Copper antiferromagnetic resonance in Gd2CuO4: Evidence for coherent crystal-structure distortions

We report on Cu2+ antiferromagnetic resonance (AFMR) measurements of gadoli nium cuprate Gd2CuO4 using far-infrared techniques coupled to high magnetic fields. The experiments were performed in the energy range 10-100 cm(-1) a t helium temperatures and in magnetic fields up to 13 T. The measurements w ere carried out in geometry H parallel to(001) plane. The AFMR mode corresp onding to "out-of-plane" spin vibrations of the omega(0)*-18.2 cm(-1) has b een studied. The observed square-root copper moments characterized by the g ap value oo magnetic-field dependence of the resonance frequency of the AFM R made omega similar to root H unambiguously shows the presence of a tiltin g of copper spins in Gd2CuO4 arising from the in-plane oxygen displacements . A value of antisymmetrical exchange interaction of Dzyaloshinskii-Moriya type associated with these distortions of tetragonal structure is determine d to be H-D=65.5 T. This allows us to infer the canting angle between the c opper moments in the CuO2 planes equals to 2.4 degrees. The observed very n arrow signal of copper AFMR displaying a linewidth less than 0.3 cm(-1) giv es strong evidence for three-dimensional coherent crystal distortions in Gd 2CuO4. The effect of considerable enhancement (approximately by a factor of 8) of static magnetic field at the copper position originating from the ga dolinium-copper exchange interaction has been detected. In frame of a simpl e model we discuss the role of the structural distortions in the forming of the magnetic ground state and the dynamic behavior of the complex magnetic system.