Thermally detected electron spin resonance of Fe2+ in LiNbO3

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Solid State Communications, Vol. 76, No. 3, pp. 299-302, 1990. Printed in Great Britain.

0038-i098/9053.00+.00 Pergamon Press plc

T H E R M A L L Y D E T E C T E D E L E C T R O N SPIN R E S O N A N C E OF FeZ+ IN LiNbOs S. Juppe and O. F. Schirmer Fachbereich Physik, Universitlit Osnabr~ick, D-4500 Osnabriick, FRG (Received 11 June 1990 by M. Cardona)

Electron spin resonance of Fe2+ in LiNbO3 has been detected thermally. The signals are caused by electrically induced Ares = ±2 transitions between the Ims = ± 1> states of an excited non-Kramers spin doublet lying - 15 cm 1 aboved the Ires = 0> groundstate. For Bllc g,= 2 is measured, pointing to an orbital singlet groundstate separated by > 650 cm-1 from an orbital doublet. Large low symmetry deviations from the trigonal crystal fields at the cation sites of the structure cause wide asymmetric resonance bands.

1. I n t r o d u c t i o n

cryostat was used, capable to deliver fields up to 6 T. The diameter of the probe chamber was r a t h e r narrow, 15 mm; in order to get the required small component dimensions, 35 GHz was chosen as the resonance frequency. The central part of the setup is shown in Fig. 1. Some features of the construction are adapted from that reported by Moore et al.5. No microwave cavity was employed. The specimen was fixed to a high purity single crystal quartz rod by vacuum grease and thread, which did not show any ESR-signals. A carbon thermistor, made from colloidal graphite, was fixed to this rod below the end of the TE01 waveguide. All measurements were performed near 6 K. The investigated specimen had dimensions of about .5 x .5 x 3 mm3. The c-axis was along one of the short dimensions. The sample was cut from a crystal grown from a congruent melt containing 0.1 wt% Fe203. In order to reduce Fe3+ to Fe2+ the specimen was embedded into Li2CO3 powder7 and heated in air at 600°C for 46 h. After this treatment ESR and TDESR of the previously observable Fe3+ signals had vanished almost entirely. The crystal was mounted in the m a x i m u m o f the electric microwave field, El, with its c-axis perpendicular to El.

Light induced intervalence transfer between Fe2+ and Fe3+ via conduction band states is the initial step of the ~hotorefraetive effect in Fe-doped LiNbO31. Under mhomogeneous illumination spatially structured charge patterns result which are translated by the electrooptic effect into the corresponding distribution of the refractive indices. The phenomenon can be used for information storage and processing2 and related applications. Knowledge on the structure of these defects is essential if one wants to elucidate the processes involved in the photorefractive effect on a microscopic basis. Fe3+ is easily accessible by ESR3, whereas information on Fe2+ so far can be taken only from optical absorptionl as well as M~ssbauer4 studies. In order to gain complementary insight into the electronic structure of this defect we investigated reduced LiNbO3:Fe using thermally detected ESR (TDESR)5. We were led to this study by a previous investigation of Fe2+ in A1203 by TDESR6. The crystal structure of A1203 is similar to t h a t of LiNbOs. In t~is communication we shall give a short renpo.rt on the method used as well as on the main expemental results obtained with LiNbO3:Fe 2+. They will be exploited and discussed with respect to the electronic structure of this system. For the groundstate of this 3d6 ion in LiNbO3 short spin lattice relaxation times, TI, are expected: Fe2+ is a non-Kramers ion strongly coupled to the lattice. Also first order spin orbit coupling, present if the orbital degeneracy of the groundstate is not completely lifted by the trigonal crystal field of LiNbO3, can lead to short T l . I n conventional ESR a shortening of Tl leads to line broadening and a corresponding s t r o n g decrease of signal intensity, whereas in TDESR the signal rises inversely proportional to T15.

3. Results and their I n t e r p r e t a t i o n a. Fe2+ in trigonal fields Under these conditions the signal shown in Fig. 2 was observed. As will be demonstrated below, the steep high field onset can be attributed to Fe2+ ions in trigonal crystal fields; perturbations of lower symmetry lead to the absorption at lower magnetic fields. The angular dependence of the high energy onsets is exhibited in Flag.3. These results can be explained on the basis of the level scheme in Fig. 4. E S R is induced between the indicated [ms = +1>-levels. The Zeeman effect of

2. E x p e r i m e n t a l In our setup a superconducting magnet with 299

300

THERMALLY DETECTED ELECTRON SPIN RESONANCE OF Fe

this axially split S = 2 system is described by the Hamiltonian:

H=

D (S: - 3

S(S+I )) + g itB SSz cos8 + g i flBSz sin8 +

+ E(S~- 8:).

