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Absorption peak for ultrasonic waves near the spin-flop transition of uniaxial antiferromagnets
Volume 24A, number 7
PHYSICS LETTERS
ABSORPTION PEAK FOR ULTRASONIC SPIN-FLOP TRANSITION OF UNIAXIAL
27 March 1967
WAVES NEAR THE ANTIFERROMAGNETS
Y. SHAPIRA National Magnet Laboratory*, Massachusetts Institute of Technology, Cambridge, Mass.
Received 21 February 1967
An absorption peak for longitudinal and shear ultrasonic waves was observed near the spin-flop transition of (Cr203)0.94-(A1203)0.06 and of MnF2.
We r e p o r t the o b s e r v a t i o n of a l a r g e a b s o r p tion peak for both longitudinal and s h e a r u l t r a sonic waves n e a r the spin-flop t r a n s i t i o n of (Cr203)0.94-(A1203)0.06 and of MnF 2. To our knowledge, this i s the f i r s t t i m e such a p h e n o m enon has been o b s e r v e d in an a n t i f e r r o m a g n e t . E x p e r i m e n t s w e r e p e r f o r m e d on single c r y s t a l s of (Cr203)0.94-(AI203)0.06 and of MnF 2. The m a g n e t i c behavior of these m a t e r i a l s at low t e m p e r a t u r e s i s well a p p r o x i m a t e d by that of a u n i a x i a l a n t i f e r r o m a g n e t in which the m a g n e t i z a tion of each sublattice at zero m a g n e t i c field i s p a r a l l e l to the c - a x i s [1]. The application of a s t r o n g m a g n e t i c field, /'/, along the c - a x i s , r e s u l t s in a rotation of the a n t i p a r a l l e l sublattice m a g n e t i z a t i o n s to a d i r e c t i o n p e r p e n d i c u l a r to !1 (spin-flop). A n t i f e r r o m a g n e t i c r e s o n a n c e e x p e r i m e n t s * * , p e r f o r m e d at 77OK, showed that the spin-flop t r a n s i t i o n in (Cr203)0.94-(A1203)0.06 o c c u r s at the field Hsf = 77 + 2 kG. F o r MnF 2 it i s known [1] that Hsf = 93 kG at 4.2°K. The u l t r a s o n i c attenuation of longitudinal and l i n e a r l y p o l a r i z e d s h e a r u l t r a s o n i c waves p r o p a gating along the c - a x i s of the s a m p l e s was m e a s u r e d as a function of magnetic field. The m e a s u r e m e n t s on (Cr203)0.94-(AI203)0.06 were p e r f o r m e d at 77 and 4.2OK u s i n g 7 to 5 6 - M c / s sound waves, and the e x p e r i m e n t s on MnF2 w e r e conducted at 4.2OK u s i n g 8 to 2 2 0 - M c / s sound waves. W i t h / ' / d i r e c t e d along the c - a x i s , a peak in the a b s o r p t i o n was o b s e r v e d in all c a s e s n e a r the spin-flop t r a n s i t i o n . An example of the data is * Supported by the U.S,Air Force Office of Scientific Research. ** The AFMR experiments were performed in collaboration with S. Foner of this laboratory.
shown in fig. 1. Fig. 2 shows photographs of an echo p a t t e r n of a 5 2 - M c / s longitudinal wave in (Cr203)0.94-(A1203)0.06 at z e r o field and at 76 kG where the a b s o r p t i o n is m a x i m u m . E x p e r i m e n t s with longitudinal sound waves showed that the magnitude of the a b s o r p t i o n peak in both m a t e r i a l s d e c r e a s e s r a p i d l y a s the angle 8 between H and the d i r e c t i o n of propagation of the sound wave (c-axis) i n c r e a s e s . As a r e s u l t , the a b s o r p t i o n peak in (Cr203)0.94-(A1203)0.06 cannot be o b s e r v e d if 0 i s l a r g e r than a few deg r e e s . To o b s e r v e the a b s o r p t i o n peak in MnF 2 the angle 0 should be s m a l l e r than ~ 1°. (Crz 03)o.94 -(AIz 03 )0.06 52 Mc LONGITUDINAL qll HII C, T= 77"K
i 40
60
1
80 H (KILOGAUSS)
t
I00
Fig. 1. Recorder tracing of the magnetic-field variation of the attenuation of a 52-Mc/s longitudinal wave in (Cr203)0.94-(A1203) 0.06. 361
Volume 24A, number 7
P H YSI C S L E T T E R S
27 March 1967
The i n t e r a c t i o n of sound w a v e s with spin w a v e s in u n i a x i a l a n t i f e r r o m a g n e t s was c o n s i d e r e d by P e l e t m i n s k i i and Savchenko [2]. T h e s e a u t h o r s p r e d i c t e d the o c c u r r e n c e of an a b s o r p t i o n peak n e a r the spin-flop t r a n s i t i o n f o r s h e a r w a v e s propagating along the p r e f e r r e d axis. H o w e v e r , t h e i r t h e o r y does not p r e d i c t the o b s e r v e d a b s o r p t i o n peak f o r longitudinal w a v e s p r o p a g a t i n g along the p r e f e r r e d exis. It t h e r e f o r e a p p e a r s that a c o m p l e t e explanation of the data r e q u i r e s e i t h e r a m o d i f i c a t i o n of the P e l e t m i n s k i i - S a v c h e n k o t h e o r y o r the introduction of additional a b s o r p t i o n m e c h a n i s m s for sound w a v e s n e a r the spin-flop transition.
