MAGNETIC BEHAVIOUR AND PHASE TRANSITION IN [ThCu3] (Mn4)O12

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MAGNETIC BEHAVIOUR AND PHASE

TRANSITION IN [ThCu3] (Mn4)O12

M. Deschizeaux, B. Bochu, J. Joubert, G. Fillion, J. Chenavas, A. Collomb,

D. Samaras, M. Marezio

To cite this version:

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JOURNAL DE PHYSIQUE Collogue CI, supplement au n° 4, Tome 38, Avril 1977, page Cl-103

MAGNETIC BEHAVIOUR AND PHASE TRANSITION IN [ThCu

3

] (Mn

4

)0

) 2

M. N. DESCHIZEAUX, B. BOCHU, J. C. JOUBERT Laboratoire de Genie Physique, I. N. P., Grenoble, France

G. FILLION

Laboratoire de Magnetisme C. N. R. S., Grenoble, France J. CHENAVAS, A. COLLOMB, D. SAMARAS, M. MAREZIO

Laboratoire de Cristallographie, C. N. R. S., Grenoble, France

Résumé. — Le composé [ThCu3] (Mru)Oi2 appartient à la série des oxydes [AC3] (B.i)Oi2 de structure pérovskite dans lesquels les sites [A] icosaédriques sont occupés par les cations Th4 +, les sites [C] plans-carrés par les cations Cu2 + et les sites (B) octaédriques par les cations Mn3 + et M n, + en proportions égales. Diverses mesures indiquent un comportement ferrimagnétique avec une tempé-rature d'ordre 7 N = 430 K, une aimantation spontanée à 4,2 K de 9 ^n/unité de formule et une transition à 7\ = 180 K. La structure ferrimagnétique, déduite des expériences de diffraction neutronique est en accord avec les valeurs des interactions magnétiques tirées des mesures d'aiman-tation et de susceptibilité.

La transition Tt a été étudiée par des mesures d'aimantation et de diffraction des rayons X. Elle est caractérisée par une variation thermique anormale du paramètre de maille et du moment magné-tique, sans changement de la symétrie cristallographique et magnétique.

Abstract. — The compound [ThCu3] (Mn4)Oi2 belongs to the series of [AC3] (B4) O12 perov-skite-like oxides in which the icosahedral sites [A] are occupied by the Th4+ cations, the square

planar sites [C] by the Cu2+ cations, and the octahedral sites (B) by the Mn3' and Mn4 r cations in

equal proportion. Previous measurements indicated a ferrimagnetic behavior with an ordering temperature 7\- = 430 K, a spontaneous magnetization of 9 #B/formula unit at 4.2 K and a magne-tic transition at Tt = 180 K. The ferrimagnemagne-tic structure, as determined from powder neutron dif-fraction data, is consistent with the values of magnetic interactions that can be estimated from magnetization and susceptibility measurements.

The low temperature transition has been studied by magnetization measurements and X-Rays techniques. It seems to be of second order with anomalies in the cubic cell parameter and of the magnetic moments. No changes in cristallographie symmetry and magnetic structure arc observed.

1. Introduction. — [Th4 + Cu3 + ] (Mn2 + M n |+) 01 2

belongs to a series of perovskite-like compounds with the general formula [AC3] (B4)01 2. They are cubic,

space groupe Im3, and a cell parameter which is double with respect to that of the undistorted perovs-kite structure. The structural arrangements of these compounds are described in detail in the preceeding article.

2. Synthesis. — Initially the compound [ThCu3]

(Mn4)Oi2 has been synthesized by using high

pressure-high temperature conditions [1]. A single phase is obtained when a stoichiometric mixture such as Th(OH)4 + 3 CuO + 2 M n 02 + M n203 is

subjected to 80 Kbar at 1 200 °C in a belt-type appa-ratus. If a few drops of water are added to the starting mixture a better cristallization is obtained and the quenched material contains very small cube-shaped single crystals ( ~ 50 u).

Since [ThCu3] (Mn4)Oi2 is stable in air up to

600 °C it can also be obtained by hydrothermal synthesis at 600 °C under 2 kbar. The same starting mixture is used together with nitric acid, which is necessary in order to prevent partial reduction of the Mn*+ cations. An excess of 5-10 % of CuO must also

be added for compensating its great solubility in the acid ambient. Under these conditions larger single crystals ( ~ 200 u) are obtained.

