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Submitted on 1 Jan 1979
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CoU2S5 - magnetic structure and properties
P. Wolfers, M. Bacmann, G. Fillion, H. Noël
To cite this version:
P. Wolfers, M. Bacmann, G. Fillion, H. Noël. CoU2S5 - magnetic structure and properties. Journal
de Physique Colloques, 1979, 40 (C4), pp.C4-194-C4-196. �10.1051/jphyscol:1979462�. �jpa-00218858�
JOURNAL DE PHYSIQUE Colloque C4, supplément au n° 4, Tome 40, avnV 1979, page C4-194
CoU
2S
5— magnetic structure and properties
P. Wolfers, M. Bacmann, G. Fillion (*) and H. Noel (**)
Laboratoire de Cristallographie, CNRS, 166X, 38042 Grenoble Cedex, France (*) Laboratoire Louis-Nee], CNRS, 166X, 38042 Grenoble Cedex, France (**) Laboratoire de Chimie Minerale B, Fac. Sciences, 35031 Rennes, France
Résumé. — Le composé CoUiS^ s'ordonne magnétiquement à 270 K avec un arrangement antiferromagnétique simple des moments du cobalt et une configuration non colinéaire des moments de l'uranium. Ces derniers forment deux réseaux d'Ising couplés par une faible interaction d'échange aux moments du cobalt.
Abstract. — CoU2S5 exhibits below 270 K a pure colinear antiferromagnetic structure for the Co moments and a canted one for the U moments. These later can be described by two Ising sublattices weakly coupled to the cobalt ones.
1. Introduction. — The new compound CoU2S5 belongs to the series MX-(AX2)„ (M = 3d-metal, A = 5f-metal, X = S or Se, n = 1,2) [1]. It crystalli- zes in space-group C2/c with a = 14.685 A, b = 6.259 A, c = 7.020 A and /3 = 96.42° as parameters of the monoclinic cell. Its crystal structure is derived from that of the Hagg carbide Fe5C2 with the 3d- metal located in the octahedral site • of • C2Fe5. The uranium coordination polyhedron is a bicapped trigonal prism [2]. Co and U atoms are respectively in position 4d and 8f and numbered as following :
Col J, J, i ; Co2 I J, 0 ; Ul x, y, z ; U2 - x, - y, - z ; U3 - x, y, \ - z ;
U4 x, - y, i + z.
2. Magnetic properties. — The magnetic suscepti- bility of CoU2S5 follows a Curie-Weiss law above 300 K with a Curie constant of 4.76 Kuem/Oe/mole and indicates an ordering with a weak ferromagnetic component below 270 K [3].
Below the Curie point, the magnetic behaviour of powder samples in high magnetic fields is quite similar to that of many compounds of the MUX, series like CrUS3 [4], CrUSe3 or VUSe3 [5]. We observe critical fields for the irreversible increase of magnetization and long after effect time constants which are characteristic of the propagation of narrow domain walls (Fig. 1). These phenomena are due mainly to the existence of a large magnetocrys- talline anisotropy [6].
When the temperature is lowered the remanent magnetization increases as indicated in figure 2 and reaches the value of 0.65 /xn by CoU2S5 formula at 4.2 K.
pg/mol .
/ 2 4 K / /
I / ^ ^ lcou2s5|
°. 5 10 15
UeAnol ^ — - " *
05. - ^ — - ^ ' ' ' " ' ^ ^ _ ^ ^ ° K
rW- L . 1 1 _
° 5 10 15 INDUCTION ( teslas)
Fig. 1. —Isothermal magnetization curves.
3. Neutron diffraction. — From the high resolu- tion diagrams obtained at 300, 125 and 4.2 K with multidetector DIB of Institute Laue-Langevin, ac- curate nuclear and magnetic intensities have been deduced. At 125 and 4.2 K all magnetic lines can be indexed in the chemical unit-cell with the condition h + k = 2 n.
The application of the macroscopic method of Bertaut [7] to the space group C2/c with q = 0 provides the four different possible models which are indicated in table I.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979462
CoU2S5 - MAGNETIC STRUCTURE AND PROPERTIES C4-195
TEMPERATURE (K)
Fig. 2 .
-
Remanent magnetization versus temperature.Table I. - Representation of C2/c ( q = 0) for axial vectors in position 8f and 4d.
