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Specific heat and a.c. susceptibility measurements of SmMn2Ge2 and UMn2Ge2
Journal of AUoys and Compounds, 178 (1992) 249-252 JAL 5025
249
Specific heat and a.c. susceptibility measurements of SmMn2Ge~ and UMn2Ge2 M. Slaski Institute of Physics, Technical University of Cracow, 30-084 Krakdw (Poland)
T. L a e g r i d a n d K. F o s s h e i m Division of Physics, The Norwegian Institute of Technology and Applied Physics SINTET, N-7034 Trondheim (Norway)
Z, T o m k o w i c z a n d A. Szytu|a Institute of Physics, Jagellonian University, 30-059 KrakSw (Poland)
(Received July 3, 1991)
Abstract The magnetic phase transitions in SmMn2Gez and UMn2Gez are investigated by specific heat and a.c. susceptibility measurements. In UMnzGe2 only one phase transition at the Curie temperature is observed. SmMn2Ge2 has two phase transitions corresponding to the ferro-antfferro and re-entrant ferromagnetism and a third phase transition occurring at the Curie temperature.
1. I n t r o d u c t i o n In r e c e n t y e a r s MT2Xz c o m p o u n d s ( w h e r e M -= r a r e e a r t h o r actinide a t o m , T ~- t r a n s i t i o n m e t a l a n d X = Si o r Ge) h a v e b e e n t h e s u b j e c t of intensive r e s e a r c h [ 1 - 7 ] . M a g n e t i c a n d n e u t r o n diffraction m e a s u r e m e n t s indicate t h e different m a g n e t i c p r o p e r t i e s of t h e s e c o m p o u n d s . Of t h e s e c o m p o u n d s , t h o s e c o n t a i n i n g m a n g a n e s e a r e a distinct g r o u p b e c a u s e , unlike o t h e r t r a n s i t i o n d - e l e c t r o n m e t a l s , for Mn localized m a g n e t i c m o m e n t s are o b s e r v e d . T h e s e c o m p o u n d s h a v e t w o critical m a g n e t i c t e m p e r a t u r e s [5]: (1) at l o w t e m p e r a t u r e s the m a g n e t i c m o m e n t s localized o n b o t h Mn a n d R a t o m s are o r d e r e d ; (2) at high t e m p e r a t u r e s t h e m a g n e t i c m o m e n t s of the Mn sublattice only are ordered. A similar situation is o b s e r v e d in t h e c a s e o f UMn2Ge2 in w h i c h the m a g n e t i c m o m e n t o f t h e Mn sublattice o r d e r s f e r r o m a g n e t i c a l l y w i t h a Curie t e m p e r a t u r e o f 3 9 0 K. B e l o w T = 150 K t h e m a g n e t i c m o m e n t s o f the U a t o m s o r d e r f e r r o m a g n e t i c a l l y [6]. Different p r o p e r t i e s are o b s e r v e d in the c a s e o f SmMn2Ge2. T h e t e m p e r a t u r e d e p e n d e n c e o f t h e m a g n e t i z a t i o n r e v e a l s f e r r o m a g n e t i s m f o r 153 K < T < 3 4 1 K, a n t i f e r r o m a g n e t i s m f o r 106.5 K < T < 153 K a n d r e - e n t r a n t f e r r o m a g n e t i s m b e l o w 106.5 K [7, 8].
