Magnetic circular and linear dichroism of iron garnets thin films

Solid State Communications,Vol. 15, pp. 767—770, 1974.

Pergamon Press.

Printed in Great Britain

MAGNETIC CIRCULAR AND LINEAR DICHROISM OF IRON GARNETS THIN FILMS J.C. Canit, J. Badoz and B. Briat* Laboratoire d’Optique Physique, E.P.C.I., 10, rue Vauquelin, 75231 Paris Cedex 05, France and R. Krishnan Laboratoire de Magnétisme, C.N.R.S., 92190— Meudon-Bellevue, France (Received 9May 1974 byM Balkanski)

Low temperature absorption, MCD and MLD spectra are reported for pure and substituted Y3Fe5O12 thin films in the 13000—33000 cm’ spectral range. The materials were obtained by lic~uidphase epitaxial growth on a Gd3Ga5O12 substrate. Above 19000 cm we demonstrate that our transmission experiments on garnet films provide the same information as dothe 1 however, reflexion former aremeasurements much more reliable on bulkand crystals. accurate. Below All our 19000 data cmindicate that the films are of excellent optical quality. MCD and MLD spectra show a considerably increased resolution as compared to the absorption spectrum and help in sorting out most of its numerous overlapping components. Finally we prove unambiguously that Clogston’s theoretical interpretation is qualitatively correct in the 13000—23000 cm’ region and even probably at higher wavenumbers.

INTRODUCTION

Important technical and research progress which have recently been made are likely to lead to new developments in this research field. For example, gamets thin films are now obtainable by liquid phase epitaxy, which are suitable for absorption, reflexion, magnetic circular and linear dichroism (MCD and MLD respectively) measurements at any temperature, in the whole spectral range including the charge transfer bands.’ MCD and MLD provide in principle the same information as does magnetic circular or linear birefringence through absorption bands. It is now widely recognized however that the former spectra are much more easy to interpret, especially when bands overlap a great deal. Besides, the MCD spectroscopy of a number of d5 ions including Fe3~or the isoelectronic Mn2~in certain non-magnetic lattices has now been reported.2’3

OPTICAL and magnetooptical properties of fernmagnetic gamets such as YIG have been widely studied in view of their practical application. Most measurements by transmission (absorption, differential absorption, Faraday rotation) have so far been limited to the edge of the near infrared or visible absorption bands, this being due to the use of bulk magnetic crystals. Measurements by reflexion (reflexion, differential reflexion, Kerr effect) have been conducted through the absorption bands and valuable empirical information has been obtained. In spite of this great deal of effort in various directions, a survey of the presently available literature shows that the microscopic origin of magnetooptical effects in garnets is not yet fully understood. *

To whom correspondence is to be addressed. 767

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FIG. 1. MCD of a 850 A thick YIG filn~(—) and MCD of a YIG crystal (— — —) as computed from our measurements of its Kerr rotation and ellipticity and of its optical constants.5 Both spectra have been takenat300K.

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In an effort to provide a better understanding of the microscopic origin of magnetooptical effects, we are carrying out a research program including MCD and eventually MLD studies on Fe3~at various point site symmetries in non-magnetic crystals and in fernmagnetic thin films. We present here a few preliminary measurements on some gamets and comment on them briefly in terms of their relationship to reflexion data and also of their spectroscopic interest, EXPERIMENTAL AND DISCUSSION MCD and MLD spectra have been run on an appartus described elsewhere.4 MCD is defined in terms of the differential absorbance txA~for two circularly polarized lights (u+ and u_) while MLD is expressed in terms of the differential absorbance 1~ALfor linearly polarized components parallel and perpendicularto the applied magnetic field respectively. Reflexion data have been obtained on a newly designed apparatus which is more reliable than that used previously.5 The thin films used in this work have been deposited on both [110] and Liii] Gd 3Ga5O12 substrates. With the samples available to us so far, spectra could be taken in the spectral range 13000—33000 cm~for YIG, Y3Fe5_~Ga~O12 (x 0.3), Y3_~Bi~Fe5_~Ga~O12 (x 0.3, y —0.1) as films and 3~(Fe also for a crystal of YAG Fe 203 i wt.%). ‘—

