Magnetic ordering in Fe-doped Gd2BaCuO5

The structural and magnetic properties of iron-doped Gd2BaCuO5 have been studied by X-ray diffractometry, Mössbauer spectroscopy and susceptibility measurements. Mössbauer data on Gd2BaCu0.8Fe0.2O5 show that at room temperature Fe is not magnetically ordered, displaying hyperfine parameters similar to those generally assigned to Fe at Cu(2) sites in the GdBa2(Cu1−xFex)3O7 superconductor. Susceptibility measurements demonstrate that Gd2BaCu1−xFexO5 behaves like a three-dimensional antiferromagnet withTN=11.9±0.1 K, independent ofx. The effective magnetic moment calculated within a mean field approximation is consistent with an ordering of the Gd sublattice.


Magnetic ordering in Fe-doped Gd2BaCuO5
S.J. Stewart, G.F. Goya, R.C. Mercader, A.E. Goeta, G. Punte 1 Departamento de Ffsica, Universidad NacionaI de La Plata, CC67, 1900 La Plata, Argentina and F. Sapinya-Navarro Departament de Qufmica lnorgimica, Facultat de Qufmica, Universitat de Valencia, 46100 Burjasso t, Valencia, Spain The structural and magnetic properties of iron-doped Gd2BaCuOs have been studied by X-ray dfffractometry, MSssbauer spectroscopy and susceptibility measurements.M6ssbauer data on Gd2BaCu0.sFe0.205 show that at room temperature Fe is not magnetically ordered, displaying hyperfine parameters similar to those generally assigned to Fe at Cu(2) sites in the GdBaz(Cua_xFex)307 superconductor.Susceptibility measurements demonstrate that Gd2BaCul_xFexO5 behaves like a three-dimensional antiferromagnet with TN = 11.9 5:0.1 K, independent of x.The effective magnetic moment calculated within a mean field approximation is consistent with an ordering of the Gd sublattice.

I n t r o d u c t i o n
The oxides REBaMO5 (R = yttrium or rare earth and M = transition metal) have been the subject of several investigations [1][2][3][4][5].For R = Y and M = Cu, structural and electron difference density studies [6] have shown that Cu sites are similar to Cu(2) sites in the YBa2Cu3OT_~ superconductor.However, reported Mfssbauer data for YEBaCu0.99Fe0m05 [7] are in contradiction with this similarity calling for further studies on Fe-doped samples.The insulating phases eventually comprise two different magnetic subsystems, namely the rare earth and the transition metal lattices, that might be coupled, like in GdzCuO4 where links between Gd and Cu sublattices have been reported [8].In this paper we analyze the influence of Fe doping on the magnetic ordering of GdzBaCuO5.
prepared by a standard solid state reaction from Gd203, CuO, CO3Ba and Fe203 in stoichiometric amounts.The X-ray diffraction characterization was performed in a Philips PW-1710 diffractometer using Cu K~ radiation.AC susceptibility measurements were performed between 4.2 and 130 K at two different fields of 1 and 10 Oe with a frequency of 333.3 Hz.Room temperature M6ssbauer data were taken in a conventional spectrometer.A non-linear least-squares program with constraints, was used to fit the spectra to Lorentzian line shapes.Isomer shifts are referred to a-Fe at 300 K.

Results and discussion
The X-ray patterns show to phases: Gd2BaCul_xFexO5 and Gd203.The estimated percentage of Gd203 is maximum for x = 0.15 and is ~ 10%.The M6ssbauer spectrum shown in fig. 1 could be fitted with three quadrupolar doublets of hyperfine parameters listed in table 1. Susceptibility measurements (fig.2) gave, for all Fe concentrations, a value of TN = 11.9 + 0.1 K as deduced from the derivative of the x(T) T curve.The maximum of the x(T) curve was found to be at 11.9 + 0.5 K.The inverse of the susceptibility, fitted with a Curie-Weiss law in the paramagnetic region, showed a smooth increment of E9 and C with x.Below the transition point, for x/> 0.05, there is an increasingly important contribution from a paramagnetic system which does not order even at 4.2 K.
The fit of the x = 0.20 M6ssbauer spectrum (fig. 1) shows a predominant occupation of Fe at a site ofhyperfine parameters comparable to those assigned to Fe at a quasi-octahedral plane-site (Fe 3+ Sz = 5/2) in YBa2Cu307 and YBa2Cu4Os  Y2BaCuO5 [1][2][3][4][5].In addition, MSssbauer measurements on Y2BaCu0.99Feo.0105yielded a TN of 15 K [7].Hence, it is plausible that the ordering temperature of the Cu magnetic subsystem be depressed by the inclusion of small amounts of Fe.Indeed, in samples of Y2BaCul_xFexOs, the TN of the Cu lattice falls from 25.4 K to less than 4.2 K for x/> 0.05 [12].The paramagnetic signals observed below TN (fig.2) are interpreted as coming from the Cu(Fe) subsystem.The susceptibility curve of the Gd2BaCuO5 (x = 0) sample shows no sign of ordering about 30 K probably because the large magnetic moments of the Gd 3+ ions mask the magnetic signal coming from the Cu atoms [5].Specific heat and MSssbauer 57Fe-enriched measurements at lower Fe concentrations are in progress.

Fig. 1 .
Fig. 1.57Fe M6ssbauer spectrum of Gd2BaCul_~FexO5 for x = 0.20 obtained at room temperature.The solid curve is the result of the least-squares fit described in the text.