Erkin Kulatov

Ab initio study of the magnetism in GaAs, GaN, ZnO, and ZnTe-based diluted magnetic semiconductors

Abstract Electronic structure and magnetic properties of Ga 1− x Mn x As, Ga 1− x Mn x N, Zn 1− x M x O, and Zn 1− x M x Te (M=V, Cr, Mn, Fe, and Co) diluted magnetic semiconductors (DMS) are calculated by the tight-binding LMTO method in the 64 atom supercell. Calculations are made at several x with varied spatial distribution of dopant atoms and codoping of DMS. The results show that stability of the ferro- and antiferromagnetic (FM and AFM) states in DMS strongly correlates with the occupation and energy position of 3D-dopant bands. Adequacy of the double exchange and superexchange mechanisms for explanation of the FM–AFM competition is discussed.

Optical Measurements and Band Calculations of FeSi.

Reflectivity measurements of high quality FeSi single crystal have been performed at 6 and 300 K over a very wide spectral region from 0.05 to 30 eV. The optical conductivity is obtained by a Kramers-Kronig transformation, and it is discussed in connection with the recent band calculation in which the relativistic spin-orbit effects are taken into account and with other measurements done on FeSi before. The result of direct photoconductivity measurement in the spectral region from 200 to 800 cm -1 is also presented.

High-temperature ferromagnetism in Si1 − xMnx (x ≈ 0.5) nonstoichiometric alloys

It has been found that the Curie temperature (TC ≈ 300 K) in nonstoichiometric Si1 − xMnx alloys slightly enriched in Mn (x ≈ 0.52–0.55) in comparison to the stoichiometric manganese monosilicide MnSi becomes about an order of magnitude higher than that in MnSi (TC ∼ 30 K). Deviations from stoichiometry lead to a drastic decrease in the density of charge carries (holes), whereas their mobility at about 100 K becomes an order of magnitude higher than the value characteristic of MnSi. The high-temperature ferromagnetism is ascribed to the formation of defects with the localized magnetic moments and by their indirect exchange interaction mediated by the paramagnetic fluctuations of the hole spin density. The existence of defects with the localized magnetic moments in Si1 − xMnx alloys with x ≈ 0.52–0.55 is supported by the results of numerical calculations performed within the framework of the local-density-functional approximation. The increase in the hole mobility in the nonstoichiometric material is attributed to the decay of the Kondo (or spin-polaron) resonances presumably existing in MnSi.

Magnetic phase transitions and electronic structure of the manganese silicides

The results of the experimental investigations of physical properties of the single crystals of Mn 5 Si 3 and MnSi≈ l.7 doped by Ge and Cr are presented. For the first time an existence in Mn 5 Si 3 the first order metamagnetic transition have been observed. From the experimental data it is found that the properties of the induced ferromagnetic state in Mn 5 Si 3 and the doped higher manganese suicides can be interpreted on the basis of the band model. Theoretical calculations conclude that the band character of magnetism in Mn 5 Si 3 is due to Mn I atoms in 4(d) positions while atoms in 6(g) positions are covalent bonded and more localized.

Electronic structure and magnetism of FeGe with B20-type structure

Abstract The electronic structure of FeGe with B20-type crystal structure is calculated by a self-consistent LMTO method. A ferromagnetic state is obtained with the magnetic moment of about 1μB per Fe at the observed lattice constant, being consistent with the observed induced moment. It is shown at smaller lattice constants that a non-magnetic semiconducting state with a very narrow band gap becomes stable, where a magnetic field-induced metamagnetic transition takes place from a non-magnetic state (semiconductor) to a ferromagnetic one (metal), similar to that in FeSi with the same crystal structure. It is concluded that the difference between the magnetic states of Fe in FeGe and FeSi comes from the band-gap width in the non-magnetic state.

