Exchange bias effect in La_0.2Ca_0.8MnO₃ antiferromagnetic nanoparticles with two ferromagnetic-like contributions

We report the structural and magnetic properties of La_0.2Ca _0.8MnO₃ nanoparticles synthesized by a chemical route, having an average diameter of 15 nm. Synchrotron experiments performed in the temperature range 80-300 K have shown that below 200 K a structural transition from room temperature orthorhombic Pnma to monoclinic P2₁/m space group, associated with orbital ordering, survives in studied particles, although this transition is highly suppressed in comparison with that of the bulk. The magnetization of these particles exhibits the appearance of a ferromagnetic contribution from the shell at T > 200 K, antiferromagnetic ordering within the particle core at T_N ∼ 140 K, and the emergence of another ferromagnetic contribution at T < 100 K. The latter appears as a result of spin canting within the antiferromagnetic core or is developed at some interfaces inside the nanoparticles. Considerable horizontal (H_EB) and vertical (M_Shift) shifts of the magnetization hysteresis loops are observed after field cooling, manifesting the exchange bias (EB) effect. We found that a nonmonotonic variation of H_EB and M_Shift as well as of the coercive field (H_C) can be caused by the changes in the temperature at which the cooling field (H_cool) is applied. The maximum values of negative H_EB and positive M_Shift are obtained when H_cool = 15 kOe is applied between 100 and 200 K. Moreover, the exchange bias field, the remanent magnetization, the coercive field, as well as the magnitude of the vertical shift depend strongly on the magnitude of H_cool. Our studies show that the volume fraction of the ferromagnetic phase, the strength of the interfacial exchange, and the topology of phase separation are altered clearly by the procedure of application of the magnetic field. The existence of two interfaces contributing to the total EB effect is proposed to explain the observed effects.