Magnetism of spinel microcrystals in a Cr3+ -doped cordierite glass : an E. S. R. study

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Magnetism of spinel microcrystals in a Cr3+ -doped cordierite glass : an E. S. R. study

C. Blanchard, A. Deville, A. Boukenter, Bernard Champagnon, E. Duval

To cite this version:

C. Blanchard, A. Deville, A. Boukenter, Bernard Champagnon, E. Duval. Magnetism of spinel mi- crocrystals in a Cr3+ -doped cordierite glass : an E. S. R. study. Journal de Physique, 1986, 47 (11), pp.1931-1937. �10.1051/jphys:0198600470110193100�. �jpa-00210389�

Magnetism ^of spinel microcrystals ^{in a} Cr3+ -doped cordierite glass : ^an

E. S. R. study

C. Blanchard, A. Deville A. Boukenter, ^B. Champagnon and E. Duval

Université de Provence, Laboratoire d’Electronique ^{des Milieux Condensés} (*), 13397 Marseille Cedex 13,

France

Université Claude Bernard, Laboratoire de Physico-Chimie des Matériaux Luminescents (**),

69622 Villeurbanne Cedex, ^France

(Requ le 17 avril 1986, accepté ^{le 17} juillet 1986)

Résumé. ^- Nous avons utilisé la Résonance Electronique pour caractériser ^{les amas} d’ions Cr3+ qui ^se

forment ^{au cours} de divers traitements thermiques ^{dans un verre de} cordiérite dopé ^{à 0,8 % en} Cr2O3. ^{Nous avons jugé} bon de comparer les propriétés magnétiques ^{de ces amas à} celles de MgCr2O4 ^en poudre. ^A température ambiante le spectre ^{R.P.E. de} MgCr2O4 ^est constitué d’une raie dipolaire ^rétrécie par

échange. ^{Nous avons trouvé} qu’à ^{T = 0 K le} champ d’échange He ^entre les deux sous-réseaux est d’environ 2 x 106 _G, ce qui explique l’impossibilité d’observer en bande X le signal ^{de résonance en} dessous de

TN = 15 K. Les résultats obtenus par R.P.E. ^à température ^ambiante confirment que les ^{amas de} Cr3+ formés

au cours d’un recuit 100 H à 825 °C ou 6 H à 850 °C ont une composition ^du type MgCr2xAl2(1-x) ^{O4, et}

permettent ^de préciser ^la valeur de x (x~0,9). ^{A basse} température, ^{ces amas} forment des grains ^fins antiferromagnétiques, ^{avec un} champ d’anisotropie élevé (Ha ~ 10 kG) . Pour les échantillons ayant ^{subi un} recuit de 4 h-875 °C et 2 h-900 °C, ^la concentration en Cr3+ dans les amas est plus ^faible. Après ^{un recuit de}

2 h-875 °C et 10 min-1 050 °C les ions Cr3+ sont distribués aléatoirement dans une matrice de Al2O3.

Abstract. ^- We have used Electron Spin ^{Resonance to} characterize the Cr3+ clusters which grow, during

various heat treatments, ^{in a} 0.8 % Cr2O3-doped cordierite glass. We found useful to compare the magnetic properties ^{of these clusters to those of} powdered MgCr2O4. ^In MgCr2O4 ^the room-temperature ^E.S.R.

spectrum consists of ^a single exchange-narrowed dipolar ^{line. An} approximate value of the exchange ^field

between the two sublattices at T ⁼ 0 K is He ~ ^{2 106 G,} explaining ^the disappearance of the E.S.R. signal

below TN ^{= 15 K.} Room-temperature E.S.R. results confirm that the Cr3+ clusters built up during ^{a 100 H-} 825 °C or a 6 H-850 °C heating ^{have a} MgCr2xAl2(1-x) ^O4 composition, ^and precise ^{the x value} (x ~ 0.9 ) .

At low temperature ^{these clusters are} antiferromagnetic ^fine grains, ^{with a} high anisotropy ^field

(Ha ~ ¹⁰ kG). ^{After a 4 h-875 °C +} 2 h-900 °C heating, ^{the Cr3+} concentration in the clusters is smaller, whereas after 2 h-875 °C + 10 min-1 050 °C heating, ^the Cr3+ ions are randomly distributed in MgAl2O4.

Classification

Physics ^Abstracts

81.40E-76.30F-75.30E

1. Introduction.

The study of nucleation of microcrystals ^{in a cordie-}

rite glass ^{has been} performed by several technics.

