Resonance Raman scattering in ferromagnetic CdCr2S4

Solid State Communications, Vol. 18, PP. 1333—1336, 1976.

Pergamon Press.

Printed in Great Britain

RESONANCE RAMAN SCATTERING IN FERROMAGNETIC CdCr2 S4 N. Koshizuka, Y. Yokoyama and T. Tsushima Electrotechnical Laboratory, Tanashi, Tokyo 188, Japan (Received 25 December 1975 by Y. Toyozawa) Large enhancement of Raman cross section for the phonon lines at 352 and 396cm’ was observed below T~with 650nm excitation light, and it is accounted for by a resonance effect related to the “red shifting” C transition in CdCr2 S4. While, for 281 cm’ line resonant cancellation and enhancement effects were observed with different excitation wavelengths. It becomes evident tharthe anomalous temperature dependence of intensity of the phonon lines depends strongly on the excitation wavelength corresponding to the electronic transitions near the band gap in CdCr2S4. RAMAN SCATTERING in CdCr2 S4 and CdCr2 Se4 was observed for the first time by Steigmeier and Harbeke (abbreviated as S.H.), and influences of the ferromagnetic ordering on the phonon lines were studied in such spinel-type magnetic semiconductors.’ They found that the intensities of certain phonon lines increase abruptly below the Curie temperatures.’ In order to explain this phenomenon, a general theory of the spin-dependent phonon Raman scattering in magnetic crystals has been developed by Suzuki and Kamimura, and they have proposed two scattering types of spin-dependent scattering mech2 The mechansims originate from the anisms. variation of the d-electron transfer and that of the nondiagonal exchange interaction with lattice vibrations, Recently, we studied dependences on excitation wavelength and temperature of phonon Raman spectra of non-magnetic Cdln2 S4 and observed a large change of intensity with temperature for one of phonon 3 the On the basis lines a certain wavelength of thiswith result, we considered thatofalight. resonance effect plays an important role in anomalous temperature dependence of the intensity of the phonon lines in CdCr2X4 (X: 5, Se). From the present work on CdCr2 S4, it turns out that thermal behaviors of the Raman cross section depend strongly on the wavelength of excitation light, and that the result obtained by S.H.’ is accounted for by a resonance effect related to 4 the magnetic “redspectra shifting” transition in the absorption and Kerr effect5 of CdCr 2 S4. Single crystals of CdCr2 S4 were grown by a closed tube vaportwo transport CrCl3 transport agent for about months.method Ramanwith scattering was observed by use of a triple monochromator (JASCO R-750) with synchronous photon counting techniques. We used twelve lines of an Ar—Kr mixed gas laser (Spectra-Physics 164) and a tunable dye laser (Spectra-Physics 375) to obtain

the excitation wavelength dependence of Raman spectra. Temperature dependence of Raman spectra in the range from 15 to 300 K was measured using a “Displex” refrigerator (Air Products and Chemicals). The temperature of the surface spot was raised where the light beam is focused compared with the other parts of the crystal. The degree of the temperature rise was estimated to be less than 5 degrees at low temperatures with 15 mW excitation light from a comparison between radiation power dependence (15—60 mW) and temperature dependence of Raman spectra. All measurements were made by the backscattering arrangement with the propagation vectors parallel to one of the trigonal axes or the cubic axes of octahedral crystals. Figure 1 shows Stokes Raman spectra of CdCr 2 S4 with various wavelengths of excitation light below the Curie temperature T~(= 84 K), where the electric vectors of incident and scattered areintensity parallel to a specific axis in the trigonal plane. light As the ofA line at 105 cm’ is too weak to get reliable data for our purpose, its spectrum is not shown in Fig. 1. CdCr 2X4 belongs to the space group O~and has 14 atoms in the primitive cell. A group-theoretical analysis for the normal spine! shows that there are 12 Raman active modes out of 42 phonon modes at k = 0: 1A ig’ lEg, and 3T2~modes. The Raman scattering tensors for A lg those and Eg have only ponents, while formodes T2g modes havediagonal only non-comdiagonal components. An assignment of the observed phonon lines has been made denoted in Fig. 1 ~6 But, 1 we also as found the breakdown of as S.H. theobserved selectionbyrule for E and F lines in the polarization properties of Raman spectra with 647.1 rim light at 15 K: The lines E and F which have nearly the same intensities are observed in both spectra with diagonal and non-diagonal components of Raman scattering tensor.

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RAMAN SCATTERING IN FERROMAGNETIC CdCr2S4 CdCr2S4

Vol. 18, Nos. 9/10

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Fig. 2. Integrated intensity ratios of the phonon Raman

Fig. 1. Raman spectra of CdCr 2 S4 with various excitation wavelengths of light at 15 K. 2i

lines D (281 cm’)’ E (352 cm’), and F (396 cm~)to the line C (258 cm’) as a function of wavelength of excitation light.

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3. Temperature dependences of the integrated intensity ratio of the phonon lines D (281 cm’) and F (396 cm~) to the line C (258 cm’) with 647.1, 568.2, and 514.5 nm excitation light.

