A critical discussion of calculated modulated structures, Fermi surface nesting and phonon softening in magnetic shape memory alloys Ni2_2Mn(Ga, Ge, Al) and Co2_2Mn(Ga, Ge)

A series of first principles calculations have been carried out in order to discuss electronic structure, phonon dynamics, structural instabilities and the nature of martensitic transformations of the Heusler alloys Ni$_2$Mn(Ga, Ge, Al) and Co$_2$Mn(Ga, Ge). The calculations show that besides electronic pecularities like Fermi--surface nesting, hybridizing optical and acoustic phonon modes are important for the stabilization of the modulated martensitic structures.Comment: 3 pages, 4 figures, JEMS-200

First-principles study of lattice instabilities in the ferromagnetic martensite Ni2_2MnGa

The phonon dispersion relations and elastic constants for ferromagnetic Ni$_2$MnGa in the cubic and tetragonally distorted Heusler structures are computed using density-functional and density-functional perturbation theory within the spin-polarized generalized-gradient approximation. For $0.9<c/a<1.06$, the TA$_2$ tranverse acoustic branch along $[110]$ and symmetry-related directions displays a dynamical instability at a wavevector that depends on $c/a$. Through examination of the Fermi-surface nesting and electron-phonon coupling, this is identified as a Kohn anomaly. In the parent cubic phase the computed tetragonal shear elastic constant, C$^\prime$=(C$_{11}-$C$_{12}$)/2, is close to zero, indicating a marginal elastic instability towards a uniform tetragonal distortion. We conclude that the cubic Heusler structure is unstable against a family of energy-lowering distortions produced by the coupling between a uniform tetragonal distortion and the corresponding $[110]$ modulation. The computed relation between the $c/a$ ratio and the modulation wavevector is in excellent agreement with structural data on the premartensitic ($c/a$ = 1) and martensitic ($c/a$ = 0.94) phases of Ni$_2$MnGa.Comment: submitted to Phys. Rev.

A first-principles investigation of tetragonal and orthorhombic deformations in the ferromagnetic Heusler alloy Ni2_2MnGa

Tetragonal and orthorhombic deformations in Ni$_2$MnGa were studied by using the Density Functional Theory formalism with US GGA pseudopotentials implemented in the VASP code in order to find an optimal geometry of the low-temperature structure. Structural and magnetic parameters of the initial L2$_1$ structure and elastic constants are in a good agreement with experimental data and previous calculations. Not all of the low-temperature martensitic structures could be reproduced by applying the tetragonal and the orthorhombic deformations. It was argued that the modulation arising from shuffling of atoms is important for these structures. A simple kind of shuffling was studied. That gives experimentally predicted structural instability associated with a soft phonon mode, but does not stabilize any new structures

Strain effects on the electric polarization of BiMnO 3

77.65.-j Piezoelectricity and electromechanical effects, 77.80.-e Ferroelectricity and antiferroelectricity, 77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials, 75.47.Lx Manganites,

First-principles study of elastic and vibrational properties of Ni 2MnIn magnetic shape memory alloys

We present the results of ab initio calculations of lattice dynamics and the second order elastic stiffness constants of nickel-based magnetic shape memory alloy Ni 2MnIn in stoichiometric composition. The plane wave basis sets and pseudopotential method within spin-polarized generalized gradient approximation (σ-GGA) scheme of the density functional theory (DFT) is applied. Elastic constants are calculated by tetragonal and monoclinic isochoric strains on cubic L2 1 structure. The calculated elastic constants agree very well with the recent ultrasonic experimental data. Phonon dispersion spectra are investigated within linear response technique of the density functional perturbation theory (DFPT). A vibrational anomaly is observed in phonon spectra at the transverse acoustic mode (TA 2) in [ζ ζ0] direction at wavevector ζ=0.3 as an indication of the structural instability of the system to shear deformation. This anomaly is also verified by the low shear modulus and large elastic anisotropy ratio. Phonon dispersion curves are in excellent agreement with the results of recent neutron diffraction experiments. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2011

Competing ground states in transition metal oxides: Behavior of itinerant Sr

The ferromagnetic (FM) phase transition of the itinerant electron-system Sr1−xCaxRuO3 can be tuned by chemical composition resulting in a quantum critical point (QCP) at the critical concentration xc ≈ 0.7. Applying epitaxial pressure at constant x leads to a reduction of the Curie temperature TC which is found to be proportional to the shrinkage of the unit-cell volume Vuc, shifting xc to higher values for tensile strained films. Surprisingly, the tetragonal distortion seems to play here only a minor role. With increasing x the critical scaling of the order parameter shows unusual behavior. The magnetic critical exponents β, γ, and δ change systematically from typical mean-field values at x = 0 with increasing x towards β = 1, γ = 0.9 and δ = 1.6 at x = 0.7. The results are discussed with respect to a crossover from mean-field-like behavior at x = 0 to a line of fixed points that might emerge in the strong-disorder limit as the system approaches the QCP at or near xc. Magnetic inhomogeneities are indeed suggested by a non-vanishing magnetic moment at xc and the evidence of a Griffiths phase as well as glass-like behavior close to xc. Although spin fluctuations certainly play an important role around xc as proposed previously, our highly accurate data of the magnetization M(T,B) and specific heat C(T,B) for x = 0.7 suggest dynamic scaling with an unusual dynamic exponent z = 1.8, incompatible with standard spin-fluctuation theories at a ferromagnetic QCP