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Submitted on 1 Jan 1980
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MÖSSBAUER SPECTROSCOPIC STUDIES OF
DIMENSIONALITY AND SPIN REDUCTION
EFFECTS IN THE ANTIFERROMAGNETIC
SYSTEMS : (AF)n-FeF3
G. Gupta, J. Baines, D. Cooper, D. Dickson, C. Johnson
To cite this version:
JOURNAL DE PHYSIQUE Colloque Cl
,
supplbment au n"
1, Tome 41, janvier 1980, page C1-
187
M~~SSBAUER
SPECTROSCOPIC
STUDIES
OF DIMENSIONALITY
AND
SPIN
REDUCTION EFFECTS
IN THE ANTI
FERROMAGNETIC
SYSTEMS
:
(AF),-F~F~
G.P. ~uptay J.A. Baines, D.M. Cooper, D.P.E. Dickson and C.E. Johnson
Department of Physics, f i e University
of
LiverpooZ, LiverpooZ, ENGLAND.The mixed fluorides (AF)n-FeF3, where A = K, Rb etc., provide good examples of magnetic systems with different dimensionality. The diamagnetic
AF layers greatly weaken the exchange path in a perpendicular direction and the magnetic dimen- sionality of the system therefore varies from three-dimensional in the case of FeF3 to quasi zero-dimensional in the case of A3FeF
6' One of the effects of the dimensionality is that although the strongest superexchange path, Fe-F-Fe, is essentially the same in all members, the antiferromagnetic ordering (NIel) temperatures,
chain to intrachain exchange constants can be obtained 131 and in the A FeF system it is found
2 5
to be approximately 5 x (compared with one for a pure three-dimensional system and zero for a pure one-dimensional system). Another effect of the one-dimensionality is that the observed saturation values of the hyperfine field (41T for K2FeF5, 43T for Rb2FeF5 and N2H6FeF5) are anomalously low for a high-spin ferric ion for which a value of about 60T would be expected (in FeF3 the saturation value of the hyperfine field is 62.5T). The hyperfine field can be reduced as a result of which depend on the strength of the exchange covalency or crystal field effects but this seems interaction in all three directions, range from highly unlikely in view of the octahedral fluorine 362K for FeF to less than 1.3K for K3FeF6 and
3 environment of the iron atoms. The low saturation
Rb3FeF6. value of the hyperfine field is interpreted as
Some of the most interesting behaviour is arising from zero-point spin reduction, which is observed in the quasi one-dimensional system essentially a consequence of the zero-point energy A2FeF (A2 = K2, Rb and N21i6), which consist of
5
2 in an antiferromagnetic system. Spin wave theory3-
chains of (FeF6) octahedra separated by the A predicts that the zero-point spin reduction is a anions /1,2/. All three compounds exhibit low function of J'/J and is considerably enhanced in a Ndel temperatures of around 9K. A pure one- one-dimensional system. The 30% spin reduction dimensional system cannot sustain long range order observed in the A2FeF system leads to a value of
s
at finite temperatures, but weak interactions about for J'/J. The Kdssbauer spectra of between the chains will lead to magnetic ordering singie crystal samples of K FeF5 and RbqFeF5 taken
2
at low temperatures. From the ratio of the NIel in applied magnetic fields show an appreciable temperature (from the K6ssbauer measurements) to dependence of the hyperfine field on both the the Curie-Weiss constant (from magnetic suscepti- value and the orientation of any applied magnetic bility measurements) the ratio J'/J of the inter- field. This highly unusual behaviour can be under-
stood in terms of spin reduction at finite
*
Present address: Department of Physics, Lucknow temperatures, which on the basis of spin wave University, Lucknow, INDIA. theory can be shown to have an applied field
CI-188 JOURNAL DE PHYSIQUE
dependence which increases with increasing temperature. In keeping with these predictions the dependence of the hyperfine field in K2FeF5 and Rb FeF on the magnitude of the applied field
2 5
at 2.2K is less than half what it is at 4.2K. Comparison of the experimental variation of hyperfine field with applied field with curves showing the spin reduction against applied field for different values of J 1 / J gives further
-3 confirmation that this ratio is around 10
.
Although this behaviour is most clearly evident in the spectra obtained from single crystal specimens it has also been observed with poly- crystalline samples of Rb2FeF5 and N2H6FeF5 in which the variation of the hyperfine field with the orientation of the applied field for different crystallites leads to an appreciably different spectral profile than that which would be obtained if this effect were absent.Although spin wave theory predicts part- icularly pronounced spin reduction effects in quasi one-dimensional antiferromagnetic systems, the behaviour should also be observable in
systems with higher dimensionality. In particular the quasi two-dimensional antiferromagnetic systems, AFeF4, should exhibit measurable effects. The relatively low saturation value of the hyperfine field in RbFeF4, 53.4 k 5T, /4/ is very likely to be a consequence of zero-point
spin reduction. The dependence of the hyperfine field on the applied field is not observable in the Miissbauer spectra of these materials at 4.2K but because the field dependent spin reduction effects are enhanced at higher temperatures it should become evident in the spectra obtained at higher temperatures nearer to the N6el tempera- ture.
In A FeF there is only a very weak super- 3 6
exchange path Fe-F-A-F-Fe in all three directions and thus this system can be considered as quasi zero-dimensional. This is confirmed by spectra obtained from K3FeF6 and Rb FeF6 in which no
3
ordering is observed at temperatures down to 1.3K. However even in the paramagnetic region the application of a large magnetic field leads to a slowing down of the spin relaxation and magnetic hyperfine splitting is observed in the K6ssbauer spectra. In the case of K3FeF the spectra
6
indicate that the saturation value of the hyperfine field is about 60T as expected. This gives further confirmation that the anomalously low values found in the one- and two-dimensional systems result from the enhanced zero-point spin reduction effects discussed above, rather than from any effects related to tkie environment of the ferric atom.
The authors are indebted to Mrs. B.M. Wanklyn of the Clarendon Laboratory, Oxford, and Dr. D. Hanzel of the J.Stefan Institute, Ljubljana, for who provided us with the samples used in this work.
References
/I/ Gupta, G.P., Dickson, D.P.E. and Johnson, C.E. J.Phys.C:Solid State Phys.
M
(1978) 215./ 2 / Gupta, G.P., Dickson, D.P.E., Johnson, C.E. and Wanklyn, B.M. J.Phys.C:Solid State Phys.
11
(1978) 3889./3/ Oguchi, T., Phys.Rev.,
9
(1964) 1098. /4/ Rush, J.D., Simopoulos, A., Thomas, M.F.and Wanklyn, B.M., Solid State Commun. 18 (1976) 1039.
