Investigation on structural electronic and magnetic properties of perovskites srmo3 m mn and co via gga and gga+u methods

In this study, motivated by observation of these remarkable properties in perovskites SrMnO3and SrCoO3, the structural, electronic and magnetic properties of perovskites SrMO3 M¼ Mn and

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Original Article

Investigation on structural, electronic and magnetic properties of

Department of Physics, College of Science, Qassim University, Buraidah 51452, Saudi Arabia

a r t i c l e i n f o

Article history:

Received 4 November 2016

Received in revised form

2 February 2017

Accepted 7 February 2017

Available online 22 February 2017

Keywords:

Magnetic materials

Perovskites

Physical properties

DFT

GGA method

a b s t r a c t

In this study, two transition-metal perovskites SrMO3(M¼ Mn and Co) of interest were studied Their structural, electronic and magnetic properties were investigated via the full-potential linear mufﬁn-tin orbital (FP-LMTO) method within a generalized gradient approximation (GGA) and GGAþ U in the framework of the density functional theory (DFT) At room temperature, both compounds of SrMnO3and SrCoO3crystallize in a cubic structure, with space group of Pm3m (no 221) in a single phase, having the lattice constants of a ¼ 3.806 Å and a ¼ 3.740 Å, respectively The calculated results are in good agreement with the experimental results DFT calculations reveal strong hybridization between Mn (3d) and Co (3d) and the O (2p) orbitals, and the conduction bands were found to be raised from the hy-bridized M (3d)eO (2p) orbitals The spin magnetic moments were systemically calculated based on double-exchange interaction M4þeOeM3þin SrMO3

© 2017 The Author Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Perovskites AMO3 have been termed as inorganic changeable

oxides due the large ﬂexibility of their crystal structure Many

different inorganic oxides take this or related crystal structure since

the parent structure easily distorts or adopts to the relative sizes of

A and M ions forming the perovskite AMO3 Consequently, the

ﬂexibility of the crystal structure of perovskite AMO3and its ability

to accommodate wide range of cations with different ionic sizes

and oxidation states in A and M sites are at the origin of the large

variety of perovskite compounds with a wide range of physical

properties The ideal perovskite AMO3crystallizes in cubic

struc-ture; it consists of MO6 octahedra connected to each other by

corner sharing oxygen, and A-cations occupy the 12-fold

coordi-nation sites surrounding by eight MO6 octahedra Therefore, the

ionic radius of the A ion is larger than that of the M ion and the

presence of the different-size cation sites enables a wide variety of

perovskite-type oxides For example, the crystal structure of

alkali-earth manganese perovskites AMnO3(A2þ¼ Ca, Sr, Ba) reﬂects the

importance of A2þ size, where the ionic radius increases from

1.98Å for Ca2þto 2.15Å for Sr2þand 2.24Å for Ba2þ In AMnO3

series, CaMnO3 crystallizes in an orthorhombic derivative of the ideal cubic SrMnO3 [1], containing intermediately sized ions, crystallizes in both cubic and hexagonal and this is a rare example

of perovskites having alternating crystal structures While BaMnO3

exhibits hexagonal [2] in which all MnO6 octahedra share faces along the c axis in the crystal Hexagonal structure in SrMnO3is a 4H-type with alternating face sharing and corner sharing along the

c axis The hexagonal modiﬁcation is stabled up to about 1035C

where it transferred into an ideal cubic in high temperature[2,3] During the last decades, transition-metal perovskites have been

a hot topic of the experimental and theoretical research due to their exclusive physical properties, such as ferroelectric in BaTiO3 [4], multiferroic in BiFeO3 [5], high Curie-temperature (TC) in SrRu1 xCrxO3 [6] and p-type conducting [7] The interest in transition-metal perovskites was derived from their potential ap-plications For example, colossal magnetoresistance (CMR) in

Pr0.7Ca0.3MnO3[8], spin-ﬁlter-type magnetic tunnel junction (MTJ) barriers in La0.1Bi0.9MnO3[9], magnetic sensors and memory de-vices in reading heads in SrTiO3[10,11], electrolytes in solid oxide fuel cells (SOFC) in Y-doped BaCeO3[12], proton conducting fuel cell (PCFC) in La0.7Sr0.3FeO3 a[13], etc Moreover, transition-metal perovskites were reported as materials with half-metallic (HM) and

