Self-assembled magnetic nanostructured alloys prepared by molecular beam epitaxy on low energy surfaces

Abstract

The objective of this work is to understand how the deposition temperature and the nature of substrate can tune the structural and magnetic properties and the morphology of MxPt1-x (M=Co and Fe) nanostructured alloys. The metallic adatoms have been co-deposited by molecular beam epitaxy at different deposition temperature on low energy surfaces as WSe2(0001) and NaCl(001). The characterization has been performed by the using of several techniques. Reflection high energy electron diffraction (RHEED) has been use to follow the epitaxy growth during the deposition; the X-ray diffraction measurements (XRD) has been performed to characterize the structure and extrapolate the long range chemical order parameter; the X-ray absorption spectroscopy (XAS) has been use to probe the local structure; the magnetic anisotropy has been investigated by the superconducting quantum interference devices (SQUID); the morphology was studied by grazing incidence small angle x-ray scattering (GISAXS) measurements, coupled to other characterization techniques (TEM, STM and Field Emission-SEM). A first result is the structural anisotropy at short range observed in the CoPt3 grown at room temperature on WSe2(0001). It is characterized by preferential Co-Co correlations in the film plane extending up to third shell that suggests the formation of Co rich thin disks in the film plane. The cobalt segregation is accompanied with a lattice distortion, where the in-plane bonds are shorter than the out-of-plane bonds. These effects disappear for the samples prepared at higher deposition temperature, Td, in which partial L12 order and larger in-plane bonds are observed. To understand the reason of such Co segregation, CoPt3 nanostructures were formed on the low energy surface, NaCl(001). No L12 ordering was observed by RHEED in nanostructures grown at 670K, and no local anisotropic order in nanostructures grown at 370K was detected by XAS. It suggests that the short range anisotropic order observed in nanostructures grown at room temperature on WSe2 would be driven by Se surfactant effect. An XRD study has been performed on the equiatomic CoPt and FePt alloys. The grown of CoPt on WSe2(0001) leads to the formation of a single crystalline fcc CoPt nanoparticles with [111] orientation even for samples prepared at room temperature. The L10 chemical order starts at 470K. However, increasing the deposition temperature to 720K does not enhance the chemical order parameter, which is only about 0.35. The segregation of Se at elevated deposition temperatures and surface disorder effects associated with a large surface-to-volume ratio might limit the long-range chemical order. Magnetic studies do not indicate the presence of an out-of-plane magnetic anisotropy as previously observed for CoPt3(111) alloys grown on WSe2(0001). The growth of FePt on NaCl(001) leads to the formation of nanoparticles oriented along the [001] direction for Td=520K; the chemical order parameter has been found to be 0.34 and increases to 0.4 for Td=620K. However at this deposition temperature a [110] growth direction is also observed. The size and morphology of the self-assembled CoPt and CoPt3 were studied. Deposits of alloys with various nominal thicknesses (t) and prepared at different Td were measured under incidence angles closed to the critical values of the substrates. The STM observations in nanostructures grown on WSe2 show a decrease of the aspect ratio r=h/l (h and l being the height and the lateral size) with increasing the growth temperature. A change from an hexagonal to triangular shape occurs above 700K in nanostructures higher than 3nm. Facetting is observed by GISAXS with a higher sensitivity to the less inclined 001 side-wall facets, even in minority. For highly dense assemblies as observed for t=1nm, information extracted from GISAXS analysis would be restricted to the upper part of the particles due to a coalescence of their lower part. The nanostructures grown on NaCl(001) have a truncated pyramid shape with an aspect ratio of 0.75 for Td=530K, i.e significantly larger than the ones grown on WSe2 (0.4). The 111 side-wall facets are well observed by GISAXS for nanostructures with lateral size of 10nm, while for smaller nanostructures (l<5nm) simulated patterns cannot differentiate the cubooctahedral shape from the spherical shape.