49 research outputs found
Effect of Fe and Co substitution on martensitic stability, elastic, electronic and magnetic properties of MnNiGa: insights from \textit{ab initio} calculations
We investigate the effects of Fe and Co substitutions on the phase stability
of the martensitic phase, mechanical, electronic and magnetic properties of
magnetic shape memory system MnNiGa by first-principles Density
functional theory(DFT) calculations. The evolution of these aspects upon
substitution of Fe and Co at different crystallographic sites are investigated
by computing the electronic structure, mechanical properties and magnetic
exchange parameters. We find that the martensitic phase of MnNiGa
gradually de-stabilises with increase in concentration of Fe/Co due to the
weakening of the minority spin hybridisation of Ni and Mn atoms occupying
crystallographically equivalent sites. The interplay between relative
structural stability and the compositional changes are understood from the
variations in the elastic modulii and electronic structures. We find that the
elastic shear modulus C can be considered as a predictor of
composition dependence of martensitic transformation temperature in substituted
MnNiGa. The magnetic properties of MnNiGa are found to be greatly
improved by the substitutions due to stronger ferromagnetic interactions in the
compounds. The gradually weaker(stronger) Jahn-Teller distortion (covalent
bonding) in the minority spin densities of states due to substitutions lead to
a half-metallic like gap in these compounds resulting in materials with high
spin-polarisation when the substitutions are complete. The substitutions at the
Ga site result in two new compounds MnNiFe and MnNiCo with very
high magnetic moments and Curie temperatures. Thus, our work indicates that
although the substitutions de-stabilise the martensitic phase in MnNiGa,
new magnetic materials with very good magnetic parameters and potentially
useful for novel magnetic applications can be obtained.Comment: Supplementary material can be obtained from the author upon reques
First-principles investigations into the thermodynamics of cation disorder and it's impact on electronic structure and magnetic properties of spinel
Recent experiments on Mn doped multiferroic indicate that a
possible distribution of Mn atoms among tetrahedrally and octahedrally
coordinated sites in the spinel lattice give rise to different variations in
the structural parameters and saturation magnetisations in different
concentration regimes of Mn atoms substituting the Cr. A composition dependent
magnetic compensation behaviour points to the role conversions of the magnetic
constituents. In this work, we have investigated the thermodynamics of cation
disorder in system and it's
consequences on the structural, electronic and magnetic properties, using
results from first-principles electronic structure calculations. We have
computed the variations in the cation-disorder as a function of Mn
concentration and the temperature and found that at the annealing temperature
of the experiment many of the systems exhibit cation disorder. Our results
support the interpretations of the experimental results regarding the
qualitative variations in the sub-lattice occupancies and the associated
magnetisation behaviour, with composition. We have analysed the variations in
structural, magnetic and electronic properties of this system with variations
in the compositions and the degree of cation disorder from the variations in
their electronic structures and by using the ideas from crystal field theory.
Our study provides a complete microscopic picture of the effects that are
responsible for composition dependent behavioural differences of the properties
of this system. This work lays down a general framework, based upon results
from first-principles calculations, to understand and analyse the
substitutional magnetic spinel oxides in
presence of cation disorder.Comment: 13 pages, 5 figures, submitted to Physical review
First principles study of the structural phase stability and magnetic order in various structural phases of MnFeGa
We investigate the structural and magnetic properties of MnFeGa for
different phases(cubic, hexagonal and tetragonal) reported experimentally using
density functional theory. The relative structural stabilities, and the
possible phase transformation mechanisms are discussed using results for total
energy, electronic structure and elastic constants. We find that the phase
transformation form hexagonal to ground state tetragonal structure would take
place through a Heusler-like phase which has a pronounced electronic
instability. The electronic structures, the elastic constants and the
supplementary phonon dispersions indicate that the transition from the
Heusler-like to the tetragonal phase is of pure Jahn-Teller origin. We also
describe the ground state magentic structures in each phase by computations of
the exchange interactions. For Heusler-like and tetragonal phases, the
ferromagnetic exchange interactions associated with the Fe atoms balance the
