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STUDY OF SrEuFeO4 AND SrGdFeO4 BY MÖSSBAUER EFFECT
D. Hanžel
To cite this version:
D. Hanžel. STUDY OF SrEuFeO4 AND SrGdFeO4 BY MÖSSBAUER EFFECT. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-159-C1-161. �10.1051/jphyscol:1980142�. �jpa-00219711�
JOURNAL DE PHYSIQUE Colloque C1, suppI&ment au no 1 , Tome41, janvier 1980, page C1-159
STUDY OF SrEuFe04 AND SrGdFe04 BY &SSBAUER EFFECT
D. ~ a n s e l
J. Stefan Instifzte, E. KardeZj Vn-Lversity of Ljt*bZjana, 61000 LjubZjana, YugosZatcia.
The oxides of the SrLnFe04 family, where Ln = rare earth, have K2NiF4 struc- ture /1/ and present an interesting examp- le of 2-D magnetic materials which have already been studied by X-ray, magnetic susceptibility and Massbauer methods.
Previous Mijssbauer measurements /2/
had shown that compounds with Ln' = La, Pr, Nd, Sm ordered magnetically in the tempe- rature interval 250 K
-
400 K, whileSrEuFe04 and SrGdFe04 ordered at tempera- tures below 80 K. The spin reorientation process /3/ had been observed in SrPrFe04 and in SrNdFe04, while magnetic suscepti- bility measurements had indicated no chan- ges even at magnetic phase transitions.
Since contradictory results for magnetic ordering temperature of SrEuFe04 have been reported /2,4/, and since the spin reori- entation process of be3+ ions in SrGdPe04 has not yet been observed, we have reexa- mined both compounds.
The samples of SrEuFe04 and SrGdFe04 were prepared by a ceramic technique by firing the mixture of oxides at 1350 K in the appropriate mole ratio with an'excess of. SrO. The samples thus obtained were checked by the X-ray powder method and a single phase of the K2NiF4 type structure was found for both samples'.
Massbauer spectra of 5 7 ~ e were ta- ken in the temperature interval from
15
K to 300 K for SrEuFe04 and from 8 K to 350 K for SrGdFe04. Some characteristic spectra of 5 7 ~ e in SrEuFe04 are given in Fig. 1. They can be fitted with a single broadened sextet below 270 K and with one broadened quadrupole doublet above 280 K.,Fig.l: Mijssbauer spectra of 5 7 ~ e in SrEuFe04 at various temperatures taken with the source of 57~0 in Rh.
:These spectra reveal a considerably larger (Nee1 temperature than reported in Ref. 2.
Due to the additional broadening of the lines and the appearance of the central doublet near the Nee1 temperature TN ili both compounds, the transition temperatn-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980142
c1-160 JOURNAL DE PHYSIQUE
res were estimated from the onset of hy- perfine fields to be:
TN = 280 2 5 K for SrEuFe04 TN = 310 2
5
K for SrGdFe04It is interesting to note that the Ordering temperatures of these compounds depend very much upon the preparation pro- cedure and the presence of impurities in the samples. For samples prepared by fi- ring at 1250 K we obtained quadrupole- split spectra even at 80 K similarly as reported in Ref. 2. The same spectra were observed in samples which contained impu- rities of Pb or Rh with concentrations less than 0.2 46. A possible explanation for the quadrupole splitting in these samples is that superparamagnetic clusters are formed due to defects in the 2-D lat- tice.
The spectra of 5 7 ~ e in SrGdFe04 gi- ven in Fig. 2 indicate that in this sample the iron spins reorient with respect to the crystallographic axes on lowering the temperature. An analysis of the spectra by diagonalizing the Hamiltonian matrix with simultaneous magnetic and electric inte- ractions give the best fit with experimen- tal data if the polar angle between Vzz and
fief
is near 90' at77
K and near 0' at 8 K. With the above values for the polar angles the spin reorientation is unambi- guously determined.In the temperature region from 55 K - 250 K, changing the polar angle by a few degrees from 90' results in a poor fit, suggesting that the principal axis Vu of the EFG tensor remains in the same
direction in this temperature region.
Fig.2: MGssbauer spectra of 5 7 ~ e in SrGdFe04 at 8 K and 58 K taken with the source of 5 7 ~ o in Rh.
Near the spin reorientation tempera- ture, which is at
45
K, splitting of the absorption lines in the spectra of 5 7 ~ e in SrGdFe04 becomes visible. These spectra can be analysed by a superposition of two sextet patterns with linewidths of 0.26 mm/s, while the widths are more than 0.58 mm/s for single six line pattern at 8 K.A superposition of two or more subspectra can be seen also in Figs. 3 and 4 of Ref. 3 corresponding to the spectra of isostructural SrNdFe04 and SrPrFe04. The- refore, we assume that the splitting of the lines in the spectra of SrGdFe04 re- presents structural properties and not the inhomogeneity of the sample.
We have performed a point charge lattice calculation by using the crystal- lographic data of SrNdFe04 and found that the positive principal axis Vzz of the EFG tensor pointed along the crystallo- graphic c-axis. Using this result for SrGdFe04 and considering the polar angles obtained above, it follows that the iron spins- in - SrG@e04- are -directed in --paralelt-
to the crystallographic c-axis below 45 K, References
45 K. However, the spectra of Fe3+ in G. le Flem, A.Daoudi and G.Ollivier, SrEuFe04 give evidence that the Fe3+ spins J. Solid State Chem.
2
(1970) 343.point along the crystallographic c-axis
/2/ M.Shimada and M.Koizumi, Mat. Res.
and the iron spins do not reorient down to
Bull.
11
(1976) 1237.the lowest temperature 15 K measured by us.
/3/ M.Shimada, M.Koizumi, M.Takano, Further heating-cooling cycle measu-
T.Shinjo and T.Takada, J. Physique rements around the spin reorientation tem-
Colloque
C2 (1979)
272.perature and the investigations of the in-
fluence of inhomogeneities on the MBssba- /4/ D.Hanie1, Fizika
5
Suppl. (1974) 33.uer spectra of 5 7 ~ e in SrGdFe04 are in progress.
Acknowledgements: The author is indepted to Dr. M.Drofenik for preparation the samples. The financial support of the re- search Community of Slovenia is also ack- nowledged.
