Doping of Mn$_2$VAl and Mn$_2$VSi Heusler alloys as a route to half-metallic antiferromagnetism

0.21 MB·English

by  I. Galanakis

Doping of Mn VAl and Mn VSi Heusler alloys as a route to half-metallic
2 2
antiferromagnetism
I. Galanakis1,∗ K. O¨zdo˜gan2,† E. S¸a¸sıo˜glu3,4,‡ and B. Akta¸s2
1 Department of Materials Science, School of Natural Sciences, University of Patras, GR-26504 Patra, Greece
2 Department of Physics, Gebze Institute of Technology, Gebze, 41400, Kocaeli, Turkey
7 3 Institut fu¨r Festk¨orperforschung, Forschungszentrum Ju¨lich, D-52425 Ju¨lich, Germany
0 4 Fatih University, Physics Department, 34500, Bu¨yu¨kc¸ekmece, I˙stanbul, Turkey
0 (Dated: February 6, 2008)
2
Half-metallic antiferromagnets are the ideal materials for spintronic applications since their zero
n magnetizationleadstolowerstrayfieldsandthustinyenergylosses. StartingfromtheMn VAland
2
a
Mn VSi alloys we substitute Co and Fe for Mn and we show by means of first-principle electronic
J 2
structure calculations that the resulting compounds are ferrimagnets. When the total number of
5 valenceelectronsreachesthemagicnumberof24theFe-dopedcompoundsaresemi-metalsandthus
2
non-magnetic while the Co-doped ones show the desirable half-metallic antiferromagnetic charac-

ter. The compounds are very likely to be synthesized experimentally since the parent compounds,
i] Mn2VAl and Co2VAl, havebeen already grown in theHeusler L21 lattice structure.
c
s
PACSnumbers: 75.47.Np,75.50.Cc,75.30.Et
-
l
r
t
m Half-metallic ferromagnets (HMF) are at the center Mn2VSi which are well known to be HMFi. The impor-
. of scientific research during the last decade due to their tance of this route stems from the existence of Mn2VAl
t
a potential applications in spintronic devices [1]. These intheHeuslerL21 phaseasshownbyseveralgroups[16].
m materials are ferromagnets where the one of the two EachMnatomhasaspinmomentofaround-1.5µ and
B
- spin-channels presents a gap at the Fermi level [2]. Sev- V atom a moment of about 0.9 µB [16]. All theoreti-
d
eralmaterialshavebeenpredictedtheoreticallybasedon calstudies onMn2VAl agreeonthe half-metallic charac-
n
first-principles calculations to present this peculiar be- ter with a gap at the spin-up band instead of the spin-
o
c havior : several Heusler alloys [3], some magnetic ox- down band as for the other half-metallic Heusler alloys
[ idesandcolossalmagnetoresistancematerials[4],diluted [11,12,13,17]. S¸a¸sıo˜gluandcollaboratorsstudiedinde-

1 magnetic semiconductors [5], transition-metal pnictides tail the exchange interactions in the Mn2VZ (Z=Al,Ge)
v andchalcogenides[6],andHeuslersemiconductorsdoped HMFi and showed that the antiferromagnetic coupling
1 with high-valent transition metal atoms [7]. Heusler al- between the V and Mn atoms stabilizes the ferromag-
1 loys are particularly attractive for applications due to netic alignment of the Mn spin moments [13]. Except
6
their very high Curie temperatures and their structural Mn2VAl, alsothe case of compounds with Ga,In, Si, Ge
1
similarity to the widely used binary semiconductors like and Sn instead of Al have been predicted to be HMFi
0
7 GaAs, InP, etc. [11].
0
Although the research on HMF is intense, the ideal In the following we will use the full–potential
/
t case would be a half-metallic antiferromagnet (HMA), nonorthogonal local–orbital minimum–basis band struc-
a
m alsoknownasfully-compensatedferrimagnet,likethehy- ture scheme (FPLO) [18] in conjunction with the local
- potheticalHeuslerMnCrSb[8]orMn3Ga[9]compounds, density approximation to study the properties of the
d since such a compound would not give rise to stray flux [Mn1−xXx]2VAl and [Mn1−xXx]2VSi compounds where
n and thus would lead to smaller energy consumption in X is Co or Fe. The coherent potential approximation is
o
devices. Unfortunately these alloys do not crystallize in employed to ensure random doping of the lattice sites.
