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Influence of the magnetic field on the electrical resistivity of U3As4 in the vicinity of the curie point
991 INFLUENCE OF THE MAGNETIC FIELD ON THE ELECTRICAL RESISTIVITY OF UaAs4 IN THE VICINITY OF THE CURIE POINT Z. H E N K I E and J. K L A M U T Institute [or Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 937, 50-950 Wroctaw, Poland
The influence of the magnetic field up to 150Oe, applied along the direction [111], on the longitudinal resistivity of U~As4 has been investigated. Results show that the electrical resistivity depends strongly on the domain structure and at lower magnetic fields the spin critical fluctuation induces sharp maximum of the resistivity at To.
The electrical resistivity of magnetic materials in the vicinity of the transition temperature depends, first of all, on the magnetic critical fluctuations. Among various other factors which apparently influence the magnetic part of electrical resistivity, the topology of the Fermi surface is also considered to be of some importance. Therefore, a large variety of behaviour of the electrical resistivity in the critical region is observed. In spite of a large number of experimental and theoretical papers devoted to this problem, the influence of the spin fluctuations on the scattering of conduction electrons requires further investigation. A peculiar dependence of the resistivity on the temperature has been found for U3As4. This compound reveals two resistivity maxima: the first below T~ and the second in the close vicinity of T~. Further investigations have shown that an external magnetic field larger than 150 Oe very strongly influences the dependence p(T) [1] in the neighbourhood of To, and moreover, in an anisotropic way. In the present paper we study the influence of a magnetic field smaller than 150Oe on the longitudinal resistivity of U3As4 in the [111] direction. U 3 A s 4 crystallizes in a b.c.c, lattice of the Th3P4-type structure. The dependence of the magnetization on the temperature below 198 K is characteristic for a ferromagnet [2]. This compound shows an extremely large magnetic anisotropy with the effective easy axis of magnetization along the direction [111] [3,4]. Both the electrical and magnetic properties point to the validity of the hypothesis that this ferromagnet forms three magnetic sublattices with mutually perpendicular magnetic axis [4,5]. This hypothesis has not been completely confirmed by neutron diffraction data. The measurements of the resistivity were carried out with the conventional four-point d.c. Physica 86-88B (1977) 991-992 O North-Holland
method. Single crystals of the dimensions 0.3 x 0.3 x 4 mm 3 with the longest dimension along the [111] axis were fastened to a copper block through an electrically insulating layer. The temperature of the block was measured with a copper-constantan thermocouple. The external field was produced by the solenoid and the earth field was compensated with Helmhoitz coils. The temperature dependence of the resistivity for various field strengths of weak magnetic fields is shown in fig. 1. It is seen that the maximum in the resistivity (below To) is strongly dependent on the magnetic field strength; the value of the resistivity at the maximum increases and moves towards low temperature when the field strength increases. Fig. 2 shows the dependence of the resistivity on the field strength below To. The resistivity increases with the increase of the field strength and tends to a saturation. It seems that the obtained dependence is related to magnetic domain processes. Two distinct maxima are
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Fig. I. Temperature dependence of the longitudinal resis-
tivity for various fieldstrengths for a single crystal of U3As,.
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0 [10"~2]
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T = I'~ 5.8
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Fig. 3. Dependence of the magnetoresistance [(AR/R)= (R(0) - R(H)/R(0)) • 100%] on the angle (a) between the current (i) and the magnetic field (H). H is rotating in the plane (710); iH[lll]; T = 78 K; H = 0.69 T.
