Miyoung Kim

X-ray absorption spectroscopy in MnxGe1−x diluted magnetic semiconductor: Experiment and theory

Accurate first-principles calculations of soft x-ray absorption spectra are compared with experimental data obtained for the ion-implanted MnxGe1−x ferromagnetic semiconductor. The well-defined features in the spectra are recognized as a signature of homogeneous Mn dilution within the Ge host, as demonstrated by comparing the Mn spectra in diluted MnGe alloys with other competing Mn–Ge crystalline phases. Moreover, provided that an efficient Mn dilution is achieved, the nature of the semiconducting host is shown to affect only slightly the Mn absorption spectrum, as shown by the similarity of the present results with those for other magnetic semiconductors. Both these findings establish the relevance of ion-implantation in the dilute magnetic semiconductor framework, emphasizing its potential impact in device technology.

Magnetism and magnetic anisotropy in FM NiFe, AFM NiMn and their interface

First-principles local density full-potential linearized augmented plane wave (FLAPW) calculations have been performed to investigate the magnetism and magnetic anisotropy in ferromagnetic NiFe, antiferromagnetic NiMn and their interface. We found that the magnetic moments of the Fe atom in NiFe and that of the Mn atom in NiMn are enhanced at their surface compared to their bulk states. The magneto-crystalline anisotropy (MCA) is also drastically influenced by the environment. Bulk NiFe and NiMn show slightly perpendicular and in-plane easy magnetic axes, respectively. However, the effects of the surface change their MCA direction and strength. For the interface between NiFe and NiMn, where the magneto-crystalline anisotropy is dominated by the NiMn layer, the interface effects may be important for discussing the exchange bias.

Magnetism, magneto-crystalline anisotropy, magnetostriction and MOKE at surfaces and interfaces

Abstract A major issue for first-principles theory in magnetism is the treatment of the weak spin–orbit coupling (SOC) and its subsequent effects in magnetic transition metal bulk, surfaces and multilayers. Using either a perturbative or a self-consistent approach for SOC, we have recently investigated important phenomena such as magnetic crystalline anisotropy (MCA), magnetostriction, and magneto-optical Kerr effects (MOKE) in various transition metal systems using the highly precise local density full-potential linearized augmented plane wave (FLAPW) method. With the aid of accurate total energy and atomic force approaches based on the LDA and GGA formalisms, the atomic structures of all the surfaces and interfaces are fully relaxed. Excellent agreement with experiment has been achieved for most of the systems investigated in terms of their equilibrium geometries, MCA energies, magnetostrictive coefficients and MOKE spectra.

Structural and magnetic phase stability of Si/MnAs superlattices: Tetragonal-distortion-induced ferromagnetism and half-metallicity

The structural and magnetic phase stabilities of a Si(001)∕MnAs superlattice have been investigated using the highly precise all-electron full-potential linearized augmented plane-wave method within the generalized gradient approximation. From a total energy and atomic force calculations, we found that the zincblende structure for MnAs is most stable over other atomic configurations, where either Mn or As layers are attached to the Si interface. The antiferromagnetic (AFM) coupling between the Mn atoms is calculated to be energetically favored over the ferromagnetic (FM) coupling by a total energy difference of 40meV∕unit cell. More interestingly, we predict that a 2% tetragonal distortion from its AFM crystal structure induces a magnetic phase transition from the AFM to a half-metallic FM phase with a 0.36eV band gap for the minority spin channel, which indicates a promising possible spintronics application.

Screened-exchange determination of the optical properties of large gap insulators: CaF2

Optical measurements have provided an extremely difficult challenge to existing electronic band structure calculations. Although CaF2, an important large gap insulator, has been intensively investigated, no parameter-free first-principles calculations have been done due to the well-known failure of the local density approximation (LDA) in treating excited states. Here, we present results of fully first-principles calculations of the electronic structure and optical properties of CaF2 with the self-consistent screened-exchange LDA method implemented in the highly precise full-potential linearized augmented plane wave approach. The calculated optical energy gap, 12.05 eV, is in excellent agreement with experiment (12.0±0.1 eV) and so greatly improves the LDA result (7.23 eV). The optical properties, including the imaginary part of the dielectric function and the reflectance determined ab initio with full matrix elements and no parameters, are found to be in good agreement with experiment.

Hybridization reduction of the magnetization for N-doped FeCo superlattices

In search of a soft magnetic material exhibiting high magnetization, we investigated the magnetic and structural properties of an (Fe3Co)4Nm superlattice (SL) with (m=2) and without (m=0) N doping via first principles full-potential linearized augmented plane wave calculations in order to examine the possible magnetic enhancement by (i) N addition (which has drawn considerable attention since the first report (in 1972) of a giant magnetic moment for quasistable α″-Fe16N2) and (ii) the lowered dimensionality of the SL. The structural optimization was fully accomplished by total energy and atomic force calculations within the generalized-gradient approximation. Despite the lattice expansion by 9% due to the N insertion, the magnetization of (Fe3Co)4N2 was found to be reduced from the value of the Fe3Co superlattice by the strong hybridization of N with Fe and Co.

