Magnetic properties of the ternary oxide glasses, Li2O-B2 O3-Fe2O3 from 57Fe Mössbauer spectroscopy

HAL Id: jpa-00209553

https://hal.archives-ouvertes.fr/jpa-00209553

Submitted on 1 Jan 1982

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

Magnetic properties of the ternary oxide glasses,

Li2O-B2 O3-Fe2O3 from 57Fe Mössbauer spectroscopy

J.P. Sanchez, J.M. Friedt

To cite this version:

1707

Magnetic

properties

of the

_ternary

oxide

_glasses,

_{Li2O-B2O3-Fe2O3}

from

57Fe

Mössbauer

_spectroscopy

J. P. Sanchez and J. M. Friedt

Centre de Recherches Nucléaires, 67037

_Strasbourg

_Cedex, France

(Reçu le 13 avril _1982, révisé le _{2 juillet, accepté} le _{22 juillet 1982)}

Résumé. 2014 L’état de valence et la coordination du fer ont été établis _par

_{spectroscopie}

Mössbauer de 57Fe dans les

oxydes

ternaires _amorphes,

_{Li2O-B2O3-Fe2O3.}

Dans _{la gamme de concentrations} étudiée (~ 8,4 at

_%

_Fe), les ions

Fe3+ sont tétra- et

hexacoordinés ;

la faible

_proportion

₍ 15

_%)

d’ions

Fe2+,

observée dans certains échantillons,

est liée aux conditions de

_{préparation.}

Les

_{propriétés magnétiques}

de ces verres, établies en combinant des mesures

macroscopiques (susceptibilité dynamique

et

_aimantation)

et locales

_{(spectroscopie Mössbauer),}

sont déterminées

par la concentration des ions fer. A faible concentration

(~

3 at _%) de _fer, des ions Fe 3+ isolés coexistent avec des

petits agrégats (2

à 3 _atomes)

_{antiferromagnétiques.}

Pour des teneurs en fer

_plus

élevées _(3,8 à _8,4 at

_%),

les verres

présentent

une transition

_{mictomagnétique}

conduisant à un ordre

_{spéromagnétique.}

Dans un échantillon étudié

en détail _(5,8 at

_%

_Fe), la variation de la

_température

de

_{gel (Tf)}

avec la

_fréquence

de mesure obéit à une loi de

Fulcher.

Abstract. 2014 57Fe Mössbauer _spectroscopy has been combined with bulk _magnetic measurements to determine

the valence state and coordination of iron as well as the

_{magnetic properties}

of the ternary oxide

_glasses,

Li2O-B2O3-Fe2O3.

In all

_{investigated glasses}

_(~ 8.4 at _{% Fe),} the Fe3+ ions coexist in tetrahedral and octahedral

coordina-tions ; the presence of Fe2+ ions ₍ 15

_%),

which is observed in some

_glasses,

is related to

_preparation

conditions. The

_magnetic

_properties

of the

_glasses

are

_governed

_by

the iron content. At low iron concentration ₍ 3 at

_%),

isolated Fe3+ ions coexist with

_{antiferromagnetic}

dimers and trimers. At

_large

iron content _(3.8 to 8.4 at

_%)

the

glasses

undergo

a

_{mictomagnetic}

transition to a

_{speromagnetic}

ordered state. The

_freezing

_temperature

_(Tf),

which is

_{frequency dependent,}

is found to follow a Fulcher law in one

_sample

_(5.8 at

_%

_Fe)

_investigated

in detail.

J.

_Physique

43

₍₁₉₈₂₎

1707-1716 NOVEMBRE 1982, Classification

Physics

Abstracts 61.40D - 75.50K - 76.80 1. Introduction. - Considerable

experimental

and theoretical work has been devoted

_recently

to the structural

_{[1, 2]}

and

_{magnetic properties [3, 4]}

of

disordered

_systems.

The

_concept

of

_{spin glass}

or

mictomagnetism [5],

corresponding

to the random

freezing

of

_spins

via a

_{progressive blocking}

of the

magnetization

of clusters or via a collective

_phase

transition,

has been extended from diluted

_magnetic

alloys

CuMn,

AuFe,

etc...

_[6])

to

_randomly

diluted

crystalline

solids such as

_{EuxSr 1 - xS [7]}

or

_{EuxGd1 - xS}

[8]’ and

to

_{amorphous compounds}

with

_{predominantly}

antiferromagnetic

interactions

_[9].

The

_{present report}

concerns the latter class of

materials. The

properties

of the

_ternary

oxide

_glasses,

L’20-B203-Fe2o3,

are inferred

_{by combining}

bulk

(ac-susceptibility, magnetization)

and local

_{(17 Fe}

Mossbauer

_{spectroscopy)}

measurements. In a

pre-vious _paper on the

_{Ba0-B203-Fe203}

_glass

_systems

_[10]

it has been shown that the

_{magnetic properties}

are

governed by

the Fe concentration.

_Here,

more

atten-tion is devoted to the dilute concentration range, where isolated

Fe3 +

ions coexist with small

_magnetic

clus-ters, and to the intermediate concentration range, where the

_glasses

exhibit a

_{mictomagnetic}

transition

owing

to

_topological

disorder and frustration effects. The nature of the transition is

_investigated

in detail for one

_{sample (5.8}

at

_{% Fe) by following}

the

depen-dence of the

_freezing

_temperature

on the time window

(0.5

to

10-8

_s)

of the

_{measuring technique.}

2.

_ExperimentaL

- The

amorphous samples

were

prepared by rapid quenching using

the roller techni-que as described in detail elsewhere

_[11].

