MnP films and MnP nanocrystals embedded in GaP epilayers grown on GaP(001): Magnetic properties and local bonding structure

MnP nanostructures embedded in GaP epilayers, and MnP polycrystalline films, grown from the vapor phase on GaP(001) substrates using metalorganic precursors are compared with bulk MnP. We observe a large increase of the low transition temperature from the ferromagnetic to the antiferromagnetic screw phase, from TN = 47 K for bulk to 82 K for nanocrystals in MnP:GaP films, while the Curie temperature TC, close to room temperature, varies only slightly. A net magnetic moment is measured in the nanocrystals and films at 5 K, as well as large coercive fields, contrary to bulk MnP. X-ray absorption spectroscopy and diffraction show that epilayers and films contain MnP grains in the nanometric range with average Mn–P bond lengths very close to those of bulk MnP. The MnP film lattice parameters are almost identical to bulk values (within 0.5%) and the main crystallographic preferential orientations are those also present in the epilayers but with different relative populations. Overall the local structures of all MnP forms are very similar, except for indications of more disorder in the nanocrystals. Such combined changes of TN and TC are in apparent contradiction with the known response of bulk MnP to strains induced by hydrostatic, uniaxial or chemical pressure. We conclude that the differences in the low temperature magnetic behavior are most probably originated by local structural disorder at the surface of the nanostructures and by finite size effects.