Preparation, optical and magneto-optical properties of ytterbium chalcogenide thin films

Solid State Communications,

Vol. 8, pp. 1853—1855, 1970.

Pergamon Press.

Printed in Great Britain

PREPARATION, OPTICAL AND MAGNETO-OPTICAL PROPERTIES OF YTTERBIUM CHALCOGENIDE THIN FILMS R. Suryanarayanan and C. Paparoditis Laboratoire de Magnétisme et de Physique du Solide C.N.R.S. 92 Bellevue France —

—

—

and

J.

Ferre and B. Briat

Laboratoire d’Optique Physique*, EPCI, 10 rue Vauquelin 75 Paris Se France —

—

(Received 9 August 1970 by M. Balkan ski)

Three compounds referred to as YbSe (I), YbSe (II) and YbTe (I) have been prepared as thin films and their optical and magneto-optical properties investigated. They are all stable and crystallise in the NaC1 sturcture. YbSe (II) is found to have an approximate composition YbSe1 ~ and is paramagnetic whereas stoichiometric YbSe (I) and 44f135d(t2g, e ) is proposed the accessible energy levels of Yb2~ in YbSe (I) an8 YbTe (I) are for diamagnetic. A tentative assignment 4f-~ YbTe (I).

THIS PAPER is intended to complete and extend previous work on YbTe and EuTeS to the chalcogenides of the stable divalent rare earth ions. Thin films have been prepared by coevaporation of the elements under vacuum on

5.65 A for YbSe Optical and magnetic circular dichroism (MCD) experiments have been conducted between 16°Kand 300°Kon single crystal films oriented parallel to the (111) face of the cleaved CaF 2 substrates as shown by electron back

heated substrates of pyrex and freshly cleaved

reflection diagrams. The MCD apparatus used in this work has been described elsewhere ~. Due to

~.

CaF2. Three compounds subsequently referred

to as YbSe (I), YbSe (II) and YbTe (I) were obtained. They are coloured green, amber and purple respectively by transmission. YbTe (I) and YbSe (I) are the stoichiometric monochalcogenides while YbSe (II) is richer in selenium: an electron microprobe analysis reveals an approximate composition YbSe1~. No similar composition was obtained in the case of the Yb—Te system. YbSe (I) and YbSe (II) crystallise in the NaCl structure with a 5.929 ±0.002 A and 5.654 ±0.002 A as lattice parameters respectively, These agree closely 2 who foundwith a those 5.931 of A landelli for YbSeand and Palenzona * Equipe de Recherche n°5 du C.N.R.S. =

=

1853

the weak MCD signal of the samples, we have used a 36 kG magnetic field obtained with a superconducting coil. Figure 1 shows the absorption and reflection spectra of the samples at 295°Kbetween 0.5 and 5eV. Reflectance measurements were made on films deposited on pyrex, these films having a [200] preferential growth. Three main absorption bands are observed for YbSe (I) and YbTe (I), located around 1.9, 3 and 4.2 eV. It was observed that the and position of and the 3.05 peakseVat (YbTe) 1.85 and 2.93 eV (YbSe) at 2.05 depends neither on the nature of the substrate nor on the

1854

YTTERBIUM CHALCOGENIDE THIN FILMS

I

The magnetic dichroic optical density {AD] standardized to 1G is shown (4) to be well described by:

WbTe(I) Z~18

Vol. 8, No. 22

ThTQW -~

~

~

!,Iy--

~I “~/

\

v

~—‘

Yb5e(1)

[AD] Dm

~---

L~ I

D÷—lX DmH

=

a~L~+(b+ di-~ \

C\f()

in the ofstand anand isolated absorption where Dm andcase f(~) respectively for of theband maximum optical density shape function the Due to the derivative, a df(i-’)/dv varies asband. l/y

EN~RGY~e~’

FIG. 1. Reflectance and absorption spectra of YbSe (I) and YbTe at room temperature.

where 7 is the half-width of the band. The paramagnetic (c/kT)f(z-’) term arises from the eventual difference in population in the sub-levels of the ground state.

ScxJ

2

400

Between 295 and 30°K no variation is observed

300 A(rvv~

YbSe~rr)

75~k

-

--

whereas the in the MCD absorption the at 3.47 MCDeV, spectrum is trongly for example, ofaffected. YbSecan (II)be Actually (Fig. understood 2),

oc

as due to a (c/kT)f(12) term (with a small bf(z-i)

05

between the extreme temperatures. conclusivel~ contribution) since [ADI varies by aThis factor of 8 demonstrates the paramagnetism of YbSe (II).

-

29~~

1~-i2~ 2

FIG. 2. Optical density D (bottom) and magnetic cir~ulardichroism (top) for YbSe (II) at various temperatures. 500

05

4b0

A(nm) 3à0

The absorption spectrum of YbSe (I) is modified at low temperature (Fig. 3). The peaks located at

‘

YL5e(I)

-Q5~

~

295k ii0’k -is 0.5 2

Z5

3

3.5

Furthermore, the MCD spectrum the3.90 presence of three absorption lines at 2.92,reveals 3.47 and eV since a (c/kT)f(ii) term is necessarely associated with each absorption line. It follows from our spectroscopic work that YbSe (II) does not 2~. We thus feel confident that it contain Yb contains Yb3~essentially, although the presence of selenium with an unknown valency cannot be ruled out.

