Rare earth manganites, La_1-xA_xMnO₃ (A=Sr, Ca,...), are of great interest due to a variety of physical phenomena such as charge and orbital ordering, electronic phase separation, spin-polarized electronic transport, intrinsic and extrinsic magnetoresistance (MR), e.t.c. The highest crystalline quality manganite films demonstrate colossal magnetoresistance meanwhile polycrystalline films were found to exhibit low-field MR due to spin-polarized tunnelling between misaligned grains.
Microwave (MW) radiation has been demonstrated as a useful tool for investigation of the origin of MR in the manganites. Our recent study of resistance response of the LCMO thin films induced by MW radiation had obviously shown the dynamical phase coexistence in the material just below the semiconductor-to-metal phase transition.
Here in this work we present the succeeding experimental study of LCMO and LSMO thin films of both epitaxial and polycrystalline quality prepared either by MOCVD and magnetron sputtering on various (NdGaO₃, LaAlO₃ and MgO) substrates. The resistance response under MW radiation (f=10 and 35GHz) as well as their resistance and MR were measured in the temperature range 78-300 K. The electrical resistance response measured for LCMO and LSMO films of epitaxial quality confirmed the phase separation in a narrow temperature range below the PM-FM transition temperature T_c. We point out significant increase of the resistance response with frequency for the films. Meanwhile, the resistance response of polycrystalline films has been indicated in a significantly wider temperature range (from T_c down to 78 K). It depended strongly on preparation conditions and microstructure of the material. A model based on nonuniform heating and formation of insulating regions in the strain fields of grain boundaries has been developed to explain the observed peculiarities of magnetoresistance and MW induced response for the polycrystalline manganite films.


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