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

Abstract

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 T N ¼ 47 K for bulk to 82 K for nanocrystals in MnP:GaP films, while the Curie temperature T C, 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 T N and T C 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.

Manganese phosphide thin films and nanorods grown on gallium phosphide and on glass substrates

“…X-ray diffraction reciprocal space maps combined with transmission electron microscopy measurements show that the orthorhombic MnP nanoclusters are oriented along specific GaP crystallographic directions, forming six well defined families, whose population is influenced by the growth temperature and the film thickness. In contrast, much less is known of polycrystalline MnP films on GaP [5,12]. Growth of MnP films on GaP (100) substrates has a substantial endotaxial component limited by diffusion of Mn into the GaP, as was reported in Ref.…”

“…Manganese phosphide (MnP) is of fundamental interest due to its unique magnetic, magneto-optical, and magneto-caloric properties [1][2][3][4][5][6][7][8][9]. Particularly at the nanoscale, MnP structures have shown significantly different properties than bulk.…”

Confinement effects on the low temperature magnetic structure of MnP nanocrystals

“…1(d) and 1(e) evidence the similarities between the films. Both show high coercive fields (H C ) in the screw and collinear ferromagnetic phases as well as high remanence in contrast to the bulk (H C^0 and no remanence, in powder or single crystal 7,9 ).…”

“…The reported hydrostatic or uniaxial strain effects in bulk MnP, 15,16 do not offer a straightforward explanation for the observed variations of T C and T N . 7 However, it is reasonable to suppose that J 1 and J 2 may vary in nanocrystals due to the important weight of the surface Mn compared to bulk. Within this framework, we have estimated the dependence of both transition temperatures on the strength of both coupling parameters.…”

“…6 The grains of the studied MnP film have around 40 nm and lattice parameters within 0.2% of bulk MnP. 7 Since the local structure and symmetry of Mn in MnP nanocrystals are found to be similar to those of bulk crystals and strain effects seem hardly compatible with the reported variations of both transition temperatures T C and T N , 7 it becomes important to determine what is at the origin of the markedly different magnetic behavior of the nanocrystals, particularly at low temperatures. A simple model including ferro-(J 1 ) and antiferro-magnetic (AFM) (J 2 ) interactions is further developed here to evaluate the effects of nanometric size on the magnetic structure of MnP.…”