Reflectivity spectra of Eu rich EuO near the absorption edge

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Reflectivity spectra of Eu rich EuO near the absorption edge

M. Escorne, A. Mauger, C. Godart, J.C. Achard

To cite this version:

M. Escorne, A. Mauger, C. Godart, J.C. Achard. Reflectivity spectra of Eu rich EuO near the absorption edge. Journal de Physique, 1979, 40 (3), pp.315-319. �10.1051/jphys:01979004003031500�.

�jpa-00209111�

Reflectivity spectra ^{of Eu rich EuO}

^near

^the absorption edge

M. Escorne, ^A. Mauger

Laboratoire de Physique ^du Solide, C.N.R.S., 1, p1. A.-Briand, 92190 Bellevue Meudon, France

C. Godart and J. C. Achard

Equipe ^{de Chimie} Métallurgique ^et Spectroscopie des Terres Rares, C.N.R.S., 1, p1. A.-Briand, 92190 Bellevue Meudon, France

(Reçu ^{le 16 août} 1978, ^{révisé le 30 octobre} 1978, accepté ^{le 9 novembre} 1978)

Résumé. ²⁰¹⁴ Les spectres de reflectivité sous incidence quasi normale d’un monocristal de EuO riche en Eu mais voisin de la st0153chiométrie ont été mesurés dans le domaine spectral (1,5-4 03BC) ^et dans le domaine de température (4,2-300 K) ^en configuration Voigt. ^Deux pics négatifs ^sont observés à 1,85 ^et 2,20 03BC ^en très bon accord avec l’em-

placement ^des pics d’absorption observés dans ce type de matériaux. Ces pics ^sont attribués dans la littérature soit à des transitions optiques d’un électron lié à la lacune d’oxygène, soit à des transitions de centres F formés

sur la lacune. Nos mesures révèlent _que ces pics ^{ont une faible} amplitude ^et présentent ^une très faible dépendance

de la position ^en énergie ^des pics négatifs ^{avec la} température, ^{dans tout} le domaine (4,2-300 K). ^Ceci plaide ^en

faveur de la première interprétation quand le matériau est isolant, ^{et d’une autre} interprétation ^{dans la} configu-

ration métallique, ^{à savoir une} transition interbande entre sous-bandes de spins opposés. ^A 4,2 K, ^deux pics supplémentaires apparaissent ^{à 1,74} 03BC ^et 2 _03BC. Le champ magnétique ^{30 kG n’affecte} pas la position ^{des différents} pics, mais affecte beaucoup ^leurs profils.

Abstract. ²⁰¹⁴ The reflectivity spectra ^{at near} normal incidence in the spectral range 1.5 03BCm ^to 4 _03BCm of EuO single crystals slightly rich in Eu but almost stoichiometric have been measured in the temperature range 4.2 to 300 K, using ^the Voigt configuration. ^Two dips ^{are observed at} 1.85 and 2.20 _03BCm in fairly good agreement with the location of the two absorption peaks observed in similar materials. These peaks ^are ascribed in the literature either to optical

transitions of an electron bound to an oxygen vacancy ^{or to} transitions into F centres formed at the vacancies.

Our measurements show a very small temperature dependence of the energy position ^{of the} dips in the whole range [4.2-300 K] ^{and a small} intensity ^{of the} dips, which is in favour of the first interpretation ^when the material is an insulator, and of an alternative one in the metallic configuration, ^{i.e. an} interband transition between _up and down spin subbands. At 4.2 K, two extra dips appear ^at 1.74 and 2 _03BCm. The magnetic field up ^to 30 kG does not affect the energy position of the different dips, ^but greatly affects their shape.

Classification Physics ^Abstracts

78.20L - 71.30

1. Introduction.

- Europium

oxide has been the

subject of extensive studies since the

discovery

^that

an insulator metal transition (IMT) ^{occurs at}

T L-- 50 K for Eu rich

samples

[1]. ^{The mechanism}

of this transition is that one of the electrons trapped

at the _oxygen vacancies delocalizes when the material becomes

ferromagnetic,

^{the Curie} temperature

being

Tc ^{69 K. At}

high

temperatures, ^however (T > 50 K),

this electron is bound on the oxygen vacancy and

polarizes

^the

spins

^of

surrounding

^{Eu atoms to form}

a bound

magnetic polaron

(BMP) [2, 3].

The

simplest

^{way to} demonstrate the IMT is to

measure the resistivity ^{which is found to decrease} by several orders of

magnitude

^{below 50 K, for} oxygen deficiencies estimated to be near 0.05 % [3].

