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The site-preference rule (SPR), which is widely used to design and study the properties of full-Heusler alloys X2YZ, is applied to determine the positions of different transition-metal elements with various numbers of valence electrons in Heusler alloys. The scandium-based full Heusler alloys should form an XA structure according to the SPR, because the transition-metal element Sc with fewer valence electrons tends to occupy the Wyckoff sites A(0, 0, 0) and B(0.25, 0.25, 0.25). As opposed to previous investigations, L21-type scandium-based full Heusler alloys, which do not follow the SPR, are also considered in this work. Theoretical computations are used to study the atomic-site preferences of 171 scandium-based full Heusler alloys, and the results indicate that most of these alloys form L21-type structures rather than XA-type structures. Two possible orderings are also studied in terms of their difference in charge density. Furthermore, the ground state, electronic structure, and magnetic characteristics of these alloys are also investigated in detail in both XA and L21 structures. The results reveal dramatic differences between XA- and L21-type alloys. This study thus presents a counterexample of the SPR rule and should provide significant guidance for the future design of full-Heusler alloys. Keywords: Full-Heusler alloys, Atomic orderings competition, First-principles, Spintronic

Spin-gapless semiconductors (SGSs) with Dirac-like band crossings may exhibit massless fermions and dissipationless transport properties. In this study, by applying the density functional theory, novel multiple linear-type spin-gapless semiconducting band structures were found in a synthesized R 3 − c -type bulk PdF3 compound, which has potential applications in ultra-fast and ultra-low power spintronic devices. The effects of spin-orbit coupling and on-site Coulomb interaction were determined for the bulk material in this study. To explore the potential applications in spintronic devices, we also performed first-principles combined with the non-equilibrium Green’s function for the PdF3/Ga2O3/PdF3 magnetic tunnel junction (MTJ). The results suggested that this MTJ exhibits perfect spin filtering and high tunnel magnetoresistance (~5.04 × 107).

A new all-[Formula: see text]-metal full-Heusler alloy (Cd2MnPd) was designed based on density functional theory. Its crystal structure, band structure, density of states, magnetism, and the possibility of martensitic transformation were studied. The calculated total energy shows that the most stable cubic structure of this alloy is the L21 type at ferromagnetic (FM) state, and the equilibrium lattice constant is 6.57[Formula: see text]Å. The band structure and density of states show that it is a FM metal with a total magnetic moment of 3.97[Formula: see text][Formula: see text], mainly originating from the Mn element. The results of the martensitic transformation investigation show that the martensitic phase has minimum energy when the c/a ratio is 1.35, and the absolute value of the energy difference ([Formula: see text]) between the martensitic and the austenitic phases is 0.07[Formula: see text]eV. This indicates that there is a high possibility that a stable martensitic phase exists. Furthermore, [Formula: see text] can be tuned by uniform strain, and the absolute value of [Formula: see text] increases as the volume increases in the range of [Formula: see text] ([Formula: see text] is the optimal volume, namely, the cubic volume under its equilibrium lattice). In conclusion, Cd2MnPd may be a good candidate for ferromagnetic shape-memory alloys (FSMAs). We expect that our theoretical prediction can provide guidance for exploring valuable FSMAs experimentally.

Recently, an increasing number of rare-earth-based equiatomic quaternary compounds have been reported as promising novel spintronic materials. The rare-earth-based equiatomic quaternary compounds can be magnetic semiconductors (MSs), spin-gapless semiconductors (SGSs), and half-metals (HMs). Using first-principle calculations, we investigated the crystal structure, density of states, band structure, and magnetic properties of a new rare-earth-based equiatomic quaternary Heusler (EQH) compound, ScFeRhP. The results demonstrated that ScFeRhP is a HM at its equilibrium lattice constant, with a total magnetic moment per unit cell of 1 &mu, B. Furthermore, upon introduction of a uniform strain, the physical state of this compound changes with the following transitions: non-magnetic-semiconductor-(NMS) &rarr, MS &rarr, SGS &rarr, HM &rarr, metal. We believe that these results will inspire further studies on other rare-earth-based EQH compounds for spintronic applications.

A hypothetical full-Heusler alloy, Sc2VGe, was analyzed, and the comparison between the XA and L21 structures of this alloy was studied based on first-principles calculations. We found that the L21-type structure was more stable than the XA one. Further, the electronic structures of both types of structure were also investigated based on the calculated band structures. Results show that the physical nature of L21-type Sc2VGe is metallic, however, XA-type Sc2VGe is a half-metal (HM) with 100% spin polarization. When XA-type Sc2VGe is at its equilibrium lattice parameter, its total magnetic moment is 3 &mu, B , and its total magnetism is mainly attributed to the V atom. The effects of uniform strain and tetragonal lattice distortion on the electronic structures and half-metallic states of XA-type Sc2VGe were also studied. All the aforementioned results indicate that XA-type Sc2VGe would be an ideal candidate for spintronics studies, such as spin generation and injection.