MAGNETIC AND STRUCTURAL TRANSITIONS IN THE 2-D SYSTEM Rb2CrxMn1-xCl4 : A RAMAN STUDY

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MAGNETIC AND STRUCTURAL TRANSITIONS IN

THE 2-D SYSTEM Rb2CrxMn1-xCl4 : A RAMAN

STUDY

P. Moch, A. Abdalian, C. Dugautier, B. Briat, M. Nerozzi

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, Supplement au no 12, Tome 49, d6cembre 1988

MAGNETIC AND STRUCTURAL TRANSITIONS IN THE 2-D SYSTEM Rb2CrxMnl-,C14: A RAMAN STUDY

P. Moch (I), A. T. Abdalian (I), C. Dugautier (I), B. Briat (2) and M. Nerozzi (')

(I) Laboratoire PMTM, CNRS, Universite' Paris-Nord, Avenue JB Clirnent, 93430 Villetaneuse, France

(2) Laboratoire d70ptique Physique, ER5 du CNRS, ESPCI, 10 rue Vauquelin, 75231 Cedex 05, France

Abstract. - The x dependence of phase transitions in RbzCr,Mnl-,C14 is studied through Raman scattering. The onemagnon and the twemagnon spectra observed in the ferromagnetic and in the antiferromagnetic phases respectively are discussed. The phonon spectra are interpreted within a model assuming a distorted-undistorted transition at x % 0.4.

Introduction

Under a gradual substitution of Mn by Cr, the RbzCr,Mnl-,C14 system is expected to suffer vari- ous structural and magnetic transitions, starting from the tetragonal (D::) easy-axis antiferromagnetic phase of RbzMnCl4 and ending to the orthorhombic (D::) Jahn-Teller distorted easy-plane ferromagnetic phase of RbzCrCl4. This mixed system has recently given rise to a number of experimental investigations [l-51 with a particular interest to the spin glass behaviour in a concentration range which is not yet fully elucidated [I, 51. From inelastic neutron scattering, the magnetic excitations consist, for all x, of a mixing of cluster modes and of propagating spin-waves 14, 51. Neutron diffraction 121 indicates a tetragonal to orthorhombic transition coinciding with the spin glass-ferromagnetic transition (at T w 0) and occurring around the nearest neigh- boar Cr-Cr percolation limit (a:

=

0.59). However, such a conclusion poorly fits more recently published results [5] as well as Raman data [6] on the parent KzCu,Znl-,F4 crystals.

The present study is devoted to magnon and phonon scattering at low temperature (W 8 K) in pure and mixedcrystals ( ~ ~ 0 . 0 1 , 0.25, 0.4, 0.5, 0.8, 0.87and 0.99). Our results for pure RbzCrCl4 have been discussed in a previous publication [7].

Our crystals have been prepared in a Bridgman fur- nace after an appropriate mixing of the two end mem- bers. The procedures used for these have been described previously [7, 81. The Mn content in the inves- tigated pieces of crystals was determined both by X-

ray measurements [2] and by absorptiometry of MnO; (more reliable).

Magnon scattering

In the Cr-rich region (x

>

0.8)

,

the one-magnon Ra-

man line (at 168.1 cm-' in pure RbzCrCla) is still observed. It lies on the wing of a phonon line at 173 cm-' which is only observed for the mixed crystals and which shows an intensity increasing roughly proportionally to the Mn concentration (Fig. 1). Except for x = 0.99,

Fig. 1. - One-magnon scattering in Rb2CrXMnl-,Clr :

for x = 0.87 and x = 0.80, the magnon contribution (m)

has been extraded from the full magnon-phonon band.

this prevents a very accurate determination of the fre- quency and shape profile of the magnon line. We note however that, under an applied magnetic field, the fre- quency variation is consistent with the previously determined [7] g = 2.1 value of the Land6 factor in the pure Cr compound. Moreover, a careful analysis -of our data indicates a small but significant broadening of the line as well as a slight shift to lower frequencies when x decreases.

Using an extended Brillouin zone scheme, this o p tic mode has t o be considered as related to a zone boundary for the single spin-wave branch obtained with this model. It has been observed with the appropriate wave-vector transfer through neutron scattering in RbzCrClr [9] (167 f 2 cm-' a t 20 K) and, more recently, in R ~ Z C ~ O . ~ S M ~ ~ . Z ~ C ~ ~ (160 f 7 cm-I) [4]. Our results thus confirm that, up t o rather high Mn concentrations, the magnetic structure and the mag-, netic excitations (except for the occurrence of cluster modes) of the mixed crystals are not significantly mod- ified [4, 51.