Absorption

spectra ^{of such}

samples

^{at room} temperature [3, 4, 5]

show two

absorption

^{lines at 1.85 and} 2.20 J.1m, which are not present ⁱⁿ stoichiometric

samples.

It has. been

suggested

[4] that these two lines are due to an exchange ^{resonance of the outer} electron of the vacancy and the

aligned

^4f

spins

^{of Eu atoms near}

the vacancy. The resonance should be

split

^{into a}

doublet by ^the

spin

^orbit interaction. In this model,

at T 50 K, ^these lines should vanish with the delo- calization of the electron. However, Helten et al. [6]

have measured the

absorption

spectra ^{in the whole}

range between 300 K and 4.2 K, and have found that the two

absorption

^{lines are} present ^{at all} tempe- ratures, ^even for T 50 K, ^{and that} they ^{are not}

affected by ^the metal insulator transition. On these

grounds, ^these authors assumed that the

absorption

lines should be attributed to transitions into F+

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01979004003031500

316

centres formed at the oxygen vacancies. In this _paper,

we intend to

provide

^{an answer to the} actual debate

on the nature of the structures at 1.85 and 2.20 _gm.

With this aim, ^{we studied and} report the detailed

reflectivity

spectra ^{of Eu rich EuO}

single crystals

in the temperature range 4.2 ^to 300 K. Our

sample

is nearer to stoichiometry than those of reference [6],

and

previous reflectivity

measurements in this spectral

range have been

performed only

^{at room} tempe-

rature [5]. ^{Our work} may hence be considered as a

complement

^{to the}

experimental

^{data on} EuO, and in

particular

^{to the}

study

^{of the}

absorption

spectra ^made by Helten et al. [6]. ^An

advantage

^of

the

reflectivity

measurements is that it is a convenient

means to

study

^the

optical properties

of the material at low temperatures (T ⁵⁰ K), ^{where the} absorp-

tion

by

free carriers makes _any

absorption

coefficient measurement difficult, ^{if not}

impossible.

2. Magnetooptic apparatus. ^{- The}

drawing

^{of the}

cryostat is shown in

figure

^{1. The main}

liquid

^helium

container (B) ^of

capacity

^{5 1 feeds the container} (A)

through

^a tap. This container (A), ^{has a}

capacity

of 1 1 at one end of which are the

sample

(L), Ge, ^C

and Pt thermometers, ^{and an Au} (0.03 % Fe) ^chromel

thermocouple

^{with a 200 Q} resistance for the elec-

Fig. ^{l. -} Cryostat ^{used for} magnetooptics. ^{Letters arc referred}

in the text.

tronic temperature

regulation.

^The

optical

^beam

enters with an

angle

^{60 on a}

sample

^{5 mm}

high

^and

3 mm wide. Four tubes (D) ^{are connected at the}

container (B) with wood’s metal solder (R) ^{with a}

chamber enclosing ^a superconducting Helmotz coil (H)

wound around forms (V) ^of dural, ^{i.e. Al} (4 % Cu) alloys. Containers (E) ^and (M) ^are of brass. An

« 0

»-ring

(N) ^at their ends prevents wood’s metal at (T) ^from

flowing along

^the

superconducting

^coil

and

shortcircuiting

^it. (E) ^and (M) ^{are soldered} by

wood’s metal to a cross machined from _copper with two tubes (J) ^and (P). ^{The tube} (J) ^{is soldered}

to a cap

allowing

^the

opening

^and

shutting

^{of the}

containers at (I).

CaF2

^windows (K)

lay on

^the

exterior

envelope

(G). ^{The coil} (H) ⁱⁿ

parallel

^with

a 1 Q shunt (U) ^is

supplied

^{with current from ter-}

minals (F). ^A teflon buffer

ring

(0) ^{serves as a}

guide

for the two chambers

containing

the coil. The screen

for

nitrogen liquid

(C) ^{and its} cap (S) ^{are of Al.}

The whole _space (Q) ^{is in vacuum. This} apparatus allows

experiments

^{between 2 K and 300} K, ^{and a} temperature

regulation

^{with a} drift of ± 0.05 K/h

for T 10 K, ± 0.1 K/h for 10 T 77 K and

± ¹ K/h ^{at T} > 77 K. The maximum magnetic

field available at the

sample

^{is 30 kG. 20 1 of}

liquid

helium are needed to cool the sample ^{to 4.2 K. Then}

the loss is 11

per

^{hour. The}

reflectivity

measurements at near normal incidence were made

using

^{a Perkin}

Elmer model 112 spectrometer

equipped

^{with a}

CaF2 prism.

