Samuel Lambert-Milot

Three Dimensional Reciprocal Space Measurement by X-ray Diffraction Using Linear and Area Detectors: Applications to Texture and Defects Determination in Oriented Thin Films and Nanoprecipitates

The authors present a method for the fast and efficient measurement of volumes of reciprocal space by x-ray diffraction using linear and area detectors. The goal of this technique is to obtain a complete overview of the reciprocal space to detect and characterize the nature and orientation of all the phases present. They first explain the detailed procedures and scan strategies required for transforming raw scattering data into three-dimensional maps of reciprocal space and present a complete open-source software package for advanced data processing, analysis, and visualization. Several case studies, chosen to highlight the overall capabilities of the technique, are then introduced. First, thermal diffuse scattering from a monocrystalline Si substrate is characterized by the presence of lines linking diffraction peaks in reciprocal space. Second, a detailed investigation of texture in multiphase thin layers permits us to reveal the unambiguous presence of fiber, axiotaxial, and epitaxial components in ori...

Metal-organic vapor phase epitaxy of crystallographically oriented MnP magnetic nanoclusters embedded in GaP(001)

Hybrid ferromagnetic-semiconductor GaP:MnP thin films were grown at 650 °C by metal-organic vapor phase epitaxy on GaP(001) using trimethylgallium, tertiarybutylphosphine, and methyl cyclopentadienyl manganese tricarbonyl (MCTMn). Overall Mn concentrations in the hybrid films, determined by Rutherford backscattering spectrometry, were found to be nearly proportional to the MCTMn precursor gas flow rate and ranged from 2 to 3.5 at. %. Cross-sectional transmission electron microscopy (TEM) analyses revealed the presence of a homogeneous distribution of 15–30 nm wide nanoclusters in a dislocation-free GaP matrix that is fully coherent with the substrate. The nanocluster facets are predominantly aligned along the (220) planes of the GaP matrix and selected-area electron diffraction patterns in TEM indicate that the nanoclusters are coherent (or semicoherent) with the single-crystal GaP matrix. The Mn:P composition ratio in the nanoclusters was determined to be 1.00±0.05 from parallel electron energy loss spec...

MnP nanoclusters embedded in GaP epitaxial films grown by organometallic vapor-phase epitaxy: A reciprocal space mapping and transmission electron microscopy study

Full three dimensional x-ray diffraction reciprocal space maps combined with transmission electron microscopy measurements provide a systematic determination of the texture of GaP epilayers containing embedded MnP nanoclusters grown on GaP(001) by organometallic vapor phase epitaxy. This approach reveals that the texture of the MnP clusters depends on the growth surface morphology and bonding configuration and on the lattice mismatch at the cluster/matrix interfaces during growth. It demonstrates 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. The clusters principally grow on GaP(001) and GaP{111} facets with a small fraction of clusters nucleating on higher-index GaP{hhl} facets. Most epitaxial alignments share a similar component: the MnP(001) plane (c-axis plane) is parallel to the GaP{110} plane family. Axiotaxial ordering between the MnP clusters and the GaP matrix is also observed. Furthermore, with this systematic approach, all phases present in these heterogeneous films can be identified. In particular, traces of hexagonal Mn2P precipitates have been observed while their formation can be avoided by lowering the growth temperature. Comparing the structural results presented here with magnetic measurement carried out on similar samples confirms that the effective magnetic properties of the heterogeneous layer can be tuned by controlling the texture of the ferromagnetic nanoclusters.

