ESR OF GdxEu 1-xBa 2 Cu 3 O 7-δ CERAMIC OXIDES

Electron spin resonance on ~ d ~ + ions in G ~ , E U ~ , B ~ ~ C U ~ O ~ ~ are presented. The dependence of the g-factor and linewidth with the concentration of magnetic ions and temperature is discussed for superconducting and non superconducting samples with 6 = 0 and 6 = 0.7 respectively. The magnetic behavior of materials can be studied with Magnetic Resonance Spectroscopy which provides information on the interaction of magnetic ions among themselves and between them and the crystal lattice. This information is conveyed mainly by two parameters: the g-factor and the linewidth of the resonance, AH. We report here experiments on ceramic powders of G ~ , E u ~ ~ B ~ ~ C U ~ O ~ S in the range 0.02 < z < 1. In this family of oxides non-magnetic E U ~ + ions are substituted for paramagnetic ~ d ~ + ions. In this system we can vary independently two parameters: the concentration of magnetic ~ d ~ + ions and the oxygen defficiency 6. For 6 = 0 this oxide shows high temperature superconductivity and antiferromagnetic ordering [I] a t T~=2.24 K for x = 1. The increase in 6 suppresses the superconductivity without affecting the antiferromagnetic transition [2]. It has been shown also that the decrease in x depresses TN at almost a linear rate [3]. Our samples were prepared in two sets: a first one, that we shall call oxygenated, was prepared by sintering a mixture of oxides at 950 OC in an oxygen atmosphere as described in [4]; a second one consist of pieces of samples of the first set that were subsequently heat treated a t 530' for 3 hs in an argon flow [5]. We call this samples deoxygenated and, from the weight loss during the heat treatment, we have estimated S = 0.7(1). All the samples were prepared for the ESR experiments by thoroughly grinding a mixture of the material with KC1 in a 1:3 proportion, in order to avoid texture and attenuation effects. All oxygenated samples were superconducting above 77 K while after deoxygenation none of them showed superconductivity down to 2 K. Calorimetric measurements, shown in figure 1, indicate that the magnetic transition remained unaffected by the heat treatment, in agreement with previous results [2]. The ESR experiments were made at *band (9.5 GHz) between 4 K and room temperature. The measured g-factor for ~ d ~ + shows small departures 'LOS Alarnos National Laboratory, Los Alan~os, NM 87545 U.S.A. an Diego State University, San Diego, CA 02182 U.S.A. from the free ion value g = 1.992, as shown in figure 2. At room temperature the average g-value for oxygenated samples is 1.98(3). For deoxygenated samples the g values are slightly higher. A possible source of this g-shift is the coupling of Gd ions to magnetic moments associated to the Cu ions present in the deoxygenated compound [6]. The observed ESR lineshape is assymetric, see figure 3. This assymetry is explained if take into account the two components of the resonance, centered at positive and negative magnetic fields, that overlap due to Fig. 1. Specific heat of GdBa2Cu307-6 showing the Xtype anomaly at the antiferromagnetic transition for (+) 6 N 0 and (0) 6 N 0.7.