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At least 703 records · Page 39

Ab Initio Study of Half-Heusler Compounds MnVZ (Z = P, As, Sb)

Ab initio study of the half-Heusler compounds MnVZ (Z = P, As, Sb) is performed in the framework of Density Functional Theory (DFT). We have investigated their structural, elastic, mechanical, electronic, magnetic, and optical properties using the full potential linearly augmented plane wave (FP-LAPW) method. The compounds have stable structure with lattice constant in the range 4.90–5.75 Å. The correlation effects are incorporated by performing the GGA + U calculations. The compounds are ductile in nature as indicated by their elastic properties like Cauchy pressure (CP), Pugh’s and Poisson’s ratios. All the three compounds MnVZ (Z = P, As, Sb) are half-metals with small indirect band gaps in the spin-down channels of the values 0.74, 0.30, and 1.29 eV respectively at Γ −X symmetry points. These compounds possess relatively small total magnetic moments of about ≈ 1μ{sub B}. The analysis of optical properties reveals that these compounds are more active at small and high energies and show negligible activity at intermediate energies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Role of the Cluster Structure of Amorphous Fe{sub 67}Cr{sub 18}B{sub 15} Alloy in Magnetism and in the Changing of Electron Scattering Mechanisms under the Influence of Ion (Ar{sup +}) Irradiation

The contribution of clusters of different sizes to magnetism and the switching of electron scattering mechanisms in amorphous Fe{sub 67}Cr{sub 18}B{sub 15} alloy during ion Ar{sup +} irradiation is studied. The cluster magnetism is found to be related to the presence of clusters of the following two types: large α-(Fe, Cr) clusters of size D = 150–250 Å and small (D = 40–80 Å) clusters in a random intercluster medium. The generation of small ferromagnetic and antiferromagnetic clusters during ion irradiation leads to the formation of cluster glass, which affects the electrical properties of the alloy and causes a magnetic frustration. The temperature dependence of the barrier height is shown to characterize the magnetic state of the alloy in low fields. On the whole, the temperature dependence of the order parameter is a universal characteristic of the system. The temperature dependence of resistivity of initial alloys in the temperature range 98–300 K (ρ(T) ∝ T{sup 2}) is determined by electron scattering by quantum defects, and the transition into a ferromagnetic state is revealed when the derivative ∂ρ/∂T ∝ T is analyzed. The increase in resistivity and the relation ρ ∝ T{sup 1/2} in strongly inhomogeneous samples after irradiation at a dose Φ = 1.5 × 10{sup 18} ions/cm{sup 2} are caused by weak localization effects, and the transition to a ferromagnetic state becomes obvious when the derivative ∂ρ/∂T ∝ T{sup –1/2} is considered. Irradiation by fluence Φ = 3 × 10{sup 18} ions/cm2 induces a giant (twofold) increase in the alloy density, restores the ferromagnetism of large clusters, decreases the resistivity by 37%, and restores the relation ρ(T) ∝ T{sup 2}, which results from the overlapping of the irradiation-induced small clusters when their concentration increases and from an increase in the alloy density. The overlapping of clusters lowers the barrier height and decreases the sensitivity of the alloy to an applied field. The relation ρ(T) ∝ T{sup 2} is valid for the entire temperature range T = 2–300 K because of the partial screening of the magnetic moments of large clusters by a medium having the properties of cluster glass.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Features of Spin Waves Focusing in Ferromagnets

Abstract—Features of focusing spin waves in ferromagnets with magnetic moment exchange interaction with the closest neighbors and second neighbors are explored. It is shown that in the long-wave approximation, no spin wave focusing occurs: it is observed only in the wavevectors in the second half of the Brillouin zone (at aq ≥ π/2, where a is the lattice constant, q is the value of wavevector). Additionally, it is shown that magnons in such system are focused along directions [110], and are defocused in directions [100]. It is found that the external magnetic field and the magnetic anisotropy field do not cause changes in directions of magnon focusing. When the parameter of exchange interaction with the second neighbors near directions [110] and [111] is negative, magnons form a caustic, near which the intensity of the spin-wave field surges.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Dynamical resilience to disorder: The dilute Hubbard model on the Lieb lattice

