Spin Polarization of Mn<Subscript>5</Subscript>Ge<Subscript>3</Subscript> in the Bulk and Thin Films

The intermetallic compound Mn₅Ge₃ is one of the promising materials for application as a source of charge carriers in spintronics. The existing experimental data on the spin polarization in Mn₅Ge₃ demonstrate significant discrepancies. All theoretical studies concern a Mn₅Ge₃ bulk crystal. At the same time, thin films are of interest for applications. In this work, ab initio calculations have been performed for a Mn₅Ge₃ thin film on a germanium substrate. The difference between the magnetic moments of manganese atoms, densities of states, and spin polarizations for the bulk crystal and thin film has been demonstrated.     

A first-principles study aided with Monte Carlo simulations of carbon doped iron-manganese alloys

We have investigated the complex magnetic properties of Fe_1?x Mn _x C _y alloys by using an iterative combination of ab initio calculations and Monte Carlo simulations. The latter gives insight into finite temperature magnetism and allows to determine the critical temperature of magnetic phase transitions. We restrict the investigation to ordered systems with 25, 50 and 75% manganese and study the influence of carbon at octahedral interstitial sites on the magnetic properties. The combination of ab initio calculations with Monte Carlo simulations turns out to be a powerful tool to determine the complex magnetic structures, which originate from the competition of ferro- and antiferromagnetic interactions in the FeMn alloys.     

Study of electronic and magnetic properties of MnS layers

<正>Self-consistent ab initio calculations,based on the density functional theory(DFT) and using the full potential linear augmented plane wave(FLAPW) method,are performed to investigate both electronic and magnetic properties of the MnS layers.Polarized spin and spin-orbit coupling are included in the calculations within the framework of the antiferromagnetic state between two adjacent Mn layers.Magnetic moments considered to lie along axes are computed. Obtained data from ab initio calculations are used as input data for the high temperature series expansion(HTSE) calculations to compute other magnetic parameters.The zero-field high temperature static susceptibility series of the spin-4.39 nearest-neighbour Heisenberg model on centred face cubic(FCC) and lattices is thoroughly analysed by a power series coherent anomaly method(CAM).The exchange interactions between the magnetic atoms,the Neel temperature,and the critical exponent associated with the magnetic susceptibility are obtained for the MnS layer.     

Structural stability,mechanical, electronic and thermal behaviour of Ru2CrZ (Z=Sb,Si, Pb,Ge) Heusler alloys

In this paper, the structural, elastic, electronic properties of Ru2CrZ (Z=Si, Ge, Pb, Sn) are explored using the generalized gradient approximation based on ab initio plane-wave pseudopotential density functional theory. With the help of the quasi-harmonic Debye model, we also investigate the variation of normalized volume V/V0, the heat capacities CV and CP, thermal expansivity, and Debye temperature of Ru₂CrZ (Z=Si, Ge, Pb, Sn). Results show that the Cu₂MnAl type structure is more stable then Hg₂CuTi type structure. The four compounds in the ground state are predicted to be nearly half-metal behavior with total magnetic moment near to the integer value. To provide a comparative and complementary study to future researches, we investigated the elastic and thermodynamic properties.     

Ab initio calculations and experimental determination of the structure of Cr2AlC

We have calculated the equilibrium volume and the density of states (DOS) of Cr₂AlC for antiferromagnetic (AFM), ferromagnetic (FM) and paramagnetic (PM) configurations by ab initio total energy calculations. Based on a comparison of the cohesive energies as well as the DOS for all three magnetic configurations we have identified the FM configuration to be metastable. Furthermore, we report the structural characterization of polycrystalline Cr₂AlC thin films grown by magnetron sputtering. Our calculated interplanar distances and equilibrium volume for the PM and AFM configurations are in good agreement with our experiment. The charge density distribution suggests that the chemical bonding between Cr and C in Cr₂AlC is very similar to the one in cubic CrC.     

TDPAC study of Cd-doped SnO

The combination of hyperfine techniques and ab initio calculations has been shown to be a powerful tool to unravel structural and electronic characterizations of impurities in solids. A recent example has been the study of Cd-doped SnO, where ab initio calculations questioned previous TDPAC assignments of the electric-field gradient (EFG) in ¹¹¹In-implanted Sn-O thin films. Here we present new TDPAC experiments at ¹¹¹In-difused polycrystalline SnO. A reversible temperature dependence of the EFG was observed in the range 295–900 K. The TDPAC results were compared with theoretical calculations performed with the full-potential linearized augmented plane wave (FP-LAPW) method, in the framework of the density functional theory. Through the comparison with the theoretical results, we infer that different electronic surroundings around Cd impurities can coexist in the SnO sample.     

