- Email: [email protected]
Pressure effect on the Curie temperature and exchange striction of LaMn2Ge2
Journal of Magnetism and Magnetic Materials 104-107 (1992) 1951-1952 North-Holiand
Pressure effect on the Curie temperature and exchange striction of LaMn 2Ge2 T. Kaneko
~, H . Y a s u i
~', T . K a n o m a t a
b and T. Suzuki
t,
a h~stitute for Materials Research, Tokohu Unicersi~., Katahira, Sendai, 980, Japan t, Department of Applied Physics, Faculty of Engineering, Tohoku Gakuin Unicersity, Tagajo, Miyagi, 985, Japan The pressure effect on the Curie temperature Tc and the thermal expansions of the lattice parameters a and c are studied for the ferromagnetic compound LaMn2Ge 2. The pressure derivative of Tc is obtained to be dTc/dP = - 0 . 2 K/kbar. Both the exchange strictions of a and c are found to be positive. The relation between the Mn-Mn distance and the Curie temperature is discussed. lntermetallic c o m p o u n d s R M 2X 2 (R is a rare-earth metal, M a 3d or 4d metal and X germanium or silicon) have a crystal structure of the ThCr2Si2-type, which can be described as u stacking of atomic layers in the direction of the c-axis with the sequence R - X - M - X - + R. In R M n 2 G e 2 c o m p o u n d s at a temperature above 100 K, the magnetic m o m e n t s of the M n atoms order ferromagnetically (F) for light R and Y, except for Sm, and antiferromagnetically (AF) for heavy R, respectively. In the A F state, the Mn magnetic m o m e n t s couple ferromagneticaily in the same c-plane and the ferromagnetic Mn layers coup ~c antiferromagnetically with adjacent c-planes. SmMn2Ge~ and Y0.7La0.3Mn2 G e , have an A F - F transition below the Curie temperature (T c) [1,2]. Fujii et al. have pointed out that the M n - M n distance, R ~ , _ M , , in the Mn intralayer has a close relation to the a p p e a r a n c e of A F or F states of the M n magnetic m o m e n t s [3]. In order to cxamine thc M n - M n distance d e p e n d e n c e of the magnctic transitions, we have studied the pressure effect on the magnetic transiton t e m p e r a t u r e s and exchange striction for D y M n z G e 2 [4] and Y0.TLa0.3Mn2Ge2 [5]. In this paper,
the pressure effect on the Curie temperature Tc and the thermal expansion of the lattice par~imeters are studied for the ferromagnetic compound LaMn2Ge 2. The specimen was prepared by melting a mixture of constituent elements and annealing at 800°C for 7 d. The pressure dependence of Tc was determined by measuring the temperature variation of the permeability under various pressures which were applied by using a piston-cylinder type apparatus. The temperature variation of the lattice p a r a m e t e r was measured by using a high-temperature X-ray diffractometer at temperatures from 280 to 600 K. Fig. 1 shows the permeability (/1) versus temperature (T) curves at various pressures. As the temperature increases, g increases first, deureases rapidly just below Tc and then maintains constant above Tc. Tc was defined as the temperature shown with an arrow in the figure. Tc thus determined at normal pressure is 326 K. In fig. 2 the Curie temperature versus pressure curve is shown. Tc decreases linearly with pressure and its pressure derivative, d T c / d P , is o b t a i n e d to be -0.2 K/kbar. Fig. 3 shows the lattice p a r a m e t e r s a, c and volume versus temperature curves. Since a and c below Tc
is--.
LoMn2Oe2
-.':"~""-;
,...-- t,,o
LaMn2Ge2
•-""f .f" fN "" .............. ~ ....'"" ..": ...- - ..
"E 4 o
...~' ""''.."'""''."." • ........- / . r .-- "..~,
.......
-'"" ;.." "
,,.."
:
•
'-...
328
I bar 3.4 kbor
326
-£
.....
,'"
." ~.
"O
6.1 kbor
~
O ~
O ......__ -
3241.-
322
2 300
310
t 320
330
3t,0
ttKi
Fig. 1. Magnetic permeability versus temperature curves of LaMn,Ge, at various pressures.
32O
l
1
.
5 10 p [kber I Fig. 2. The Curie temperature versus pressure curve of LaMn.Ge..
