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Magnetic interaction in EuS, EuSe, and EuTe
~rolume 2, number 5
PHYSICS
MAGNETIC
INTERACTION
LETTERS
IN EuS,
1 October 1962
EuSe,
AND
EuTe
S. VAN HOUTEN Philips Research Laboratories, N.V. Philips' Gloeflampenfabrieken, Eindhoven, Netherlands Received 3 September 1962
In t h i s l e t t e r the p r e p a r a t i o n , c r y s t a l s t r u c t u r e and m a g n e t i c p r o p e r t i e s of EuS, EuSe a n d E u T e a r e d e s c r i b e d . We have found EuS a n d EuSe to be p u r e l y f e r r o m a g n e t i c , i n s u l a t i n g c o m p o u n d s with C u r i e p o i n t s of 16°K a n d 6 ° K , r e s p e c t i v e l y , w h e r e a s E u T e p r o b a b l y i s w e a k l y a n t i f e r r o m a g n e t i c with a n a s y m p t o t i c C u r i e p o i n t of - 5 ° K and a N~el t e m p e r a t u r e of a b o u t 6°K. EuO h a s a l r e a d y b e e n d e s c r i b e d to be f e r r o m a g n e t i c with a C u r i e p o i n t of 7 7 ° K by M a t t h i a s et al. 1). T h i s h a s r e c e n t l y b e e n c o n f i r m e d by m e a n s of n e u t r o n d i f f r a c t i o n 2). T h e s e c o m p o u n d s w e r e p r e v i o u s l y m a d e by K l e m m and Senff 3) f r o m EuC12 a n d b y D o m a n g e et a l . 4) a n d Q u i t t a r d et al. 5) f r o m E u 2 0 3. We p r e p a r e d EuS, EuSe and E u T e b y a d i r e c t r e a c t i o n b e t w e e n the e l e m e n t s i n e v a c u a t e d , s e a l e d q u a r t z t u b e s at t e m p e r a t u r e s of 4 4 5 o c , 6 9 0 o c a n d 1 2 3 0 o c , respectively. The products obtained were single p h a s e and have the r o c k s a l t s t r u c t u r e . The l a t t i c e p a r a m e t e r s a, d e t e r m i n e d f r o m p o w d e r d i a g r a m s , a r e g i v e n i n t a b l e 1. T h e s e v a l u e s a r e s o m e w h a t l a r g e r t h a n t h o s e r e p o r t e d in the l i t e r a t u r e 3-5).
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/000
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Table 1 Compound
a (A)
0 (OK)
P
saturation moment PB
EuO 1) EuS EuSe EuTe
5.141 5.968 + 0.001 6.197 + 0.002
77 16 6
7.9 8.0 8.2
7 6.5 7.0
6.603 + 0.001
-5 (TN = 6)
7.4
-
The e l e c t r i c a l r e s i s t i v i t y at r o o m t e m p e r a t u r e , a s m e a s u r e d on p r e s s e d p o w d e r s , w a s found to be about 10 q 12cm f o r a l l t h r e e c o m p o u n d s . M e a s u r e m e n t s of the m a g n e t i c s u s c e p t i b i l i t y a s a f u n c t i o n of the t e m p e r a t u r e a r e shown i n fig. 1. W e l l above the C u r i e p o i n t the s u s c e p t i b i l i t y v a r i a t i o n c a n be d e s c r i b e d b y the C u r i e - W e i s s law X = C / ( T - O) with C = N p 2 ~B2/3k. F o r E u S and E u S e the e x p e r i m e n t a l p o i n t s l a y w e l l on s t r a i g h t l i n e s p o i n t i n g to r e s p e c t i v e l y O = 16OK and 6OK, i n d i c a t i n g that t h e s e c o m p o u n d s a r e f e r r o m a g n e t i c . F o r E u T e , h o w e v e r , a m a r k e d d e v i a t i o n at low t e m p e r -
-10
0
20
40
60
80
I00
~_T(O~O
