Spinodal Nano-decomposition Research Articles

Magnetism of Eu-doped GaN modulations by spinodal nanodecomposition

By the Monte Carlo method, magnetic properties have been investigated for Eu-doped GaN involving internal nanostructures induced by nanoscale spinodal decomposition, where these nanostructures are spontaneously or artificially formed. In the present simulations, hysteretic and nonhysteretic magnetization curves are observed in the systems with the large-sized and small-sized nanostructures, respectively. These nanostructures affect the blocking temperatures as well. Furthermore, they influence temperature-dependent energy barriers of spin flipping; therefore, the simulations suggest that the magnetization is thermally stable. However, we observe that the blocking temperatures are smaller than the experimental values, which may be due to atomic vacancies.

First-principles calculations on the origin of ferromagnetism in transition-metal doped Ge

Many researchers have shown an interest in Ge-based dilute magnetic semiconductors (DMSs) due to potential advantages for semiconductor spintronics applications. There has been great discussion about mechanisms of experimentally observed ferromagnetism in (Ge,Fe) and (Ge,Mn). We investigate the electronic structures, structural stabilities, magnetic exchange coupling constants, and Curie temperature of Ge-based DMSs, and clarify origins of the ferromagnetism, on the basis of density functional theory calculations. In both the (Ge,Fe) and (Ge,Mn) cases, the inhomogeneous distribution of the magnetic impurities plays an important role to determine the magnetic states; however, physical mechanisms of the ferromagnetism in these two materials are completely different. By the spinodal nanodecomposition, the Fe impurities in Ge gather together with keeping the diamond structure, so that the number of the first-nearest-neighbor Fe pairs with strong ferromagnetic interaction increases. Therefore, the Curie temperature drastically increases with the progress of the annealing. Our cluster expansion method clearly reveals that the other ordered compounds with different crystal structures such as ${\mathrm{Ge}}_{3}{\mathrm{Mn}}_{5}$ and ${\mathrm{Ge}}_{8}{\mathrm{Mn}}_{11}$ are easily generated in the (Ge,Mn) system. The estimated Curie temperature of ${\mathrm{Ge}}_{3}{\mathrm{Mn}}_{5}$ is in agreement with the observed Curie temperature in experiments. It should be considered that the precipitation of the ferromagnetic ${\mathrm{Ge}}_{3}{\mathrm{Mn}}_{5}$ clusters is an origin of high Curie temperature in (Ge,Mn).

Theoretical investigation of phase separation in thermoelectric AgSbTe2

The pseudo-binary alloy (GeTe)x(AgSbTe2)1−x, which is one of the highly efficient thermoelectric materials, shows the anomalous lattice thermal conductivity and figure of merit ZT, when the GeTe concentration (x) is around 0.8. These singularities are considered to be due to the inherent instability of AgSbTe2, derived from the antibonding contributions at the valence band maximum. Here, we investigate the defect physics in AgSbTe2 by first-principles electronic structure calculations. It is found, from our calculations, that one must carefully control the atomic chemical potentials of Ag and Sb to grow AgSbTe2 in thermal equilibrium, and that a defect complex 2VAg+SbAg, which has a low formation energy, is easily generated under the Ag-poor crystal growth condition. Additionally, we propose spinodal nanodecomposition between AgSbTe2 and ordered defect compounds using 2VAg+SbAg, which is a crucial rule for the experimentally observed unusual thermoelectric properties.

Computational nano-materials design of self-organized nanostructures by spinodal nano-decomposition in Eu-doped GaN

We propose that nanostructures spontaneously generated by spinodal decomposition can be used as an efficiently luminescent material. The doping of Eu into GaN beyond the solubility limit forms EuN nanostructures, whose forms depend on the crystal growth method and conditions. The three-dimensional crystal growth generates the Dairiseki phase constructed of EuN quantum dots. These nanostructures are suitable for emission of red light and laser. The two-dimensional layer-by-layer crystal growth leads to the Konbu phase consisting of nanorods. The Konbu phase can be applied to the bottom-up construction of distributed feedback semiconductor lasers, which is currently built by the top-down nanotechnology such as photolithography.

Spinodal nanodecomposition in semiconductors doped with transition metals

The occurrence of high-temperature ferromagnetism in transition metal semiconductors holds promise for devices with spintronic functionalities. This review focuses on transition metal aggregation process, which leads to random patterns of high-temperature material doped beyond the solubility limit. A computational description of nanodecomposition and the relevant experimental results are presented for a range of semiconductor compounds. The correlation of high-temperature ferromagnetism with spinodal nanodecomposition points to promising nanotechnology utilizing materials with transition metal rich nanoclusters coherently embedded within semiconducting hosts.

Computational nano-materials design for circularly polarized luminescence in (Eu,Mg,O)-codoped GaN

We describe the production of circularly polarized luminescence from a luminescent center, which is generated by the spontaneous spinodal nano-decomposition of Eu, Mg, and O impurities doped into GaN. In this material, Zener’s p–f exchange interaction creates an electronic structure suitable for the production of circularly polarized luminescence. Moreover, the 7P2 ground state causes finite magnetic moments due to the reduction of the local symmetry around the Eu atom. These factors provide us with an expectation of not only circularly polarized luminescence but also potential control of the lifetime, color, and intensity, using the self-organized magnetic nano-structures.