Vol. 18, Nos. 9/10

RAMAN SCATTERING IN FERROMAGNETIC CdCr2S4

Although the intensity of C line changes with excitation wavelength and temperature especially when excited with red light, the change is small compared 1 we with that E, 2and lines. As wassection done byofS.H., showofinD,Fig. theFRaman cross D, E, and F lines normalized by that of C line as a function of excitation wavelength. The following facts should be noted from Fig. 2: (1) Strong excitation light dependence of the Raman spectra at low temperatures, that is, large enhancement of the cross section for E and F lines with 650 nm light, while, disappearance of the cross section forD line with 530.9 and 568.2 rim light at 105 and 15 K, respectively. (2) Strong temperature dependence at low temperatures, that is, enhancement of the cross section with decreasing temperature forD, E, and F lines with red excitation light and for D line with green light. We also measured detailed temperature dependences of the Raman spectra with various wavelengths of excitation light between 15 and 300 K. In Fig. 3, the cross sections of D and F lines normalized by that of C line are shown as a ft nction of temperature with different excitation light. The temperature dependences of the

intensity for E line are almost the same as those for F line. Our result with 637.2 rim is consistent with those obtained by S.H.’ who carried out with use of 632.8 rim He—Ne laser light. However, thermal behaviors of Raman spectra with other wavelengths of light are quite different from those by S.H. On the basis of these results, it is suggested that the anomalous temperature dependence of intensity of certain phonon lines are strongly related to the energy level structure near the energy of excitation light. As shown in the low part of Fig. 2, there are several crystal field transitions from 4A 2 to ~ 1’2, T~of 3” ions in the octahedral field of 2ST2, ionsand in the visible Cr region of the absorption4 and Kerr effect5 spectra of CdCr 2 S4. In addition to these transitions, a new transition C appears at low temperatures near T~and 4’5 shows aasprominent red shift with decreasing temperature denoted by E~in Fig. 2. On comparison of the excitation wavelength dependence of the Raman cross section and the energy level structure, the large enhancement of the cross section for E and F lines observed ‘~

1335

with red excitation light may be accounted for by a resonance Raman effect from the factthat the wavelength whichwithEr corresponds the temperature intensity maximum at 15K agrees at the to same in Fig. 2. That is, the resonance enhancement of E and F lines are possibly caused by the decrease of energy difference between incident or scattered light and “red shifting” Clevel in the denominator of the scattering process at low temperatures below T~.The origin of the C transition is explained by “magnetic F’ center” model which consists of two electrons trapped at a vacancy of a S2 ion with their spins keeping parallel.7 It seems that the spin-dependent behavior of the temperature dependence of the cross section for E and F lines is due to the “magnetic red shift” of E~with decreasing temperature below T~.Furthermore, the breakdown of the polarization selection rule for E and F lines may be ascribed to the resonance with the transition of the trapped electrons at a vacancy in a low symmetry field of C 7 3~. The other interesting results are resonant cancellation and enhancement effects of the cross section for D line. The thermal behavior of Raman spectra of CdCr 2 S4 with 514.5 rim light is similar to that of CdCr2 Se4 with 647.1 nm light, though the former shows the intensity maximum near 20K. On the other hand, the direct band gap E,~which shows blue shift with the decrease of temperature is estimated to be about 2.4 eV (520 nm) for CdCr2 S4 and 2.0 8eVand (630 mm) for photoconducCdCr2 Se4 from the reflectance tivity9”°measurements. Therefore, it is possible that the intensity increase of D line with 514.5 nm light at low temperatures originates from a resonance effect related with the thermal variation of Ed. The abrupt intensity increase below T~,however, cannot be ex-

plained by other the resonance effect such only.asInthe addition to this effect, the interpretation spin-dependent mechanisms for phonon Raman scattering2 may be necessary for understanding this thermal behavior. We consider that the results presented herescattering” can be explained as “spin-lependent resonance Raman from phonons in magnetic semiconductors. Acknowledgements We would like to thank Dr. M. Kusunoki for helpful discussions, and H. Hiruma and M. Suzuki for their technical assistances. —

REFERENCES 1. 2.

STEIGMEIER E.F. & HARBEKE G.,Phys. Kond. Mat. 12, 1(1970). SUZUKI N. & KAMIMURA H., J. Phys. Soc. Japan 35, 985 (1973).

3.

KOSHIZUKA N., YOKOYAMA Y., HIRUMA H. & TSUSHIMA T., Solid State Commun. 16, 1011(1975).

4.

BERGER S.B. & EKSTROM L., Phys. Rev. Lett. 23, 1499 (1969).

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RAMAN SCATTERING IN FERROMAGNETIC CdCr2 S4

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5. 6.

WITTEKOEK S. & RINZEMA G.,Phys. Status Solidi (b) 44, 849 (1971). BRUESCH P. & D’AMBROGIO F.,Phys. Status Solidi(b) 50, 513 (1972).

7.

NATSUME Y. & KAMIMURA H., Solid State Commun. 11, 875 (1972).

8.

AHRENKIEL R.K., MOSER F., LYU S. & PIDGEON C.R.,J. Appl. Phys. 42, 1452 (1971).

9. 10.

LARSEN P.K. & WITTEKOEK S., Phys. Rev. Lett. 29, 1597 (1972). SATO K. & TERANISHI T.,J. Phys. Soc. Japan 29, 523 (1970).