0312-8853/02/$05.00 (C 1002 - Elsevier Science Publishers B.V. All rights rese~,ed
T. Kaneko et al. / Curie temperature and ¢~'changestriction of LaMn ,Ge,
1952
...o
LaMn~Ge2
196
~. O~ 0"~0 ,~,~ 0 ~0~0
195
..%.oo 194 193
/o~.~o/°'°"
11,06
O
ll.0t~
0" 0 0"~''~'0 10/01
u I1.00
~° 10.9B
oo,3.o/°°
o~ 421 f . ,t,'9 °"°a'%°'°'°°-°'°-°"~-°-°'°-°-°"°-° 360
,~60
T(K)
s~0
Gc 2 (P > 4 kbar) [5] and SmMn2Gc 2 [6] is positive and that of Tc, d T c / d P , of Y0.TLa,.3Mn2Ge 2 is negative like that of LaMn2Gc 2. The exchange strictions along the a- and c-axis below T N are negative for DyMn~Gc 2 and thosc bc!ow Tc. are positive for Y,.TLa..3Mn2Gc~. These results imply that the M n - M n distance dependence of the exchange interaction is dJ,,/da < 0 and dJ,./dc > 0 (J,, < 0) for the AF compound and dJ,,/da > 0 and dJ,./dc > 0 for the F compound. The above results show that the pressure dependence of the magnetic transition temperature for the RMn2Ge 2 compound system cannot be interpreted on the basis of a simple smooth interaction curve. It was already pointed out that the pressure dependence of the A F - F transition temperature TI and the change of a and c at Tt cannot be explained in terms of the interaction curve
[3,51.
660
Fig. 3. Thermal expansion curves of the lattice parameters a, c and volume V of LaMn,Ge,. expand to the smooth extrapolations (dotted lines) of their thermal expansion curves above Tc, both a and c have positive exchange strictions. The volume exchange striction is also positive. The change of the volume thermal expansion coefficients at T c, ~a,. = a , . ( T < Tc) - a,,(T > To), is estimated to be --. 15 × 10- 5 K i The positive sign of the volume exchange striction is consistent with that expected from the negative value of d T c / d P obtained above. Whcn wc take J,, along thc a-axis and J, along the c-axis as exchange interactions, ,I,, and J, arc positive in the ferromagnetic compound LaMn,Gc 2. The positive signs of thc cxchange strictions of a and c obtained above mean dJ,,/da > 0 and dJ,,/dc > 0 on the basis of the interaction curve, where the exchange interaction or magnetic transition temperature is plotted as a function of the distance between the magnetic atoms. On the other hand, the interaction curve for RMn2Ge 2 by Fujii et al. [3], in which the magnetic transiton temperatures arc plotted against the M n - M n distances in the intralayer or between the layers, imply dJ,,/da < 0 , dJ,,/d,. < 0 and d T c / d P > O. Thcreforc, the present results on the exchange strictions of a and c and the pressure effect on Tc are inconsistent with those expected from the interaction curves in rcf. [3]. The previous results for the pressure effect on the magnetic transition temperatures show that the pressure derivative of T N, dTN/dP, of DyMn2Gc 2 [4], Y,.TLau3Mn2
As mentioned above, the M n - M n distance dependence of the magnetic properties of the F compound and the AF ones has an opposite tendency which is not .simply explained in terms of the interaction curve assuming localized magnetic moments on Mn sites. According to the recent investigations for Laves compounds RMn:,, the magnetic properties of Mn in those compounds are interpreted on the basis of itinerant electron magnetism [7]. Therefore, the magnetic propertics of Mn in the present compounds are also considered to be discussed in terms of itinerant electron magnetism. Further studies of the pressure effcct on the magnetic moments of thc Mn atom as well as that on Tc arc desirable to clarify the nature of the magnetism of Mn in this compound system. In order to examine the pressure effect on the magnetization, rncasurcmcnts of forced striction are in progress for F compounds YI ..,.La., Mn 2Gc 2-
References [1] S. Siek, A. Szytula and J. Leciejewics, Solid State Commun. 39 (1981) 863. [2] A. Szytula and !. Szott, Solid State Commun. 40 (1981) 199. [3] H. Fujii, T. Okamoto, T. Shigeoka and N. lwata, Solid State Commun. 53 (1985) 715. [4] T. Kaneko, H. Yasui, T. Kanomata, H. Kobayashi and H. Onodera, J. de Phys. 49 (1988) C8-441. [5] T. Kaneko, T. Yasui, T. Kanomata, T. Shigeoka and N. iwata, to be published. [6] M. Duraj, R. Duraj, A. S~t,_.!a and Z. Tomkowicz, J. Magn. Magn. Mater. 73 (1988) 24(I. [7] M. Shiga, It. Wad;l, Y. Nakamura, J. Deportes and K.R.A. Zicbcck, J. dc Phys. 49 (,1988) C8-241.