0925-8388/92/$5.00
© 1992- Elsevier Sequoia. All rights reserved
250
The present work reports results of the specific heat and a.c. susceptibility measurements of UMn2Ge2 and SmMn2Ge2. 2. E x p e r i m e n t a l d e t a i l s and r e s u l t s
Experiments were carried out on samples synthesized as reported in previous papers [6, 7]. The specific heat was measured using an a.c. technique [9]. The a.c. magnetic susceptibilities X' and X" were measured at a frequency of 118 Hz using a mutual inductance bridge of the Hartshorn type down to liquid nitrogen temperature. The temperature dependence of the susceptibilities X' and X" of UMn2Ge2 shows no anomaly in the temperature range 8 0 - 3 5 0 K (Fig. 1). The temperature dependence of the specific heat measured in the temperature range 8 0 - 4 1 0 K shows an anomaly only at T = 3 2 0 K, which is the Curie temperature of the Mn sublattice (Fig. 2). Different results were obtained for SmMn2Ge2. The temperature dependence of the a.c. susceptibility gives two phase transitions with thermal hysteresis (106.5 K and 153 K on heating and 101.5 K and 147 K on cooling (Fig. 3). The third phase transition at 341 K does not show any thermal hysteresis. The data correlate with the temperature dependence of the magnetization, indicating that (with increasing temperature) ferro-, antiferro- and re-entrant ferromagnetism are observed [7, 8]. The curve of specific heat vs. temperature shows a strong anomaly at the Curie temperature and very small anomalies at the temperatures of the phase transitions F - A F and A F - F (Fig. 4). 3. D i s c u s s i o n
The results obtained show the different properties of UMn2Ge2 and SmMn2Ge2. The results for the specific heat of UMn2Ge2 are in agreement
P. 6oo
s
360
380 T[K ]/DO0
3OO
100
150
200
2.50
300
T[.K]
1 •0 0
'
'
'
' 2 0'0
'
'
'
J
300. . . . .
TfK]
Fig. 1. The temperature dependence of the susceptibilities X' and X" 01' UMn2Ge2. Fig. 2. Specific heat
vs.
temperature for UMn2Ge2.
400
251
1-
80
120
200
160
240
280
320
T [
Fig. 3. Temperature dependence of the susceptibilities
1200
o
. . . . . . . .
I
. . . .
"~
K] X'
31~0
and X" of SmMn2Ge2.
'
"
800
40(I
*
,
,
,
,
,
,
60
L
1 240
,
,
'
,
,
,
' 400
T[K]
Fig. 4. Specific heat
vs.
temperature for SmMn2Ge2.
with t h o s e r e p o r t e d in ref. 10. T h e t e m p e r a t u r e d e p e n d e n c e o f the specific h e a t in the t e m p e r a t u r e r a n g e 1.6--150 K has n o distinct anomaly. The lowt e m p e r a t u r e d a t a lead t o a n e l e c t r o n i c specific h e a t c o n s t a n t ~/equal to 30.3 m J m o l - 1 K - 2 a n d a D e b y e t e m p e r a t u r e 0 e q u a l t o 341 K. At high t e m p e r a t u r e s t h e specific h e a t s h o u l d also c o n t a i n a spin-wave c o n t r i b u t i o n f r o m the o r d e r e d Mn a n d U sublattices. C o m p a r i s o n o f the Kerr effect o b s e r v e d at r o o m t e m p e r a t u r e in UMn2Ge2 a n d i s o s t r u c t u r a l LaMn2Ge2 indicates t h a t t h e U sublattice magnetization is n o n z e r o owing to its c o u p l i n g t o t h e m a g n e t i c a l l y o r d e r e d 3d sublattice [ 11 ]. T h e results d e s c r i b e d in refs. 10 a n d 11 a n d h e r e indicate that the U and Mn sublattices o r d e r at the s a m e t e m p e r a t u r e . In the case o f SmMn2Ge 2 t h e transition b e t w e e n the ferro- and a n t f f e r r o m a g n e t i c p h a s e s is first order. T h e v a l u e s o f t h e p h a s e t r a n s f o r m a t i o n e n t h a l p i e s and o f the e n t r o p y c h a n g e s AS for b o t h p h a s e transitions w e r e c a l c u l a t e d using the C l a p e y r o n e q u a t i o n
252 TABLE 1 Thermodynamic parameters of the first-order phase transition in SmMn2Ge 2 Temperature of p h a s e transition
AS (cal kmo1-1)
AQ (cal g - l )
0.246(15) 0.248(3)
0.063(3) 0.091 (2)
(IO 106.5 113
dTF_AF
- -
dp
V F - VAF =
SF--SAF
V F - VAF
--
_
_
L
TF_AF
Using the values of dTF_AF/dpand the corresponding change in the unit cell volume reported in ref. 7, the thermodynamic parameters of the first transition in SmMn2Ge2 were calculated (see Table 1). It is seen that the heats of transition for the two phase transitions are small and close together. This is in agreement with the experimental data.