3. Magnetic behaviour and lattice parameter varia-tion. — The paramagnetic susceptibility of [ThCu3]

(Mn4)01 2 has been measured with an automatic

vertical translating balance in the temperature range of 400-900 K [2]. The reciprocal susceptibility versus temperature curve (Fig. 1) shows a hyperbolic behaviour which is characteristic of a ferrimagnetic compound above its N6el temperature, consistent with a two-sublattices model. The two kinds of

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Cl-104 M. N. DESCHIZEAUX ef al.

RG. 1. - Reciprocal susceptibility 1/x versus temperature. tic cations are : the Cu2+ ones which occupy the square- planar sites and the

<

Mn:.;, Mn$.<

>

cations which occupy the octahedral sites.

The reciprocal susceptibility is related to the temperature by :

I T 1

Y

- = - + - - -

X

C Xo T - 8

where C is the average Curie constant, x0, y and 8 are parameters deduced from the curve. The experimental Curie constant is 10.45 uem/mole which is in good agreement with the theoretical value

FIG. 2. - Magnetization c ( H ) versus applied field for T = 4.2Kand T = 290K.

calculated with the spin-only values. The molecular field coefficients n,,,,, n,,,, and n,,,, deduced from the experimental values of

X,

and 8 are :

nMnMn = 3 Oe/uem/mole, ncUcu =

-

162 Oe/uem/ mole and n,,,, = - 228 Oe/uem/mole, respectively. The large negative value of n,,,, is in agreement with the ferrimagnetic configuration proposed for this compound. The manganese intrasublattice interaction coefficient has a nearly zero value, while the copper one has a large negative value of the same order of magnitude as that found in [CaCu,] (Ti4)OI2 (n,,,, =

-

138 Oe/uem/mole) [3] namely, a compound which belongs to the same subseries.

The magnetization of a powder sample of [ThCu,] (Mn4)O12 was measured by the axial extrac- tion technique in high magnetic field coils and by a vibrating sample magnetometer. The o(H) curve is shown in figure 2. The spontaneous magnetization at 4.2 K is 9 ,u,/formula unit. The o(T) curve obtained for a constant magnetic field of 9 560 Oe (Fig. 3) shows a transition at 180 K.

A low temperature X-rays powder diffractometer (Cr Ka,) was used in the temperature range of 80- 300 K in order to determine the variation of the cell parameter (Fig. 4). A large discontinuity is observed at about 190 K. No change in symmetry has been detect- ed at this temperature. Since the powder method gives only the metric symmetry, X-rays diffraction experiments on a single crystal are in progress in

FIG. 3.

-

Magnetization o(T) versus temperature for

H = 9 650 Oe.

1

100 200 300 T K

FIG. 4.

-

Cell parameter a versus temperature.

order to confirm both the symmetry and the space group.

4. Neutron diffraction. - The magnetic structure of this compound, as determined from neutron diffraction powder data taken at 4.2 K, consists of a colinear ferrimagnetic configuration of the

c u 2 + and

<

~ n : : Mn;.:

>

moments along the [111] ternary axis. The experimental values of the magnetic moments are

Pcu2 + = 1 f 0.05 pB = 3.1 f 0.1 pB. Mn,35 ~ n d : : )

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MAGNETIC BEHAVIOUR AND PHASE TRANSITION IN [ThCu3] (Mn4)Olz C1-105 rally observed for CuZ' (pCu2+ = 0.96 pB) on one

side and for

~ n (pwn3+ ~ '= 3.5 pa) and Mn4+ (p,,4+ = 2.6 pB) on the other. The total magnetization along the [ I l l ] axis is : o = 9.4

+

0.5 pdformula unit. A neutron diffraction scan taken at 300 K is consistent with the same spin arrangement and a magnetization of

o = 7.4

+

0.7 p,/formula unit. The corresponding

value deduced from magnetic measurements at T = 290 K is C.T = 5.5 _+ 0.1 ,u,/forrnula unit. The

discrepancy can be explained by an anisotropy of the magnetization. Magnetic measurements on a single crystal must be performed in order to verify this assumption. Other experiments such as ferrimagnetic resonance and X-rays diffraction at low temperature are presently being performed on single crystals in order to define the nature of the transition.

References

[l] DESCHIZEAUX, M. N., JOUBERT, J. C., VBGAS, A., COL- LOMB, A., CHENAVAS, J., MAREZIO, M., J. Solid State Chem. 19 (1976) 45.

[2] AUBERT, J. J., DELANNOY, T., Rapport de stage. Labora- toire de Genie Physique. INPG, Grenoble 1975. [3] COLLOMB, A., TheSe d'Etat Grenoble (1976) CNRS-AO.