c
: the desired structure. In the absence of HMA a lot of We will show that doping with either Fe or Co keeps the
v
studies have been focused on the half-metallic ferrimag- HMFi of the ideal alloys respecting the Slater-Pauling
i
X nets (HMFi) which yield lower total spin moments than (SP) rule (the total spin moment in the unit cell is the
r HMF. Van Leuken and de Groot have shown that dop- number of valence electrons minus 24 [12])[22]. When
a ing of the semiconductor FeVSb results in such a mate- the concentration x is such that there are exactly 24 va-
rial [8]. Also some other perfect Heusler compounds like lence electrons the Co-doped compounds show the de-
FeMnSb [10] and Mn2VAl [11, 12, 13] are predicted to sirable HMA character contrary to the Fe-doped ones
be HMFi. Recently other routes to half-metallic ferri- whichloosetheirmagneticcharacterandaresimplesemi-
magnetism have been studied like the doping of diluted metals. The Co-doped compounds are very likely to be
magneticsemiconductors[14]andtheinclusionofdefects synthesized experimentally since the parent compounds
in Cr pnictides [15]. Mn2VAl and Co2VAl [19] already exist.
Inthisletterwewillstudyanotherrouteleadingtothe WewillstartourdiscussionfromtheCo-doping. Prior
desirable HMA, the doping with Co of the Mn2VAl and tothepresentationofourresultswehavetonotethatdue
2
05 x=0.05 [MnM1n-xC1o-xC]xo2]VxA2lVSi Mn Co 04 tvtohaelethngecaepSePilsecrlutorlcoeant[s1e2dh]a,avttehtenhseeegacsotpimivnep-uotpoutnabdlasnspwdi.inthmMloeomsrseeotnvhteasrnatn2hd4e
-5 [ -4 spin-up electrons correspond to the minority-spin elec-
x=0.1 trons and the spin-down electrons to the majority elec-
5 x=0.10 x=0.20 V Z 2
trons contrary to the other Heusler alloys [12]. We have
V) 0 0 substituted Co for Mn in Mn2V(Al or Si) in a random
s/e-5 -2 way and in the upper panel of Fig. 1 we present the
e
at total density of states (DOS) as a function of the con-
DOS (st 05 x=0.05 [MnM1n-Fx1e-Fx]xe2]VxA2VlSi Mn Fe 04 c0[Me.2nnt(1rl−aetfxtiCocnooxlx]u2mVinnS)i[M(arnned1d−txdhCaesohaxet]do2VmlAi-nrlees)(osfolovlireddxbD=laO0c.S0k5f,loinr0e.x1)=aa0nn.dd1
-5 [ -4
inthetoprightpanel. Theperfectcompoundsshoware-
x=0.1
5 x=0.10 x=0.20 V Z 2 gion of low spin-up DOS (we will call it a “pseudogap”)
instead of a real gap. Upon doping the pseudogap at
0 0
the spin-upbandpersistsandthe quaternaryalloyskeep
-5 -2 the half-metallic character of the perfect Mn2VAl and
-4 -2 0 -4 -2 0 -4 -2 0 -4 -2 0 Mn2VSicompounds. Coatomsarestronglypolarizedby
E (eV) E (eV)
the Mnatomssincethey occupythe samesublatticeand
theyformCo-Mnhybridswhichafterwardsinteractwith
FIG. 1: (Color online) Top panel : total DOS as a func-
tion of the concentration x (left column) and atom-resolved theVandAlorSistates[12]. Thespin-upCostatesform
DOS for x = 0.1 (right panel) for the [Mn1−xCox]2VAl and a common band with the Mn ones and the spin-up DOS
[Mn1−xCox]2VSi compounds. Note that the atomic DOS’s for both atoms has similar shape. Mn atoms have less
have been scaled to one atom and Z corresponds either to weight in the spin-down band since they accommodate
Al or Si. The Fermi level has been chosen as the zero of the less charge than the heavier Co atoms.
energy axis, and positive values of DOS correspond to the
In Table I we have gathered the total and atom-
spin-up(minority)electronswhilenegativevaluescorrespond
resolved spin moments for all the Co-doped compounds
to thespin-down (majority) electrons.