Fig. 1. Magnetic susceptibility as a function of the temperature. The measurements on EuS were made with a field of 640 Oe up to 40OK, with 1370 Oe from 50°K to 61OK and with 1820 Oe up from 62OK. The measurements on EuSe with a field of 1820 Oe up to llOK, with 2700 Oe from 12OK to 40°K and with 4550 Oe up from 44°I{ and those on EuTe with 3080 Oe. a t u r e s i s o b s e r v e d . T h i s m a y be e x p l a i n e d b y a s s u m i n g E u T e to be w e a k l y a n t i f e r r o m a g n e t i c , with a N6el t e m p e r a t u r e of a b o u t 6OK and an a s y m p t o t i c C u r i e p o i n t 6 = - 5°K. The v a l u e s found f o r the C u r i e p o i n t s O and f o r the effective m o m e n t s p a r e l i s t e d in t a b l e 1, t o g e t h e r with the l i t e r a t u r e d a t a f o r EuO 1). The v a l u e s o b t a i n e d f o r p a r e i n good a g r e e m e n t with the t h e o r e t i c a l v a l u e p = g = 7.94, c a l c u l a t e d a s s u m i n g e u r o p i u m to b e in the divalent s t a t e (g = 2, S = ½). The magnetisation a s a function of the t e m p e r a -
215
Vohtme 2, number 5
PHYSICS
ture f o r s e v e r a l fieldstrengths (ranging f r o m 8.2 kOe to 32.1 kOe) in the t e m p e r a t u r e region 4OK 100°K was m e a s u r e d for both EuS and EuSe. It a p p e a r s that the t e m p e r a t u r e and f i e l d - s t r e n g t h dependence of the magnetisation can be described s a t i s f a c t o r i l y by the Weiss field theory. Values f o r the Curie t e m p e r a t u r e T c of 16°K and 6°K have h e r e b y been found, which a r e equal to those found f o r the Curie points f r o m the susceptibility m e a s u r e m e n t s . F o r the saturation magnetisation per Eu-ion at 0°K values of 6.5 NB and 2.0 ~B have been obtained, which a r e in good a g r e e m e n t with the t h e o r e t i c a l value KS = 7.0. These m e a s u r e m e n t s and their interpretation will be described extensively in the Philips R e s e a r c h Reports 6). It is interesting to relate the i n c r e a s e of the lattice p a r a m e t e r going f r o m the oxide to the t e l luride to the d e c r e a s e of the positive interaction and even the change of sign of this interaction for the telluride. In our opinion this provides strong evidence that t h e r e is in these compounds a c o m p e tition between a positive direct exchange interaction and a weaker negative indirect exchange interaction, the f o r m e r being dominant in the oxide, sulfide and selenide. It is likely that the d i r e c t exchange will d e c r e a s e strongly with i n c r e a s i n g lattice p a r a m e t e r .
INCREASE
LETTERS
1 October 1962
On the other hand it is generally found that the ind i r e c t exchange i n c r e a s e s with i n c r e a s i n g covalent c h a r a c t e r going f r o m the oxide to the telluride. Recently we learned that Busch et al. 7) a r e also working on the magnetic p r o p e r t i e s of these c o m pounds. We wish to thank Dr. J. Smit f o r many stimulating d i s c u s s i o n s and M e s s r s . J. F. F a s t , Th. P. M. Meeuwsen and H. A. M. Paping f o r their invaluable experimental a s s i s t a n c e .
References i) B.T. Matthias, R.M. Bozorth and J. H. Van Vleck, Phys. Rev. Letters 7 (1961) 160. 2) N.G. Nerson, C.E. Olsen and G. P. Arnold, Phys. Rev., to be published. 3) W. K l e m m and H. Senff, Z. anorg, u. allgem. Chem. 241 (1939) 259. 4) L.Domange, J. Flahaut and M.Quittard, Compt. rend. 249 (1959) 697. 5) M. Quittard and A. Benacenaf, Compt. rend. 248 (1959) 2589. 6) U.Enz, J.F. Fast, S.Van Houten and J.Smit, to be published in Philips Research Reports. 7) G. Busch, P. Junod, M. Risi and O. Vogt, Proc. of the Int. Conf. on the Physics of Semiconductors, Exeter 1962, to be published.