RFfF~'~ZCeS Fig. 2. Echo patterns of a 52-Mc/s longitudinal wave propagating along the c-axis of (Cr203) 0 94-(A1203)0 06 at 77°K. The upper photograph was taken at H = 0. " The lower photograph shows the same echo train at 76 kG.
1. S. Foner, in Magnetism, eds. G.T.Rado and H.Suhl (Academic Press, Inc., New York, 1963). Vol. I, p. 383 and references therein. 2. S.V. Peletminskii, Soviet Physics-JETP 10 (1960) 321; M. A. Savchenko, Soviet Physics-Solid State 6, (1964) 666.
SUPERCONDUCTING ENERGY GAP OF BULK BY POINT CONTACT TUNNELING
LANTHANUM
H. J. LEVINSTEIN, V. G. CHIRBA and J. E. K U N Z L E R
Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey
Received 28 February 1967
The superconducting energy gaps of both cubic and hexagonal lanthanum, obtained by point contact tunneling to bulk samples, arc reported. The values are somewhat higher than predicted by BCS theory as is typical of strong coupling superconductors.
The f i r s t r e p o r t e d e l e c t r o n tunneling o b s e r v a t i o n s of the e n e r g y gap of lanthanum by E d e l stein and Toxen [1] yield an e n e r g y gap v a l u e that i s about one half of the value p r e d i c t e d by the BCS theory. Since t h e o r i e s had p r e v i o u s l y been s u g g e s t e d [2, 3] that could lead to an en362
ergy gap significantly smaller than the B C S value, the observations apparently were considered reasonable andwere cited as evidence that "superconductivity in lanthanum m a y be stronly influenced by electronic interactions other than the electron-phonon interaction of the B C S theory'[I]
PHYSICS LETTERS
ABSORPTION PEAK FOR ULTRASONIC SPIN-FLOP TRANSITION OF UNIAXIAL
27 March 1967
WAVES NEAR THE ANTIFERROMAGNETS
Y. SHAPIRA National Magnet Laboratory*, Massachusetts Institute of Technology, Cambridge, Mass.
Received 21 February 1967
An absorption peak for longitudinal and shear ultrasonic waves was observed near the spin-flop transition of (Cr203)0.94-(A1203)0.06 and of MnF2.
We r e p o r t the o b s e r v a t i o n of a l a r g e a b s o r p tion peak for both longitudinal and s h e a r u l t r a sonic waves n e a r the spin-flop t r a n s i t i o n of (Cr203)0.94-(A1203)0.06 and of MnF 2. To our knowledge, this i s the f i r s t t i m e such a p h e n o m enon has been o b s e r v e d in an a n t i f e r r o m a g n e t . E x p e r i m e n t s w e r e p e r f o r m e d on single c r y s t a l s of (Cr203)0.94-(AI203)0.06 and of MnF 2. The m a g n e t i c behavior of these m a t e r i a l s at low t e m p e r a t u r e s i s well a p p r o x i m a t e d by that of a u n i a x i a l a n t i f e r r o m a g n e t in which the m a g n e t i z a tion of each sublattice at zero m a g n e t i c field i s p a r a l l e l to the c - a x i s [1]. The application of a s t r o n g m a g n e t i c field, /'/, along the c - a x i s , r e s u l t s in a rotation of the a n t i p a r a l l e l sublattice m a g n e t i z a t i o n s to a d i r e c t i o n p e r p e n d i c u l a r to !1 (spin-flop). A n t i f e r r o m a g n e t i c r e s o n a n c e e x p e r i m e n t s * * , p e r f o r m e d at 77OK, showed that the spin-flop t r a n s i t i o n in (Cr203)0.94-(A1203)0.06 o c c u r s at the field Hsf = 77 + 2 kG. F o r MnF 2 it i s known [1] that Hsf = 93 kG at 4.2°K. The u l t r a s o n i c attenuation of longitudinal and l i n e a r l y p o l a r i z e d s h e a r u l t r a s o n i c waves p r o p a gating along the c - a x i s of the s a m p l e s was m e a s u r e d as a function of magnetic field. The m e a s u r e m e n t s on (Cr203)0.94-(AI203)0.06 were p e r f o r m e d at 77 and 4.2OK u s i n g 7 to 5 6 - M c / s sound waves, and the e x p e r i m e n t s on MnF2 w e r e conducted at 4.2OK u s i n g 8 to 2 2 0 - M c / s sound waves. W i t h / ' / d i r e c t e d along the c - a x i s , a peak in the a b s o r p t i o n was o b s e r v e d in all c a s e s n e a r the spin-flop t r a n s i t i o n . An example of the data is * Supported by the U.S,Air Force Office of Scientific Research. ** The AFMR experiments were performed in collaboration with S. Foner of this laboratory.
