%B.T. Matthias

A.M. Clogston

B. Drittler

B. Johansson

D. Riegel

D.D. Johnson

G. Moroni

H. L. Skriver

H.L. Skriver

I. A. Abrikosov

I.A. Abrikosov

J. Friedel

M. Akai

O.K. Andersen

P. Lang

S. Mirbt

S.H. Vosko

V.I. Anisimov

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Magnetic properties of Fe embedded in V and Cr:ff Thin films and dilute alloys

We have calculated the magnetic moment of Fe as a function of the number of nearest V and Cr atoms. We find that the Fe magnetic moment disappears in a neighborhood of exclusively V atoms, but remains larger than 1.85 mu(B) in a Cr neighborhood. This behavior, which is in excellent agreement with experiment, is a result of the pseudogap formed in the bce state density

Mirbt, S.

Abrikosov, I.A.

Johansson, B.

Skriver, Hans Lomholt

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Magnetic properties of Fe embedded in V and Cr: Thin films and dilute alloysMirbt, S.; Abrikosov, I.A.; Johansson, B.; Skriver, Hans LomholtPublished in:Physical Review B Condensed MatterLink to article, DOI:10.1103/PhysRevB.55.67Publication date:1997Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Mirbt, S., Abrikosov, I. A., Johansson, B., & Skriver, H. L. (1997). Magnetic properties of Fe embedded in V andCr: Thin films and dilute alloys. Physical Review B Condensed Matter, 55(1), 67-69. DOI:10.1103/PhysRevB.55.67Magnetic properties of Fe embedded in V and Cr: Thin films and dilute alloysS. Mirbt,* I. A. Abrikosov, and B. JohanssonDepartment of Physics, Uppsala University, Box 530, S-75121 Uppsala, SwedenH. L. SkriverCenter for Atomic-scale Materials Physics and Department of Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark~Received 18 June 1996!We have calculated the magnetic moment of Fe as a function of the number of nearest V and Cr atoms. Wefind that the Fe magnetic moment disappears in a neighborhood of exclusively V atoms, but remains largerthan 1.85mB in a Cr neighborhood. This behavior, which is in excellent agreement with experiment, is a resultof the pseudogap formed in the bcc state density. @S0163-1829~97!01501-4#In the early 1960s Matthias et al.1 and Clogston et al.2found that Fe impurities are magnetic in Mo, but nonmag-netic in Nb. These experimental findings were confirmed inthe late 1980s by Riegel et al.,3 who found that Fe impuritiesare generally magnetic in group-VIb metals, but nonmag-netic in group-Vb metals. Ab initio calculations have repro-duced these findings for Fe impurities in Mo and Nb,4 but weare not aware of any satisfactory explanation of these trends.In the present paper we investigate the magnetic propertiesof Fe atoms embedded in V and Cr. These systems haveattracted considerable attention, but so far only calculationsfor dilute Fe-based alloys5–7 or Fe alloys with intermediateconcentrations8,9 have been carried out, while we are inter-ested in the limit of small Fe concentrations.In an earlier publication, the decrease of the Fe magneticmoment at an Fe surface covered by V or Cr overlayers wasexplained in terms of the hybridization between the Fe hostand the V and Cr d bands.10 Here, we extend this work bycalculating the magnetic moment of Fe as a function of thenumber of V or Cr atoms in its nearest neighborhood. Tounderstand the magnetic behavior of Fe surrounded by eitherV or Cr, we performed two different sets of calculations.First, we studied a sandwich geometry, where we embeddedone or two @100# layers of either pure Fe or an Fe xV12xalloy between two semi-infinite body-centered-cubic crystalsof V. The Fe-Cr system was treated in a similar manner. Bychanging the number of Fe monolayers ~ML! or/and alloyconcentration we modify the number of V or Cr nearestneighbors in the first shell of the Fe atom. For example, inthe case of a single Fe ML embedded in a V host all of theeight nearest neighbors of Fe are host ~5V! atoms, while fortwo ML of Fe this number is reduced to 4. Second, we stud-ied bulk Fe xV12x and Fe xCr 12x alloys. In the dilute limit ofsmall x the probability of finding an Fe-Fe pair can be ne-glected, and thus the number of host V or Cr neighbors in thefirst Fe shell is again 8.We have performed self-consistent electronic structurecalculations by means of the scalar relativistic spin-polarizedGreen’s-function technique11 based on the linear muffin-tinorbital ~LMTO! method12,13 within the tight-binding,14–16frozen core, and atomic-sphere approximations together withthe local spin density approximation as parametrized byVosko, Wilk, and Nusair.17 The alloy was treated within thecoherent potential approximation ~CPA!.18 Our calculationsdo not take lattice relaxations into account and we assumedatomic spheres of equal radii. For the V-rich and Cr-richcalculations we assumed lattice constants of 3.03 Å and 2.88Å, respectively, corresponding to the experimental bcc latticeconstants of the hosts.We find, in accordance with earlier calculations and ex-periments, that the Fe magnetic moment vanishes if all itseight nearest neighbors are V atoms, whereas Fe retains amagnetic moment in a Cr surrounding. In Fig. 1 we show theFe magnetic moment as a function of the number of its for-eign nearest neighbors. The Fe magnetic moment is seen tovanish in the case of one monolayer ~ML! of pure Fe or 1ML of an Fe 50V50 alloy embedded into bulk V, as well asfor a dilute bulk Fe 1V99 alloy, i.e., when the number of hostV atoms in the first Fe shell is 8. In contrast, when all nearestFIG. 