"K
s5.6
H[ 0 4 i
i
I
~_
~o 5o Fig. 2. Dependence of the resistivity on the field strength at the constant temperature (T = 193.8 K). s e e n in the c u r v e s p(T) f o r the fields o f the s t r e n g t h 15, 26.5, 30 a n d 4 0 O e . O n e o f t h e m o c c u r s at t h e t e m p e r a t u r e c o r r e s p o n d i n g to t h e m a x i m u m in the f i e l d - f r e e c a s e . T h e l a t t e r m a x i m u m is u s u a l l y o b s e r v e d in f e r r o m a g n e t i c s as a c o n s e q u e n c e o f s c a t t e r i n g of t h e c o n d u c tion e l e c t r o n s b y the critical m a g n e t i c fluctuations. T h e i m p o r t a n t role o f t h e d o m a i n s t r u c t u r e in the i n t e r p r e t a t i o n c o n f i r m s , in o u r o p i n i o n , t h e results of the measurement of the dependence of the m a g n e t o r e s i s t a n c e on the angle b e t w e e n t h e v e c t o r s o f t h e c u r r e n t a n d t h a t of m a g n e t i c field. Fig. 3 s h o w s this d e p e n d e n c e f o r t h e c a s e w h e n the v e c t o r of t h e c u r r e n t i n t e n s i t y is p a r a l l e l to the d i r e c t i o n [111] a n d t h a t o f t h e field is r o t a t i n g in t h e p l a n e (710) at the c o n s t a n t t e m p e r a t u r e ( T = 78 K) a n d the c o n s t a n t field s t r e n g t h (0.69 T). T h e l a t t e r r e s u l t s i n d i c a t e that the e l e c t r i c a l
r e s i s t i v i t y o f U3As 4 is s t r o n g l y a n i s o t r o p i c in a d o m a i n . If the m a g n e t i c m o m e n t s a r e d i r e c t e d a l o n g t h e e a s y axis [111], the r e s i s t i v i t y in this d i r e c t i o n is a b o u t 20% g r e a t e r t h a n in t h e c a s e with the m a g n e t i c m o m e n t s d i r e c t e d in a d i f f e r e n t d i r e c t i o n . A s w e m e n t i o n e d , it is the n o n - c o l l i n e a r m a g n e t i c s t r u c t u r e w h i c h is, a m o n g s t o t h e r f a c t o r s , r e s p o n s i b l e f o r such c o n s i d e r a b l e a n i s o t r o p y in t h e d o m a i n . On the b a s i s of r e s u l t s s h o w n in figs. 2 a n d 3 it s e e m s that a p a r t f r o m t h e p o s s i b l e c a u s e s o f the a n i s o t r o p y in the d o m a i n , t h e d o m a i n s t r u c t u r e is o f p r i m e i m p o r t a n c e f o r the b e h a v i o u r o f e l e c t r i c a l r e s i s t i v i t y b e l o w Tc a n d at w e a k fields. References
[1] Z. Henkie and J. Klamut, Phys. Status solidi (a) 20 (1973) K69. [2] W. Trzebiatowski, A, Sgpichowska and A. Zygmunt, Bull. Acad. Polon. Sci., S6r. Sci. Chim. 12 0963) 661. [3l C. Buhrer, J. Phys. Chem. Solids 30 (1969) 1273. [4] K.P. Belov, Z. Henkie, A.S. Dmitrievsky, R.Z, Levitin and W, Trzebiatowski Zh. Eks. Teor. Fiz. 64 (1973) 1552. [5] Z. Henkie and C. Bazan, Phys. Status Solidi (a) 5 (1971) 259.
The influence of the magnetic field up to 150Oe, applied along the direction [111], on the longitudinal resistivity of U~As4 has been investigated. Results show that the electrical resistivity depends strongly on the domain structure and at lower magnetic fields the spin critical fluctuation induces sharp maximum of the resistivity at To.
The electrical resistivity of magnetic materials in the vicinity of the transition temperature depends, first of all, on the magnetic critical fluctuations. Among various other factors which apparently influence the magnetic part of electrical resistivity, the topology of the Fermi surface is also considered to be of some importance. Therefore, a large variety of behaviour of the electrical resistivity in the critical region is observed. In spite of a large number of experimental and theoretical papers devoted to this problem, the influence of the spin fluctuations on the scattering of conduction electrons requires further investigation. A peculiar dependence of the resistivity on the temperature has been found for U3As4. This compound reveals two resistivity maxima: the first below T~ and the second in the close vicinity of T~. Further investigations have shown that an external magnetic field larger than 150 Oe very strongly influences the dependence p(T) [1] in the neighbourhood of To, and moreover, in an anisotropic way. In the present paper we study the influence of a magnetic field smaller than 150Oe on the longitudinal resistivity of U3As4 in the [111] direction. U 3 A s 4 crystallizes in a b.c.c, lattice of the Th3P4-type structure. The dependence of the magnetization on the temperature below 198 K is characteristic for a ferromagnet [2]. This compound shows an extremely large magnetic anisotropy with the effective easy axis of magnetization along the direction [111] [3,4]. Both the electrical and magnetic properties point to the validity of the hypothesis that this ferromagnet forms three magnetic sublattices with mutually perpendicular magnetic axis [4,5]. This hypothesis has not been completely confirmed by neutron diffraction data. The measurements of the resistivity were carried out with the conventional four-point d.c. Physica 86-88B (1977) 991-992 O North-Holland
method. Single crystals of the dimensions 0.3 x 0.3 x 4 mm 3 with the longest dimension along the [111] axis were fastened to a copper block through an electrically insulating layer. The temperature of the block was measured with a copper-constantan thermocouple. The external field was produced by the solenoid and the earth field was compensated with Helmhoitz coils. The temperature dependence of the resistivity for various field strengths of weak magnetic fields is shown in fig. 1. It is seen that the maximum in the resistivity (below To) is strongly dependent on the magnetic field strength; the value of the resistivity at the maximum increases and moves towards low temperature when the field strength increases. Fig. 2 shows the dependence of the resistivity on the field strength below To. The resistivity increases with the increase of the field strength and tends to a saturation. It seems that the obtained dependence is related to magnetic domain processes. Two distinct maxima are
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o° ,
izo
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I
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~o
,
I
zoo
l-
T['K]
Fig. I. Temperature dependence of the longitudinal resis-
tivity for various fieldstrengths for a single crystal of U3As,.