The size of nuclei was determined by ^{a small} angle

neutron scattering (SANS) [1] ^{or small} angle X-ray scattering (SAXS) [2]. ^{The chemical and} physical

structure was determined by ^laser spectroscopy, electron-spin ^resonance (E.S.R.) [1, 3] ^{and also}

electron microscopy [2]. As it is well known, ^the (*) ^{U.A. 784.}

(**) ^{U.A. 442.}

Cr3 + ion is a good nucleating agent ⁱⁿ glasses. ^{It was}

observed that nucleation occurs at temperatures higher ^than 800 °C, ^{when ions can diffuse into the} glass. At the first stage ^of the nucleation process there is a clustering ^of Cr3 ’ ions and it appears very small microcrystals. ^E.S.R. spectra ^{showed a} strong exchange coupling ^between Cr3 + _ions, and it was

deduced that the structure of microcrystals ^{is close to}

the MgCr2o4 spinel [1, 3]. ^{After a heat treatment} during ^a relatively long ^{time at a} temperature higher

than 900 °C, the E. S. R. signal ^{of the Cr3 + ion in the} MgA1204 spinel progressively ^built up, and ^a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198600470110193100

1932

decrease of the Cr3 + signal ^{in the} MgCr2o4-like microcrystals ^was simultaneously observed. The almost totality ^of chromium is imbedded in MgA1204 microcrystals ^{after a} heating ^{at 1 050 °C.}

The nucleation _process can be described as fol- lows. At first there is clustering ^{of Cr3 + ions and}

formation of a spinel ^{close to} MgCr2o4, ^{which is}

insoluble in glass ^{and demixes.} Microcrystals ^{of the} MgCr204 spinel appear and grow with the heating

time. By epitaxy, ^the spinel MgAI204 crystallizes

around a MgCr2o4 microcrystal ; ^then MgCr2o4

dissolves partially ^into MgA’204; finally complete

dissolution of MgCr2o4 ^into MgA’204 ^{occurs at}

temperatures higher ^{than 1000 °C.} Actually ^{there is}

first a nucleation of MgCr :zxA12 ( I - x) ^{04 with x near}

to 1, ^then during ^{the heat} treatment x decreases down to x = 0.02.

The aim of the research presented ⁱⁿ this paper

was to confirm the presence ^{of a}

MgCr:zxA12 (I - x) 04 spinel, ^{with x near to 1, in the}

first stages ^of nucleation, by comparing the thermal evolution of the nucleated glass ^E.S.R. spectrum with that of the MgCr204 spinel. Furthermore infor- mations on the magnetic order in the microcrystals

in glass ^were expected from these E.S.R. measure- ments.

2. Experimental conditions.

The experiments ^were performed ^{with a X band} spectrometer (9.4 GHz) (Varian E 112), ^{and a}

variable temperature gas flow crystat (Oxford ESR 9). ^{Two AuFe} thermocouples, ^{one near the}

output of the helium flow, and the other inside the

sample powder, ^{were used} for the determination of the temperature. ^The intensity measurements of the

resonance signal ^{were obtained} by comparing ^the signal ^{from the} sample, ^at temperature T, ^{with that} given by ^{a Varian reference} sample ^{at room} tempera-

ture (double rectangular cavity). ^{In the} experiments,

a signal proportional ^to the derivative dX "/dH ^of

the absorption ^{curve is} recorded. When T is varied,

if general conditions are satisfied (given lineshape,

constant amplitude ^{of the} modulation field, ^constant

value of the product: amplitude ^{of the r.f. field.}

gain ^{of the detection} chain), ^{then the area under the} absorption curve, which we call I, ^is proportional ^to y’ AH 2 ^{(ym : amplitude} of the recorded signal,

åHpp: ^{its peak to peak} linewidth). ^In usual parama- gnets, ^{I. T is} nearly ^{constant in our} temperature

range (Curie law).

The selected base glass, prepared ^at the laborato- ries of Pilkington ^Brothers p.l.c., ^{has a} composition

close to that of mineral cordierite : 52 Si02 ^34.7 A1203,12.5 MgO ^{with 0.8} Cr203. ^In

this paper ^we report the results from E.S.R. ^measu- rements on four samples ^{of this} glass : A, ^heated

100 h at 825 °C, A2 heated 6 h at 850 °C, A3 prehea-

ted 4 h at 875 °C and heated 2 h at 900 °C, A4 preheated ^{2 h at 875 °C} and heated 10 min at

1050 °C.

Table I.

In table I are given the diameters of microcrystals

in glass ^as determined by SANS, ^{SAXS or electron} microscopy. ^The MgCr2o4 ^or MgAI204 spinels ^used

were polycrystalline samples.

