- Email: [email protected]
Comparative study of the complex magnetic properties of RRu2X2 (R = rare earth; XSi,Ge)
Journal of Magnetism and Magnetic Materials 157/158 (1996) 389-390
journal of magnetism and magnetic materials
N
ELSEVIER
Comparative study of the complex magnetic properties of R R u z X 2 ( R -- rare earth; X = Si, Ge) A. Gamier a, D. Gignoux a, D. Schmitt a,* , T. Shigeoka b Laboratoire Louis N&l, CNRS, BP i66, 38042 Grenoble Cedex 9, France b Faculty of Science, Yamaguchi University, Yamaguchi 753, Japan Abstract The magnetic properties of R R u 2 X 2 compounds are characterized by long period commensurate a n d / o r incommensurate structures, anisotropic multistep metamagnetic processes, and complex field-temperature phase diagrams. These properties originate from the competition between antagonistic RKKY interactions in the presence of magnetocrystalline anisotropy. Keywords." Anisotropy; Field-temperature phase diagram; Incommensurate structures; Intermetallic compounds; Long period commensurate structures
Magnetization, susceptibility and neutron diffraction experiments have been carried out on single crystals for many compounds of the series R R u 2 X 2 (R = Pr, Nd, Gd, Tb, Dy, Ho, Er; X = Si, Ge) [1-8] (ThCrzSiz-type structure, space group 1 4 / m m m ) . The magnetic characteristics of these compounds are summarized in Table 1. Propagation vectors Q are localized in a limited region of the first Brillouin zone, mainly in the basal plane, except for NdRu2Si 2. Compounds with light rare earths are ferromagnetic or exhibit a ferro-antiferromagnetic transition at low temperature. The propagation vectors of the antiferromagnetic phase lie along [110], not far from Q0 = 0, which explains the locking of Q towards Q0 when the temperature decreases for Nd compounds. For heavy rare earth compounds, propagation vectors are along the [100] direction and their magnitude varies from 0.235 (Tb) to 0.2 (Ho), i.e. values higher than in Nd and Pr compounds. Therefore, when the temperature decreases, more complex magnetic structures with long commensurate period are stabilized instead of ferromagnetic structures. The magnetic cell includes from 5 to 26 moments. Tb and Dy compounds have very similar behaviour, they both exhibit a magnetic transition at very low temperature with no detectable change of Q. The ordering temperatures do not follow de Gennes law; in particular, Tb compounds order at a much higher temperature than Gd compounds. Replacing Si by Ge systematically lowers the ordering temperature, except for
* Corresponding author. Fax: [email protected].
+33-76-88-11-91;
email:
Pr and Ho. Study of T t / T N separates these compounds into two groups. In the first group (Gd, Nd), this ratio is about 0.5-0.7, a value usually observed in intermetallic compounds. Replacing Si by Ge does not drastically modify this behaviour. The situation differs strongly for Tb and Dy compounds, where T t / T N may reach values as low as 0.1 and Tt is only weakly affected by the substitution of Table 1 Magnetic properties of RRn2X 2 Compound
Tt
TO
(K)
(K)
PrRu2Si 2 PrRu 2Ge 2 NdRu2Si 2
10
14 18 24
NdRu2Ge 2
10
17
GdRu 2Si 2 GdRu 2Ge 2 TbRu2Si 2 TbRu2Ge 2 DyRn 2Si 2 DyRu2Ge 2 HoRu2Si 2 HoRu2Ge 2 ErRu2Si 2 ErRu 2Ge 2
40 29 5 4.3 3.5 3.4
47 33 57 37 29 15 19 20 6 4
Q
M~
Ref.
(/zB/fu) Ferro Ferro Ferro +(0.13 0.13 0) a +(0.13 0.13 0) b Ferro a (0.19 0.05 0.125) +(0.12 0.12 0) b AF AF (3/13 0 0) (4/17 0 0) (2/9 0 0) AF (1/5 0 0) (0.22 0.11 0) (1/5 0 0) AF
2.7 1.8 2.8
[1] [2] [3]
3.27
[4]
7.1 7.1 8.9 8.9 9.6 9.8 9.25 d 6.6 d 6.5
[5] c [6] [6] [7] c [8] [2] ° [2]
To, ordering temperature; Tt, transition temperature; Q, propagation vector; Ms, magnetic moment. a Below Tt. b Above Tt. ° This work. d Neutron results.
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSD1 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 1 1 8 6 - 2
A. Garnier et al. /Journal of Magnetism and Magnetic Materials 157/158 (1996) 389-390
390 ._.,
I
I
I
9 -TbRuzGe2.. .........
•
.
