Copper antiferromagnetic resonance inGd2CuO4:Evidence for coherent crystal-structure distortions

Abstract

We report on ${\mathrm{Cu}}^{2+}$ antiferromagnetic resonance (AFMR) measurements of gadolinium cuprate ${\mathrm{Gd}}_{2}{\mathrm{CuO}}_{4}$ using far-infrared techniques coupled to high magnetic fields. The experiments were performed in the energy range $10--100 {\mathrm{cm}}^{\ensuremath{-}1}$ at helium temperatures and in magnetic fields up to 13 T. The measurements were carried out in geometry $\mathbf{H}\ensuremath{\Vert}(001)$ plane. The AFMR mode corresponding to ``out-of-plane'' spin vibrations of the copper moments characterized by the gap value ${\ensuremath{\omega}}_{0}^{*}=18.2 {\mathrm{cm}}^{\ensuremath{-}1}$ has been studied. The observed square-root magnetic-field dependence of the resonance frequency of the AFMR mode $\ensuremath{\omega}\ensuremath{\sim}\sqrt{H}$ unambiguously shows the presence of a tilting of copper spins in ${\mathrm{Gd}}_{2}{\mathrm{CuO}}_{4}$ 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 determined to be ${H}_{D}=65.5 \mathrm{T}.$ This allows us to infer the canting angle between the copper moments in the ${\mathrm{CuO}}_{2}$ planes equals to $2.4\ifmmode^\circ\else\textdegree\fi{}.$ The observed very narrow signal of copper AFMR displaying a linewidth less than $0.3 {\mathrm{cm}}^{\ensuremath{-}1}$ gives strong evidence for three-dimensional coherent crystal distortions in ${\mathrm{Gd}}_{2}{\mathrm{CuO}}_{4}.$ The effect of considerable enhancement (approximately by a factor of 8) of static magnetic field at the copper position originating from the gadolinium-copper exchange interaction has been detected. In frame of a simple 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.