The final

compositions

of the

_ternary

oxides were checked

_by

standard chemical

_analysis.

The

_{non-crystalline}

nature

of the

_products

was established

by X-ray

and

elec-tron diffraction as well as

_by

transmission electron

microscopy. Dc-magnetic

data will be

_reported

in

detail

_by

Chaumont and Bernier in a

forthcoming

paper [ 12].

Ac-susceptibility

measurements were

_performed

at

the C.R.T.B.T.

_{(Grenoble) using}

a mutual inductance

bridge operating

between 17 Hz and 11 kHz

_[13].

The "Fe

Mossbauer measurements were

perform-ed as a function of

_temperature

between 1.4 and 295 K

against

a

57Co/Rh

source. Some

_experiments

were

carried out in a

longitudinal

external

magnetic

field

up to 80 k0e

_{provided by}

a

_{superconducting}

coil.

Absorbers

_containing

about 5

_mg/cm2

of natural iron were used. The Mossbauer

_spectra

were

_computer

analysed

as described in detail in a

_preceding

_paper

[10].

3.

_Experimental

results. -

57Fe

Mossbauer data

are

_reported

for the

_ternary

oxide

_glasses,

L’20-B203-Fe2o3,

of nominal

_{compositions given}

in table I. Table I. - Nominal

compositions

of

the _ternary oxide

glasses, Li20-B203-Fe203’ investigated

in this work.

Complementary ac-susceptibility

measurements as

a function of

_temperature

and

frequency

are

_reported

for one

_{specific sample}

e

_(5.8

at

_%

_Fe),

in which the

mictomagnetic

transition is established in detail.

3. 1 M6SSBAUER MEASUREMENTS. - At 77

K the

samples

b,

c and

f,

with

_respectively

_1.8,

3.0 and 6.9

Fig. 1. - Mossbauer

spectra of _samples b _(1.8 at

_%

_Fe)

and h _(8.4

_{at %}

_Fe) at 77 K. The full curves

_correspond

to a fit

assuming Lorentzian

_lineshapes

_{(see text).}

Table II. -

Mössbauer data at 77 K

_of

the _ternary oxide

_{glasses, Li20-B203-Fe203’ bjs}

is the isomer

_shift

versus

iron metal at RT.

_AL

and

_WL

_represent

_respectively

the

_{quadrupole splitting}

and the resonance width

_assuming

Lorentzian

_lineshapes.

P is the

_fractional

_spectral

area

of

iron in a

_given

coordination and

(or)

valence state. x is

1709

at

_%

_{Fe, present}

two

_{slightly asymmetric}

Mossbauer lines

_{(Fig. 1).}

_Assuming

Lorentzian

_lineshapes,

the

spectra

are

_{satisfactorily analysed}

as a

_{superposition}

of two broadened

_quadrupole

doublets with different isomer shifts and

_{quadrupole splittings characterizing}

Fe3 +

ions in both octahedral

_(Oh)

and tetrahedral

(Td)

coordinations

_{(Table II).}

By

contrast, the Mossbauer

_spectra

of

_samples

_d,

_e, g and h with

respectively

3.8, 5.8,

7.6 and 8.4 at

_%

Fe

present

at 77 K a third resonance line at about 2.1

_mm/s

superposed

upon the

previous

two main resonances

(Fig.

1).

The

_spectra

are

_decomposed

into three

sym-metrical broadened

_{doublets, indicating}

the _presence of

Fe3 +

ions,

both

_octahedrally

and

_{tetrahedrally}

coordinated,

and of a small amount of

Fe 21

ions

(Fig.

1,

Table

_II).

The observation of

Fe2 +

ions in these

_glasses

is related to

_preparation

conditions rather than to

_{compositional}

or other effects.

The Mossbauer _spectrum of

_sample

a

_(0.6

at

%

Fe)

reveals at 77 K

_(Fig.

₂₎

a

_{superposition}

of a central

doublet and of a

_{magnetic hyperfine}

structure

_(see

below).

At 4.2

_{K, except}

for

_sample

c

(3.0

at

_{% Fe),}

the

Mossbauer spectra of all

_samples

under

_study

reveal

a fraction of the iron ions

_experiencing

a

_magnetic

hyperfine

field.

_However,

two classes of behaviour

Fig. 2. -

Temperature

dependence

of the Mossbauer

spec-tra of _sample a _(0.6 at _{% Fe).}

Fig. 3. -

4.2 K Mossbauer _spectra of

_{Li20-B203-Fe203}

glasses

at different iron concentrations (lower concentration range,

samples

a,

b).

can be

_{distinguished, according}

to the iron

concen-tration range. At iron content x 3.0 at

_%,

the fraction of iron ions

_showing

a

_magnetic

_hyperfine

structure decreases with

_increasing

iron

concentra-tion ;

the

_opposite

trend is observed for x > 3.0 at

_%

(Figs.

3 and

_4).

The former behaviour is _{characteristic for}

para-magnetic

relaxation effects. In addition to the above mentioned concentration

_dependence,

this

attribu-tion is further

_supported

from the

_temperature

dependence

of the

_spectra,

which reveal a

progres-sive

_broadening

and

_collapse

of the

_{« magnetic»}

lines

_(see

_Fig.

2 at 77 and 295

_K).

