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The first order paramagnetic ferromagnetic phase transition in MnAs whiskers
Voluem 35A, number 5
PHYSICS LETTERS
The s e c o n d m a j o r r e s u l t i s : H a l l effect quant u m o s c i l l a t i o n p e r i o d s could b e m e a s u r e d f o r the f i e l d at a l l a n g l e s b e t w e e n z e r o and 87 d e g r e e s f r o m 'C'. T h e r a t i o of the p e r i o d at z e r o to that at 87 d e g r e e s i s g r e a t e r than 13, thus i n d i c a t i n g the F e r m i s u r f a c e i s e x t r e m e l y elongated. Soule found the r a t i o at z e r o to that at 90 d e g r e e s to b e 9 in a s i n g l e c r y s t a l . F r o m b o r o n doping Soule [5] c o n c l u d e d that the r e s p o n s i b l e c a r r i e r s w e r e h o l e s . T h i s s u g g e s t s that they o r i g i n a t e n e a r the 'H' p o i n t s of the B r i l l o u i n zone. T h i s was a l s o W i l l i a m s o n ' s c o n c l u s i o n [3] c o n c e r n i n g the c a r r i e r s with l a r g e r p e r i o d s . Thus, both p e r i o d b r a n c h e s shown in fig. 2 could o r i g n a t e n e a r the ~ ' point (~ and B in fig. 1). M c C l u r e [7] h a s s u g g e s t e d that t r i g o n a l a s y m m e t r y of the m a j o r i t y e l e c t r o n s could be the s o u r c e of m i n o r i t y e l e c t r o n e f f e c t s o b s e r v e d in the low f i e l d H a l l effect [8]. H o w e v e r , s e p a r a t e m i n o r i t y e l e c t r o n s (fig. 1) could be p r o d u c e d if
THE
FIRST
ORDER
28 June 1971
r 3 w e r e t h r e e t i m e s l a r g e r than p r e s e n t l y a c c e p t e d [4, 5]. The a u t h o r would like to acknowledge helpful c o n v e r s a t i o n s with D r s . M c C l u r e , Spain and Soule.
R efe~'ence$ [1] J.A. Woo[lam, Phys.Rev. B3 (1971) 1148; and references therein. [2] J. C. Slonczewski and P. R. Weiss, Phys. Rev. 109 (1958) 272. [3] S.J. Wiltiamson, S. Foiler and M. S. Dressethaus, Phys.Rev. 140 (1965) A1429. [4] P. Nozieres, Phys. Rev. 109 (1958) 1510. [5] D.E.Soule, I.B.M.J. Res.Dev. 8 (1964) 268. [6] S.J. Williamson, private communication. [7] J.W. McClure, Phys. Rev. 101 (1956) 1642. [8] L L. Spain, A.R. Ubbelohde and D. A. Young, Phih Trans. Roy. Soc. 262 (1967) 345.
PARAMAGNETIC FERROMAGNETIC IN MnAs WHISKERS
PHASE
TRANSITION
K. B ~ R N E R IV.Physikalisches Institut der Universitiit GOttingen, G~ttingen, Germany
Received 11 May 1971 In comparison to polycrystals resistivity and magnetic susceptibility of MnAs whiskers change very rapidly (several/~sec) at the transition temperature. However the whiskers are not destroyed by the mechanical strains arising from the discontinuous volume change.
