation. W o r k a t Los A l a m o s was p e r f o r m e d u n d e r t h e a u s p i c e s o f t h e U.S. D e p a r t m e n t o f E n e r g y . T h e h i g h field m e a s u r e m e n t s were made at the Francis Bitter National Magnet Laboratory (supported by the NSF).
F i g u r e 2. T h e low t e m p e r a t u r e p h a s e dia g r a m o f U B e l 3 in t h e H - T p l a n e . T h e solid line is a g u i d e t o t h e eye.
REFERENCES [1] G. M . S c h m i e d e s h o f f , P h i l . M a g . 711 (1992). [2] G. M. S c h m i e d e s h o f f et al., a n d ences therein, to be published. [3] B. E l l m a n e t al., P h y s . R e v . 12074 (1991). [4] G. M. S c h m i e d e s h o f f et al., P h y s . B 45, 10544 (1992).
B 66, referB 44, Rev.
Physica B 194-196 (1994) 245-246 North-Holland
Anomalous
magnetic
torque
in t h e h e a v y - f e r m i o n
superconductor
UBela
G. M. S c h m i e d e s h o f f a, Z. Fisk b a n d J. L. S m i t h b a D e p a r t m e n t of Physics, Occidental College, Los Angeles, California 90041, USA bLos A l a m o s N a t i o n a l L a b o r a t o r y , Los Alamos, New Mexico 87545, USA M e a s u r e m e n t s of t h e m a g n e t i c t o r q u e a c t i n g u p o n a single c r y s t a l of t h e h e a v y - f e r m i o n s u p e r c o n d u c t o r U B e l 3 have b e e n m a d e at t e m p e r a t u r e s f r o m 0.5 K to 30.0 K a n d in m a g n e t i c fields to 23 T using a capacitive m a g n e t o m e t e r . We find t h a t a large, a n o m a l o u s c o n t r i b u t i o n to t h e m a g n e t i c t o r q u e a p p e a r s in at low t e m p e r a t u r e s a n d in h i g h fields. The a n o m a l o u s t o r q u e coexists w i t h t h e s u p e r c o n d u c t i n g s t a t e at low t e m p e r a t u r e . We propose t h a t t h e a n o m a l o u s t o r q u e reflects t h e existence of a f i e l d - i n d u c e d m a g n e t i c phase transition. T h e m a g n e t i c t o r q u e on a single crystal s a m p l e of U B e l 3 has b e e n m e a s u r e d w i t h a capacitive m a g n e t o m e t e r described elsewhere [1]. In high fields t h e sign of t h e m a g n e t i c t o r q u e changes w i t h falling temp e r a t u r e suggesting two c o m p e t i n g t o r q u e m e c h a n i s m s . We believe t h a t t o r q u e resulting f r o m s h a p e effects (as e x p e c t e d [1]) coexists, at low t e m p e r a t u r e s a n d high m a g n e t i c fields w i t h t o r q u e r e s u l t i n g f r o m the a p p e a r a n c e of a t r a n s v e r s e c o m p o n e n t of t h e m a g n e t i z a t i o n M z . T o r q u e resulting f r o m an M±is e x p e c t e d to be m u c h larger t h a n t h a t r e s u l t i n g f r o m shape effects [2]. T h e t e m p e r a t u r e a n d field dep e n d e n c e of M i i s shown in Fig. 1. T h e change in sign of M ± ( T ) w i t h falling temp e r a t u r e is a p p a r e n t in Fig. l a . T h e behavior of M ± ( H ) shown in Fig. l b suggests t h e p r e s e n c e of an onset field of 3-5 T above which M ± a p p e a r s (the oscillatory b e h a v i o r is discussed elsewhere). We can e s t i m a t e t h e m a g n i t u d e of M± by c a l i b r a t i n g t h e m a g n e t o m e t e r u n d e r the a s s u m p t i o n t h a t t h e high t e m p e r a t u r e d a t a results f r o m s h a p e effects only. Calib r a t i n g t h e m a g n e t o m e t e r f r o m t h e change in c a p a c i t a n c e o b s e r v e d at 20 K a n d 20 T, we find t h a t , at 1 K a n d 20 T (the maxi m u m value of M ± i n Fig. l b ) , M± ~ 0.23