The concentration and

_temperature

_{dependences correspond}

to

spin-spin

and

spin-lattice coupling

relaxation mechanisms

respectively. By

reference to

_computed

relaxation line

shapes,

the

_{temperature dependence (Fig.}

₂₎

_actually

indicates a distribution of relaxation _rates, which is

probably

connected to

_{inhomogeneity}

of the

_magnetic

environment of the

Fe3 +

ions,

as

_already

inferred from the

_quadrupole

interaction distribution.

Micro-crystalline heterogeneities (e.g. precipitated

ferrites

etc)

in the

_proportion

which would be indicated

_by

the Mossbauer data are ruled out from electron

microscopy

and bulk

_{magnetization}

results.

In the

_larger

Fe _{concentration range}

_(x

> 3.0

at

_%),

the iron concentration

dependence

of the 4.2 K Mossbauer

_{spectra (Fig.}

₄₎

shows an increase

in resolution of the

magnetic hyperfine

structure

with

_increasing

x. Detailed

temperature dependence

Fig.

4. - 4.2 K Mossbauer

spectra of

_{Li,O-B,O,-Fe2o3 glasses}

at different iron concentrations _(upper concentration range, samples c to _h).

indicates that the

_{magnetic hyperfine}

structure sets

in at 7 _± 0.5 K

_{(Fig. 5).}

The

_{magnetic hyperfine}

splitting

increases and the lines become less broadened when the

_temperature

decreases.

_Magnetic

saturation is achieved at 1.4 K

_only

for the more concentrated

sample

h

_(8.4

at

_%)

for which

_{magnetic broadening}

occurs at 14 ₊ 2 K.

For this

sample

h

_(8.4

at

_%

_Fe),

the saturated magne-tic

_spectrum

_(1.4

_K)

is

_analysed

in the

_hypothesis

of

an «

amorphous » magnetic

_structure,

_using

a

fitting

procedure

described in detail in

_{[10] (Fig. 6) :}

a

"

random orientation is assumed for the

_angle

0

bet-ween electric field

_gradient

and

_hyperfine

field

prin-cipal

axes. This

_results,

to first

_order,

in a

_symmetrical

magnetic spectrum.

The effects of the above

_angular

distribution and of the distribution of

_quadrupole

splitting

are

_{represented by}

a uniform

_broadening

of

all the

_magnetic

_{components given}

from a

perturba-tion calculation as

[10, 14] :

where

_{WL is}

the linewidth observed in the

parama-gnetic

state

_{(77 K)}

and

_6Wo

= - A,

with

d

_being

-,,15-the

_{quadrupole splitting}

deduced from the

para-magnetic

data. The distribution function of the

hyperfine

field is then fitted

_directly

from the data.

The _average fields and the standard deviations for both the

_octahedrally

and

_{tetrahedrally}

coordinated

Fe3 +

ions are collected in table III. It should be noticed that the average

hyperfine

fields,

are lower than those measured in

crystalline

iron-based

oxides

_{(e.g. Fe3 +(Oh)=}

537.5

_k0e,

_Fe3+(Td)=521

kOe in

_LiFe508

at 4.2

_K)

or than that

_{(530 kOe)}

deter-mined for isolated

Fe3 +

ions under a

_large

external field at 4.2 K

_(see

_below).

3.2 SUSCEPTIBILITY MEASUREMENTS. -

Preli-minary

dc-susceptibility

and

_{magnetization}

measure-ments have been

_{reported by}

Chaumont et al.

_[1,1].

A more detailed account of their work will _be

pu-blished

_[12].

The main features of their data are

summarized here.

Reciprocal susceptibilities,

x -1,

of the

_glasses

under

_study

follow a Curie-Weiss law above some

characteristic

_temperature

_TD

_(e.g.

100 K for

_sample

h with 8.4 at

_{% Fe)}

and show a

_negative

curvature

below this

_temperature.

The latter feature indicates

Curie-1711

Fig. 5.

_{- Temperature dependence}

of the M6ssbauer

spectra of _sample e _(5.8 at

_%

Fe) around the

_magnetic

transition.

0

Fig. 6. -

Mossbauer spectrum of _sample h _(8.4 at _{% Fe)} at 1.4 K. The full curve _corresponds to a fit as discussed in the text.

Table III. -

Hyperfine

interaction data

_from

the

analysis of

the Mössbauer _spectrum

_{of sample}

h

_(8.4

at

_{% Fe)}

at 1.4 K.

_Hhf

and a are

_respectively

the

mean value and the standard deviation

_of

the Gaussian

distribution

_of

the

_{hyperfine field.}

6 is assumed to be the same

_for

the tetrahedral and octahedral

_hyperfine

field

distributions. Other

_symbols

are

_defined

as in

Table II.

* _{The random orientation of the}

_{angle B}

_{between the}

EFG and _{Hb axes is taken into} account

_{by introducing}

an uniform

broadening

of the lines and

_setting

the

quadru-pole splitting

L1

_equal

to zero _{(see text).}

Weiss _constants,

_CM,

are in

_general

_{significantly}

lower than the free ion

Fe3 +

value

_(Ct

=

4.38); they

increase with

_decreasing

Fe

_{concentration,}

_reaching

C,

for the less concentrated

_samples

₍

0.6 at

_{% Fe).}

The modulus of

_0p

decreases

_(from

115 K to

_zero)

with

_decreasing

iron content. In none of the

_samples

does

_{x -1}

show a

_minimum;

absence of remanence

effects is observed within the

_investigated

tempera-ture _{range, i.e.} above 4.2 K.

Magnetization (per

at _g

_Fe)

decreases with

increas-ing

iron content and does not saturate even for the

less-concentrated iron

_{sample (up}

to 150 kOe at

4.2

_K).