In itinerant systems, electron-electron interactions may lead to the formation of local magnetic moments and their effective exchange coupling, which in turn gives rise to long-range magnetic order. Therefore, when moment formation is weakened, such as in the single-band Hubbard model on a square lattice with the on-site repulsion being randomly switched off on a fraction $\textit{x}$ of sites, magnetic order is suppressed beyond some critical $x_c$, which was found to lie below the classical percolation threshold $x^{\text{(perc,sq)}}_c$. Here we study dilute magnetism in flat band systems, namely, in the Hubbard model on a “Lieb” lattice. Interestingly, we show that magnetic order persists to x almost twice as large as the classical percolation threshold for the lattice, thus emphasizing the central role of electron itinerancy to the magnetic response. Further, the analysis of the orbital-resolved order parameters reveals that the contribution of the fourfold coordinated $\textit{“d”}$ sites to magnetism is dramatically affected by dilution, while the localized $\textit{“p”}$ states of the flat band provide the dominant contribution to long-range correlations. We also examine the transport properties, which suggest the existence of an insulator-to-metal transition in the same range of the critical magnetic dilution.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Electromagnetic moments of 215,217 Bi: Probing shell evolution beyond N = 126

The nuclear properties of bismuth isotopes (Z = 83) , with just one valence proton above the closed spherical shell at (Z = 82) , are expected to be governed by a single unpaired proton. However, already in semimagic 209 Bi (Z = 83, N = 126) , , the magnetic moment (μ) strongly deviates from the single-particle Schmidt value. A near linear decrease in μ with the increase of N after the N = 126 magic number was observed up to N = 130 . In order to test whether this trend is kept at N > 130 and to reveal the underlying mechanisms, an investigation of 215,217 Bi (N = 132, 134) has been undertaken. The magnetic dipole and electric quadrupole moments of the I π = 9/2 - nuclear ground states in these isotopes have been measured for the first time using the in-source resonance-ionization spectroscopy technique at ISOLDE (CERN). It has been shown that the linearly decreasing trend of μ( 209,211,213 Bi g ) is broken in 215,217 Bi with a nearly constant value of μ observed. Experimental data have been compared to calculations in the framework of the configuration-interaction shell model with the monopole-based universal V MU +LS interaction. The peculiarities in the behavior of μ(Bi, 9/2 - ) with increasing neutron number are explained as being due to the shell evolution, change of the neutron orbitals occupancies and strong configuration mixing beyond N = 130 . Also, the difference in the μ trends for bismuth (Z = 83) and astatine (Z = 85) isotopes with N > 126 are reproduced by the shell-model calculations. It is shown that monopole interaction plays noticeable role in the description of the peculiarities of the μ behaviour. Additionally, the extension of the application of the V MU interaction to the μ isotopic trends for heavy nuclei is important for further study of the capabilities of this promising version of the shell-model calculations.

Dipole magnetic moments↗

Investigation on Physical Properties of Sn-Modified Cubic Cu{sub 2}O Nanostructures

Cubic Cu{sub 2−x} Sn{sub x}O (0 ≤ x ≤ 0.4) nanostructures have been developed with the solvothermal method. X-ray diffraction data analysis confirms the formation of a pristine Cu{sub 2}O phase below x=0.3 Sn{sup 2+} substitution, and SnO-related phases appear in 0.3 substitute sample indicating phase segregation starts at x≥0.3. The magnetization versus applied field (M − H) curves have been determined using a vibrating sample magnetometer at 300 K which reveals that ferromagnetic behavior of synthesized nanomaterials is reliant on the substituent concentration and enhanced gradually with its addition in the material. Photoluminescence spectra show that ferromagnetism originates from the induced magnetic moment in defects created as cation vacancies in the host material. Beside these, the dielectric measurements were performed at various frequencies (20 Hz–1 MHz) and in the temperature range of 100 to 350 K.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

Electronic Structures and Ferromagnetic Properties of 3d (Cr)-Doped BaSe Barium Selenide

In this study, we have employed the first-principle methods based on spin-polarized density functional theory to investigate the structural parameters, the electronic structures, and the half-metallic ferromagnetic behavior of chromium (Cr)-doped barium selenide (BaSe) such as Ba{sub 1 − x}Cr{sub x}Se at concentrations x = 0.25, 0.5, and 0.75. The exchange and correlation potential is described by the generalized gradient approximation of Wu and Cohen (GGA-WC). The calculated structural parameters of BaSe are in good agreement with theoretical data. Our findings reveal that the p-d exchange coupling is ferromagnetic for Ba{sub 0.75}Cr{sub 0.25}Se and Ba{sub 0.5}Cr{sub 0.5}S, but it becomes anti-ferromagnetic for Ba{sub 0.25}Cr{sub 0.75}S. The electronic structures exhibit that the Ba{sub 1 − x}Cr{sub x}Se materials for all concentrations are half-metallic ferromagnets with spin polarization of 100% and total magnetic moment per Cr atom of 4 μ{sub B}. Therefore, the Ba{sub 1 − x}Cr{sub x}Se compounds are suitable candidates for possible spintronics applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Cyclotron resonance features in a three-dimensional topological insulators

The specific features of the cyclotron resonance in topological insulators of the Bi{sub 2}Te{sub 3} family are analyzed. The energy spectrum of electron excitation on the surface of these compounds in a magnetic field has the C{sub 3V} symmetry. With the use of quasiclassical equations of motion, which take into account the Berry curvature and magnetic moment, it is found that the cyclotron frequency spectrum contains additional harmonics due to spectrum anisotropy. An analysis of these harmonics makes it possible to reveal the contribution from the topological characteristics to the observables.

