Magnetic and structural properties of iron-based oxide glasses, Fe 2O3-BaO-B2O3, from 57Fe Mössbauer spectroscopy

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Submitted on 1 Jan 1982

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Magnetic and structural properties of iron-based oxide

glasses, Fe 2O3-BaO-B2O3, from 57Fe Mössbauer

spectroscopy

A. Bonnenfant, J.M. Friedt, M. Maurer, J.P. Sanchez

To cite this version:

Magnetic

and structural

_properties

of iron-based oxide

_glasses,

Fe2O3-BaO-B2O3,

from

57Fe

Mössbauer

_spectroscopy

A.

Bonnenfant,

J. M.

Friedt,

M. Maurer and J. P. Sanchez

Centre de Recherches _Nucléaires, 67037 _{Strasbourg Cedex,} France

(Reçu le _{28 janvier 1982,} révisé le 6 mai, accepté le _{7 juin 1982)}

Résumé. 2014 La

configuration

électronique du fer, l’ordre structural à courte distance et les

_{propriétés magnétiques}

de verres de

_composition

_{(Fe2O3)x(BaO)y(B2O3)z}

sont _{établis par}

_{spectroscopie}

Mössbauer de 57Fe en combi-naison avec des mesures

_{macroscopiques.}

La coordinance moyenne du fer est déterminée par le rapport

atomique

Fe/O dans le milieu tandis que la

configuration

de valence du fer

_dépend

_{principalement}

des conditions de

prépa-ration. Un ordre structural à courte distance très strict est mis en évidence autour des atomes de Fe ; il

_correspond

à des fluctuations aléatoires des _angles et

_longueurs

de liaison Fe2014O. A faible concentration en

_Fe2O3,

le verre reste

_{paramagnétique jusqu’à}

_{1,5 K;} la mesure locale de moment révèle

_cependant

la coexistence d’ions isolés et

_{d’agrégats}

à l’échelle

_atomique.

A concentration moyenne en _{Fe2O3 (~} 25

_%),

le verre

_présente

une transition

mictomagnétique.

A concentration élevée en _{Fe2O3 (~ 50 %),} le verre

_présente

un _comportement

quasi-super-paramagnétique,

révélant une

inhomogénéité

de la

répartition

du fer dans le milieu. Les

phases

formées lors de la

cristallisation du verre à concentration moyenne en _Fe2O3 sont _{identifiées par}

_{spectroscopie}

Mössbauer.

Abstract. 2014 57Fe Mössbauer spectroscopy has been combined with bulk results to determine the electronic

configu-ration of iron, the _short-range structural order and the _magnetic

_properties

of _glasses in the _system

(Fe2O3)x(BaO)y(B2O3)z.

The average coordination of the iron atoms is determined _by the _Fe/O atomic ratio while the valence state of Fe seems to be affected

_{essentially by}

the _preparation conditions. Structural _short-range order around Fe atoms is concluded to be very strong, with random

_bonding

_angle and distance fluctuations. At low

Fe2O3 content, the _glass is

_paramagnetic

down to 1.5 K. Local moment measurements reveal the coexistence of isolated Fe ions and of atomic scale clusters. At intermediate _Fe2O3 concentration _(~ 25 _%), the

_glass

_presents a

mictomagnetic

transition. At _{large Fe2O3} content _(~

_50%),

the

_glass

behaves like a _{superparamagnet,}

_indicating

inhomogeneous

distribution of the Fe atoms in the solid. Mössbauer spectroscopy is also

_applied

to the _{investigation} of the _{crystallization} of a _glass with intermediate Fe2O3 concentration.

Classification

Physics Abstracts

61.40D - 75.50K - 76.80

1. Introduction. - The

investigation

of the

struc-tural and

_magnetic

_{properties specific}

to

_amorphous

systems is of much current interest

_[1-3].

The

under-standing

of these

_properties

in

_{insulating compounds}

differs from that in metallic

_{glasses owing}

to dominant

superexchange

interactions.

Therefore,

the

_magnetic

properties

of

_amorphous

insulators will reflect rather

directly

the distributions of

_{bonding angles}

and distances. The atomic structure is best

_approximated

by

a random coordination model with

preferred

cation-anion bonds in a manner which ensures local

charge

neutrality

[1, 4].

It would be

_expected

that these features should

_permit

an easier theoretical

approach

of

_amorphous

insulators as

_compared

with

metals.

Moreover,

there are

only

few

amorphous

materials which

_{display negative exchange}

interac-tions

_leading

to an ordered state with random

_spin

freezing (speromagnetism)

[5].

Considerable work has indeed been devoted

recen-tly

to

_amorphous

insulators

_[1-8].

_Simple

ionic

com-pounds

are

_generally

difficult to obtain in the

amor-phous

state

_[9,10].

Extensive literature exists on

_binary

or _ternary

_oxides,

which are easier to prepare but

usually

cannot be

_compared

to

_{corresponding}

crys-talline _systems. Current interest in

_amorphous

oxide

glasses

primarily

concerns the

following

_{aspects :}

a)

The electronic structure and

_{bonding properties}

of the cations

_present

in the

_glass.

In the case of the

widely investigated

iron

_glasses,

this

_corresponds

more

_precisely

to the determination of the valence

state and the coordination of the Fe

ions,

in

connec-tion with the

_glass

former or

glass

modifier behaviour

[11, 12].

