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MAGNETIC HYPERFINE FIELD DISTRIBUTIONS
IN FERRIMAGNETIC SPINELS Fe2(1-y)Mg1+yTiyO4
WITH y
≤ 0.5
E. de Grave, R. Vanleerberghe, C. Dauwe, J. de Sitter, A. Govaert
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 12, Tome 37, Décembre 1976, page C6-97
MAGNETIC HYPERFINE FIELD DISTRIBUTIONS
IN FERRIMAGNETIC SPINELS Fe
2(1_
y)Mg
1+yTi
y0
4WITH y ^ 0.5
E. DE GRAVE, R. VANLEERBERGHE, C. DAUWE, J. DE SITTER and A. GOVAERT University of Ghent, Laboratory of Magnetism, Proeftuinstraat 86, 9000 Ghent, Belgium.
Résumé. — Un nombre d'échantillons, riches en Fe, avec formule générale Fe2(i-j/)Mgi+ï/Ti3,04,
a été étudié par effet Môssbauer à des températures au-dessus de 77 K. Nous avons trouvé que la structure hyperfine est fortement influencée par le nombre des ions diamagnétiques dans le réseau. Même pour des valeurs du paramètre y égales à 0,2 ou 0,3 des effets de relaxation sont observés à des températures inférieures à celle du point de Néel correspondant. Pour des valeurs plus élevées, les spectres montrent simultanément une configuration à six raies fortement élargies ou relaxées et un doublet quadrupolaire. L'origine des propriétés observées est discutée.
Abstract. — Mossbauer effect measurements at temperatures above 77 K have been performed on a number of Fe-rich samples with general formula Fe2u-wMgi+s/Ti!/04. It is found that the
magne-tic hyperfme structure is strongly influenced by the number of diamagnemagne-tic ions present in the lattice. Even for rather low diamagnetic substitution rates (y = 0.2, 0.3), relaxation effects are observed at temperatures far below the corresponding Neel temperatures. For higher values of y, the spectra show the simultaneous presence of a strongly broadened or relaxed six-lines pattern and a quadrupole doublet. The origin of the observed properties is discussed.
1. Introduction. — In the solid-solution series y
Mg2Ti04-(l - y) MgFe204, the M g2 + and Fe3 +
ions are distributed among the A- and B-sites of the spinel structure, while the Ti4 + ions are situated on the
B-sites. All previous investigations on this spinel were concerned with the determination of the cation distri-bution in these materials by means of magnetic [1, 2] or X-ray diffraction measurements [3]. All authors agree in that the ionic distribution, at least for composi-tions y > 0.5, depends on the thermal treatment of the samples. This dependence has been widely studied for M g F e204 (y = 0.0) using different techniques :
magne-tization measurements [4], X-ray diffraction [5, 6] and, more recently. Mossbauer effect [7].
As far as we know, only the end-member y = 0.0, i. e. MgFe204, has been investigated with the
Moss-bauer effect technique [8, 9, 10]. From these studies, especially from the measurements in high external magnetic fields [10], it follows that both A- and B-site magnetic hyperfine patterns are strongly broadened which has been ascribed to the locally varying cation distributions, giving rise to a magnetic hyperfine field distribution.
In this paper, we report and explain our Mossbauer effect measurements, carried out at temperatures above liquid nitrogen temperature, on slowly cooled samples with compositions y = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5. The aim of our measurements was to investigate the influence of the diamagnetic substitution on the beha-viour of the magnetic hyperfine fields of the ferric ions which are the only magnetic ions in the samples.
2. Results. — The influence of the diamagnetic
substitution rate y on the magnetic hyperfine structure is illustrated in figure 1 in which we have represented one Mossbauer spectrum of each of the six investigated compounds. The respective temperatures have been chosen in such a way that the macroscopic T/rN-effect
is nearly equal for all samples. In the spectrum of MgFe304, two six-lines patterns may be recognized.
fewiwff
„ jf : ^ = 0 . 0 - •• i "l .. y = 0 3 -09 8 1 S 0 9 / l- } ' T=300K 4 -T=150K cI f f f 1
s' 0 0 - < ^ , / f c ^ / ^ J *1- - * * . A * - - L O O S if ; : ? 5 J - ? U > -099 0 9 8 - •; < » 1 - w t • • y=01 il y=0 4 . : T=235K T=125K T T T 0 9 8 - •••• . : : •; ••> •..- - \ \ ' • :•' • 5 :-" ':.' - \ t -0.98 • i ' 5 <y=02 j y=05 0.96- f { ' T='90l<- I T=86K_0.97 I 1 I I I I I I • ' i -10 -5 0 5 10 -10 -5 0 5 10velocity Imm/s) ' velocity (mm/s)
*- »-FIG. 1. — Mossbauer spectra of six compounds of the solid-solution series Fe2(i_»)Mgi+!/Ti3/04 at temperatures T
cor-responding to T/Tv = 0.40 ± 0.02.
