arXiv subjects

W. H. Wang

Publications and source records attributed to W. H. Wang.

At least 37 records · Page 2Linked to original sources

arXiv2016

Windows open for highly tunable magnetostructural phase transitions

An attempt was made to tailor the magnetostructural transitions (MSTs) over a wide temperature range under the principle of isostructural alloying. A series of wide Curie-temperature windows (CTWs) with a maximal width of 377 K between 69 and 446 K were established in the Mn1-yCoyNiGe1-xSix system. Throughout the CTWs, the magnetic-field-induced metamagnetic behavior and giant magnetocaloric effects are obtained. The (Mn,Co)Ni(Ge,Si) system shows great potential as multifunctional phase-transition materials that work in a wide range covering liquid-nitrogen and above water-boiling temperatures. Moreover, general understanding to isostructural alloying and CTWs constructed in (Mn,Co)Ni(Ge,Si) as well as (Mn,Fe)Ni(Ge,Si) are provided.

cond-mat.mtrl-sci↗

arXiv2016

Correlations Between Local Elastic Heterogeneities and Overall Elastic Properties in Metallic Glasses

The common notion suggests that metallic glasses (MGs) are a homogeneous solid at the macroscopic scale; however, recent experiments and simulations indicate that MGs contain nano-scale elastic heterogeneities. Despite the fundamental importance of these findings, a quantitative understanding is still lacking for the local elastic heterogeneities intrinsic to MGs. On the basis of Eshelby's theory, here we develop a micromechanical model that correlates the properties of the local elastic heterogeneities, being very difficult to measure experimentally, to the measurable overall elastic properties of MGs, such as shear/bulk modulus and Poisson's ratio. Our theoretical modeling is verified by the experimental data obtained from various MGs annealed to different degrees. Particularly, we revealed that the decrease of Poisson's ratio upon annealing of MGs is associated with a much large shear softening over hydrostatic-pressure softening, and $vice$ $versa$ in local elastic inhomogeneities. The relative extent of the bulk versus shear modulus softens is extracted for different MGs, and is found to closely depend on the specific composition and their ductility. The implication of our results on the Poisson's ratio criterion on the ductility as well as the aging dynamics in MGs is discussed.

cond-mat.mtrl-sci↗

arXiv2016

Anti-sites disordering suppression of the possible phase transition in Mn2CrGa

Theoretical and experimental characterizations of Mn2CrGa compound in regard to the possibility of phase transformation have been carried out in this work. Under a high ordering L21 structure, this compound has the potential to be a martensite phase transition material. However, experimental results show a severe disordering took place in this system, which forbids the occurring of the phase transition. This work provides important reference for the design of new phase transition materials in Heusler alloys.

cond-mat.mtrl-sci↗

arXiv2015

New spin injection scheme based on spin gapless semiconductors: A first-principles study

Spin injection efficiency based on conventional and/or half-metallic ferromagnet/semiconductor is greatly limited by the Schmidt obstacle due to conductivity mismatch, here we proposed that by replacing the metallic injectors with spin gapless semiconductors can significantly reduce the conductive mismatch to enhance spin injection efficiency. By performing first principles calculations based on superlattice structure, we have studied the representative system of Mn2CoAl/semiconductor spin injector scheme. The results showed that a high spin polarization was maintained at the interface in systems of Mn2CoAl/Fe2VAl constructed with (100) interface and Mn2CoAl/GaAs with (110) interface, and the latter is expected to possess long spin diffusion length. Inherited from the spin gapless feature of Mn2CoAl, a pronounced dip was observed around the Fermi level in the majority-spin density-of-states in both systems, suggesting fast transport of the low-density carriers.

cond-mat.mtrl-sci↗

arXiv2015

Realization of multifunctional shape-memory ferromagnets in all-d-metal Heusler phases

