X.F. Dai

Superelasticity of CoNiGa : Fe single crystals

We have fabricated CoNiFeGa single crystals with excellent superelasticity. The superelastic strains of 4% and 6.7% in compression have been obtained along the [001] and [110] directions, respectively. These single crystals show strong anisotropy in strains, superelastic parameters, and even transformation path related to the different crystalline directions. A large superelastic strain up to 11% has been obtained in tension test. The perfect superelasticities have also displayed in bending and torsion tests.

Martensitic transformation and magnetic properties of Co-Ni-Al shape memory alloy ribbons

We report on the magnetic and martensitic transformation properties of Co39Ni33Al28 ferromagnetic shape memory alloy ribbons. We found that the phase formation of Co39Ni33Al28 is strongly dependent on the method of preparation. The conventional as-cast ingot sample contains a large amount of the γ phase embedded in the primary β phase, while melt-spun ribbons contain the pure β phase. Co39Ni33Al28 ribbons exhibit a perfect thermoelastic martensitic transformation from a cubic to a tetragonal structure at 240 K during cooling. The martensite structure can be well described by the L10 lattice, similar to that of Ni–Al alloys. The martensitic phase at 5 K exhibits a saturation magnetization of 48.67 emu g −1 and saturates at about 8000 Oe. A large increase in coercive force after ageing at 500uC for 1 h has been found due to the change in atomic chemical ordering. The temperature dependence of the saturation magnetization indicates that magnetization can be well interpreted by spin-wave theory at temperatures lower than 200 K. The material shows a recoverable strain of 500 ppm upon the martensitic transformation.

First-principles study on quaternary Heusler compounds ZrFeVZ (Z = Al, Ga, In) with large spin-flip gap

First-principles calculations were used to systematically investigate the structural, electronic and half-metallic properties of newly designed quaternary Heusler compounds ZrFeVZ (Z = Al, Ga, In). The calculated results show that these three compounds have an excellent half-metallicity in their ferrimagnetic ground state. ZrFeVZ (Z = Al, Ga, In) compounds exhibit the large spin-flip gaps of 0.348 eV, 0.428 eV and 0.323 eV at their equilibrium lattice constants, respectively. The total spin magnetic moment is 2 μB for all the three compounds, which is in agreement with the Mt = Zt − 18 rule. The half-metallic properties of these three compounds are quite robust to the hydrostatic and tetragonal strain, and can also be kept when the electron correlation (U) is considered. For the ZrFeVIn compound, the spin-flip band gap achieves the maximum value (0.57 eV) under a small strain. The calculated Curie temperatures based on the mean field approximation (MFA) method are 818.04 K, 826.66 K, and 751.70 K for the ZrFeVAl, ZrFeVGa, and ZrFeVIn compounds, respectively. We hope that our current work may trigger Heusler compounds containing 4d transition metal elements and with quite large spin-flip band gaps for application in future spintronics devices.

Topological Type-II Nodal Line Semimetal and Dirac Semimetal State in Stable Kagome Compound Mg3Bi2

Topological type-II nodal line semimetal (NLS) was proposed quite recently and exhibits distinct properties compared with conventional type-I NLS. To date, no ambient-condition stable candidate material has been reported. Here we propose that a stable Kagome compound Mg3Bi2 can host a type-II nodal line state with the protection of time reversal and spatial inversion symmetries. Similar to type-I NLSs, the type-II nodal line in Mg3Bi2 is characterized by the drumhead surface states, which has not been observed in the previous type-II NLSs. The nodal line in Mg3Bi2 can open a minor gap, and a pair of 3D Dirac points occurs when SOC is included. The SOC-induced gap around the nodal line is quite small, and the formation of 3D Dirac points is independent of the nodal line. Therefore, the Mg3Bi2 compound is expected to be a good candidate to investigate the exotic properties of both type-II NLS and 3D Dirac semimetal states.

Rare earth-based quaternary Heusler compounds MCoVZ (M = Lu, Y; Z = Si, Ge) with tunable band characteristics for potential spintronic applications

Under uniform strain, there are natural physical transitions from spin-filter magnetic semiconductor (MS) to spin-gapless semiconductor (SGS) to half-metal (HM) for rare earth-based equiatomic quaternary Heusler (EQH) compounds with the formula LuCoVZ, and from HM → SGS → MS → SGS → HM for EQH compounds with the formula YCoVZ.