For

_instance,

for

_sample

e

_(5.8

at

_%)

the

magnetization

at 4.2 K

_{(20 kOe),}

which amounts to

2 100

_emu/at

_{g Fe,}

is far below the

Fe3 +

free-ion saturation value of 27 924

_emu/at

_g Fe. At 4.2

K,

the

M(H)

curve of this same

_sample

_(e)

shows a

_slight

curvature

_except

in the low field

_region

₍

5

_kOe)

where a _very weak

_{ferrimagnetic}

_component

_(LiFe508

crystallites)

saturates. The

_shape

is similar to those found in

_amorphous

_manganese aluminosilicates

[16].

The

_{ac-susceptibility}

of

_sample

e

_(5.8

at

_%)

exhibits

a somewhat broad _cusp below 4.2 K which is fre-quency

dependent;

it takes

place

at

_temperatures

of

3.25,

3.35 and 3.45 K for

_{respective measuring}

fre-quencies

of

17.3,

197.6 and 2 103 Hz

(Fig. 7).

This cusp defines the

spin freezing temperature Tf.

In the

frequency

range accessible to

_{ac-susceptibility}

_{T f}

varies

_linearly

with

_log

_v; the

_slope

of the relative shift of

_Tf

_against

decimal

_logarithm

of

_frequency,

is an order of

_{magnitude larger}

than in the canonical

metallic

_{spin glass}

CuMn

_[17].

Note that the assump-tion of an Arrhenius law for the

_frequency

_dependence

of

_Tf

_{T -1 - -}

k

In v

leads to

_unphysical

values

f

f

Fig.

7. -

Ac-susceptibility

of

_sample

e _(5.8 at

_%

_Fe) as a function of _temperature for various _measuring

_frequencies.

for both the characteristic

_frequency

and the activation energy

(Ea/k ~

270

K).

4.

_Magnetic

_properties

of the

_ternary

oxide

_{glasses :}

Li2O-B2O3-Fe2O3.

-

Combining

the

_present

M6ss-bauer and

_{ac-susceptibility}

data with results from

dc-susceptibility

and

_{magnetization}

measurements

_[11,

12],

it _appears that the

_investigated

iron-based oxide

glasses

exhibit two distinct classes of

_magnetic

behaviour,

according

to the concentration range of the

_magnetic

iron ions. The

_boundary

between the

two

_regimes

_{lies in the range 3.0-3.8} at

_%

Fe. 4.1 UPPER CONCENTRATION RANGE. - Consider

first

_sample

e

_{(5.8 at % Fe)}

which is

_{representative}

of this concentration range. Above 7 ± 0.5

_K,

the Mossbauer

_{spectra present}

_quadrupole

doublets

revealing

«

_{paramagnetic »}

behaviour

_(Fig.

_5).

Below

7 K the doublet smears out and

_develops

progressi-vely

into a

_{magnetic lineshape (Fig. 5).}

In the

pre-sence of an intense external field

₍₄₀

and 80

_kOe)

at

4.2

_K,

the

_spectral

resolution is

_{significantly}

enhanced and the

_spectral

_splitting

increases

_{(Fig. 8).}

Several

_explanations

_may be considered in order

to account for this behaviour.

_{Superparamagnetic}

effects can

_hardly

be

_{invoked; indeed,}

neither

elec-tron

_microscopy

nor

_{magnetization}

data indicate

the presence of

superparamagnetic particles.

More-over, the Mossbauer

spectra

do not _present the

tem-perature

dependence

characteristic for

superpara-magnets

i.e. a

_fully

_{developed magnetic}

_spectrum

coexisting

with a

paramagnetic

doublet over a wide

temperature

range

[18].

Also,

magnetic

field

satura-tion,

which should be

_easily

achieved for small

inde-pendent magnetic particles

even in a

relatively

small

external

field,

is not observed.

Paramagnetic

relaxation

_effects,

_{corresponding}

to

Fig.

8. -

Applied

field

_dependence

of the Mossbauer

spec-tra of

_sample

e _(5.8

_{at %}

_Fe) at 4.2 K _(external field

_parallel

to y-ray

propagation

axis).

spin flipping

of individual

Fe3 +

_ions,

are

_reasonably

ruled out

from

the relative narrowness of the

tempe-rature _range where these effects would occur

_(compare

figures

2 and

₅₎

and from the

_temperature

_dependence

of the

_spectral

_{shape (by}

reference to established

examples

and

_{calculations).}

The observed

_temperature

and external field

depen-dences of the Mossbauer

_spectra

of

_sample

_e,

_along

with the

_{ac-susceptibility}

_cusp, rather _suggest a

mictomagnetic

transition

_[5].

Over a

_temperature

range below

T f,

the Mossbauer

spectral shapes

can

be

_{reproduced qualitatively by}

a

spherical

relaxation

model

[8,

19,

20],

when

_assuming

some distribution

of relaxation rate. This latter

_assumption

is

neces-sary in

particular

to

_represent

the

_apparent

persis-tence of the

_paramagnetic

doublet,

while a

_unique

isotropic

relaxation rate would

_imply

a fast

_smearing

out of these

_{peaks [8,}

19,

20].

The alternative

expla-nation of

_temperature

_{dependent exchange}

field distribution cannot be

_definitively

ruled out from the

present

results. Notice that the

_proposed

interpre-tation is consistent with the one demonstrated in

another established concentrated

_{spin glass}

_system,

i.e.

_Eu1-xGdxS,

where a narrow

_frequency

1713

The

_{mictomagnetic}

transition is understood in

terms of a

_progressive

_freezing

of the

_isotropic

fluc-tuations of

_{magnetization}

of clusters

_[5,

8,

21].

