Observation and interpretation of negative remanent magnetization and inverted hysteresis loops in a thin film of La_0.7Sr_0.3MnO₃

The observation of inverted magnetic hysteresis loops and negative magnetic remanence (NRM) in a 7.6 nm thin film of La_0.7Sr_0.3MnO₃ grown on SrTiO₃ substrates is reported. The film was grown employing pulsed laser deposition and characterized by reflection high-energy electron diffraction during growth and using x-ray reflectivity measurements post-growth. Magnetic properties of the film were measured from 5 K to 400 K under both the field-cooled (FC) and zero-field-cooled (ZFC) conditions. The observed results of inverted magnetic hysteresis loops and NRM are interpreted in terms of the co-existence of a magnetically inhomogeneous region consisting of superparamagnetic spin clusters with a blocking temperature T _B  =  240 K and the ferromagnetic state with an ordering temperature T _C  =  290 K. Hysteresis loop inversion is observed in the temperature region of T _B  <  T  <  T _C whereas NRM appears in the mixed superparamagnetic and ferromagnetic states for T  <  T _C down to 5 K. These observations of hysteresis loop inversion and NRM are related to the magneto-static interaction between the superparamagnetic and ferromagnetic phases leading to anti-alignment of spin of both magnetic phases with respect to each other. (paper)