Semiconducting (Half-Metallic) Ferromagnetism in Mn(Fe) Substituted Pt and Pd Nitrides

0.23 MB·English

by  Abdesalem Houari

Semiconducting (Half-Metallic) Ferromagnetism in Mn(Fe) Substituted Pt and Pd
Nitrides
Abdesalem Houari∗
Laboratoire de Physique Th´eorique, D´epartement de Physique, Universit´e de B´ejaia, B´ejaia, Alg´erie
Samir F. Matar
CNRS, Universit´e de Bordeaux, ICMCB, 87 avenue du Docteur Albert Schweitzer, 33600 Pessac, France
Volker Eyert†
1
Center for Electronic Correlations and Magnetism,
1
Institut fu¨r Physik, Universita¨t Augsburg, 86135 Augsburg, Germany
0
2 (Dated: January 11, 2011)

n Using first principles calculations as based on density functional theory, we propose a class of so
a farunexploreddilutedferromagneticsemiconductorsandhalf-metals. Here,westudytheelectronic
J properties of recently synthesized 4d and 5d transition metal dinitrides. In particular, we address

9 Mn-andFe-substitutioninPtN2 andPdN2. Structuralrelaxation showsthattheresultingordered
compounds, Pt0.75(Mn,Fe)0.25N2 and Pd0.75(Mn,Fe)0.25N2, maintain the cubic crystal symmetry
] of the parent compounds. On substitution, all compounds exhibit long-range ferromagnetic order.
i
c While both Pt0.75Mn0.25N2 and Pd0.75Mn0.25N2 are semiconducting, Fe-substitution causes half-
s metallic behavior for both parent materials.
-
l
r PACSnumbers: 71.15.Mb,71.15.Nc,71.20-b,75.10.Lp,74.25.Ha,73.43.Cd
mt Keywords:
.
t
a Being known since the beginning of the 20th century, (OsN ,RuN andRhN )havebeenalsoobtainedandare
2 2 2
m transition metal nitrides are considered as an exciting shown to crystallize in marcasite type structure10,11.
- class of materials due to a wide range of technologi- Platinumdinitridehasbeenpredictedtohaveexcellent
d
cal applications. Traditionally, the great advantages of
n mechanical properties. The calculated hardness (bulk
these compounds concern their hardness and refractory
o modulus, shear modulus and other elastic constants)
nature1,2. However,much attention is currently directed
c shows that it is harder than many known hard mate-
[ towards their electronic, magnetic, and optical proper- rials like TiN and SiC13,14. The electronic properties of

1 ties, where fascinating applications are expected. Many PtN2 are also very interesting. Contraryto other transi-
v efforts, experimental as well as theoretical, have been tion metal nitrides, which are almost all metallic, PtN2
5 made to study the transition metal nitrides3–6. Until isfoundtobe semiconducting,andthiscouldmakeitan
3 recentlynoneofthe noblemetalnitridesorthe platinum importantmaterialforoptoelectronicapplications. Band
6 group nitrides were known. The first synthesis of plat-
structurecalculationsasbasedondensityfunctionalthe-
1 inum nitride (Pt-N), under extreme conditions of pres- ory and the local density approximation(LDA) show an
1. sureandtemperature,wasreportedonlyfewyearsago7,8. indirect band gap of ∼ 1.5 eV, which is probably some-
0 Lateron, several other nitrides of different elements (Ir, whatsmallerthantheexperimentalvalueduetotheten-
1 Os, Ru and Pd) were also obtained9–11. dency of the LDA to underestimate the band gap13–15.
1
: Therewasadebateaboutthecrystalstructureandthe In general, pyrite-type compounds have attracted at-
v
stoichiometry of platinum nitride. While a zinc-blende tention since long. The dinitrides AN (A=C,Si,Ge)
i 2
X structure was first proposed, Crowhurst et al. demon- were devised in assumed pyrite-type structures leading
r stratedthatthisnitridecrystallizesneitherinzinc-blende to compounds with peculiar properties, such as the ex-
a (PtN:mononitride)norinfluorite(PtN : dinitride)type treme hardness obtained for CN with a bulk modulus
2 2
structures, which are highly unstable at the synthesis of 405GPa. For these systems characterized as wide
conditions (P =50GPa and T =2000K)8. Instead, the band gap semiconductors, strong hybridization of the
authorsrevealedthatthecoumpoundisadinitride,hence N 2p states with the A p states results in a depression
PtN , and the ground state structure is a cubic pyrite of the optical band gap along the C, Si, Ge series16,17.
