Particle size effects on charge ordering and exchange bias in Nanosized Sm_0.43Ca_0.57MnO₃

Size dependence of magnetic properties of Sm_0.43Ca _0.57MnO₃ nanoparticles with average size of 15-60 nm has been investigated. Charge ordering, characteristic for the bulk form of this material, was found to be gradually suppressed with decreasing particle size and fully disappearing in 15 nm nanoparticles. Onsets of ferromagnetic contributions to the magnetization appear below 90 K, independently on the particle size, while spontaneous magnetization at 10 K increases with decreasing particle size. Magnetic hysteresis loops exhibit size dependent exchange bias effect manifested by horizontal H_EB and vertical M_EB shifts, appearing after field cooling. Variation of H_EB and M _EB shows opposite size dependences; H_EB decreases while M_EB increases with decreasing particle size. For large 60 nm particles, H_EB decreases monotonously with increasing temperature, while for the smallest 15 nm particles it shows surprising nonmonotonic temperature dependence. Size and temperature dependences are discussed in terms of magnetic coupling between antiferromagnetic cores and shells containing ferromagnetic clusters in a frustrated spin configuration.