Field-induced spin reorientation in Eu2CuO4:Gd studied by magnetic resonance

We report a magnetic-resonance study of Gd-doped Eu2CuO4 single crystals. Cooling the samples in a magnetic field HFC, induces weak ferromagnetism (WF), with a strong out-of-plane anisotropy determined by the Dzyaloshinsky-Moriya (DM) interaction. In addition, there is in-plane anisotropy with an easy-axis parallel to the [110] crystal axis closest to HFC. An intense resonance mode is observed at the X band (9.5 GHz) when HFC is applied parallel to one of the 110 axes and the measuring field is rotated by 90°in the CuO2 plane. At the Q band (35 GHz), the in-plane resonance modes strongly depend on angle and temperature. We analyze the experimental results in terms of a phenomenological model for the magnetic free energy, which predicts a reorientation transition of the WF component of the magnetization mWF induced by the external field. Associated with this transition, a softening of the WF magnetic resonance mode occurs when the external field is applied perpendicular to the easy magnetization axis. The resulting angular variation of the resonance modes depends on whether the energy gap for the magnetic excitations is larger or smaller than the microwave energy. From the resonance data we have determined both the out-of-plane and in-plane anisotropy fields, HDM(T) and Hax(T), respectively. The extrapolated values for T=0 are HDM(0)=3.5(5)×105 G and Hax(0)=12(2) G. Both anisotropy fields decrease with increasing T, vanishing around TN243 K. The temperature dependence of the peak-to-peak linewidths, ΔHpp, measured at the X and Q bands is explained in terms of a temperature-independent frequency linewidth, Δω1/2/γ=1.6(2) kG. Nonresonant absorption losses around the maxima and minima of the ω/γ vs H curves are also described in terms of this finite width for the resonance modes. © 1993 The American Physical Society