On the magnetoelastic contribution to the magnetic anisotropy of thin epitaxial Permalloy films: an ab initio study

Permalloy films are often used in the magnetic thin film technology. It is expected that in this material the magnetoelastic coupling of the magnetization to the epitaxial film strain does not produce undesired magnetic anisotropy, because the linear magnetoelastic bulk coefficient B₁ of Permalloy is near-zero. It is shown by means of the ab initio density functional electron theory that the nonvanishing nonlinear magnetoelastic couplings also do not lead to a considerable anisotropy. Explicit values for two of the nonlinear magnetoelastic coupling coefficients are given.     

Optical spectrum and local lattice structure for ruby

We perform ab initio calculations using a pseudo-potential plane-wave method based on density functional theory, within the local density approximation and generalized gradient approximation, in order to determine and predict the pressure dependence of structural and elastic properties of spinel compounds: MgAl₂O₄, MgGa₂O₄ and MgIn₂O₄. The results are in agreement with the available experimental data and other theoretical calculations.     

Electronic structure and magnetic coupling in CaV<Subscript>2</Subscript>O<Subscript>5</Subscript>: spin dimer versus spin ladder

The electronic structure and magnetic exchange interactions of the ladder vanadate CaV₂O₅ have been studied by ab initio electronic structure calculations based on density functional theory (DFT). Geometry optimization and electronic structure calculations are performed using spin-polarized generalized gradient approximation (GGA) exchange-correlation functionals for four possible spin-ordered states. The experimentally observed insulating behavior has been reproduced successfully in the framework of the band theory by considering the magnetic ordering. Calculated results reveal that the true magnetic ground state of CaV₂O₅ is the antiferromagnetic (AFM) state with AFM exchange interactions both inside the rungs and along the ladder legs. Calculated exchange parameters indicate that the ladder structural vanadate CaV₂O₅ should be described as weakly coupled dimer system rather than as spin ladder compound. The AFM interactions inside the dimer are crucial to the insulating ground state and magnetic characteristics of CaV₂O₅.     

The effect of the impurities on the magnetic,electronic and optical properties of Mn5Ge3

Earlier, we experimentally showed a significant effect of oxygen on the magnetic and structural properties of Mn₅Ge₃ due to the formation of a Nowotny phase of Mn₅Ge₃O_x. Here, in continuation of this study, we present a theoretical study of the magnetic and electronic properties of Mn₅Ge₃ and Mn₅Ge₃D_x (D = B, C, O). It was found that hexagonal Mn₅Ge₃ is a ferromagnetic metal with two nonequivalent manganese atoms in the structure. Our ab initio calculations also predict the existence of a spin-crossover in Mn₅Ge₃ under pressure. Impurities reduce saturation magnetization and electrical and thermal conductivity; however, the magnetic susceptibility and Curie temperature increase. Microscopic mechanisms of the effect of the impurities on the magnetic and electronic properties Mn₅Ge₃ are discussed.     

Photoluminescence of crystalline and disordered BTO:Mn powder: Experimental and theoretical modeling

Disordered and crystalline Mn-doped BaTiO₃ (BTO:Mn) powders were synthesized by the polymeric precursor method. After heat treatment, the nature of visible photoluminescence (PL) at room temperature in amorphous BTO:Mn was discussed, considering results of experimental and theoretical studies. X-ray diffraction (XRD), PL, and UV-vis were used to characterize this material. Rietveld refinement of the BTO:Mn from XRD data was used to built two models, which represent the crystalline BTO:Mn (BTO:Mn_c) and disordered BTO:Mn (BTO:Mn_d) structures. Theses models were analyzed by the periodic ab initio quantum mechanical calculations using the CRYSTAL98 package within the framework of density functional theory at the B3LYP level. The experimental and theoretical results indicated that PL is related with the degree of disorder in the BTO:Mn powders and also suggests the presence of localized states in the disordered structure.     

Control of the boiling crisis: analysis of a model system

Ab initio calculations of the magnetic properties and site preference of Mn in Fe_3-xMn_xSi were carried out using density functional theory and employing the TB-LMTO-ASA method. Qualitative agreement with the experimental results is obtained for site preference as well as for total and local magnetic moments. It is postulated that short-range order would explain observed discrepancies between theoretical and experimental results.     

Preserving the half-metallicity at the Heusler alloy Co2MnSi(001) surface: a density functional theory study

We have studied the stability, the electronic, and the magnetic properties of Co2MnSi(001) thin films for 15 different terminations using density functional theory calculations. The phase diagram obtained by ab initio atomistic thermodynamics shows that in practice the MnSi, pure Mn, or pure Si terminated surfaces can be stabilized under suitable conditions. Analyzing the surface band structure, we find that the pure Mn termination, due to its strong surface-subsurface coupling, preserves the half-metallicity of the system, while surface states appear for the other terminations.     