Pressure effect on the Curie temperature and exchange striction of LaMn 2Ge2 T. Kaneko
~, H . Y a s u i
~', T . K a n o m a t a
b and T. Suzuki
t,
a h~stitute for Materials Research, Tokohu Unicersi~., Katahira, Sendai, 980, Japan t, Department of Applied Physics, Faculty of Engineering, Tohoku Gakuin Unicersity, Tagajo, Miyagi, 985, Japan The pressure effect on the Curie temperature Tc and the thermal expansions of the lattice parameters a and c are studied for the ferromagnetic compound LaMn2Ge 2. The pressure derivative of Tc is obtained to be dTc/dP = - 0 . 2 K/kbar. Both the exchange strictions of a and c are found to be positive. The relation between the Mn-Mn distance and the Curie temperature is discussed. lntermetallic c o m p o u n d s R M 2X 2 (R is a rare-earth metal, M a 3d or 4d metal and X germanium or silicon) have a crystal structure of the ThCr2Si2-type, which can be described as u stacking of atomic layers in the direction of the c-axis with the sequence R - X - M - X - + R. In R M n 2 G e 2 c o m p o u n d s at a temperature above 100 K, the magnetic m o m e n t s of the M n atoms order ferromagnetically (F) for light R and Y, except for Sm, and antiferromagnetically (AF) for heavy R, respectively. In the A F state, the Mn magnetic m o m e n t s couple ferromagneticaily in the same c-plane and the ferromagnetic Mn layers coup ~c antiferromagnetically with adjacent c-planes. SmMn2Ge~ and Y0.7La0.3Mn2 G e , have an A F - F transition below the Curie temperature (T c) [1,2]. Fujii et al. have pointed out that the M n - M n distance, R ~ , _ M , , in the Mn intralayer has a close relation to the a p p e a r a n c e of A F or F states of the M n magnetic m o m e n t s [3]. In order to cxamine thc M n - M n distance d e p e n d e n c e of the magnctic transitions, we have studied the pressure effect on the magnetic transiton t e m p e r a t u r e s and exchange striction for D y M n z G e 2 [4] and Y0.TLa0.3Mn2Ge2 [5]. In this paper,
the pressure effect on the Curie temperature Tc and the thermal expansion of the lattice par~imeters are studied for the ferromagnetic compound LaMn2Ge 2. The specimen was prepared by melting a mixture of constituent elements and annealing at 800°C for 7 d. The pressure dependence of Tc was determined by measuring the temperature variation of the permeability under various pressures which were applied by using a piston-cylinder type apparatus. The temperature variation of the lattice p a r a m e t e r was measured by using a high-temperature X-ray diffractometer at temperatures from 280 to 600 K. Fig. 1 shows the permeability (/1) versus temperature (T) curves at various pressures. As the temperature increases, g increases first, deureases rapidly just below Tc and then maintains constant above Tc. Tc was defined as the temperature shown with an arrow in the figure. Tc thus determined at normal pressure is 326 K. In fig. 2 the Curie temperature versus pressure curve is shown. Tc decreases linearly with pressure and its pressure derivative, d T c / d P , is o b t a i n e d to be -0.2 K/kbar. Fig. 3 shows the lattice p a r a m e t e r s a, c and volume versus temperature curves. Since a and c below Tc
is--.
LoMn2Oe2
-.':"~""-;
,...-- t,,o
LaMn2Ge2
•-""f .f" fN "" .............. ~ ....'"" ..": ...- - ..
"E 4 o
...~' ""''.."'""''."." • ........- / . r .-- "..~,
.......
-'"" ;.." "
,,.."
:
•
'-...
328
I bar 3.4 kbor
326
-£
.....
,'"
." ~.
"O
6.1 kbor
~
O ~
O ......__ -
3241.-
322
2 300
310
t 320
330
3t,0
ttKi
Fig. 1. Magnetic permeability versus temperature curves of LaMn,Ge, at various pressures.
32O
l
1
.
5 10 p [kber I Fig. 2. The Curie temperature versus pressure curve of LaMn.Ge..
