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Magnetic properties of RMg compounds
95 MAGNETIC PROPERTIES OF RMg C O M P O U N D S R. A L I ~ O N A R D , P. M O R I N , J. P I E R R E and D. S C H M I T T Laboratoire de Magn~tisme, CNRS, 166X, 38042-Grenoble-Cedex, France
The magnetic properties of equiatomic RMg compounds were investigated by magnetization measurements on single crystals (R = Tb, Dy, Ho, Er, Tm) and neutron diffraction. Magnetic structures are antiferromagnetic with Ce, Pr, Nd, but result from a competition between ferro- and antiferromagnetism with heavy rare earths.
Table I. Summary of magnetic properties for RMg compounds
1. Introduction Equiatomic rare e a r t h - m a g n e s i u m c o m p o u n d s crystallize within the cubic CsCl structure. Buschow [1] investigated their properties and found ordering temperatures much lower than in isoelectronic RZn compounds. We have studied previously the magnetic structures and magnetization on some h e a v y rare earth c o m p o u n d s [2]; we report here new results obtained for other c o m p o u n d s by neutron diffraction, magnetization on single crystals and neutron spectroscopy. 2. Light rare earth compounds Neutron diffraction experiments were undertaken on CeMg and PrMg. As in the case of N d M g [3], the ordered structure is antiferromagnetic (a.f.) with A (or (0, 0, I1)) type. The m o m e n t s at 4.2 K are 1.85 ft a for Ce and 2.70 P~B for Pr collinear with the c axis of the magnetic cell, with large quadratic distorsions (c/a - 1 1.3% and 1.4% respectively). The ordering temperatures are close to those for corresponding RZn c o m p o u n d s , due to the same kind of ordering [4]. =
3. Heavy rare earths compounds Neutron diffraction experiments show peculiar features of magnetic structures. TbMg has a canted structure with orthogonal sublattices [2], which is a superposition of the ferromagnetic and the A-type structures (table I). The ferromagnetic c o m p o n e n t ftt and the antiferromagnetic one /~a.f. are nearly equal to 5.17--0.1/z B, giving a resulting m o m e n t of 7.3/~a. D y M g has the A-type structure, but with a modulation; neutrons lead to a mean moment/~a.t. = 5 - 0.7/z B, whereas the M6ssbauer effect shows a local m o m e n t close to 10/~B- H o M g orders with a ferromagnetic c o m p o n e n t t~t=5.0_+0.1ttR at 4.2 K, but neutron spectra show a diffuse a.f. Physica 86-88B (1977)95-96 (~ North-Holland
R
Ce
Tordcr(K)
20
0p (K)
Structure*
Moment (/~B) at 4.2 K
-10
A A A** NC NC mod. A NC
!.85-+0.1 2.70-+0.1 2.09 -+ 0.04** p.f = 4.54 -+ 0.05 /.t,f = p.f = 5.17_+0.1 t t , f = 5.0 -+ 0.7 /.tf = 4.97 -+ 0.1
Pr Nd
45 64
- 1I -9
Gd Tb Dy Ho Er Tm
119
108
81
83
22 21 5.5 < 1.4
25 24 9 - I
* Structures - A : A-type antiferromagnetic; NC: non collinear; mod. A: modulated A-type. ** From ref. 3.
scattering corresponding to a (0 0 ~) propagation vector. The neutron data are confirmed by the study of magnetization and susceptibility. Below their Curie point (table I), GdMg, T b M g and H o M g show a spontaneous magnetization and a large superimposed susceptibility mainly due to the antiferromagnetic component. The ferromagnetic m o m e n t of G d M g reaches only 4.54 ft a at 4.2 K, and varies a s T 2 up to 100 K. The low field susceptibility of DyMg shows an antiferromagnetic behaviour with T N = 22 K; its anomalous variation in the paramagnetic state is attributed to short range order. The susceptibility of ErMg also shows a m a x i m u m at 5.5 K, but some spontaneous magnetization seems to appear near the ordering point. Finally, no ordering is found in T m M g down to 1.4 K. The magnetic structures of h e a v y rare earth c o m p o u n d s are thus complex and not yet solved completely. It is suggested that they arise f r o m indirect interactions intermediate between those for the light rare earths c o m p o u n d s and the ferromagnetic h e a v y rare earths RZn compounds. Due to the presence of 5d conduction
96 electrons, energy bands are found very flat at the Fermi level in YMg [5]; slight modifications of the Fermi surface may cause drastic variations of indirect interactions. Some structures may be stabilized by crystal field or anisotropic interactions of orbital origin.
