Effect of size on the magnetic properties and crystal structure of magnetically frustrated DyMn2O5 nanoparticles

We synthesized magnetically frustrated ${\mathrm{DyMn}}₂{\mathrm{O}}₅$ nanoparticles in pores of mesoporous silica, with particle sizes ranging from 7 to 20 nm, and investigated their magnetostructural correlation. We found that the lattice constants of the ${\mathrm{DyMn}}₂{\mathrm{O}}₅$ nanoparticles deviated from those of the bulk crystal below $\ensuremath{\sim}12$ nm and their crystallographic structures at the unit cell level were distorted. The size dependences of the blocking temperature and coercive field drastically change at $\ensuremath{\sim}12$ nm. In addition, the Weiss temperature depends strongly on particle size, and its sign changes at $\ensuremath{\sim}12$ nm. It is considered that such features can be realized owing to the distortion caused by the ligand atoms at the surface. The orbital structures of the magnetic sites are easily modified due to the distortion of the ligand ions at the surface, so that the correlation between the crystal structure and magnetic properties can be enhanced. Moreover, magnetization of the nanoparticle results in quasi-superparamagnetic behavior. Monte Carlo calculation of the nanoparticles indicates that such a feature is realized due to the quasi-free spins induced at the surface by magnetic frustration.