Con-trary to the

_original

model of Tholence and Tournier

[21],

which assumed the existence of

_essentially

independent

magnetic

clouds

(clusters),

the external field

_dependence

of the Mossbauer

_spectra

indicates

strong interactions _among the clusters via

_dipolar

and/or

exchange

interactions.

Indeed,

very

large

fields are

_required

to block the relaxation and

satu-ration is not even reached at 80 kOe. The same

conclusion is ascertained from the

_frequency

depen-dence of the

_freezing

_{temperature Tf [22-25]. Defining}

the Mossbauer

_freezing

_temperature

as the

tempe-rature of _appearance of the

_{wings coexisting}

with the

_quadrupole

doublet

_{(Fig. 5),}

the

_frequency

dependence

of

_Tf

can be extended from the _{lac range}

to

_{10’ Hz,}

i.e. the characteristic

_frequency

of

57 Fe

Mossbauer

_{spectroscopy.}

An Arrhenius

_plot,

_{T f 1}

vs. In

v,

shows a _curvature,

_indicating

that such a law

cannot _represent the

_{frequency dependence.}

A better

description

of the

_{frequency dependence}

of

_{T f}

is achieved

_{by using}

a Fulcher

law,

where

_To

is a

_{phenomenological parameter}

which

takes into account the interactions among the clusters

[22-25].

The

_plot

of

_Tf

vs.

1/(ln

v - In

_vo)

is linear

with a characteristic

frequency

_vo =

109

Hz

(Fig. 9).

The interaction energy,

To,

and the activation energy,

Ea/k,

are 2.7 and 9.9 K

_{respectively.}

These values are

_comparable

to those _{obtained in manganese and} cobalt aluminosilicate

_{glasses [23].}

At

_temperatures

well below

_{T f}

the

_{magnetization}

fluctuation is

frozen,

resulting

into a static «

amor-phous »

type

Mossbauer

_spectrum

as found for

sample

h at 1.4 K

_(Fig.

_6).

In summary,

although

the

_dynamics

involved at

the

_{mictomagnetic}

transition cannot be elucidated

unambiguously

from the

_present

results,

it is

qualita-tively argued

that

_freezing

takes

_place

at

_Tf

via a

Fig. 9. -

Frequency

dependence

of _T

_for

_sample e _(5.8 at

% Fe) using a Fulcher law. _To and

_Ea/k

are deduced from the _plot of _Tf

_{vs. 1/(In vo - In}

_v) with _vo = 109 s-1.

spherical

relaxation mechanism. The fluctuations of

magnetization

of individual clusters are

_strongly

coupled.

This is deduced from the

_frequency

depen-dence of

_Tf

and from the

relatively

narrow _range of

relaxation rates.

_Indeed,

in the _presence of such

interactions between

clusters,

the relaxation rate

will be less

_dependent

on cluster size than for instance in a true

_{superparamagnet.}

The

_temperature

_dependence

of the Mossbauer

spectra

of the other

_{samples belonging}

to the

_large

concentration

regime

(3.8

to 8.4 at

_%)

has been

investigated

in less detail. It _appears that the

_freezing

temperature

is

_{approximately proportional}

to the iron atomic fraction. The increase of

_Tf

is also evident from the evolution of the

_spectral

_shape

at 4.2 K with iron

concentration,

which reveals an

_increasing

fraction of

_{magnetic component (Fig. 4).}

The reduction of saturation

_hyperfine

field in the ordered

_{(speromagnetic)}

state as

_compared

to the

extrapolated paramagnetic

value

_(evaluated

from the external field

_{measurements)}

or to the value in

comparable crystalline compounds

cannot be attri-buted to

_{changing covalency}

effects

_[26]

in view of the similar

_bjs.

More

_reasonably,

one _may invoke

largely

different

_{supertransferred}

and

_dipolar

contri-butions to the

_hyperfine

field in

_amorphous

materials

as a _{consequence of}

_positional

disorder.

Alternatively,

zero-point spin

deviations

_might

be

_responsible

for

Hh f

and moment reduction

[26],

as established for

Fig. 10. - Mossbauer

crystalline

one-dimensional

_{antiferromagnets [27].}

However,

the existence of

_spin

waves in frustrated

amorphous magnets

is still debated

_[28].

The frustration

_{implied by}

the combination of

negative superexchange

interactions and of

topolo-gical

disorder leads well below

_Tf

to a new

_type

of

magnetic

order

_{(speromagnetism) :}

the

_spins

are

frozen in random directions and

_{antiferromagnetic}

correlations

_extend,

at the most, over a few inter-atomic distances

_{[29]. Speromagnetic}

structure is proven at 4.2 K for the

_present

_glasses

_{by investigating}

the

_{magnetic polarization}

induced

_by

an external

field. For

_sample

h

_(8.4

at

_{%), application}

at 4.2 K of a field of 80 kOe has no effect on the _average

linepositions

while minor

_changes

affect the relative line areas and linewidths

_{(Fig. 10).}

Both effects reveal

negligible

moment

_polarization

even

_by

such

_large

external fields in accordance with the

_strong

_negative

exchange

revealed

_by

the

_large

value of _the

para-magnetic

Curie

_temperature,

i.e.

