Magnetic circular dichroism study on YIG films

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Magnetic circular dichroism study on YIG films

S. Visnovsky, J.C. Canit, B. Briat, R. Krishnan

To cite this version:

S. Visnovsky, J.C. Canit, B. Briat, R. Krishnan. Magnetic circular dichroism study on YIG films.

Journal de Physique, 1979, 40 (1), pp.73-77. �10.1051/jphys:0197900400107300�. �jpa-00208886�

Magnetic circular dichroism study ^{on YIG films}

S. Visnovsky (*), ^{J. C.} Canit, ^{B. Briat}

Laboratoire d’Optique Physique, EPCI,10, ^rue Vauquelin, 75231 Paris Cedex 05, France

and R. Krishnan

Laboratoire de Magnétisme, C.N.R.S., 92190 Meudon-Bellewe, ^France

(Reçu ^{le 20} juillet 1978, accepté ^{le 27} septembre 1978)

Résumé.

Nous présentons ^la dépendance ^en temperature (300 2014 1,8 K) ^des spectres de dichroisme circulaire

magnétique (DCM) de films de YIG entre 19 000 et 33 000 cm-1. Nos résultats sont analyses ^à l’aide d’un modele à deux sous-réseaux dont les contributions sont additives. Nous montrons ainsi que les transitions optiques ^du

YIG mettent en jeu simultanément des excitations sur les deux sous-réseaux. Par ailleurs, ^nos spectres ^{de DCM} permettent ^{de localiser avec} precision des bandes non observées dans le spectre d’absorption.

Abstract.

This paper presents ^the temperature dependence (300 2014 ^1.8 K) ^{of the} magnetic circular dichroism

(MCD) spectrum of YIG in the 19 000-33 000 cm-1 _{energy range.} MCD data are analysed ^{in terms of a model}

of two sublattices with additive contributions. This analysis provides strong support ^{to the current} picture ^that optical transitions involve simultaneous excitations on the two sublattices. Furthermore, ^{our MCD} data enable several new excited states to be located accurately.

1. Introduction.

Since the magnetooptic ^Kerr

effect measurements on Y3Fes012 (YIG) by ^Kahn

et al. [1] much work has been done in the investigation

of the magnetooptical spectrum ^{of this} compound

in the near ultra-violet spectral region. However, with only ^{a few} exceptions [2, 3], ^these experiments

were limited to room temperature [4]. ^{The aim of}

these investigations ^was essentially ^to explain ^the origin ^{of the} magnetooptic ^{effects and the influence}

of various substitutions on their enhancement. It

was also expected ^{that the} magnetooptical spectra ⁱⁿ this spectral region (where ^{the Fe3 + states are} strongly perturbed by surrounding ions) ^would provide ^a

better insight ^{into the nature of the} exchange coupling

which gives ^{rise to the} ferrimagnetic ordering ^below

the Néel temperature, i.e., ^{553 K.}

The purpose of this _paper is twofold : (i) present the magnetic ^{circular dichroism} (MCD) spectra ^of

YIG measured at various temperatures ^and explain

them in the frame of the model of two lattice contri- butions ; (ii) ^{shed some} light ^{on the} intensity gaining

mechanism of optical transitions in this material.

2. Experimental.

YIG thin films were prepared by liquid phase epitaxy ^on Gd3GaS012 ^substrates

with a (110) orientation. Their MCD spectra ^were measured between 19 000 and 33 000 cm-1 ^at tempe-

ratures ranging ^{from 1.8 to 300 K in a} magnetic

field of 0.7 tesla. All our spectra ^were taken with a

12 A bandwidth. We checked at 1.8 K that our data

were unafi’ected by ^a reduction of the bandwidth down to 2 Á. Figure ^{1 shows our MCD data} (Da)

at various temperatures ^{between 10 K and 300 K.}

Weak lines originating ^{from the substrate} (around

32 000 cm -1 ) ^were removed from these data. A spectrum ^{taken at 34 K has not} been included since it differed only negligibly ^{from that at 10 K. Da is} simply ^{related to the measured} quantity ^{AA via the} relationship ^Aoa

AA/log e ^{x 1} (1, sample ^thickness éxpressed ⁱⁿ cm). Here, dA stands for the difi’erential absorbance between left and right circularly polarized lights respectively.

We also measured the 1.6 K spectrum. ^{It shows} little difference with respect ^{to that at 10 K and is} therefore not reported ^here. Actually, ^{in view of our} past experience ^{of these} materials, ^we believe that the slight changes ^{observed when} comparing ^the

data at 1.6 K and 10 K, ^{are due to} the fact that we

used samples which have been prepared separately,

(*) On leave from the Institute of Physics, ^Charles University,

12116 Prague 2, Czechoslovakia.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0197900400107300

74

Fig. ^1.

^MCD spectra ^at saturation of a YIG film 850 A thick at various temperatures.

e.g., they might ^{contain different amounts} of residual lead. This is substantiated by ^a comparison ^{of the}

data in references [2] ^and [3].

In thé case of high absorbance (A > 1.2), ^the light

scattered by the monochromator _may cause a reduc- tion of Aa. However, ^{even for the} peaks ^{around 30 000}

and 32 000 cm -1 (Fig. 1), this reduction cannot reach more than a few percent ^{and does not lead to}

any displacement ^{of the} peaks. Altogether, ^{the rela-}

tive amplitudes ^{of these at different} temperatures ^and for a given sample ^{are estimated to be correct within}

1 %. 0

The physically important quantity is the MCD _per unit length (Ax). ^It is therefore _necessary to deter- mine 1 precisely. ^{This was done} by ^two independent

methods.

First, ^we compared ^the sample absorbance A

(taken ^{on a} Cary ¹⁷ instrument) and its extinction index k measured previously [5]. ^We recall that k and A are related via the relationship klA = 1/4 ^nla log e

where a is the wave number expressed ^{in cm -1. The}

measurement of A was performed ^{in a} spectral region

where 0.7 A 1 in order to avoid complications

due to interference within the film. Moreover, ^reflec- tion losses have been fully considered. We obtained 1 = (85 ± 6) ^{10-’ cm.}

Second, ^we compared ^our sample ^MCD (Ax) ^to

the data derived from the measurement of the Kerr rotation (QK)’ ^{the Kerr} ellipticity (03C8K), and the refrac- tion and extinction indices (n ^and k). ^{This was done} by using ^the following expressions :

e’ ^and G1 ^{are the real and} imaginary parts ^{of the}

off-diagonal element Gl ^{of the} permittivity ^tensor.

Both ({JK and 03C8K ^are expressed ^{in radians.}

The above equations ^{are valid for} any value of n and k. When k is small however (e.g., ^k 0.3), ^a great simplification ^{occurs since one then has :}

({JK is known to be (155 :t 3) 10- 5 rad. at 23 700 cm-l

and 300 K [5-7]. ^The corresponding n ^{has been}

determined by several authors to be 2.40 [1], ^2.52 [5]

and 2.64 [8]. ^{It seems} therefore reasonable to adopt

This leads to Da

(2.47 ± 0.34) 101 cm-1. Since

we determined AA at 23 700 cm -1 and 300 K as

(88 ± 5) 104, ^{one finds 1}

(82 ± 18) ^10-’ cm, ^a result remarkably consistent with that already

obtained. The value 1 = 85 ^x 10-7 ^cm has been retained for the forthcoming calculation.

Let us now briefly summarize the main features of the observed spectra. ^On figure 1, ^the positions ^of

the peaks change only slightly ^with temperature, those at 20 700 (b) and 32 000 cm -1 (d) shifting

towards low energies ^when temperature increases. So does the minimum in the amplitude ^{at 24 600 cm-1} (i).

The peak ^{at 22 400 cm-1} (h) falls faster with increasing temperature than do the others, ^{while the} amplitude

of the peak ^{at 27 600 cm-1} (j) ^remains practically

constant when the temperature ^is changed.

3. Discussion.

Fe3 + ions in YIG as well as in

a number of ferrimagnetic spinels ^or diamagnetically

substituted garnets, occupy both octahedral (A) ^and

tetrahedral (B) sites. There are several reasons to

believe that it is the number of iron ions in each site of a given compound ^which plays ^{the most} important

role as far as its magnetooptical properties ^are concerned, ^the specific ^structure of the material

being ^{of less} importance. ^For example, ^the polar

Kerr rotation data on Y, ^{Eu and Er iron} garnets [1]

present very striking analogies. Similarly, ^{the near IR} Faraday ^rotation spectra of YIG and magnetic

oxides with the spinel ^structure [14] ^are closely ^corre-

lated. These facts probably originate in the fact that the _oxygen surrounding ^{Fe3 + ions in} Y3Fes012 ^and

e.g., Lio._5Fe2.-504, ^have approximately ^{the same}

dimensions and symmetry, ^both exhibiting ^{a similar}

geometry ^{of the} Fe3 +-02 --Fe3 + units and exchange paths.

It is therefore not surprising ^{that a number of}

workers [9-15] ^have provided ^{a tentative} interpreta-

tion of their measurements by assuming ^{that each}

where T is the absolute temperature ^{and :}

nA and _nB stand for the number of Fe3 + _{ions per} cm3 in the octahedral and tetrahedral sites respectively ;

MA(T)/I MA(0) ) ^and MB(T)/I MB(O) ^{are the ratios}

of the octahedral and tetrahedral sublattice moments at the temperature ^{T to} the absolute values of the

corresponding moments at zero Kelvin. The values of MA ^and MB should be taken with the appropriate algebraic sign in accordance with the orientation of the sublattice moment in the applied magnetic

field. Eq. (5) implicitely ^{assumes that the role of the}

external field is only ^to orient the domains. Actually,

this is fully justified ^{since we} checked that the MCD

quickly ^saturates upon increasing B and ^{is then field} independent up ^to about 4 teslas.

Our goal ^is twofold here : (i) ^{show that} eq. (5)

may ^{serve to} explain ^the temperature dependence ^of

our MCD spectra ; (ii) get ^some insight ^{into the} microscopic origin ^{of the} absorption and, thus, MCD of YIG and related substances, by comparing E 1 A(Q) ^and 81B(U)’

The determination of both _ElA and _GIB requires ^a comparison ^{of the} properties of two materials contain-

ing ^{different amounts of Fe3 + ions} in sites A and B.

We have chosen YIG and lithium ferrite since the ratio of the iron concentration in B and A sites increases by ^{a factor of 2.25} from the former to the latter. Keeping ^{in mind} that E1A and _81B are complex quantities, ^{we need a set of four} equations ^{to deter-}

mine EIA, eïA, EiB ^and E1B ^{at 300 K} through eq. (5).

The necessary experimental information originates

as follows : (i) 81 ^and E i ^{for LF are} taken from refe-

rence [16] ; (ii) e1,’ ¹ and e" 1 ^{for YIG are those of refe-}

rence [7] ^{for Q} > 23 000 cm-1, ^{while at lower} energies E1 ^was determined from our MCD data via the rela-

tionship eï ^{= - n} Aa/nu [17] ; (iii) ^the ratios of the sublattice moments have been deduced from the data of references [18] ^and [19] for YIG and LF respecti- vely. Our derivations for e’1A, E 1 A, e’lB ^and EïB ^are given ⁱⁿ figures ^{2 and 3.}

ârx(a, T) ^{has then been} computed ^from eqs. (1)

and (5) and is shown in figure ^{4. In} view of our absorp-

tion spectra, ^{we have assumed in} this calculation that k and n were practically temperature independent.

Comparison ^of experimental ^and theoretically ^esti-

mated MCD data usually proceeds ^{via a moment} analysis ^{of the spectra} [20]. ^{In our} situation, ^the normalized zeroth order moment

Fig. ^2.

Spectra of the sublattices components £’lA ^and 8’lB’

Fig. ^3.

Spectra of the sublattices components e1A’,, ^and E’lB’

has been retained as a significant parameter. ^Measured (Fig. 1) and estimated (Fig. 4) Mo ^{values are shown}

in table I. The agreement ^{between the two sets of} figures appears satisfactory and is taken to indicate that the phenomenological ^model employed ^{is indeed}

reasonable. This is further supported by ^{the fact} that,

as observed experimentally, ^the peak at ^{22 400 cm -1} (h) (in Fig. 4) ^increases significantly ^{more than that at}

20 700 cm-1 (b) upon cooling.

A closer examination of figures ^{1 and 4 indicates} significant differences _among the details of the

l’able I.

Relative normalized zeroth order moment

of ^our experimental and calculated MCD curves versus temperature. Both have been taken equal ^{to 1}

at 300 K so as to facilitate comparison.

76

Fig. ^4.

Calculated MCD spectra ^{of YIG at various} temperatures.

spectra. ^For example, the MCD for peaks ^{a, c,} f, j

and _g in figure ^{1 does not} completely follow the

predictions ^{shown in} figure 4. This is expected ^since dipole strengths corresponding ^to individual lines are, ^{as we} verified, practically temperature indepen- dent, ^whereas _81A ^and _E1B ^do vary due to the change

in line widths. The effect of such a modification on

the MCD is illustrated in figure ^{5a in the case of}

two bands P and Q separated by, e.g., 60 arbitrary

units in energy. It is assumed that their band width goes from 57 to 45 units _upon cooling, ^while magne- tization increases by ^a factor 1.16. The resulting ^MCD presents ^a spectral dependence ^{similar to that noted}

in region ^{a-c of} figure ^1.

