- Email: [email protected]
Unusual magnetic structures in Ce3Al11
Journal of Magnetism and Magnetic Materials 140-144 (1995) 1229-1230
~H
journal of magnetism and magnetic materials
ELSEVIER
Unusual
magnetic
s t r u c t u r e s in Ce3A1 u
J.X. Boucherle, F. Givord *, G. Lapertot, A. Mufioz, J. Schweizer CEA, DRFMC / SPSMS-MDN, C.E.N.G., 38054 Grenoble Cedex 9, France
Abstract The orthorhombic Kondo Ce3AI~ compound shows two magnetic phases: below 6.2 K it is ferromagnetic, but below 3.2 K, it becomes modulated, with an incommensurate propagation vector k = (0 0 kz). In both phases, Ce moments are along b. Their values are different on the two crystallographic sites, one being strongly reduced.
I. Introduction Ce3AII~ is one of the Ce-A1 compounds in which the coexistence of Kondo and magnetic interactions has been observed. Up to now, only studies on polycrystalline samples have been carried out. Kondo-type behavior was deduced from the resistivity [1] and specific heat [2] results. Magnetic properties [3,4] are rather striking: in the temperature range 3.2-6.2 K, the compound is ferromagnetic, but below 3.2 K, a sine-wave modulated structure with a propagation vector k = (0, 0, 1 / 3 ) was found. Recently, we have succeeded in the fabrication of large single crystals. Cc3AI1] is orthorhombic (space group Immm), with two sites for the Ce atoms, site 2a (mmm) and site 4g (m2m). However, the relation b = 3a between the lattice parameters led to twinned crystals, with interchanged a and b directions. The twin population of the crystals has been determined by neutron diffraction in order to correct the following measurements [5]. Magnetization and neutron diffraction experiments were carried out on these single crystals in order to confirm the unusual transition from a ferromagnetic phase to a modulated structure as the temperature is decreased, and to clarify the Ce atom behavior on both sites.
K) along the b axis (inset of Fig.la). A large anisotropy is present in the whole ordered region with b as the easy magnetization direction. At 4 K, the spontaneous magnetization is 1.60(10)/Xg/Ce3Alll formula. Neutron diffraction studies were performed on a twoaxis spectrometer with lifting counter at various temperatures ranging from 12 to 1.6 K. The onset of ferromagnetic order at T = 6.2 K is confirmed by a general increase in the intensities of Bragg reflections. The magnetic moments are along b, but their values on each site are quite different: 1.27(6)/x B for site 2a and 0.24(3)/x n for site 4g.
'Ce3AIll '
('a)
=
o't *: 2
4
.....
2K
6
H (T) .
2. Experimental results co
.
.
.
i
80
.
.
.
i
Ce3lll 1
.
.
.
.
(b)
¢--....,
=
The magnetization measurements, performed by the extraction method, confirm the existence of two magnetic phases: a ferromagnetic one between 6.2 and 3.2 K, and a phase with no spontaneous magnetization below 3.2 K (Figs. la, b). In this latter phase, transitions are induced by the applied field, especially in a very low field (0.17 T at 2
.
I
.~
~
HIIb
1
g
40
I r
I
i ,
,
i
5
i
,
10
15
T (K)
* Corresponding author. Fax: +33 76 88 51 09; email: [email protected].
