Investigation of spin waves above TNin RbFeCl3 by neutron scattering

Physica 107B{1981) 75-76 North.Holland Publishing Company

BF 11

INVESTIGATION OF SPIN WAVES ABOVE T N IN RbFeCI 3 BY NEUTRON SCATTERING D. Petitgrand and B. Hennion Laboratoire L~on Brillouin*, C.E.N.Saclay, 91191Gif-sur-Yvette Cedex, France and C. Escribe Institut Laue Langevin, 156 X Centre de Tri, 38042 Grenoble, France We have studied the temperature dependence of spin-waves around and above 2N = 2.55 K in RbFeCI 3 using inelastic scattering of neutrons. Though spin waves propagating in the chain are found to behave like l-d paramagnons, those propagating perpendicularly show that correlations between chains exist up to 8 K. These experiments reveal that RbFeCI 3 is less l-dimensional than previously reported and open the question of the persistence of l-d like properties in a poor l-d system.

I.

INTRODUCTION

RbFeCI 3 has been extensively studied these last years partly because l-d magnetic properties were expected in this material. Susceptibility measurements (I-2) M~ssbauer (2-3) and neutron diffraction (4) experiments have been reported which were found consistent with ferromagnetic l-d behaviour of a planar-Heisenberg system. On the other hand investigations of the spin dynamics by infrared (5) and neutron scattering (6, 7) have given unexpected results which may question the l-d feature. In particular we have shown (6) that the low temperature spin wave dispersion (Fig.l) leads to a value of the inter/ intrachain interaction ratio (J'/J=0.29) larger than previously estimated. In order to clarify this problem we have undertaken the investigation of spin-waves around and above T N by neutron inelastic scattering which results are reported here. The preparation and the growing of RbFeCI 3 single crystal have already been described (6). The experiments have been carried out at the ILL on triple axis INI2 and IN3. Incoming neutrons were in the range 1.2-2.3 A-l 'The best energy resolution was 0.02 THz.

to say magnons propagating along the chain with chains in phase and 2) the ~ m ( ~ 0 ) magnons for near I/3 or 2/3 i.e.magnons near the low temperature magnetic Bragg reflections. (Reciprocal lattice points are indexed in the nuclear-hexagonal cell). 2.1 Spin waves propagating in the chain Measurement of neutron groups at ~ - (II0) for temperatures below and above T N clearly show that the transition has no observable effect on this magno~ As the temperature is increased up to 1 5 K t h e magnon weakens and slightly shifts to a lower energy. A similar behaviour is observed At ai~¢erent points along the (ll~) line up to zone boundary~ Such a behaviour is consistent with paramagnons in a planar l-d ferromagnet and similar observations in CsNiF 3 have been reported (8). From a theoretical point ofviewthe persistence at high temperature of magnons peaks having no broadening nor energy shift but an intensity decreasing with temperature has been attributed to the out of plane component of spin fluctuations (8-9). 2.2 Spin waves propagating perpendicularly to the chains Spin waves with wave-vectors perpendicular to the chain show a much more puzzling behaviour, especially near reciprocal points (±I/3 ± I/3 2~) where magnetic Bragg peaks developp at T < T N. Around such points, low temperature spin waves consist of two sheets one going linearly to zero and the other exhibiting a small gap of O.l THz (Fig.l). By carefully following the temperature dependence of the Q = (2/3 2/3 O) magnon peak we find that the gap decreases while approaching T N from below and seems to disappear at T N (Fig.2).

Figure I : Low temperature spin-wave dispersion in the (a*+b*,c*) plane. 2.

EXPERIMENT AND DISCUSSION

The spin-wave dispersion at low temperature (6) (l.5K) is summarized in Fig. l. For the present investigation we have focussed on the temperature dependence of I) the ~ = (ll~) magnons, that is

0378-4363/81/0000~3000/$02.,50

© North-Holland Publishing Company

Since at this point only the upper branch can be seen we have also made experiments at neighbouring points like ( 2 / 3 + ~ 2/3+ ~ O) and (2/3 2/3~) which corroborate the previous behaviour and indicate that the lower branch do not move. This suggests that the upper sheet merges into the lower one at T N. But the most surprising result is that the gap at (2/3 2/3 O) reappears above T N and that well defined spin waves with wave vector perpendicular to

75

76

1 3

0

01

(r. r. 2t)

02

0.3

(hh~)

0.4

03'~'°'°"°~

T=1.52K / o

at 2.55 K is not a 3d-]d one and dynamical correlations between chains exist up to 8K. This is probably a consequence of the rather large value of J'/J which is intermediate between l-d and 3-d. Since RbFeCI 3 exhibits other properties w h ~ h are l-d like it would be interesting to know the range in J'/J where one would expect 1-d like behaviour. Other open questions are the nature of the transition at 2.55K and that of the phase above this temperature. ACKNOWLEDGMENTS

0.1

We wish to thank Dr. J.Bouillot for his assistance during the experiments and S. Legrand who has grown the RbFeCI 3 single crystals. REFERENCES *CEA-CNRS Laboratory

T=1,98K 0.1 I

I

I

I

I

I

I

I

I

T=4K 0.1

J

X•<

(~I

(~. ~. 20 i

i

i

I

L

1

I

£

I

7 - H.Yoshizawa, W.Kozukue and K.Hirakawa, the Phys.Soc.Jap. 49 (1980) 144.

T=6K (hht:) I

0

Figure 2 : Temperature dependence of spin wave dispersion in the neighborhood of

(±1/3 ± 1/3 2~). the chain can be observed between 3 K and 8 K . Furthermore at 6 K it seems that two branches can be seen at (2/3 2/3 O) which probably means that the lower branch also has a gap above T N, These last experimental findings are inconsistent with the behaviour of a 1-d system undergoing a 3d-ld phase transition. In particular the observation of undamped spin-waves propagating in the hexagonal plane r e v e a l the existence of a - at least short range - magnetic order between chains above 2.55 K. 3.

I - N.Achiwa, J.Phys.Soc.Jap. 27 (1969) 561. 2 - M.Eibsch~tz, M.E.Lines and R.C.Sherwood, Phys.Rev.B II (1975) 4595. 3 - P.A.Montano, E.Cohen and H.Shechter, Phys. Rev.B 7 (1973) 1180. 4 - G.R.Davidson, M.Eibsch~tz, D.E.Cox and V.J.Minkiewicz in Seventeenth Conf. on Magn. and Magnetic Materials, Chicago, 1971, AlP Conf.Proc. 5 (1972) 436. 5 - G.A.Prinz, Proc.of the Int.Conf.on Magnetism, Munich, 1979, J.Magn.Magn.Mat. 15-18 (1980) 839. 6 - D.Petitgrand, B.Hennion, P.Radhakrishna and C.Escribe, 1980 Conference of the Condensed Matter Division of the European Physical Society, Antwerpen, April 1980, Proc.published by Plenum Publ.Corp. (New-York).

CONCLUSION

Our neutron scattering experiments have displayed some features of RbFeCI 3 which are inconsistent with a 1-d ferromagnet : the magnetic transition

8 - M.Steiner, J.Villain and C.G.Windsor, ces in Physics 25 (1976) 87. 9 - J.M.Loveluck and S.W.Lovesey, J.Phys.C Sol.State Phys. 8 (1975) 3857.

J.of Advan: