PIEZOTRANSMISSION BEHAVIOR OF FERROMAGNETIC EuO UNDER MAGNETIC FIELD

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PIEZOTRANSMISSION BEHAVIOR OF

FERROMAGNETIC EuO UNDER MAGNETIC FIELD

J. Lascaray, J. Nadai, J. Diouri, M. Averous

To cite this version:

J. Lascaray, J. Nadai, J. Diouri, M. Averous. PIEZOTRANSMISSION BEHAVIOR OF FERRO-

MAGNETIC EuO UNDER MAGNETIC FIELD. Journal de Physique Colloques, 1980, 41 (C5),

pp.C5-47-C5-50. �10.1051/jphyscol:1980509�. �jpa-00219944�

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?IEZOTRANSMISSION BEHAVIOR OF FERROMAGNETIC EuO UNDER MAGNETIC FIELD J.P. Lascaray, J.P. Nadai, J. Diouri and M. Averous

Centre d'Etudes dlEZectronique des Solides, Universite' des Sciences e t Techniques du Languedoc, P I . E . Bataillon, 34060 MontpeZZier, France.

R6sumb.- Dans un prscsdent article, nous avons d6crit le comportement du signal de pi'ezotransmission de l'oxyde dqEuropium lorsque la temperature varie. Ce signal, A Tr/Tr, qui est directement relie au coefficient de pression de la transition optique observee change son signe 3 la temperature de Curie2 Dans la region ferro- magn6tique la comparaison des mesures d'absorption et de piezotransmission met en evidence deux contributions du spectre "d'absorption apparente". Un faible champ magnetique supprime l'une d'elles. Les s?ectres de pi6zotransmission ont alors la mEme allure dans la region ferromagngtioue H > B et dans la region paramagnstique.

Cette forme est caracfgristique d'une transitionsban& a bande. Les spectres d'absorption et de piezotransmission sont fortement influencgs dans la region ferromagnGtique, par la diffusion de la lurniGre due aux domaines magnstiques et doivent donc &re analysgs avec attention.

Abstract.- It was reported in a previous paper that piezotransmission signal (ATrPr) changes its sign at the Curie temperature T

.

It can be shown that the piezotransmission signal is directly related to the p%essure coefficient of the optical tran-

sition observed (4f 5d). Transmission and piezotransmission measurements have been carried out and in the ferromagnetic state on other contribution to the "apparent optical absorption" appears. Under a small magnetic field this other contribution disappears. Indeed the piezotransmission spectra have the same shape in the

ferromagnetic state under a magnetic field than in the p a r ~ g n e t i c state.a-ds shape is characteristic of a band to band transition.The piezotransmission and optical absorption in the ferromagnetic range are thus strongly influenced by light diffusion by domains and must be carefully analysed.

Introduction.- A critical behavior of piezo- transmission and a sign inversion at the Curie Temperature has been observed in Europium oxide /I/. In the paramagnetic range the change of the piezotransmission coefficient could be correlated to the temperature dependence of the exchange contribution to the (f-d) energy interval through the spin correlation function (1)

.

In the ferromagnetic range the observed anomaly could not be attributed to this simple phenomenon.In order to understand the shape of the piezotransmission spectrum in the whole temperature range, the effect of the magnetic field in the ferromagnetic region has been investigated.

Experimental Results

.-

The stress nodulation technique, like all modulation spectroscopy, permits to obtain the derivative of the absorption spectrum under the condition that the studied transition is stress depen- dent.

The modulated transmission ATr and the piezotransmission

- iF

can be related to

the pressure coefficient of the observed transition dE/dP by the following expres- sions :

dE d Tr dE

ATr=Tr (Ei-AP) _dp -Tr (E) =- _dE

-

_{d P}^Ap A Tr da dE

and

-

Tr = -1

-

dE dp

-

Ap

where : E the incident photon energy a the absorption coefficient 1 the sample thickness assuming that Tr = exp (-a1)

The generally observed quantity is ATr/Tr since it is related to da/dE which characterizes the observed transition / 2 / .

If the absorption spectrum represents seve- ral contributions, the problem becomes more complex ; particularly, it is of interest, to examine the effects of the magnetic field and temperature on the spectrum in order to separate the different contributions.

It is* assumed in a first approximaticn that the absorption edge in Europium oxide, moves without deformation toward lower energies when the temperature decreases /3/.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980509

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JOURNAL DE PHYSIQUE

This suggeststhat the amplitude evolution of ATr/Tr is identical to the one of dE/dp.

~t is of interest to examine the temperature dependence of the inflexion point a(E) which corresponds to the maximum of ATr/Tr.

However examining figure 1, it can be seen that this is equivalent to examine the temperature dependence of ATr permitting

the determination of dE/dp at the inflexion point of transmission spectrum.

Fig.1 : Typical spectra in EuO in paramagnetic range. la) Transmission spectra. lb) Elodulated part of transmission. lc) Piezo- transmission spectra.

Figure 2 shows transmission and modulated transmission ATr spectra as a fonc- tion of the photon energy for various magnetic field intensity. When an increasing magnetic field parallel to the sample plane, is applied the followings can be observed :

There is a large increasing of ATr under weak magnetic field which reaches a maxi-

mum at Hm and after it decreases with a sign inversion. Furthermore ATr reaches a saturation at a magnetic field intensity Hs. It can be seen that for H smaller than Hs, ATr is different from d Tr/dE. When H is larger than Hs, ATr is proportional to d Tr/dE as it is the case in the paramagnetic region without magnetic field (Fig .l)

.

The observed phonomenon without magnetic field is complex and does not follow eq.(l).

If H > Hs the well known transition 4f+5d is observed.

Fig.2 : Full line : transmission spectra on EuO at 15K for various magnetic field intensities. Dotted line : piezotransmission spectra in same conditions.

The piezotransmission spectra have the same shape on figure 3 as well in the paramagnetic range without magnetic field as in the ferromagnetic range with gn applied magnetic field ( H > Hs)

.

On figure 4, the optical density and piezotransmission spectra with and without magnetic field can be compared. The two linear parts, AB and BC of the OD spectrum (Fig .4) without magnetic field indicate two contributions.

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of two subbands /4/ (5d-6s) or to the split- ting of the 5d t level in the case of

2g

decreasinq temperature. However, the very small value of the applied field make these hypothesis not very convincing. Another 1 tentative explanation can be proposed, based

on the well known phenomenon which is the 4

2 0

liqht diffusion induced by the magnetic

116 K

-

_F"^X.

-/-

a'

P

-

+.+-+.+-

I I I

-

Fig .3 : Piezotransmission spectra Full line : domains in ferromagnetic tenperature range.

for T > T ; H=O, dotted line : for T<Tc;

H=3500 0 g . The domains light diffusion gives an

0.8 0.9 1

"apparent absorption" represented by the AB

O.D.

c ,o'

- A

Tr

Eu 0 4 6 K Tr

Fig.4 : Optical density spectra (0, 0 ) and respectively Piezotransmission spectra

(a ,

I) for H=O aad H=3500 Oe.

The ATr/Tr due to BC is positive, whereas the ATr/Tr due to AB is negative

.

^{The maxi-}

mum of the ATr/Tr corresponds to the transition betvieen these two contributions and not to the inflexion point of OD. When a magnetic field H > Hs is applied the OD

spectrum shows that the contribution indi- cated by BC becomes predominant, and the corresponding ATr/Tr is always positive as

part in the optical density spectrum without magnetic field. When H > Hs, there is only

a single domain (ferromagnetic) and this apparent absorption disappears..

In order to verify this idea, the dependence of magnetic moment on the magnetic field intensity at various temperature

(see Fig .S) is examined

.

Fig.5 : Magnetization curves for different temperatures.

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JOURNAL DE PHYSIQUE

It can be seen in this figure that the moments of the magnetic domains are oriented at a magnetic field intensity FI = Hs as defined above. Therefore it can be conclude5 that the sign inversion of ATr/Tr observed in the ferromagnetic range is due to the pressure effect on the magnetic domains /5/.

Conclusion.- A detailed analysis of the optical and piezotransmission spectra as a function of magnetic field intensity and temperature permitsto explain the anomaly of the piezotransmission spectra in the low temperature range. The effect of the pressure on the magnetic domains has been investigated. The pressure dependence of the 4f- 5d piezotransmission could be followed in all temperature range, when the contribution of the magnetic domains is removed by a magnetic field.

References

/1/ Lascaray, J .P., Merle, P., Mathieu, EL, Averous, M

.,

Leroux-Hugon, P

.,

^Phys.

Rev. B> (1977) 358.

/ 2 / Semiconductors and semimetals,

(Academic Press, Kew York, London) , Vol.9 197 2.

/3/ Freiser, M.S

.,

Holtzberg, F.,

Methfessel, S., Pettit, G.D., Shafer, M.W., Suits, J .C., Helv. Phys. Acta.

41 (1968) 832.

-

/4/ Lascaray, J .P., Merle, P., Kadai, J .P.

Segui, F

. ,

IEEE Trans

.

^Plagn

. 14

⁽¹⁹⁷⁸⁾

4 5 1 .

/5/ Lascaray, J .P

.,

^ThSse d'Etat, 1979, Montpellier

.

Acknowledgments

.-

^The*authors are grateful to Mr. Desportes Porteseil and Barbara for their contribution.