_0p

= - 115 K for

sample

h

_(8.4

at

_{% Fe).}

The

_{slight (5 %)}

decrease of

linewidth is attributed to the

_quenching

of relaxation

persisting

at 4.2 K in the absence of external field. This conclusion is consistent with the

_temperature

dependence

in the absence of

field,

which shows

between 4.2 and 1.4 K a

_slight

increase of the outer

line

_{intensity (Figs.}

_{4, 6).}

_Also,

the deduced

_magnetic

ordering

agrees with the small value of the induced

magnetization (a

few

_%

of theoretical

Fe3 +

saturation

value)

and with the absence of saturation at fields

as

_large

as 150 k0e

_{[11, 12].}

4.2 LOWER CONCENTRATION RANGE. - A

quite

different

_magnetic

behaviour is observed for the lower concentration _range

_(Figs.

2 and

_{3). Sample}

c

(3.0

at

_%)

shows a

_quadrupole

doublet down to 4.2

K,

while at the same

_temperature

a

_{magnetic hyperfine}

structure

_component

coexists with a

quadrupole

doublet in

_samples

a and b

_{(with respectively}

0.6 and 1.8 at

_%).

The fraction of the

_{magnetically split}

component

increases with

_decreasing

iron

concen-tration

_{(Fig. 3).}

As

_already

_discussed,

this

concen-tration

_{dependence, along}

with the

_temperature

dependence

observed for each

sample (Fig. 2),

cha-racterize the occurrence of

_paramagnetic

relaxation

effects. The coexistence of slow- and fast

_relaxing

Fe3 +

ions reveals a non-statistical distribution of

these ions within the

_glass

matrix. The

_hyperfine

field at 4.2 K

_{corresponding}

to the _±

_5/2

state of

the slow

_relaxing

Fe3 +

ions amounts to 520 kOe.

Application

of a

large longitudinal magnetic

field

(up

to 80

_kOe)

on

_sample

c

_(3.0

at

_%)

reveals a

magne-tic

_splitting

for a fraction of the

Fe3+

ions

_{(Fig. 11).}

The field

_dependence

of the outer lines

_(c)

follows a

Brillouin law

_(S

=

5/2, p

= 5

JlB)’

indicating

that

this

_component

arises from isolated

Fe3 +

ions. The saturation value of

_{Hh f equals -}

530 kOe. The central

_part

of the

_spectra

_(a)

does not

_split

and

merely corresponds

to the

_applied

field.

Thus,

this

component is attributed to

_{antiferromagnetically}

Fig.

11. -

Applied

field

_{(longitudinal) dependence}

of the

Mossbauer _spectra of

_sample

c

_(3.0

at

_{% Fe)}

at 4.2 K. The

bar

_diagram

refers to the three

_{subspectra corresponding}

to isolated Fe3 + ions _(c),

_{antiferromagnetically coupled}

dimers _(a) and

_larger

iron clusters

_(b).

coupled

dimers which as a whole behave as

non-magnetic.

The lines

_(b),

between the outer set and

the central

_portion

of the

_{spectra, correspond}

to an

effective field of - 250 kOe and are

_tentatively

assigned

to

_{antiferromagnetically coupled}

trimers

_[30].

The above discussion is consistent with the observed

reduction of Curie constant for this

_composition

(CM

= 3.43 _vs. free ion value of

4.38).

The

proportion

of

_coupled

clusters decreases with

_decreasing

Fe

concentration,

with

_mainly

isolated

Fe3 +

ions at

x = 0.6 at

%.

5. Structural

_properties

of the

_ternary

oxide

_{glasses :}

L’20-B203-Fe2o3’

- The

infor-1715

mation available from the Mossbauer data is the coordination and valence state of the iron ions. This

type

of information is

_generally

obtained

_by

compar-ing

the measured isomer shifts

(or

hyperfine fields)

to those established in

_{crystalline compounds [31].}

The

_bjs

allows a

_{straightforward}

attribution to

_high

spin

ferric or ferrous ions and can also

generally

provide

their coordination. For

_instance,

the

_bls

(at

77

_K)

of 4-coordinated

Fe3 +

ions falls in the range from 0.26 to 0.43

_{mm/s (vs.}

iron at

_RT)

while

bjs

for 6-coordinated

Fe3 +

ions is in the range from 0.38 to 0.63

_{mm/s [32].}

From the 77 K Mossbauer data of the

_present

glasses (Table II),

it is concluded that

Fe3 +

ions are

present

in both tetrahedral and octahedral coordi-nations. This is also confirmed from

_{crystallization}

study [12],

which initiates with the formation of

LiFe508

where the

Fe3 +

ions

_present

both coordi-nations. The 4-coordinated

Fe3 +

ions

_experience

a

larger

electric field

_gradient

than the 6-coordinated

ones. Both the _average isomer shift and

_quadrupole

splitting

of the

Fe3 +

ions increase with

_decreasing

Li20

content

_(Tables

I and

II)

as

_{already reported}

by

Raman et al.

_[33].

These authors account for this

behaviour in terms of a

change

of coordination from

tetrahedral to octahedral with

_{decreasing Li20}

con-tent.

_However,

this

_explanation

is inconsistent with the

_present

results which show that the

_Li20

pro-portion

does not affect the

_{Fe 3’(O,)/Fe 3 ’ (Td)}

ratio.

Combining

the two sets of

data,

it is

_suggested

that the

_{glasses including}

Li as a cation differ

_structurally

from those with other

_{(Na, K)}

alkali cations. This conclusion is

_{supported by}

the monotonous decrease of average

ðlS

as a function of

_{Li20 proportion}

reported by

Raman et al.

_[33],

while for

_Na20

or

K20 glass

modifiers a

_{sharp anomaly}

is observed

for

_6js

at a

specific

alkali oxide concentration.