Heusler ferromagnetic shape-memory alloys (FSMAs) normally consist of transition-group d-metals and main-group p-elements. Here, we report the realization of FSMAs in Heusler phases that completely consist of d metals. By introducing the d-metal Ti into NiMn alloys, cubic B2-type Heusler phase is obtained and the martensitic transformation temperature is decreased efficiently. Strong ferromagnetism is established by further doping Co atoms into the B2-type antiferromagnetic Ni-Mn-Ti austenite. Based on the magnetic-field-induced martensitic transformations, collective multifunctional properties are observed in Ni(Co)-Mn-Ti alloys. The d metals not only facilitate the formation of B2-type Heusler phases, but also establish strong ferromagnetic coupling and offer the possibility to tune the martensitic transformation.

cond-mat.mtrl-sci↗

arXiv2015

Hand in Hand Evolution of b-relaxation and Boson Peak in Metallic Glasses

Boson peak and beta-relaxation are two intrinsic and markedly different dynamic modes of glasses, and their structural origins are long-standing issues. Through tuning atomic packing of a model metallic glass with microalloying of different types of elements, we find that low-temperature boson heat capacity peak evolves hand in hand with high-temperature beta-relaxation. A linear correlation between the boson peak temperature and the activation energy of beta-relaxation is disclosed. The coupling of the boson peak and the beta-relaxation indicates their common structural origin of the loosely packed regions in metallic glasses.

cond-mat.mtrl-sci↗

arXiv2015

Structural and magnetic properties of MnCo1-xFexSi alloys

The crystal structures, martensitic structural transitions and magnetic properties of MnCo1-xFexSi (0 <= x <= 0.50) alloys were studied by differential scanning calorimetry (DSC), x-ray powder diffraction (XRD) and magnetic measurements. In high-temperature paramagnetic state, the alloys undergo a martensitic structural transitions from the Ni2In-type hexagonal parent phase to the TiNiSi-type orthorhombic martensite. Both the martensitic transition temperature (TM) and Curie temperatures of martensite (T_C^M) decrease with increasing Fe content. The introduced Fe atoms establish ferromagnetic (FM) coupling between Fe-Mn atoms and destroy the double spiral antiferromagnetic (AFM) coupling in MnCoSi compound, resulting in a magnetic change in the martensite phase from a spiral AFM state to a FM state. For the alloys with x = 0.10, 0.15 and 0.20, a metamagnetic transition was observed in between the two magnetic states. A magnetostructural phase diagram of MnCo1-xFexSi (0 <= x <= 0.50) alloys was proposed.

cond-mat.mtrl-sci↗

arXiv2015

First-principle prediction of Martensitic transformation and magnetic properties of Heusler-type Pt2-xMn1+xGa alloys

The electronic structure, magnetism and phase stability of Pt2-xMn1+xGa(x=0, 0.25, 0.5, 0.75, 1) alloys are studied by first-principle calculations. The calculations reveal that a potential magnetic martensitic transformation can be expected in all the series. In addition, a large magnetic-field-induced strain is likely to appear in Pt2-xMn1+xGa(x=0, 0.25, 0.75, 1) alloys. The electronic structure calculations indicate that the tetragonal phase is stabilized upon the distortion because of the pseudogap formation at the Fermi Level. The magnetic structure is also investigated and the total magnetic moment of the tetragonal phase is a little larger than that of the cubic austenite phase in all the series.

cond-mat.mtrl-sci↗

arXiv2014

Spin injection based on the spin gapless semiconductor(SGS)/semiconductor heterostructures

Spin injection efficiency based on conventional ferromagnet (or half-metallic ferromagnet) /semiconductor is greatly limited by the Schmidt obstacle due to conductivity mismatch, here we proposed that by replacing the metallic injectors with spin gapless semiconductors can significantly reduce the conductive mismatch while conserve high spin polarization. By performing first principles calculations based on superlattice structure, we have studied the representative system of Mn2CoAl/semiconductor spin injector scheme. The results showed that high spin polarization were maintained at the interface in systems of Mn2CoAl/Fe2VAl constructed with (100) interface and Mn2CoAl/GaAs with (110) interface, and the latter is expected to possess long spin diffusion length. Inherited from the spin gapless feature of Mn2CoAl, a pronounced dip was observed around the Fermi level in the majority-spin DOS in both systems, suggesting fast transport of the low-density carriers.