OF MAGNETIC SATURATION INDUCTION OF HIGHLY SATURATED IRON
N. M I T R O F A N O V Instituto de Ffsica y Matem~ticas, Universidad de Chile, Santiago
Received 5 September 1962
The aim of this work is the study of the magnetic p r o p e r t i e s of highly saturated iron (with hydrogen contents up to 2~ atom.). It has been suggested by Galperin 1), that an inc r e a s e in the lattice p a r a m e t e r of iron would cause an i n c r e a s e in the atomic magnetic moment. M a r b u r g e r 2) found that atomic magnetic m o ment in iron (ferromagnetic region) i n c r e a s e s linearly with the t e m p e r a t u r e , showing in this way the dependence of atomic magnetic moment upon lattice spacing. T h e r e f o r e , the author has suggested that the magnetic saturation induction of a highly saturated iron should augment with the amount of absorbed hydrogen because interstitially entered hydrogen (in the f o r m of protons) p r o d u c e s some change of the lattice spacing. 216
The f i r s t stage in this s e r i e s of experiments was the saturation of the iron until the value 1% 2°~ atom. of hydrogen was reached. The samples were 1 m m thick laminated sheets of c o m m e r c i a l iron with 0.05% carbon content. The difficulty of the iron saturation with such a high quantity of hydrogen was o v e r c o m e by an electrolytic method. The electrolytic solution consisted of 20% H2SO4 and a small quantity of A s 2 0 3 dissolved in hot water. The c u r r e n t density was relatively high (0.16 A/em2) 3). X - r a y d i a g r a m s (fig. 1) show b r o a d e r lines in the hydrogenated sample than in the sample that did not contain hydrogen, thus showing the lattice distortion due to the high proportion of hydrogen atoms present. The quantity of hydrogen absorbed by the iron
PHYSICS
MAGNETIC
INTERACTION
LETTERS
IN EuS,
1 October 1962
EuSe,
AND
EuTe
S. VAN HOUTEN Philips Research Laboratories, N.V. Philips' Gloeflampenfabrieken, Eindhoven, Netherlands Received 3 September 1962
In t h i s l e t t e r the p r e p a r a t i o n , c r y s t a l s t r u c t u r e and m a g n e t i c p r o p e r t i e s of EuS, EuSe a n d E u T e a r e d e s c r i b e d . We have found EuS a n d EuSe to be p u r e l y f e r r o m a g n e t i c , i n s u l a t i n g c o m p o u n d s with C u r i e p o i n t s of 16°K a n d 6 ° K , r e s p e c t i v e l y , w h e r e a s E u T e p r o b a b l y i s w e a k l y a n t i f e r r o m a g n e t i c with a n a s y m p t o t i c C u r i e p o i n t of - 5 ° K and a N~el t e m p e r a t u r e of a b o u t 6°K. EuO h a s a l r e a d y b e e n d e s c r i b e d to be f e r r o m a g n e t i c with a C u r i e p o i n t of 7 7 ° K by M a t t h i a s et al. 1). T h i s h a s r e c e n t l y b e e n c o n f i r m e d by m e a n s of n e u t r o n d i f f r a c t i o n 2). T h e s e c o m p o u n d s w e r e p r e v i o u s l y m a d e by K l e m m and Senff 3) f r o m EuC12 a n d b y D o m a n g e et a l . 4) a n d Q u i t t a r d et al. 5) f r o m E u 2 0 3. We p r e p a r e d EuS, EuSe and E u T e b y a d i r e c t r e a c t i o n b e t w e e n the e l e m e n t s i n e v a c u a t e d , s e a l e d q u a r t z t u b e s at t e m p e r a t u r e s of 4 4 5 o c , 6 9 0 o c a n d 1 2 3 0 o c , respectively. The products obtained were single p h a s e and have the r o c k s a l t s t r u c t u r e . The l a t t i c e p a r a m e t e r s a, d e t e r m i n e d f r o m p o w d e r d i a g r a m s , a r e g i v e n i n t a b l e 1. T h e s e v a l u e s a r e s o m e w h a t l a r g e r t h a n t h o s e r e p o r t e d in the l i t e r a t u r e 3-5).
