Spin dynamics and relaxation in 7.6 nm thin film of La0.7Sr0.3MnO3/SrTiO3: ac magnetic susceptibility and magnetic viscosity investigations

Analysis of the results from our investigations of the temperature dependence of ac susceptibilities ( chi ' and chi ' ') and magnetic viscosity in a 7.6nm thin film of La0.7Sr0.3MnO3 grown on SrTiO3 (001) substrate is presented. The Curie temperature (T-C) of this film is magnetic field (H) dependent, varying as [ T C ( H ) - T C ( 0 ) ] similar to H n with T-C(0)=270K and n=0.15. Temperature dependence of chi ' and chi ' ' measured at ac frequencies f(m) in the range of 0.1-10kHz shows a broad peak near 230K associated with the blocking temperature ( T B ) of spin clusters present in the 1.4nm surface layer and a frequency dependent peak near 270K associated with T C. The mean relaxation time tau determined from the Cole-Cole analysis of the temperature dependent chi ' and chi ' ' is shown to fit the Vogel-Fulcher law: tau = tau 0 exp [ Delta E / k B ( T - T 0 ) ] with T-0=245K, Delta E/k(B)=270K=T-C, and tau (0)=1.2x10(-9)s. Magnetic viscosity S (measured in H=0 for a sample cooled in H=50Oe) determined from the fit to the magnetization: M (t)=M (0) - S ln t, with time t up to 2h, shows a peak at 230K above which M (2h) switches to negative values for temperatures up to T-C(0). It is argued that this negative magnetization results from an interaction between the superparamagnetic spin clusters and the ferromagnetic phase, both being present in the temperature range of T-B