7
C6-98 E. DE GRAVE, R. VANLEERBERGHE, C. DAUWE, J. DE SITTER AND A. GOVAERT
Mossbauerparameters of octahedral ( B ) and tetrahedral (A) ferric ions in MgFe204
* Versus Pd.
The two overlapping subspectra are, as one expects, generated by the two crystallographically different groups of ferric ions. Moreover, the absorption lines are markedly broadened and show, as observed from the computer fittings, some asymmetry. These features may be ascribed to the local variations in the number of magnetic superexchange interactions per ~e~ ion due to the random distribution of the diamagnetic ions in both octahedral and tetrahedral sublattices. The asymmetric broadening is expected to decrease with decreasing temperature. In table I we have listed the full width at half maximum (FWHM) of the outer absorption lines of both subspectra of MgFe,04 at four different temperatures.
The subspectra have been assigned to the respective groups of ferric ions on the basis of their hyperfine parameters. Hyperfine field and isomeric shift have also been included in table I. Although the results for the hyperfine parameters become less reliable at higher temperatures, the obtained values show the normal temperature behaviour. The observed differences between the A- and B-site hyperfine data are consistent with the results usually obtained for ferrites El 11. The A site absorption lines are about 30
%
broader than the B site ones which is due to the larger range of local varia- tions in the B site cation distribution. The ratio of the intensities of the two subspectra was found to be 0.80+
0.05 at 77 K whereas it is expected to be 0.86+
0.03 on the basis of the cation distribution determined with X-ray diffraction.As expected, the asymmetric broadening increases with increasing amount of diamagnetic ions. The A-
and B site magnetic hyperfine patterns are not longer resolved and the absorption lines may not be described as a superposition of two Lorentzian lines. For Fe,.,Mg,.,Ti,.,04, it was found that at least six Lorentzian lines, with FWHM of about 0.4 mm/s, were needed to describe each of the two outer absorp- tion lines of a spectrum recorded at 77 K. The fitting is shown in figure 2a. Due to the symmetric position, relative to the point of zero velocity, of lines with nearly equal intensities, an approximative magnetic hyperfine field distribution may be derived (Fig. 2b). A connection between this distribution and the possible cation arrangements could not be observed.
FIG. 2. - (a) Outer absorption lines of the Mossbauer spectrum
at 77 K for Fe1.4Mg1.3Ti0.304 and the fitted sum of twelve Lorentzian lines ; (6) Approximative magnetic hyperfine field
distribution corresponding to the fitted spectrum.
Another interesting property of the y = 0.3 sample appears, as illustrated in figure 3, in the high tempe- rature spectra. From about 270 K on, the spectra show typical relaxation features [12, 131. An analogous behaviour was observed for the sample with composi- tion y = 0.2, however over a smaller temperature range.
The Mossbauer spectra of compounds with y 2 0.4 are clearly characterized by the simultaneous presence of a central quadrupole doublet and a strongly broadened or relaxed six-lines pattern. The intensity
Np, relative to the total intensity N,,, of the spectrum,
MAGNETIC HYPERFINE l?IELD DISTRIBUTIONS IN FERRIMAGNETIC SPINELS Fe211-Y)Mgl+YTiY04 (26-99 1.00 m Z 0.50 - 100 025- - a98 1 I I I I I I 0 50 100 150 200 250 300 T I K )
,
'and 0 for y = 0.5). 099 330K-10 -5 0 5 10 -10 -5 0 5 10 3. Discussion.