The p r e p a r a t i o n of MnAs single c r y s t a l s i s difficult not only b e c a u s e of the high v a p o r p r e s s u r e of A s but a l s o b e c a u s e of the a b r u p t v o l u m e change at the f i r s t o r d e r t r a n s i t i o n , which t e n d s to d e s t r o y the s a m p l e m e c h a n i c a l l y [1]. M i c r o c r a c k s c a n be d e t e c t e d by m i c r o s c o p i c i n s p e c t i o n , but the i r r e v e r s i b l e i n c r e a s e in r e s i s t a n c e of the s p e c i m e n if t e m p e r a t u r e c y c l e s p a s s i n g the transition temperature are applied is a more s e n s i t i v e i n d i c a t o r [2]. MnAs w h i s k e r s (fig.l) g r o w at 950 o c in A s v a p o r on p o l y c r y s t a l l i n e M n - A s s u b s t r a t e . To realize these conditions glass ampules (Supremax) f i l l e d with p o w d e r e d Mn and A s ( e x c e s s ) a r e s e a l e d in i r o n p i p e s and e x p o s e d to the following t e m p e r a t u r e p r o g r a m : 6h 750 ° C , 24h 850 °C, 24h 950 o c , 6h 600 °C. To p r e v e n t oxide f r o m
Fig.1. MnAs whisker, diameter 19 ~ m coating MnAs, Mn ( g r a i n s i z e 50-100 /~m) i s f r e s h l y p o w d e r e d and A s f r e e d f r o m oxide by s u b l i m a t i o n . The w h i s k e r s u s u a l l y f o r m a s p i r a l with the tangent to the s c r e w line p a r a l l e l to the h e x a g o n a l a x i s Ch[00.1 ] ( d e t e r m i n a t i o n by X - r a y s - r o t a t i n g c r y s t a l method). The t r a n s i t i o n t e m p e r a t u r e Tu s i m p l y can be d e t e r m i n e d by o b s e r v i n g the motion of the slowly h e a t e d ( w a t e r bath) w h i s k e r in an inhomogeneous m a g n e t i c 333
Volume 35A, number 5
PHYSICS LETTERS
f i e l d H. We have m e a s u r e d the m a g n e t i c s u s c e p t i b i l i t y of one s p e c i m e n (TuT = 26 o c , t e m p e r a t u r e h y s t e r e s i s ATu = 6 o c , g r a d i e n t d T u l / d H = 2.38 k O e / ° C ) by setting the weight equal to bouyancy p l u s the f o r c e due to the c a l i b r a t e d m a g n e t i c f i e l d [3]. T h e p a r a m a g n e t i c s u s c e p t i b i l i t y did not d i f f e r f r o m the v a l u e m e a s u r e d on p o l y c r y s t a l l i n e m a t e r i a l ×v = 9.38 × 10 -4 c g s (30 ° C , density 6.17 g / c m 3 ) . The ' i n i t i a l ' s u s c e p t i b i l i t y Xo = 0.758 c g s (23 ° C , H = 130 - 520 Oe) is a l r e a d y c o r r e c t e d f o r the d e m a g n e t i z i n g field. C o r r e s p o n d i n g v a l u e s f o r p o l y c r y s t a l s a r e : TuT = 45 °C , A Tu = 13 o c , d T u T / d H = 2.57 kOe/OC, [4] and ×o = 0.262 c g s (20 o c , 760 Oe), [5]. In c o n t r a s t to p o l y c r y s t a l s the w h i s k e r a p p e a r e d to t r a n s f o r m at a v e r y definite t e m p e r a t u r e , the t r a n s i t i o n s t a r t i n g f r o m one n u cl eu s only. We did not find any c r a c k s on the s u r f a c e of r e g u l a r l y g r o w n s p i r a l s a f t e r w a r d s ( m a g n i f i c a t i o n 500 - fold). R e s i s t i v i t y m e a s u r e m e n t s (four p r o b e method) a r e m o s t suitable to c o n f i r m the o b s e r v a t i o n s made in m a g n e t i c fie lds. H o w e v e r c a r e m u s t be taken in making c o n t a c t s to the s p e c i m e n s , although only the t h i c k n e s s d of the w h i s k e r s c h a n g e s abruptly (Ad/d = 0.8%; ¢ h i s not a l t e r e d [3]). The e l e c t r i c a l c o n t a c t s using s i l v e r - a d h e s i v e l a q u e r m i x t u r e D e g u s s a L e i t s i l b e r 200 a r e found to be s t a b le , but s o m e t i m e s an additional