Dynamic superparamagnetic state in nano-clustered Nd0.5Sr0.5MnO3 films

The structural and the magnetic properties of Nd0.5Sr0.5MnO3 films with column-like nano-clustered microstructure have been investigated. The high-resolution electron microscopy, X-ray diffraction and electron diffraction reveal that the average cluster diameter was about 10 nm, and that the charge-ordered state, typical for this composition, was not observed down to 87 K. The films manifest a superparamagnetic behavior with increasing temperature, and an additional antiferromagnetic transition at T-N <= 50K. The analysis of the temperature-dependent magnetization shows that an increase in the applied magnetic field from 0.01 to 1T leads to a drastic decrease from 3000 to 150 mu B in the effective magnetic moment of superparamagnetic particles, which is simply described by mu(eff)similar to 1/H. Therefore, the superparamagnetic particles in these films are not rigidly bound with the crystalline clusters and the size is controlled by applied magnetic field. The greatly asymmetric hysteresis loops at low temperatures are explained by the exchange-bias interaction between the ferro- and the antiferro-magnetic clusters. (c) 2006 Elsevier B.V. All rights reserved.