Griffiths phase and colossal magnetoresistance in Nd0.5Sr0.5MnO3 oxygen-deficient thin films

Abstract

This work is devoted to study the influence of the Griffiths phase in colossal magnetoresistance manganites. Griffiths-phase-like behavior of the paramagnetic susceptibility χ0 is observed in Nd0.5Sr0.5MnO3 oxygen-deficient thin films fabricated by magnetron sputtering deposition. In Nd0.5Sr0.5MnO3−δ films with oxygen deficiency for ТG≈260–280K>T>TC=138K (ТG and ТС—Griffiths and Curie temperatures, respectively), paramagnetic matrix consists of a magnetic phase with short-range order (∼1–1.5nm) (which is responsible for the colossal magnetoresistance (CMR) above ТС), and is embedded in this matrix region with long-range ferromagnetic order (⪢10nm), responsible for the Griffiths phase-like behavior of the paramagnetic susceptibility. Electrical resistivity is caused by carrier tunneling between the localized states and obeys the Efros–Shklovskii law. Magnetic resistivity is caused by change of the localized state sizes under the magnetic field. The temperature and magnetic field dependencies of size of the phase inhomogeneity inclusions, found from measurements of magneto-transport properties, can be satisfactorily described by the model of thermodynamic phase separation into metallic droplets of small radius in a paramagnetic matrix. Intrinsic nanoscale inhomogeneities caused by thermodynamic phase separation, rather than the Griffiths phase, determine the electrical resistivity and colossal magnetoresistance of the films. In half-doped manganites, the nature of long-range ordered magnetic phases may be related, besides the chemical heterogeneity, to proximity to a ferromagnetic–antiferromagnetic boundary at the phase diagram as well. The results are in good agreement with the model of existence of an analog of Griffiths phase temperature in half-doped manganites.