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Pressure effect on the Néel temperature and exchage striction of the ordered alloy Au5Mn2
MI
Journal of Magnetism and Magnetic Materials 104-1117 (1992) 21161-.116_ "~ "~ North-t lolland
ii
Pressure effect on the NEel temperature and exchange stricticm of the ordered alloy AusMn 2 M. Matsumoto a S, Abe b, H. Yoshida b, S. Mori ':, T. Kanomata " and T. Kancko h "Research lnsitute of Mineral Dressing and Metallurgy, Tohoku Unit'emit),, Sendai, 080 Japatt t, Institute .for Materials Research, Tohoku University, Sendai, 980 Japan ' Department of Applied P~3'sics, Faculty oJ" Engineering, Tohoku Gakuin Unit'er~'ity, Tagajo, M#'agi, 985 Japan
The pressure effect on the N~el temperature T N, the thermal expansion and the specific heat are studied for the ordered alloy AusMn 2. T N decreases linearly with pressure, d T N / d p is obtained to be 11.31 K/kbar. The jumps in the volume thermal expansion coefficient t~ and the specific heat C v at TN are determined to the Aat. = 11.5x Ill s K t and A('t, = 8.2 cal/mol K, respectively. The atomic distance dependence of TN is discussed.
It is known that the a p p e a r a n c e of magnctic order in m a n g a n c s e alloys and c o m p o u n d s has a close rclation with the M n - M n atomic distances. Many ordered alloys formed between Mn and Au metals show intercsting magnetic properties in grcat varicty and thc occurrcncc of various magnctic order in them was extensively studied from the point of vicw of the M n Mn atomic distancc d c p c n d c n c c of the exchangc interactions [2-4]. Very little is known about thc magnetic propcrtics of thc ordcrcd alloy A u s M n 2 having a monoclinic cry:;tal structurc with the space group of C 2 / m [5]. Prcvious rcsults of magnetic mcasuremcnts have shown that Au.sMne is antifcrromagnctic with a N6el t c m p c r a t u r e TN of 348 K, a paramagnctic Curie tcmpcrauturc Op of 1211 K and an effective magnctic m o m e n t /.t~, of 5.56# u in rcf. [5] and T x = 353 K, (-)p = I'll) K and ,tt,:H = 5.5btn in rcf. [6]. According to the rcsults of neutron diffraction studics, the magnetic unit cell is identical to thc chcmical onc and each ccll contains four mangancse at.oms with a magnctic momcnt of 5/.t n pcr Mn arranged in a colinear antifcrromagnetic array. In this paper, thc magnctic susccptibility, the prcssurc cffcct on thc N6ei tcmpcraturc, the thermal cxpansion and thc specific hcat of thc ordered alloy A u s M n 2 arc mcasurcd in order to cxaminc thc M n - M n atomic distancc d e p e n d e n c e of its magnetic propertics. The specimcn was p r e p a r e d by melting the consituent cicmcnts in a high-frcqucncy furnace and the product was anneaicd at 400°C for 40 d. X-ray diffract,on study showed that the prcparcd spccimcn is in singlc phasc with a monoclinic crystal structure. The lattiCeo p m a m c t e r s arc found to bc a = 9.21 A, b = 3.96 A, c = 6.48 A and /3 = 97.6 °, which are in a g r c c m c n t with thosc in rcf. [1]. The magnctic susccptibility was m c a s u r c d using a magnctic balancc. Thc prcssurc change of thc Ndel t c m p c r a t u r c was d c t c r m i n c d by m c a s u r i n g the tcmpcraturc dependcnce ot the electri-
cal resistance under various pressures, which were applied by using a piston-cylinder type apparatus. The measurements of the specific heat and the thermal expansion were carried out by using a pulsed calorimeter and a dilatometer, respectively. Fig. 1 shows the magnetic susceptibility (X,,) versus temperature ( T ) curve of A u s M n 2 , where a peak of Xv is observed at 354 K ( = T N). The temperature dependence of reciprocal susceptibility follows the C u r i e Weiss law and the effective magnetic moment is found to be 4 . 0 u u / M n . Fig. 2 shows thc Nt3el temperature (T x) versus prcssure ( p ) curve of A u s M n 2. T N increases linearly with applied pressure and the pressure derivative of T x is found to bc d T x / d p = 1!.31 K / k b a r . Fig. 3 shows the linear thermal expansion curve of AusMn_~. The A I / I curvc below about 360 K deviates downward from the smooth extrapolation (dotted line) of the curve for high temperatures, which indicates the exchange striction to be negative. From the above
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1 Temperature der.:ndence of the magnetic susceptibilit) Xv, of AusMn 2. •
0312-88:';3/92/$05.01; S~; 1992 - Elsevier Science Publishers B.V. All rights reserx,ed
2062
M. Matsumoto et al. / Pressure el'/'cot cm ttw N&'I temperature and erclumge striction o.f tlw ordered alloy Au.~Mn 2 358
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Fig. 2. Pressure dependence of the N6cl temperature 7'N of AusMn 2.