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C8 - 1504 JOURNAL DE PHYSIQUE

For x

<=

0.5, one is concerned with a spin glass or with an antiferromagnetic phase. One-magnon Raman scattering is not expected and we did not find it but we observed two-magnon Raman scattering in RbzMnCl4 as predicted for an antiferromagnet. The band shows the expected (D4h) character and its shape fits per- fectly theoretical predictions [lo] using the Heisenberg exchange constant (J = -4.63 cm-l) previously determined through neutron scattering [ll]. The band is still present in mixed crystals for x = 0.01 but not for x = 0.25. This is consistent with a recently published phase diagram [5] which suggests a tranSition to a spin glass phase around x = 0.25. In contrast, antiferromagnetic spin-waves have been reported up to x = 0.3 [3].

P h o n o n scattering

As previously discussed [7] for pure RbzCrClr, in the Jahn-Teller distorted structure the vibrational modes which, strictly speaking, have to be enumerated within the othorhombic D:lf space group are approximately described using a Dih space group. The corresponding unit cell is obtained via the doubling of the D::

cell related to the undistorted structure [A = a

-

b,

B = a

+

b,

C

= c]. Such a doubling can be viewed as a folding of the D:: Brillouin zone bringing an M point regarded in D:: towards a

I'

point in Dih. Within this scheme [7], at the concentration x, corresponding to the presumed undistorted-distorted phase transition of the mixed crystals, some non-zero wavevector modes of the RbzMnCl4 arrangement (corner modes) become Raman active. This gives rise to additional lines in the distorted (x

>

2,) structure, connected with previously (x

<

x,) k

#

0 corner modes. The &man spec- trum is then expected to consist of 4 lines (2rf

+

2r:)

which persist over the whole concentration range and of 8 lines ( r f

+

I?$

+

3 r f

+

3 r i ) related t o these corner modes and disappearing when x

<

x,. Such a behaviour is observed for KzCu,Znl-,F4, although the number of Raman lines found for this material [6] is significantly lower than that predicted. By contrast, it follows from our data on RbzCrC14 that only one line

(r:)

_. _.is missing.

The above model is not fully satisfactory as far as the mixed Cr-Mn compounds are concerned. Among the 4 lines appearing as predicted in Rb2MnClc, 3 of them ( 2 r t

+

I??) show the expected smooth variation over the 0-1 interval of z while the last one (I?? at 205 and 233 cm-l for x = 0 and 1 respectively) shows an unexplained splitting in the 0.5-0.75 range. The remaining lines observed in pure RbzCrClr disap- pear more or less progressively but a detailed analysis

based on the intensity variations versus x supports an x, value around 0.4 as for the CU-ZI~ compounds.

Finally two extra lines a t 173 and 321 cm-', which are absent in the pure crystals, slhow an intensity roughly proportional to the Mn co~~centration. It is known that k

#

0 singularities of the phonon density of states in the pure crystals can provide Raman al- lowed lines through the breaking of the translational invariance in the mixed crystals. With such an inter- pretation however, the relatively small observed width of the extra lines would be surprising. They should better be described in terms of clustler modes.

In conclusion, a simple model based upon a structural transition occurring around x,, near the p e r m lation limit for the Cr-Cr next nearest neighbours, is a reasonably satisfactory first approach to interpret our Raman scattering data on Rb2Cr,Mnl-,Cl4. We stress however that such details as the existence of a few extra lines or of splittings are not relevant to this model and would probably be better understood in terms of local vibrational modes i.nvolving clusters. This would bring consistency with the fact that cluster magnetic excitations have been seen for these crystals

13, 41.

[I] Miinninghoff, G., Hellner, E., Treutmann, W., Lehner, N. and Heger, G., J. Phys.

C

17 (1984) 1281.

[2] Grieb, Th., Wacker, K., Naunlann, E., Treut- mann, W., Heger, G. and Geick, R., Phys. Status Solidi B 136 (1986) 585.

[3] Sieger, D., Tietze, H., Geick, R., Bates, S., Cow- ley, R. A., Treutmann, W. and Siteigenberger, U., Solid State Commun. 60 (1986) 847.

[4] Sieger, D., Tietze, H., Geick, 'R., Schweiss, P., Treutmann, W. and Frings, P., Solid State Corn- mun. 6 4 (1987) 1413.

151 Katsumata, K., Tuchlender, J., IJemura, Y. J. and Yoshizawa, H., Phys. Rev. B 37 (1988) 356. [6] Natsume, Y. and Yamada, I., J. Phys. Soc. Jpn

54 (1985) 4410.

[7] Abdalian, A. T., Briat, B., Dilgautier, C . and Moch, P., J. Phys. C 20 (1987) 2465.

[8] Briat, B., Vervoitte, A., Canit, J. C. and Francke, E., Solid State Commun. 50 (19'84) 229.

[9] Hutchings, M. T., Als-Nielsen, J., Lindgard, P. A. and Walker, P. J., J. Phys. C 14 (1981) 5327.

[lo]

Parkinson, J. B., J. Phys. C 2 (1969) 2012.

[Ill Schroder, B., Wagner, V., Lehner, N., Keshar- wani, K. M. and Geick, R., Phy:;. Status Sotidi B