Coupling of Small Lattice Polarons to Magnetic Field in Magnetoresistive Manganites

We report Extended X-Ray Absorption Fine Structure measurements at the Mn K absorption edge on hole doped manganese oxides ${La}_{1-x}Ca_x{MnO}_3$ in a wide temperature range above and below the metal insulator transition temperature in presence of a magnetic field. In accordance with previous studies an anomalous increase in the Debye Waller factors is detected at the phase transition in the first coordination shell made up of O atoms. This is due to a local structural deformation around $Mn^{3+}$ atoms associated with the Jahn-Teller effect and the formation of polarons. The application of a magnetic field results in a variation of this deformation with a thermal behaviour that closely resembles that of the magnetoresistance. This confirms the relevance of the polarons on the conductivity properties of such materials. The magnetic field has a huge effect on the second coordination shell probably related with the change of the Mn-O-Mn angle upon application of the magnetic field