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Table 1 Compound
a (A)
0 (OK)
P
saturation moment PB
EuO 1) EuS EuSe EuTe
5.141 5.968 + 0.001 6.197 + 0.002
77 16 6
7.9 8.0 8.2
7 6.5 7.0
6.603 + 0.001
-5 (TN = 6)
7.4
-
The e l e c t r i c a l r e s i s t i v i t y at r o o m t e m p e r a t u r e , a s m e a s u r e d on p r e s s e d p o w d e r s , w a s found to be about 10 q 12cm f o r a l l t h r e e c o m p o u n d s . M e a s u r e m e n t s of the m a g n e t i c s u s c e p t i b i l i t y a s a f u n c t i o n of the t e m p e r a t u r e a r e shown i n fig. 1. W e l l above the C u r i e p o i n t the s u s c e p t i b i l i t y v a r i a t i o n c a n be d e s c r i b e d b y the C u r i e - W e i s s law X = C / ( T - O) with C = N p 2 ~B2/3k. F o r E u S and E u S e the e x p e r i m e n t a l p o i n t s l a y w e l l on s t r a i g h t l i n e s p o i n t i n g to r e s p e c t i v e l y O = 16OK and 6OK, i n d i c a t i n g that t h e s e c o m p o u n d s a r e f e r r o m a g n e t i c . F o r E u T e , h o w e v e r , a m a r k e d d e v i a t i o n at low t e m p e r -
-10
0
20
40
60
80
I00
~_T(O~O
Fig. 1. Magnetic susceptibility as a function of the temperature. The measurements on EuS were made with a field of 640 Oe up to 40OK, with 1370 Oe from 50°K to 61OK and with 1820 Oe up from 62OK. The measurements on EuSe with a field of 1820 Oe up to llOK, with 2700 Oe from 12OK to 40°K and with 4550 Oe up from 44°I{ and those on EuTe with 3080 Oe. a t u r e s i s o b s e r v e d . T h i s m a y be e x p l a i n e d b y a s s u m i n g E u T e to be w e a k l y a n t i f e r r o m a g n e t i c , with a N6el t e m p e r a t u r e of a b o u t 6OK and an a s y m p t o t i c C u r i e p o i n t 6 = - 5°K. The v a l u e s found f o r the C u r i e p o i n t s O and f o r the effective m o m e n t s p a r e l i s t e d in t a b l e 1, t o g e t h e r with the l i t e r a t u r e d a t a f o r EuO 1). The v a l u e s o b t a i n e d f o r p a r e i n good a g r e e m e n t with the t h e o r e t i c a l v a l u e p = g = 7.94, c a l c u l a t e d a s s u m i n g e u r o p i u m to b e in the divalent s t a t e (g = 2, S = ½). The magnetisation a s a function of the t e m p e r a -
215
Vohtme 2, number 5
PHYSICS
ture f o r s e v e r a l fieldstrengths (ranging f r o m 8.2 kOe to 32.1 kOe) in the t e m p e r a t u r e region 4OK 100°K was m e a s u r e d for both EuS and EuSe. It a p p e a r s that the t e m p e r a t u r e and f i e l d - s t r e n g t h dependence of the magnetisation can be described s a t i s f a c t o r i l y by the Weiss field theory. Values f o r the Curie t e m p e r a t u r e T c of 16°K and 6°K have h e r e b y been found, which a r e equal to those found f o r the Curie points f r o m the susceptibility m e a s u r e m e n t s . F o r the saturation magnetisation per Eu-ion at 0°K values of 6.5 NB and 2.0 ~B have been obtained, which a r e in good a g r e e m e n t with the t h e o r e t i c a l value KS = 7.0. These m e a s u r e m e n t s and their interpretation will be described extensively in the Philips R e s e a r c h Reports 6). It is interesting to relate the i n c r e a s e of the lattice p a r a m e t e r going f r o m the oxide to the t e l luride to the d e c r e a s e of the positive interaction and even the change of sign of this interaction for the telluride. In our opinion this provides strong evidence that t h e r e is in these compounds a c o m p e tition between a positive direct exchange interaction and a weaker negative indirect exchange interaction, the f o r m e r being dominant in the oxide, sulfide and selenide. It is likely that the d i r e c t exchange will d e c r e a s e strongly with i n c r e a s i n g lattice p a r a m e t e r .