2
structure. Lateron, these authors succeeded in synthe- Pyrite-type disulfides have also been of considerable in-
sizing IrN and PdN , where the first one is found to terest for different reasons18. Semiconducting FeS has
2 2 2
be in the monoclinic baddeleyite structure9. Yet, PdN , found widespread attention for its application in photo-
2
which could by synthesized at high pressures but was voltaic energy conversion19. ZnS is a diamagnetic insu-
2
not stable at ambient conditions, was proposed to also lator. Substitution of Zn for Fe in iron pyrite has thus
crystallize in the pyrite structure. In a recent theoreti- been used to tune the optical band gap in order to en-
calinvestigationitwasshownthattetragonaldistortions hance the response to the solar spectrum20. While FeS
2
maystabilizePdN atambientpressure12. Othernitrides is a van Vleck paramagnet, metallic CoS displays long-
2 2
2
range ferromagnetic order. In contrast, NiS is an anti- projected densities of states as arising from the FPASW
2
ferromagnetic insulator, where the insulating behaviour calculations. However, for the Mn-substituted systems
has been attributed to the presence of strong electronic the LDA results bear some ambiguity. To be specific,
correlations. we obtain semiconducting behavior for Pd Mn N ,
0.75 0.25 2
Ourpresentworkisfocusedespeciallyontheelectronic whereas Pt0.75Mn0.25N2 is at the verge of being a semi-
and magnetic properties of substituted PtN and PdN . conductorbutdisplaysa smallbandoverlap. Inorderto
2 2
We demonstrate that substitution of the non-magnetic check these findings, we additionally performed a set of
4d-and5d-transitionmetalionsby the magnetic 3d-ions calculationsbasedontheGGA25. Theyresultedinsemi-
MnandFemayleadtosemiconductingandhalf-metallic conducting ferromagnetic ground states for both com-
ferromagnetism, respectively. poundswithindirectbandgapsof0.17eVand0.42eVfor
Pt Mn N and Pd Mn N , respectively. The
In our investigation, we first consider Mn-substitution 0.75 0.25 2 0.75 0.25 2
correspondingpartial densities of states (DOS) are illus-
in PtN and PdN . For the latter compound, we as-
2 2
trated in Figs. 1 and 2. The spectrum falls essentially
sumed a cubic pyrite crystalstructure as for PtN . This
2
assumption is based on the fact that experimentally
PdN is actually synthesized in cubic pyrite structure
2
4
at high pressure conditions, even though it is not sta-
ble at ambient pressure9. Replacing one of the four
2
Pt and Pd by magnetic Mn leads to Pt Mn N
0.75 0.25 2
and Pd0.75Mn0.25N2, respectively. To check if the cu- V) 0
btuicmsymmomleecturlyarisdmynaainmtaicisnerdelaoxnaMtionn-suhbasstbiteuetniopne,rafoqrumaend- S (1/e -2
using the Siesta ab initio simulation package with norm- DO -4 MMnn--33dd--te2g
conserving pseudopotentials21,22. Both atomic positions Pt-5dg
andcellshapewereincludedintherelaxationprocess. As -6 N
aresult,neitherPt Mn N norPd Mn N dis-
0.75 0.25 2 0.75 0.25 2 -8
play any deviations from cubic symmetry and the atoms
remain nearly at the positions of the pure compound. -10 -8 -6 -4 -2 0 2 4 6
In particular, the internal nitrogen parameters are al- (E - EV) (eV)
mostunchangedafterMn-substitutioninbothPtN and
2
PdN . The changes in the nitrogen positions are within
2
0.07˚A. To be specific, in PtN the internal nitrogen pa- FIG. 1: (Color online) Partial DOS of Pt0.75Mn0.25N2. Here
rameter changes from 0.415 (a2s given in Ref. 8) to 0.416 andinallsubsequentfigures,t2gandegorbitalsarereferredto
arotatedcoordinatesystemwiththeCartesian axespointing
after the relaxation of the substituted system.
along themetal-nitrogen bonds.