Computational materials design of ferromagnetic Fe–Cu alloy by phase separation

Using ab initio electronic structure calculations by the Korringa–Kohn–Rostoker coherent potential approximation and Monte Carlo simulation of the two-dimensional spinodal nano-decomposition, we propose a new materials design for ferromagnetic Fe–Cu alloys. We demonstrate that Fe nano-wires of the size of 3–5 nm are surrounded by Cu matrix under a modified layer by layer crystal growth. Ferromagnetism is enhanced by the inhomogeneous distribution of Fe atoms in the alloy.

Spinodal nano decomposition in perovskite three-way catalysts: First-principles calculations and Monte Carlo simulations

We show that self-forming perovskite three-way catalysts might occur fluctuation distribution of precious metal concentrations owing to spinodal nano-decomposition (SND). We perform ab initio calculations to evaluate the free energy of La(Fe1−x,Pdx)O3 and La(Fe1−x,Rhx)O3. In addition the fluctuation distribution is simulated by applying the Monte Carlo method mapped to the Ising model with realistic ab initio chemical pair interactions between precious metal impurities in perovskite catalysts. It is found that the SND inherently occurs in La(Fe1−x,Pdx)O3, but not in La(Fe1−x,Rhx)O3. Calculated inhomogeneous distributions of Pd atoms in La(Fe1−x,Pdx)O3 are consistent with the high-resolution X-ray energy dispersive spectroscopy measurements.

Computational Nano-Materials Design of Low Cost and High Efficiency Cu2ZnSn[Se1-xSx]4Photovoltaic Solar Cells by Self-Organized Two-Dimensional Spinodal Nanodecomposition

Computational Nano-Materials Design of Low Cost and High Efficiency Cu₂ZnSn[Se_1-<i>x</i>S_<i>x</i>]₄Photovoltaic Solar Cells by Self-Organized Two-Dimensional Spinodal Nanodecomposition

Computational Nano-Materials Design of Low Cost and High Efficiency Cu2ZnSn[Se1-xSx]4 Photovoltaic Solar Cells by Self-Organized Two-Dimensional Spinodal Nanodecomposition

We propose the possibility of spinodal nanodecomposition in Cu2ZnSn[Se1-xSx]4 (CZTSeS) for high efficiency photovoltaic solar cells, based on the first-principles calculations within the self-interaction-corrected local density approximation. By using the Korringa–Kohn–Rostoker coherent potential approximation method, electronic structures of CZTSeS are calculated. Due to the calculated positive mixing energy and type II band alignment between Cu2ZnSnSe4 and Cu2ZnSnS4, we can expect that the efficiency of CZTSeS becomes higher by spinodal nano-decomposition. Then we simulate the self-organized two-dimensional spinodal nanodecomposition by Monte Carlo method using the Ising model with chemical pair interactions calculated from the first principles.

Self-Organized Nanostructures and High Blocking Temperatures in MgO-Based d0 Ferromagnets

We propose a new mechanism explaining the magnetic properties of MgO-based d0 ferromagnets determined from multi-scale simulations. Chemical pair interactions between N atoms in Mg(O,N) and Mg vacancies (VMg) in (Mg,VMg)O were calculated using a generalized gradient approximation and the VASP code. Monte Carlo simulations of the crystal growth were performed, using the Ising model, to predict the favored configurations of dopant distribution. It was found that self-organized nanowires can be formed both in Mg(O,N) and (Mg,VMg)O under layer-by-layer crystal growth, which suggests high blocking temperatures can be obtained in these d0 ferromagnets by spinodal nanodecomposition.

Computational nano-materials design for high- [formula omitted] ferromagnetism in wide-gap magnetic semiconductors

We propose materials design of high- T C wide band-gap dilute magnetic semiconductors (DMSs) based on first-principles calculations by using the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) method. First, we discuss a unified physical picture of ferromagnetism in II–VI and III–V DMSs and show that DMS family is categorized into two groups depending on the electronic structure. One is the system where Zener's double exchange mechanism dominates in the ferromagnetic interaction, and in the other systems Zener's p–d exchange mechanism dominates. Next, we develop an accurate method for T C calculation for the DMSs and show that the mean field approximation completely fails to predict Curie temperature of DMS in particular for wide-gap DMS where the exchange interaction is short-ranged. The calculated T C of homogeneous DMSs by using the present method agrees very well with available experimental values. For more realistic material design, we simulate spinodal nano-decomposition by applying the Monte Carlo method to the Ising model with ab initio chemical pair interactions between magnetic impurities in DMS. It is found that by controlling the dimensionality of the decomposition various characteristic phases occur in DMS such as 3 D Dairiseki-phase and 1 D Konbu-phase, and it is suggested that super-paramagnetic blocking phenomena should be important to understand the magnetism of wide-gap DMS. Based on the present simulations for spinodal nano-decomposition, we propose a new crystal growth method of positioning by seeding and shape controlling method in 100 Tera-bit density of nano-magnets in the semiconductor matrix with high- T C (or high- T B ).