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ELECTRICAL CONDUCTIVITY OF THE MAGNETIC SPINEL SERIES Mn1-xCuxCr2S4
T. Groń, M. Gogolowicz, J. Warczewski, H. Lutz
To cite this version:
T. Groń, M. Gogolowicz, J. Warczewski, H. Lutz. ELECTRICAL CONDUCTIVITY OF THE MAG-
NETIC SPINEL SERIES Mn1-xCuxCr2S4. Journal de Physique Colloques, 1985, 46 (C6), pp.C6-
441-C6-444. �10.1051/jphyscol:1985685�. �jpa-00224947�
ELECTRICAL CONDUCTIVITY OF THE MAGNETIC SPINEL SERIES Mnl-xCuxCs2S4
T . Groii, M. GogoTowicz, J. Warczewski and H.D. ~ u t z +
SiZesian University, I n s t i t u t e of Physics, u2. Uniwersytecka 4 , 40007 Katmice, ,PoZand
+ ~ n s t i t u t f 7 b Anorganische Chemie der Universittit Siegen, 0-5900 Siegen, F. R. G.
Rksum6
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L ' B t u d e d e s p r o p r i g t g s Q l e c t r i q u e s d e l a s 6 r i e d e s s p i n e l l e s m a g n g t i q u e s c o n c e r n g e m o n t r e que l a c o n d u c t i v i t g p a r t r o u s a u g m e n t e a v e c l a t e m p g r a t u r e e t l a c o n c e n t r a t i o n e n c u i v r e . C e t e f f e t e s t r e l i 6 a u c h a n g e m e n t d e v a l e n c e d e s i o n s chrome d e 3 + 2 4 + e t r g s u l t e du c h a n g e m e n t d e c o n d u c t i v i t 6 d e l ' g t a t s e m i - c o n d u c t e u r & l ' d t a t m g t a l l i q u e . Un m o d s l e d e s t r u c t u r e . d e b a n d e , c o h e r e n t a v e c l a t r a n s i t i o n d e p h a s e n-p d a n s MnCr2S4 e s t proposd.A b s t r a c t
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The s t u d y of t h e e l e c t r i c a l p r o p e r t i e s of t h e mag- n e t i c s p i n e l s e r i e s under i n v e s t i g a t i o n shows t h a t i n t h i s se- r i e s h o l e c o n d u c t i v i t y i n c r e a s e s w i t h t h e i n c r e a s e of b o t h t e m p e r a t u r e and Cu composition. T h i s e f f e c t i s connected w i t h 'the change of v a l e n c e of chromium i o n s from 3+ t o 4+ and r e -s u l t s i n t h e change of t h e c o n d u c t i v i t y from a semiconductor t o a metal. A band s t r u c t u r e model c o n s i s t e n t w i t h t h e n-p phase t r a n s i t i o n i n MnCr2S4 i s proposed.
I
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INTRODUCTIONThe compounds Mnl,xCuxCr2S4 a r e s o l i d s o l u t i o n s of t h e two s p i n e l s MnCr2S4 and CuCr2Sp, which d i f f e r d r a s t i c a l l y i n t h e i r e l e c t r i c a l and magnetic p r o p e r t i e s . MnCr2S4 i s a f e r r i m a g n e t i c semiconductor w i t h TN- 100 K /I/ and a n e l e f t r l c a l r e s i s t i v i t y of I O ~ - I O ~ J L ~ ~ I at
300 K, depending on t h e sample technology d e s c r i b e d i n s e v e r a l pa- p e r s /see 2/. From n e u t r o n d i f f r a c t i o n s t u d i e s t h e magnetic moments a r e found t o be 4.7 PB/bh2+ and 3.0 irg/~r3+ /I/. CuCr2S4 shows mag- n e t i c p r o p e r t i e s t y p i c a l of a ferromagnet w i t h TC=390-420 K and p 4 . 2 ~ 4 . 6 8 pg /3/. T h i s compound h a s a p-type m e t a l l i c c o n d u c t i v i t y of about 1 0 - ~ ~ c m a t 300 K /4/. T h e r e f o r e i t w a s i n t e r e s t i n g t o s t u d y t h e f o r m a t i o n as w e l l as t h e magnetic and e l e c t r i c a l proper- t i e s of t h e s o l i d s o l u t i o n s of MnCr2S4 and CuCr2S4. I n r e f .5 i t w a s proved, t h a t f o r x c 0 . 4 t h e s e compounds a r e f e r r i m a g n e t i c w i t h a com- p l e x magnetic s t r u c t u r e . For x=0.8 i t w a s found, t h a t t h e magnetiza- t i o n c u r v e i s t y p i c a l o f a ferromagnet. I n r e f . 6 , t h e compounds Mnl,xCuxCr2S4 have been s t u d i e d and a complex f e r r i m a g n e t i c s t r u c t u - r e h a s been proposed a l s o f o r ~ ~ 0 . 8 5 , 0.90, 0.95. Recently a n-p phase t r a n s i t i o n h a s been observed i n MnCr2S4 a t a b o u t 326 K /7/.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985685
C6-442 JOURNAL DE PHYSIQUE
I1
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EXPERIMENTAL AND RESULTSThe p r e p a r a t i o n of t h e compounds i s d e s c r i b e d i n ref.8. The e l e c t r i - c a l c o n d u c t i v i t y measurements were o a r r i e d out i n t h e t e m p e r a t u r e r a n g e of 70-500 K. The accuracy of t h e s e measurements was about