asafunctionofthe concentration. Wehavegoneupto a
Bottom Panel : similar to the top panel for the
[Mn1−xFex]2VAl and [Mn1−xFex]2VSicompounds. concentration which corresponds to 24 valence electrons
intheunitcell,thusuptox=0.5forthe[Mn1−xCox]2VAl
and x=0.25 for the [Mn1−xCox]2VSi alloys. In the last
6 Mn2VAl Mn 4 columnwehaveincludedthetotalspinmomentpredicted
MnVSi
2 bytheSlater-Pauling(SP)rulefortheperfecthalf-metals
0
3 [12]. A comparison between the calculated and ideal to-
-4
tal spin moments reveals that all the compounds under
0
V Z 2 study are half-metals with very small deviations due to
-3 the existence of a pseudogap instead of a real gap. Ex-
0
V) actlyfor24valenceelectronsthe totalspinmomentvan-
e-6
s/ -2 ishes as we will discuss in the next paragraph. Co atoms
e
stat [Mn Co ]VAl Mn Co 6 haveaspinmomentparalleltotheVoneandantiparallel
S ( 6 [Mn0.5Co0.52]VSi 3 totheMnmoment,andthusthe compoundsretaintheir
O 0.75 0.252
D 0 ferrimagnetic character. As we increase the concentra-
3
-3 tion of the Co atoms in the alloys,each Co has more Co
0 -6 atomsasneighbors,ithybridizesstrongerwiththemand
V Z 2
its spin moment increases while the spin moment of the
-3
0 Mn atom decreases (these changes are not too drastic).
-6 ThespatomshaveaspinmomentantiparalleltotheMn
-2
atoms as already discussed in Ref. 11.
-4 -2 0 -4 -2 0 -4 -2 0
E (eV) E (eV) The most interesting point in this substitution proce-
dureisrevealedwhenweincreasetheCoconcentrationto
FIG. 2: (Color online) Top panel : total DOS (left column) a value corresponding to 24 valence electrons in the unit
andatom resolvedDOS(rightpanel)fortheperfect Mn2VAl cell, thus the [Mn0.5Co0.5]2VAl and [Mn0.75Co0.25]2VSi
and Mn2VSicompounds. alloys. SP rulepredicts forthese compoundsa zerototal
Bottom panel : similar to the top panel for the Co-
spin moment in the unit cell and the electrons popula-
based half-metallic antiferromagnetic [Mn0.5Co0.5]2VAl and tion is equally divided between the two spin-bands. Our
[Mn0.75Co0.25]2VSicompounds.
first-principlescalculationsrevealthatthisisactuallythe
3
TABLE I: Atom-resolved spin magnetic moments for the TABLEII:Sameastable1fortheFedopingoftheMn-sites.
[Mn1−xCox]2VAland[Mn1−xCox]2VSicompounds(moments Notethatwhenthetotalnumberofvalenceelectronsis24we
have been scaled to one atom). The two last columns are get a non magnetic semi-metal.
the total spin moment (Total) in the unit cell calculated as
2×[(1−x)∗mMn+x∗mCo]+mV+mAlorSiandtheidealto- [Mn1−xFex]2VAl
x Mn Fe V Al Total Ideal
talspinmomentpredictedbytheSPruleforhalf-metals(see
0 -1.573 – 1.082 0.064 -2.000 -2.0
Ref. 12). Thelatticeconstantshavebeenchosen0.605nmfor
0.1 -1.604 -0.179 1.054 0.071 -1.799 -1.8
Mn VAl and 0.6175 for Mn VSi for which both systems are
2 2
0.2 -1.602 -0.222 0.987 0.066 -1.599 -1.6
half-metals (see Ref. 11) and have been kept constant upon
0.4 -1.572 -0.242 0.827 0.054 -1.199 -1.2
Co doping.
0.6 -1.498 -0.210 0.616 0.039 -0.796 -0.8
[Mn1−xCox]2VAl 0.8 -1.315 -0.136 0.337 0.020 -0.387 -0.4
x Mn Co V Al Total Ideal 1.0 non-magnetic semi-metal
0 -1.573 – 1.082 0.064 -2.000 -2.0 [Mn1−xFex]2VSi
0.025 -1.587 0.406 1.102 0.074 -1.899 -1.9 x Mn Fe V Al Total Ideal
0.05 -1.580 0.403 1.090 0.073 -1.799 -1.8 0 -0.960 – 0.856 0.063 -1.000 -1.0
0.1 -1.564 0.398 1.067 0.069 -1.600 -1.6 0.1 -0.979 0.487 0.808 0.055 -0.800 -0.8
0.2 -1.522 0.412 1.012 0.059 -1.200 -1.2 0.2 -0.961 0.437 0.718 0.045 -0.600 -0.6
0.3 -1.484 0.456 0.953 0.047 -0.804 -0.8 0.3 -0.899 0.367 0.605 0.034 -0.400 -0.4
0.4 -1.445 0.520 0.880 0.034 -0.404 -0.4 0.4 -0.726 0.257 0.446 0.021 -0.200 -0.2
0.5 -1.388 0.586 0.782 0.019 ∼0 0 0.5 non-magnetic semi-metal
[Mn1−xCox]2VSi
x Mn Co V Si Total Ideal
0 -0.960 – 0.856 0.063 -1.000 -1.0
0.025 -0.958 0.716 0.870 0.062 -0.900 -0.9 to be synthesized.