The effect of

_Li20

addition on the structural

evo-lution of the borate network has been

_investigated

in detail from

"B

nuclear

_magnetic

resonance in

B203-Li20 glasses by Bray

and his coworkers

_[34].

For a molar ratio R of alkali oxide to boron oxide smaller than

0.5,

they

observe in addition to

_B03

units a

_growing

fraction of boron atoms in tetrahedral coordination

_{(B04 units)}

for

increasing

Li20

content.

For 0.5 R

1, i.e.,

the _{concentration range of} interest

_here,

the

_proportion

of both

_symmetrical

B03

and

_B04

units decreases whereas a fraction of

asymmetric B03

units

_(with

one or two

_non-bridging

oxygens)

grows with R. It has been

recently argued

that the decrease of both

_bls

and the

_quadrupole

splitting

of

Fe3+

ions with

_increasing

alkali oxide

content

_might

be connected with the formation of

non-bridging

oxygens in the

neighbourhood

of these ions

_{[35]. However,}

further

_{investigations}

are needed

to confirm such a

_proposal

and no

_arguments

can

be inferred from the

_present

results to

_support

this

hypothesis.

It is felt that no

_{strictly significant}

infor-mation can be inferred from the

_present

results with

respect to the

_glass

former or

glass

modifier

_property

of the iron atoms in the considered

glasses.

Such

type

of

_{investigation}

would

require

a detailed

inves-tigation

of the iron coordination

dependence

on the

ratio of alkali to boron elements and on the

_iron

concentration

_{respectively.}

Moreover,

the

_B203

glas-ses

_might

not

_represent

the best choice for such studies since the coordination

_geometry

of boron

already changes

from

_triangular

to tetrahedral in

binary

alkali oxide-boron oxide

_glasses.

A distribution of

_{quadrupole splittings,}

deduced from the Lorentzian

linebroadenings,

is observed in all of the

_{investigated samples.}

As discussed

exten-sively

previously,

this is attributed to fluctuations around _average values of

_{bonding angles}

and

dis-tances within

_nearly

constant structural units

(octa-hedra and

_{tetrahedra) [10].}

Relevant information about the

_{heterogeneity}

of the iron distribution within the

_glass

host is inferred from the local

_magnetic

data revealed

_by

Mossbauer

spectroscopy.

The observation in the lower Fe concentration range of small

magnetic

clusters

_(dimers

and

_{trimers) coexisting}

with isolated

Fe3 +

ions agrees with earlier

reports [10, 30]

and turns out to

represent

a common feature for iron-based oxide

glasses. According

to

_{susceptibility}

data

_(Curie

cons-tant),

the

Fe3 +

ions occur

_{predominantly}

as isolated

species

in the lower iron concentration

_{glass (0.6}

at

_{%) [11, 12].}

However,

the

_{inhomogeneous}

para-magnetic

relaxation behaviour observed for this

sample

indicates that the

Fe3 +

ions are not

_randomly

distributed in the

_glass

matrix. 6. Conclusion. -

The valence and coordination of the iron ions in

_{L’20-B203-Fe2o3}

_{glasses (}

8.4

at

_{% Fe)}

have been determined

_by

57Fe

Mossbauer

spectroscopy.

The

Fe3+

ions occur in both

tetra-hedral and octatetra-hedral

_{coordinations;}

_{the presence} of

Fe2 +

_ions,

in some

_glasses,

_corresponds

determi-nantly

to

_equilibrium

conditions in the

_preparation

process.

The

_{magnetic properties,}

ascertained from

combin-ed bulk

_{(ac-susceptibility, magnetization)}

and local

(Mossbauer)

measurements, are determined

prima-rily by

the iron content in the

_glasses.

At low iron concentration

_{( 3}

at

_%)

Mossbauer studies in external fields demonstrate the coexistence of

_magnetic

clusters on the atomic scale with isolated

Fe3 +

ions. At 0.6 at

_%

_Fe,

the

Fe3 +

ions occur

_mainly

as isolated

ions; however,

they

are distributed

_{non-statistically}

in the

_glass.

In the _upper concentration _range

_(3.8

to 8.4 at

_%),

the

_{glasses present}

a

mictomagnetic

transition at

_Tf

as characterized from Mossbauer and

ac-susceptibi-lity

measurements. The

_temperature

_Tf

increases

monotonously

with the iron concentration.

_Strong

interactions _among the clusters are evidenced

_through

the

_{frequency dependence}

of

_Tf

which follows a

random _arrangement,

_resulting

in a

_{speromagnetic}

structure.

_Although

a

temperature dependent

distri-bution of

exchange

interaction or distribution of

transition

_temperatures

via chemical

heterogeneities

cannot be ruled out

_definitively

from

_present

results,

it is

_suggested

_that,

similar to

_Eu,

_{_xGdxS,}

the

micto-magnetic

transitions at

_{T f}

_proceed

via a

progressive

freezing

of random fluctuations of cluster

magnetiza-tion.

_However,

one should notice that the above

basic

_ambiguity

about the actual nature of the

_spin

glass

transition affects most of the

_existing

experi-mental results and affords a

_{particularly challenging}

question.

Acknowledgments.

- We thank C. Chaumont and

J. C. Bernier for

_providing

the

_samples

and for communication of their

_{dc-susceptibility}

and magne-tization data.

References

[1]

GASKELL, P. _H., J. _Non-Cryst. Solids 32 _{(1979) 207 ;} J.

_Phys.

C 12

₍₁₉₇₉₎

4337.