These

_problems

have been

_extensively

studied in _{many systems,} in

_{particular by spectroscopic}

methods,

e.g.,

Mossbauer, NMR, ESR,

etc... In the

Fe203-BaO-B203

glasses

considered

here,

general

trends

_emerging

from

_previous

studies indicate that the coordination of the

Fe3 +

ions is

_governed

_by

the

Fe2o3/BaO

ratio

_(i.e.,

tetrahedral network when

Fe203/BaO

1 and both tetrahedral and octahedral coordination when

Fe203/BaO

>

₁₎

_[13].

For

com-parison,

11B

NMR in alkali-boron oxide

_glasses

demonstrates that tetrahedral coordination of boron is

_preferred

to the 3-coordinate _oxygen

neighbour-hood when the

_{proportion (up}

to 33

_%)

of alkali oxide

increases,

i.e. for

_increasing

_O/B

atomic ratio

[14].

Thus,

the

_metal-oxygen

coordination number is

_{governed by}

the atomic ratio of the elements

pre-sent in the

_glass.

_{The presence of}

Fe2 +

ions in some

glasses

is related to the conditions of

_preparation.

b)

The

_{peculiar magnetic}

transitions and

_orderings

occurring

in such

_glasses

are a _{consequence of}

frus-tration connected with

_topological

disorder

_[5-10].

Speromagnetism

has been demonstrated in a few

insulating amorphous

materials and

_{mictomagnetism}

or

_spin

_glass

transitions have been characterized in a

variety

of concentrated oxide

glasses

[5-7,

15,

16].

c)

Additional

_problems

of interest concern the

possibility

of chemical

short-range

order and

com-positional heterogeneities [17].

In oxide

_glasses

of

composition

(1 -

x) [0.2

BaO - 0.8

_B2O3]xFe2O3

the

occurrence of small iron clusters

_(dimers,

trimers,

etc...)

has been claimed at low Fe concentration

_[18],

while

_{superparamagnetic}

behaviour has been demon-strated at

_high

Fe content.

d)

The

_{crystallization}

_mechanisms,

the

_phase

ana-lysis

and the evolution of the

_{magnetic properties}

on

_{crystallization}

is of intrinsic interest as well as

from the

_point

of view of

_{applications.}

For

_example,

microcrystalline (possibly

monodomain)

precipitates

of barium hexaferrite evolve within a

_non-magnetic

host;

such _{systems may} be considered for

_permanent

magnet

applications [19,

20].

2.

_{Experimental.}

- The

amorphous samples

were

prepared by quenching

from the melt between rollers

as described

previously

[19].

The final

composition

was checked

_by

chemical

_analysis.

The

_{non-crystalline}

nature of the materials was established

_{by X-ray}

and electron diffraction halos as well as from

crystalliza-tion on thermal

annealing

as

_{investigated by}

DTA

and

_X-ray

_analyses.

The evolution of the bulk

_magnetic

properties

on

_{crystallization}

has been

_reported

sepa-rately

[ 19, 20].

The

57Fe

Mossbauer

_experiments

were

_performed

as a function of _temperature between 1.5 and 800 K

using

a

57Co/Rh

source. Some

_experiments

were

performed

at _temperatures between 4.2 and 300 K

in a

longitudinal magnetic

field up to 100 k0e. Thin absorbers

_(2-5

_mg

_Fe/cm’)

were used for

measure-ments in order to minimize

_intensity

saturation effects

_[21].

The data were _computer

analysed

for the

hyperfine

interaction _parameters and their

_possible

distributions;

in the case of non-collinear

_quadrupole

and

_magnetic

interactions,

the calculation of the

spectral shape

is

_performed

_{numerically by}

diagonali-zation of the full nuclear Hamiltonians rather than

using

perturbation

calculations. A

_{comprehensive}

discussion of the method of

_analysis

is

_developed

in

the text.

3.

_Experimental

results on the

_{amorphous samples.}

-

5’ Fe

Mossbauer measurements are

reported

for

amorphous ternary

oxides of nominal molar compo-sitions :

They

are discussed in combination with

_previously

published

[19, 20]

static

_{magnetization}

and

suscep-tibility

results. In

addition,

dynamic susceptibility

measurements were

_performed

on

_sample

A

_{(the only}

one

_presenting

a

_unique

valence state and coordina-tion of the Fe

_cations)

in which the

_magnetic

transi-tion and the nature of the ordered state are

investi-gated

in detail

_[22].

The Mossbauer _spectra of

_sample

A reveal a

broa-dened

_quadrupole

doublet in the _{temperature range} from 670 to 42 K

_(Fig.

_1).

The isomer shift characte-rizes

Fe3+

ion in tetrahedral environment. The

ave-rage

quadrupole splitting (J L)’

isomer shift

(ðIs)

Fig.

1. - Mossbauer

spectra in the _{paramagnetic phase} of

sample A

_{(Fe203)30(ÐaO)4S(Ð203)2S.}

The full line

and resonance width

_{( WL)}

deduced under the

assump-tion of two Lorentzian lines are summarized in table I.

The linewidths are found to be

_equal

for both

compo-nents and are

comparable

to those

_{reported previously}

in other oxide

_glasses

_{[11-13, 15].}

The

_spectra

broaden

sharply

below 42

K,

presenting

complex

shapes

bet-ween 42 and 29 K

(Fig.

2).

Below 29

K,

resolved

magne-Table I. - Isomer

shifts

(b,s), quadrupole

splitting

(dL),

resonance width

(WL)

_assuming

Lorentzian

line-shapes.

_

Mean value

_(:1)

and deviation

(a)

assuming

Gaussian distribution

_{of quadrupole splitting}

in the

_{paramagnetic spectra}

_of

the

_glass

(Fe203)30-(Bao)45(B203)25

(sample

A).