cond-mat.mes-hall↗

arXiv2014

Structural transitions, magnetic properties, and electronic structures of Co(Fe)-doped MnNiSi compounds

The structural transitions, magnetic properties, and electronic structures of Co(Fe)-doped MnNiSi compounds are investigated by x-ray powder diffraction (XRD), differential scanning calorimetry (DSC), magnetic measurements, and first-principles calculations. Results indicate that all samples undergo a martensitic transition from the Ni2In-type parent phase to TiNiSi-type orthorhombic phase at high temperatures. The substitution of Co(Fe) for Mn in Mn1-xCoxNiSi (x = 0.2, 0.3, 0.4) and Mn1-yFeyNiSi (y = 0.26, 0.30, 0.36, 0.46, 0.55) samples decreases the structural transition temperature (Tt) and Curie temperature of martensite (TCM). The martensite phases show a typical ferromagnetic behavior with saturation field (HS) being basically unchanged with increasing Co(Fe) content, while the saturation magnetization (MS) shows a decreasing tendency. The theoretically calculated moments are in good agreement with the experimentally measured results. The orbital hybridizations between different 3d elements are analyzed from the distribution of density of states.

cond-mat.mtrl-sci↗

arXiv2014

A coupling of martensitic and metamagnetic transitions with collective magneto-volume and table-like magnetocaloric effects

A coupling of the first-order paramagnetic-to-induced-ferromagnetic martensitic and the second-order antiferromagnetic-to-ferromagnetic metamagnetic transitions was found in MnNi0.8Fe0.2Ge alloy. Based on the coupling, a magneto-volume effect driven by the martensitic transition and a table-like magnetocaloric effect generated by the successive magnetic phase transitions arise collectively. By using the magneto-volume effect, the internal pressure stress in the volume-expansion martensitic transition was determined at 350 MPa. The magnetocaloric effect, with a wide working temperature range of 26 K around room temperature, shows a small hysteresis loss (5 J/kg) and a large net refrigerant capacity (157 J/kg).

cond-mat.mtrl-sci↗

arXiv2014

Magneto-transport properties of oriented Mn2CoAl films sputtered on thermally oxidized Si substrates

Spin gapless semiconductors are interesting novel class of materials by embracing both magnetism and semiconducting. Its potential application in future spintronics requires realization in thin film form. In this letter, we report a successful growth of spin gapless Mn2CoAl films on thermally oxidized Si substrates by magnetron sputtering deposition. The films deposited at 673K are well oriented to (001) direction and display a uniform-crystalline surface. Magnetotransport measurements on the oriented films reveal a semiconducting-like resistivity, small anomalous Hall conductivity and linear magnetoresistance (MR) representative of the transport signatures of spin gapless semiconductors. The magnetic properties of the films have also been investigated and compared to that of bulk Mn2CoAl, with small discrepancy induced by the composition deviation.

cond-mat.mtrl-sci↗

arXiv2014

Observations and analysis of phase scintillation of spacecraft signal on the interplanetary plasma

Aims. The phase scintillation of the European Space Agency's (ESA) Venus Express (VEX) spacecraft telemetry signal was observed at X-band (λ= 3.6 cm) with a number of radio telescopes of the European VLBI Network (EVN) in the period 2009-2013. Methods. We found a phase fluctuation spectrum along the Venus orbit with a nearly constant spectral index of -2.42 +/-0.25 over the full range of solar elongation angles from 0° to 45°, which is consistent with Kolmogorov turbulence. Radio astronomical observations of spacecraft signals within the solar system give a unique opportunity to study the temporal behaviour of the signal's phase fluctuations caused by its propagation through the interplanetary plasma and the Earth's ionosphere. This gives complementary data to the classical interplanetary scintillation (IPS) study based on observations of the flux variability of distant natural radio sources. Results. We present here our technique and the results on IPS. We compare these with the total electron content (TEC) for the line of sight through the solar wind. Finally, we evaluate the applicability of the presented technique to phase-referencing Very Long Baseline Interferometry (VLBI) and Doppler observations of currently operational and prospective space missions.