MR nature, such as in PrMnO3 [14], Pr1 xSrxMnO3 [15],

La0.7Ca0.3MnO3[16]and La1 xSrxCoO3[17]

In doped manganite with mixed Mn3þ(3d4; t32ge1) and Mn4þ (3d3; t3

2ge0) valence-states, the hopping of eg electrons occurs

* Fax: þ966 163800911.

E-mail address: musa.1964@gmail.com

Peer review under responsibility of Vietnam National University, Hanoi.

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Journal of Science: Advanced Materials and Devices 2 (2017) 115e122

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between two partially ﬁlled 3d orbitals of neighboring

Mn3þeMn4 þ This can be simpliﬁed by the pddsorbital overlap

Mn3þ(eg)eO (2ps)eMn4þ(eg) and the strong onsite Hund coupling

between t2gcore spins and the egelectrons Such mechanism is

known as double-exchange (DE) interaction; it brings about the

simultaneous onset of ferromagnetic (FM) and metallic nature

Also, another mechanism in doped manganites is the

antiferro-magnetic (AFM) super-exchange (SE) interaction via the pddp

orbital overlap Mn3þ ðt4

2gÞeO (2pp)eMn4 þ ðt3

2gÞ Therefore, the presence of FM-DE and AFM-SE interactions along with the

pres-ence of disorder plays an important role in determining the

elec-tronic and magnetic properties of doped manganites

Transition-metal perovskites have been the subject of many

research studies due to their unique properties, such as

semi-conducting behavior, spin-polarization, metallic or half-metallic

(HM), ferromagnetic (FM), antiferromagnetic (AF), ferroelectric

(FE), etc Among this family, the pure and doped SrMO3containing

Mn and Co have attracted much attention in many fundamental

and appliedﬁelds of solid-state physics, solid-state chemistry and

material science because of the unusual combination of their

structural, electronic and magnetic properties [2,3] SrMO3

(M¼ Mn, Co) compounds cover a variety of technological

appli-cations, such as use in semiconductor products, solid oxide fuel

cells, design magnetic memories, read head in hard disks, tunnel

magnetic junctions, multiferroics applications, gas diffusion

membranes, etc.[10e15] In this study, motivated by observation of

these remarkable properties in perovskites SrMnO3and SrCoO3, the

structural, electronic and magnetic properties of perovskites SrMO3

(M¼ Mn and Co) are investigated using the full potential linear

mufﬁn-tin orbital method according to density functional theory

(DFT) Moreover, an attempt is made to explore the effect of

exchange-correlation energy U and M4þ-site with 3d

transition-metal ions, M4þ ¼ Mn4þ and Co4þ, on the structural, electronic

and magnetic properties The electronic and magnetic results will

provide some information on the DE and SE interactions of

M4þeOeM3þin both of the 90and 180arrangements in SrMO3

2 DFT calculations

The calculation of the properties for SrMO3(M¼ Mn and Co)

was based on theﬁrst-principles density functional theory (DFT)

[18], via the full-potential linear mufﬁn-tin orbital (FP-LMTO)

method [19] Electronic and magnetic properties were obtained

from the generalized gradient approximation (GGA)[20], and the

exchange-correlation parameterization of PerdeweWang (PW91)

[20] was employed in GGA Unluckily, GGA cannot successfully

describe the 3d states, so, this lack was corrected by adding

exchange-correlation terms to GGA using the GGAþ U method[21]

