Temperature-dependent magnetic interactions and their effects on the macroscopic magnetism of CoFe2O4/La0.7Sr0.3MnO3 composites

Abstract

Hard CoFe2O4/soft La0.7Sr0.3MnO3 polycrystalline composites with different mass ratios were chosen as studying objects, and the effects of interparticle dipolar interaction (IPDI), exchange coupling (EC) and magnetic anisotropy K on the saturation magnetization Ms, remanence ratio Mr/Ms and coercivity Hc in different temperature regions were investigated. The importance that IPDI, EC and K affects the magnetic property differs in different temperature region. The temperature T dependent Ms showed that the IPDI dominates Ms below 250 K but the EC does above 250 K where Ms follows the Bloch T3/2 law. K monotonically decreases with increasing T, however, at a certain T, K does not decrease monotonically, as is commonly believed, with increasing the ratio of La0.7Sr0.3MnO3. In the temperature region 10 K–300 K, Mr/Ms and Hc of CoFe2O4 and composites monotonously decrease with increasing T, while they slightly increase above 300 K. At T = 10 K, the EC occurring inside the domain wall and among particles was detected, and the inter-particle EC disappears at 250 K and 390 K. Further analyses revealed that in the temperature region 10 K–390 K the magnetic grain size increases with increasing T, but it is much smaller than the particle size. The magnetic moments are ordered just within the region of 2–3 domains, but they are disordered in a larger scale inside the particle. These results provided the experimental reference for the practical applications of bi-magnetic materials.