Temperature Evolution Of Crystal Field Interactions Across The Jahn-Teller Transition In A La7/8sr1/8mno3 Single Crystal

We present electron-spin-resonance (ESR), x-ray-diffraction, and dilatometry studies in a La7/8Sr1/8MnO3 single crystal. ESR experiments were performed in the paramagnetic regime (T-C approximate to180 K), from 220 K to 570 K at a microwave frequency of 9.4 GHz. We measured the ESR linewidth as a function of temperature with the static magnetic field H parallel to the crystallographic directions [100] and [001], referred to as the orthorhombic (Pbnm) axes. Detailed angular variations at constant temperature for H rotating in the (001), (010) and ((1) over bar 10) planes were also performed. The temperature dependence of the linewidth for fixed directions clearly reflects the changes in the symmetry of the crystal. At the Jahn-Teller (JT) transition, a very noticeable decrease of the linewidth is observed. The angular dependence of the linewidth reveals an anisotropy that is temperature dependent. The amplitude of the variations decreases with T and almost disappears above T(JT)approximate to280 K. However, there is a small remnant anisotropy present up to 400 K. We have carefully investigated the temperature dependence of this anisotropy across the Jahn-Teller transition. We showed that it is related with the evolution of the crystal field interactions. The symmetry of the ESR spectra is explained in terms of a spin Hamiltonian that includes isotropic Heisenberg superexchange, single-ion-crystal field, and Dzyaloshinsky-Moriya antisymmetric exchange. The experimental angular variation of the ESR linewidth and its temperature dependence were fitted using the calculated expression for the linewidth as obtained from a perturbative Kubo-Tomita approach. From the ESR data we have obtained a ratio (E) over tilde/(D) over tilde=-0.4(1) for the spin Hamiltonian coefficients associated with the orthorhombic ((E) over tilde) and tetragonal ((D) over tilde) Jahn-Teller distortions of the MnO6 octahedra. The temperature dependence of the parameters of the model is presented and the correlation with the dilatometry results are discussed.