-
The relaxation spectra and theveloci ty(mm/s) velocity (mmls) superimposed central doublet are an indirect conse- quence of the large amount of diamagnetic ions in the
FIG. 3. - Mossbauer spectra of Fei.4Mg1.3Ti0.304 at different lattice. The number of AB superexchange linkages per
temperatures. ferric ion is, for a homogeneous sample, randomly
distributed throughout the lattice and varies between zero and the maximal number, the average being deter- in figure 4 in which we have collected some ~~~~b~~~~ mined by the substitution rate. Ferric ions which have spectra of Fe, .,Mgl .,Ti,.,O, at different tempera- none or only one magnetic linkage with magnetic ions tures. The doublet lines and the two central lines of the of another coordination are suggested to behave para- Zeeman pattern may be well described by one Lorent- magnetically at temperatures above a certain threshold zian line each. In this way, the rate N,IN,,, could be temperature T t well below .the corresponding ~ 6 e l determined and is plotted against temperature in point. The magnitude of Tt depends on the number and figure 5 for compositions y = 0.4 and y = 0.5. strength of the sublattice superexchange interactions. From the statistical treatment of Gilleo [14], in which the role of the intrasublattice interactions is neglected, the amounts of so-called paramagnetic centers in our samples were calculated to be 3
%,
8%
and 15%,
respectively for compositions y = 0.3, 0.4 and 0.5. The consequence of the presence of these paramagnetic centers is to limit, in a random way, the dimensions of the chains built up by Fe3+ ions which have two or more intersublattice superexchange linkages. So, the099 length of these chains varies randomly throughout the
specimen and chains, having a total number of ferric
100 K
;
,
ions below a certain critical value Nc generate a quadru- pole doublet due to the fast relaxation of the spins.
$7;
\;
v
The critical number Nc decreases with increasingo 99 temperature and therefore, the inverse behaviour is to
I Z ~ K 275 K be expected for the doublet intensity. At 0 K the ratio I . O
*:
~ri;i;Tl~
Np/Ntot would be equal to the statistically calculatedG : s
bour amounts of paramagnetic centers if the nearest-neigh- AB interactions would be the only magnetic interactions. However, due to the intrasublattice o 98 I ~ O K 300 K interactions and the longer-range magnetic interac--10 - S o , 5 lo -10 -5 o 5 10 tions, the spectra recorded at very low temperatures
veloclty (mmls) velocity (mmls) will probably show a well defined six-lines pattern,
FIG. 4. - Mossbauer spectra of Fe1.2Mg1.4Ti0.404 at different without central doublet.
C6-100 E. DE GRAVE, R. VANLEERBERGHE, C. DAUWE, J. DE SITTER AND A. GOVAERT
they are short or long) remain coupled to each other by paramagnetic doublet may be understood on the same the weaker intrasublattice interactions and the whole basis.
system must be treated as a coupled system. For such
compounds, i. e. Fe, .,Mg, .,Ti, .,O,, the spin fluctua- Acknowledgments. - The authors wish to thank tions give rise to Mossbauer spectra exhibitting ferro- Prof. Dr. G. Robbrecht for his continuous interest in magnetic relaxation features. The relaxed six-lines this work. They are very grateful to F. K. F. 0. patterns observed in those samples which also show a and I. W. 0. N. L. for financial support.
References
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[2] TELLER, J. and LENSEN, M., Bull. Soc. Chim. 1966 2502. Acta Phys. Pol. 28 (1965) 423.
131 DE GRAVE, E., DE SITTER, J. and VANDENBERGHE, R., ~ p p l . [91 WIESER, E., SCHRODER, H. and KLEINSTUCK, K., ~ h y s . stat.
Phys. 7 (1975) 77. Sol. (a) 1 (1970) 749.
[4] N ~ E L , L., Annls de Phys. 3 (1948) 137. [lo] SAWATZKY, G. A., VANDER WOUDE, F. and MORRISH, A. H.,
Phys. Rev. 187 (1969) 747. 151 CORLISS, L. M., HASTING% J. M. and BROCKMAN, F. G., VAN LoEF, J. J . , PhySica 32 (1966) 2102.
Phys. Rev. 90 (1953) 1013. [12] VAN DER WOUDE, F. and DEKKER, A. J., Phys. Stat. Sol. 9 [6] KRIESMAN, C. J. and HARRISON, S. E., Phys. Rev. 103 (1965) (1965) 775.
857. [13] NOWICK, I. and WICKMAN, H. H., Phys. Rev. Lett. 17 (1966) [7] DE GRAVE, E., DAUWE, C., GOVAERT, A. and DE SITTER, J., 949.
