En las fronteras de la Materia Condensada

Electron spin resonance (ESR) measurements has been performed in a single crystal of La_7/8Sr_1/8MnO₃. The experiments were carried out in the paramagnetic regime (T_C ~ 180K), from 220K to 570K and at a microwave frequency of 9.4GHz. Data of the ESR linewidth as a function of temperature were taken for the magnetic field (H₀) along the crystallographic directions [100] and [001], referred to the orthorhombic (Pbnm) axes. Detailed angular variation measurements at constant temperature for H_o rotating in the (001), (010) and (-110) 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 noticeable decrease of the linewidth is clearly observed. The angular variation of the linewidth reveals an anisotropy that is also temperature dependent, decreasing as T goes up and almost dissapears above T_JT ~ 280K, although a small remnant anisotropy can be observed up to 400K.
We showed that the temperature dependence of this anisotropy across the Jahn-Teller transition can be related to the evolution of the crystal field interactions. The symmetry of the ESR spectra can be explained in terms of a spin Hamiltonian that includes isotropic Heisenberg superexchange, single ion-crystal fields, and Dzyaloshinsky-Moriya antisymmetic interactions. The experimental angular variation of the ESR linewidth and its temperature dependence were fitted using the calculated expressions for the linewidth as obtained from the Kubo-Tomita description of the ESR absorption. We obtained a ratio E/D = -0.4 for the spin Hamiltonian coefficients associated with the orthorhombic (E) and tetragonal (D) Jahn-Teller distortions of the MnO₆ octahedral units. The temperature dependence of the parameters of the model are presented, and the correlation with the dilatometry results are discussed.