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

Abstract

The observation of inverted magnetic hysteresis loops and negative magnetic remanence (NRM) in a 7.6 nm thin film of LaSrMnO grown on SrTiO 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 = 240 K and the ferromagnetic state with an ordering temperature T = 290 K. Hysteresis loop inversion is observed in the temperature region of T < T < T whereas NRM appears in the mixed superparamagnetic and ferromagnetic states for T< T 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.

Effects of Oxygen Modification on the Structural and Magnetic Properties of Highly Epitaxial La0.7Sr0.3MnO3 (LSMO) thin films

“…By increasing temperature, M switches sign. Therefore, we observed negative remanent magnetization (NRM) 35 for T < 150 K for all LSMO thin films synthesized at different oxygen partial pressures. Magnetization increases with an increase of temperature and then decreases above bifurcation temperature (T bif ).…”

“…This NRM has been observed only when the measuring field is less the coercive field (H c ) since for applied H > H c only positive values of M are observed in ZFC. This observed NRM is attributed to the coexistence of magnetically inhomogeneous regions consisting of spin clusters of a lower oxidation state and the ferromagnetic order in LSMO thin films 35,39,[41][42][43] . The growth pressure dependence of NRM is plotted in Fig.…”

“…A two-layer model was found to fit well with the XRR data. The two-layer model has been used previously in LSMO and other films [34][35][36][37] . Based on the reflectivity analysis, we determined the thickness and the surface roughness of the individual layers and the results extracted from the fitting are presented in Table 1.…”

“…Magnetization increases with an increase of temperature and then decreases above bifurcation temperature (T bif ). The NRM effect appears in a material when the direction of M is reverse to the direction of the applied H to that material 35 . NRM has been already observed and reported by our group in LSMO thin films grown in 100 mTorr 35,39 .…”

“…The NRM effect appears in a material when the direction of M is reverse to the direction of the applied H to that material 35 . NRM has been already observed and reported by our group in LSMO thin films grown in 100 mTorr 35,39 . Lee et al have also observed NRM in LSMO films via X-ray magnetic circular dichroism (XMCD) technique 34 .…”