astro-ph.EP↗

arXiv2014

First-principles investigation of half-metallic ferromagnetism of half-Heusler compounds XYZ

We investigate the electronic structure and magnetism of half-Heusler compounds XYZ (X, Y=V, Cr, Mn, Fe, Co and Ni; Z=Al, Ga, In, Si, Ge, Sn, P, As, and Sb) using the ab initio density functional theory calculations. Nine half-metals with half-Heusler structure have been predicted with the half-metallic gap of 0.07-0.67 eV. The calculations show that the formation energies for these nine half-Heusler compounds range from -1.32 to -0.12 eV/f.u., and for CoCrSi, CoCrGe, CoFeGe, CoMnSi, CoMnGe, FeMnGe and FeMnAs, the total energy differences between the half-Heusler structure and the corresponding ground-state structure are small (0.07-0.76 eV/f.u.), thus it is expected that they would be realized in the form of thin films under metastable conditions for spintronic applications. The stability of the half-metallicity of CoCrGe and FeMnAs to the lattice distortion is also investigated in detail.

cond-mat.mtrl-sci↗

arXiv2013

Microscopic dynamics perspective on the relationship between Poisson ratio and ductility of metallic glasses

In metallic glasses a clear correlation had been established between plasticity or ductility with the Poisson ratio and alternatively the ratio of the elastic bulk modulus to the shear modulus, K/G. Such a correlation between these two macroscopic mechanical properties is intriguing and is challenging to explain from the dynamics on a microscopic level. Such a microscopic explanation is provided in this paper.

cond-mat.mtrl-sci↗

arXiv2013

A new class of topological insulators from I-III-IV half-Heusler compounds with strong band inversion strength

In this paper, by first principle calculations, we investigate systematically the band topology of a new half-Heusler family with composition of I(A)-III(A)-IV(A). The results clearly show that many of the I-III-IV half-Heusler compounds are in fact promising to be topological insulator candidates. The characteristic feature of these new topological insulators is the naturally strong band inversion strength (up to -2eV) without containing heavy elements. Moreover, we found that both the band inversion strength and the topological insulating gap can be tailored through strain engineering, and therefore would be grown epitaxially in the form of films, and useful in spintronics and other applications.

cond-mat.mtrl-sci↗

arXiv2013

The glass transition diagram in model metallic glasses

We report a strain rate (equivalent to experimental observation time) induced glass transition in model SrCaYbMg(Li)Zn(Cu) metallic glasses at room temperature. A critical strain rate, equivalent to glass transition temperature, is found for the strain rate induced a glassy state to liquid-like viscoplastic state translation. The results show that the observation time, equivalent to temperature and stress, is a key parameter for the glass transition. A three-dimension glass transition phase diagram involved in time, temperature and stress in metallic glasses is established for understanding the nature of the metallic glasses.

cond-mat.dis-nn↗

arXiv2013

Crossover of magnetoresistance in the zerogap half-metallic Heusler alloy Fe2CoSi

This work reports on the band structure and magneto-transport investigations of the inverse Heusler compound Fe2CoSi. The first-principles calculations reveal that Fe2CoSi has a very peculiar band structure with a conducting property in the majority spin channel and a nearly zero bandgap in the minority spin channel. The synthesized Fe2CoSi sample shows a high-ordered inverse Heusler structure with a magnetic moment of 4.88 μB at 5 K and a high Curie temperature of 1038 K. With increasing temperature, a crossover from positive to negative magnetoresistance (MR) is observed. Complemented with the Hall effect measurements, we suggest the intriguing crossover of MR can be ascribed to the dominant spin carriers changing from the gapless minority spin channel to the majority spin channel at Fermi level.

cond-mat.mtrl-sci↗