Magnetic Properties of La_0.81Sr_0.19Mn_0.9Fe_{0.1-xZnxO3 (x=0, x=0.05)}

Magnetic properties of polycrystalline La0.81Sr0.19Mn0.9Fe0.1-xZnxO3 (x = 0, 0.05) have been investigated by means of electron spin resonance, magnetic susceptibility, and Mossbauer measurements. Both samples show a clear ferromagnetic transition. The Curie temperature T-C decreases on increasing Fe content (x = 0.05-T-C = 222 K; x = 0-T-C = 148 K). Mossbauer studies indicate that Fe in these compounds is in the trivalent high-spin state. The temperature evolution of the Mossbauer spectra at low temperatures (T < T-C) is typical for ferromagnetic clusters with a wide distribution in size and magnetic correlation length. The inverse susceptibility of all the samples deviates from the Curie-Weiss law above T-C, indicating the presence of fluctuations on approaching magnetic order. An anomalous downturn of the inverse susceptibility for x = 0.05 significantly above T-C and the concomitant observation of ferromagnetic resonance signals coexisting with the paramagnetic resonance up to approximately room temperature, is caused by a Griffiths-like behavior. This regime is characterized by the coexistence of ferromagnetic entities within the globally paramagnetic phase.