The isomer

_{shift refers}

to

Fe metal at 300 K.

Fig.

2. -

Temperature

dependence

of the _spectra of _sample A in the

_vicinity

of the _magnetic transition.

tic

_hyperfine

_spectra are observed

_(Fig.

_3).

For the

orientation of the data

_analysis

it is of interest to note

that the

magnetic

spectra appear

symmetrical,

in

apparent

contradiction with the

quadrupolar splitting

at

_higher

_temperature.

_Assuming

Lorentzian

line-shapes

for the individual

_spectral

_components, the

analysis

of the.4.2 K data

_provides

linewidths for the

inner,

middle and outer lines

of 0.75,1.08

and 1.50 _mm/s while the relative areas are almost in the theoretical

ratio of 1 : 2 : 3 for a thin

_sample

₍₂

_mg

_Fe/cm2).

Proper

analysis

of the distribution of

_hyperfine

para-meters from these data is discussed below

_(section

_4).

Magnetization

curves for this

_{sample (A)}

are linear

down to 4.2 K in fields _up to 50 k0e _except in the low field

_region

_«

5

_k0e)

where a _very weak

ferroma-gnetic

component appears

[19,

20,

23].

The latter behaviour

_probably

arises from a minor contami-nant

_{of ferrimagnetic}

_BaFe12019

which is

undetect-ed

_by

_X-ray,

electron diffraction and Mossbauer

analyses.

Recent

_high

field

_{magnetization}

_(,

150

_k0e)

measurements confirm

_linearity

above 80 K and reveal

_slight

curvature below this _{temperature [24].} It should be noticed that the

_high

field

_{magnetization}

amounts to

_only

a few _percents of the calculated saturation value. The

_{susceptibility}

deduced from the

slope

of the linear

_part

of the

_{magnetization}

curves

follows a Curie-Weiss law above 80 K with C = 1.52 and

_9p

= - 95 K. The molar Curie-Weiss _constant C is

_{significantly}

below the value

_(4.38)

_predicted

for

Fig.

3. - M6ssbauer

spectra of _sample A at 4.2 K.

_(a)

In the absence of external _field; the full line

_corresponds

to a fit as

discussed in the text _{(with equal}

_weights

of + and - _signs of the L1

_{distributions). (b)}

In _{the presence of} an external

free

_{Fe3+ ions;}

the

_{negative paramagnetic}

Curie

temperature

0p

and

_{magnetization}

indicate

predomi-nant

_{antiferromagnetic exchange}

interactions. Below

80 K the

_{reciprocal susceptibility}

vs. T curve deviates

from

_linearity

and reaches a minimum at 12 K

_[19].

The increase in

_{susceptibility}

below 80 _{K may} be due to

_{large magnetic}

moments localized in small clusters

_[8].

Thermoremanent effects are observed

below 14 K for a

_sample

cooled in a field of 18.8 kOe

[19, 23].

Unidirectional remanence and

_displaced

hysteresis

loops

observed at 4.2 K are

_comparable

to those

_reported

for metallic

_mictomagnets

[25].

In order to elucidate the

_discrepancy

between the appearance of

magnetic

hyperfine

interactions in Mossbauer _spectroscopy at 42 K and the unusual

temperature

dependence

of

_{susceptibility (suggesting}

a transition _temperature of 12

_K),

_dynamic

suscep-tibility

measurements were

_performed

in a low field

of 6 Oe at a

_frequency

of 70 Hz

_{[22]. They}

reveal a

cusp characteristic of a

mictomagnetic

transition at a

_temperature

of 38 K.

Sample

B of nominal molar

_composition

(Fe203)30(BaO)17.5(B203)52.5

(i.e.

increasing

the

B203/BaO

ratio at constant

_Fe203

content vs.

sample A) presents

three Mossbauer resonance lines above 24 K

_(Fig.

_4).

The deconvolution of these

spec-tra into two broadened

_quadrupole

doublets charac-terizes the simultaneous _presence of

Fe"

_(~

76

_%)

and

Fe 21

_(-

24

_%)

ions

_(Table

_II).

The onset of

magnetic hyperfine splitting

occurs at 22 + 1

_K,

resulting

at 4.2 K in a broadened

_magnetic

_pattern

Fig. 4. - M6ssbauer spectra of sample B in the

_paramagnetic

and ordered _phases.

Table II. -

Hyperfine

data

_from

the

_{analysis of}

the

s7Fe

Mossbauer spectra

of

the

_glass

(Fe203)30-(BaO)17.5(B203)52.5

(sample B). Hhf

and a are

respectively

the mean value and the deviation

of

the

Gaussian distribution

_of

the

_hyperfine

_field.

Other

symbols

are

_defined

as in table I.

(*)

The random orientation of the _{angle ()} between the EFG and _Hhf axes has been taken into account

_by

introdu-cing

an uniform

_broadening

of the lines and _setting J = 0

(see

text, _{Eq. (2)).}

(**) 0

= 800 (see text).

which can be resolved into two

_magnetic

sextets

(Fig.

4).

Because of the coexistence of two valence

states of

iron,

bulk

_magnetic

measurements have not

been carried out in detail on this

_sample.

Sample

C,

(Fe203)50(BaO)35(B203)15,

presents

at

_high

_temperature

₍₂₀₀

K T 300

_K)

a

broaden-ed and

_asymmetric

doublet

_(Fig.

_5).

A

_magnetic

spec-trum,

coexisting

with the

_doublet,

_{progressively}

develops

below 200

_{K ;}

the

_paramagnetic

_component vanishes below 55 K

_(Fig.

_5).