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Figure 1 illustrates a comparison between our reflexion and transmission measurements. The dotted curve required four sets of experiments (Kerr rotation and ellipticity, refractive and extinction indices) as well as some computational work while with the use of a film, the MCD data (full curve) were obtained

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spectra of a 1 .8 ~m thick YIG 111 film at room (MCD and MLD) and low-temperature (MCD) in the spectral range 13000—195000 cm~.MCD spectra have been measured under 0.7 Tesla and saturation was reached. MLD has been measured under 0.027 Tesla. (b) detail of the low-temperature MCD spectrum in the 13100—14200 crni region. The spectral band-width used in this expenment was about 2.5 cm~’.

with one single experiment. In view of this difference in the treatment of the signal, we believe that the agreement within less than 10 per cent between the two spectra, is very satisfactory, peaks occuring at roughly the same wave number in the two cases. This proves that both kinds of measurements do provide the same information (as expected) when they are feasible. We note at this stage that Kerr rotation measurements do not depend upon the sample thickness, unlike MCD which is proportional to it. In the 19000~33000cm_i region for YIG, both techniques are usable and equivalent with respect to the signal to noise ratio since the Kerr rotation is large. However the base line is more reliable in the case of direct MCD since it can be established easily in zero external field and checked by applying the field in either sense parallel to the wave vector. The situation is by far different at lower energies (13000— 19000 cm_i)

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CIRCULAR AND LINEAR DICHROISM OF IRON

where the Kerr rotation is small and the accuracy very 6 poor as established first by Kahn et aL and reproduced by us. In comparison, our direct MCD spectrum at 300K on a film [Fig. 2(a)] allows a gain of about 100 on the signal/noise ratio. It is worth pointing out that we have obtained similar spectra for samples of different thickness, thus ruling out possible artifacts due to interference Figures 2(a and b) further illustrates some of our magnetooptical data for YIG on the visible absorption bands edge. The major consequence of cooling upon MCD [Fig. 2(a)] is to reveal a large number of significant bumps and shoulders on the main room temperature peaks. This is particularly well illustrated in Fig. 2(b) which covers a limited spectral range at an expanded scale, the noise level being still negligible. For example two closely lying (45 cm~’)peaks are found in the 13340 and 13580 cm_i regions. Most of the structure shown in Fig. 2(b) has never been reported before. This is so since an MCD instrument is in many respects a very sensitive spectrophotometer (sensitivity A 10_6); further the relative magnitudes of peaks has no reason to be the same in MCD and absorption experiments. The observance of a fine structure in the MCD spectrum of our film certainly demonstrates that it is of very good quality (e.g., homogeneity). This is consistent with the data in Fig. 1 and the fact that we confirmed the shape of the MCD at 300K in Fig. 2(a) (l3000—i6500cm~’region) on a thin sample cut from a bulk crystal, this shape being then identical to that reported previously.7 Figure 3 shows some of our MCD, MLD and absorption spectra in the 19000~33000cm_i spectral range. Here again, the MCD and MLD spectra show a considerably increased resolution as compared to the absorption spectrum The 19000—22000 cm_i region for YIG can be analysed easily. MCD bell-shaped signals correspond to the absorption peak 2 at 20720 cm~’.These are equivalent to the so-called “paramagnetic” c and C terms encountered in the low-temperature spectra of many paramagnetic materials,8 and predicted in Clogston’s original paper9 for transitions within the d5 configuration. From a close examination of our MCD, MLD and absorption data we can infer that similar terms (thus absorption peaks, noted 1, 3—5) are also present around 19800, 21000, 21400 and 21600 cmi for both YIG and YIG : Ga: Bi. The 21200—22800 cm~region also

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FIG.3.Absorption(. .),MCD(—)andMLD (— —) spectra of a 850 A thick YIG 110 film at 22 K. The MCD spectrum of a YIG—Ga, Bi film at 7K(—.—.) is also shown for comparison. (A —0.12 at 20800 cm_i). Saturation was reached in MCD experiments only. .