shown in fig. 1. Fig. 2 shows photographs of an echo p a t t e r n of a 5 2 - M c / s longitudinal wave in (Cr203)0.94-(A1203)0.06 at z e r o field and at 76 kG where the a b s o r p t i o n is m a x i m u m . E x p e r i m e n t s with longitudinal sound waves showed that the magnitude of the a b s o r p t i o n peak in both m a t e r i a l s d e c r e a s e s r a p i d l y a s the angle 8 between H and the d i r e c t i o n of propagation of the sound wave (c-axis) i n c r e a s e s . As a r e s u l t , the a b s o r p t i o n peak in (Cr203)0.94-(A1203)0.06 cannot be o b s e r v e d if 0 i s l a r g e r than a few deg r e e s . To o b s e r v e the a b s o r p t i o n peak in MnF 2 the angle 0 should be s m a l l e r than ~ 1°. (Crz 03)o.94 -(AIz 03 )0.06 52 Mc LONGITUDINAL qll HII C, T= 77"K
i 40
60
1
80 H (KILOGAUSS)
t
I00
Fig. 1. Recorder tracing of the magnetic-field variation of the attenuation of a 52-Mc/s longitudinal wave in (Cr203)0.94-(A1203) 0.06. 361
Volume 24A, number 7
P H YSI C S L E T T E R S
27 March 1967
The i n t e r a c t i o n of sound w a v e s with spin w a v e s in u n i a x i a l a n t i f e r r o m a g n e t s was c o n s i d e r e d by P e l e t m i n s k i i and Savchenko [2]. T h e s e a u t h o r s p r e d i c t e d the o c c u r r e n c e of an a b s o r p t i o n peak n e a r the spin-flop t r a n s i t i o n f o r s h e a r w a v e s propagating along the p r e f e r r e d axis. H o w e v e r , t h e i r t h e o r y does not p r e d i c t the o b s e r v e d a b s o r p t i o n peak f o r longitudinal w a v e s p r o p a g a t i n g along the p r e f e r r e d exis. It t h e r e f o r e a p p e a r s that a c o m p l e t e explanation of the data r e q u i r e s e i t h e r a m o d i f i c a t i o n of the P e l e t m i n s k i i - S a v c h e n k o t h e o r y o r the introduction of additional a b s o r p t i o n m e c h a n i s m s for sound w a v e s n e a r the spin-flop transition.
RFfF~'~ZCeS Fig. 2. Echo patterns of a 52-Mc/s longitudinal wave propagating along the c-axis of (Cr203) 0 94-(A1203)0 06 at 77°K. The upper photograph was taken at H = 0. " The lower photograph shows the same echo train at 76 kG.
1. S. Foner, in Magnetism, eds. G.T.Rado and H.Suhl (Academic Press, Inc., New York, 1963). Vol. I, p. 383 and references therein. 2. S.V. Peletminskii, Soviet Physics-JETP 10 (1960) 321; M. A. Savchenko, Soviet Physics-Solid State 6, (1964) 666.
SUPERCONDUCTING ENERGY GAP OF BULK BY POINT CONTACT TUNNELING
LANTHANUM
H. J. LEVINSTEIN, V. G. CHIRBA and J. E. K U N Z L E R
Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey
Received 28 February 1967
The superconducting energy gaps of both cubic and hexagonal lanthanum, obtained by point contact tunneling to bulk samples, arc reported. The values are somewhat higher than predicted by BCS theory as is typical of strong coupling superconductors.
The f i r s t r e p o r t e d e l e c t r o n tunneling o b s e r v a t i o n s of the e n e r g y gap of lanthanum by E d e l stein and Toxen [1] yield an e n e r g y gap v a l u e that i s about one half of the value p r e d i c t e d by the BCS theory. Since t h e o r i e s had p r e v i o u s l y been s u g g e s t e d [2, 3] that could lead to an en362
ergy gap significantly smaller than the B C S value, the observations apparently were considered reasonable andwere cited as evidence that "superconductivity in lanthanum m a y be stronly influenced by electronic interactions other than the electron-phonon interaction of the B C S theory'[I]