^The

light

^{source was a}

globar

^rod.

A standard front surface aluminum mirror was

used for reference.

3. Experiments. ^{- The EuO}

sample

^{used has been} prepared by ^usual

polishing techniques,

^{and is the}

same as

sample

^{n° 3} of reference [7]. ^The

resistivity

p is shown in

figure

^{2. The dark}

conductivity

^shows

that this

sample

^{is an} insulator, ^the

resistivity being

too

high

^to be measured at

Fig. ^{2. -} Resistivity p ^vs. temperature. ^{At T} > 150 K, p ^{is the} resistivity with and without illumination. The left part ^{at T 150 K} is the resistivity ^{when the} sample is illuminated in the conditions

given ^{in the text.}

However, ^{when the}

sample

^is illuminated

by

^{a laser}

beam of 1 mW power, 6 328 A

wavelength,

^{and 1 mm2}

impact

^{surface, an IMT can be seen, the}

resistivity decreasing

up ^to 5 ^x 1010 Q.cm at low temperature, and

probably

^{to a} lower value since the surface area

of the laser beam and its

penetration depth

^were

neglected

^{in the} determination of _p [7]. ^This

sample

can be considered at the frontier between EuO

samples exhibiting

^{the IMT and}

samples

^which

remain

always

insulators.

Reflectivity

spectra ^{are shown in}

figure

^{3 at}

high

temperatures tT > 50 K). ^All spectra ^{exhibit two}

dips,

^{which, for an} intermediate

température

T ⁼ 129 K are located at 1.85 and _{2.20 gm.} These structures

obviously correspond

^{to the two} absorp-

tion

peaks

^{at the same}

wavelengths.

These lines are

not present in stoichiometric

samples,

^{and their}

presence corroborates the fact that this

sample

^is

Eu rich, _contrary ^{to the} assertion made in reference [7],

that the

sample

^was stoichiometric. This assertion

was based on the fact that the

sample

^{does not exhibit}

the IMT when not illuminated. This fact, however,

only

proves that the concentration

of oxygen

^vacancies

does not exceed that of the Mott transition. Moreover, the presence of the

dips

^{in the}

reflectivity

spectra ^is

expected

^{to be}

imputed

^{to the} presence of such vacan-

cies,

involving

^{BMP. The interest of this comment}

is that it

questions

the existence of molecular

magnetic polarons

ⁱⁿ EuO,

expected

by Kasuya [8], ^{since it}

Fig. ^{3. -} Reflectivity spectra of Eu-rich EuO at zero magnetic

field and various temperatures, ^{T = 300 K} (left scale), ^{T = 128 K} (left scale), ^{T = 84 K} (right scale) ^{and T = 60.2 K} (right scale).

Vertical arrows point ^{out the} dips.

was assumed so in reference [7] ^on the basis of the

stoichiometry

^{of the}

sample.

The _energy

position

^{of the two}

dips

ⁱⁿ

figure

³

apparently

^{remains constant between room} tempe-

rature and 50 K. However, ^{there is a}

slight

^{blue shift}

of the hollow at 1.85 pm, and a red shift of the

dip

at 2.20 J.1m

by

^{an amount of} 0.10 ym. This is in

good

agreement with the shift of the two

absorption peaks

^with temperature in other Eu rich EuO

single crystals [6,

9]. ^{It is}

important

^to notice that the

magnitude

^{of the}

dips

^is very small, like that of the

absorption peaks

^{on the more Eu rich}

samples

[6].

The different structures between 2.5 and _{3.1 gm}

can be due to

absorption by CO2

^and

H20,

^{and we}

could not get

reproducible

results in this

spectral

range. At such

wavelengths -

2.8 ym ^a

large absorp-

tion

peak

^{has also been observed,} attributed in refe-

rence [6] ^to

europium hydroxide

^{at the}

sample

^surface.

In any case, these structures are not intrinsic _pro-

perties

^{of the} material and do not

require

^further

comment.

Reflectivity

spectra ^{at 4.2 K are shown in}

figure

^4.