References 1 Z. Ban and M. Sikirica, Acta Crystallogr., 18 (1965) 594. 2 T. T. M. Palstra, Ph.D. Thesis, Leiden University, Netherlands, 1986. 3 J. Leciejewicz and A. Szytula, Magnetic Ordering in Ternary Intermetallic MT~X2 Systems, Pahstwowe Wydawnictwo Naukowe, Warsaw, Cracow, 1987. 4 A. Szytuta and J. Leciejewicz, in K. A. Gschneidner, Jr. and L. Eyring (eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol. 12, North-Holland, Amsterdam, 1989, p. 133. 5 A. Szytuta and I. Szott, Solid State Commun., 40 ( 1 9 8 1 ) 199. 6 A. Szytuta, S. Siek, J. Leciejewicz, A. Zygmunt and A. Ban, J. Phys. Chem. Solids, 49 ( 1 9 8 8 ) 1113. 7 M. Dur~, R. Duraj, A. Szytuia and Z. Tomkowicz, J. Magn. Magn. Mater., 73 (1988) 240. 8 E. M. Gyorgy, B. Batlogg, J. P. Remeika, R. B. v a n Dover, R. M. Fleming, H. E. Bai L G. P. Espinosa, A. S. Cooper and R. G. Maines, J. Appl. Phys., 51 ( 1 9 8 7 ) 4237. 9 S. Stokka, J. Phys. E, 15 (1982) 123. 10 T. Endstra, A. J. Dirkmaat, S. A. M. Mentink, A. A. Menovsky, G. J. Nieuwenhuys and J. A. Mydosh, Physica B, 163 (1990) 309. 11 P. P. J. v a n Engelen, D. B. de Mooij, K. H. J. Buschow, IEEE Trans. Magn., 24 ( 1 9 8 8 ) 1728.
249
Specific heat and a.c. susceptibility measurements of SmMn2Ge~ and UMn2Ge2 M. Slaski Institute of Physics, Technical University of Cracow, 30-084 Krakdw (Poland)
T. L a e g r i d a n d K. F o s s h e i m Division of Physics, The Norwegian Institute of Technology and Applied Physics SINTET, N-7034 Trondheim (Norway)
Z, T o m k o w i c z a n d A. Szytu|a Institute of Physics, Jagellonian University, 30-059 KrakSw (Poland)
(Received July 3, 1991)
Abstract The magnetic phase transitions in SmMn2Gez and UMn2Gez are investigated by specific heat and a.c. susceptibility measurements. In UMnzGe2 only one phase transition at the Curie temperature is observed. SmMn2Ge2 has two phase transitions corresponding to the ferro-antfferro and re-entrant ferromagnetism and a third phase transition occurring at the Curie temperature.