992
j~
0 [10"~2]
l i
i
I-
55.7 O*
T = I'~ 5.8
II0 °
i
b i
/,R R
I0%
- --i I I l i I
.... 20~o
~
360 °
Fig. 3. Dependence of the magnetoresistance [(AR/R)= (R(0) - R(H)/R(0)) • 100%] on the angle (a) between the current (i) and the magnetic field (H). H is rotating in the plane (710); iH[lll]; T = 78 K; H = 0.69 T.
"K
s5.6
H[ 0 4 i
i
I
~_
~o 5o Fig. 2. Dependence of the resistivity on the field strength at the constant temperature (T = 193.8 K). s e e n in the c u r v e s p(T) f o r the fields o f the s t r e n g t h 15, 26.5, 30 a n d 4 0 O e . O n e o f t h e m o c c u r s at t h e t e m p e r a t u r e c o r r e s p o n d i n g to t h e m a x i m u m in the f i e l d - f r e e c a s e . T h e l a t t e r m a x i m u m is u s u a l l y o b s e r v e d in f e r r o m a g n e t i c s as a c o n s e q u e n c e o f s c a t t e r i n g of t h e c o n d u c tion e l e c t r o n s b y the critical m a g n e t i c fluctuations. T h e i m p o r t a n t role o f t h e d o m a i n s t r u c t u r e in the i n t e r p r e t a t i o n c o n f i r m s , in o u r o p i n i o n , t h e results of the measurement of the dependence of the m a g n e t o r e s i s t a n c e on the angle b e t w e e n t h e v e c t o r s o f t h e c u r r e n t a n d t h a t of m a g n e t i c field. Fig. 3 s h o w s this d e p e n d e n c e f o r t h e c a s e w h e n the v e c t o r of t h e c u r r e n t i n t e n s i t y is p a r a l l e l to the d i r e c t i o n [111] a n d t h a t o f t h e field is r o t a t i n g in t h e p l a n e (710) at the c o n s t a n t t e m p e r a t u r e ( T = 78 K) a n d the c o n s t a n t field s t r e n g t h (0.69 T). T h e l a t t e r r e s u l t s i n d i c a t e that the e l e c t r i c a l
r e s i s t i v i t y o f U3As 4 is s t r o n g l y a n i s o t r o p i c in a d o m a i n . If the m a g n e t i c m o m e n t s a r e d i r e c t e d a l o n g t h e e a s y axis [111], the r e s i s t i v i t y in this d i r e c t i o n is a b o u t 20% g r e a t e r t h a n in t h e c a s e with the m a g n e t i c m o m e n t s d i r e c t e d in a d i f f e r e n t d i r e c t i o n . A s w e m e n t i o n e d , it is the n o n - c o l l i n e a r m a g n e t i c s t r u c t u r e w h i c h is, a m o n g s t o t h e r f a c t o r s , r e s p o n s i b l e f o r such c o n s i d e r a b l e a n i s o t r o p y in t h e d o m a i n . On the b a s i s of r e s u l t s s h o w n in figs. 2 a n d 3 it s e e m s that a p a r t f r o m t h e p o s s i b l e c a u s e s o f the a n i s o t r o p y in the d o m a i n , t h e d o m a i n s t r u c t u r e is o f p r i m e i m p o r t a n c e f o r the b e h a v i o u r o f e l e c t r i c a l r e s i s t i v i t y b e l o w Tc a n d at w e a k fields. References
[1] Z. Henkie and J. Klamut, Phys. Status solidi (a) 20 (1973) K69. [2] W. Trzebiatowski, A, Sgpichowska and A. Zygmunt, Bull. Acad. Polon. Sci., S6r. Sci. Chim. 12 0963) 661. [3l C. Buhrer, J. Phys. Chem. Solids 30 (1969) 1273. [4] K.P. Belov, Z. Henkie, A.S. Dmitrievsky, R.Z, Levitin and W, Trzebiatowski Zh. Eks. Teor. Fiz. 64 (1973) 1552. [5] Z. Henkie and C. Bazan, Phys. Status Solidi (a) 5 (1971) 259.