The spin correlations in YBa2(Cu1-xFex)(3)O-y ceramics at T=15-500 K investigated by the small-angle scattering of polarized neutrons

The ceramic YBa2(Cu1-xFex)(3) O-y system was studied by means of the small- angle scattering of polarized neutrons in a wide range of iron concentratio n 0 less than or equal to x less than or equal to 0.2 in both oxygen-rich y greater than or equal to 7 and oxygen-reduced y less than or equal to 6.7 states. The measurements were carried out in the range of temperatures 15 l ess than or equal to T less than or equal to 550 K and magnetic fields 0 < H <less than or equal to> 4500 Oe. It was obtained that magnetic phase tran sition of the spin-glass like type occurs for all oxygen-rich samples and o xygen-reduced sample with x = 0.1 at T < 45 K. Moreover, it was found that in the high-temperature region the magnetic ordering was observed for the o xygen-reduced sample with x = 0.1 and y = 6.36 at T-m2 <approximate to> 100 K and for the oxygen-rich one with x = 0.2 and y = 7.21 at T-m2 approximat e to 280 K, respectively. The observed anomalies of the temperature depende ncies of the scattering intensity l(T,q), polarization P(T) of the transmit ted beam and magnetic-nuclear interference term Delta (T, q) (where q is mo mentum transfer) are in consequence of scattering of neutrons from spin-den sity fluctuations. The characteristic scale R, of the spin correlations obt ained from the fitting of the observed scattering described as: l(q) = A/(q (2) + kappa (2))(2) (where kappa = 1/R-c) are estimated as few hundred angs trom. After vacuum annealing of the oxygen-reduced sample with s = 0.1 and y = 6.36 at T = 550 K the magnetic ordering occurred previously were not fo und but the anomalies of the temperature dependencies of I(T,q) and P(T) we re observed in the range of 350 < T < 500 K. (C) 2001 Elsevier Science B.V. All rights reserved.