0.05 -0.944 0.749 0.860 0.059 -0.800 -0.8 In the second part of our study we have investigated
0.1 -0.925 0.819 0.847 0.054 -0.600 -0.6 the effect of using Fe instead of Co. In the bottom panel
0.2 -0.905 0.907 0.839 0.046 -0.201 -0.2
ofFig. 1weincludetheDOS’sforseveralconcentrations
0.25 -0.899 0.935 0.839 0.041 ∼0 0
and in Table II the total and atomic spin moments. The
conclusions already drawn for the case of Co-doping are
validalsoforthecaseofFedoping. Inthecaseofdoping
case. The interest arises from the fact that although the of Mn2VAl the Fe moment is parallelto the Mn one and
total moment is zero, these two compounds are made very small (∼0.2 µB) while the case of [Mn1−xFexVSi]
up from strongly magnetic components. Mn atoms have is similar to the Co case with Fe moment antiparallel
a mean spin moment of ∼-1.4 µB in [Mn0.5Co0.5]2VAl to the Mn one. As we increase the concentration in Fe
and ∼-0.9 µB in [Mn0.75Co0.25]2VSi. Co and V have and reach Fe2VAl and [Mn0.5Fe0.5]2VSi, which have 24
spin moments antiferromagnetically coupled to the Mn valence electrons, the total spin moment vanishes. But
ones which for [Mn0.5Co0.5]2VAl are ∼0.6 and ∼0.8 µB, ourcalculationsindicatethatinsteadofaHMAwegeta
respectively, and for [Mn0.75Co0.25]2VSi ∼0.9 and ∼0.8 non magnetic compound. To make the origin of this dif-
µ . Thus these two compounds are half-metallic fully- ferent behavior clear we present in Fig. 3 the calculated
B
compensated ferrimagnets or as they are best known in DOS’s for these compounds together with non-magnetic
literature half-metallic antiferromagnets. To confirmthe calculationsfortheCocompounds. InFecompoundsthe
HMA character of these two compounds in Fig. 2 we Fermilevelfallswithinapseudogapandthealloysactas
have drawn the total and atom resolved DOS of both the usual semi-metals (these results agree with previous
compounds(bottompanel)togetherwiththeDOSofthe calculations by Weht and Pickett [20] while experiments
parent Mn2VAl and Mn2VSi compounds (upper panel). suggest that Fe2VAl exhibits heavy-fermionic behavior
The substitution of 50% of the Mn atoms by Co ones in being at the edge of becoming magnetic [21] but such a
Mn2VAl leads to a smoothening of both the total and discussionexceeds the scope of the present paper). Con-
atom-projected DOS due to the hybridization between trary, in the case of the non-magnetic Co-compounds,
the Mn and Co atoms. Overall the energy position of theFermilevelfallswithinaregionofhighDOSanddue
the Mn states does not change and the Fermi level falls to the Stoner criterionthe alloys prefer energetically the
within the pseudogap in the spin-up band and in a re- magnetic configuration. In the right column of Fig. 3
gionofhigh-DOSinthespin-downband. Alltheremarks we presentalsothe atomic DOS.V atomshavethe same
drawn in the previous paragraphsare still valid. A simi- behavior in both cases and the high DOS for the Co-
lar picture occurs also when substituting 25% of the Mn compounds arises from the Co-Mn hybrids. In Ref. 12
atomsbyCoinMn2VSi. Thecaseof[Mn0.5Co0.5]2VAlis it was shown that the gap arises between the occupied
ofparticularinterestsincebothMn2VAl[16]andCo2VAl t1u and the unoccupied eu states which are exclusively
[19] exist experimentally in the L21 structure of Heusler localized in space at the higher valent transition metal
alloys and this quaternary compounds seems very likely atoms,heretheFe-MnorCo-Mnsites. InthecaseofFe-
4
Fe2VAl Mn [ M n 0 . 5 F e 0 . 5 ] 2 V S i Fe 8 mental research in the emerging field of spintronics.
15 [Mn Fe ]VSi
0.5 0.52
4
10
0
V Si
6 ∗ Electronic address: [email protected]
5 † Electronic address: [email protected]
V) 3 ‡ Electronic address: [email protected]
e
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grown experimentally we expect these results to stimu- element.
late a strong interest in both the theoretical and experi-