Magnetic structures and interactions in RZn12 and RCu4Al8 compounds

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Magnetic structures and interactions in RZn12 and RCu4Al8 compounds

J. Déportes, D. Givord, B. Kebe, C. Fuller

To cite this version:

J. Déportes, D. Givord, B. Kebe, C. Fuller. Magnetic structures and interactions in RZn12 and RCu4Al8 compounds. Journal de Physique Colloques, 1979, 40 (C5), pp.C5-154-C5-155.

�10.1051/jphyscol:1979558�. �jpa-00218975�

JOURNAL DE PHYSIQUE Colloque C5, supplément au «° 5, Tome 40, Mai 1979, page C5-154

Magnetic structures and interactions in RZn 12 and RCu 4 Al s compounds

J. Deportes, D. Givord, B. Kebe

Laboratoire Louis-Neel, C.N.R.S., 166X, 38042 Grenoble-cedex, France

and C. Fuller

Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, Mo. 65401, U.S.A.

Résumé. — Les propriétés et les structures magnétiques des composés RCu

Al

et TZn

sont déduites des mesures d'aimantation et des diagrammes de diffraction de neutrons. Pour rendre compte des structures magné- tiques dans un modèle d'Heisenberg, des interactions à longue distance ( > 10 Â) doivent être considérées.

Abstract. — The magnetic properties and structures of RCu 4 Al g and RZn 12 compounds are deduced from magnetic measurements and neutron diffraction experiments. To explain the observed structures in the Hei- senberg exchange model, long range interactions (> 10 A) must be considered.

1. Introduction. — In RZn 12 and RCu 4 Al 8 inter- metallics, the magnetic rare earths are diluted in a non magnetic matrix (Zn, Cu or Al). Distances between rare earths are large and such compounds are espe- cially suitable for a study of long distance magnetic interactions.

2. Experiments and results. — In the two series, we have prepared alloys with the heavy rare earths.

They crystallize in the I4/mmm body-centered tetra- gonal structure, R atoms being in special positions, (000) and (i-§-§). Magnetic susceptibilities were mea- sured from 1.4 K to 300 K; magnetization measu- rements were performed up to 150 kOe at the S.N.C.I., Grenoble; neutron diffraction studies (1 N = 1.14 A) at the C.E.N.-Grenoble allowed us to determine the magnetic structures.

The thermal variation of the initial susceptibility exhibits a maximum which is characteristic of an antiferromagnetic behaviour at a Neel temperature respectively below 33 K for RCu 4 Al 8 and 15.5 K for RZn 12 . At high temperatures the reciprocal suscep- tibility obeys a Curie-Weiss law. The paramagnetic moment is in good agreement with the theoretical value of the corresponding ion. The paramagnetic temperatures are negative giving evidence for pre- dominant negative interactions. Deviations from the Curie-Weiss law can be observed at temperatures between T N and 2 T" N . At low temperature, meta- magnetic transitions are observed on the magneti- zation curves except for GdZn 12 and GdCu 4 Al 8

where the field dependence of the magnetization is linear (figure 1). leading to saturation fields of respec- tively 400 kCe and 300 kOe. For all compounds the value of magnetization under 150 kOe is much smaller

T=1.4K J^/ ^ ^ \ ^

W / . / / / ^ GdOj,AI

^^<^T '

55 i5o i5o

APPLIED FIELD (W> )

Fig. 1. — Field dependence of the magnetization at 1.4 K for some RCu^Alg and RZn

compounds, R = Gd, Tb, Dy.

than the maximum value g 3 J/x v of the corresponding free ion.

On the neutron diffraction patterns, no ferroma- gnetic contribution appears on the nuclear lines below the ordering temperatures. In RCu 4 Al 8 when R = Dy, Ho, Er, extra magnetic lines appear with the condition h + k + I = 2n + 1, the propagation vector of the magnetic structure is Q = 1001 |. The magnetic arrangement is simply antiferromagnetic, the moments at the corners and those at the center of the cell are antiparallel. For these three compounds, the magnetic moments are along the c-axis.

The magnetic pattern of TbCu 4 Al 8 is indexed with the use of a propagation vector Q = | 000.84 |, the moments being in the basal plane. The structure can be described by a stacking of ferromagnetic planes ; since the crystallographic c-axis is parallel to the propagation vector of the magnetic structure, it is

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979558

MAGNETIC STRUCTURES AND INTERACITONS IN RZn,, AND RCu,Al, COMPOUNDS C5- 155

not possible to distinguish a modulated structure from and helimagnetic one.

In RZn,, compounds the patterns at low tempe- rature can be indexed with the use of one or two propagation vectors such as (4:23 [I]. The structures are formed by a stacking of antiferromagnetic planes.

The amplitude of the moments on each of these planes is modulated along c as cos (2 nz, 2 ) . The large value of the mean magnetic moment calculated for TbZn,, (( o _{) =} 8.5 pB) would lead to a maximum magnetlc moment o = $ ^{( o ) =} 12 pB larger than the maxi- mum value of the free ion : an antiphase modulation is then deduced along the c-axis [2].

3. Discussion. - A modulated or an antiphase structure results from the crystal field acting on a compound where the magnetic interactions would favour an helimagnetic arrangement.

Smart [3] has calculated all the possible magnetic structures in a body-centered quadratic cell, taking into account interactions associated with the three shortest interatomic distances. Accordingly, in RCu,Al,, the magnetic structures are stabilized by negative inter- actions J, coupling magnetic moments of atoms at the corner of the cell to those at the center. The heli- magnetic structure observed on TbCu,Al, must then result from competition between J, and negative

interactions along c coupling corner atoms 1, (figure 2).

In RZn,, compounds, the magnetic structure cannot be explained in this 3-interactions model.

Introducing the interactions J; between atoms second neighbours along c, the exchange energy can be writ- ten :

E(Q) = 8 J, ^COS n ~ , ^cos n ~ , ^COS nz, +

+ 2 J,(cos 2 IIrX + cos 2 IIzJ

z,, z,, ^2, being the components of the propagation vector: by minimizing this energy with respect to Q,

Fig. 2. - Interactions in RCu,AI, and RZn,, compounds accord- ing to Smart's notations J,, J , , J,, Ji are respectively the interactions between Ist, 2nd, 3rd, 4th neighbours. The corresponding inter- atomic distances are 5.1 A, 6.7 A, 8.8 A, 10.2 A.

a propagation vector (iiz,) is shown to appear if IJ,I > IJ, ( a n d ( 4 J j ( > J3,(cos2l7z, = - J3/4J;).

In conclusion, in intermetallics, the exchange inter- actions occur via conduction electrons. According to the RKKY model such interactions are oscilla- toring and rapidly decreasing,with distance. In rare earth rich compounds, interatomic distances being short, interactions between first neighbour atoms are then predominant. For large distances, the Ruderman- Kittel function is less dependent on distance, and interactions between atoms at different distances may have similar magnitude.

In our compounds, the complex magnetic struc- tures result from competition between interactions associated with interatomic distances varying from 5 a ^{to 10} a. In such structures, numerous interactions are not satisfied and other configurations of close energy may exist. The field induced transitions can then result from the change of an antiferromagnetic structure to another one.

References

[I] DBPORTES, J., GIVORD, D. and KEBE, B., Inst. Phys. Conf.

Ser. 278 (1978) No 37.

[2] -BE, B., These 3e Cycle (1978) University of Grenoble.

[3] SMART, J. S., Eflective$eld theories of magnetism (W. S. Saun-

ders Co, Philadelphia and London) (1966).