Dependence of the coercivity of La_0.67Ca_0.33MnO₃ films on substrate and thickness

We investigate the effects of strain and film thickness on the coercivity of La_0.67Ca_0.33MnO₃ films grown on SrTiO₃ and LaAlO₃ substrates. Textured microstructure with the grain 001 pseudo-cubic axis normal to the film surface is obtained. The average grain size decreases with decreasing film thickness t. The magnetic anisotropy depends upon the substrate-induced strain. Firstly, the intrinsic coercivity _iH_C increases as t varies from 5 nm to 10 nm, then attains a maximum at t～10—25nm. A further increase in t yields a gradual reduction of _iH_C. By the analysis of the dependence of _iH_C on the measured direction of magnetic field, the coercivity mechanism is assumed to be mainly determined by the nucleation of reversed domain and the domain wall pining for t≤10nm and t≥25nm, respectively, which were further verified by the investigation of the initial magnetization curves for the LCMO/STO films with t=5, 10, 25 and 400nm, and the minor magnetic loops for the LCMO/LAO films with t=5 and 50nm. Moreover, the size of the inhomogeneous region in which the nucleation or pinning takes place is discussed.