_Using

Lorentzian

line-shapes,

the

_paramagnetic

_spectra were

_analysed

as a

superposition

of two

_symmetrical

doublets with

diffe-rent isomer shifts and

_{quadrupole splittings}

_{(Table III).}

The

_magnetic

_spectra are

_represented

as a

superposi-tion of two sextets with different isomer shifts and

hyperfine

fields

_together

with

_{vanishing quadrupole}

effects

_(see

discussion

_below).

Both the

_£5Is

and

_Hhf

values of these two sites characterize the coexistence of tetrahedral and octahedral coordination of

Fe3 +

ions.

Table III. -

Hyperfine

data

_from

the

_analysis

_of

the

5’Fe

Mössbauer spectra

of

the

_glass

(Fe203)50-(BaO)35(B203)15,

sample

C.

_{Approximately}

51

_{% of}

the

Fe 3’

are

octahedrically

coordinated whereas 49

%

of the

ions are in tetrahedral coordination.

Fig. 5. -

Temperature

dependence

of the M6ssbauer spectra

of

_sample

C

_{(Fe203)50(BaO)35(B203)15*}

Magnetization

and

_{susceptibility}

results for this

sample

C indicate a minor

ferrimagnetic

_component

(saturated

at low

_field)

and show a broad maximum

of

_{susceptibility}

at 70 K. Remanence is observed below

70 K in a field of 18.8 kOe. The

_high

_temperature Curie-Weiss _parameters are C =

4.31,

9p

= - 194 K

[20].

The Mossbauer _spectra of

_sample

_D,

reveal in the _{temperature range from 300} to 4.2 K

three lines which are

decomposed

into two

symme-trical doublets

_{characterizing}

the _presence of

Fe2 +

(-

12

_%)

and

_{Fe3 + (-}

88

_°%)

valence states

_(Fig.

_6,

Table

_IV).

At 1.5 K, a broad

magnetic

spectrum

co-exists with the

_paramagnetic

_pattern.

Isothermal

magnetization

is linear above _{20 K up} to fields of

20 kOe and deviates from

_linearity

at

_higher

fields without

_reaching

saturation at 150 kOe. The

recipro-cal

_{susceptibility}

v,s. _temperature shows a

Curie-Weiss behaviour

_(C

=

3.08, Op

= - 9

K)

above 20 K

and _presents no

_singularity

down to 1.5 K.

Fig.

6. - M6ssbauer

spectra at 4.2 and 1.5 K of _sample

Table IV. -

M6ssbauer parameters

of

4.

_Analysis

of

_hyperfine

interaction distributions in the

_glasses

_{Fe203-BaO-B203- -}

The

_large

linewidths observed in the Mossbauer _spectra of

_amorphous

materials arise from distribution of

_hyperfine

inter-actions. The

_analysis

of these distributions is useful for

_probing

the local structure and for

_establishing

the electronic and

_magnetic

_properties

_by

reference,

for

_instance,

to _computer

_{modelling predictions}

of

amorphous

structures

_{[1, 4].}

Let us recall some

_general

features of the

57Fe

resonance

_shapes

in the _presence

of such distributions and introduce the _computer

analysis

of the

_experimental

results obtained in this work.

By

reference to the

_systematics

of isomer shifts

(3js)

in

_crystalline

materials

_[26],

the distribution of

3js

will be small, in

_comparison

to the resonance

_width,

in the case of a

_single

valence state, coordination

number,

and nature

_{of ligands}

of the Fe atoms. On the

contrary,

differing 3js

are associated with different

valence states or coordination numbers and these

will be correlated to their

_quadrupole

and

_magnetic

hyperfine

interactions. A reliable

_analysis

of distribu-tions of

_hyperfine

interactions from Mossbauer data

will be

_{possible only}

in the former situation of

negli-gible

3,s

distribution. In other cases, some correlations

Assuming

a

unique

value for

bls,

the distribution

function of the

_{quadrupole splitting,}

can be

_directly

deduced from

_paramagnetic

data. It is

noteworthy

that

_quadrupole

doublets will be symme-trical under the above

_assumption,

whereas

_they

will be

_asymmetric

in the case of correlated

distribu-tions

_{of l5ls}

and L1

_[27].

In the _presence of additional

_magnetic

interaction and in the absence of

_single

ion

_{anisotropy (e.g.}

the

6S

_{configuration}

of

_{F e3 +)}

one does not _{expect any}

directional correlation between local

_{anisotropy (i.e.}

electric field

_gradient,

EFG )

axes and

hyperfine

field axes. A

_perturbation

treatment of the

_quadrupole

interaction on a

_magnetic

interaction

predicts

an

apparent cancellation of

_quadrupole

effect in the case

of random orientation

₍₀₎

between EFG and

_Hhf

axes.

_Indeed,

the

_angular

_average of the

quadrupole

perturbation

of the nuclear

_magnetic

_energy levels

can be written to first order :

(The

same result is obtained for non-axial EFG

tensor.)

This means that the

_{bar)’centres}

of the six

_magnetic

resonance _components are

_{arranged symmetrically}

with _respect to the isomer shift. However numerical calculation of the

_{spectral shape}

reveals that each of the six lines

_actually

is

_asymmetric

in _{the presence of} a

defined

_sign

of J and when the _{asymmetry parameter}

(q)

is small

_(Fig.

_7).

Similar conclusions are reached

from

_analytical

calculations

_[28].