dominating antiferromagnetic interactions between the Mn atoms leading to
collinear magnetic structures. In the hexagonal phase, the direction of atomic
moment are completely in the planes with a collinear like structure, in stark
contrast to the well known non-collinear magnetic structure in the hexagonal
phase of MnGa, another material with similar structural properties. The
overwhelmingly large exchange interactions of Fe with other magnetic atoms
destroy the possibility of magnetic frustration in the hexagonal phase of
MnFeGa. This comprehensive study provides significant insights into the
microscopic physics associated with the structural and magnetic orders in this
compound.Comment: Supplementary material can be obtained on reques
Site occupancies and their effects on the physical properties of spinel : an {\it ab initio} study
Recent experimental studies on Fe substituted spinel CoCrO have
discovered multiple functional properties in the system such as temperature and
composition dependent magnetic compensation, tunable exchange bias and
magnetostriction. These properties are attributed to the renormalisation of the
inter-atomic magnetic exchange interactions arising due to the non-regular site
occupancies of the magnetic cations in the system. In this work, we perform
{\it ab initio} electronic structure calculations by DFT+U method and combine
with a generalised thermodynamic model to compute the site occupancy patterns
of the magnetic cations, the structural properties and the magnetic exchange
interactions of CoO for the entire
composition range . We find that the substituting Fe atoms prefer to
occupy the tetrahedral sites of the spinel structure for the entire range of
, in agreement with the experimental inferences. Our results on the
variations of the structural parameters with compositions agree very well with
the experiments. By computing the variations of the various inter-atomic
magnetic exchange interactions, we provide a microscopic picture of the
evolution of a collinear structure from a non-collinear one due to substitution
of Fe in CoCrO. The computed results are analysed in terms of the
elements of the crystal field theory, and the features in the atoms and
orbital-projected densities of states. The results and analysis presented in
this work is the first comprehensive study on this system which would help
understanding the complexities associated with the site occupancies, the
electronic structures and the magnetic interactions in this multi-functional
material.Comment: Submitted in Journal of Physics
Site occupancy, composition and magnetic structure dependencies of martensitic transformation in MnNiSn
A delicate balance between various factors such as site occupancy,
composition and magnetic ordering seems to affect the stability of the
martensitic phase in MnNiSn. Using first-principles DFT
calculations, we explore the impacts of each one of these factors on the
martensitic stability of this system. Our results on total energies, magnetic
moments and electronic structures upon changes in the composition, the magnetic
configurations and the site occupancies show that the occupancies at the 4d
sites in the Inverse Heusler crystal structure play the most crucial role. The
presence of Mn at the 4d sites originally occupied by Sn and it's interaction
with the Mn atoms at other sites decide the stability of the martensitic
phases. This explains the discrepancy between the experiments and earlier DFT
calculations regarding phase stability in MnNiSn. Our results
qualitatively explain the trends observed experimentally with regard to
martensitic phase stability and the magnetisations in Ni-excess, Sn-deficient
MnNiSn system
Half-metallicity in quaternary Heusler alloys with 3 and 4 elements: observations and insights from DFT calculations
In this work, we provide important insights into the evolution of
half-metallicity in quaternary Heusler alloys. Employing {\it ab initio}
electronic structure methods we study 18 quaternary Heusler compounds having
the chemical formula CoXYAl, where Y = Mn, Fe; and
X a 4 element. Along with the search for new materials for
spintronics applications, the trends in structural, electronic, magnetic
properties and Curie temperature were investigated. We have made comparative
studies with the compounds in the quaternary series
CoXYSi with X materials from 3 and 4
transition metal series in the periodic table. We observe that the
half-metallic behaviour depends primarily on the crystal structure type based
on atomic arrangements and the number of valence electrons. As long as these
two are identical, the electronic structures and the magnetic exchange
interactions bear close resemblances. Consequently, the materials exhibit
identical electronic properties, by and large. We analysed the roles of
different transition metal atoms in affecting hybridisations and correlated
them with the above observations. This work, therefore, provides important
perspectives regarding the underlying physics of half-metallic behaviour in
quaternary Heusler compounds which goes beyond specifics of a given material.