In a second step, full potential augmented spherical
wave(FPASW)calculationswerecarriedout23,24inorder
toaddresstheelectronicpropertiesofallcompoundsun-
3
derstudy. Tostartwith,werecalculatedtheequilibrium
lattice constant. From non-spin polarized LDA calcula- 2
tions, we obtained a lattice parameter of aNM = 4.79˚A 1
for Pt Mn N . Taking into account spin polariza-
0.75 0.25 2
tion led to a slightly larger value of a = 4.82˚A, with V) 0
FM e
the ferromagnetic state being more stable than the non- S (1/ -1
magnetic one. It is important to note that the values O
obtained for the lattice constant of Pt Mn N re- D -2 Mn-3d-t2g
0.75 0.25 2 Mn-3d-eg
semble that of PtN2, which is a = 4.80˚A, and confirm -3 Pd-4d
the molecular dynamics result. To conclude from both N
-4
sets of calculations, not only the cubic symmetry is pre-
-5
served after Mn-substitution, but even the lattice con-
-10 -8 -6 -4 -2 0 2 4 6
stant is almost not affected. The negligible changes of
(E - E ) (eV)
V
the structure can be understood from the fact that only
one out of four Pt atoms is replaced and thus the plat-
inum network is affected by the substitution only to a FIG. 2: (Color online) Partial DOS of Pd0.75Mn0.25N2.
small degree. Motivated by these findings, we decided
toperformthecalculationsforPd0.75Mn0.25N2 usingthe in four parts. While the low-energy range from −9 to
same lattice constant as for PdN2, i.e. a = 4.75˚A (see −6eV and from −8 to −4eV for Pt0.75Mn0.25N2 and
also the discussion below). Pd Mn N , respectively, is dominated by the N 2p
0.75 0.25 2
Subsequently, the electronic structures of the Mn- states, the upper valence band is formed mainly by the
substituted compounds were analyzed in terms of the t manifolds of the transition metal d states. For the
2g
3
spin-minority bands, the situation is slightly more com- netic calculations are illustrated in Figs. 3 and 4. The
plicated. WhereasthePd4dandPt5dstatesarefullyoc-
cupiedandfoundinthe sameenergyrangeastheir spin-
majority counterparts, the Mn 3d t2g states experience 6
strongexchangesplitting. Asaresult,theMn3dt spin-
2g
4
down states form the lower conduction band of this spin
channelandamagneticmomentof3µB isfoundatthese 2
atoms. In contrast, spin polarizations of Pd, Pt, and N V)
e
are negligible. Finally, the remaining conduction band S (1/ 0
states can be attributed to the transition metal d states O -2
D
othfeegNs2ypmsmtaettersy,.wSeinficnedthaecloantstiedrefroarbmleσa-dtmypixetbuorendosfbwoitthh -4 FFee--33Pddt---t5e2dgg
types of states in the lower valence and upper conduc- -6 N
tionband. Thisadmixtureismuchsmallerforthe bands
between −6 and +1eV and −4 to +2eV, respectively, -8
-10 -8 -6 -4 -2 0 2 4 6
whichareoft symmetry andformlessstrongπ bonds.
2g (E - E ) (eV)
F
In passingwe mentionthe albeit smallband gaps,which
make both Mn-substituted compounds semiconducting.