3. Experimental ^results.

In order to get ^{a better} understanding of the E.S.R.

spectra ^{of annealed} cordierite samples, ^{we first}

recorded the E.S.R. spectra ^{of three reference} samples : ^a cordierite glass sample ^before annealing,

a crystalline powder ^of MgAI204 doped ^{with 1 %} Cr3 + , ^{and a} MgCr204 crystalline powder ; ^{we then}

studied the E.S.R. of four annealed (A1 ^to

A4

cordierite glass samples (see ^Table I) :

3.1 ROOM TEMPERATURE E. S. R. SPECTRA OF REFE- RENCE SAMPLES. ^- The E.S.R. spectrum ^{of an}

unannealed cordierite glass sample is shown in

figure ^{1. It shows a} peak around 1250 G

( geff - 5.3) ;

kind of spectrum ^{has been} interpreted by Landry

et al. [4] assuming ^{that the Cr3 + ions are in orthor-}

hombic sites, randomly oriented relative to the external field. The spin Hamiltonian of a Cr3 + ion in

a given ^{site is :}

X, Y, ^Z being the local cristalline axes.

However Landry’s interpretation ^{assumes that}

E « D ; the line with geff ^{= 5.3 is then} expected ^{to be} only weakly allowed, its intensity

bein g E D

Fig. ^{1. 2013} Room-temperature ^E.S.R. spectrum ^of Cr3 ’

ions in cordierite glass.

smaller than that of an allowed transition. This is not

experimentally observed. On the contrary, ^{if it is} assumed that E > D, ^one expects ^a spreading ^{out of}

the spectrum ^between two extreme values of the

magnetic field, corresponding respectively ^to geff ~

The powder spectrum ^of MgAl20 4: ^{1 % Cr3 + is} presented ⁱⁿ figure ^2. The E.S.R. spectrum ^of

Cr3 + in a MgA1204 single crystal has been studied by

Stahl-Brada et al. [6] ^and by ^{Atsarkin et al.} [7]. ^In

this material, ^the Cr3 + ions substitute to Al3+ ions in B sites ; ^{there are four non} equivalernt ^{B sites in}

the unit cell ; ^{the local} symmetry ^is axial, ^{the axes} corresponding ^to crystallographic directions [111], [111], [111], [111] ; ^the spin Hamiltonian for a given

site can be written :

where g = 1.98 and Z is the symmetry axis ; ^{for X} band, ^D gl3 H (D > 0.9 cm 1 [7]). When the Zee-

man term is considered a perturbation of the fine structure, ^the ground ^level splits ^{into two Kramers} doublets, ± 1/2 ) (energy - D) and ± 3/2 ) (energy ⁺ D). The random distribution of the orien- tation of the symmetry axis results in a powder spectrum ^with gll = g and g_ = 2 g. There then appears ^a peak ^{in the} spectrum, located around The E.S.R. spectrum of Cr3 + ions in a MgCr2o4 powder consists of ^a single nearly Lorentzian line, around g ^{= 2} (Fig. 3), ^{and with} AHpp ^{= 255 G,}

which agrees with previous ^results [8]. ^The position

of the line, its Lorentzian shape and the small value of the linewidth are the signature ^{of a} dipolar ^line

with strong exchange narrowing. ^{The fine structure}

term has a minor importance ^{in this material, where}

the Cr3 + concentration is high.

3.2 ROOM-TEMPERATURE E.S.R. SPECTRA OF ANNEALED CORDIERITE SAMPLES. ^- We now dis-

cuss the room-temperature ^E.S.R. spectra given by

c2 + -doped cordierite glass samples after various

annealings : ^these samples, ^called AI, A2, A3, A4 ^are presented in table I :

The room-temperature ^E.S.R. spectra ^of Al, A2, A3 ^are mainly characterized by ^a nearly ^Lorentzian

line around geff = 1.98 and by ^a much weaker signal

around geff ^{= 5.3} (Fig. 3). ^Our previous results with the reference samples suggest that these signals ^are

the sum of two contributions : the intense line around 1.98 (AHpp ^{= 490, 470, 710 G for A,, A2, A3}

respectively) ^is associated with Cr3 + clusters ; ^{it is}

possible ^{to correlate} this line with that given by

(1) We assume g = 1.98 for Cr3 ’ in cordierite, ^{a value} typical ^{for Cr3 +} [5].