Fig. 1. (a) Isothermal magnetization curves. Inset: detail of the low-field region. (b) Thermal evolution of the remanent magnetization M R.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00657-1
1230
J.X. Boucherle et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1229-1230
The moment for one Ce3Alll formula is then 1.75(12)/x n, in agreement with the spontaneous magnetization along b. Below T = 3.2 K, magnetic satellites appear at positions given by the propagation vector k = (0, 0, 1 / 3 ) , whereas the intensities of the Bragg reflections decrease, as shown in Fig. 2 for the (0, 0, - 2 ) reflection. Another small magnetic peak is observed at the (0, 0, - 1) position, a reflection which is forbidden by the space group Immm (h + k + l = 2 n + 1). It corresponds to the propagation vector 3k. Actually, this peak is composed of the two satellites 3 k and - 3 k associated with the Bragg reflections (0, 0, - 2 ) and (0, 0, 0), respectively. Its width at 2.8 K indicates that k. is slightly different from 1 / 3 , because, in such a case, the two satellites should coincide. The analysis of the temperature dependence of the k and 3 k satellites [5] shows that k: decreases towards 1 / 3 as temperature is lowered (Fig. 3a). The magnetic structure is modulated with the moments parallel to b for both sites. The presence of third-order satellites implies that the magnetic structure is not purely sinusoidal and can be of square-type. In the latter case, the ratio between the intensities of the 3 k and k satellites is 1 / 9 for magnetic atoms all in phase. Because there are two Ce sites, this ratio is more complex and depends on the value of A ~0 (phase difference between the modulation of the two sites) [5]. The experimental thermal variation of this ratio for the satellites of the (0, 0, - 2 ) reflection is reported on Fig. 3(b). It increases as the temperature is lowered, but remains much smaller than 1 / 9 . Its value at 2 20000
,, ', 10000
,'
.~ • , ~
0
C e3Ai1
i
T = 4.8 K
I
2, ,,,.4', ;-
,-;
~----,----.----.----.----.----~----.-
Z
~ 10000
=,
>-
~_
T=2.8K
,~
~
8~
, o
,
,
, --:J' .... ~--:-- : - -.....
~,
10000
o
T = 1.6 K
,,#"~
o
-~'
-2.2
, ~'-2
-J, -1.8
,~+~" ~
,k- ..................... -1.6
-1.4
-1.2
-1
C e3AI1
1
0.35
,
g
Q_ E
e
e
+
L
;
q,
(a) 0.34
,3 113
............................... 0.33 v
J
[
i
I
i
I
i
0.04 0.03
(b)
,, l o
0.02 0.01
i
1
,
i
2
,
i
3
T (K)
Fig. 3. Thermal evolutions (a) of k z, and (b) of the ratio between the intensities of the satellites (0, 0, 2) 3k and (0, 0, - 2) k. K is 0.04, which would correspond to A r k = 34 ° for a square-type structure. For A ~ ~ 34 °, the structure is then intermediate between a square one and a sinusoidally modulated one. The maximum values of the moments are between the values calculated for a square structure (1.16(6)/x B and 0.49(3)/~ B) and those corresponding to a sinusoidal modulation (1.48(8)/x B and 0.62(4)/XB). 3. C o n c l u s i o n s
Magnetization and neutron measurements are in agreement for the values and directions of the moments. The large observed anisotropy is related to the effects of the crystalline electric field acting on the Ce 3+ ions and splitting the J = 5 / 2 level into three doublets. However, these effects cannot explain the drastic moment reduction on site 4g (0.24/Xn), the smallest expected value being 0.43/x n for J = 1/2. Other effects, such as Kondo interactions, have to be invoked, especially on the 4g site. The competition between Kondo and magnetic exchange interactions, the presence of the two Ce sites with quite different behaviors, must be at the origin of this unusual situation: the stability of a modulated structure at low temperatures and a ferromagnetic one at higher temperatures. References
-.--,,"7"~,
, ,~
0.36
-0.8
[0 0 q Fig. 2. hkl scans through the (0, 0, - 2) Bragg reflection along the [001] direction at various temperatures.
[1] H.J. Van Daal and K.H.J. Buschow, Phys. Lett. 31A (1970) 103. [2] A. Berton, J. Chaussy, B. Cornut, J.L. Lasjaunias, J. Odin and J. Peyrard, J. Magn. Magn. Mater. 15-18 (1980) 379. [3] G. Chouteau, J. Flouquet, J.P. Keradec, J. Palleau, J. Peyrard and R. Tournier, J. Physique Lett. (Paris) 39 (1978) L-461. [4] A. Benoit, J.X. Boucherle, J.L. Buevoz, J. Flouquet, B. Lambert, J. Palleau and J. Schweizer, J. Magn. Magn. Mater. 14 (1979) 286. [5] J.X. Boucherle, F. Givord, G. Lapertot, A. Mufioz and J. Schweizer, to be published.