4

E(eV)

2.93 andincreases, 4.26 eV sharpen andit the maximumtooptical density although is difficult be mor&definite since the peaks superimpose on a very inaccurately known background. MCD peaks [AD]/Dm ratio varies little between 295 and 16°K, around 2.90 eV are likely to arise from bf(v) terms or a cimbination of a and b terms. Since the the presence of (c/kT) f(~i)term is ruled out and our results demonstrate the existence of a dia-

FIG. 3. Optical density D (bottom) and magnetic circular dichroism (top) for YbSe (I) at various temperatures.

magnetic phase for YbSe (I). Essentially similar conclusions have been reached from our study of YbTe (I). The fact that temperature dependent

film thickness. YbSe (II) is transparent up to 3.5 eV (Fig. 2). Figures 2 and 3 show the temperature deperidance of MCD and absorption for these samples.

MCD terms are observed for YbSe (I) around 3.3 and 3.9 eV in Fig. 3 [as in Fig. 2 for YbSe (II)] indicate that -a slight amount of the paramagnetic phase of YbSe might be present in YbSe (I).

Vol. 8, No. 22

YTTERBIUM CHALCOGENIDE THIN FILMS

The diamagnetism of YbSe (I) as well as its

1855

with a crystal field effect due to the difference in

high absorption suggest the assignment )-. 4f ~5d for the electric dipole allowed 2~.The average energy observed transitions of Yb of the 4f ‘35d configuration would then be different from that of the free ion.5 Similar results were obtained in Eu-chalcogenides. The effect of the crystalline field can also be considered tentatively as follows. In an octahedral field, the five-fold

4f ~

tdegeneracy of the d-orbital is lifted and one obtains 29 and e9 with t29 the lowest. Actually we suggest

the extreme levels arising from 4f 13 Sd(t20) and 4f ‘~ Sd (eu) to be located around 1.85 and 4.5 eV respectively. Two arguments support this assignment. Firstly, around 1.85 eV, the MCD peak moves towards lower energies at low temperatures; this might be due to stresses (which modify the crystal field) consistent with Secondly, the presence a 3 5dand (t is) excited level.6 the of energy

4f’ difference between the bands in the visible (~1.9 eV) and in the near UV increases from YbTe to YbSe (I) (Fig. 1). This again is consistent

the ionic radii of selenium and tellurium. In the intermediate region (~3 eV) there is a mixing of t 7 20 and e0 character in the excited states. The fact that the two main bands at 2.93 eV and 4.26 eV are separated by 10.700 cm’ suggest their assignment to the transitions 4f’4(l~)-~ 4f13(~F 7,~ ) 5d(e0) and 2F 8 ~ 13 ( 512 ) 5d(e0) respectively. In conclusion, two phases were obtained in the Yb—Se system, the stoichiometric composition YbSe is diamagnetic (as for YbTe) while YbSe 1 (approximate composition) is paramagnetic.

Acknowledgements — We thank Mr. Delvalle (Laboratoire de Microscopie Electronique, C.N.R.S., 92 — Bellevue — France) for the electron back reflection diagrams and Mr. Rouy for the electron microprobe analysis.

REFERENCES 1.

SURYANARAYANAN R. and PAPARODITIS C., Colloque intern. C .N.R.S. Les éléments des Terres rares — Paris-Grenoble, 5—10 mai 1969, n°l80; BRIAT B., BILLARDON M., SURYANARAYANAN R. and PAPAROL~ITISC., Phys. Status. Solidi 35, 983 (1969).

2.

IANDELLI A. and PALENZONA A., Colloque Intern. C.N.R.S. Les propriétés des dérivés semimétalliques — Paris, 397, (1967).

3.

BILLARDON M., RIVOAL J.C. and BADOZ

4.

BADOZ

J., Rev. Phys. App!. 4, 353 (1969).

J., BILLARDON M., BOCCA.RA A.C. and BRIAT B., Symp. Faraday Soc. 3, 27 (1969).

S.

BRYANT B.W., J. Opt. Soc. Am. 55, 771 (1965).

6.

GUNTHERODT G., SCHOENER

7.

PIPER T.S., BROWN J.P. and Mc CLURE D.S., J. Chem. Phys. 46, 1353 (1967).

8.

DIEKE G.H., Spectra and Energy Levels of Rare Earth ions in Crystals. p.l42. Interscience Publishers, New York (1968).

J. and WACHTER P., J. app!. Phys. 41, 1083, (1970).

Nous avons préparé des couches minces de YbSe (I), YbSe (II) et YbTe (I) et leurs propriétés optiques et magneto optiques ont été étudiées. Les trois composes sont stables et cristallisent dans Ia structure de NaC1. YbSe (II), avant la composition approximative YbSe ~ est paramagnétique tandis que YbSe (I) et YbTe (I) qui ont une composition stoechiométriq ue, sont diamagnetiques. L’assignation 4f14 .~ 4f 13 Sd (t e )YbTe est proposée pour les transitions 2~dans YbSe 20, (I) et (I). observées de Yb