DISORDER ENHANCED MAGNETIC MOMENT IN Fe2CrAl RIBBONS

The electronic structure and magnetic properties of Fe2CrAl have been investigated theoretically and experimentally. The theoretical calculations show that there is an energy gap in the minority-spin band in the electronic structure of ordered Fe2CrAl with the L21 structure and that the spin-polarization ratio is as high as 93%. The calculated total spin moment is 0.97 μB/f.u. However, in melt-spun Fe2CrAl ribbons, the B2 type structure, an atomic-site disordered L21 structure, is found and this atomic site disorder gives rise to a magnetic moment of 2.02 μB in the ribbons which is much higher than the theoretical value. The calculations prove that the enhanced magnetic moment is mainly due to the Fe–Cr and Fe–Al types of disorder. In contrast, the Cr–Al type disorder has little influence on the total spin moment and the spin-polarization ratio.

Structural, electronic, magnetic, half-metallic, mechanical, and thermodynamic properties of the quaternary Heusler compound FeCrRuSi: A first-principles study

In this paper, we have investigated the structural, electronic, magnetic, half-metallic, mechanical, and thermodynamic properties of the equiatomic quaternary Heusler (EQH) compound FeCrRuSi using the density functional theory (DFT) and the quasi-harmonic Debye model. Our results reveal that FeCrRuSi is a half-metallic material (HMM) with a total magnetic moment of 2.0 μB in agreement with the well-known Slater-Pauling rule Mtu2009=u2009Ztu2009−u200924. Furthermore, the origin of the half-metallic band gap in FeCrRuSi is well studied through a schematic diagram of the possible d-d hybridization between Fe, Cr and Ru elements. The half-metallic behavior of FeCrRuSi can be maintained in a relatively wide range of variations of the lattice constant (5.5–5.8u2009Å) under uniform strain and the c/a ratio (0.96–1.05) under tetragonal distortion. The calculated phonon dispersion, cohesive and formation energies, and mechanical properties reveal that FeCrRuSi is stable with an EQH structure. Importantly, the compound of interest has been prepared and is found to exist in an EQH type structure with the presence of some B2 disorder. Moreover, the thermodynamic properties, such as the thermal expansion coefficient α, the heat capacity CV, the Grüneisen constant γ, and the Debye temperature ΘD are calculated.

Intermetallic Ca3Pb: a topological zero-dimensional electride material

Based on first-principles calculations, we report the concept of topological electride materials in this paper. It is found that the intermetallic Ca3Pb compound is a zero-dimensional electride, where the excess electrons are well localized at the 1b (0.5,0.5,0.5) lattice voids. These localized electrons serve as anions in the positively charged crystalline framework. Besides the electride properties, intermetallic Ca3Pb is also a topological material containing various types of fermionic states. The topological and electride properties of Ca3Pb are robust upon lattice distortions, which are available for practical applications. The quantum phenomena/effects from the fermionic states (e.g. quantum magnetoresistance and momentum-space Klein tunneling effect in Dirac and Weyl materials) in topological electrides are expected to be more pronounced than in conventional topological materials, because of their loosely bound conducting electrons. The current work opens a new research perspective by combining the nature of topological and electride materials.

Half-metallicity of the bulk and (001) surface of NbFeCrAl and NbFeVGe Heusler compounds: a first-principles prediction

Using first-principles calculations based on density-functional theory, the structural, electronic and magnetic properties in the bulk and (001) surfaces of quaternary Heusler compounds NbFeCrAl and NbFeVGe are investigated. For the bulk, the two compounds exhibit half-metallicity with band gaps of 0.4xa0eV and 0.2 eV in the majority-spin direction at their equilibrium lattice constants. The total magnetic moments are 2 μB following the Slater–Pauling formula: Mt = 24 − Zt rule. The half-metallicity can be maintained in the range of 5.76–6.07 A and 5.96–6.10 A (lattice constant), and 0.971–1.035 and 0.962–1.033 (c/a) for NbFeCrAl and NbFeVGe compounds, respectively. The half-metallicity is destroyed on the Fe–Cr, Nb–Al, Fe–V, and Nb–Ge terminated (001) surfaces, and the spin-polarization ratio sharply decreases below 50% for NbFeCrAl and NbFeVGe compounds.

The effect of internal and external stress on two-way shape-memory behaviour in Co49Ni21.6Ga29.4 single crystals

The effect of the internal stress on the two-way shape memory in Co49Ni21.6Ga29.4 single crystals has been investigated. We found that the internal stress generated natively by the solidifying process works as a tensile force along the growth direction. Applying different compressive pre-stresses along the [0 0 1] direction, the shape-memory strain can be continuously changed from +1.0% to −2.3%. In the [1 1 0] direction, the strain monotonically increases from −2.0% to −4.0% due to a strong detwinning produced by the consistent effect of the external and internal stresses.