GGAþ U yields quite satisfying results for correlated perovskites by

exploiting correlation parameters, Coulomb repulsion (U) and

Hund exchange (J)[22,23] For M (3d) ions, the reasonable values

U ¼ 4.0 eV and J ¼ 0.96 eV were employed [22e24], and their

settings were examined by the total energy convergence reached in

the calculations

The cutoff-energy was set to 450 eV and 6 6 6 k-points grids

were set in the irreducible part of Brillouin zone Toﬁnd the stable

state, the energy convergence criterion for the electronic

self-consistent calculation was set to 0.001 meV The WignereSeitz

radii of the mufﬁn-tin (MT) spheres were set to 2.50, 2.0 and 1.5 a.u

for Sr, M and O atoms, respectively In addition, the effect of

spineorbit coupling (SOC) was included in the calculations by using

the scalar relativistic method (GGAþ SOC) and (GGA þ SOC þ U)

based on Dirac equation [19] The full relativistic effects were

calculated by using the Dirac equation for core states, whereas the

scalar relativistic approximation was used for the valance states

[25] Including SOC effect is important for investigating the elec-tronic and magnetic properties of SrMO3due to the presence of the relatively heavy atoms The SOC was included in a self-consistent manner by solving the radial Dirac equation for the core electrons and evaluated based on the second variation treatment[25,26]; thus, the total angular momentum coupled the orbital angular momentum to the spin of the valence and semi-core states of M (3d)

3 Results and discussion 3.1 Structural properties

In crystalline material, the large ion O2(R¼ 1.44 Å) is com-bined with a transition-metal having small ionic radius, such Mn2þ (R¼ 0.67 Å), Co2þ(R¼ 0.65 Å) and Ni2þ(¼ 0.69 Å), the resulting structure can be looked upon as close packed oxygen ions with transition-metal ions in the interstitials This is observed for many inorganic compounds with oxygen ions and transition-metal of valenceþ2, e.g Mn2þO2, Co2þO2and Ni2þO2 In these crystal structures, the O2ions form a cubic close packed (ccp) lattice with the transition-metal ions in the octahedral interstitials, i.e the rock-salt (NaþCl) structure Replacing one-fourth of the oxygen with a cation of approximately the same radius as O2, e.g alkali-earth element (A¼ Ca2 þ, Sr2 þor Ba2 þ) reduces the number of

octahe-dral voids, occupied by a small M-cation, to one-fourth The resulted formula can be written as AMX3and this crystal repre-sented the perovskite structure, where the anion X is often O2but also other large ions such as Fand Clare possible This mineral is stable perovskite type, which is one of the fundamental crystal structures

Perovskite SrMO3is a centrosymmetric solid with a typical cubic perovskite structure in Pm3m space group, in which the Sr2þ cat-ions are 12-fold coordinated forming cuboctahedral SrO12clusters, whereas the M4þcations are 6-fold coordinated[27] The octahe-dral MO6clusters form the framework of the cubic structure, where the Sr2þ, M4þand O2atoms occupy the cube corners and the edge centers, respectively, as shown inFig 1 From GGA calculations, the obtained equilibrium lattice constants are a¼ 3.806 Å for M ¼ Mn, and a¼ 3.740 Å for M ¼ Co in a good agreement with the previous theoretical values[28,29], as well as with the experiments[30,31] Moreover, similar calculations are repeated by the GGA þ U method; however, the crystal structure is only slightly changed with respect the GGA one.Table 1shows the structural properties

of the unit cell for perovskites SrMO3(M¼ Mn4þand Co4þ) Crystal symmetry, space group, lattice constant, unit cell volume, ionic radius, tolerance factor, atomic sites and positions, bond distance and bond angle

3.2 Electronic properties

In this part, the effects of M-site substitution, spineorbit coupling (SOC) and exchange-correlation energy U on electronic properties of SrMO3(M¼ Mn and Co) are described Therefore, the total and partial density of states (DOS) are calculated within the DFT by using four methods GGA, GGA þ SOC, GGA þ U and GGAþ SOC þ U, and plotted inFigs 2e4, in the range8.0 eV

toþ12.0 eV relative to the Fermi energy (EF; dashed line) First,

Fig 2represents the total density of states (TDOS) of perovskites SrMO3(M ¼ Mn and Co) along their cubic symmetry axes The large TDOS at EFin four methods suggest that two compounds have a metallic behavior, which is similar with the results obtained

by LSDA method [30,32] There are some bands with different TDOS cross the EFin both spin-up and spin-down directions with

no energy-gaps From GGA and GGAþ SOC, it is clearly seen that

M Musa Saad H.-E / Journal of Science: Advanced Materials and Devices 2 (2017) 115e122 116

