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 La1-xCaxMnO3 in a wide temp erature range above and below the metal-insulator transition temperature in presence of a magnetic field. In accordance with previous studies an anoma lous increase in the Debye-Waller factors is detected at the phase transiti on in the first coordination shell made up of O atoms. This is due to a loc al structural deformation around Mn3+ 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 cl osely resembles that of the magnetoresistance. This confirms the relevance of the polarons on the conductivity properties of such materials. The magne tic field has a huge effect on the second coordination shell probably relat ed with the change of the Mn-O-Mn angle upon application of the magnetic fi eld.