Optically detected magnetic resonance of Fe4+OI in KTaO3

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Solid State Communications, Vol. 98, No. 5, pp. 445-447, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/96 $12.00 + .00

1P e r g a m o n

0038-1098(95)00806-3 OPTICALLY DETECTED M A G N E T I C RESONANCE OF Fe4+-Oi IN KTaO3 B. Faust, H.-J. Reyher and O.F. Schirmer Fachbereich Physik, Universit/it Osnabrfick, 49069 Osnabriick, Germany

(Received and accepted 27 November 1995 by J. Kuhl) The electron spin resonance of Fe4+-Oi in KTaO 3 has been observed optically (ODMR) via the magnetic circular dichroism (MCD) of absorption. The charge state Fe 3+ - O I was present simultaneously. Fe4+-Oi was created metastably at low temperatures by light used to detect the MCD. Keywords: A. insulators, C. point defects, E. electron paramagnetic resonance.

IN PEROVSKITE-TYPE oxides like KNbO3, K T N and BaTiO 3, iron is discussed as an important ion impurity, possibly influencing the photorefractive effect. In contrast to these substances, the cubic KTaO 3 has minor importance for technical applications. However, it may serve as a model material to study defect properties, like charge states or defect structures of, e.g. iron centres, which should have similar properties to the related ferroelectrics. Reports on iron centres in KTaO 3 appear very often in literature [I, 2], and we have recently studied [3] the ODMR-tagged [4] MCD-spectra of Fe3+-OI as well as the linear dichroism due to optical alignment of this axial centre. In this report, we present O D M R results yielding evidence for the (4+)-charge state of the same centre. The KTaO3 crystal has been grown in an oxidizing atmosphere and was doped with 5000 ppm iron in the melt. The sample was prepared as a [1 1 0]-cut, meaning the polished surfaces for the incidence of the detection light were (1 1 0)-planes. The [0 0 1]-direction was the axis of rotation in the measurements of the angular dependence of the ODMR. Magnetic field and the propagation vector of the detection light were parallel and perpendicular to the axis of rotation. All measurements were performed at 1.5 K using a standard O D M R set-up described elsewhere [3]. In conventional EPR measurements at 9 and 35 GHz at room temperature, three paramagnetic centres were found in the same specimen which was used in the O D M R studies. These three centres have been observed before in nominally pure KTaO 3 by

Glinchuk et al. [2] and were identified as axial Fe 3+, axial Fe Z and cubic Gd 3+. The parameters given in [2] lead to an excellent fit to our experimental EPR data. Also as in [2], no r e r3+ a - V o was present, which can be distinguished easily from Fe3+-OI at 35GHz. This finding is also consistent with the fact that our sample was in an oxidized state. The continuous line in Fig. 1 shows the MCD spectrum obtained from this crystal. All the bands could be assigned to Fe3+-Oi by O D M R [3]. The dashed curve represents the spectrum observed after illuminating the crystal by near UV light (3.5 eV) and applying a subsequent heating-cooling cycle: 1.5approx. 120-1.5 K. An additional weak but clearly resolved band at 1.8 eV is observed. Also, the rest of the spectrum has changed to some extent. This may be caused either by additionally created bands or by linear dichroism from the Fe3+-OI centres, which may become aligned to some extent by residual polarisation of the intentionally unpolarised UV light [3]. Since the origin of the deviation between the two spectra above 2 eV is uncertain until now, we confine ourselves to the band at 1.8 eV in the following. The illumination by the detection light below 3.5 eV causes this band to decrease continuously and it disappears completely by heating the crystal to temperatures well above 120 K. Within experimental error, no change of the intensity of the O D M R signals from Fe3+-Oi[3] could be found when the new band was created or quenched. In the metastable MCD band at 1.8eV, weak O D M R signals were observed which we will now