Fig. ^{2. -} Room-temperature ^{E. S. R.} spectra ^of Cr3+ ions in : Polycrystalline MgAlzO 4 (1 ^% Cr3 ’ ) spinel ; ^cordierite glass ^{after a} 2 h-875 °C and a 10 min 1050 °C heating

M

Fig. ^{3. -} Room-temperature ^E.S.R. spectra ^of Cr3+ ions in: Polycrystalline MgCr204 spinel (,IHPP = 255 G) ;

Cordierite glass ^{after a} 100 h-825 °C heating

(A1 i1H pp ^{= 490} G) ; Cordierite glass ^{after a 6 h-850 °C}

heating (A2 &Hpp ^{= 470} G ) ; Cordierite glass ^{after a 4 h-}

Cr3+ in MgCr204 ; ^we will however see that there may appear differences in their thermal behaviour.

The much weaker shoulder around geff ⁼ 5.3 is due to a small proportion of isolated Cr3 + ions in the

glass ^matrix (cf. Fig. 1).

Figure 2 shows that the room-temperature ^E.S.R.

signal ^for A4 ^is nearly ^{the same as that for} MgAl20 4 :

1 % Cr’ +. This comparison suggests that, ^{after a} strong heat treatment, Cr3 + ions are imbedded into

MgA1204 microcrystals.

In the following, ^we will look for a better characte- rization of the Al, A2 ^and A3 samples ; ^{as we think}

that in these samples ^most Cr3+ ions belong ^to clusters, ^we will compare the thermal variation of the E.S.R. parameters (linewidth, intensity) ^for

these samples and for the MgCr2o4 powder, ^which represents ^{an extreme case.}

1934

Fig. ^{4. - 1 TII T} ( 295 K ) ^versus temperature ^{for :} a) polycrystalline MgCr2o4 spinel . ; b) ^nucleated glass samples (A1 ^{0, A2 + , A3} 0 ).

3.3 TEMPERATURE DEPENDENCE OF E.S.R. SPEC- TRA. ^- MgCr204 ^{is known to be an} antiferromagnet

with TN ^{= 15 K} [9]; ^when lowering ^the temperature from 300 K, ^we observed that the E.S.R. line in the

polycrystalline MgCr2o4 sample broadens and that its intensity decreases. The E.S.R. signal finally disappears ^{at 15} K, within the precision ^{of our} experiments ; the thermal variation of AHPP ^{is given}

in figure 5 for T - TN > 1 K (it ^{was not} possible ^to

obtain significant ^{results for T -} TN ^{1 K since then}

âHpp - ^{Ho and moreover the intensity goes to zero).}

The temperature dependence ^{of the}

ratio is reported ⁱⁿ figure ^4a.

The signal ^of samples Al ^and A2 disappears ^at

15 K, ^{as for} MgCr2o4. The linewidth for these

samples also increases when the temperature ^decrea-

ses down to TN, but much less strongly ^{than for} MgCr2o4, ⁱⁿ particular ^near TN (Fig. 5).

The IT / IT (295 K) ^{ratio is} given ⁱⁿ figure 4b ; ^it

goes ^{to zero} when T - TN.

The signal ^of sample A3 ^can still be detected at the lowest temperature (5 K) ; the linewidth is seen to

increase down to this temperature (Fig. 5).

Fig. ^{5. -} åHpp (T) - åHpp ^{( 295 K ) versus tempera-}

ture for polycrystalline MgCr2o4 spinel (.) ( OH ( 295 K ) ^{= 255} G ) ^and different nucleated glass samples.

4. Discussion.

4.1 MAGNETIC PROPERTIES. ^- We will successively

discuss the origin ^{of the linewidth at} room-tempera-

ture for MgCr2o4 ^{and for} sample A, ^and A2, ^then

the reason of the absence of E.S.R. signal ^below TN. ^{We will} finally briefly discuss the magnetic

behaviour of sample A3.

Before going ^{on, we} will first make two comments :

- As far as we know, the E.S.R. of Cr3+ in

MgCr2o4 ^{has not} previously been studied between 15 and 300 K ; the aim of this paper is ^not the study

of the E.S.R. of MgCr2o4 ^near TN; ^{we however}

mention that we have not been able to fit the linewidth increase near TN ^{with a} power law ^as

predicted by ^Kawasaki [10] : ^this theory predicts

that /1Hpp(T) -/1Hpp(oo)oc

T - TN y

y ⁼ 5/3 ; ^{such a} critical behaviour has been observed

for instance in MnF2 [18], ^{but not with the} expected

y value (y - 1.2).

- We have already stated that in MgCr2o4 ^at

room-temperature, ^{the fine structure} energy is negli- gible against ^the exchange ^and dipolar couplings ; ⁱⁿ

the following ^we will consider that this is also true for Al, A2 ^and A3 ^as suggested by ^{the room-} temperature spectra.

