FIELD-INDUCED SPIN REORIENTATION IN EU2CUO4GD STUDIED BY MAGNETIC-RESONANCE

We report a magnetic-resonance study of Gd-doped Eu2CuO4 single crysta ls. Cooling the samples in a magnetic field H(FC), induces weak ferrom agnetism (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 close st to H(FC). An intense resonance mode is observed at the X band (9.5 GHz) when H(FC) is applied parallel to one of the [110] axes and the m easuring field is rotated by 90-degrees in the CuO2 plane. At the Q ba nd (35 GHz), the in-plane resonance modes strongly depend on angle and temperature. We analyze the experimental results in terms of a phenom enological model for the magnetic free energy, which predicts a reorie ntation transition of the WF component of the magnetization m(WF) indu ced by the external field. Associated with this transition, a softenin g of the WF magnetic resonance mode occurs when the external field is applied perpendicular to the easy magnetization axis. The resulting an gular variation of the resonance modes depends on whether the energy g ap for the magnetic excitations is larger or smaller than the microwav e energy. From the resonance data we have determined both the out-of-p lane and in-plane anisotropy fields, H(DM)(T) and H(ax)(T), respective ly. The extrapolated values for T = 0 are H(DM) (0) = 3.5(5) X 10(5) G and H(ax)(0) = 12(2) G. Both anisotropy fields decrease with increasi ng T, vanishing around T(N) congruent-to 243 K. The temperature depend ence of the peak-to-peak linewidths, DELTAH(pp) measured at the X and Q bands is explained in terms of a temperature-independent frequency l inewidth, DELTAomega1/2/gamma = 1.6(2) kG. Nonresonant absorption loss es around the maxima and minima of the omega/gamma vs H curves are als o described in terms of this finite width for the resonance modes.