Peaks f and _g in figure ^{1 deserve} special ^attention

since the MCD decreases upon cooling, ^{in contra-}

diction with our predictions ⁱⁿ figure ^{4. The} expla-

nation rests upon ^our previous observation [2] ^of

two magnetic ^{linear dichroism} peaks precisely ^at

26 900 cm-1 and 28 700 cm-1, ^these being ^asso-

ciated with hidden absorption components. ^There

are corresponding positive ^MCD peaks ^{at these} energies ^but they ^are also hidden under the negative

contributions due to neighbouring optical transitions.

This effect is illustrated in figure 5b which resembles

figure 5a, except ^{that a third} positive peak ^R (2.5 ^times

less intense than P and Q ^{but the same} width) ^has

Fig. ^5.

^Scheme illustrating ^{the MCD at 300 K} (- - -) ^{and 0 K} ( ) resulting ^{from :} a) ^two negative contributions (P ^and Q) ; b) ^two negative (P-Q) ^{and one} positive (R) components. ^{All units are} arbitrary.

been added between the two major negative compo- nents. The resulting temperature dependence ^{of the}

MCD closely ^{follows that observed} experimentally.

The two sublattice model therefore provides ^{a satis-} factory phenomenological ^{account for our} experi-

mental data.

Turning ^{now to the} microscopic consequences of this study, ^the important conclusion is that _E1A and _Sis (Figs. ^{2 and} 3) ^{have an} almost similar spectral dependence, i.e., peaks appear ^at the same energy..

This demonstrates that absorption ^{bands cannot be}

correlated to only ^{one or} the other of the two ions.

Actually, they ^do necessarily ^involve simultaneous excitations on the two sublattices, e.g., exciton- magnon and exciton-magnon-phonon transitions. Our

study ^therefore strongly supports ^{current views} [3, 21, 22], regarding ^the interpretation ^{of the} magneto-

optical properties ^of YIG and related ferrimagnetic

materials.

Acknowledgments.

^{One of us} (S.V.) ^{would like}

to thank C.I.E.S., France, for financial assistance to

support ^his study. ^{We also} acknowledge ^useful

comments on our manuscript, from J. Ferré and R. H. Petit.

References

[1] KAHN, ^F. J., PERSHAN, ^P. S., REMEIKA, ^J. P., Phys. ^{Rev. 186} (1969) 891.

[2] CANIT, ^J. C., BADOZ, J., BRIAT, B., KRISHNAN, R., Solid State Commun. 15 (1974) ^767.

[3] SCOTT, ^G. B., LACKLISON, ^D. E., RALPH, H. I., PAGE, ^J. L., Phys. ^{Rev. B 12} (1975) ^2562.

[4] ^{For a review see} WETTLING, W., ^J. Magn. Magn. ^{Mater. 3}

(1976) 147.

[8] WEMPLE, ^S. H., BLANK, ^S. L., SEMAN, ^J. A., BIOLSI, ^W. A., Phys. ^{Rev. B 9} (1974) ^2134.

[9] ^{MATTHEWS, H.,} SINGH, S., LE CRAW, R. C., Appl. Phys. ^{Lett. 7} (1965) 165.

[10] CROSSLEY, W. A., COOPER, ^R. W., PAGE, ^J. L., VAN STA- PELE, R. P., Phys. ^{Rev. 181} (1969) ^896.

[11] ABULAFYA, G., LE GALL, H., Solid State Commun. 11 (1972)

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[12] VISNOVSKY, S., PROSSER, V., ZVARA, M., POLIVKA, P., Phys.

Status Solidi (a) ²⁶ (1974) ^513.

[13] KRISHNAN, R., ^{TRAN KHANH} VIEN, CANIT, ^J. C., ^VISNOV-

SKY, S., IEEE Trans. Magn. ^MAG-13 (1977) ^1577.

[17] In view of our own 0394A and Kerr effect measurements below 23 000 cm-1 we believe that the results in reference [7]

are in serious quantitative ^error (~ ⁵⁰ %) ^{for this} region.

[18] LITSTER, ^J. D., BENEDEK, ^G. B., ^J. Appl. Phys. ³⁷ (1966) ^1320.

[19] PRINCE, E., ^J. Physique ²⁵ (1964) ^509.

[20] BRIAT, ^B. in Electronic States of Inorganic Compounds : ^New Experimental Techniques, ^edited by ^P. Day, ^{D. Reidel} (Dordrecht, Holland) ^1975.

[21] SCOTT, ^G. B., PAGE, ^J. L., Phys. Status Solidi (b) ⁷⁹ (1977)

203.

[22] ANDLAUER, B., SCHNEIDER, J., WETTLING, W., Appl. Phys. ¹⁰

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