This, thus, paves way for smart prediction of new half-metals. This work also
figures out an open problem of understanding how different ternary Heuslers
with different electronic behaviour may lead to half-metallic behaviour in
quaternary Heuslers with 4 transition metal elements.Comment: Supplementary material available on reques
Giant magnetocaloric effect driven by first-order magneto-structural transition in cosubstituted Ni-Mn-Sb Heusler compounds: predictions from \textit{Ab initio} and Monte Carlo calculations
Using Density Functional Theory and a thermodynamic model [Physical Review B
86, 134418 (2012)], in this paper, we provide an approach to systematically
screen compounds of a given Heusler family to predict ones that can yield giant
magnetocaloric effect driven by a first-order magneto-structural transition. We
apply this approach to two Heusler series
NiFeMnCuSb and
NiCoMnCuSb, obtained by cosubstitution at
Ni and Mn sites. We predict four new compounds with potentials to achieve the
target properties. Our computations of the thermodynamic parameters, relevant
for magnetocaloric applications, show that the improvement in the parameters in
the predicted cosubstituted compounds can be as large as four times in
comparison to the off-stoichiometric Ni-Mn-Sb and a compound derived by single
substitution at the Ni site, where magnetocaloric effects have been observed
experimentally. This work establishes a protocol to select new compounds that
can exhibit large magnetocaloric effects and demonstrate cosubstitution as a
route for more flexible tuneability to achieve outcomes, better than the
existing ones.Comment: Supplementary Materials can be obtained on request by sending e-mail
to [email protected]
Systematic analysis of structural and magnetic properties of spinel (B=Cr,Mn and Fe)compounds from their electronic structures
The structural and magnetic properties of spinel compounds
(B=Cr,Mn and Fe) are studied using the DFT+U method and generalized gradient
approximation (GGA). We concentrate on understanding the trends in the
properties of these materials as the B cation changes, in terms of relative
strengths of crystal fields and exchange fields through an analysis of their
electronic densities of states. We find that the electron-electron correlation
plays a significant role in obtaining the correct structural and electronic
ground states. Significant structural distortion in CoMnO and
"inverted" sublattice occupancy in CoFeO affects the magnetic
exchange interactions substantially. The trends in the magnetic exchange
interactions are analysed in terms of the structural parameters and the
features in their electronic structures. We find that the Fe states in
CoFeO are extremely localised, irrespective of the symmetry of the
site, which makes it very different from the features of the states of the B
cations in other two compounds. These results provide useful insights into the
trends in the properties of CoBO compounds with variation of B
cation which would help in understanding the results of recent experiments on
doping of Mn and Cr in multiferroic CoCrO.Comment: 10 pGES, 7 FIGURES, To appear in Journal of Physics Condensed Matte
Anti-site disorder and improved functionality of MnNi{\it X} ({\it X}= Al, Ga, In, Sn) inverse Heusler alloys
Recent first-principles calculations have predicted MnNi{\it X} ({\it
X}=Al, Ga, In, Sn) alloys to be magnetic shape memory alloys. Moreover,
experiments on MnNiGa and MnNiSn suggest that the alloys deviate
from the perfect inverse Heusler arrangement and that there is chemical
disorder at the sublattices with tetrahedral symmetry. In this work, we
investigate the effects of such chemical disorder on phase stabilities and
magnetic properties using first-principles electronic structure methods. We
find that except MnNiAl, all other alloys show signatures of martensitic
transformations in presence of anti-site disorder at the sublattices with
tetrahedral symmetry. This improves the possibilities of realizing martensitic
transformations at relatively low fields and the possibilities of obtaining
significantly large inverse magneto-caloric effects, in comparison to perfect
inverse Heusler arrangement of atoms. We analyze the origin of such
improvements in functional properties by investigating electronic structures
and magnetic exchange interactions
A new first principles approach to calculate phonon spectra of disordered alloys
The lattice dynamics in substitutional disordered alloys with constituents
having large size differences is driven by strong disorder in masses,
inter-atomic force constants and local environments. In this letter, a new
first-principles approach based on special quasi random structures and
itinerant coherent potential approximation to compute the phonon spectra of
such alloys is proposed and applied to NiPt alloy. The
agreement between our results with the experiments is found to be much better
than for previous models of disorder due to an accurate treatment of the
interplay of inter-atomic forces among various pairs of chemical species. This
new formalism serves as a potential solution to the longstanding problem of a
proper microscopic understanding of lattice dynamical behavior of disordered
alloys.Comment: 10 pages, 2 figure