1. I n t r o d u c t i o n In r e c e n t y e a r s MT2Xz c o m p o u n d s ( w h e r e M -= r a r e e a r t h o r actinide a t o m , T ~- t r a n s i t i o n m e t a l a n d X = Si o r Ge) h a v e b e e n t h e s u b j e c t of intensive r e s e a r c h [ 1 - 7 ] . M a g n e t i c a n d n e u t r o n diffraction m e a s u r e m e n t s indicate t h e different m a g n e t i c p r o p e r t i e s of t h e s e c o m p o u n d s . Of t h e s e c o m p o u n d s , t h o s e c o n t a i n i n g m a n g a n e s e a r e a distinct g r o u p b e c a u s e , unlike o t h e r t r a n s i t i o n d - e l e c t r o n m e t a l s , for Mn localized m a g n e t i c m o m e n t s are o b s e r v e d . T h e s e c o m p o u n d s h a v e t w o critical m a g n e t i c t e m p e r a t u r e s [5]: (1) at l o w t e m p e r a t u r e s the m a g n e t i c m o m e n t s localized o n b o t h Mn a n d R a t o m s are o r d e r e d ; (2) at high t e m p e r a t u r e s t h e m a g n e t i c m o m e n t s of the Mn sublattice only are ordered. A similar situation is o b s e r v e d in t h e c a s e o f UMn2Ge2 in w h i c h the m a g n e t i c m o m e n t o f t h e Mn sublattice o r d e r s f e r r o m a g n e t i c a l l y w i t h a Curie t e m p e r a t u r e o f 3 9 0 K. B e l o w T = 150 K t h e m a g n e t i c m o m e n t s o f the U a t o m s o r d e r f e r r o m a g n e t i c a l l y [6]. Different p r o p e r t i e s are o b s e r v e d in the c a s e o f SmMn2Ge2. T h e t e m p e r a t u r e d e p e n d e n c e o f t h e m a g n e t i z a t i o n r e v e a l s f e r r o m a g n e t i s m f o r 153 K < T < 3 4 1 K, a n t i f e r r o m a g n e t i s m f o r 106.5 K < T < 153 K a n d r e - e n t r a n t f e r r o m a g n e t i s m b e l o w 106.5 K [7, 8].
0925-8388/92/$5.00
© 1992- Elsevier Sequoia. All rights reserved
250
The present work reports results of the specific heat and a.c. susceptibility measurements of UMn2Ge2 and SmMn2Ge2. 2. E x p e r i m e n t a l d e t a i l s and r e s u l t s
Experiments were carried out on samples synthesized as reported in previous papers [6, 7]. The specific heat was measured using an a.c. technique [9]. The a.c. magnetic susceptibilities X' and X" were measured at a frequency of 118 Hz using a mutual inductance bridge of the Hartshorn type down to liquid nitrogen temperature. The temperature dependence of the susceptibilities X' and X" of UMn2Ge2 shows no anomaly in the temperature range 8 0 - 3 5 0 K (Fig. 1). The temperature dependence of the specific heat measured in the temperature range 8 0 - 4 1 0 K shows an anomaly only at T = 3 2 0 K, which is the Curie temperature of the Mn sublattice (Fig. 2). Different results were obtained for SmMn2Ge2. The temperature dependence of the a.c. susceptibility gives two phase transitions with thermal hysteresis (106.5 K and 153 K on heating and 101.5 K and 147 K on cooling (Fig. 3). The third phase transition at 341 K does not show any thermal hysteresis. The data correlate with the temperature dependence of the magnetization, indicating that (with increasing temperature) ferro-, antiferro- and re-entrant ferromagnetism are observed [7, 8]. The curve of specific heat vs. temperature shows a strong anomaly at the Curie temperature and very small anomalies at the temperatures of the phase transitions F - A F and A F - F (Fig. 4). 3. D i s c u s s i o n
The results obtained show the different properties of UMn2Ge2 and SmMn2Ge2. The results for the specific heat of UMn2Ge2 are in agreement
P. 6oo
s
360
380 T[K ]/DO0
3OO
100
150
200
2.50
300
T[.K]
1 •0 0
'
'
'
' 2 0'0
'
'
'
J
300. . . . .
TfK]
Fig. 1. The temperature dependence of the susceptibilities X' and X" 01' UMn2Ge2. Fig. 2. Specific heat
vs.
temperature for UMn2Ge2.
400
251
1-
80
120
200
160
240
280
320
T [
Fig. 3. Temperature dependence of the susceptibilities
1200
o
. . . . . . . .
I
. . . .
"~
K] X'
31~0
and X" of SmMn2Ge2.
