Properties of the Ferrimagnetic Double-Perovskites A 2 FeReO 6 (A=Ba and Ca)
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a, C. GALLEY a, R.L. GREENE a, K.
of Physics, University of Maryland, UMR 6508, 6 Bd. du Mardchal Juin, of Science, Bangalore 560012, India.
Author to whom correspondence should be addressed : [email protected].
ABSTRACT We have synthesized ceramics of A2FeReO 6 double-perovskites A2FeReO 6 (A=Ba, Ca). Structural characterizations indicate a cubic structure with a=8.0854(l) A for Ba 2FeReO 6 and a distorted monoclinic symmetry with a=5.396(l) A, b=5.522(l) A, c=7.688(2) A and 13=90.4' for Ca 2FeReO 6 . The barium compound is metallic from 5K to 385K, i.e. no metal-insulator transition has been seen up to 385K, and the calcium compound is semiconducting from 5K to 385K. Magnetization measurements show a ferrimagnetic behavior for both materials, with T,=315 K for Ba 2FeReO 6 and above 385K for Ca 2FeReO 6 . At 5K we observed, only for Ba 2FeReO 6 , a negative magnetoresistance of 10% in a magnetic field of 5T. Electrical, magnetic and thermal properties are discussed and compared to those of the analogous compounds Sr 2Fe(Mo,Re)0 6 recently studied. INTRODUCTION Perovskite manganites exhibiting a variety of exotic electronic properties [1] that include a spectacular decrease of electrical resistance in a magnetic field [2], the so-called colossal magnetoresistance (CMR) [3], have attracted wide attention in recent years. A significant feature of the electronic structure of the ferromagnetic CMR manganites, revealed by recent experiments [4-6] and theory [7], is that the charge carriers are almost completely spin-polarized at the Fermi level EF. These materials are half-metallic ferromagnets, where the majority spin states near EF are delocalized and the minority spin channel is effectively localized. Since half-metallic ferromagnetism and magnetoresistance (MR), especially at low fields, seem to be intimately related to each other [8] - the latter arising from the former- there is an intense search for halfmetallic magnets which could be candidate materials for the realization of MR applications. While several double-perovskite oxides of the kind A 2BB'0 6 (A being an alkaline earth or rare earth ion and B, B' being d-transition metal ions) have been theoretically predicted to be halfmetallic antiferromagnets [9], one such material, Sr 2FeMoO 6 [10], has recently [11-13] been shown to be a half-metallic ferrimagnet exhibiting a significant tunneling-type magnetoresistance at room temperature. More recently, it has also been shown that it is possible to grow by Pulsed Laser Deposition, thin films of Sr2FeMoO 6 [13-15]. Ba 2FeReO 6 [16] and Ca 2FeReO 6 are ordered double-perovskites whose structure and properties are quite similar to those of Sr 2FeReO 6. In both materials a valence degeneracy between the B-site cation occurs, giving rise to the observed metallic and magnetic properties ; in the Sr 2FeReO 6 case, the valence-degeneracy is between 23 Mat. Res. Soc. Symp. Proc. Vol. 602 © 2000 Materials Research Society
Fe 3++Mo5+ -- Fe 2++Mo6+ states, while in the Ba 2FeReO 6 case, the degenerate oxidation states
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