The detection of such

_spectral

_asymmetry should allow information

Fig. 7. - Simulated Mossbauer

spectrum

(full line)

for a

random orientation between EFG and _Hhf axes _{(,J = 1 mm/s,}

Hhf = 400 k0e, component width : 0.25 mm/s). The dotted

line _represents the

_spectral

_shape in the absence of

quadru-pole interaction, in order to _point out the _lineshape

asym-metry and the asymmetry in

_peak

maxima _energies

cor-responding

to the above situation. These features will be washed out in an

_experimental

situation via the inherent distributions in modulus of 4 and Hhf.

to be inferred

_concerning

the

_sign

of the

_principal

component of the field

_gradient

_and/or

the _asymmetry

parameter.

In

_practice,

such

_asymmetries

will

unfor-tunately

be difficult to

_detect,

_owing

to the

_relatively

small field

_{gradients acting}

at the nucleus of an

S-state ion

_{(d (Fe3 + )}

1.5

_mm/s)

and to the line

broadening

which will be induced

_by

the concomitant

L1 and

_{Hh f}

distributions. The latter contributions were

neglected

in the

_lineshape

calculation of

_figure

7 in order to illustrate the

_principal

effect of the

_angular

distribution alone.

In summary, the random distribution of 0

_provides

to first order a-

_symmetrical

_arrangement of the

barycentres

of the six resonance

lines,

each of these

being

equally

broadened

_by

a value

_AWO

estimated

as

_{[29] :}

Adding

an isomer shift distribution to the random

angular

distribution

_produces

a uniform

broadening

if the two distributions are

_independent.

If

_they

are

correlated,

asymmetry is induced

_[27].

An additional distribution in the modulus of the

_quadrupole

_splitting

gives

rise to a

_broadening

which will be

_equal

for all

the

_magnetic

_components. This contribution sums

up to the above-mentioned

_angular

distribution effect. The

_resulting

linewidth can be calculated from the sum of second moments of the two distributions or more

_simply

as :

where

_WL

is the linewidth observed _{in the}

parama-gnetic phase.

An additional

_independent

distribution of

_Hhf

produces

non-uniform

line

_broadenings,

in the ratio of the _average transition

_energies

of the _components.

Thus,

unequal

linewidths of the

_magnetic

_spectral

components

unequivocally

characterize distribution

in

_Hhf.

The

_reliability

of the

_analysis

of combined distri-butions of

_hyperfine

interactions is

_clearly

limited and

requires

data of

_good

statistical

_quality.

The

_hyperfine

interaction distributions have been

analysed numerically

from the _present

_experimental

data

_according

to the

_guidelines

mentioned above. The Lorentzian _component linewidth is constrained

to an

_experimental

value of 0.25 mm/s

(measured

against

a thin reference absorber under identical

conditions)

and

_{spectral shapes}

are calculated

_by

computer summation

_according

to the relevant

hyper-fine interaction distribution. For

_sample

A, the

paramagnetic

spectra

(42

T 670

_K)

are very

satisfactorily represented

when it is assumed that there is constant isomer shift and a Gaussian

Fig.

8. -

Temperature

dependence

and _shape of the dis-tribution of

_{quadrupole splitting}

in _sample A.

The

_broadening

_appearing

at 42 K

_corresponds

to the

onset of

_{magnetic hyperfine}

effects

_(Fig.

_8).

The

_{nearly symmetric magnetic}

_spectra

which are

observed at

_{4.2, K}

are

_{successfully analysed using}

the

full Hamiltonians of nuclear interaction with :

_a)

a constant isomer

shift,

b)

the Gaussian distribution of

_{quadrupole splitting}

obtained from the

parama-gnetic phase ( r

=

0),

c)

_a

spherical

distribution for

the

_{polar angle}

0 between the EFG and

_Hhf

axes,

d)

a Gaussian distribution of

_{Hhf, e)}

no correlations

between these distributions

_(Fig.

9,

Table

_V).

_Figure

3 represents the data

_{analysis by}

_{assuming equal weights}

for both

_signs

of L1

_{and q}

= 0. The

quality

of the fit

is

_comparable

when

_assuming

one

_{sign only}

for the L1

distribution.

Thus,

as

already pointed

out

_above,

the

sign

of L1 as well as the

_magnitude

of L1 cannot be

reliably

deduced from combined

_quadrupole

and

magnetic

spectra because of the

_relatively

small value

of 4.

Fig.

9. - Deduced

hyperfine

field distribution at 4.2 K in

sample A.

Table V. - _{Mdssbauer parameters}

_of

(Fe203)30-(BaO)45(B203)25,

sample

A,

at 4.2 K.

_Upper

line

from

model

_a)

assuming

a Gaussian distribution

_of

_L1,

a

spherical

distribution

of

0,

an

_independent

Gaussian

distribution

_{of Hhf.}

Lower

_{line from}

a

simplified

version

of

model

_a);

the

_{quadrupole splitting}

and

_angular

distribution

_effects

are taken into account via

_uniform

broadening of

the linewidths

_{(see text).}

_{aA and CH}

are the deviation

of

the Gaussian distribution

of

L1

and

_Hhf

f

respectively.

(*) Fixed values.

In order to lift this

_ambiguity

an

_experiment

was

performed

under

_applied

field at a

_temperature

which is

_{sufficiently high}

so that the

susceptibility

is

negli-gible.

The

_experimental

_spectrum

obtained in a field

of 100 kOe at a _temperature of 300 K

_[30]

is

_compared

to _computer simulations

_(Fig.

_10).

These have been

performed by

numerical

_integration

in order to

represent the random orientation between effective field at the nucleus and EFG

_principal

axis. The Gaussian distribution of L1 is included in the calcu-lations

_(a.1

= 0.30

_mm/s).