Giant magneto-optical Faraday effect in GaP epilayers containing MnP magnetic nanoclusters

We report a giant magneto-optical (MO) effect in a GaP(011) epilayer with embedded MnP nanoclusters (GaP:MnP) grown on a GaP(011) substrate using metal-organic vapor phase epitaxy. The MO effect exhibits a hysteretic behavior when sweeping the applied magnetic field, thereby indicating a clear dependence upon the magnetization of the sample. The measured Faraday rotation per unit length is 60 times higher in the epilayer than that of GaP at room temperature. The role of the magnetization in the MO effect makes it possible to increase this factor even further by lowering the temperature well below the Curie point (TC) of 292 K for those samples. We observed an increase of the Faraday rotation at longer wavelengths, indicating that those GaP:MnP epilayers could potentially be useful for enhanced MO effects in the visible and infrared regions of the electromagnetic spectrum.

Magnetic anisotropy in GaP(001) epilayers containing MnP nanoclusters observed by angle dependent ferromagnetic resonance measurements

We report on the angular dependence of ferromagnetic resonance (FMR) of GaP(001) epilayers with embedded MnP nanoclusters (GaP:MnP) grown on a GaP(001) substrate using metal-organic vapor phase epitaxy (MOVPE). Angle dependent FMR spectra obtained at 292K show several peaks and indicate a strong magnetic anisotropy. The measured angular dependence of resonance fields can be modeled assuming that the MnP clusters possess a strong biaxial magnetocrystalline anisotropy whose axes are oriented along specific GaP crystallographic directions. Dominant populations of MnP clusters have their b-axis along GaP [111] and [111¯] directions, while a smaller population has the b-axis along the GaP [001] direction. In comparison, angle dependent FMR spectra of a polycrystalline MnP thin film grown on a GaP(001) substrate using MOVPE suggest that the crystals b-axis is oriented favorably along the GaP [001] direction.

Adjusting the magnetic properties of semiconductor epilayers by the crystallographic orientation of embedded highly anisotropic magnetic nanoclusters

GaP:MnP samples, which consist of GaP(001) epilayers with embedded MnP nanoclusters occupying approximately 7% of the epilayer volume, were grown at three different substrate temperatures (600, 650, and 700 °C) using metal-organic vapor phase epitaxy. Angle dependent ferromagnetic resonance (FMR) spectroscopy indicates that, in all samples, MnP clusters are crystallographically oriented along specific GaP directions and possess high magnetic anisotropy fields. FMR results also suggest that the growth temperature significantly modifies the distribution of clusters among the possible orientations. This is verified from the measured angular dependence of the remanent magnetization, which shows a different crystallographic orientation of the GaP:MnP effective magnetic easy axis for each growth temperature. Modeling of the remanent magnetization allowed the determination of the relative volume fraction of clusters corresponding to each MnP c-axis orientation at a given growth temperature. These results support...

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.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 al...

Confinement effects on the low temperature magnetic structure of MnP nanocrystals

The low temperature magnetic properties of MnP nanocrystals (15-40 nm), both in GaP:MnP epilayers and MnP films, are significantly different compared to bulk and cannot apparently be explained by differences in the structure. A simple model of localized spins is used to describe the magnetic screw structure confined to nanocrystals. The results indicate that the observed magnetic behaviour is related to the nanometric size and to changes in the coupling constants most probably localized at an external grain shell. The nucleation of helical regions at the surface of the ferromagnetic grains is proposed as a possible mechanism for the reversal of the magnetization.

Surface induced magnetization reversal of MnP nanoclusters embedded in GaP

We investigate the quasi-static magnetic behavior of ensembles of non-interacting ferromagnetic nanoparticles consisting of MnP nanoclusters embedded in GaP(001) epilayers grown at 600, 650 and 700{\deg}C. We use a phenomenological model, in which surface effects are included, to reproduce the experimental hysteresis curves measured as a function of temperature (120-260 K) and direction of the applied field. The slope of the hysteresis curve during magnetization reversal is determined by the MnP nanoclusters size distribution, which is a function of the growth temperature. Our results show that the coercive field is very sensitive to the strength of the surface anisotropy, which reduces the energy barrier between the two states of opposite magnetization. Notably, this reduction in the energy barrier increases by a factor of 3 as the sample temperature is lowered from 260 to 120 K.