Phase Separation and Microwave Response of Epitaxial and Polycrystalline Manganite Films

The resistance, magnetoresistance, and resistance response under microwave irradiation (f=10 and 35 GHz) were measured for epitaxial and polycrystalline La$\text{}_{0.67}$Ca$\text{}_{0.33}$MnO$\text{}_{3}$ and La$\text{}_{0.67}$Sr$\text{}_{0.33}$MnO$\text{}_{3}$ thin films in the temperature range 78÷300 K. The microwave induced resistance increase observed for the epitaxial films in a narrow temperature range below the ferromagnetic to paramagnetic transition temperature T$\text{}_{c}$ certifies coexistence of low resistance (ferromagnetic) and high resistance (paramagnetic) regions in the manganites. Resistance of polycrystalline films decreased under microwave irradiation in a wide temperature range below T$\text{}_{c}$. The effect was explained in terms of microwave assisted hopping of carriers in high resistance regions formed at grain boundaries of the polycrystalline films.