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the TDOS split into three main bands, valence band, between

7.0 eV and 3.0 eV, conduction band through EF, between

3.0 eV and þ4.0 eV, and a band above the EF, betweenþ4.0 eV

andþ11.0 eV In GGA þ U and GGA þ SOC þ U,Fig 2(c) and (d),

these bands have more splitting due to the U energy The main

contribution in the spin-up and spin-down TDOS of SrMO3

(M ¼ Mn and Co) in the valence bands, between 7.0 eV and

1.0 eV, is mainly due to the Sr (5s), Mn (3d)/Co (3d) and O (2p)

states The broad O (2p) bands between3.0 eV and 6.0 eV can

be seen to have almost identical structure in SrMnO3and SrCoO3,

as do the 3d bands, which lie above them In addition, both bands

of Mn (3d) in SrMnO3 and Co (3d) in SrCoO3 occupy the same

energy levels As is well known, SOC is the relativistic effect and as

such increases strongly with the nuclear charge of atoms

con-taining partially ﬁlled bands [25] In 3d perovskites SrMO3

(M¼ Mn and Co), since they contain atoms with a low nucleus

charge, Z¼ 25 for Mn (3d) and Z ¼ 27 for Co (3d), the effect of SOC

is regularly small compared with the effect of correlation energy U,

whereas, for 5d perovskites containing heavier atoms with a high

nucleus charge, such as Z¼ 75 for Re (5d), Z ¼ 76 for Os (5d) and

Z¼ 77 for Ir (5d), the SOC effect is much larger The 3d localized

electrons in Mn (3d3) and Co (3d5) have less extended electronic

orbitals in the valence bands, leading to signiﬁcant increases in the

electronic correlations and decreases in orbital overlap in the crystal structures

Moreover, to explain the contribution of different states to the TDOS, the partial density of states (PDOS) of Sr (5s), Mn (3d)/Co (3d) and O (2p) in SrMO3with M¼ Mn and M ¼ Co are calculated and shows inFigs 3 and 4, respectively From the GGA and GGAþ SOC for SrMnO3inFigs 3 and 4(a) and (b), the broad O (2p) bands be-tween2.5 eV and 7.0 eV can be seen with the Mn (3d) and Co (3d) bands above them, which they must place at the turn of the EF Two sharp spin-up and spin-down peaks of 3.26 and 2.79 expand from1.20 eV to þ0.50 and from 0.92 eV to þ0.74 corresponding

to the Mn (3d) and Co (3d) states, respectively, are clearly visible through EF Whereas, these peaks split in spin-up for M¼ Mn and spin-down for M¼ Co and decline to 2.77 and 0.90, respectively, in PDOS from GGAþ U and GGA þ SOC þ U methods,Figs 3 and 4(c) and (d) Moreover, in all PDOS there is an electronic hybridization between M (3d)/Co (3d) and O (2p) bands, which is expected to be occurred between2.0 eV and þ2.0 eV, and of course leads to the

DE interaction in magnetic perovskites

Once more, the Sr (5s) bands can be seen to have considerable dispersion suggesting that it is involved in covalent bonding, and the Mn (3d) and Co (3d) bands overlap with tiny orbitals of partially occupied Sr (5s) states The large TDOS at the EFsuggests that the cubic FM structure is stable, and that a lower energy structure could be achieved by allowing spin-polarization of the conduction-electron The TDOS and PDOS of SrMnO3are similar to that previously published [33e35], and agree well with the experiment[28,36] The top of the valence bands is composed of O (2p)eMn (3d) and O (2p)eCo (3d) hybridized orbitals, while the bottom of the conduction bands is mainly composed of Mn (3d) and Co (3d) orbitals,Figs 3 and 4 The modest contributions of Sr (5s) orbitals are located in the middle of the top of valence bottom

of the conduction bands For SrMnO3, the calculated energy-gap in spin-down band is 0.52 eV,Fig 3, which compares well with the reported value, i.e 0.75 eV[34] The underestimation with respect

to the experimental values 3.2e3.4 eV [28,36]is a well-known consequence of the incomplete cancellation of the self-interaction in the local exchange functional of GGA Moreover, similar calculations are repeated by using the GGA þ U and GGAþ SOC þ U methods for SrMO3(M¼ Mn and Co) However, the energy-gap is only considerably changed with respect to the GGA