445

DETECTED M A G N E T I C RESONANCE OF Fe4+-Oz IN KTaO3

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times. The O D M R lines had a full width at half maximum of 12 + 3 mT, independent of the orientation. In Fig. 2, also the angular dependence of the O D M R signals of Fe3+Oi (observed via the MCD at 2"46 eV) is depicted fOr the sake Of cOmpleteness" The pattern as well as the characteristic effective gvalue of the new metastable centre is nearly identical to that observed for Fe4+-Vo in SrTiO3 [5]. Consequently, we assume axial Fe 4+ centres, most probably Fea+-Vo or Fe4+-OI, and describe the magnetic behaviour of this centre by a spin Hamiltonian reflecting tetragonal symmetry (3d 4, S = 2)

7-[ =- gflHS + (D/3)O ° + (a/120)(O ° + 504). Fig. 1. MCD-spectrum of Fe3+-Oi in KTaO 3 (solid), the dashed line was obtained after illumination of the crystal by light with about 3.5 eV. It shows an additional band at 1.8 eV which we attribute to Fe4+-OI. attribute to axial Fe 4+ by interpreting their angular dependence shown in Fig. 2. For approximately perpendicular incidence of the MCD probe beam on the crystal plate (corresponding to the [1 1 0J-direction), we found a signal to noise ratio (S/N) of 3. Since for the other angles the crystal has to be tilted substantially with respect to the probe beam, an effective linear dichroism arises due to the then anisotropic reflection from the crystal's surface. As a result, the MCD was weakened leading to S/N ~1 for the O D M R at these angles (Fig. 2, lower left branch). The O D M R signals in the upper branches showed an even worse S/N, presumably because of decreasing transition probabilities, and became unobservable for more than 25 degrees off [1 1 0] for reasonable scan

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Fig. 2. Angular dependence of the O D M R lines of Fe4+-Oi (filled circles) and Fe3+-Oz (open circles) in KTaO3 for the magnetic field B in a (1 00)-plane at 36.16GHz and 1.5K, O being the angle between [1 00] and B. The solid lines show fits obtained by diagonalising the appropriate spin Hamiltonian.

(1)

The g-tensor has been assumed isotropic, since the difference between g± and gH, allowed by symmetry, is expected to be so small for the orbital singlet state under consideration, that it will not affect the fit to the experimental data significantly. For the same reason, also the tetragonal contribution proportional to 04 (F-term) has been neglected. As in [5], one finds agreement with experiment only if one ascribes the observed resonances to the "forbidden" transitions between the Ms = 4-2 levels [6], which must be lowest in energy (D < 0) as the experiment was performed at 1.5 K. Satisfactory fits to the data, such as the one shown in Fig. 2, were obtained using g = 2.02 and D < - 5 c m 1. The cubic crystal field term with parameter a allows a zero field splitting of these levels, but the fits to the data only yielded an upper limit of a < 0.1cm -~. Future measurements using a second microwave frequency will give more precise information on the spin Hamiltonian parameters. We now want to discuss the nature of the observed Fe4+-centre, i.e. to differentiate between the two most plausible cases: Fe4+-Vo and Fe4+-O1. As the axial Fe 4+-centre was created metastably by light illumination, one has to take into account the neighbouring charge states Fe3+-X or FeS+-X as precursors, where X stands for Vo or Ot. Fe3+-Vo must be excluded a priori because its well-known EPR signal was not observed. Both FeS+-X centres seem unlikely, since such centres should be detectable by EPR as, e.g., in SrTiO3 [7]. Furthermore, because of the size of the g-values, the corresponding signals should be easily distinguishable from Fe + [2], a similar axial S = 3/2 center, which was present in the sample investigated here. Since Fe3+-Oi was found as a dominant defect by EPR, Fe4+-Oi is the most likely candidate for the axial centre under consideration. The fact that after illumination heating is needed to create Fe4+-Ol can only be understood by assuming that Fe 3+-O~ is not photoionized directly. Rather, electrons and holes are generated by bandgap