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illustrates well the kind of additional information which can be gained from both MCD and MLD measurements at low temperature with respect to less sophisticated techniques. It is quite certain that the MCD and absorption peaks noted 6 at 22500 cm_i in YIG actually covers two close components since two peaks of opposite sign are shown in the MLD spectrum. In the case of YIG : Ga: Bi these cornponents are likely to be hidden under the lowest energy MCD term (also noted 6) in the same region. More work is certainly required to understand the rest of the spectra at higher energies. We note however that our presently available data does not provide any experimental support for the presence of S-shaped MCD terms assumed by, e.g. Kahn et al.6 in order to interpret their magnetooptical data on ferric oxide compounds. One major conclusion of our research on such materials is certainly that the number of their absorption components is very large and that it has currently been underestimated in the past. Thus many previous assignments are very questionable. Finally a brief comparison with Faraday rotation measurements is indicated in this paper. Bearing in mind that an S-shaped like magnetic circular or linear birefningence shape is associated with an MCD or MLD bell shape, we can understand that there is little hope to sort out any reliable spectroscopic information

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CIRCULAR AND LINEAR DICHROISM OF IRON

on iron garnets from birefringence spectra in absorption regions and that dichroism on films certainly offers a unique chance towards this goal.

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Acknowledgements — The authors are highly grateful to Dr. W. Tolksdorf of Philips Research Laboratories, Hamburg, for help with the substrate material.

REFERENCES 1. 2.

See for example KRISHNAN R., LE GALL H. and KHANH VIEN TRAN,Phys. Status Solidi (a) 17, K65, (1973); WEMPLE S.H.,Surf Sci. 37, 297 (1973). RIVOAL J.C. and BRIAT B.,Mol. Phys. 27, 1051 (1974).

3.

KATO H.,J. Chem. Phys. 58, 1964, (1973);ibid. 59, 1732 (1973).

4.

BADOZ J., BILLARDON M., BOCCARA A.C. and BRIAT B.,Symp. Farad. Soc. 3,27 (1969).

5. 6.

CANIT J.C., BILLARDON M. and KRISHNAN R.,Phys. Status Solidi (a) 14,229 (1972). KAHN F.J., PERSHAN P.S. and REMEIKA J.P.,Phys. Rev. 186, 891 (1969).

7. 8.

EGASHIRA K., MANABE T. and KATSURAKI H.,J. App!. Phys. 42,4334(1971). BRIAT B., Proc. Advanced Study Institute for Optical Rotatory Dispersion and Circular Dichroism, (edited by CIARDELLI F. and SALVADORI P.) Heyden (1973).

9.

CLOGSTON A.M.,J. Phys. Radium 20, 151 (1959).

Nous présentons les spectres d’absorption et de dichroismes circulaire et linéaire magnétiques (DCM et DLM) de Y3Fe5Oi2 pur ou substitué, en

couche mince, dans le domaine 13000—33000 cm~ia basse temperature. Les matériaux ont été préparés par croissance épitaxiale en phase liquide sur des cristaux de Gd3GasOi2. Pour a> 19000 cm~’,nous prouvons que les experiences par transmission (films) et par réflexion (cristaux), apportent les mémes informations spectroscopiques dans le cas des grenats. Ii n’en est plus de méme pour a < 19000 cm’, la mesure du dichroisme étant alors de beaucoup la plus sensible et la plus precise. Toutes nos mesures indiquent que la qualité optique des couches est excellente. Les spectres de DCM et de DLM sont beaucoup mieux résolus que celui d’absorption et us permettent de localiser la plupart des nombreuses composantes de ce dernier. En definitive, nous prouvons sans awbiguité que l’interprétation theorique proposée par Clogston est certainement correcte qualitativement dans le domaine 13000—23000 cm~et méme probablement au-delà.