(l)

The last term, describing orthorhombicity of the crystal field, has been included anticipating later use. The energy distance between the |ms = ± 1>states is8

3(g±flBsine)2 )2]v2 AW = 2 [ (g.BB cod)) ~ + ( 3E +

(2)

2WI0

2+

IN LiNbO 3

Vol. 76, NO. 3

where Wlo is the energy difference between the I+1> and[0> levels for B = 0 and E = 0. Using the resonance condition AW = by, this expression was fitted to the observed angular dependence. It was found that glt = 2.0 , 0.1, consistent with the quenched orbital angular momentum of the orbital singlet groundstate. For 0 near ±90°, the angular dependence is dominated by a second order Zeeman effect, expressed by the last term in eq. 2. This leads to the rounding off of the curve in Fig. 3 near 0 = ±90". Also g± will be close to 2 on account of the orbital nondegeneracy of the groundstate. A little positive Ag± is expected by admixture of the excited orbital doublet: Ag± = - 2A/A. Using ~ = - 100 cm-1, the free ion value, which is expected to be reduced somewhat by orbital reduction, and A > 650 cm-l, to be motivated below, we find Ag± < 0.23. This leads to 15 cm-I < W]0 < 20 cm-]. Using the derived values of g, and W]0 the experimental angular dependence in Fig. 3 is well reproduced. In a general survey of the level structure of Fe2+ in a trigonally distorted octahedral crystal field, Meshcheryakov et al.9 have calculated, among other things, the gi0 value of the groundstate as well as the splitting WlO by diagonalizing the Hamiltonian

H = Vcub + Vtri&+ HSO + Hzeem (in self-evident notation) in the spin-orbit manifold of the free ion groundstate 5D. A cubic field splitting of 10000 cm-! was assumed, close to the value corresponding to LiNbO3:Fe2+, ~ 9000 cm- ! (Ref. 1). The spin orbit coupling constant ~kwas taken to be -100 cm-l. The trigonal potential was varied. As expected, got ~ 2 is calculated if the orbital singletlies lowest and if it is separated from the orbital doublet by A > 650 cm-l. This leads to the lower bound for A indicated in Fig. 4. It is furthermore shown in Ref. 9 that the Ims = + 1 >-states lie about 15 cm- I,by W ] 0, above the [ms = 0 > spin groundstate. These theoretical results support our above interpretation of the experimental data.

AT

Fe2*

10 mK [ 1 Central part of the TDESR spec_trometer: . . 1: rectangular TE01 waveguide, stai.'nle~ steeL. 2: vacuum puml~.ut for waveguide and measuring cell. 3:LINbO3 crystal. 4: high purity crystalline quartz (0.5 x 0.5 ram2)..5: plastic film, d = 0.75 pm. 6: stainless smez cyunuer, two parts. 7: carbon resistor, temperature reference. 8: thermistors (colloidal graphite). 9: feedthrough. - The outer diameter of the arrangement Is 13 mm.

~~1 0

I 0.5

I I 1

I

' 1.§

Fe 3.

J D/T

TDESR signal of reduced LiNbO3:Fe at 0 = 0 ° measured for E1 ± c. The broad background is supposed to be caused by magnetores[stance of the thermistor. The superimposed three weak lines are due to TDESR of remnants of Fe3 +. T 6 K, v = 34.855 GHz.

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THERMALLY DETECTED ELECTRON SPIN RESONANCE OF Fe 2+IN LiNbO 3

301

E9

3

5D.~...4¢I_

T2g =I0000cm-111),1-1)

/ \ L

i

&~6,50¢m-1

-

80

-40

0

J

40

mS -1

I0)

octohedrol bigonal crystat field

i

8O

120

3 Dependence of the high energy onsets of the TDESR bands on polar angle 0.

mog.field

coul~ing

4 Level scheme for Fe2+ in trigonally distorted octahedral crystal field.

b. Fe2 + in fields of lower symmetry So far we have treated the TDESR of Fe2+ in trigonal symmetry, as represented by the steep high field onset of the band in Fig. 2. Most of the signal lies at lower fields. This has to be attributed to the influence of low symmetry crystal field components8, which can be described by the addition of terms of the type E(Sx2 - Sy2) to the Hamiltonian as indicated above. Under such interactions the Ires = ± 1 >-levels in Fig. 4 show a distribution of zero field splittings corresponding to the distribution of the parameter E. It is seen that for E = 0 the resonance occurs at the highest possible field, i. e. at the high field onset of the band. For the observed low field onset in Fig. 2 one calculates E ~ 0.2 cm-1. For comparison it should be stated that the axial field parameter in (1) is: D ~ 15 cm-1. The shape of t h e b a n d cannot be explained by a Gaussian distribution of Es. The TDESR signals become increasingly broad when the magnetic field is rotated from 0 = 0 to 0 = ,90 °. This is caused by the decrease of the effective -value under this rotation, see Fig. 3: The high eld onset of the band shifts to the corresponding higher fields, as indicated in Fig. 3, whereas resonances influenced by E, occuring at lower fields, change less. As mentioned above, the resonances were only observed for the microwave electric field El J- c. In this orientation, the E1 field can induce transitions between the ]ms = ±1>-levels because the point symmetry of cations is only C3, lacking m i r r o r symmetry parallel to c. The El field can thus modulate the position of Fe2 + with respect to that of its neighb~.~6,10, leading to time dependent low symmetry hems E(t)(Sx - Sv2). These, alone or in conjunction with the stati6 low symmetry perturbations, can induce the observed (Ares = ±2)-transitional0. No such transitions are taking place for El l[ e, although there is also no mirror plane perpendicular to c. Here the axial symmetry is not aestroyed by the modulation of theposition of Fe2+ and therefore no transitions are induced]0.