[OOll
_
l
i
•
6-
6 K
-
:
[0~
= 4
1 2 Magnetic
3 field
4 (T)
5
Fig. 1. Magnetization versus field at T = 1.5 and 6 K for TbRuaGe 2 . Ge for Si. Moreover, other magnetic features are anomalous in the low temperature phases, such as the variation of specific heat. This behaviour [6,7] is not yet fully understood. The magnetization processes are very similar in both series; therefore, the microscopic behaviour can likely be explained in the same way. For instance, Figs. 1 and 2 show the results for TbRu2Ge 2. At low temperature, multistep metamagnetic behaviour, with a huge hysteresis for the high field transitions, is observed. A change in the low field magnetic behaviour can clearly be observed on both sides of Tt: below Tt, the plateaus are more horizontal, suggesting that the periodicity is locked onto a long period commensurate value. This change is also observed in Dy compounds. A huge magnetocrystalline anisotropy (Fig. 2) can be observed in TbRu2Ge 2, as in other rare earth compounds. At 300 K the Curie-Weiss law is not yet fully attained. This behaviour may originate in residual crystal
,--1.6
1
I
TbRu2Ge 2 ~
1 2
[001] leo
--
.."
.."T "'o.
....""
3m
16
~i'"
TN
". -
oe
--
2
•
o~
E
--
7
I
.
s
,.0 o~
,,.'0.4 •
ev)
0 10
-
[100]
0 "-"'"T
0
20
10
30
4o
-
T (~)
. . . . . . . . . T. . . . . . . " T - " - - ' f
20 30 Temperature
Oo
~£aauG; .
,
.
~,2F~ .....
~
,
2--,
,
.
' ,
J
•
'; .... ,
-
,
'
~
,
•
t
1
"1
%'''1'o"'2'o"'3'o'''4o 1
t
I
t
2 3 4 5 Magnetic field (T)
6
Fig. 3. Magnetization versus fieId at T = 1.5 K for GdRu2Ge 2 along and perpendicular to the c-axis. Inset: magnetic specific heat versus temperature. field (CEF) effects and a noticeable Ru contribution to the susceptibility. Saturated magnetizations for Tb and Dy are close to the theoretical values. For the other compounds, reduced values arise from a mixing of the CEF eigenstates. For Gd compounds, the magnetization process is very similar. Compared with neighbouring compounds, the propagation vector is likely incommensurate. Fig. 3 clearly emphasizes several magnetic transitions and the strongly anisotr0Pic character of the magnetization process. This suggests a noticeable anisotropic bilinear exchange interaction at q =g 0 since no anisotropy can exist due to CEF. A theoretical model [9], developed in our laboratory, explains how the isotropic bilinear exchange interaction coefficients J(nQ) contribute to the magnetization process. This model, extended to anisotropic exchange, should be very efficient for studying anisotropy, since all J(nQ) contributions are included in the calculations.
References
I
~"~,,
::k ~0.8
l|
100
It/G
m
1 0
,
. . . . . 22.
]
__
=
,[
A-- ~''-
y//
1,5
"~=5
6-
I
GdRu2Ge2 / ~ . o - ~ _
~-~
/
I
......... 40 (K)
50
Fig. 2. Temperature variation of the magnetization for an applied field of 0.1 T for TbRuzGe 2. Inset: field-temperature diagram for the [001] direction.
[1] T. Shigeoka, N. Iwata and H. Fujii, J. Magn. Magn. Mater. 104-107 (1992) 1229. [2] A. Szytula, in: Handbook of Magnetic Materials, Vol. 6, ed. K.H.J. Buschow (North-Holland, Amsterdam, 1991) ch. 2. [3] T. Shigeoka, M. Saeki, N. Iwata, T. Takabatake and H. Fujii, J. Magn. Magn. Mater. 90-91 (1990) 557. [4] A. Gamier, D. Gignoux, N. Iwata, D. Schmitt, T. Shigeoka and F.Y. Zhang, J. Magn. Magn. Mater. 140-144 (1995) 897. [51 A. Gamier, D. Gignoux, N. Iwata, D. Schmitt, T. Shigeoka and F.Y. Zhang, J. Magn. Magn. Mater. 140-144 (1995) 899. [6] A. Garnier, D. Gignoux, D. Schmitt and T. Shigeoka, Physica B 212 (1995) 343. [7] B. Andreani, G. Fraga, A. Gamier, D. Gignoux, D. Maurin, D. Schmitt and T. Shigeoka, J. Phys. Condensed Matter 7 (1995) 1889. [8] M. Slaski, J. Leciejewicz and A. Szytula, J. Magn. Magn. Mater. 39 (1983) 268. [9] A. Ball, Thesis, University J. Fourier, Grenoble, 1993, unpublished.