STRUCTURAL STABILITY, MAGNETISM, AND SURFACE MAGNETO-OPTIC KERR EFFECT SPECTRA OF MNAG(001) SURFACE ALLOYS

The stability of a MnAg surface alloy was investigated employing the total energy and atomic force full-potential linearized augmented plane wave method based on the local density approximation for: (i) a clean Ag(001), (ii) 1 ML Mn overlayer [1 Mn/Ag(001)], (iii) 1 and 2 ML MnAg alloys as overlayers on Ag(001) [1(MnAg)/Ag(001) and 2(MnAg)/Ag(001)], and (iv) 1 ML Mn diffused into Ag(001) substrate [Ag/1 Mn/Ag(001)]. Results obtained show that 2(MnAg)/Ag(001) is much more stable than 1 Mn/Ag(001) (by a large energy difference of 150 meV), whereas 1(MnAg)/Ag(001) is marginally more stable (by a slight energy difference of 8 meV) compared to a separate phase of Ag and Mn atoms. Surface Mn and subsurface Mn atoms in 2(MnAg)/Ag(001) were found to be coupled antiferromagnetically with magnetic moments of 3.96 and −3.55 μB, respectively. The surface corrugation (Δz=0.05 a.u.) of 2(MnAg)/Ag(001) was found to be much smaller than that (Δz=0.5 a.u.) of another magnetically stabilized surface alloy system, MnCu/Cu(0...

Excited states and optical properties entirely from first principles: Extended FLAPW method and its graphical user interface

This review serves to explain in some detail the nature and unique treatment of optical properties entirely from first-principles within the powerful FLAPW code and its accompanying sophisticated GUI. One unique feature of the method and its code is the viewing and printing of the various calculated optical properties – Im (eps), Re (eps), N, K, R, EELS and alpha – in the same way as the band structure, charge and spin densities and the density of states were done in the previous version of the FLAPW code. Another unique feature is the ability to calculate the optical properties in several modes, with the LDA and/or the model GW and the sX-LDA approaches (each with and without the inclusion of spin-orbit coupling). The code is also designed to carry out accurate calculations of the optical properties of dilutely doped semiconductors employing several dilute limit simulation tools: these include k-mesh refinement around a certain k-point within a given radius, edge refinement (which refines the tetrahedra around the Fermi energy to get an accurate absorption edge), a further k-mesh refinement to take account the user-specified (or calculated) Fermi level that reflects the dopant or defect concentration, simulation of temperature effects with the Fermi-Dirac statistics, etc.

Orientation and structure dependence of interface magnetocrystalline anisotropy of Co/Cu overlayers and superlattices

The full potential linearized augmented plane wave method and atomic force approach are employed for the theoretical determination of interface magnetocrystalline anisotropy (MCA) for superlattice systems of Co/Cu in (001), (110), and (111) orientations, and overlayer systems of the monolayer Co on Cu (111) substrate adsorbed by different further coverage of Cu. It is found in superlattices that the interface MCA is sensitive to the geometry arising from different orientations. In good agreement with experiment, the interface MCA with Cu overlayers is found to peak at 1 monolayer of Cu‐coated Co/Cu(111) and then to decrease with further Cu deposition. In addition to the hybridization of electronic states at the Co/Cu interface, the interaction between the interface layers and the next‐to‐interface layers in superlattices and structure relaxation in overlayers may have a significant influence on the MCA of the Co layer.

Phase stability of the intermetallic L21 Heusler alloys of A2(Hf1−xZrx)Al (where A=Pd and Pt) for an Nb-based high-temperature materials design

First principles phase stability calculations are used to predict the lattice mismatches between Nb and the A2(Hf1−xZrx)AlL21 Heusler phases and the L21 phase formation energies, where A=Pd and∕or Pt, and x=0, 0.25, 0.75 and 1. The calculated L21 phase mixing energy demonstrates that the Hf-Zr solution phases in the form of A2(Hf1−xZrx)Al (x≠0 and 1) are energetically favored, although the Zr-rich alloys exhibit a smaller lattice mismatch than the Hf-rich alloys. The introduction of Pt reduces the lattice mismatch, and forms the energetically favorable (PtPd)XAl Heusler phase, where X=Hf and Zr. A number of critical diffusion couple experiments confirm the phase stability predictions and establish new microstructural design parameters.