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

Abstract

We report a magnetic-resonance study of Gd-doped ${\mathrm{Eu}}_{2}$${\mathrm{CuO}}_{4}$ single crystals. Cooling the samples in a magnetic field ${\mathbf{H}}_{\mathrm{FC}}$, 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 ${\mathbf{H}}_{\mathrm{FC}}$. An intense resonance mode is observed at the X band (9.5 GHz) when ${\mathbf{H}}_{\mathrm{FC}}$ is applied parallel to one of the 〈110〉 axes and the measuring field is rotated by 90\ifmmode^\circ\else\textdegree\fi{} in the ${\mathrm{CuO}}_{2}$ 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 ${\mathbf{m}}_{\mathrm{WF}}$ 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, ${\mathit{H}}_{\mathrm{DM}}$(T) and ${\mathit{H}}_{\mathrm{ax}}$(T), respectively.The extrapolated values for T=0 are ${\mathit{H}}_{\mathrm{DM}}$(0)=3.5(5)\ifmmode\times\else\texttimes\fi{}${10}^{5}$ G and ${\mathit{H}}_{\mathrm{ax}}$(0)=12(2) G. Both anisotropy fields decrease with increasing T, vanishing around ${\mathit{T}}_{\mathit{N}}$\ensuremath{\simeq}243 K. The temperature dependence of the peak-to-peak linewidths, \ensuremath{\Delta}${\mathit{H}}_{\mathrm{pp}}$, measured at the X and Q bands is explained in terms of a temperature-independent frequency linewidth, \ensuremath{\Delta}${\mathrm{\ensuremath{\omega}}}_{1/2}$/\ensuremath{\gamma}=1.6(2) kG. Nonresonant absorption losses around the maxima and minima of the \ensuremath{\omega}/\ensuremath{\gamma} vs H curves are also described in terms of this finite width for the resonance modes.