Magnetic behaviour of cobaltite SrCoO2.5 with brownmillerite-type structure.

PABLO M. BOTTA; VÍCTOR PARDO; DANIEL BALDOMIR; CRISTINA DE LA CALLE; JOSÉ A. ALONSO; JOSÉ RIVAS

Londres, Reino Unido

Conferencia; MC8: Advancing Materials by Chemical Design; 2007

Polycrystalline samples of SrCoO2.5 were prepared by a citrate technique, using thermal treatments under high oxygen-pressure and quenching the reactive mixture from 900ºC to room temperature to avoid the decomposition of the product. X-ray diffraction patterns confirm the crystalline structure of brownmillerite (space group Ibm2) for the synthesized compound. The lattice parameters and atomic positions were refined by the Rietveld method. Differential scanning calorimetry shows a transition at 540 K, which is attributed to the Néel temperature. dc and ac magnetic susceptibilities at several applied fields as a function of temperature were measured. The dc susceptibility curves show a clear maximum between 120 and 170 K, which appears at lower temperatures for higher applied fields. Under ac magnetic field this peak does not show any shift with frequency variation (0.1- 10 kHz), suggesting that it could be due to a magnetic transition rather than to a relaxation phenomenon. Curves of magnetization as a function of applied field reveal a noticeable hysteresis at temperatures lower than that observed for the susceptibility peak. This indicates the existence of magnetic ordering with a non-zero net magnetic momentum, which does not agree with the antiferromagnetic state reported at T < 540 K. By means of ab initio calculations the electronic properties of the antiferromagnetic SrCoO2.5 are analyzed. The description of the electronic structure is given; at low temperatures all the Co cations are in a high spin state with a small unquenched orbital angular momentum. The existence of a spin-transition that explains the observed magnetic properties is discussed.