4. Anisotropic magnetization and crystal field The magnetization has been measured on single crystals for R = Tb, Dy, Ho, Er, Tm in fields up to 140 kOe. It is strongly anisotropic in high fields, and largest along [111] direction in TbMg and ErMg, [100] in DyMg and [110] in HoMg and TmMg. From these data, crystal field parameters could be evaluated: the fourth order term A4(r 4) is positive and the sixth order one A6(r 6) negative. The values obtained agree with a determination from neutron spectroscopy [6] in the case of HoMg and ErMg, respectively: A4(r4) = +43 K,
A6(r 6) = - 13 K,
and
A4(r 4) = +
4 K,
A6(r 6) = -
10.8 K.
The sign change of A4(r 4) in RMg compounds relative to that in isomorphous phases with transition metals may be understood from the spatial distribution of d-band electrons within and around the rare earth sphere, which is influenced by the l o c a l i z a t i o n - o r a b s e n c e - of d electrons on the alloyed metal.
References [1] K.H.J. Buschow,J. Less CommonMetals 33 (1973) 239. [2] R. AI6onard,P. Morin, J. Pierre and D. Schmitt, Sol. Stat. Commun. 17 (1975) 599; J. Phys. F 6 0976) 1361. 13] K.H.J. Buschow, J.P. de Jong, H.W. Zandbergen and B. van Laar, J. Appl. Phys. 46 0975) 1352. [4] P. Morin and J. Pierre, Phys. Stat. Sol. (a) 30 (1975) 549. [5l D. Schmitt, J. Pierre and M. Belakhovsky, J. Phys. F 6 (1976) 789. [6] P. Morin, J. Pierre and D. Schmitt, J. de Physique 37 (1976) 611.
The magnetic properties of equiatomic RMg compounds were investigated by magnetization measurements on single crystals (R = Tb, Dy, Ho, Er, Tm) and neutron diffraction. Magnetic structures are antiferromagnetic with Ce, Pr, Nd, but result from a competition between ferro- and antiferromagnetism with heavy rare earths.
Table I. Summary of magnetic properties for RMg compounds
1. Introduction Equiatomic rare e a r t h - m a g n e s i u m c o m p o u n d s crystallize within the cubic CsCl structure. Buschow [1] investigated their properties and found ordering temperatures much lower than in isoelectronic RZn compounds. We have studied previously the magnetic structures and magnetization on some h e a v y rare earth c o m p o u n d s [2]; we report here new results obtained for other c o m p o u n d s by neutron diffraction, magnetization on single crystals and neutron spectroscopy. 2. Light rare earth compounds Neutron diffraction experiments were undertaken on CeMg and PrMg. As in the case of N d M g [3], the ordered structure is antiferromagnetic (a.f.) with A (or (0, 0, I1)) type. The m o m e n t s at 4.2 K are 1.85 ft a for Ce and 2.70 P~B for Pr collinear with the c axis of the magnetic cell, with large quadratic distorsions (c/a - 1 1.3% and 1.4% respectively). The ordering temperatures are close to those for corresponding RZn c o m p o u n d s , due to the same kind of ordering [4]. =
3. Heavy rare earths compounds Neutron diffraction experiments show peculiar features of magnetic structures. TbMg has a canted structure with orthogonal sublattices [2], which is a superposition of the ferromagnetic and the A-type structures (table I). The ferromagnetic c o m p o n e n t ftt and the antiferromagnetic one /~a.f. are nearly equal to 5.17--0.1/z B, giving a resulting m o m e n t of 7.3/~a. D y M g has the A-type structure, but with a modulation; neutrons lead to a mean moment/~a.t. = 5 - 0.7/z B, whereas the M6ssbauer effect shows a local m o m e n t close to 10/~B- H o M g orders with a ferromagnetic c o m p o n e n t t~t=5.0_+0.1ttR at 4.2 K, but neutron spectra show a diffuse a.f. Physica 86-88B (1977)95-96 (~ North-Holland
R
Ce
Tordcr(K)
20
0p (K)
Structure*
Moment (/~B) at 4.2 K
-10
A A A** NC NC mod. A NC
!.85-+0.1 2.70-+0.1 2.09 -+ 0.04** p.f = 4.54 -+ 0.05 /.t,f = p.f = 5.17_+0.1 t t , f = 5.0 -+ 0.7 /.tf = 4.97 -+ 0.1
Pr Nd
45 64
- 1I -9
Gd Tb Dy Ho Er Tm
119
108
81
83
22 21 5.5 < 1.4
25 24 9 - I
* Structures - A : A-type antiferromagnetic; NC: non collinear; mod. A: modulated A-type. ** From ref. 3.