Basis Vectors Represen- Generators- 8f (U) 4d (Co)
tations e 2 y 1 x y z x y z
- -
- -r:
I 1 1 C ~ C G F Gr:
1 - 1 1 ~ C ~ F G Fr;
1 1 - 1 a g ar;
1 - 1 - 1 g a g With the moments configurations :The referring axes are chosen so that X 11 a, Y 11 b, Z = X A Y.
At 125 and 4.2 K the magnetic structure belongs to
r:
but the absence of the 200 reflections eliminates the Fy contribution on Co-atom. So we observe a pure colinear antiferromagnetic structure (Gx,, modes) for the cobalt moments and a canted one for the uranium moments (c,,, and fy modes) (Fig. 3).Whereas in FeU2S, a reorientation occurs at 20 K
with a sharp increase of uranium moments [8], in CoU2S, there is no significant change of the direc- tions of moments from T , down to 4.2 K (table 11).
Co, x : 3/4.
sheet of octahedra
) double sheet of prisms (uranium 1
* / Co,x.l/4.
sheet of
J J
octahedraX
Co: G, and Gz
I/Y
o U : cx , fy and czFig. 3. -Magnetic structure of CoU2S5.
Table 11.
-
Magnetic structures at 125 K and 4.2 K .4. Discussion.
-
The observed ferromagnetic component is only due to the uranium moments which are expected to exhibit a very large magneto- crystalline anisotropy as a result of a strong crystalli- ne field acting on strongly spin-orbit coupled Sf electrons [9].In fact, the observed values of the powder magne- tization are very close to one half the values found by neutron diffraction. The behaviour of the rema- nent magnetization is a proof that each particle of the powder remains a single domain in the remanent
Components
Temperatures Atoms (PB)
-
- -
x 2.3, Co
125 K
x - 0.1, Y 0.16 z 0.2,
Conf igu- Moment Veracity
rations (PB) factor
-
--
C4-196 P. WOLFERS, M. BACMANN, G. FILLION AND H. NOEL
state and half the spontaneous magnetization is just the expected value for an assembly of particles with a quasi-infinite uniaxial anisotropy. As the direc- tions of the moments do not vary significantly, the curve of spontaneous magnetization versus tempe- rature reflects almost exactly the variation of the uranium moment with the temperature.
On the other hand, the value of the Co moment is in good agreement with a Co2' state and the Curie constant per U atom deduced from the experimental' one is C , = 1.44. Assuming an Ising-like behaviour for uranium with an effective spin S =
:,
it leads to an effective g factor of 3.92 and an ordered moment , of 1.96 p, at 0 K, in good agreement with theobserved value : 1.77 p B at 4.2 K. The exchange interaction between U and Co atoms can be esti- mated as 32 K in terms of U level splitting at 0 K.
The variation of the spontaneous magnetization ver- sus temperature computed in a simple molecular field model is very close to the observed one. The uranium-uranium interaction seems therefore to be very small and probably negligible.
5. Conclusion. - The compound CoU,S, can be described in first approximation as a colinear Co antiferromagnet with strong internal interactions giving a high ordering temperature, weakly coupled to a paramagnetic-like system of Ising-like uraniums with two inequivalent easy directions.
References
[I] NOEL, H., C.R. Hebd. Sian. Acad. Sci. C 277 (1973) 463. [6] BARBARA, B . , J. Physique 34 (1973) 1039.
121 NOEL, H., POTEL, M . , PADIOU, J . , Acta Cryst. B 32 (1976) 605. 171 BERTAUT, E. F . , Magnetism, Rado and Shull Ed. (Academic [3] NOEL, H., Thesis Rennes (1977) unpublished. Press N.Y.) 1963, vol. 111, p. 149.
[41 WOLFERS, P., FILLION, G . , BACMANN, M., NoBL, H., J. [8] WOLFERS, P., BACMANN, M . , Proceedings 2nd Int. Conf. on Physique 37 (1976) 233. the Electron Struct. of the Actinides, Wroclaw (1976) [S] NOEL, H . , WOLFERS, P., FILLION, G., Proceedings 2nd Int. 483.
Conf. on the Electron Struct. of the Actinides, Wroclaw [9] WOLFERS, P . , Thesis Grenoble (1977) unpublished.
(1976) 489.