Yet,wenotethatLDAandGGAunderestimatetheopti- FIG. 3: (Color online) Partial DOS of Pt0.75Fe0.25N2.
calbandgap,whichmightthus be considerablylargerin
reality. In order to check this, we performed additional
GGA+U calculations for Pt Mn N . While there
0.75 0.25 2
4
were no qualitative changes, both the exchange splitting
oftheMn3dt statesandtheopticalbandgapincreased
2g
considerably. 2
The second substitution that we considered was the
replacementofPtandPdbyiron,leadingtothe ordered V) 0
e
compoundsPt Fe N andPd Fe N . Ourpro- 1/
cedure was th0e.7s5am0e.25as2for Mn-s0u.7b5stit0u.2t5ion2. Molecu- OS ( -2
D
lar dynamics relaxations using the Siesta code (with the Fe-3d-t
2g
same calculations details cited above)w were performed Fe-3d-eg
-4 Pd-4d
includingrelaxationofboththeatomicpositionsandthe
N
cell shape. As in the Mn-case we found that the cubic
symmetryisnotbrokenonFe-substitutionandthateven -6
-10 -8 -6 -4 -2 0 2 4 6
the internal nitrogenparameterremainedessentially un-
(E - E ) (eV)
changed. F
For the FPASW calculations performed in a second
stepinordertoaddresstheelectronicandmagneticprop- FIG. 4: (Color online) Partial DOS of Pd0.75Fe0.25N2.
erties, we followed the procedure already adopted for
PdN and used the lattice constants of the pure systems
2 gross features of the partial densities of states are the
also for the substituted materials. In this case, our pro-
same as for the Mn-substituted compounds. Differences
cedure wasjustified by anadditionalrecalculationofthe
areduetothesmallermagneticmomentsoftheFeatoms,
equilibrium lattice constant for Pd0.75Fe0.25N2. As a re- which lead to reduced exchange splittings of the 3d t2g
sult, values of a = 4.742˚A and a = 4.749˚A were
NM FM states. As a consequence, the respective spin-majority
obtainedas arising fromnon-spinpolarizedand spin po-
bands are shifted to higher energies as compared to the
larized calculations, respectively. The latter value is al- d states of the Pt and Pd matrix. In addition, the Fe
most identical to the value of a=4.75˚A of pure PdN .
2 spin-minoritybandsareshiftedtolowerenergiesascom-
Again, the LDA results bear some ambiguity as they pared to the Mn-systems due to the increased electron
led to half-metallic behavior for Pd0.75Fe0.25N2 but count. As a result, the semiconducting behavior is lost
metallicityofbothspinchannelsforPt0.75Fe0.25N2. Yet, andametallicspin-downchannelfound. Again,thesere-
the spin-majority density of states at the Fermi energy sultswerequalitativelyconfirmedbyadditionalGGA+U
was found to be very small. The problemcould againbe calculations, which revealed an increase of the exchange
resolvedbyGGAcalculations,whichrenderbothsubsti- splitting of the Fe 3d t states as well as the band gap
2g
tuted materials half-metallic. Both compounds exhibit of the spin-majority channel.
stable magnetic order with magnetic moments of 2.0µB Inpassing,wementionadditionalspinpolarizedcalcu-
located almost completely at the iron atoms. lations,whichwereperformedPt Mn N inorderto
0.75 0.25 2
The electronic structure and partial DOS of the two check for long range antiferromagnetic order. For these
compounds as arising from the spin-polarized ferromag- calculations, we used a tetragonalstructure arising from
4
doubling the cubic cell along the c axis. As a result, netic order results with magnetic moments of 3µ and
B
an antiferromagnetic and again semiconducting solution 2µ ,whicharewelllocalizedattheMn-andFe-sites,re-
B
was found albeit with a total energy, which by about 7 spectievly. While Mn-substitution leads to semiconduct-
mRy/f.u. higher than that of the ferromagnetic ground ing behavior, introduction of iron causes the substituted
state. compounds to be half-metallic. Our results still await
Insummary,basedonourfirstprinciplesinvestigation experimental confirmation.
weproposetheexistenceofsofarunexploreddilutedfer-
romagnetic semiconductors and half-metals. These ma-
terialsarisefromsubstituting magnetic 3dions (Mn, Fe)
in the non-magnetic dinitrides PtN and PdN . Accord-
2 2 Acknowledgments
ingtomoleculardynamicscalculations,theorderedcom-
poundsA B N ,whereA=Pt,PdandB=Mn,Fe,
0.75 0.25 2
preserve the cubic pyrite structure of their parent com- ThisworkwassupportedbytheDeutscheForschungs-
pounds. On substitution, stable long-range ferromag- gemeinschaft through TRR 80.
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