The two

previous dips

^{are still} present, ^{and are not} affected by ^the IMT. Moreover two other

dips

appear at 1.74 and 2.00 _pm. These two extra structures

exactly correspond

^{to the two small extra}

peaks

reported by ^{Helten et al. at 5 750 cm - 1 and 5 005 cm - 1}

and the

absorption

spectra ^{of their}

sample

^{n° 59}

at low temperatures. ^It is noticeable that these struc-

Fig. ^{4. -} Reflectivity ^curves of Eu-rich EuO at 4.2 K and various

magnetic fields in a Voigt configuration : ^{H = 0} (left scale),

H ⁼ 4.7 kG (right scale), ^{H = 9.35 kG} (left scale) ^{and H = 27.88 kG} (right scale). ^{The curves at 9.35} and 27.88 kG are plots ^{of the} reflectivity ^R (in %) ^{vs. the} wavelength ^À. However, ^{to avoid} crossing

of the curves, the two other curves are shifted and are plots ^of

R (in %) ⁺ 1 % ^vs. À. Vertical arrows point ^{out the} dips.

318

tures which were not seen on their

sample

46, ^have

an

amplitude

^as

large

^{as the two other structures}

at 1.85 and 2.20 _pm on our nearly stoichiometric

sample. Figure

⁴ also shows the influence of

magnetic

fields _up to 30 kG on the spectra, ^{in the}

Voigt configu-

ration. The _energy

positions

of the four lines are

not

significantly

^affected by ^the

magnetic

^field.

However, ^{there is a} considerable decrease of the

intensity

^{of the}

dips

^at 1.85 ym and 2.20 JlIIl correlated with an

intensity

increase of the

dips

^at 1.74 Nm and 2.00 ym when the field is increased. On the

absorption

spectra however, Helten et al. [6] could observe the lines at 1.85 gm and _{2.20 gm} even at very

high magnetic

fields (50 kG) ^{in the} Faraday

configuration.

^Other- wise, these authors have noticed an increase of the

intensity

^{of the} structure at 1.85 pm correlated with

a

sharpening

of this line when the temperature ^is decreased. We did not observe such a behaviour in our

reflectivity

spectra. ^This

large disagreement

between

absorption

^and

reflectivity

^data

regarding

the evolution of the

shape

^{of the different structures}

either with temperature ^or

magnetic

field, may be due to the too low resolution of the monochromator available _{to us,} which does not allow an accurate determination of the profiles ^{of the} structures. More accurate measurements of the

reflectivity using

^a

spectrometer

equipped

^with

grating

instead of a

prism

^{are needed to} discuss this

point

any further.

4. Discussion. ^- The energy

positions

^{of the}

dips

in our

reflectivity

spectra ^{or in}

absorption

spectra [6, 9]

at 1.85 and _{2.20 ym} and T > 50 K are consistent with both distinct

interpretations

^{found in the lite-}

rature [4, 6]. ^We have found that the location of these structures

only slightly

shifts in _energy, in agreement with references [6] ^and [9]. Lascaray

et al. [9] ^have asserted that it was an evidence for the

validity

^{of the}

interpretation

of reference [4],

according

to which the structures are the result of an exchange

resonance of the outer electron of the vacancy in a

bound

magnetic polaron.

This assertion was rejected by Helten et al. [6], ^who

argued

that any

optical

transition due to a F+ centre has the same property.

The temperature

dependence

^of the energy

position

of the structures is thus also consistent with both

interpretations.

^We have found that the structures

are still present ^{at T 50 K in} agreement ^with reference

[6].

^{It is not sure that our}

sample

^exhibits

a IMT transition in the

experimental

^conditions

used to make the

reflectivity

measurements, since the intensity ^and

wavelength

^{of the}

light

^{are different}

from those used in the

conductivity

measurements.

However,

samples

of reference [6] ^{which deviate}

more

significantly

^from

stoichiometry obviously

^show

that the structures are still present ^{in the metallic}

phase

^of

samples exhibiting

^{the IMT.} This led Helten

et al. to reject ^the

interpretation

of reference [4].

However, ^{we believe that the result}

according

^to

which the IMT does not affect the two structures

at 1.85 and _{2.20 gm} does not

imply

^{as it is} supposed

in reference

[6]

that the electron

responsible

^{for the}

IMT is not

capable

^{of the}

optical

transitions observed.

Let us first consider the BMP

(insulating phase).

According

^{to the usual}

picture,

^{it is a} ferromagnetic

cluster around the oxygen vacancy, and the

spin

^of

the

loosely

bound electron on the _vacancy is

aligned

with the

neighbouring spins

of Eu ions of the cluster.