The Lorentzian _component width is _{0.30 mm/s,} i.e.

_slightly

broadened

_owing

to

the

_fringing

_magnetic

field

_occurring

at the source.

A numerical fit of

_experimental

data to the above model is

_{impracticable}

because of

_prohibitive

com-puter time.

However,

a visual

_inspection

of the cal-culated

_{spectral shapes}

reveals that a

_spectral

asym-metry

unequivocally

demonstrates a

weighting

in

sign

of the L1 distribution and a value of the EFG

asymmetry parameter

differing significantly

from

unity (Fig.

10).

The _present

_experimental

result

(Fig.

10a)

is

_{adequately represented}

when

_assuming

a

_positive

_sign

_only

for the L1 distribution and

_{q = 0.5}

(Fig.

10b).

Nevertheless,

spectral

shape

simulations illustrate

_clearly

that the deduced

_weight

of the +

and -

_wings

in the L1 distributions is _strongy

corre-lated with the value of _q. For

instance,

an

_asymmetric

distribution in

_sign

of L1 and a value

of q

differing

from

_unity

is

_certainly

consistent with the

measure-ment.

_Summarizing,

it _may be

_pointed

out that the external field measurement in the

_paramagnetic

amorphous

phase

demonstrates

_{unequivocally}

a

signi-ficant

_weighting

to

_positive

_sign

of the L1 distribution and a value

_{of q}

_deviating

from

_unity.

This

informa-tion cannot be extracted

_reliably

from the

_magnetic

hyperfine

data in the

_{speromagnetic phase.}

_However,

the external field method does not allow simultaneous

analyses

of the

_sign

distributions of L1 and of the

Fig.

10. -

a) M6ssbauer _spectrum of _sample A measured

at 300 K in an external field of 100 k0e. The solid lines

represent

spectral lineshapes

computed for a random

orientation between the EFG and effective field (95 k0e)

axes and for a Gaussian distribution of L1

_(Ã

= 1.08 mm/s, Oj = 0.30 mm/s, Lorentzian width of 0.30 mm/s); b) L1

distribution of

_{positive sign}

_{only, 17} = 0.5 ; c) _as in b), 17 = 0; d) _as in b), 17 = 1; e)

equal weights

of

positive

and

negative signs

of _{L1, 17} = 0.

The

_experimental

data obtained on the other three

samples (B,

C,

D)

have not been

_{computer-analysed}

in detail since the isomer shift distribution revealed

by

the

_{paramagnetic phase}

data renders a

_complete

analysis prohibitive;

in _any case, a detailed

under-standing

of the

_magnetic

behaviour will be

_prevented

by

the additional

_complexity

_{in the presence of}

distributions of valence states and coordination of the iron ions. This restriction

_applies

both to local

mea-surements, e.g.,

hyperfine

interactions,

and even to a

larger

extent to

_macroscopic

results

_{(magnetization}

and

_{susceptibility}

_{measurements).}

For

_sample

B,

the

_paramagnetic

data

reveal the occurrence of both

Fe"

and

Fe3+

ions

from the

_respective

isomer shifts of the two doublets.

The

_QS

distributions are well

represented

by

broa-dened Lorentzian lines

_{(Table II).}

The

_magnetic

data at 4.2 K are

_{satisfactorily}

fitted

using

a

pertur-bation calculation

_(Eq.

_{(2)) :}

the

_b,s

difference between the two _components was constrained to the value determined in the

_{paramagnetic phase.}

For the

Fe3 +

component the effect of the

_{quadrupole splitting}

distribution and of the random orientation of the

EFG and

_Hhf

_axes was introduced as a uniform line

broadening

(Eqs.

(1)

and

₍₂₎₎

_{(Table II).}

For the

F e2 +

ions,

owing

to the

_single

ion

_anisotropy,

the

EFG and

_Hhf

axes are

_strongly

_{correlated ;}

thus 0 is

expected

to _present a defined value _contrary to

Fe 3+

ions.

A

_satisfactory

fit is achieved

_(Fig.

₄₎

even with a

single

Fe2 +

_magnetic

_component

_{by letting}

the

angle

0 for this site as a free

_parameter

(Table

_II).

However,

a wide distribution of

_Hhf

would

be

expected

for

Fe 2+

ions as a result of the

sensitivity

of this

parameter to orbital effects. Because of the low

inten-sity

of this site and the lack of

_spectral

resolution the fit is

_actually

insensitive to such effects.

Thus,

we

believe that the deduced standard deviation of the

Fe3 +

Gaussian

_Hhf

f distribution is reliable.

The

_{decomposition}

of the

_paramagnetic

data of the more concentrated

_sample

C into two broadened

symmetric quadrupole

doublets establishes the occur-rence of

Fe3+

ions

_only;

these are distributed _among

octahedral and tetrahedral coordination sites as

shown

_by

the 0.13 mm/ s difference in isomer shifts.

Consistent with the

_paramagnetic

_data,

the

satura-tion

_magnetic

data

_{(4.2 K)}

are

analysed

(using

the

perturbation

calculation

_{(Eq. (2)),}

in terms of two

magnetic subspectra

with

_randomly

distributed EFG and

_{Hh f}

axes. The _average fields are

Hh f(Oh)

= 500 kO

and

_{Hh f(Td)}

= 445 kOe while the deviations of their

Gaussian distributions are

_respectively

23 and 9.5 k0e

(Table

III,

Fig. 5).

The values of the

_hyperfine

fields further confirm the attribution of the

_subspectra

to

octahedral and tetrahedral coordination of

Fe3 +.

5.

