Weak ferromagnetism and spin-glass-like behavior in the rare-earth cupratesR2CuO4(R=Tb, Dy, Ho, Er, Tm, and Y)

Dc magnetization, ac magnetic susceptibility, and microwave magnetoabsorption measurements on rare-earth cuprates, ${\mathit{R}}_2$ ${\mathrm{CuO}}_4$ (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 ${\mathit{T}}_0$. From the maximum in the amplitude of the microwave absorption, we have determined ${\mathit{T}}_0$=295(3) K for R=Tb. The values of ${\mathit{T}}_0$ decrease monotonically along the series down to ${\mathit{T}}_0$=260(3) K for R=Tm. For R=Y we have measured ${\mathit{T}}_0$=265(3). The in-phase component of the ac magnetic susceptibility χ’(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, χ’ ’(T), suggests the presence of relaxation phenomena. The dc magnetization of ${\mathrm{Tb}}_2$ ${\mathrm{CuO}}_4$, 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 ${\mathit{M}}_{\mathrm{Cu}}$ related to the appearance of weak ferromagnetism and spin-glass-like features in the Cu lattice, and (iii) an effective internal field ${\mathit{H}}_{\mathit{i}}$ acting on the Tb ions, which is linearly dependent on ${\mathit{M}}_{\mathrm{Cu}}$.