Atomic disorder and the magnetic, electrical, and optical properties of a Co2CrAl Heusler alloy

Two Co₂CrAl alloy samples subjected to different heat treatment regimes are studied. An exact distribution of atoms over the sublattices in the samples is determined by X-ray diffraction and neutron diffraction methods. These data are used to perform ab initio density of states calculations and to calculate the magnetic moments of the samples in a coherent potential approximation. The calculated magnetic moments are compared to the experimental values. The effect of atomic ordering on the electronic structure near the Fermi level is analyzed using optical methods. The possible causes of the detected temperature dependence of the electrical resistivity, unusual for metallic alloys, are discussed.     

Phase transformation in Ni–Mn–Sn ferromagnetic shape memory alloy thin films induced by dense ionization

The effects of 200 MeV Au ions irradiation on the structural and magnetic properties of Ni–Mn–Sn ferromagnetic shape memory alloy (FSMA) thin films have been systematically investigated. In order to understand the role of initial microstructure and phase of the film with respect to high energy irradiation, the two types of Ni–Mn–Sn FSMA films having different phases at room temperature were irradiated, one in martensite phase (Ni_58.9Mn_28.0Sn_13.1) and other in austenite phase (Ni₅₀Mn_35.6Sn_14.4). Transmission electron microscope (TEM) and scanning electron microscope (SEM) images along with the diffraction patterns of X-rays and electrons confirm that martensite phase transforms to austenite phase at a fluence of 6×10¹² ions/cm² and a complete amorphization occurs at a fluence of 3×10¹³ ions/cm², whereas ion irradiation has a minimal effect on the austenitic structure (Ni₅₀Mn_35.6Sn_14.4). Thermo-magnetic measurements also support the above mentioned behaviour of Ni–Mn–Sn FSMA films with increasing fluence of 200 MeV Au ions. The results are explained on the basis of thermal spike model considering the core and halo regions of ion tracks in FSMA materials.     

Electronic and spatial structure of nanoclusters of Sn2P2S6 ferroelectric crystals

The results obtained from ab initio calculations of the optimized configuration and electronic structure of the cluster models of Sn₂P₂S₆ ferroelectric crystals are presented. The calculations have been performed using the spin-restricted Hartree-Fock method and the density functional theory in the DZVP basis set. It has been shown that the clusters are stable and retain the topology of the simulated crystal. The influence of the cluster sizes on the properties under investigation has been analyzed.     

第一原理研究Sn(O1-xNx)2材料的光电磁性质

基于第一原理的密度泛函理论, 以量子化学从头计算软件 为平台研究了Sn(O_1-xN_x)₂材料的光电磁性能, 分析了体系的态密度、 能带结构、 磁性、 介电虚部及折射率. 计算结果表明, N替代O后, 随着掺杂浓度的增加, 体系的带隙先减小后增大, 掺杂量为12.50%时带隙最窄. 由于N 2p轨道电子的贡献, 在0.55-1.05 eV范围内产生了浅受主能级, 价带和导带处的能级均出现了劈裂及轨道的重叠现象, Sn-O键的键强大于N-O键的键强. 从磁性来看, N原子决定了磁矩的大小. 从介电虚部可知, 掺杂后体系的光学吸收边增宽, 主跃迁峰发生红移, 反射率和介电谱相对应, 各峰值与电子的跃迁吸收有关.     

Half-metallicity and magnetic properties of half-Heusler type Mn2Sn: Ab initio predictions

Ab initio calculations have been carried out to investigate the electronic structure and magnetism of the compound Mn₂Sn with the bcc half-Heusler structure. For the equilibrium lattice parameter 5.69 Å, Mn₂Sn is predicted to be a half-metallic fully compensated ferrimagnet (also called half-metallic antiferromagnet) with zero total spin moment. This zero moment agrees well with the Slater-Pauling curve and mainly comes from the compensated Mn (A) and Mn (B) spin moments in antiparallel configuration. The half-metallicity of Mn₂Sn is stable in a wide lattice-parameter range from 5.6 Å to 5.9 Å. Upon contraction of the lattice, a transition from half-metallicity to semimetallicity is observed.     

Structural,electronic and magnetic properties of negative thermal expansion material Mn3Cu1−xSnxN

We have investigated the structural, electronic and magnetic properties of Mn₃Cu_1?xSn_xN(x=0, 0.5) using first-principles density functional theory within the generalized gradient approximation (GGA) + U schemes. The crystal structure of the compounds is tetragonal crystal for x=0 while it is a cubic crystal for x=0.5. Our spin-polarized calculations give a metallic ground state for the x= 0, 0.5 in agreement with experiments. From the charge density and density of states(DOS), the coupling between Sn 5p with Mn 3d and spin geometrical frustration effect are the main reasons for magnetic transition in Mn₃Cu_1?xSn_xN.