journal of magnetism and magnetic materials
N
ELSEVIER
Comparative study of the complex magnetic properties of R R u z X 2 ( R -- rare earth; X = Si, Ge) A. Gamier a, D. Gignoux a, D. Schmitt a,* , T. Shigeoka b Laboratoire Louis N&l, CNRS, BP i66, 38042 Grenoble Cedex 9, France b Faculty of Science, Yamaguchi University, Yamaguchi 753, Japan Abstract The magnetic properties of R R u 2 X 2 compounds are characterized by long period commensurate a n d / o r incommensurate structures, anisotropic multistep metamagnetic processes, and complex field-temperature phase diagrams. These properties originate from the competition between antagonistic RKKY interactions in the presence of magnetocrystalline anisotropy. Keywords." Anisotropy; Field-temperature phase diagram; Incommensurate structures; Intermetallic compounds; Long period commensurate structures
Magnetization, susceptibility and neutron diffraction experiments have been carried out on single crystals for many compounds of the series R R u 2 X 2 (R = Pr, Nd, Gd, Tb, Dy, Ho, Er; X = Si, Ge) [1-8] (ThCrzSiz-type structure, space group 1 4 / m m m ) . The magnetic characteristics of these compounds are summarized in Table 1. Propagation vectors Q are localized in a limited region of the first Brillouin zone, mainly in the basal plane, except for NdRu2Si 2. Compounds with light rare earths are ferromagnetic or exhibit a ferro-antiferromagnetic transition at low temperature. The propagation vectors of the antiferromagnetic phase lie along [110], not far from Q0 = 0, which explains the locking of Q towards Q0 when the temperature decreases for Nd compounds. For heavy rare earth compounds, propagation vectors are along the [100] direction and their magnitude varies from 0.235 (Tb) to 0.2 (Ho), i.e. values higher than in Nd and Pr compounds. Therefore, when the temperature decreases, more complex magnetic structures with long commensurate period are stabilized instead of ferromagnetic structures. The magnetic cell includes from 5 to 26 moments. Tb and Dy compounds have very similar behaviour, they both exhibit a magnetic transition at very low temperature with no detectable change of Q. The ordering temperatures do not follow de Gennes law; in particular, Tb compounds order at a much higher temperature than Gd compounds. Replacing Si by Ge systematically lowers the ordering temperature, except for
* Corresponding author. Fax: [email protected].
+33-76-88-11-91;
email:
Pr and Ho. Study of T t / T N separates these compounds into two groups. In the first group (Gd, Nd), this ratio is about 0.5-0.7, a value usually observed in intermetallic compounds. Replacing Si by Ge does not drastically modify this behaviour. The situation differs strongly for Tb and Dy compounds, where T t / T N may reach values as low as 0.1 and Tt is only weakly affected by the substitution of Table 1 Magnetic properties of RRn2X 2 Compound
Tt
TO
(K)
(K)
PrRu2Si 2 PrRu 2Ge 2 NdRu2Si 2
10
14 18 24
NdRu2Ge 2
10
17
GdRu 2Si 2 GdRu 2Ge 2 TbRu2Si 2 TbRu2Ge 2 DyRn 2Si 2 DyRu2Ge 2 HoRu2Si 2 HoRu2Ge 2 ErRu2Si 2 ErRu 2Ge 2
40 29 5 4.3 3.5 3.4
47 33 57 37 29 15 19 20 6 4
Q
M~
Ref.
(/zB/fu) Ferro Ferro Ferro +(0.13 0.13 0) a +(0.13 0.13 0) b Ferro a (0.19 0.05 0.125) +(0.12 0.12 0) b AF AF (3/13 0 0) (4/17 0 0) (2/9 0 0) AF (1/5 0 0) (0.22 0.11 0) (1/5 0 0) AF
2.7 1.8 2.8
[1] [2] [3]
3.27
[4]
7.1 7.1 8.9 8.9 9.6 9.8 9.25 d 6.6 d 6.5
[5] c [6] [6] [7] c [8] [2] ° [2]
To, ordering temperature; Tt, transition temperature; Q, propagation vector; Ms, magnetic moment. a Below Tt. b Above Tt. ° This work. d Neutron results.
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSD1 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 1 1 8 6 - 2
A. Garnier et al. /Journal of Magnetism and Magnetic Materials 157/158 (1996) 389-390
390 ._.,
I
I
I
9 -TbRuzGe2.. .........
•
.