INCREASE
LETTERS
1 October 1962
On the other hand it is generally found that the ind i r e c t exchange i n c r e a s e s with i n c r e a s i n g covalent c h a r a c t e r going f r o m the oxide to the telluride. Recently we learned that Busch et al. 7) a r e also working on the magnetic p r o p e r t i e s of these c o m pounds. We wish to thank Dr. J. Smit f o r many stimulating d i s c u s s i o n s and M e s s r s . J. F. F a s t , Th. P. M. Meeuwsen and H. A. M. Paping f o r their invaluable experimental a s s i s t a n c e .
References i) B.T. Matthias, R.M. Bozorth and J. H. Van Vleck, Phys. Rev. Letters 7 (1961) 160. 2) N.G. Nerson, C.E. Olsen and G. P. Arnold, Phys. Rev., to be published. 3) W. K l e m m and H. Senff, Z. anorg, u. allgem. Chem. 241 (1939) 259. 4) L.Domange, J. Flahaut and M.Quittard, Compt. rend. 249 (1959) 697. 5) M. Quittard and A. Benacenaf, Compt. rend. 248 (1959) 2589. 6) U.Enz, J.F. Fast, S.Van Houten and J.Smit, to be published in Philips Research Reports. 7) G. Busch, P. Junod, M. Risi and O. Vogt, Proc. of the Int. Conf. on the Physics of Semiconductors, Exeter 1962, to be published.
OF MAGNETIC SATURATION INDUCTION OF HIGHLY SATURATED IRON
N. M I T R O F A N O V Instituto de Ffsica y Matem~ticas, Universidad de Chile, Santiago
Received 5 September 1962
The aim of this work is the study of the magnetic p r o p e r t i e s of highly saturated iron (with hydrogen contents up to 2~ atom.). It has been suggested by Galperin 1), that an inc r e a s e in the lattice p a r a m e t e r of iron would cause an i n c r e a s e in the atomic magnetic moment. M a r b u r g e r 2) found that atomic magnetic m o ment in iron (ferromagnetic region) i n c r e a s e s linearly with the t e m p e r a t u r e , showing in this way the dependence of atomic magnetic moment upon lattice spacing. T h e r e f o r e , the author has suggested that the magnetic saturation induction of a highly saturated iron should augment with the amount of absorbed hydrogen because interstitially entered hydrogen (in the f o r m of protons) p r o d u c e s some change of the lattice spacing. 216
The f i r s t stage in this s e r i e s of experiments was the saturation of the iron until the value 1% 2°~ atom. of hydrogen was reached. The samples were 1 m m thick laminated sheets of c o m m e r c i a l iron with 0.05% carbon content. The difficulty of the iron saturation with such a high quantity of hydrogen was o v e r c o m e by an electrolytic method. The electrolytic solution consisted of 20% H2SO4 and a small quantity of A s 2 0 3 dissolved in hot water. The c u r r e n t density was relatively high (0.16 A/em2) 3). X - r a y d i a g r a m s (fig. 1) show b r o a d e r lines in the hydrogenated sample than in the sample that did not contain hydrogen, thus showing the lattice distortion due to the high proportion of hydrogen atoms present. The quantity of hydrogen absorbed by the iron