Minimal number of atoms to constitute a magnet:Suppression of magnetic order in spherical MnS nanoparticles

We have studied the paramagnetic-to-antiferromagnetic phase transition in spherical β-MnS nanoparticles of well defined diameters in the range of 3-11 nm. The MnS nanoparticles were obtained by intrapore synthesis inside mesoporous silica matrices. Electron spin resonance and magnetization measurements reveal that no antiferromagnetic order is established in MnS spheres of 3 nm down to 2 K and that the antiferromagnetic order is gradually recovered on increasing the particle diameter to 11 nm. Photoluminescence excitation spectroscopy proves that in all MnS nanostructures the nearest-neighbor coupling between the Mn ions remains the same as in bulk suggesting that the suppression of the phase transition arises due to geometric restrictions alone.