_Magnetic

transitions and

_orderings

in the

amor-phous

oxides

_{Fe203-BaO-B203. - Combining}

the

present Mossbauer data with the

_published

bulk

magnetic

results

_{[19, 23],}

three _types of

_magnetic

behaviour can be

_identified,

_{corresponding}

respec-tively

to low -

(sample

D),

intermediate -

(samples

A, B) and

_high

-

(sample C)

concentration _range of

magnetic

ions.

Sample

A is most

_easily

discussed since here the iron occurs in a

_single

valence state

_{(Fe 31 )}

and

coor-dination

_{(tetrahedral).}

Above 42

_K,

the Mossbauer

spectra reveal

_paramagnetic

behaviour. Between 42

and 29

_K,

relaxation effects are observed in terms of

coexisting

unresolved

_magnetic

and central

para-magnetic subspectra (Fig.

2),

with the relative

inten-sity

of the latter

_decreasing

as _temperature is lowered.

merely

a line

_broadening

both in the intermediate

temperature range

(40-34

K,

Fig.

11)

and at 4.2 K

(Fig.

3).

In

_particular,

at the latter _temperature, no

polarization

of

_{magnetization}

is induced since the relative areas of the

_spectral

_components are unaf-fected

_by

the external field

_(Fig.

_3).

Among

the conceivable

_explanations

for the

measur-ed

_temperature

_dependence,

small

_particle

super-paramagnetism

is

_definitely

ruled out on the basis

of the

_following

_{arguments :} electron

_microscopy

(resolution : -

50 A)

does not reveal small

_particles

and

_{magnetization}

data do not show the Q vs.

_{HI T}

superposition

behaviour,

typical

for

_{non-interacting}

superparamagnetic particles [20, 23].

In a true

super-paramagnet,

the

_nearly

saturated

_magnetic

com-ponent should coexist in the Mossbauer _spectrum with the

_paramagnetic

doublet

_[31],

_contrary to

experiment. Finally,

in the _{temperature range} from 35

to 45

K,

a

large magnetic splitting

would be

_predicted

for small

_{magnetic particles}

at an external field of

45

kOe,

which is not observed

_{(Fig. 11). Paramagnetic}

relaxation effects are ruled out because of the narrow

temperature

range of the smeared spectra and from the

_{spectral shape}

evolution.

Magnetic

ordering

with a wide distribution in

transition _temperatures cannot account for the results since the _components which would order at the

higher

temperature

(40 K)

should

_already

be resolved

at 35 K. Distribution of molecular field from site to

site

_according

to the near

neighbour

atomic

environ-ment induces also

_significant

line

_broadening

in the

vicinity

of a

_homogeneous

_ordering

_temperature

_[32].

Fig. 11. - Mossbauer

spectra of _sample A measured at 35 K without _(a) and with an external field of 45 k0e (b).

However, in this

model,

the

_broadening

should extend to reduced _temperatures

_T/T,,

much lower than the

_{experimentally}

observed value of 0.75.

All the above cited Mossbauer

results,

along

with the irreversible

_{magnetization}

_effects,

the external field

_dependence

of the dc

_{susceptibility [23]}

and the

ac

_{susceptibility}

_cusp characterize a

_{mictomagnetic}

transition

_[25].

The _temperature

_dependence

of

Mbss-bauer

_spectra

between 40 and 30 K is _very similar

to that

_predicted

from a model of

_spherical

_relaxation,

with a relaxation rate

_decreasing

at lower

tempe-rature

_{[33, 34].}

However, tentative fits with such a

model show that a distribution of relaxation rates

should be considered.

A

_spherical

relaxation mechanism with a narrow

distribution in relaxation rate

_(or

_freezing

tempe-ratures)

has

_already

been concluded from Mossbauer results in an established

_{spin glass}

_{system :}

EU1 -xGdxS

[35].

The

_{mictomagnetic}

transition

occur-ring

in the _present oxide

_glass

is understood in terms

of a

_{progressive freezing}

of

_magnetic

clusters

_arising

from frustration

effects,

similar to the

_picture

ori-ginally

introduced

_by

Tholence and Tournier

_[36].

Contrary

to

_early

_{assumptions [36],}

_strong

interactions

couple

the

_magnetic

clusters via

_dipolar

or

_(and)

frustrated

_exchange

interactions,

as indicated

by

the

negligible

effect of the external field in the

_vicinity

of the

_{spin freezing}

_{temperature ( T f).} The

_frequency

dependence

of

_Tf,

which is revealed

_by

the difference between the ac

susceptibility

_cusp and the Mossbauer

transition _temperature, further confirms a

_freezing

mechanism with

_significant

interactions _among clusters. Such interactions between

_magnetic

clusters have been

_repeatedly

invoked to

_explain

the

_peculiar

properties

of the

_{mictomagnetic}

transition,

in

parti-cular to account for the Fulcher law

_dependence

of the relaxation rate in the

_vicinity

of the

_spin

_freezing

temperature [37-40].

Because of frustration

_effects,

which are intrinsic to an

_amorphous

solid with

antiferromagnetic

exchange,

the

_magnetic

structure is

_{speromagnetic,}

i.e. random

_freezing

of the

_spins

_{[5] ;}

antiferromagne-tic-like order extends at the most over a few inter-atomic distances. The

_{speromagnetic}

structure at

4.2 K is revealed

_by

the absence of _any detectable

polarization

in an external field of 45 kOe. This _type of order is

_already

indicated

_by

the _{very low} induced

magnetization

(~ 10-2

of saturation

_value)

and

_by

the absence of saturation _up to 150 k0e.