Fig 1 Crystal structure of perovskites AMO 3 (a) The cubic unit cell, A-atoms (grey) at the corners, M-atoms (blue) in the centers and O-atoms (red) in the face-centers (b) The corner-sharing octahedral structure, where A-atoms occupy every void, which is created by eight MO 6 octahedra, giving an 12-fold oxygen coordination for A-atoms and 6-fold oxygen coordination for M-atoms.

Table 1

DFT calculations of the structural properties for SrMO 3 (M ¼ Mn and Co).

Atomic sites and positions (x, y, z)

Bond distances and bond angles

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and GGAþ SOC methods, the energy-gap is 4.51 eV for M ¼ Mn,

which is signiﬁcantly increased, while for M ¼ Co, there is a tiny

spin-up band cross the EF and two splitting spin-down bands

around the EF Co results in SrCoO3agreement with the previous

results[37,38] In addition, no signiﬁcant changes appear in the

TDOS and PDOS shapes for SrMO3compounds

Furthermore, clearly, the PDOS for Mn (3d) and Co (3d) states are

expanded and responsible for the metallic behavior of the

perov-skites SrMnO3and SrCoO3, respectively Since the 3d orbitals split

into t2gand egstates, the 3d-t2gstates are more localized than the

3d-egstates Thus, the M (3d-eg)eO (2p) bond in SrMnO3is stronger as

compared with the M (3d-t2g)eO (2p) Consequently, the partial

states of M (3d-eg) and O (2p) hybridize and cross the EFin the

electronic TDOS, due to which the perovskites show metallic

behavior It is also clear that M (3d-t2g) and M (3d-eg) in GGAþ U and

GGAþ SOC þ U results,Figs 3 and 4(c) and (d), are shifted toward the

valence bands as compared to GGA and GGAþ SOC, which is due to

the fact that GGA method is not appropriate to treat 3d states exactly

3.3 Magnetic properties

In order to describe the magnetic properties of transition-metal perovskites, weﬁrst investigate the magnetic interaction and its mechanisms in SrMO3(M¼ Mn and Co) As is well known, DE is a type of a magnetic interaction that was originally proposed by C Zener[39]for FM manganites with perovskite structure Then it applied to SrMO3by K Kubo and N Ohata[40]to account for the simultaneous appearance of its FM order and metallic behavior To describe DE mechanism in perovskites SrMO3, a little must be ﬁrst understood about the electronic and magnetic structures about the transition-metals Mn (3d) and Co (3d) sites In an ideal crystal lattice, the 3d orbital splits into two sub-states, a triple t2g

and double egsub-orbitals, due to the crystalﬁeld created by the cubic symmetry surrounding the M (3d) sites with Mn4þ (3d3;

t3 2ge0) and Co4þ(3d5; t3

2ge2) The doublet egsub-orbitals in SrMO3

usually lie 2e4 eV above the triplet t2gsub-orbitals in energy, see

Fig 5

Fig 2 Total density of states (TDOS) for the perovskites SrMO 3 (M ¼ Mn 4þ and Co4þ), computed within DFT using (a) GGA, (b) GGA þ SOC, (c) GGA þ U and (d) GGA þ SOC þ U The horizontal solid lines indicate the zero TDOS and the vertical dashed lines indicate the Fermi level (E F ).

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In octahedral crystal ﬁeld, Mn4þ (3d3) ions have only one

possible spin state, t3

2ge0(S¼ 3/2) with three unpaired electrons, whereas Co4þ (3d5) ions have three possible spin states, t5

2ge0

(S¼ 1/2), t4

2ge1(S¼ 3/2) and t3

2ge2(S¼ 5/2) with one, three and ﬁve unpaired electrons, respectively We designated these spin states in

Fig 5as low spin (LS) state with one unpaired electron,

interme-diate spin (IS) state with two or three unpaired electrons and high

spin (HS) state withﬁve unpaired electrons for Mn4 þand Co4 þions.