e m u / g . M±is t h u s a b o u t 4% of MII at 20 T as m e a s u r e d by a v i b r a t i n g s a m p l e magn e t o m e t e r at t h e same t e m p e r a t u r e . We p r o p o s e t h a t a f i e l d - i n d u c e d magnetic p h a s e t r a n s i t i o n , p e r h a p s a m e t a m agnetic t r a n s i t i o n , is responsible for M±. A l t h o u g h we do n o t find a s h a r p f e a t u r e in a n y of t h e d a t a p r e s e n t e d in this p a p e r (a c h a r a c t e r i s t i c of m e t a m a g n e t i c transitions), t h e onset of this b e h a v i o r at low t e m p e r a t u r e s correlates quite well w i t h o t h e r i n d i c a t i o n s of p h a s e t r a n s i t i o n s at low t e m p e r a t u r e s a n d in high m a g n e t i c fields: T h e onset field is in r e a s o n a b l e agreement with a phase b o u n d a r y near 2 T observed up to a b o u t 0.5 K in field dependent specific h e a t m e a s u r e m e n t s [3]. T h e onset field is also in g o o d a g r e e m e n t w i t h a sharp kinks in H c 2 ( T ) o b s e r v e d in t h e same s a m p l e [4]. A p h a s e d i a g r a m for our p r o p o s e d fieldi n d u c e d m a g n e t i c t r a n s i t i o n is shown in Fig. 2, w h e r e T is p l o t t e d on a logarithmic scale. T h e small solid circles r e p r e s e n t m e a s u r e m e n t s of H c 2 ( T ) r e p o r t e d elsewhere [4]. T h e large o p e n circles denote t h e positions of sharp kinks in Hc2 (T) which are too small to be easily visible in Fig. 2. T h e large closed circles represent t h e onset of t h e a n o m a l o u s m a g n e t i c
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)E0702-I
246
i0
i2
UBeta alngla crystal
UBea
t0
H U [~.00]
sample #3
"'3.
@ ,..~
d
sample #3
£ 2 Ca)
-2
•
i
'
i
i
t0
20
T (K)
i
J
I
30
T
t.t 0.9
4
Cb)
0.7 D m
!
0.5
/
0.3
/ =
I
t "l
0
singla crystal
/
0.t -0.t o
UBe~a
0.1
s a ~ l e #3
i'o
=
|
t5
=
'
10
T (K)
1.OK
=
i
25
H (T)
F i g u r e 1. (a) T h e t r a n s v e r s e m a g n e t i z a t i o n (M_L, see t e x t ) p l o t t e d as a f u n c t i o n o f t e m p e r a t u r e a t f i x e d fields o f 10 T ( s q u a r e s ) , 15 T ( t r i a n g l e s ) , a n d 20 T (circles). T h e solid lines a r e g u i d e s t o t h e eye. (b) M ± p l o t t e d as a f u n c t i o n o f field a t 1.0 K.
t o r q u e d e t e r m i n e d in a m a n n e r d e s c r i b e d e l s e w h e r e [2]. T h e solid line is a g u i d e t o the eye. T h i s w o r k was s u p p o r t e d b y t h e National Science Foundation under DMR-9019661 and by a Cottrell College Science Grant from the Research Corporation. W o r k a t Los A l a m o s was p e r f o r m e d u n d e r t h e a u s p i c e s o f t h e U.S. D e p a r t m e n t o f E n e r g y . T h e h i g h field m e a s u r e m e n t s were made at the Francis Bitter National Magnet Laboratory (supported by the NSF).
F i g u r e 2. T h e low t e m p e r a t u r e p h a s e dia g r a m o f U B e l 3 in t h e H - T p l a n e . T h e solid line is a g u i d e t o t h e eye.
REFERENCES [1] G. M . S c h m i e d e s h o f f , P h i l . M a g . 711 (1992). [2] G. M. S c h m i e d e s h o f f et al., a n d ences therein, to be published. [3] B. E l l m a n e t al., P h y s . R e v . 12074 (1991). [4] G. M. S c h m i e d e s h o f f et al., P h y s . B 45, 10544 (1992).
B 66, referB 44, Rev.