WEAK FERROMAGNETISM AND SPIN-GLASS-LIKE BEHAVIOR IN THE RARE-EARTH CUPRATES R2CUO4 (R = TB, DY, HO, ER, TM, AND Y)

dc magnetization, ac magnetic susceptibility, and microwave magnetoabsorption measurements on rare-earth cuprates, R2CuO4 (R = Tb, Dy, Ho, Er, Tm, and Y), synthesized under high pressure, indicate the presence of antiferromagnetic order with a weak-ferromagnetic component below a characteristic temperature T0. From the maximum in the amplitude of the microwave absorption, we have determined T0 = 295(3) K for R = Tb. The values of T0 decrease monotonically along the series down to T0 = 260(3) K for R = Tm. For R = Y we have measured T0 = 265(3). The in-phase component of the ac magnetic susceptibility chi'(T) presents a peak whose shape and intensity are dependent on both the amplitude and the frequency of excitation of the magnetic field. The simultaneous observation of a nonzero and also frequency-dependent out-of-phase term, chi"(T), suggests the presence of relaxation phenomena. The dc magnetization of Tb2CuO4, measured in magnetic fields up to 50 kOe, shows: (i) a linear and reversible part associated with the polarization of the rare-earth ions by the applied magnetic field, (ii) a contribution M(Cu) related to the appearance of weak ferromagnetism and spin-glass-like features in the Cu lattice, and (iii) an effective internal field H(i) acting on the Tb ions, which is linearly dependent on M(Cu). Both M(Cu) and H(i) present a strong dependence on the magnetothermal history of the samples, below a characteristic temperature T(irr)(H). The maximum values of M(Cu) observed in Tb2CuO4 were 2.4(7) x 10(-3) and 4.7(14) x 10(-3) mu(B)/mole Cu, for two samples prepared under different conditions. A mean-field analysis of the proportionality between H(i) and M(Cu) gives an exchange interaction J(Tb-Cu) = 0.19(3) meV. The frequency dependence of the ac susceptibility, a strong magnetothermal irreversibility, and the time relaxation of the dc magnetization, point to the existence of a disordered magnetic state with spin-glass-like behavior. We discuss the possible relation between microscopic structural distortions and the observation of weak ferromagnetism in the heavier rare-earth compounds. Maxima in the ac susceptibility and the dc magnetization observed at T(L) almost-equal-to 9-10 K for Tb2CuO4 and Dy2CuO4 indicate possible spin reorientations of the Cu moments or long-range magnetic order of the rare-earth ions.