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Russian Journal of General Chemistry,Vol. 72, No. 5,2002, pp. 8143815. Translated from Zhurnal Obshchei Khimii, Vol. 72, No. 5,2002,pp. 8713872.Original Russian Text CopyrightC 2002 by Chezhina,Kuz’mich.

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Magnetic Susceptibilityof Dilute La0.67Sr0.33MnO33LaAlO 3 Solid Solutions

N. V. Chezhina and M. V. Kuz’mich

St. Petersburg State University, St. Petersburg, Russia

Received June 28, 2001

The complex oxide La0.67Sr0.33MnO3 with theperovskite structure is a so-called[colossal magneto-resistor,] one of the series of La0.67A0.33MnO3 oxides(A = Ca, Sr, Ba, Pb). At present these oxides areextensively studied with the aim to obtain the bestelectrophysical characteristics by optimizing theircomposition [1, 2]. The basis of magnetoresistance asa cooperative phenomenon is formed by exchangeinteractions between manganese atoms in differentoxidation states [Mn(III) and Mn(IV)]. To reveal thenature of their exchange interactions and the effects ofsubstituents (Ca and Sr), we earlied studied magneticdilution in the La0.67Ca0.33MnO33LaAlO3 system [3].In this work we focused on the magnetic susceptibilityof manganite solid solutions in which lanthanum waspartially substituted by strontium.

Solid solutions La0.67Sr0.33MnO33LaAlO3(La130.33xSr0.33xMnxAl13xO3) were obtained by theceramic procedure as described in [4]. The manganesecontent was determined colorimetrically by the colorof permanganate ions with the accuracy~3%. Themagnetic susceptibility was measured in the range773400 K.

The plots of the paramagnetic susceptibility cal-culated per 1 mol of manganese atoms vs. the con-centration of the latter in the solid solutions havea maximum in the region ofx ~0.025 over the entiretemperature range. The effective magnetic momentmeff is temperature-dependent for all the solid solu-tions studied. For very dilute solutions (x 0.008 and0.011) this dependence manifests itself in the fact thatmeff, being constant up to~300 K, begins to decreasenoticeably as the temperature is raised further. Asimilar meff3T dependence is observed for an indefini-tely dilute solid solution. Since for any oxidation stateof manganese, which can occur in the solid solutionsunder study [Mn(III), Mn(IV), or even Mn(II)], theground states are either nondegenerate [Mn(IV)4A2g,Mn(II) 6A1g] or doubly degenerate [Mn(III)5Eg], and

the effective magnetic moment for the octahedralsurrounding of the paramagnetic atom is independentof temperature, we are to suggest that even on infinitedilution the manganese atoms do not disaggregatecompletely. Thus, even atx 0 some aggregates ofmanganese atoms linked by ferromagnetic superex-change are present in the solid solutions. The natureof the superexchange is evidenced by, first, the valuesof meff, which are higher than those for Mn(III) andMn(IV) and, second, by the decrease inmeff withincreasing temperature.

The temperature changes inmeff for x > 0.02 are themost interesting. At low temperatures,meff in theregion of the maximum in the magnetic susceptibilityisotherms attains 6.9 BM and decreases with increas-ing temperature. Such highmeff values point toformation of rather large ferromagnetically linkedclusters of manganese atoms, in which the exchangeparameter appears to be positive regardless of the twomanganese atoms have different [Mn(III)3O3Mn(IV)]or the same [Mn(III)3O3Mn(III)] valences. The samephenomenon has also been observed in the La0.67.Ca0.33MnO33LaAlO3 solid solutions [3], however, instrontium-substituted solid solutions the magneticmoment much less varied with temperature, from 6.8to 5.6 BM in the range 773400 K against 73 4.5 BMfor calcium-substituted systems. Moreover, themeff3Tdependence is nonmonotic and shows two regions of asufficiently abrupt decrease inmeff: 773200 and 3003400 K, between whichmeff decreases only slightly.All this suggests that the nature and energy of themagnetic superexchange in manganese clusters varywith temperature.

It is interesting that the region of the plateau in themeff3T plot coincides, according to [1], with thetemperature range where in a pure lanthanummanganite doped with strontium, as well as in solidsolutions containing very little Al (up to 3 mol%),the low-temperature coherent ferromagnetism passes

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MAGNETIC SUSCEPTIBILITY 815

to the high-temperature incoherent ferromagnetismresponsible for colossal magnetoresistance.

Therefore, we can point out two main differencesbetween calcium- and strontium-substituted solidsolutions. First, on dilution of strontium-substitutedmanganite its manganese atoms do not disaggregatecompletely, i.e. the interaction between manganeseatoms in this compound is characterized by a higherenergy incompatible with the energy of local magneticexchange. Second, while the exchange parameter doesvary with temperature in the strontium-substitutedsolid solution, this variation does not change the signof the exchange parameter (J > 0, the exchangeremains ferromagnetic over the entire temperaturerange studied). The latter can be accounted for by thefact that strontium atoms, being more sizable thanlanthanum and calcium atoms, more rigidly fixdistortions of MnOMn angles, and these distortionsare not completely taken off by increasing temperature,leaving room for ferromagnetic exchange betweenmanganese atoms with the same oxidation state. Thepresence of the plateau in themeff3T plot in the veryregion of the transfer between different metal ferro-magnetic states in the pure magnetoresistor points toa certain correlation between changes in the exchange

parameters of relatively small clusters and the pheno-menon of colossal magnetoresistance, which testifiesthat research into dilute solid solutions does allowsthe nature of this phenomenon to be understood on amolecular level.

ACKNOWLEDGMENTS

The work was financially supported by the RussianFoundation for Basic Research (project no. 99-03-32687).

REFERENCES

1. Takenaka, K., Sawaki, Y., Osuka, A., Shiozaki, R., andSugai, S., J. Phys. Chem. Solids,2001, vol. 62,pp. 3133316.

2. D’Souza, C.M. and Sammes, N.M.,J. Am. Ceram. Soc.,2000, vol. 83, no. 1, pp. 47351.

3. Chezhina, N.V., Mikhailova, M.V., and Osipova, A.S.,Zh. Obshch. Khim.,2001, vol. 71, no. 9, pp. 143131434.

4. Chezhina, N.V., Bobrysheva, N.P., and Mikhailo-va, M.V., Zh. Obshch. Khim.,2000, vol. 70, no. 1,pp. 1583159.