r e s i s t a n c e a r i s i n g f r o m the finite width of the p o te n t ia l c o n t a c t s h a s to be c o n s i d e r e d . Fig.2 shows that the r e s i s t a n c e R of the w h i s k e r s i s p r o p o r t i o n a l to t h e i r g e o m e t r y f a c t o r . Th e r e s i s t i v i t y of 6.2 ± 0.5 × 10"4 f~ c m (24°C, p a r a m a g n e t i c state) is c l o s e to the v a l u e 6.7 × 10 -4 f~ c m (50 °C) f o r p o l y c r y s t a l l i n e m a t e r i a l g i v e n by [6]. The r e s i s t i v i t y i n c r e a s e s at the t r a n s i t i o n (TuT = 23 ° C , AT u = 3 - 6 °C) by a f a c t o r 2.5. A p o s i t i v e t e m p e r a t u r e c o e f f i c i e n t of r e s i s t a n c e ~ = 7 × 10 -3 o c - 1 is o b s e r v e d in the f e r r o m a g n e t i c p h a s e (~ = 5 × 10 -3 o c ' l [6]). In fig. 2, this r e v e r s i b l e p a r t r is a u t o m a t i c a l l y c o m p e n s a t e d in o r d e r to a m p l if y the discontinuous r e s i s t i v i t y i n c r e a s e . The s a m e r e s i s t a n c e v a l u e s
334
28 June 1971
r..~-Rp? - r(24"c)
1
5
o 0
25
t
50 °c
Fig.2. Discontinuous change in resistance at the first order phase transition, r = to(1 + st ) is subtracted. r o = 1.28 ~, ~ = 7 x 10-3 oc-1 ; constant current: 2/.tA ac, 130 s - l ; L length, D diameter, h inclination of spiral. a r e obtained a f t e r one t e m p e r a t u r e c y c l e , thus excluding m i c r o c r a c k s in the p o r t i o n u s e d f o r m e a s u r e m e n t . P r e l i m i n a r y e x p e r i m e n t s lead to t r a n s i t i o n t i m e s of 5 - 10 ~s. The author w i s h e s to thank P r o f . Dr. H. -U. H a r t e n f o r v a l u a b l e support.
References [1] z. S. Basinski and W. B. Pearson, Can. J. Phys. 36 (1958) 1017. [2] L . F . B a t e s , Phil.Meg. 7, Voi.8 (1929) 714. [3] K. Bttrner, dissertation, GSttingen, 1970. [4] R.W.de Blois and D.S.Rodbell, J.Appl.Phys. 34 4 (2) (1963) 1101. [5] L . F . B a t e s , Phil.Meg. 7, Vol.17 (1934)783. [6] G. Fischer and W. B. Pearson, Can. J. Phys., Vol 36 (1958) 1010.
PHYSICS LETTERS
The s e c o n d m a j o r r e s u l t i s : H a l l effect quant u m o s c i l l a t i o n p e r i o d s could b e m e a s u r e d f o r the f i e l d at a l l a n g l e s b e t w e e n z e r o and 87 d e g r e e s f r o m 'C'. T h e r a t i o of the p e r i o d at z e r o to that at 87 d e g r e e s i s g r e a t e r than 13, thus i n d i c a t i n g the F e r m i s u r f a c e i s e x t r e m e l y elongated. Soule found the r a t i o at z e r o to that at 90 d e g r e e s to b e 9 in a s i n g l e c r y s t a l . F r o m b o r o n doping Soule [5] c o n c l u d e d that the r e s p o n s i b l e c a r r i e r s w e r e h o l e s . T h i s s u g g e s t s that they o r i g i n a t e n e a r the 'H' p o i n t s of the B r i l l o u i n zone. T h i s was a l s o W i l l i a m s o n ' s c o n c l u s i o n [3] c o n c e r n i n g the c a r r i e r s with l a r g e r p e r i o d s . Thus, both p e r i o d b r a n c h e s shown in fig. 2 could o r i g n a t e n e a r the ~ ' point (~ and B in fig. 1). M c C l u r e [7] h a s s u g g e s t e d that t r i g o n a l a s y m m e t r y of the m a j o r i t y e l e c t r o n s could be the s o u r c e of m i n o r i t y e l e c t r o n e f f e c t s o b s e r v e d in the low f i e l d H a l l effect [8]. H o w e v e r , s e p a r a t e m i n o r i t y e l e c t r o n s (fig. 1) could be p r o d u c e d if
THE
FIRST
ORDER
28 June 1971
r 3 w e r e t h r e e t i m e s l a r g e r than p r e s e n t l y a c c e p t e d [4, 5]. The a u t h o r would like to acknowledge helpful c o n v e r s a t i o n s with D r s . M c C l u r e , Spain and Soule.