G ?0
,x I 0 4 lfl ~
L
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1
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t
t
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.
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.
.
.
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v
250
4.50
650
T(KI Fig. 4. "l'empcralurc dependence of lhc spccil'ic heal (), of Au 5M n 2.
o
350
4.00
T(K]
Fig. 3. Linear thermal expansion curve of AusMn 2. results the difference in tl~c volume expansion coefficient at T n is roughly estimated to bc ~ r t = 0.5 × 10 s K i Fig. 4 shows the specific heat (Cp) versus lcmpcraturc curve of AusMn 2. As the temperature increases there is a sharp decrease of Cp observed at 350 K (= Tn), which is ascribed to the disappearance of magnetic order. Tht jump of Cp at T n is found to be ACt, = 8.82 c a l / m o l K. Ehrenfcst's equation for a magnetic transition of the second ordcr at T N is d T N / d p = ( T N / p ) ( A a l . / AC/,). lnscrtion of the valucs of T N, Aa=. and ACt, obtained above and density p = 15.6 g / c m 3 into the cquation givcs d T N / d p = 0.3 K / k b a r , which is in good agreement with the above value obtained from the direct mcasurcnlent. T!!c ,',',',-.,0,; r~u~v=u, suggests L*'l l a l " the cxchangc intcraction (j~) bctwccn thc ncarcstncighbouring Mn atoms wi~h intcratomic distance of 2.91 A is antifcrrornaguct~c and that (J-,) bctwccn ncxt-ncarcst-ncighbouring o~cs with spacing 3.95 ,~, is . . . . . .
. .....
ferromagnetic. If wc assume that the exchange interaction between Mn atoms depends only on the M n - M n distance, the positive sign of d T n / d p seems to bc duc to the atomic distance dependence of ,I~ as a first approximation. On the other hand, it is reported that the atomic distance dependence of the exchange interactions in the A u - M n ordered alloys Au4Mn and Au ~Mn is anisotropic [7,8]. It is, thcrcforc, desirable to examine the exchange strictions of the lattice parameters a, b, c and [3 in order to get more detailed understanding of the distance dependence of the nlagnctic properties. The authors would like to express their sincere thanks to Prof. A. lnoue for the thermal expansion measurements. References
[1] [2] [3] [4] [5]
S.G. tlumble, Acta. Ct'ystallogr. 17 (1964) 1485. !1. Sato, J. Appl. Phys. 32 (1961) 53S. R,L. Cohen and K.W. Wesi, J. de Phys. 32 (1971) C1-781. E. Vogt, Phys. Slat. Sol. (a) 28 (1975) II. K. Inoue, Y. Nakamura. K. Yamamoto and S. Maruyama, J. Phys. Soe. Jp,q. 2 t, (1968) 646. [6] J.H. Smith and P. Wells, J. Phys. C 2 (1969) 356. [7] T. Kaneko, M. Matsumoto and N. Kazama, J. Phys. Soc. Jpn. 24 (19c~8) 647. [8] T. Kaneko and M. Matsumoto. J. Phys. Soc. Jpn. 27 (1969) 1141.
