High-field ESR spectroscopy of the spin dynamics in ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3$ $(x<~0.175)$

High-field electron-spin-resonance (ESR) experiments have been carried out in single crystals of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3$ in the concentration range $0<~x<~\mathrm{0.175.}$ Different quasioptical arrangements have been utilized for frequencies $40<~\nu <~700\hspace{0.5em}\mathrm{GHz}$ and for magnetic fields $B<~12\hspace{0.5em}\mathrm{T}.$ A splitting of the antiferromagnetic resonance (AFMR) mode is observed in the magnetic field for the parent compound ${\mathrm{LaMnO}}_3,$ in agreement with the antiferromagnetic structure of this material. Abrupt changes in the AFMR frequencies have been observed around $x\simeq 0.025,$ and were attributed to a transition between a pure antiferromagnetic and a canted state. For increasing Sr doping the observed AFMR modes are split even in a zero field, which can be naturally explained using a concept of a canted magnetic structure for $x<\mathrm{}\mathrm{0.1.}\mathrm{}$ In ${\mathrm{La}}_{0.825}{\mathrm{Sr}}_{0.175}{\mathrm{MnO}}_3$ the ESR spectra are consistent with the ferromagnetic and metallic state. The lines of ferromagnetic resonance and ferromagnetic antiresonance can be clearly observed. For intermediate concentrations $0.1<~x<~0.15$ complicated ESR spectra were observed, which can be well explained by a single ferromagnetic resonance mode and taking electrodynamic effects into account.