Ab Initio Study of Magnetism in III-V- and II-VI-Based Diluted Magnetic Semiconductors

Electronic structure and magnetic properties of Ga1−xMnxAs, Ga1−xMnxN, Zn1−xMxO, and Zn1−xMxTe (M=V, Cr, Mn, Fe, and Co) diluted magnetic semiconductors (DMS) are calculated by the tight-binding LMTO method in the 64-atom supercell. Calculations are made at several x with varied spatial distribution of dopant atoms and codoping of DMSs. The results show that stability of the ferro- and antiferromagnetic (FM and AFM) states in DMSs strongly correlates with the occupation and energy position of 3d-dopant bands. Adequacy of the double exchange and superexchange mechanisms for explanation of the FM vs. AFM competition is discussed.

Photo-thermal excitation gas-source MBE growth of super-doped Si:Mn for spin-photonics applications

Abstract Crystalline Si thin films doped with transition metal elements far beyond the solid solubility are named here as ‘super-doped Si:TM’ (TM=transition metals). Super-doped Si:Mn thin films with doping levels of more than 10% have been grown by using a photo-thermal excitation gas-source molecular beam epitaxy (GSMBE) technique. The GSMBE apparatus used here was equipped with a ‘hot W-filament cell’, and hence it is called photo-thermal excitation GSMBE. The hot W-filament cell decomposed quite effectively the source of SiCl 2 H 2 molecules and enabled the growth of polycrystalline super-doped Si:Mn solid-solution films at substrate temperatures as low as 300°C. Auger electron spectroscopy has revealed the characteristic valence electron spectra arising from the valence-electron hybridization between 3d electrons of Mn and s–p electrons of Si.

Spin, orbital moments and magneto-optical properties of MPt3 (M = Cr, Mn, Fe, Co) compounds

Abstract We have studied trends in the ab initio electronic structure, spin moments, orbital moments and magneto-optical (MO) properties of MPt 3 compounds. The calculated quantities agree well with the experimental data. It was found that values of the local orbital moments are closely connected with the predominant spin character of electronic states near the Fermi energy ( E F ). Features of the MO spectra of compounds are primarily defined by the absorptive off-diagonal conductivity σ 2, xy ( ω ). The spin decomposition of this quantity showed that at the MO resonance energy σ 2, xy ( ω ) is formed mainly by spin-up electronic transitions to an unoccupied part of the hybridized {M3d ↑-Pt5d ↑} band. These transitions are intense for CrPt 3 and MnPt 3 , having high MO activity. A reduction of the unoccupied part of {M3d ↑-Pt5d ↑} band in FePt 3 and CoPt 3 results in the weak electronic transitions and in small calculated Kerr rotation values for these compounds.

Magnetic phase transitions in single crystals of Mn5Si3 and (Mn, Fe)5Si3

Experiments indicate that the AF 1 phase in Mn 5 Si 3 can be suppressed by 5 kbar. Then, the AF 2 phase undergoes a transition to a ferromagnetic state. In (Mn 1−x Fe x ) 5 Si 3 samples a first-order metamagnetic transition in high magnetic fields at T < 62 K and the existence of a FM state with T c = 74 and 250 K have been observed

Ferromagnetism of MnxSi1-x(x ∼ 0.5) films grown in the shadow geometry by pulsed laser deposition method

The results of a comprehensive study of magnetic, magneto-transport and structural properties of nonstoichiometric MnxSi1-x (x ≈ 0.51-0.52) films grown by the Pulsed Laser Deposition (PLD) technique onto Al2O3(0001) single crystal substrates at T = 340°C are present. A highlight of used PLD method is the non-conventional (“shadow”) geometry with Kr as a scattering gas during the sample growth. It is found that the films exhibit high-temperature (HT) ferromagnetism (FM) with the Curie temperature TC ∼ 370 K accompanied by positive sign anomalous Hall effect (AHE); they also reveal the polycrystalline structure with unusual distribution of grains in size and shape. It is established that HT FM order is originated from the bottom interfacial self-organizing nanocrystalline layer. The upper layer adopted columnar structure with the lateral grain size ≥50 nm, possesses low temperature (LT) type of FM order with Tc ≈ 46 K and contributes essentially to the magnetization at T ≤ 50 K. Under these conditions, AHE ...