In the exchange ^resonance model between the electron and

neighbouring

^Eu

spins,

^{the electron}

flips

^its

spin,

and the

exchange

^resonance energy

required

^is

f"OooI 0.6 eV, ^the

optical

transition

being split

^{into a}

doublet

by spin

orbit interaction. In the metallic

configuration,

^{on the contrary, the}

previously weakly

bound electron is in the conduction band which is

spin split owing

^to the indirect

exchange

interaction

superimposed

^{on the}

superexchange

interaction. ^s is the

spin

of the conduction electron,

S.

^the

spin

^of

the Eu atom at site Rn. ^It has been shown

[10]

^that

due to this indirect exchange interaction calculated up ^to the second order [11] ^{with some}

approximations,

the material in the metallic

configuration

^is

always ferromagnetic

^{so that} the electron _{gas is}

completely polarized,

^{for the} electron concentration available

involving

^a metal insulator transition. In other words, only ^the

spin

up subband is

populated,

^{and the}

spins

of the electrons

responsible

^{for the} metal insulator transition are still

parallel

^{to the}

spins

^{of Eu atoms.}

Moreover, ^the exchange energy

required

^{to invert}

the

spin

of these electrons is still

[11] -

^0.6 eV, which is a manifestation of the fact that exchange interactions, ^{even if} they ^{have a} strong ^{effect on} transport properties ^{and on the} localization of electrons, ^have very small effect on the

magnetic properties

^{of the material}

[10]

for such electron concentrations.

This leads us to

give

^a

possible interpretation

^that

the structures observed in the

optical

spectra ^are due to

exchange

^{resonance of the outer} electron which

reverses its

spin

^with respect ^{to the Eu}

spins,

^{of the}

magnetic polaron

^when this electron is localized,

or of the whole crystal ^when this electron is delo- calized. This

interpretation

^can be considered as an

extension of the model

given

in reference [4] ^{to the}

metallic

configuration.

However, ^{the nature of the}

optical

transition is

quite

différent in the

insulating configuration

(transition ^{with an} intraatomic cha-

racter) ^{and in the metallic}

configuration

^(interband

transition between _up and down

spin

subbands).

At this stage ^{of the} discussion, it is rather difficult to decide which

interpretation

is valid. Nevertheless,

the small

intensity

^{of the} structures in both

absorption

and

reflectivity

spectra would be in favour of our

interpretation,

because the

intensity

ought ^{to be}

much stronger ^{in the case of} F+ centres invoked in reference [6].

References

[1] OLIVER, ^M. R., ^{Ph. D.} Thesis, ¹⁹⁷⁰ (unpublished); OLIVER, M. R., DIMMOCK, T. O. and REED, ^J. B., ^{IBM J. Res.}

Develop. 14 (1970) 276 ;

OLIVER, ^M. R., KAFALAS, ^J. A., DIMMOCK, ^{J. O. and} REED, T. B., Phys. ^{Rev. Lett. 24} (1970) ^1064.

[2] TORRANCE, ^J. B., SHAFER, ^{M. W. and} MCGUIRE, ^T. R., Phys.

Rev. Lett. 29 (1972) 1168 ;

LEROUX-HUGON, P., Phys. Rev. Lett. 29 (1972) ^939.

[3] SHAFER, ^M. W., TORRANCE, ^{J. B. and} PENNEY, T., ^J. Phys.

Chem. Solids 33 (1972) ^2251.

[4] TORRANCE, ^J. B., SHAFER, ^{M. W. and} MCGUIRE, ^T. R., ^Abstract in Bull. Amer. Phys. ^{Soc. 17} (1972) ^315.

[5] SCHOENES, ^{J. and} WACHTER, P., Phys. ^{Rev. B 9} (1974) ^3097.

[6] HELTEN, M., GRUNBERG, ^{P. and} ZINN, W., Physica ^89B (1977)

63.

[7] DESFOURS, ^J. P., NADAI, ^J. P., AVEROUS, M., GODART, C., Solid State Commun. 20 (1976) ^691.

[8] KASUYA, T., Solid State Commun. 18 (1976) ^51.

[9] LASCARAY, ^J. P., DESFOURS, ^{J. P. and} AVEROUS, M., Solid State Commun. 19 (1976) ^677.

[10] MAUGER, A., GODART, C., ESCORNE, M., ACHARD, ^J. C., DESFOURS, ^J. P., ^J. Physique ³⁹ (1978) ^1125.

[11] MAUGER, A., Phys. ^Status Solidi 84b (1977) ^761.