4.1.1 Room temperature E.S.R. results. ^- As it is difficult to calculate the linewidth for a spinel exactly

even when T > TN, ^we will make an approximate

determination of the parameters ^of interest, using

the results established for a simple ^cubic lattice, ^and considering ^nearest neighbours only ; ^{this last} approximation ^is certainly the cruder for our mate-

rials, ^{as it leads to} TN ^{= 0} (0 : ^Curie temperature ^for the susceptibility ^{in the} paramagnetic region).

We have seen that at room temperature ^the absorption ^{line in} MgCr2o4 is Lorentzian with a

width at half-height AHw. = J3 AHpp ⁼ 430 G. It is

clearly ^a dipolar exchange-narrowed line : if there

were the dipolar coupling only, the line would be Gaussian and much broader : an approximate ^value

of the dipolar ^linewidth AHd ^{can be found} by using

the result established for ^a simple ^cubic crystal:

neighbour distance) ^{in our} case ro = 2.94 A,

S ⁼ 3/2 ^and AHd ^{= 6} 000 G. When there is an

exchange energy 2: ^{Iii Si s,} it is well known

i ~ j,j

(Anderson ^{and Weiss} [13]) ^{that if y} OHd T c 1

yHo rc (moderate narrowing ; rc is the correlation time of the dipolar field in the presence of exchange, Ho ^{the resonance} field and _y the gyromagnetic factor) then the line is Lorentzian with a width when the exchange ^is impor-

tant and such that y Ho T c $ ¹ (extreme narrowing),

then the width of the Lorentzian line is

We conclude from the discussion and our experimen-

tal results that in MgCr2o4 ^{we have an extreme} exchange narrowing, with Tc ⁼ 4 x 10-13 s. We can

find an approximate ^{value of} Jij ^{for nearest}

neighbours, called ^{J, using the expression}

established for a simple ^cubic

lattice : J/k - 4 K. We will now propose ^an explana-

tion for the experimental fact that the Lorentzian line in samples Al ^and A2 is broader than in

MgCr2o4 (2). We ascribe this behaviour to the fact that the magnetic ions concentration in samples A, ^and A2 is smaller than in MgCr2O4, leading ^{to two} opposite effects : the dipolar ^linewidth AHd ^decrea-

ses, and the correlation time rc increases ; ^our experimental ^results suggest that the latter effect is the more efficient. We will now show that this

viewpoint ^seems quantitatively sound ; ^{we consider}

that the relation !1H1f2 ^{m oc}

1 ,

still true for samples Al ^and A2, ro being ^{now an}

effective distance between nearest nei hbours,

which is slightly lower than that ^- 2.94 - in

MgCr2o4. We suppose that J ^oc e - Àro _(see for ins- tance Griffiths [14]). ^{In order to} get ^an approximate

value for A, ^{we use} the numerical data given by

Mollenauer et al. [22] for the first and second

Cr3 + neighbours ⁱⁿ A1203, getting then A =. 12 Å - 1.

Àro

When ro varies, !1H1/2

oc e ,A ro

r6

6 6 ⁼ 0.5 A. As in our materials ro > rom, ^{an increase}

A

of ro will lead to a line broadening : the effect of

exchange, ^described by eÀro, dominates. Our experi-

mental results

lead to ro ^{= 3.01} A for A, ^and A2 ^{and to a relative}

concentration variation

From the above discussion we found that x - 0.9;

this represents only ^an approximate value, ^{as it rests}

on the following assumptions : ^an homogeneous

distribution on the Cr3 ’ ions in the cluster, ^{and an} exchange coupling ^{between nearest} neighbours only.

4.1.2 Behaviour below TN. ^- We will first discuss the case of MgCr2o4, which will help ^{us for the} understanding ^{of the more} complex ^{situation met in}

the antiferromagnetic ^fine grains present ⁱⁿ samples A19 Az.

In MgCr2o4, ^the exchange coupling ^{has two}

different effects. When T > TN, ^{it leads to fluctua-}

tions of the local dipolar field, ^described by ^{a finite}

Tc; ^{when T} TN it ^{leads to} the appearance of ^a spontaneous magnetization inside each sublattice

(MA ^and MB respectively) ; MA for instance is then submitted to a strong effective field coming ^from

both the spins ^inside sublattice

B (-

MB

those inside A

W’ MA

there appears ^a torque ^when MA for instance is removed from the direction of antiferromagnetism,

a fact described by ^an anisotropy ^field Ha. ^{We will}

now explain ^the disappearance of the E.S.R. signal

(2 ) ^{A change in} T c caused by ^{disorder in} samples A,

and A2 ^can’t explain this : T would ^be shorter and the line

narrower (for instance when melting ^a solid, ^{a motional} narrowing ^is observed).