+0.6 %. The h i g h - r e s i s t i v i t y samples were measured by t h e 2-point do
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method w i t h t h e use of a semiautomatic e l e c t r o n i c r e s i s t a n c e b r i d g e /9/. The l o w - r e s i s t i v i t y samples were measured a l s o by t h e 2-point d c method w i t h a c u r r e n t of i mA. P o l y c r y s t a l l i n e samples i n t h eFig. I
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C o n d u c t i v i t y vs, t e m p e r a t u r e f o r d i f f e r e n t v a l u e s of X.F i g , 2
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A c t i v a t i o n energy ( s o l i d c i r c l e s , c r o s s e s ) and temperature c o e f f i c i e n t of r e s i s t i v i t y (open c i r c l e s ) v s composition.form of p a r a l l e l e p i p e d s ( 0 . 3 ~ 3 ~ 6 mm 3 ) w e r e p r e s s e d (11 MPa), heated a t 473 K and t h e n slowly cooled t o o b t a i n r e p r o d u c i b l e r e s u l t s , The e l e c t r i c a l c o n t a c t s were made of s i l v e r p a s t e . The r e s u l t s a r e p r e - s e n t e d i n F i g u r e s 1 and 2. I t was confirmed t h a t f o r 0 . 0 g x < 0 . 2 5 t h e s p i n e l s a r e s e a i c o n d u c t o r s w i t h an a c t i v a t i o n energy d e c r e a s i n g and a r e s i s t i v i t y temperature c o e f f i c i e n t i n c r e a s i n g w i t h i n c r e a s i n g x. Moreover, a t r a n s i t i o n from a semiconductor t o metal was observed f o r t h e samples w i t h x=0.30, 0.85, 0.90, 0.95 a t t e m p e r a t u r e s of 462 K, 300
K,
261 K and 232 K, r e s p e c t i v e l y .I11
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DISCUSSIONThe small v a l u e s of t h e e l e c t r i c a l c o n u u c t i v i t y , a c t i v a t i o n energy ( s e e Figs. 1, 2 ) and t h e Seebeck c o e f f i c i e n t /7/ f o r MnCr2S4 suggest, t h a t i n t h i s s p i n e l mixed c o n d u c t i v i t y a p p e a r s , which i s confirmed by t h e observed change of t h e c o n d u c t i v i t y from n i n t o p type a t about 326 K. The observed e l e c t r i c a l p r o p e r t i e s of MnCrzS4 seem t o p o i n t t h e f o l l o w i n g mechanism of c o n d u c t i v i t y ( F i g . 3a):
d t
low tem-p e r a t u r e s t h e e l e c t r o n s from a donor l e v e l ( ~ 0 . 0 8 eV below t h e con- d u c t i o n band) a r e a c t i v a t e d , g i v i n g a weak n type c o n d u c t i v i t y . A t h i g h e r t e m p e r a t u r e s a n a c c e p t o r l e v e l (-0.32 eV above t h e v a l e n c e band) c o n t r i b u t e s t o t h e c o n d u c t i v i t y and g i v e s t h e c h a r a c t e r i s t i c bending of t h e c o n d u c t i v i t y curve. F u r t h e r temperature i n c r e a s e l e a d s t o t h e change of n t o p-type c o n d u c t i v i t y . These impurity l e - v e l s might be due t o a small d e v i a t i o n from s t o i c h i o m e t r y . They might be r e l a t e d t o S
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and Mn-
vacancies. The e x i s t e n c e of such vacancy l e v e l s and a change of t h e type of c o n d u c t i v i t y connected t o them have been observed i n t h e s p i n e l ZnCr2Se4 /i0/. S u b s t i t u t i o n ofC6-444 JOURNAL DE PHYSIQUE
Fig. 3
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Band structure model for: MnCr2S4 (a), Mni-xCuxCr2S4 (b), CuCr2S4 /li/ (0).manganese by copper causes a gradual change of valence of chromium
ions from 3+ into 4 + according to the formula
Mnf
~ x ~ ~ [ , r ~ ~ x ~ r $ ~sZ-.
This is oonfirmed by the magnetic studies /6/. In the mixed valence band of chromium (see Fig. 3b) the hole concentration increases with x, which causes a sudden increase of the hole conductivity and a de- crease of the Seebeck coefficient /7/. This leads, for xb0.30, to a transition from a semiconducting to a metallic state. The above mo- del becomes the Lotgering model /ll/ for CuCr2Sq ( ~ i g . 30).
(Work partly supported through Project MB.1.5 )
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REFERENCES
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