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DETECTED MAGNETIC RESONANCE OF Fe4+-Ol IN KTaO3

illumination. They are immobilised by shallow traps at the temperature of 1.5 K. Heating then allows the holes to migrate to Fe3+-Oi centres while the electrons stay trapped. If the temperature is further increased the trapped photoelectrons also become mobile and recombine with the generated Fe 4+centres quenching the ODMR and MCD signals. Looking at the parameters of the spin Hamiltonian, one might be surprised that a D-parameter of approximately the same magnitude (yet with opposite sign) as that found for Fe 3+-O! is observed for Fe 4+OI, although the electronic orbital state is quite different. It is also astonishing that the sign of D, the magnitude of a, as well as the positive g-shift agrees with the findings for Fe4+-Vo in SrTiO3 [5], although the effectively positively charged vacancy is replaced by a negative oxygen ion here. We do not want to comment on these findings further by hand-waving arguments, since the interpretation of the D-parameter of Fe 3÷-OI [8] has shown that elaborate calculations are needed to relate experimental parameters of the spin Hamiltonian to a specific atomic scale model. Similar calculations would be even more complicated for the charge state (4+) due to the different free ion orbital state (L = 2 instead of L = 0). To comment on the optical properties, i.e., the MCD-bands related to the investigated Fe4+-center is also difficult at this stage. Due to the weakness of the ODMR signals it was impossible to measure a tagged MCD [4] spectrum of Fe4+-Ox and, consequently, only the band at 1.8eV can be attributed to this center. To describe this MCD-band by model or even ab initio calculations will be a complicated task, and we will confine ourselves on some qualitative considerations: Absorption bands in SrTiO3 at 2.82 and 2.09 eV have been attributed to charge transfer transitions of cubic Fe 4+ [9], and these bands were compared with the spectral dependence of the creation and quenching rate of Fe4+-Vo in the same material [5]. Based on this comparison it was argued that the presence of the oxygen vacancy merely leads to a splitting of the bands of the cubic centre, as to be

447

expected by symmetry, while a substantial shift is not found. One might now speculate that the same is true for the case when an interstitial oxygen ion is present, and interpret the MCD band at 1.8eV as a wing resulting from the 2.09 eV charge transfer transition. However, the tagged MCD studies of Fe3+-Ox [3] have clearly shown that the optical properties of Fe 3+ are strongly modified by the presence of OI, and this may apply to Fe4+-OI too. Acknowledgement - - We thank the Deutsche Forschungsgemeinschaft for financial support (SFB 225/ C4). REFERENCES 1.

2. 3. 4. 5. 6.

7.

8. 9.

M.M. Abraham, L.A. Boatner, D.N. Olson & U,T. H6chli, J. Chem. Phys. 81, 2528 (1984); S.Q. Fu, W.-K. Lee, A.S. Nowick, L.A. Boatner & M.M. Abraham, Solid State Chem. 83, 221 (1989). M.D. Glinchuk, V.V. Laguta, I.P. Bykov, J. Rosa & L. Jastrabik, J. Phys.: Condens. Matter 7, 2605 (1995). H,-J. Reyher, B. Faust, M. K~iding, H. Hesse, E. Ru£a & M. W6hlecke, Phys. Rev. BS1, 6707 (1995). J.-M. Spaeth & F. Lohse, J. Phys. Chem. Solids 51, 861 (1990). O.F. Schirmer, W. Berlinger & K.A. Miiller, Solid State Commun. 16, 1290 (1975). As stated in [5], the observation of AM~ = 2 transitions is based on components of the microwave field, which are usually suppressed by typical EPR cavities but should be present in this set-up using no cavity at all. In SrTiO 3, Fe 5+ centres have been observed in a cubic [K.A. Miiller, Th. von Waldkirch, W. Berlinger & B.W. Faughnan, Solid State Commun. 9, 1097 (1971)] and an axial [Th. Kool & M. Glasbeek, Solid State Commun. 22, 193 (1977)] local field on a Ti4+ site. H. Donnerberg, M. Exner & C.R.A. Catlow, Phys. Rev. B47, 14 (1993). K.W. Blazey & H. Weibel, J. Phys. Chem. Solids 45, 917 (1984).