[2] LINES, M. E.,

Phys.

Rev. B 20 _{(1979) 3729 ;} ibid. B 21

(1980) 5793 ; J. Non-Cryst. Solids 46 ₍₁₉₈₁₎ 1.

[3]

Amorphous

Magnetism I, ed. H. O.

_Hooper

and A. M. de Graff

_(Plenum

Press, New

_York)

1973.

[4]

Amorphous Magnetism II, ed. R. A. _Levy and R. Hase-gawa

(Plenum

Press, New

_York)

1977.

[5] BECK, P. _A., in

_Liquid

and

_Amorphous

Metals, ed. E. _Lüscher, H. Coufal

_(Sijthoff

and Noordhoff,

Alphen

van den

_{Rijn, Holland)}

_{1980, p. 545;}

Prog. Mat. Sci. 23 ₍₁₉₇₈₎ 1.

[6] MYDOSH, J. A., J. _{Magn. Magn.} Mater. 7 (1978) 237.

[7] MALETTA, H., FELSCH, W., Phys. Rev. B 20 ₍₁₉₇₉₎ 1245.

[8] LITTERST, F. J., FRIEDT, J. _{M., THOLENCE,} J. L., HOLTZBERG, F., J. _Phys. C 15 ₍₁₉₈₂₎ 1049.

[9] VERHELST, R. A., KLINE, R. _W., DE _GRAFF, A. _M.,

HOOPER, H. _O.,

_Phys.

Rev. B 11

_{(1975) 4427.}

[10] BONNENFANT, A., FRIEDT, J. _{M., MAURER, M.,} SAN-CHEZ, J. P., J.

_Physique

_{43 (1982)} 1475.

[11] CHAUMONT, C., DERORY, A., BERNIER, J. _C., Mat. Res. Bull. 15 ₍₁₉₈₀₎ 771.

[12] CHAUMONT, C., BERNIER, J. _C., to be

_published

in the

Proceedings

of the «5e _Congrès International sur la _Physique des Solides non cristallins »

Montpellier, juillet

1982, to _{appear in J.}

Phy-sique

Colloq.

[13] The collaboration of J. L. Tholence for

_performing

the

ac-susceptibility

measurements is

_gratefully

ack-nowledged.

[14] HANG NAM OK, MORRISH, A. _H.,

_Phys.

Rev. B 22

(1980) 4215.

[15] MORGOWNIK, A. F. _{J., MYDOSH,} J. A., WENGER, L. E.,

J.

_{Appl. Phys.}

53 ₍₁₉₈₂₎ 2211.

[16]

FERRÉ, J., POMMIER, J., RENARD, J. P., KNORR, K.,

J.

_Phys.

C 13

₍₁₉₈₀₎

3697.

[17] MULDER, C. A. _M., VAN DUYNEVELDT, A. J., MYDOSH,

J. _{A., Phys.} Rev. B 23 ₍₁₉₈₁₎ 1384.

[18] MORUP, S., DUMESIC, J. A., TOPSOE, H., in

Applica-tions

_of

Mössbauer _{Spectroscopy,} ed. R. L. Cohen

(Academic

Press, New

_York)

1980, Vol. 2, p. 1.

[19]

DATTAGUPTA, S., BLUME, M., Phys. Rev. B 10 ₍₁₉₇₄₎ 4540.

[20]

MANYKIN, E. A., ONISHCHENKO, E. V., Sov.

_Phys.

Solid State 18 ₍₁₉₇₆₎ 1870.

[21] THOLENCE, J. L., TOURNIER, R., J.

_Physique

_Colloq. 35 ₍₁₉₇₄₎ C4-229.

[22] THOLENCE, J. L., Solid State Commun. 35 ₍₁₉₈₀₎ 113.

[23] MEERT, A. _{T., WENGER,} L. _E., J. _{Magn. Magn.} Mater. 23 ₍₁₉₈₁₎ 165.

[24] MALETTA, H., J. _{Magn. Magn.} Mater. 24 ₍₁₉₈₁₎ 179.

[25] SHTRIKMAN, S., WOHLFARTH, E. _{P., Phys.} Lett. 85A

(1981) 467.

[26] VARRET, F., HENRY, M., Revue _Phys.

_Appl.

15 ₍₁₉₈₀₎ 1057.

[27]

GUPTA, G. _{P., DICKSON,} D. P. _{E., JOHNSON,} C. _E., J. _Phys. C 11 ₍₁₉₇₈₎ 215.

[28]

VILLAIN, J., J. _{Magn. Magn.} Mat. 15-18

₍₁₉₈₀₎

105.

[29]

COEY, J. M. D., J.

_Appl.

Phys. 49 ₍₁₉₇₈₎ 1646.

[30] MOON, D. _{W., AITKEN,} J. _M., MCCRONE, R. _K.,

CIELOSZYK, G. _{S., Phys.} Chem. Glasses 16 ₍₁₉₇₅₎ 91.

[31] GREENWOOD, N. N., GIBB, T. C., Mössbauer

spectros-copy (Chapman and Hall,

London)

1971.

[32]

COEY, J. M. _D., J.

_{Physique Colloq.}

35

₍₁₉₇₄₎

C6-89 ;

Atomic _Energy Rev. 18 ₍₁₉₈₁₎ 1.

[33] RAMAN, T., RAO, G. _{N., CHAKRAVORTY, D.,} J.

Non-Cryst. Solids 29

₍₁₉₇₈₎

85.

[34]

YUN, Y. _{H., BRAY,} P. J., J. _Non-Cryst. Solids 44

(1981) 227.