~H
journal of magnetism and magnetic materials
ELSEVIER
Unusual
magnetic
s t r u c t u r e s in Ce3A1 u
J.X. Boucherle, F. Givord *, G. Lapertot, A. Mufioz, J. Schweizer CEA, DRFMC / SPSMS-MDN, C.E.N.G., 38054 Grenoble Cedex 9, France
Abstract The orthorhombic Kondo Ce3AI~ compound shows two magnetic phases: below 6.2 K it is ferromagnetic, but below 3.2 K, it becomes modulated, with an incommensurate propagation vector k = (0 0 kz). In both phases, Ce moments are along b. Their values are different on the two crystallographic sites, one being strongly reduced.
I. Introduction Ce3AII~ is one of the Ce-A1 compounds in which the coexistence of Kondo and magnetic interactions has been observed. Up to now, only studies on polycrystalline samples have been carried out. Kondo-type behavior was deduced from the resistivity [1] and specific heat [2] results. Magnetic properties [3,4] are rather striking: in the temperature range 3.2-6.2 K, the compound is ferromagnetic, but below 3.2 K, a sine-wave modulated structure with a propagation vector k = (0, 0, 1 / 3 ) was found. Recently, we have succeeded in the fabrication of large single crystals. Cc3AI1] is orthorhombic (space group Immm), with two sites for the Ce atoms, site 2a (mmm) and site 4g (m2m). However, the relation b = 3a between the lattice parameters led to twinned crystals, with interchanged a and b directions. The twin population of the crystals has been determined by neutron diffraction in order to correct the following measurements [5]. Magnetization and neutron diffraction experiments were carried out on these single crystals in order to confirm the unusual transition from a ferromagnetic phase to a modulated structure as the temperature is decreased, and to clarify the Ce atom behavior on both sites.
K) along the b axis (inset of Fig.la). A large anisotropy is present in the whole ordered region with b as the easy magnetization direction. At 4 K, the spontaneous magnetization is 1.60(10)/Xg/Ce3Alll formula. Neutron diffraction studies were performed on a twoaxis spectrometer with lifting counter at various temperatures ranging from 12 to 1.6 K. The onset of ferromagnetic order at T = 6.2 K is confirmed by a general increase in the intensities of Bragg reflections. The magnetic moments are along b, but their values on each site are quite different: 1.27(6)/x B for site 2a and 0.24(3)/x n for site 4g.
'Ce3AIll '
('a)
=
o't *: 2
4
.....
2K
6
H (T) .
2. Experimental results co
.
.
.
i
80
.
.
.
i
Ce3lll 1
.
.
.
.
(b)
¢--....,
=
The magnetization measurements, performed by the extraction method, confirm the existence of two magnetic phases: a ferromagnetic one between 6.2 and 3.2 K, and a phase with no spontaneous magnetization below 3.2 K (Figs. la, b). In this latter phase, transitions are induced by the applied field, especially in a very low field (0.17 T at 2
.
I
.~
~
HIIb
1
g
40
I r
I
i ,
,
i
5
i
,
10
15
T (K)
* Corresponding author. Fax: +33 76 88 51 09; email: [email protected].