These spin conﬁgurations of Mn4 þ (3d3) and Co4þ (3d5) ions in

octahedral crystalﬁeld are schematically represented inFig 5

Moreover, the Coulomb-repulsion energy U forces each

3d-electron to lay on a lonely level, and the Hund's rule coupling is

strong enough to ensure that all the 3d electrons on the Mn (3d)

and Co (3d) sites are FM aligned In Mn (3d) ions, three electrons

drive toﬁll the lower lying t3

2gstates forming and inert core-spin of

S¼ 3/2, whereas the remaining electrons lie in a superposition of

the e0states While in Co (3d) ions also three electrons drive toﬁll

the t3

2gstates forming S¼ 3/2 and the remaining electrons lie in the

e2states These electrons may move through the lattice subject to

the constraint that the hopping-electron always has its spin aligned

with its host's core-spin The hopping from Mn4þto Mn3þor Co4þ

to Co3þis mediated by the O2ions between them, and DE orga-nizes this hopping interaction As the temperature is lowered and spin ﬂuctuations decreased, the crystal lowers its energy by FM aligning the M (3d) core-spins allowing the egelectrons to gain kinetic energy and move about the crystal The hopping transfer integral calculated from such DE theory is a very sensitive function

of the M4þeOeM3þ bond angle, deviated from the ideal 180 resulting in much reduced hopping probability This agrees with experimental evidence, where different divalent dopants with different atomic radii cause about large change in the Curie tem-perature (TC) Dopants with small atomic radii cause a buckling in the M4þeOeM3 þ bond angle, decreasing the single electron

bandwidth and consequently reducing the TC The effect of dopant size on the structural and electronic properties can be described using the tolerance factor, which describes the degree of deviation from ideal cubic symmetry, as it discussed extensively in Section3.1

for crystal structure properties

Then, the magnetic properties of perovskites SrMO3(M¼ Mn and Co) were investigated in detail, where the spin magnetic mo-ments were calculated by using the four different approximations: GGA, GGAþ SOC, GGA þ U and GGA þ SOC þ U, within DFT method

Fig 3 Total and partial [Sr (5s), Mn (3d) and O (2p)] density of states of SrMnO 3 , computed within DFT using (a) GGA, (b) GGA þ SOC, (c) GGA þ U and (d) GGA þ SOC þ U The vertical dashed lines indicate the Fermi level (E F ).

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Fig 4 Total and partial [Sr (5s), Co (3d) and O (2p)] density of states of SrCoO 3 , computed within DFT using (a) GGA, (b) GGA þ SOC, (c) GGA þ U and (d) GGA þ SOC þ U The vertical dashed lines indicate the Fermi level (E F ).

Fig 5 Splitting of 3d levels into t 2g and e g states due to the octahedral crystal ﬁeld and different possible spin states for (a) Mn 4þ (3d 3 ) and (b) Co4þ(3d 3 ) in SrMO 3 (M ¼ Mn and Co).

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Table 2shows the calculated partial and total spin magnetic

