FIR ABSORPTION SPECTRA OF LINEAR CHAIN ISING ANTIFERROMAGNET CsCoCl3

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FIR ABSORPTION SPECTRA OF LINEAR CHAIN

ISING ANTIFERROMAGNET CsCoCl3

H. Ohta, K. Fukuda, N. Kitamura, M. Motokawa

To cite this version:

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JOURNAL DE PHYSIQUE

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

FIR ABSORPTION SPECTRA OF LINEAR CHAIN ISING ANTIFERROMAGNET

CsCoC13

H. Ohta, K. Fukuda, N. Kitamura and M. Motokawa

Department of Physics, Faculty of Science, P-be University, Rokkodai, Nada, Kobe, 657 Japan

Abstract.

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The absorption spectra of CsCoC13 are observed using FIR lasers (103.6 and 97.1 cm-l) and pulsed high magnetic field up to 17 T. Obtained g-values for H

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c vary from 4.2 t o 4.8 depending on the absorption lines. Including Zeeman and exchange interaction terms in the Abragam-Pryce model we are able to interpret the differences between the g-values obtained by optical measurement and by neutron diffraction or magnetization measurement.

CsCoC13 is the good example of quasi-1D Ising- like antiferromagnet and has attracted much interest in connection with soliton excitation predicted by Villain [I]. CsCoCls is antiferromagnetic along the c-chain with spins preferring the c axis and these chains make a triangular lattice. It has two transition tempera- tures, T N ~ = 21.5 K and T N ~ = 9.2 K, and one third of sublattices is disordered due to frustration between T N ~ and TN2. Below TN2 spins make a ferrimagnetic configuration in the c-plane. Many experiments, for instance neutron diffraction [2], inelastic neutron scattering [3], magnetization [4] and ESR measurements [5], have been done on CsCoCls. By these measurements magnetic configuration and the existence of soliton mode were confirmed. Raman scattering experiments have been performed by Lehrnann et al. [6] and

far-infrared spectroscopic measurements on CsCoC13 have also been performed by Brun et'al. [7]. In spite of

many experiments on CsCoCls, there still remain some ambiguities, for instance the discrepancies of g values. The purpose of this paper is to report the experimental results of submillimeter wave ESR on CsCoCls and to give a qualitative explanation for the reduction of g-value obtained by this experiment.

We have measured absorption spectra of magnetic excitations a t 4.2 and 15 K using FIR laser lines 103.6 and 97.1 cm-I and pulsed high magnetic field up t o 17 T. The advantage of our experiment in comparison with the FIR spectroscopic method is that only the field dependent part of the spectrum, i.e. magnetic excitations, can be shown and vague and broad phonon modes can be eliminated. Single crystals of CsCoCl3 were grown by the conventional Bridgman method. The crystals were cut into wedged shape in order t o avoid the interference effect. The measurements were done in H

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c and H l c configurations. Figure 1

shows transmission spectra for H

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c at 97.1 cm-I at 4.2 and 15 K. We can see seven strong absorptions and many weak absorptions at 4.2 K. We attribute these strong absorptions to spin flip of Ising like co2+ spins. The complicated structure of the absorptions is apparently due t o n-fold cluster excitations as was

H ( Tesla 1

Fig. 1.

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Absorption spectra of CsCoC13 as a function of applied field H.

discussed first by Shiba [8] and by Brun et al. [7],

but it is still difficult t o identify these absorptions in correspondence with the energy levels including both intra-and inter- exchange interactions. The weak absorptions are not identified a t this stage. Since we first thought they might be due to ~ n ' + impurity, we tried intentionally ~ n doped specimen, but no ~ + significant change of these weak absorptions has been observed. The temperature dependence of absorptions above and below T N ~ in figure 1 shows that all the absorptions are broader at 15 K than at 4.2 K and the absorption intensities at 3.7 and 4.7 T are stronger but those at 1.8, 4.0 and 6.4 T are weaker at 15 K. Sim- ilar results were also obtained at 103.6 cm-l. From the temperature dependence of absorptions at 103.6 and 97.1 cm-', we were able to connect each lines at 103.6 and 97.1 cm-I. We also used the results of infrared spectroscopy by Breitling et al. [9]. Plotting

these absorption fields, we obtain the frequency field relation as is shown in figure 2. It is found that the g-value varies from 4.2 t o 4.8 depending on absorption. The g-values obtained here are rather small compared to those obtained by Lehmann et al. [6]. Moreover

the numbers of absorption lines above and below T N ~

are completely different between our results and those

obtained by Lehmann et al. Our results show seven

strong lines below T N ~ and a few lines above it as is

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C8 - 1458 _{JOURNAL DE PHYSIQUE}

shown in figure 2, while only three strong lines below T N ~ and eleven lines above T N ~ were observed in the Raman case due to the partially disordered spin configuration. It is considered to be due to the difference of the selection rule between FIR absorption and Raman scattering.

As mentioned previously, the pvalues are somewhat different between measurements. The g values ob-

tained from the magnitude of magnetic moment by neutron diffraction [2] or by magnetization measurement [4] are about 6. On the other hand those obtained by Raman scattering [6] or by our experiment are about 4 t o 5. Here we briefly discuss the reason of this discrepancy. As is well known, energy level of co2+ splits into six Kramers doublets in a crystalline field [lo]. We introduce exchange interaction and Zee- man terms t o the Abragam-Pryce Hamiltonian and obtain eigen values for the ground and the first excited states. Similar to the nonlinear Zeeman effect of high field EPR [ll], large exchange interaction and high magnetic field cause crossover effect between both states. The exchange interaction separates the ground state by about 100 cm-' and the upper level has mix-

Fig. 2. - Frequency-field relation of the absorption fields at 4.2 K.

ing with the first excited state. This crossover effect gives rise t o reduction of field depmdence for the u p per level. The lower level, however, is affected little by the excited state. Therefore the field dependence of the transition from the lower level to the upper one shows the reduced 9-value, but the magnetic moment which reflects the lowest energy level gives original g-

value. Our g-value calculated using parameters ac- cording to [lo] and assuming a molecular field which makes 100 cm-' separation a t zero field is approxi- mately 5, which is qualitatively consistent with the experimental result.

Acknowledgment

We would like t o thank Professor Shiba of university of Tokyo for valuable discussions.

[I] Villain, J., Physica B 79 (1975) 1.

[2] Mekata, M. and Adachi, K., .I. Phys. Soc. Jpn 44 (1978) 806.

[3] Yoshizawa, H. and Hirakawa, K., J. Phys. Soc.

Jpn 46 (1979) 448.

[4] Amaya, K., et al., submitted to 2nd Int. Symp. on High Field Magnetism (1988).

[5] Adachi, K., J. Phys. Soc. Jpn 50 (1981) 3904. [6] Lehmann, W. P., Breitling, W . and Weber, R., J.

Phys. C 14 (1981) 4655.

[7] Brun, A., Meyer, P. and Briat, B., J. Phys. C 13 (1980) 5775.

[8] Shiba, H., Prog. Theor. Phys. 64 (1980) 466. [9] Breitling, W., Lehmann, W., Strinivasan, T. P.,

Weber, R. and Diirr, U., Solid (State Commun. 24 (1977) 267.

[lo] Abragam, A. and Pryce, M. H. L., Proc. Roy.

Soc. A 206 (1951) 173.