R efe~'ence$ [1] J.A. Woo[lam, Phys.Rev. B3 (1971) 1148; and references therein. [2] J. C. Slonczewski and P. R. Weiss, Phys. Rev. 109 (1958) 272. [3] S.J. Wiltiamson, S. Foiler and M. S. Dressethaus, Phys.Rev. 140 (1965) A1429. [4] P. Nozieres, Phys. Rev. 109 (1958) 1510. [5] D.E.Soule, I.B.M.J. Res.Dev. 8 (1964) 268. [6] S.J. Williamson, private communication. [7] J.W. McClure, Phys. Rev. 101 (1956) 1642. [8] L L. Spain, A.R. Ubbelohde and D. A. Young, Phih Trans. Roy. Soc. 262 (1967) 345.
PARAMAGNETIC FERROMAGNETIC IN MnAs WHISKERS
PHASE
TRANSITION
K. B ~ R N E R IV.Physikalisches Institut der Universitiit GOttingen, G~ttingen, Germany
Received 11 May 1971 In comparison to polycrystals resistivity and magnetic susceptibility of MnAs whiskers change very rapidly (several/~sec) at the transition temperature. However the whiskers are not destroyed by the mechanical strains arising from the discontinuous volume change.
The p r e p a r a t i o n of MnAs single c r y s t a l s i s difficult not only b e c a u s e of the high v a p o r p r e s s u r e of A s but a l s o b e c a u s e of the a b r u p t v o l u m e change at the f i r s t o r d e r t r a n s i t i o n , which t e n d s to d e s t r o y the s a m p l e m e c h a n i c a l l y [1]. M i c r o c r a c k s c a n be d e t e c t e d by m i c r o s c o p i c i n s p e c t i o n , but the i r r e v e r s i b l e i n c r e a s e in r e s i s t a n c e of the s p e c i m e n if t e m p e r a t u r e c y c l e s p a s s i n g the transition temperature are applied is a more s e n s i t i v e i n d i c a t o r [2]. MnAs w h i s k e r s (fig.l) g r o w at 950 o c in A s v a p o r on p o l y c r y s t a l l i n e M n - A s s u b s t r a t e . To realize these conditions glass ampules (Supremax) f i l l e d with p o w d e r e d Mn and A s ( e x c e s s ) a r e s e a l e d in i r o n p i p e s and e x p o s e d to the following t e m p e r a t u r e p r o g r a m : 6h 750 ° C , 24h 850 °C, 24h 950 o c , 6h 600 °C. To p r e v e n t oxide f r o m
Fig.1. MnAs whisker, diameter 19 ~ m coating MnAs, Mn ( g r a i n s i z e 50-100 /~m) i s f r e s h l y p o w d e r e d and A s f r e e d f r o m oxide by s u b l i m a t i o n . The w h i s k e r s u s u a l l y f o r m a s p i r a l with the tangent to the s c r e w line p a r a l l e l to the h e x a g o n a l a x i s Ch[00.1 ] ( d e t e r m i n a t i o n by X - r a y s - r o t a t i n g c r y s t a l method). The t r a n s i t i o n t e m p e r a t u r e Tu s i m p l y can be d e t e r m i n e d by o b s e r v i n g the motion of the slowly h e a t e d ( w a t e r bath) w h i s k e r in an inhomogeneous m a g n e t i c 333
Volume 35A, number 5
PHYSICS LETTERS
f i e l d H. We have m e a s u r e d the m a g n e t i c s u s c e p t i b i l i t y of one s p e c i m e n (TuT = 26 o c , t e m p e r a t u r e h y s t e r e s i s ATu = 6 o c , g r a d i e n t d T u l / d H = 2.38 k O e / ° C ) by setting the weight equal to bouyancy p l u s the f o r c e due to the c a l i b r a t e d m a g n e t i c f i e l d [3]. T h e p a r a m a g n e t i c s u s c e p t i b i l i t y did not d i f f e r f r o m the v a l u e m e a s u r e d on p o l y c r y s t a l l i n e m a t e r i a l ×v = 9.38 × 10 -4 c g s (30 ° C , density 6.17 g / c m 3 ) . The ' i n i t i a l ' s u s c e p t i b i l i t y Xo = 0.758 c g s (23 ° C , H = 130 - 520 Oe) is a l r e a d y c o r r e c t e d f o r the d e m a g n e t i z i n g field. C o r r e s p o n d i n g v a l u e s f o r p o l y c r y s t a l s a r e : TuT = 45 °C , A Tu = 13 o c , d T u T / d H = 2.57 kOe/OC, [4] and ×o = 0.262 c g s (20 o c , 760 Oe), [5]. In c o n t r a s t to p o l y c r y s t a l s the w h i s k e r a p p e a r e d to t r a n s f o r m at a v e r y definite t e m p e r a t u r e , the t r a n s i t i o n s t a r t i n g f r o m one n u cl eu s only. We did not find any c r a c k s on the s u r f a c e of r e g u l a r l y g r o w n s p i r a l s a f t e r w a r d s ( m a g n i f i c a t i o n 500 - fold). R e s i s t i v i t y m e a s u r e m e n t s (four p r o b e method) a r e m o s t suitable to c o n f i r m the o b s e r v a t i o n s made in m a g n e t i c fie lds. H o w e v e r c a r e m u s t be taken in making c o n t a c t s to the s p e c i m e n s , although only the t h i c k n e s s d of the w h i s k e r s c h a n g e s abruptly (Ad/d = 0.8%; ¢ h i s not a l t e r e d [3]). The e l e c t r i c a l c o n t a c t s using s i l v e r - a d h e s i v e l a q u e r m i x t u r e D e g u s s a L e i t s i l b e r 200 a r e found to be s t a b le , but s o m e t i m e s an additional r e s i s t a n c e a r i s i n g f r o m the finite width of the p o te n t ia l c o n t a c t s h a s to be c o n s i d e r e d . Fig.2 shows that the r e s i s t a n c e R of the w h i s k e r s i s p r o p o r t i o n a l to t h e i r g e o m e t r y f a c t o r . Th e r e s i s t i v i t y of 6.2 ± 0.5 × 10"4 f~ c m (24°C, p a r a m a g n e t i c state) is c l o s e to the v a l u e 6.7 × 10 -4 f~ c m (50 °C) f o r p o l y c r y s t a l l i n e m a t e r i a l g i v e n by [6]. The r e s i s t i v i t y i n c r e a s e s at the t r a n s i t i o n (TuT = 23 ° C , AT u = 3 - 6 °C) by a f a c t o r 2.5. A p o s i t i v e t e m p e r a t u r e c o e f f i c i e n t of r e s i s t a n c e ~ = 7 × 10 -3 o c - 1 is o b s e r v e d in the f e r r o m a g n e t i c p h a s e (~ = 5 × 10 -3 o c ' l [6]). In fig. 2, this r e v e r s i b l e p a r t r is a u t o m a t i c a l l y c o m p e n s a t e d in o r d e r to a m p l if y the discontinuous r e s i s t i v i t y i n c r e a s e . The s a m e r e s i s t a n c e v a l u e s
334
28 June 1971
r..~-Rp? - r(24"c)
1
5
o 0
25
t
50 °c
Fig.2. Discontinuous change in resistance at the first order phase transition, r = to(1 + st ) is subtracted. r o = 1.28 ~, ~ = 7 x 10-3 oc-1 ; constant current: 2/.tA ac, 130 s - l ; L length, D diameter, h inclination of spiral. a r e obtained a f t e r one t e m p e r a t u r e c y c l e , thus excluding m i c r o c r a c k s in the p o r t i o n u s e d f o r m e a s u r e m e n t . P r e l i m i n a r y e x p e r i m e n t s lead to t r a n s i t i o n t i m e s of 5 - 10 ~s. The author w i s h e s to thank P r o f . Dr. H. -U. H a r t e n f o r v a l u a b l e support.
References [1] z. S. Basinski and W. B. Pearson, Can. J. Phys. 36 (1958) 1017. [2] L . F . B a t e s , Phil.Meg. 7, Voi.8 (1929) 714. [3] K. Bttrner, dissertation, GSttingen, 1970. [4] R.W.de Blois and D.S.Rodbell, J.Appl.Phys. 34 4 (2) (1963) 1101. [5] L . F . B a t e s , Phil.Meg. 7, Vol.17 (1934)783. [6] G. Fischer and W. B. Pearson, Can. J. Phys., Vol 36 (1958) 1010.