0312-8853/02/$05.00 (C 1002 - Elsevier Science Publishers B.V. All rights rese~,ed
T. Kaneko et al. / Curie temperature and ¢~'changestriction of LaMn ,Ge,
1952
...o
LaMn~Ge2
196
~. O~ 0"~0 ,~,~ 0 ~0~0
195
..%.oo 194 193
/o~.~o/°'°"
11,06
O
ll.0t~
0" 0 0"~''~'0 10/01
u I1.00
~° 10.9B
oo,3.o/°°
o~ 421 f . ,t,'9 °"°a'%°'°'°°-°'°-°"~-°-°'°-°-°"°-° 360
,~60
T(K)
s~0
Gc 2 (P > 4 kbar) [5] and SmMn2Gc 2 [6] is positive and that of Tc, d T c / d P , of Y0.TLa,.3Mn2Ge 2 is negative like that of LaMn2Gc 2. The exchange strictions along the a- and c-axis below T N are negative for DyMn~Gc 2 and thosc bc!ow Tc. are positive for Y,.TLa..3Mn2Gc~. These results imply that the M n - M n distance dependence of the exchange interaction is dJ,,/da < 0 and dJ,./dc > 0 (J,, < 0) for the AF compound and dJ,,/da > 0 and dJ,./dc > 0 for the F compound. The above results show that the pressure dependence of the magnetic transition temperature for the RMn2Ge 2 compound system cannot be interpreted on the basis of a simple smooth interaction curve. It was already pointed out that the pressure dependence of the A F - F transition temperature TI and the change of a and c at Tt cannot be explained in terms of the interaction curve
[3,51.
660
Fig. 3. Thermal expansion curves of the lattice parameters a, c and volume V of LaMn,Ge,. expand to the smooth extrapolations (dotted lines) of their thermal expansion curves above Tc, both a and c have positive exchange strictions. The volume exchange striction is also positive. The change of the volume thermal expansion coefficients at T c, ~a,. = a , . ( T < Tc) - a,,(T > To), is estimated to be --. 15 × 10- 5 K i The positive sign of the volume exchange striction is consistent with that expected from the negative value of d T c / d P obtained above. Whcn wc take J,, along thc a-axis and J, along the c-axis as exchange interactions, ,I,, and J, arc positive in the ferromagnetic compound LaMn,Gc 2. The positive signs of thc cxchange strictions of a and c obtained above mean dJ,,/da > 0 and dJ,,/dc > 0 on the basis of the interaction curve, where the exchange interaction or magnetic transition temperature is plotted as a function of the distance between the magnetic atoms. On the other hand, the interaction curve for RMn2Ge 2 by Fujii et al. [3], in which the magnetic transiton temperatures arc plotted against the M n - M n distances in the intralayer or between the layers, imply dJ,,/da < 0 , dJ,,/d,. < 0 and d T c / d P > O. Thcreforc, the present results on the exchange strictions of a and c and the pressure effect on Tc are inconsistent with those expected from the interaction curves in rcf. [3]. The previous results for the pressure effect on the magnetic transition temperatures show that the pressure derivative of T N, dTN/dP, of DyMn2Gc 2 [4], Y,.TLau3Mn2
As mentioned above, the M n - M n distance dependence of the magnetic properties of the F compound and the AF ones has an opposite tendency which is not .simply explained in terms of the interaction curve assuming localized magnetic moments on Mn sites. According to the recent investigations for Laves compounds RMn:,, the magnetic properties of Mn in those compounds are interpreted on the basis of itinerant electron magnetism [7]. Therefore, the magnetic propertics of Mn in the present compounds are also considered to be discussed in terms of itinerant electron magnetism. Further studies of the pressure effcct on the magnetic moments of thc Mn atom as well as that on Tc arc desirable to clarify the nature of the magnetism of Mn in this compound system. In order to examine the pressure effect on the magnetization, rncasurcmcnts of forced striction are in progress for F compounds YI ..,.La., Mn 2Gc 2-
References [1] S. Siek, A. Szytula and J. Leciejewics, Solid State Commun. 39 (1981) 863. [2] A. Szytula and !. Szott, Solid State Commun. 40 (1981) 199. [3] H. Fujii, T. Okamoto, T. Shigeoka and N. lwata, Solid State Commun. 53 (1985) 715. [4] T. Kaneko, H. Yasui, T. Kanomata, H. Kobayashi and H. Onodera, J. de Phys. 49 (1988) C8-441. [5] T. Kaneko, T. Yasui, T. Kanomata, T. Shigeoka and N. iwata, to be published. [6] M. Duraj, R. Duraj, A. S~t,_.!a and Z. Tomkowicz, J. Magn. Magn. Mater. 73 (1988) 24(I. [7] M. Shiga, It. Wad;l, Y. Nakamura, J. Deportes and K.R.A. Zicbcck, J. dc Phys. 49 (,1988) C8-241.