Critical behavior in double-exchange ferromagnets of Pr0.6Sr0.4MnO3 nanoparticles

In recent years, perovskite-type manganites (REMnO₃ with RE = rare-earth elements) have attracted considerable interest due to their complex magnetic and transport properties. Though REMnO₃ is an antiferromagnetic (AFM) insulator, substituting the RE site by a divalent alkali earth element (A) in order to form RE_1-xA_xMnO₃ compounds makes these materials exhibiting the ferromagnetic (FM)-paramagnetic (PM) phase transition at the Curie temperature (T_C), and a FM metallic state below T_C. It has been found that RE_1-xA_xMnO₃ compounds usually exhibit unusual magneto-electro effects, such as colossal magnetoresistance (CMR) and magnetocaloric (MC) effects [1, 2]. Basically, a close interplay between magnetic and transport properties in CMR and MC materials is ascribed to the competition between Mn³⁺-Mn⁴⁺ FM double-exchange interactions and AFM super-exchange interactions of Mn³⁺-Mn³⁺ and Mn⁴⁺-Mn⁴⁺ pairs. Particularly, when the particle size of manganites is reduced to the nanometer scale, a number of outstanding physical properties (such as low-field magnetoresistance, surface spin-glass behavior, exchange bias effect, etc.) would appear. The inter-particle interaction has been also found to be strong, modifying the magnetic response of nanoparticles [3]. To further understand the magnetic properties in manganite nanoparticles, it is necessary to consider the influence of the crystallite size on the nature of their magnetic phase transition and FM interactions.