36 MATERIALS SCIENCE↗

First-Principle Investigation of V and Cr Doping on the Dilute Magnetic Semiconducting Properties of Rutile SnO{sub 2}: a Modified Becke-Johnson (TB-mBJ) Exchange Potential Study

Theoretical investigations on rutile SnO{sub 2} doped with V and Cr were performed by using recently implemented Tran and Blaha’s modified Becke-Johnson exchange potential model (TB-mBJ) based on density functional theory. The electronic, magnetic, dielectric and optical properties of V and Cr-doped tin oxide have been calculated by full-potential linearized augmented plane wave method (FP-LAPW) by WIEN2k code. The present works demonstrate accurate prediction of band structures and properties using the TB-mBJ model. The calculated values of magnetic moment, dielectric constant and refractive index of doped and undoped compounds agree well with the experimental results. The TB-mBJ model is a direct method without using empirical corrections and hence, the furnished data are useful in material modelling for spintronics applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Response of a magnetic nanoparticle lattice to a magnetic field pulse near the stability boundary

The dynamic response of a system being near the stable equilibrium boundary to an external magnetic field pulse is studied for 2D lattices of magnetic nanoparticles with cubic crystallographic anisotropy. The conditions under which magnetic moment oscillations from individual dipoles propagate to the entire system are revealed. This effect results in the lattice response are significantly larger in the external pulse duration and with an amplitude rather weakly depending on initial conditions and external field parameters, the processes during which the pulse results in reorientation of only individual lattice dipoles.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

Nanocrystalline Nickel-Substituted Lanthanum Cobaltites Synthesized by Urea Combustion Method: Magnetic and Transport Properties

We have systematically investigated the effect of Ni substitution on the structural, magnetic and transport properties of LaCoO{sub 3}. Single-phase rhombohedral structure with space group R-3c is confirmed by the Rietveld refinement of X-ray diffraction data in all samples. The unit cell parameters were found to increase with Ni doping. The FM and AFM were observed to coexist as confirmed by M–H hysteresis. Positive values of Θ for nickel-doped samples are consistent with the existence of FM interactions. The effective magnetic moment decreases with Ni substitution. All the samples exhibit temperaturedependent semiconducting behaviour. The electrical resistivity decreases considerably with increasing Ni content and with increasing temperature. The transport mechanism has been investigated by fitting a nearest-neighbour small polaron hopping (SPH) model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Electronic and Magnetic Structure of Intercalated Graphene Films

Ab initio calculations of the electron spectrum of the graphene–cobalt–nickel system were performed in the slope of the spin density functional theory (SDFT). Dispersion curves E{sub σn}(k) are presented; they were used to determine partial and total densities of valence electron states, and also magnetic moments of all atoms in the supercell. Energy position of the “Dirac cone” defined by p{sub z} states in graphene is shown to depend only slightly on the number of Co layers intercalated into the gap between the cobalt and graphene layers.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

Influence of Cation Size on the Electronic and Magnetic Structure of M CoO{sub 3} (M = Bi, La) from Ab Initio Calculations

Using the first-principle DFT method and LSDA+ U calculations, we have performed a detailed investigation on the structural and magnetic ground states of strongly correlated M CoO{sub 3} (M= Bi, La) perovskites. Magneticphase stabilities, structural behaviors and electronic properties of both materials have been investigated. LSDA+ U calculations show that the antiferromagnetic (AFM)-C phase in the tetragonal La/BiCoO{sub 3} is energetically more favorable than the other configurations, and both rhombohedral materials are nonmagnetic. All structural data of tetragonal BiCoO{sub 3} and rhombohedral LaCoO{sub 3} compounds are found in excellent agreement with the experiment for U{sub eff}= 6 eV. We have predicted also band gaps and magnetic moments with a good accuracy via the band structure calculations. From the total and partial densities of states, we have shown the important role of O p Co d and Bi/La s-O p hybridizations in both systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Multi-Level Kondo Effect and Enhanced Critical Temperature in Nanoscale Granular Al