[OOll
_
l
i
•
6-
6 K
-
:
[0~
= 4
1 2 Magnetic
3 field
4 (T)
5
Fig. 1. Magnetization versus field at T = 1.5 and 6 K for TbRuaGe 2 . Ge for Si. Moreover, other magnetic features are anomalous in the low temperature phases, such as the variation of specific heat. This behaviour [6,7] is not yet fully understood. The magnetization processes are very similar in both series; therefore, the microscopic behaviour can likely be explained in the same way. For instance, Figs. 1 and 2 show the results for TbRu2Ge 2. At low temperature, multistep metamagnetic behaviour, with a huge hysteresis for the high field transitions, is observed. A change in the low field magnetic behaviour can clearly be observed on both sides of Tt: below Tt, the plateaus are more horizontal, suggesting that the periodicity is locked onto a long period commensurate value. This change is also observed in Dy compounds. A huge magnetocrystalline anisotropy (Fig. 2) can be observed in TbRu2Ge 2, as in other rare earth compounds. At 300 K the Curie-Weiss law is not yet fully attained. This behaviour may originate in residual crystal
,--1.6
1
I
TbRu2Ge 2 ~
1 2
[001] leo
--
.."
.."T "'o.
....""
3m
16
~i'"
TN
". -
oe
--
2
•
o~
E
--
7
I
.
s
,.0 o~
,,.'0.4 •
ev)
0 10
-
[100]
0 "-"'"T
0
20
10
30
4o
-
T (~)
. . . . . . . . . T. . . . . . . " T - " - - ' f
20 30 Temperature
Oo
~£aauG; .
,
.
~,2F~ .....
~
,
2--,
,
.
' ,
J
•
'; .... ,
-
,
'
~
,
•
t
1
"1
%'''1'o"'2'o"'3'o'''4o 1
t
I
t
2 3 4 5 Magnetic field (T)
6
Fig. 3. Magnetization versus fieId at T = 1.5 K for GdRu2Ge 2 along and perpendicular to the c-axis. Inset: magnetic specific heat versus temperature. field (CEF) effects and a noticeable Ru contribution to the susceptibility. Saturated magnetizations for Tb and Dy are close to the theoretical values. For the other compounds, reduced values arise from a mixing of the CEF eigenstates. For Gd compounds, the magnetization process is very similar. Compared with neighbouring compounds, the propagation vector is likely incommensurate. Fig. 3 clearly emphasizes several magnetic transitions and the strongly anisotr0Pic character of the magnetization process. This suggests a noticeable anisotropic bilinear exchange interaction at q =g 0 since no anisotropy can exist due to CEF. A theoretical model [9], developed in our laboratory, explains how the isotropic bilinear exchange interaction coefficients J(nQ) contribute to the magnetization process. This model, extended to anisotropic exchange, should be very efficient for studying anisotropy, since all J(nQ) contributions are included in the calculations.
References
I
~"~,,
::k ~0.8
l|
100
It/G
m
1 0
,
. . . . . 22.
]
__
=
,[
A-- ~''-
y//
1,5
"~=5
6-
I
GdRu2Ge2 / ~ . o - ~ _
~-~
/
I
......... 40 (K)
50
Fig. 2. Temperature variation of the magnetization for an applied field of 0.1 T for TbRuzGe 2. Inset: field-temperature diagram for the [001] direction.
[1] T. Shigeoka, N. Iwata and H. Fujii, J. Magn. Magn. Mater. 104-107 (1992) 1229. [2] A. Szytula, in: Handbook of Magnetic Materials, Vol. 6, ed. K.H.J. Buschow (North-Holland, Amsterdam, 1991) ch. 2. [3] T. Shigeoka, M. Saeki, N. Iwata, T. Takabatake and H. Fujii, J. Magn. Magn. Mater. 90-91 (1990) 557. [4] A. Gamier, D. Gignoux, N. Iwata, D. Schmitt, T. Shigeoka and F.Y. Zhang, J. Magn. Magn. Mater. 140-144 (1995) 897. [51 A. Gamier, D. Gignoux, N. Iwata, D. Schmitt, T. Shigeoka and F.Y. Zhang, J. Magn. Magn. Mater. 140-144 (1995) 899. [6] A. Garnier, D. Gignoux, D. Schmitt and T. Shigeoka, Physica B 212 (1995) 343. [7] B. Andreani, G. Fraga, A. Gamier, D. Gignoux, D. Maurin, D. Schmitt and T. Shigeoka, J. Phys. Condensed Matter 7 (1995) 1889. [8] M. Slaski, J. Leciejewicz and A. Szytula, J. Magn. Magn. Mater. 39 (1983) 268. [9] A. Ball, Thesis, University J. Fourier, Grenoble, 1993, unpublished.