3.3 Origin of Half-metallicity and magnetism
First principle calculations on electronic structure plays a pivotal
role in understanding the multi-scale modelling of materials not only it
revails accurately the physical as well as chemical properties of
materials but also supports the adjustment of parametres (or potentials)
in higher scale methods such as classical molecular dynamics, cluster
dynamics etc. In a similar manner, electronic properties provides a
wayout to define the possible applications of the materials in various
subject areas of research. Therefore paves an adequate interests
regarding their excellent properties. In this systematic report we have
choosen exact and accurate relaxed lattice constants from their
structural unit cell optimisations to forecast and justify the
electronic nature in RE2SnFeO6
(RE=Ca,Ba) perovskite systems. The present study provides an superb
description about the electronic properties of these alloys. The
description of electronic nature of both these complex oxides within GGA
and GGA+U functionals illustrates the half-metallic band characters with
spin up-channels form as metal-type spectrum and the presence of gap
demonstrates the semiconducting nature in spin-dn channel as displayed
in Fig. (3, 4) . Seperatively, from the majority-up channels in
case of Ca2SnFeO6 the valence band (top)
and conduction band (bottom) in both of the correlation schemes are
located at Г and X symmetry directions in the Brillouin zone with a
indirect band gap of 0.81 eV and 1.51 eV respectively. Similarly, from
the band structures of Ba2SnFeO6 the
bottom of the conduction band and top of the valence band resides at X
and Г within the two calculated functional schemes GGA and GGA+U thus
describing the character of owing direct band gap with an approximate
values of 0.81eV and 1.44 eV. Here in both the materials on the
employment of GGA+U correction band gap increases due to the shifting of
energy levels from the Fermi-level. Hence the over all trend revails its
half-metallic nature in these present alloys. However, previous study
divulges that these materials are reported theoretically as the
semimetal ferromagnetic perovskites through LDA approximation [30].
Now showing the feature of band structures of these compounds interms of
total density of states (TDOS) shown in Fig. 5 (a, b) which
also incorporates its half-metallic nature in various exploiting
schemes. In order to understand the elemental activeness of various
energy states potrayed in Fig. 5 (c, d) which also points the
occupancy of bands at the Fermi level. Insighting towards the partial
density of states of Ca2SnFeO6 and
Ba2SnFeO6 alloys. Crystal filed
associated with FeO6 cage, generated by the Coloumb
interactions are responsible for the spilliting of 3d degenerate
state of Fe into non-degenerate states:dxy,dyz,dzx calledeg states anddx2-y2 ,dz2 are dt2g states.
From the projected density of states (pDOS) for both the materials
clealy predicts the presence of Fe-deg and O-p states
being of their strong hybridization at the Fermi Level and are
responsible to show the metallic behavior in all the up-spin cases,
while trending down implies semiconducting behavior. However the low
lying sates Ca-s, Ba-s, Sn-s , p which are
far away from the Fermi level depicts negligible appearance. Hence the
partial density of states potrays exclusively a clear environment of
states being of active or deactive at the Fermi-level and also
designates that p -d hybridization is predominant in these
perovskites. Next we have tried to explore the magnetic properties and
also the mechanisms which are responsible to describe the magnetism so
far as these alloys is concerned. Hence this important property is
directly linked to their structures especially electronic configuration.
Within this study two exchange-correlation approximations were used in
practice to know the appropriate knowledge among the interaction between
various constituents which therefore enhances the magnetic character in
these oxide based compounds. Here, we can see from Table 6 the
magnetism was found and is found to be the integral value equivalent to
4.00 μB arises mostly from Fe atoms in both these perovskite systems.
The obtained values of magnetic character of various atoms (Ca, Ba, Sn,
Fe and O) is positive which hints the ferromagnetic interaction within
the crystal structures of these alloys. The half-metallicity along with
100% spin polarization and quantized magnetism of these alloys projects
a better stand in a new class of spintronics devices, spin filters, high
performance electronic devices and spin injectors to meet the necessary
demands of new spin based technologies.