The

_magnetic

transitions

_occurring

in the other

glasses

have been

_investigated

in less

detail,

owing

to

the additional

_complexity

introduced

_by

the _presence of several coordinations and valence states of the

iron ions. The narrow _{temperature range} of the

tran-sition observed in the Mossbauer spectra of

_sample

B suggests a

_{mictomagnetic}

transition similar to that

reported

for

_sample

A. The simultaneous _appearance of the

_magnetic

_hyperfine

interaction at both

Fe2 +

distri-bution of the two valence states within the

_amorphous

solid.

The _temperature

_dependence

of the Mossbauer

spectra at

_high

concentration in

_magnetic

ions

(sample C)

is

_suggestive

of small

_particle

super-paramagnetism

(Fig.

5). Indeed,

one observes

super-posed paramagnetic

and

_magnetic

_spectra over a

wide _{temperature range}

_{(60-200 K)}

and the

_nearly

saturated

_hyperfine

field _{component appears}

_already

at

_high

_temperature. The observed _temperature

depen-dence is attributed to a

_progressive

_freezing

of

magne-tization

_relaxation,

with however much smaller inter-actions between clusters than in the intermediate concentration _range

_(samples

_A,

_B).

The latter

con-clusion contradicts the usual behaviour

_involving

an

increase of intercluster interactions with

concen-tration of

_magnetic

ions as found for instance in

EuxSr 1 - xS [39].

The observation is attributed to

amorphous phase segregation

in the _present oxide

glasses,

i.e.,

the iron ions are no

_{longer statistically}

distributed within the

_glass

in

_sample

C. Clusters of

relatively

high

Fe concentration coexist with

_regions

depleted

in this element, so that the interaction _among clusters becomes

_negligible.

The broad distribution

of relaxation rate

_(or

cluster

_size)

_may be connected with the occurrence of both tetrahedral and octahedral

coordinations of the

Fe3 + ions,

thus

_inducing

more severe frustration effects than in

_sample

A for instance.

In the less concentrated

_{sample (D),}

the _appearance,

at 1.5 K, of a

_magnetically

_split

_component

super-imposed

on the

_{paramagnetic quadrupole}

doublet in

the M6ssbauer _spectra is attributed to

_paramagnetic

relaxation effects for some of the iron ions

_(Fig.

_6).

Measurements at 4.2 K in

_large

external

_magnetic

fields

_(up

to 80

_kOe)

reveal a

_magnetic

_splitting

for

a fraction of the

Fe3 +

ions

_(Fig.

_12).

For one

compo-nent, the field

dependence

follows a Brillouin

beha-viour

_(,u

= 5

,uB)

indicating

that the _outer lines

correspond

to isolated

_paramagnetic

Fe3+

ions. Another fraction

_(the

central _part of the

_spectra)

does not

_split

at all and

_merely

feels the external field. This _component is attributed to

_{antiferromagnetically}

coupled

dimers. The intermediate

_spectral

lines

cor-respond

to an effective field of

_{approximately}

280

kOe ;

these are

_tentatively

attributed to

_{antiferromagnetic}

coupled

trimers

_[18].

The

_{magnetic properties}

discussed above concern

mainly

the low _temperature data. Little attention has been

_paid

to the

_{high temperature}

behaviour i.e. the _strong reduction of the calculated Curie constant

(or

effective

_paramagnetic

_moment)

when

_compared

to the

_expected

free ion value. For

_sample

A,

for

example,

C = 1.52 while the

Fe3+

free ion value

amounts 4.38.

_Ferey

et al.

_[16]

_explain

the

_lowering

of the Curie constant in

_{amorphous FeF3}

under the

assumption

of a

_spread

in

_magnitude

of the

anti-ferromagnetic

exchange

interactions as a _consequence

of frustration effects. It is

_surprising

that in other

systems, e.g. the cobalt and manganese

alumino-Fig.

12. - Mossbauer

spectra of

_sample

D measured at 4.2 K in external fields of 30 and 80 k0e, The bar _diagram refers to the three

_{subspectra corresponding}

to isolated _(a), dimer _(b) and other iron clusters (c).

silicate

_glasses,

the

_paramagnetic

Curie constants are found to be in _agreement with the free ion values

_[41].

We _suggest that the reduction of the Curie constant in the _{present systems} arises from

short-range antiferromagnetic

coupling persisting

up

to _temperatures much

_higher

than the

_spin

_freezing

temperature, so that some of the

Fe3+

moments would

not contribute to the Curie constant. A similar

explanation

has been

_already

invoked to account for

the

_magnetic

behaviour of CuMn

_alloys

_[25]

and of the

_{Fe2Ti05 spin glass}

_[42].

6. Electronic and structural

_properties

of the

amor-phous

oxides :

_{Fe2o3-BaO-B203- -}

_By

_comparing

the

5’Fe

isomer _{shifts and average}

_hyperfine

fields data to those established in

_crystalline

_{systems [26],}

it is concluded that

_sample

A contains

_only

_{Fe 31(6S)}

ions in tetrahedral coordination. It is of interest to

notice that the

_{phase initially crystallized}

from this

sample

(BaFe204)

similarly

presents a tetrahedral

Fe3 +

coordination unit

_(See

below Section

_7).

In

samples

B and

_D,

both

_{Fe 2’ (’D)}

and

_{Fe 31(6S)}

valence states coexist. In

_sample

C,

one observes the

Fe3 +

valence state

_only,

but in both octahedral and tetrahedral coordinations. The _average iron to _oxygen

coordination number is

definitely

correlated with the atomic ratio of the elements

_present

in the

_glass,

in

agreement with earlier conclusions

_[13].