The normal state conductance of high resistivity granular Al films, having an enhanced critical temperature above 3 K, shows a non-monotonous temperature dependence. Upon cooling the conductance first increases, reaches a broad maximum, then decreases, and finally increases sharply again at low temperatures. This behavior bears a striking resemblance to that predicted for the conductance of quantum dots in a regime where the broadening of discrete energy levels, due to coupling of the dot to the leads reservoirs, is larger than the level separation. In this regime, a multi-level Kondo temperature can be larger than the effective Coulomb charging energy, leading to a metallic-like behavior at high temperatures. This is followed by a Coulomb dominated regime, while at yet lower temperatures, the single level Kondo behavior can be recovered. We believe that this multi-level Kondo resonance model for single dots may apply to our 3D network of nanoscale grains. The multi-level resonance prevents the insulating state from setting-in already at high temperatures, which is favorable for superconductivity. This interpretation of the experimental conductance data is in line with the previously reported presence of magnetic moments in these films. High-resolution electron microscopy shows that intergrain coupling can take place through atomic size Sharvin contacts, which is consistent with the typical values of the intergrain resistance of the films as the metal to insulator transition is approached.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Structural, Electronic, Magnetic and Optical Investigations of Half-Heusler Compounds YZSb (Z = Cr, Mn): FP-LAPW Method

Half-Heusler compounds YZSb (Z = Cr, Mn) are theoretically explored in the framework of density functional theory (DFT). We have investigated a number of different properties with the help of the WIEN2k simulation package. Equilibrium lattice constants fall in the range 6.0–7.0 Å for both compounds. Elastic properties are also studied and it is found that the compound YCrSb is of brittle and YMnSb is of ductile nature as indicated by a number of parameters including Cauchy pressure, Pugh’s ratio and Poisson’s ratio. Electronically, these compounds are found to be half-metals as their spin-up states are semiconducting and hence 100% spin polarized at the Fermi level while spin-down states are conducting. The total magnetic moments are equal to 4μ{sub B} and 3.15μ{sub B} for YCrSb and YMnSb, respectively. The optical properties including dielectric function, reflectivity, refractive index, conductivity and absorption coefficients are calculated and discussed. Optical characteristics of the compounds hint that they are more active in the infrared region.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Structural, Electronic, Magnetic, and Optical Properties of Half-Heusler Alloys RuMnZ (Z = P, As): a First-Principle Study

Half-Heusler alloys RuMnZ (Z = P, As ) are studied in the framework of Density Functional Theory (DFT). Structural, electronic, magnetic, and optical properties are calculated and analyzed using the WIEN2k simulation code. All the calculations are done using the full potential linearized augmented plane wave (FP-LAPW) method. Equilibrium lattice parameters are found to be in the range 5.5–5.6 Å. Band gaps of the compounds and density of states (DoS) analysis reveal that the minority spin-down states are semi-conducting while the majority spin-up states are conducting confirming the half-metallic nature of the compounds. Hence, at Fermi level, states are 100 % polarized. The value of the total magnetic moment is found to be 2, i.e., M{sub Tot} = 2μ{sub B}. Several optical properties, including dielectric function, reflectivity, refractive index, conductivity, and absorption coefficient are calculated as well. It is revealed from the imaginary part of the dielectric function that the compounds are optically metallic.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Electronic Structure of GdCuGe Intermetallic Compound

The electronic structure of GdCuGe intermetallic compound has been studied. Spin-polarized energy spectrum calculations have been performed by the band method with allowance for strong electron correlations in the 4f-shell of gadolinium ions. Antiferromagnetic ordering of GdCuGe at low temperatures has been obtained in a theoretical calculation, with the value of the effective magnetic moment of gadolinium ions reproduced in fair agreement with experimental data. The electronic density of states has been analyzed. An optical conductivity spectrum has been calculated for GdCuGe; it reveals specific features that are analogous to the ones discovered previously in the GdCuSi compound with a similar hexagonal structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Magneto-electronic and Thermodynamic Properties of Quaternary NiFeMnZ (Z = In, Sn) New Spin Gapless Semiconductors

This paper deals with investigation of magneto-electronic and thermodynamic properties of new spin gapless semiconductor quaternary NiFeMnZ (Z = In, Sn) Heusler alloys. In addition, electronic properties such as band structure and density of states were studied in detail. The obtained results are based on first-principles calculations, using the full potential linearized augmented plane wave (FP-LAPW) method within well-known generalized gradient approximation (GGA) coupling with spin-orbit calculation (SOC) in framework of the density functional theory (DFT) implemented in the WIEN2k package. As results, the alloys were found to be new spin gapless semiconductors (SGSs), and both compounds in LiMgPdSn-type crystal structure are stable. The magnetic moment of NiFeMnIn and NiFeMnSn was 4.39 and 4.99 μ{sub B} per formula unit, respectively. This result is in agreement with the Slater-Pauling rule (M{sub tot} = 24 −Z{sub tot}). Furthermore, the thermodynamic properties were also predicted. All these multifunctionalities lead to crucial interest for developing future spintronics device generations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