Journal of Magnetism and Magnetic Materials 104-1117 (1992) 21161-.116_ "~ "~ North-t lolland
ii
Pressure effect on the NEel temperature and exchange stricticm of the ordered alloy AusMn 2 M. Matsumoto a S, Abe b, H. Yoshida b, S. Mori ':, T. Kanomata " and T. Kancko h "Research lnsitute of Mineral Dressing and Metallurgy, Tohoku Unit'emit),, Sendai, 080 Japatt t, Institute .for Materials Research, Tohoku University, Sendai, 980 Japan ' Department of Applied P~3'sics, Faculty oJ" Engineering, Tohoku Gakuin Unit'er~'ity, Tagajo, M#'agi, 985 Japan
The pressure effect on the N~el temperature T N, the thermal expansion and the specific heat are studied for the ordered alloy AusMn 2. T N decreases linearly with pressure, d T N / d p is obtained to be 11.31 K/kbar. The jumps in the volume thermal expansion coefficient t~ and the specific heat C v at TN are determined to the Aat. = 11.5x Ill s K t and A('t, = 8.2 cal/mol K, respectively. The atomic distance dependence of TN is discussed.
It is known that the a p p e a r a n c e of magnctic order in m a n g a n c s e alloys and c o m p o u n d s has a close rclation with the M n - M n atomic distances. Many ordered alloys formed between Mn and Au metals show intercsting magnetic properties in grcat varicty and thc occurrcncc of various magnctic order in them was extensively studied from the point of vicw of the M n Mn atomic distancc d c p c n d c n c c of the exchangc interactions [2-4]. Very little is known about thc magnetic propcrtics of thc ordcrcd alloy A u s M n 2 having a monoclinic cry:;tal structurc with the space group of C 2 / m [5]. Prcvious rcsults of magnetic mcasuremcnts have shown that Au.sMne is antifcrromagnctic with a N6el t c m p c r a t u r e TN of 348 K, a paramagnctic Curie tcmpcrauturc Op of 1211 K and an effective magnctic m o m e n t /.t~, of 5.56# u in rcf. [5] and T x = 353 K, (-)p = I'll) K and ,tt,:H = 5.5btn in rcf. [6]. According to the rcsults of neutron diffraction studics, the magnetic unit cell is identical to thc chcmical onc and each ccll contains four mangancse at.oms with a magnctic momcnt of 5/.t n pcr Mn arranged in a colinear antifcrromagnetic array. In this paper, thc magnctic susccptibility, the prcssurc cffcct on thc N6ei tcmpcraturc, the thermal cxpansion and thc specific hcat of thc ordered alloy A u s M n 2 arc mcasurcd in order to cxaminc thc M n - M n atomic distancc d e p e n d e n c e of its magnetic propertics. The specimcn was p r e p a r e d by melting the consituent cicmcnts in a high-frcqucncy furnace and the product was anneaicd at 400°C for 40 d. X-ray diffract,on study showed that the prcparcd spccimcn is in singlc phasc with a monoclinic crystal structure. The lattiCeo p m a m c t e r s arc found to bc a = 9.21 A, b = 3.96 A, c = 6.48 A and /3 = 97.6 °, which are in a g r c c m c n t with thosc in rcf. [1]. The magnctic susccptibility was m c a s u r c d using a magnctic balancc. Thc prcssurc change of thc Ndel t c m p c r a t u r c was d c t c r m i n c d by m c a s u r i n g the tcmpcraturc dependcnce ot the electri-
cal resistance under various pressures, which were applied by using a piston-cylinder type apparatus. The measurements of the specific heat and the thermal expansion were carried out by using a pulsed calorimeter and a dilatometer, respectively. Fig. 1 shows the magnetic susceptibility (X,,) versus temperature ( T ) curve of A u s M n 2 , where a peak of Xv is observed at 354 K ( = T N). The temperature dependence of reciprocal susceptibility follows the C u r i e Weiss law and the effective magnetic moment is found to be 4 . 0 u u / M n . Fig. 2 shows thc Nt3el temperature (T x) versus prcssure ( p ) curve of A u s M n 2. T N increases linearly with applied pressure and the pressure derivative of T x is found to bc d T x / d p = 1!.31 K / k b a r . Fig. 3 shows the linear thermal expansion curve of AusMn_~. The A I / I curvc below about 360 K deviates downward from the smooth extrapolation (dotted line) of the curve for high temperatures, which indicates the exchange striction to be negative. From the above
I
-I
F
.,
.0
• ~.
