Haiqiang Yang

Crystalline phase and orientation control of manganese nitride grown on MgO(001) by molecular beam epitaxy

The phase and orientation of manganese nitride grown on MgO(001) using molecular beam epitaxy are shown to be controllable by the manganese/nitrogen flux ratio as well as the substrate temperature. The most N-rich phase, θ phase (MnN), is obtained at very low Mn/N flux ratio. At increased Mn/N flux ratio, the next most N-rich phase, the η phase (Mn3N2), is obtained having its c axis normal to the surface plane. Further increasing the Mn/N flux ratio, the η phase (Mn3N2) having its c axis in the surface plane is obtained. Finally, the e phase (Mn4N) is obtained at yet higher Mn/N flux ratio. The structural phase variation with Mn/N flux ratio is due to the kinetic control of the surface chemical composition, which determines the energetically most favorable phase. For a given Mn/N flux ratio, the phase is also found to be a function of the substrate temperature, with the less N-rich phase occurring at the higher substrate temperature. The change of phase with temperature is attributed to the change in the ...

Structural and magnetic properties of η-phase manganese nitride films grown by molecular-beam epitaxy

Face-centered tetragonal (fct) η-phase manganese nitride films have been grown on magnesium oxide (001) substrates by molecular-beam epitaxy. For growth conditions described here, reflection high energy electron diffraction and neutron scattering show primarily two types of domains rotated by 90° to each other with their c axes in the surface plane. Scanning tunneling microscopy images reveal surface domains consisting of row structures which correspond directly to the bulk domains. Neutron diffraction data confirm that the Mn moments are aligned in a layered antiferromagnetic structure. The data are consistent with the fct model of G. Kreiner and H. Jacobs for bulk Mn3N2 [J. Alloys Compd. 183, 345 (1992)].

Ga/N flux ratio influence on Mn incorporation, surface morphology, and lattice polarity during radio frequency molecular beam epitaxy of (Ga,Mn)N

The effect of the Ga/N flux ratio on the Mn incorporation, surface morphology, and lattice polarity during growth by rf molecular beam epitaxy of (Ga,Mn)N at a sample temperature of 550 °C is presented. Three regimes of growth, N-rich, metal-rich, and Ga-rich, are clearly distinguished by reflection high-energy electron diffraction and atomic force microscopy. Using energy dispersive x-ray spectroscopy, it is found that Mn incorporation occurs only for N-rich and metal-rich conditions. For these conditions, although x-ray diffraction in third order does not reveal any significant peak splitting or broadening, Rutherford backscattering clearly shows that Mn is not only incorporated but also substitutional on the Ga sites. Hence, we conclude that a MnxGa1−xN alloy is formed (in this case x∼5%), but there is no observable change in the c-axis lattice constant. We also find that the surface morphology is dramatically improved when growth is just slightly metal rich. When growth is highly metal-rich, but not G...

Incorporation of manganese into semiconducting ScN using radio frequency molecular beam epitaxy

The incorporation of manganese into semiconducting ScN, using radio frequency molecular beam epitaxy, has been investigated. X-ray diffraction and reflection high energy electron diffraction measurements show the face-centered tetragonal rocksalt-type crystal structure with Sc and Mn cations and N anions. In addition to the solute incorporation into the lattice, which is clear from the positions of the diffraction peaks, atomic force microscopy images show that the surface of the alloy grown at TS⩽518°C contains dot-like features, indicating surface accumulation. The areal dot density is found to decrease as the growth temperature increases, whereas the Mn incorporation increases at 518 °C. This behavior is suggestive of a thermally activated process, and it is well explained by an Arrhenius law, giving an activation energy (diffusion barrier) of 0.67 eV. Increasing the growth temperature to 612 °C leads to an increased desorption rate, resulting in little Mn incorporation. It has been found that the grow...

Energy-dependent contrast in atomic-scale spin-polarized scanning tunneling microscopy of Mn3N2 (010): Experiment and First-Principles Theory

The technique of spin-polarized scanning tunneling microscopy is investigated for its use in determining fine details of surface magnetic structure down to the atomic scale. As a model sample, the row-wise anti-ferromagnetic Mn3N2(010) surface is studied. It is shown that the magnetic contrast in atomic-scale images is a strong function of the bias voltage around the Fermi level. Inversion of the magnetic contrast is also demonstrated. The experimental SP-STM images and height profiles are compared with simulated SP-STM images and height profiles based on spin-polarized density functional theory. The success of different tip models in reproducing the non-magnetic and magnetic STM data is explored.

Atomic-resolution study of Mn tetramer clusters using scanning tunneling microscopy

A manganese nitride surface containing a well-ordered array of MnN-bonded manganese clusters is investigated. The clusters are composed of a quadrant array of Mn atoms forming a tetramer. Scanning tunneling microscopy is used to image and resolve the clusters into their constituent atoms and their structure and arrangement is presented. The Mn–Mn and Mn–N bond lengths are estimated from the experimental data and compared with theoretical predictions by Rao and Jena [Phys. Rev. Lett. 89, 185504 (2002)] for free, N-doped Mn clusters. The possible effect of the bond lengths on the magnetic properties of the clusters is discussed.

Bias-Voltage Dependence in Atomic-Scale Spin Polarized Scanning Tunneling Microscopy of Mn3N2 (010)

Atomic-scale spin-polarized scanning tunneling microscopy results on the manganese nitride Mn3N2 (010) surface are presented. The images show the row-wise antiferromagnetic structure of the surface. It is shown that the bias voltage between tip and sample affects both the magnetic and non-magnetic components of the height profile. In particular, a reversal of the magnetic contrast is shown to occur at a certain bias voltage.