Fig. 1. (a) Isothermal magnetization curves. Inset: detail of the low-field region. (b) Thermal evolution of the remanent magnetization M R.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00657-1
1230
J.X. Boucherle et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1229-1230
The moment for one Ce3Alll formula is then 1.75(12)/x n, in agreement with the spontaneous magnetization along b. Below T = 3.2 K, magnetic satellites appear at positions given by the propagation vector k = (0, 0, 1 / 3 ) , whereas the intensities of the Bragg reflections decrease, as shown in Fig. 2 for the (0, 0, - 2 ) reflection. Another small magnetic peak is observed at the (0, 0, - 1) position, a reflection which is forbidden by the space group Immm (h + k + l = 2 n + 1). It corresponds to the propagation vector 3k. Actually, this peak is composed of the two satellites 3 k and - 3 k associated with the Bragg reflections (0, 0, - 2 ) and (0, 0, 0), respectively. Its width at 2.8 K indicates that k. is slightly different from 1 / 3 , because, in such a case, the two satellites should coincide. The analysis of the temperature dependence of the k and 3 k satellites [5] shows that k: decreases towards 1 / 3 as temperature is lowered (Fig. 3a). The magnetic structure is modulated with the moments parallel to b for both sites. The presence of third-order satellites implies that the magnetic structure is not purely sinusoidal and can be of square-type. In the latter case, the ratio between the intensities of the 3 k and k satellites is 1 / 9 for magnetic atoms all in phase. Because there are two Ce sites, this ratio is more complex and depends on the value of A ~0 (phase difference between the modulation of the two sites) [5]. The experimental thermal variation of this ratio for the satellites of the (0, 0, - 2 ) reflection is reported on Fig. 3(b). It increases as the temperature is lowered, but remains much smaller than 1 / 9 . Its value at 2 20000
,, ', 10000
,'
.~ • , ~
0
C e3Ai1
i
T = 4.8 K
I
2, ,,,.4', ;-
,-;
~----,----.----.----.----.----~----.-
Z
~ 10000
=,
>-
~_
T=2.8K
,~
~
8~
, o
,
,
, --:J' .... ~--:-- : - -.....
~,
10000
o
T = 1.6 K
,,#"~
o
-~'
-2.2
, ~'-2
-J, -1.8
,~+~" ~
,k- ..................... -1.6
-1.4
-1.2
-1
C e3AI1
1
0.35
,
g
Q_ E
e
e
+
L
;
q,
(a) 0.34
,3 113
............................... 0.33 v
J
[
i
I
i
I
i
0.04 0.03
(b)
,, l o
0.02 0.01
i
1
,
i
2
,
i
3
T (K)
Fig. 3. Thermal evolutions (a) of k z, and (b) of the ratio between the intensities of the satellites (0, 0, 2) 3k and (0, 0, - 2) k. K is 0.04, which would correspond to A r k = 34 ° for a square-type structure. For A ~ ~ 34 °, the structure is then intermediate between a square one and a sinusoidally modulated one. The maximum values of the moments are between the values calculated for a square structure (1.16(6)/x B and 0.49(3)/~ B) and those corresponding to a sinusoidal modulation (1.48(8)/x B and 0.62(4)/XB). 3. C o n c l u s i o n s
Magnetization and neutron measurements are in agreement for the values and directions of the moments. The large observed anisotropy is related to the effects of the crystalline electric field acting on the Ce 3+ ions and splitting the J = 5 / 2 level into three doublets. However, these effects cannot explain the drastic moment reduction on site 4g (0.24/Xn), the smallest expected value being 0.43/x n for J = 1/2. Other effects, such as Kondo interactions, have to be invoked, especially on the 4g site. The competition between Kondo and magnetic exchange interactions, the presence of the two Ce sites with quite different behaviors, must be at the origin of this unusual situation: the stability of a modulated structure at low temperatures and a ferromagnetic one at higher temperatures. References
-.--,,"7"~,
, ,~
0.36
-0.8
[0 0 q Fig. 2. hkl scans through the (0, 0, - 2) Bragg reflection along the [001] direction at various temperatures.
[1] H.J. Van Daal and K.H.J. Buschow, Phys. Lett. 31A (1970) 103. [2] A. Berton, J. Chaussy, B. Cornut, J.L. Lasjaunias, J. Odin and J. Peyrard, J. Magn. Magn. Mater. 15-18 (1980) 379. [3] G. Chouteau, J. Flouquet, J.P. Keradec, J. Palleau, J. Peyrard and R. Tournier, J. Physique Lett. (Paris) 39 (1978) L-461. [4] A. Benoit, J.X. Boucherle, J.L. Buevoz, J. Flouquet, B. Lambert, J. Palleau and J. Schweizer, J. Magn. Magn. Mater. 14 (1979) 286. [5] J.X. Boucherle, F. Givord, G. Lapertot, A. Mufioz and J. Schweizer, to be published.