scattering corresponding to a (0 0 ~) propagation vector. The neutron data are confirmed by the study of magnetization and susceptibility. Below their Curie point (table I), GdMg, T b M g and H o M g show a spontaneous magnetization and a large superimposed susceptibility mainly due to the antiferromagnetic component. The ferromagnetic m o m e n t of G d M g reaches only 4.54 ft a at 4.2 K, and varies a s T 2 up to 100 K. The low field susceptibility of DyMg shows an antiferromagnetic behaviour with T N = 22 K; its anomalous variation in the paramagnetic state is attributed to short range order. The susceptibility of ErMg also shows a m a x i m u m at 5.5 K, but some spontaneous magnetization seems to appear near the ordering point. Finally, no ordering is found in T m M g down to 1.4 K. The magnetic structures of h e a v y rare earth c o m p o u n d s are thus complex and not yet solved completely. It is suggested that they arise f r o m indirect interactions intermediate between those for the light rare earths c o m p o u n d s and the ferromagnetic h e a v y rare earths RZn compounds. Due to the presence of 5d conduction
96 electrons, energy bands are found very flat at the Fermi level in YMg [5]; slight modifications of the Fermi surface may cause drastic variations of indirect interactions. Some structures may be stabilized by crystal field or anisotropic interactions of orbital origin.
4. Anisotropic magnetization and crystal field The magnetization has been measured on single crystals for R = Tb, Dy, Ho, Er, Tm in fields up to 140 kOe. It is strongly anisotropic in high fields, and largest along [111] direction in TbMg and ErMg, [100] in DyMg and [110] in HoMg and TmMg. From these data, crystal field parameters could be evaluated: the fourth order term A4(r 4) is positive and the sixth order one A6(r 6) negative. The values obtained agree with a determination from neutron spectroscopy [6] in the case of HoMg and ErMg, respectively: A4(r4) = +43 K,
A6(r 6) = - 13 K,
and
A4(r 4) = +
4 K,
A6(r 6) = -
10.8 K.
The sign change of A4(r 4) in RMg compounds relative to that in isomorphous phases with transition metals may be understood from the spatial distribution of d-band electrons within and around the rare earth sphere, which is influenced by the l o c a l i z a t i o n - o r a b s e n c e - of d electrons on the alloyed metal.
References [1] K.H.J. Buschow,J. Less CommonMetals 33 (1973) 239. [2] R. AI6onard,P. Morin, J. Pierre and D. Schmitt, Sol. Stat. Commun. 17 (1975) 599; J. Phys. F 6 0976) 1361. 13] K.H.J. Buschow, J.P. de Jong, H.W. Zandbergen and B. van Laar, J. Appl. Phys. 46 0975) 1352. [4] P. Morin and J. Pierre, Phys. Stat. Sol. (a) 30 (1975) 549. [5l D. Schmitt, J. Pierre and M. Belakhovsky, J. Phys. F 6 (1976) 789. [6] P. Morin, J. Pierre and D. Schmitt, J. de Physique 37 (1976) 611.