Spin-orbit

4. C o m p a r i s o n with AI203:Fe2 + It is worthwhile to compare the TDESR results of Fe2+ in LiNbO3 with those of Fe2÷ in A12036. There are striking differences between the I~,-values in both cases (git = 3.43 in A1203). This gti Is consistent with a rather low value olD, ~ 4 cm-l (Ref. 11) and of A, ~ 40 cm-] (Ref. 9). In this case orbital angular momentum is not quenched and J, representing the total angular momentum, approximately is a good quantum number. The value of g, is then determined by the properties of the J = 1 groundstateS, which is slightly axially split into two components, ImJ = 0> and [mj = + 1>, separated by D. Bates et a l . n attribute the low axial splittings to a dynamic Jahn-Teller-Effekt. For LiNbO3:Fe2+ the static tri~onal crystal field apparently is stron~ enough to be *mmune against Jahn-Teller quenching. - It is remarkable that in LiNbO 3 the TDESR lines are much broader than in A1203. While A1203 can be considered to be stoichiometric, LiNbO3 in its congruently melting composition shows a high Li deficit and thus a high concentration of intrinsic defects, which also take part in charge compensating Fe2+(Ref. 12). The resulting lattice disorder can cause the observed large number of Fe2+ ions influenced by strong low symmetry crystal fields. Concluding, we have shown that TDESR is a diagnostic tool for the investigation of Fe2+ in LiNb03, a system important for the optical applications of the material. The analysis of the results elds information on the electronic structure of the west states. It turns out that the orbital groundstate is a singlet. Spin lattice relaxation via first order spin orbit coupling therefore is expected to be of minor importance.

~

Acknowledgements - We thank Dr. H. J. Reyher for very helpful comments. The experimental helpofW. Koslowski and T. Dollinger is gratefully acknowlegded. - The research reported here has been supported by DFG, Sonderforschungsbereich 225.

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THERMALLY DETECTED ELECTRON SPIN RESONANCE OF Fe 2+IN LiNbO3

Vol. 76, NO. 3

tteferences 1 see, e.g.: E. Kriitzig and O.F. Schirmer, in "Photorefractive Materials and their Applications I", P. Gttnter and J.-P. Huignard, eds., Topics in Applied Physics, Vol. 61, Springer 1988, p. 131 2 see, e.g.: "Photorefractive Materials and their Applications H", P. Gttnter and J.-P. Huignard, eds., Topics in Applied Physics, Vol. 62, Springer 1989 3 H.H. Towner, Y.M. Kim and H.S. Story, J. Chem. Ph~,s. ~ 3676 (1972) J.B. Hemngton, B. Dischler and J. Schneider, Solid State Commun. 10,509 (1972) 4 W. Keune, S.K. Date,'~. Deszi and U. Gonser, J. Appl. Phys. ~ 3914 (1975) 5 W.S. Moore and T.M. A1-Sharbati, J. Phys. D6_, 367 (1973)

6 W.S. Moore, C.A. Bates and T.M. A1-Sharbati, J. Phys. C6, L209 (1973) 7 W. Phi-Iips and D.L. Staebler, J. of Elec. Materials 3, 601 (1974) 8 A. Abragam and B. Bleaney, "Electron Paramagnetic Resonance of Transition Ions , Clarendon Press, Oxford, 1970, p. 211 9 V.F. Meshcheryakov, B.N. Grechushnikov and I.N. Kalinkina, Soy. Phys. JETP 39, 920 (1974) 10 J.W. Culvahouse, D.P. Schinke and D.L. Foster, Phys. Rev. Lett. 18, 117 (1967). - In thispaper the problem has been formulated using Self = 1/2. Here we use the true spin S = 2. 11 C.A. Bates and P. Steggles, J. Phys. C 8, 2283 (1975) 12 H.J. Donnerberg, S. Tomlinson, C.R.A. Catlow, J. Phys. Chem. Solids (in print)