~.
a
2t-
•
-. *
,b.
4 2
%
.
.
.
.
.
.
".
I F
,
':'
o
050
250
4-50
650
TIK)
|
,2.
1 Temperature der.:ndence of the magnetic susceptibilit) Xv, of AusMn 2. •
0312-88:';3/92/$05.01; S~; 1992 - Elsevier Science Publishers B.V. All rights reserx,ed
2062
M. Matsumoto et al. / Pressure el'/'cot cm ttw N&'I temperature and erclumge striction o.f tlw ordered alloy Au.~Mn 2 358
,
i
.
,"
,
,'"
60
. . . .
t
. . . .
w
. . . .
-
357
356 I---
355
w
354-
.1(.
353
/,,
50
-
,
,
,
,
0
I 5
p [kbar
4.0
, .I nn ¢J
!
...'
30 i
Fig. 2. Pressure dependence of the N6cl temperature 7'N of AusMn 2.
G ?0
,x I 0 4 lfl ~
L
50
1
.
,
,
t
t
.
.
.
.
I
.
.
.
.
.
v
250
4.50
650
T(KI Fig. 4. "l'empcralurc dependence of lhc spccil'ic heal (), of Au 5M n 2.
o
350
4.00
T(K]
Fig. 3. Linear thermal expansion curve of AusMn 2. results the difference in tl~c volume expansion coefficient at T n is roughly estimated to bc ~ r t = 0.5 × 10 s K i Fig. 4 shows the specific heat (Cp) versus lcmpcraturc curve of AusMn 2. As the temperature increases there is a sharp decrease of Cp observed at 350 K (= Tn), which is ascribed to the disappearance of magnetic order. Tht jump of Cp at T n is found to be ACt, = 8.82 c a l / m o l K. Ehrenfcst's equation for a magnetic transition of the second ordcr at T N is d T N / d p = ( T N / p ) ( A a l . / AC/,). lnscrtion of the valucs of T N, Aa=. and ACt, obtained above and density p = 15.6 g / c m 3 into the cquation givcs d T N / d p = 0.3 K / k b a r , which is in good agreement with the above value obtained from the direct mcasurcnlent. T!!c ,',',',-.,0,; r~u~v=u, suggests L*'l l a l " the cxchangc intcraction (j~) bctwccn thc ncarcstncighbouring Mn atoms wi~h intcratomic distance of 2.91 A is antifcrrornaguct~c and that (J-,) bctwccn ncxt-ncarcst-ncighbouring o~cs with spacing 3.95 ,~, is . . . . . .
. .....
ferromagnetic. If wc assume that the exchange interaction between Mn atoms depends only on the M n - M n distance, the positive sign of d T n / d p seems to bc duc to the atomic distance dependence of ,I~ as a first approximation. On the other hand, it is reported that the atomic distance dependence of the exchange interactions in the A u - M n ordered alloys Au4Mn and Au ~Mn is anisotropic [7,8]. It is, thcrcforc, desirable to examine the exchange strictions of the lattice parameters a, b, c and [3 in order to get more detailed understanding of the distance dependence of the nlagnctic properties. The authors would like to express their sincere thanks to Prof. A. lnoue for the thermal expansion measurements. References
[1] [2] [3] [4] [5]
S.G. tlumble, Acta. Ct'ystallogr. 17 (1964) 1485. !1. Sato, J. Appl. Phys. 32 (1961) 53S. R,L. Cohen and K.W. Wesi, J. de Phys. 32 (1971) C1-781. E. Vogt, Phys. Slat. Sol. (a) 28 (1975) II. K. Inoue, Y. Nakamura. K. Yamamoto and S. Maruyama, J. Phys. Soe. Jp,q. 2 t, (1968) 646. [6] J.H. Smith and P. Wells, J. Phys. C 2 (1969) 356. [7] T. Kaneko, M. Matsumoto and N. Kazama, J. Phys. Soc. Jpn. 24 (19c~8) 647. [8] T. Kaneko and M. Matsumoto. J. Phys. Soc. Jpn. 27 (1969) 1141.