Ferromagnetism versus charge ordering in the Pr_0.5Ca_0.5MnO₃ and La_0.5Ca_0.5MnO₃ nanocrystals

We have investigated by means of neutron diffraction and magnetic measurements the particle size effects on the structure and low-temperature spin arrangement in so-called half doped manganites. The study shows that the Mn³⁺/Mn⁴⁺charge ordering and CE-type antiferromagnetic structure characteristic for bulk are completely suppressed when particle size is decreased down to 20 nm, and a ferromagnetic state is stabilized instead. The reason is not in a lower energy of the latter state, but in the hindering of displacive processes through which the charge ordering develops. Our work is of a general importance since the room temperature crystal structures in present manganite particles are found not to deviate from bulk, disproving thus former speculations about the important role of surface stress.

The ZFC (solid symbols) and FC (open symbols) susceptibilities for nanoparticles and bulk samples of La_0.5Ca_0.5MnO₃ and La_0.75Sr_0.25MnO₃. The right panel shows the low-field details of hysteresis loops at 5 K.

¹Institute of Physics, Czech Academy of Sciences, Cukrovarnická 10, Prague 6, Czech Republic
²Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University, Břehová 7, Prague 1, Czech Republic