'
"
800
40(I
*
,
,
,
,
,
,
60
L
1 240
,
,
'
,
,
,
' 400
T[K]
Fig. 4. Specific heat
vs.
temperature for SmMn2Ge2.
with t h o s e r e p o r t e d in ref. 10. T h e t e m p e r a t u r e d e p e n d e n c e o f the specific h e a t in the t e m p e r a t u r e r a n g e 1.6--150 K has n o distinct anomaly. The lowt e m p e r a t u r e d a t a lead t o a n e l e c t r o n i c specific h e a t c o n s t a n t ~/equal to 30.3 m J m o l - 1 K - 2 a n d a D e b y e t e m p e r a t u r e 0 e q u a l t o 341 K. At high t e m p e r a t u r e s t h e specific h e a t s h o u l d also c o n t a i n a spin-wave c o n t r i b u t i o n f r o m the o r d e r e d Mn a n d U sublattices. C o m p a r i s o n o f the Kerr effect o b s e r v e d at r o o m t e m p e r a t u r e in UMn2Ge2 a n d i s o s t r u c t u r a l LaMn2Ge2 indicates t h a t t h e U sublattice magnetization is n o n z e r o owing to its c o u p l i n g t o t h e m a g n e t i c a l l y o r d e r e d 3d sublattice [ 11 ]. T h e results d e s c r i b e d in refs. 10 a n d 11 a n d h e r e indicate that the U and Mn sublattices o r d e r at the s a m e t e m p e r a t u r e . In the case o f SmMn2Ge 2 t h e transition b e t w e e n the ferro- and a n t f f e r r o m a g n e t i c p h a s e s is first order. T h e v a l u e s o f t h e p h a s e t r a n s f o r m a t i o n e n t h a l p i e s and o f the e n t r o p y c h a n g e s AS for b o t h p h a s e transitions w e r e c a l c u l a t e d using the C l a p e y r o n e q u a t i o n
252 TABLE 1 Thermodynamic parameters of the first-order phase transition in SmMn2Ge 2 Temperature of p h a s e transition
AS (cal kmo1-1)
AQ (cal g - l )
0.246(15) 0.248(3)
0.063(3) 0.091 (2)
(IO 106.5 113
dTF_AF
- -
dp
V F - VAF =
SF--SAF
V F - VAF
--
_
_
L
TF_AF
Using the values of dTF_AF/dpand the corresponding change in the unit cell volume reported in ref. 7, the thermodynamic parameters of the first transition in SmMn2Ge2 were calculated (see Table 1). It is seen that the heats of transition for the two phase transitions are small and close together. This is in agreement with the experimental data.
References 1 Z. Ban and M. Sikirica, Acta Crystallogr., 18 (1965) 594. 2 T. T. M. Palstra, Ph.D. Thesis, Leiden University, Netherlands, 1986. 3 J. Leciejewicz and A. Szytula, Magnetic Ordering in Ternary Intermetallic MT~X2 Systems, Pahstwowe Wydawnictwo Naukowe, Warsaw, Cracow, 1987. 4 A. Szytuta and J. Leciejewicz, in K. A. Gschneidner, Jr. and L. Eyring (eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol. 12, North-Holland, Amsterdam, 1989, p. 133. 5 A. Szytuta and I. Szott, Solid State Commun., 40 ( 1 9 8 1 ) 199. 6 A. Szytuta, S. Siek, J. Leciejewicz, A. Zygmunt and A. Ban, J. Phys. Chem. Solids, 49 ( 1 9 8 8 ) 1113. 7 M. Dur~, R. Duraj, A. Szytuia and Z. Tomkowicz, J. Magn. Magn. Mater., 73 (1988) 240. 8 E. M. Gyorgy, B. Batlogg, J. P. Remeika, R. B. v a n Dover, R. M. Fleming, H. E. Bai L G. P. Espinosa, A. S. Cooper and R. G. Maines, J. Appl. Phys., 51 ( 1 9 8 7 ) 4237. 9 S. Stokka, J. Phys. E, 15 (1982) 123. 10 T. Endstra, A. J. Dirkmaat, S. A. M. Mentink, A. A. Menovsky, G. J. Nieuwenhuys and J. A. Mydosh, Physica B, 163 (1990) 309. 11 P. P. J. v a n Engelen, D. B. de Mooij, K. H. J. Buschow, IEEE Trans. Magn., 24 ( 1 9 8 8 ) 1728.