Electronic Structure and Magnetism of Pristine, Defected, and Strained Ti2N MXene

From first principles electronic structure calculations, we unravel the evolution of structural, electronic, and magnetic properties of pristine, defected, and strained titanium nitride MXene with different functional groups (-F, -O, -H, and -OH). The formation and cohesive energies reveal their chemical stability. The MAX phase and defect free functionalized MXenes are metallic except for oxygen terminated (Ti 2 NO 2 ) one which is 100% spin polarized half-metallic ferromagnet. The spin-orbit coupling significantly influences the bare MXene (Ti 2 N) to exhibit Dirac topology and band inversion near the high symmetry directions and Fermi level. The strain effect sways the Fermi level thereby shifting it toward lower energy state under compression and toward higher energy state under tensile strain in Ti 2 NH 2 . The Ti 2 NO 2 exhibits exotic electronic structure and magnetic states not only in pristine but also in strained and defected structures. Its half-metallic nature changes to semi-metallic under 1% compression and it is completely destroyed under 2% compression. In single vacancy defect, its band structure remarkably transforms from half-metallic to semi-conducting with large band gap in 12.5% Ti, weakly semi-conducting in 5.5% Ti, and semi-metallic in 12.5% O. The 25% N defect changes it’s half-metallic characteristic to metallic. Further, the 12.5% Co substitution preserves it’s half-metallic character, whereas Mn substitution allows it to convert half-metallic characteristic into weak semi-metallic characteristic preserving ferromagnetism. However, Cr substitution converts half-metallic ferromagnetic state to half-metallic anti-ferromagnetic state. The understanding made here on collective structural stability, and electronic band structure, and magnetic phenomena in novel 2D Ti 2 N derived MXenes open up their possibility in designing them for synthesis and thereby taking to applications.

Zeroth-Order Nucleation Transition under Nanoscale Phase Separation

Materials with nanoscale phase separation are considered. A system representing a heterophase mixture of ferromagnetic and paramagnetic phases is studied. After averaging over phase configurations, a renormalized Hamiltonian is derived describing the coexisting phases. The system is characterized by direct and exchange interactions and an external magnetic field. The properties of the system are studied numerically. The stability conditions define the stable state of the system. At a temperature of zero, the system is in a pure ferromagnetic state. However, at finite temperature, for some interaction parameters, the system can exhibit a zeroth-order nucleation transition between the pure ferromagnetic phase and the mixed state with coexisting ferromagnetic and paramagnetic phases. At the nucleation transition, the finite concentration of the paramagnetic phase appears via a jump.

Helical phase in two-dimensional magnets due to four-spin interactions

We demonstrate that in ferromagnets with the D3h point group of symmetry a possible origin of phase transition from a collinear ferromagnetic state to a non-collinear state can be the fourth order contributions to the free energy density that are allowed by this point group of symmetry. At the same time, Dzyaloshinskii-Moria interaction vanishes in such materials. Via symmetry analysis we derive seven possible fourth order contributions to the free energy density with respect to the unit vector of the local magnetization direction but only two of them can be considered as independent. Moreover, for two-dimensional systems only one survives. Considered symmetry class is essential because a large group of two-dimensional intrinsic ferromagnets belongs to it, for example a monolayer Fe3GeTe2. The four-spin chiral exchange does also manifest itself in peculiar magnon spectra and favors spin waves.

Visualizing band selective enhancement of quasiparticle lifetime in a metallic ferromagnet

Electrons navigate more easily in a background of ordered magnetic moments than around randomly oriented ones. This fundamental quantum mechanical principle is due to their Bloch wave nature and also underlies ballistic electronic motion in a perfect crystal. As a result, a paramagnetic metal that develops ferromagnetic order often experiences a sharp drop in the resistivity. Despite the universality of this phenomenon, a direct observation of the impact of ferromagnetic order on the electronic quasiparticles in a magnetic metal is still lacking. Here we demonstrate that quasiparticles experience a significant enhancement of their lifetime in the ferromagnetic state of the low-density magnetic semimetal EuCd2As2, but this occurs only in selected bands and specific energy ranges. This is a direct consequence of the magnetically induced band splitting and the multi-orbital nature of the material. Our detailed study allows to disentangle different electronic scattering mechanisms due to non-magnetic disorder and magnon exchange. Such high momentum and energy dependence quasiparticle lifetime enhancement can lead to spin selective transport and potential spintronic applications.

Effect of argon annealing method on structural and ferromagnetic properties in Fe-doped SnO2 powders

Nanocrystalline powders of Fe-doped SnO2 (Sn1-xFexO2) (x = 0.00, 0.01, 0.03, 0.05) were prepared by a hydrothermal method. The powders were calcined in argon atmosphere at 600 °C for 2 h, causing phase transition from diamagnetic and weak ferromagnetic behavior to a ferromagnetic state. No trace and other magnetic impurity phases was detected in the samples with Fe content up to 3%. The calcined samples of Fe-doped SnO2 revealed the room temperature ferromagnetism with highest magnetization values of 434.07 memu/g at 15 kOe for x = 0.05. The room temperature ferromagnetism of samples originated from oxygen vacancies that occurred in the argon calcination process. In particular, oxygen vacancy shows a significant role in ferromagnetic coupling corresponding to F-center interaction.