mo-ments for the cubic symmetry in two compounds First, it is

remarked that when the Coulomb repulsion-energy U was used,

the obtained spin magnetic moments have an extensive increase,

whereas the SOC term has a small effect since the partiallyﬁlled 3d

states in Mn and Co ions are weak in their compounds It is seen

from these results, the spin magnetic moments are overestimated

by both GGAþ U and GGA þ SOC þ U methods, but less that the

GGAþ U calculation gives the biggest values, especially the partial

spin magnetic moments of Mn (3d) and Co (3d) ions, which are

more important than others The electronic conﬁguration of the Mn

(Z¼ 25) and Co (Z ¼ 27) atoms are [Ar]183d54s2and [Ar]183d74s2,

respectively, which indicate the dissimilar conﬁgurations of these

elements in their compounds The obtained values are 3.6688mBfor

M¼ Mn (3d) and 3.4630mBfor M¼ Co (3d), which suggest the

electronic conﬁguration of the 3d ions in-between Mn4 þ(3d3) and

Mn3þ(3d4), and Co4þ(3d5) and Co3þ(3d6) states in SrMnO3and

SrCoO3, respectively These results agree well with the partial and

total spin magnetic moments of SrMnO3 and SrCoO3 calculated

using the GGA method[16,31,41] In those studies, it carried out the

values of 2.50mBand 2.967mB, respectively, for the FM con

ﬁgura-tion of the cubic structures

On the other hand, the spin magnetic moment of the O2ions is

very small; approximately equal to zero in two compounds, with

small contribution to the total spin magnetic moment The same

remark is revealed for the Sr2þions, and found that its spin

mag-netic moment is negligibly smaller than the other ions in each of

SrMnO3and SrCoO3 Moreover, both GGA and GGAþ SOC

calcu-lations produce similar results for the M (3d) ions, around 2.5mB

and 1.7mBfor M¼ Mn and for M ¼ Co, respectively, whereas the

GGAþ U and GGA þ SOC þ U calculations overestimated the spin

magnetic moment of the M (3d) ions, in comparison with the

ob-tained values from GGA and GGA þ SOC The GGA þ U and

GGA þ SOC þ U calculations give bigger values than GGA and

GGAþ SOC ones, which show the effect of U parameter, especially

on the spin magnetic moment of the M (3d) ions Thus, the

exchange-correlation methods, GGAþ U and GGA þ SOC þ U, are

more accurate than others used in the calculations, which they will

be indispensable for the electronic and magnetic structure

calcu-lations, particularly for the transition-metal materials

4 Conclusion

The structural, electronic and magnetic properties of two

interested transition-metal perovskites SrMO3(M¼ Mn and Co)

were investigated via the full potential linear mufﬁn-tin orbital

(FP-LMTO) method within generalized gradient approximation (GGA)

and GGAþ U based on the density functional theory (DFT) First, the

structural properties of SrMO3perovskites were investigated; two

compounds crystallize in a cubic symmetry with space group of

Pm3m (no 221) The increasing of occupation-number on M-site in perovskites SrMO3 (M ¼ Mn4 þ and Co4 þ) decreased the lattice

constant and the unit cell volume, and increased the tolerance factor of SrMO3 Then the electronic properties of perovskites SrMO3(M¼ Mn and Co) are also investigated by calculating the total (TDOS) and partial (PDOS) density of states In electronic TDOS, there are some bands with different DOS cross the EFin both

of the spin-up and spin-down directions We found that the PDOS

of Mn (3d) and Co (3d) states, which hybridize with O (2p) states, are responsible for the metallic behavior of the perovskites SrMnO3

and SrCoO3, respectively Finally, the magnetic properties of transition-metal perovskites SrMO3 (M ¼ Mn4 þ and Co4 þ) were

studied in detail Also, the possible spin states of Mn4þ(3d3) and

Co4þ(3d5) ions in octahedral crystalﬁeld were discussed, where

Mn4þ(3d3) splitting into t3

2ge0(S¼ 3/2) with three unpaired elec-trons, whereas Co4þ(3d5) states have t5

2ge0(S¼ 1/2), t4

2ge1(S¼ 3/2) and t3

2ge2 (S ¼ 5/2) with one, three and ﬁve unpaired elec-trons, respectively The magnetic double-exchange interaction

M4þeOeM3þand its mechanisms in SrMO3were described The partial and total spin magnetic moments for the cubic symmetry in SrMO3(M¼ Mn4 þand Co4 þ) were determined by using the above

mentioned four different approximations, within the DFT method Acknowledgments

The author gratefully acknowledges Deanship of Scientiﬁc Research, Qassim University, Saudi Arabia forﬁnancial support of this research study (Grant number 3000)

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DFT method GGA GGA þ SOC GGA þ U GGA þ SOC þ U

SrMnO 3

SrCoO 3

Trang 8

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