Anomalous Nernst effect dependence on composition in Fe100-X Rh X alloys

We studied the anomalous Nernst effect in CsCl-type Fe100-X Rh X (X = 45, 48, 50, 52, 54, 60) with a thickness of 50 nm deposited on a thermally oxidized Si substrate. Samples with X < 48 certainly have a ferromagnetic phase, exhibiting the anomalous Nernst effect. The composition dependence of the anomalous Nernst coefficient S yx agreed with the transverse thermoelectric conductivity α yx. |S yx | and |α yx | were maximized at X = 48, which has a ferromagnetic state close to the phase transition state. The maximization of |α yx | at X = 48 can be explained using band structure-based calculations , where |α yx | rapidly increases near the phase transition.

Exact ground states and domain walls in one dimensional chiral magnets

We determine exactly the phase structure of a chiral magnet in one spatial dimension with the Dzyaloshinskii-Moriya (DM) interaction and a potential that is a function of the third component of the magnetization vector, n3, with a Zeeman (linear with the coefficient B) term and an anisotropy (quadratic with the coefficient A) term, constrained so that 2A ≤ |B|. For large values of potential parameters A and B, the system is in one of the ferromagnetic phases, whereas it is in the spiral phase for small values. In the spiral phase we find a continuum of spiral solutions, which are one-dimensionally modulated solutions with various periods. The ground state is determined as the spiral solution with the lowest average energy density. As the phase boundary approaches, the period of the lowest energy spiral solution diverges, and the spiral solutions become domain wall solutions with zero energy at the boundary. The energy of the domain wall solutions is positive in the homogeneous phase region, but is negative in the spiral phase region, signaling the instability of the homogeneous (ferromagnetic) state. The order of the phase transition between spiral and homogeneous phases and between polarized (n3 = ±1) and canted (n3 ≠ ±1) ferromagnetic phases is found to be second order.

Electronic structure, Mechanical and Thermodynamic properties of CoYSb (Y= Cr, Mo, W) half-Heusler compounds as potential spintronic materials

We used Density Functional Theory (DFT) calculations to investigate the structural, electronic, magnetic, mechanical and thermodynamic properties of CoYSb (Y = Cr, Mo and W) compounds. These are XYZ type half-Heusler alloys, which also exist in the face centred cubic MgAgAs-type structure and conform to F¯43m space group. We computed these properties in three different atomic arrangements known as Type-I, Type-II and Type-III phases. In all these phases, the alloys were found to be in the ferromagnetic state. Furthermore, the calculated electronic band structure and the total electronic density of states indicated a metallic behaviour in CoWSb, nearly half-metallic in CoMoSb and half-metallic in CoCrSb, with a minority-spin band gap of 0.81 eV. Furthermore, the calculated mechanical properties predicted an anisotropic behaviour of these alloys in their stable phase. Finally, due to its high Debye temperature value, CoCrSb shows stronger covalent bonding than CoMoSb and CoWSb, respectively.

Carbothermal reduction of mill scales formed on steel billets during continuous casting

A billet is a bar made from crude steel which surface contains scales which are rich in iron oxides. This study presents the carbothermal reduction of the scales formed in steel billets. The process included the reaction of the iron oxides contents with carbon (in ratio 5:1) and annealing in a tubular furnace under argon atmosphere. The occurred reactions are discussed using thermodynamic calculations and thermal analysis which indicate a three-stage reduction process Fe3O4 ➔ FeO ➔ Fe3C ➔α-Fe with intermediate reactions at the interval temperature 960 and 1300 °C. The X-ray diffraction confirms the reduction to α-Fe with minor presence of unreacted C, magnetite and wustite. Mössbauer spectroscopy analysis was performed at room temperature where a typical sextet corresponding to the dominant α-Fe is shown as well as wustite, magnetite and cementite to a lesser extent. The magnetization measurements confirm the ferromagnetic state corresponding to the α-Fe.

Predicting ferromagnetism and thermoelectric characteristics in bulk spinels ZnCr2X4 (X = S, Se) using density functional theory

To have control over the properties of electronic devices with the help of the spin of electrons is considered an amalgamation field of innovative technology. The thermoelectric and fero-magnetic characteristics of bulk ZnCr2X4 (X = S, Se) spinels have been investigated by the BoltzTraP and Wien2k codes. The comparative analysis of the energies that emerged from nonmagnetic and ferromagnetic states shows that the lower energy state is the ferromagnetic state. The modified Becke–Johnson (mBJ) functional has been brought into use for the computation of the density of states (DOS) and precise band structures (BS), which authenticates the ferromagnetic semiconducting behavior. The calculation of exchange splitting energies, John-Teller energy, and crystal field energy explored the origin of ferromagnetism. The strong hybridization resulting in decomposition in Cr, the magnetic moment and creates the magnetic moments at the nonmagnetic sites. Consequently, the thermoelectric characteristic has been explored by the BoltzTraP code that reveals that the increasing temperature increases the power factor, the thermal conductivity, and the electrical conductivity whereas the Seebeck coefficient reduces with it. However, the compounds in our study prove to be suitable for being used in thermoelectric devices for alternative energy resources.