1. The Fe magnetic moment as a function of the number ofits foreign nearest neighbors given by 4 ~2 ML!, 4.8 ~2 ML,c50.8), 5.2 ~2 ML, c50.7), 6.0 ~2 ML, c50.6), 6.4 ~2 ML,c50.5), 6.8 ~2 ML, c50.4), 8.0 (c50.01), and 8.0 ~1 ML! wherec is the Fe concentration. The solid ~dotted! line corresponds to Feembedded in V ~Cr!. The small difference in moment between Fe-Vand Fe-Cr for no foreign nearest neighbors reflects the difference inthe host lattice parameter.PHYSICAL REVIEW B 1 JANUARY 1997-IVOLUME 55, NUMBER 1550163-1829/97/55~1!/67~3!/$10.00 67 © 1997 The American Physical Societyneighbors are Cr atoms Fe retains a finite magnetic momentof 1.85mB . It is also seen in the figure that the Fe magneticmoment increases linearly with decreasing number of Crnearest neighbors, whereas in V the Fe magnetic momentshows almost a steplike behavior when the number of for-eign nearest neighbors decreases.In order to explain this change in behavior of the Fe mag-netic moment, we present in Figs. 2 and 3 the Fed-projected paramagnetic densities of state ~DOS! for pureFe ~top panel!, pure V and Cr ~bottom panel!, and for Featoms embedded into V or Cr ~middle panels! with differentnumbers of foreign nearest neighbors. The most prominentfeature of these densities of state is the pseudogap which inbcc metals separate the bonding and antibonding states. Inpure Fe the pseudogap, indicated by the dashed line in Figs.2 and 3, is below the Fermi level which falls at a peak in theantibonding part of the density of states. Hence, the Stonercriterion is satisfied and Fe is ferromagnetic. In pure Cr theFermi level falls in the middle of the pseudogap and theStoner criterion is not satisfied. Instead, Cr is antiferromag-netic. Finally, in pure V the pseudogap is above the Fermilevel and this metal is neither ferromagnetic nor antiferro-magnetic.We first discuss the densities of state for Fe surrounded byV shown in Fig. 2. Here, we observe that for 1 ML of Fe andthe dilute alloy the position of the pseudogap has changedsuch that the density of states at the Fermi level falls wellbelow the value necessary to satisfy the Stoner criterion.Hence, the ferromagnetic moment disappears ~we have so farnot investigated the possibility of an antiferromagnetic solu-tion!. The difference in the DOS between 1 ML of Fe and thedilute ~1% Fe! bulk alloy is due to the different numbers offoreign next-nearest-neighbor atoms, i.e., those atoms whichbelong to the second shell. Comparing the DOS for the 2ML, the 1 ML, and the dilute bulk alloy, we find that theentire Fe d band shifts towards higher energies with increas-ing number of V neighbors. This shift is especially pro-nounced for the antibonding states and as a result thepseudogap moves up through the Fermi level thereby sup-pressing ferromagnetism.In Fig. 3 we show the paramagnetic d-DOS for Fe in a Crsurrounding. In contrast to the V case, the Fe antibondingpeak now remains at the Fermi level both for the case of 1ML of Fe and the 1% alloy. As a result, the Stoner criterionremains satisfied and the magnetic moment at the Fe sitesdoes not vanish.The change in the position of the pseudogap in both Fe-Vand Fe-Cr follows a smoth interpolation between the cases ofpure Fe and pure V or Cr. This is not entirely trivial becausebulk V and Cr do not correspond to the dilute limit of Feimpurities. The position of the Fe d states may, however, beexplained in terms of a two-step model, similar to one usedby Friedel.19 First we observe that the formation of thepseudogap is caused by the geometry of the bcc structure andthat the bonding-antibonding splitting of the Fe states re-mains when Fe is surrounded by V or Cr. Then we note thatthe potentials of pure V and Cr are less attractive in com-parison with bulk Fe, because V and Cr have less nuclearcharges than Fe. As a result the perturbed potential on an Fesite will be less attractive compared to pure bulk Fe. The Fed states will therefore experience a repulsion in a V or Crsurrounding and they will shift to higher energies. This shiftin the position of the Fe d band implies that d states occu-pied in bulk Fe become empty. However, this would lead toan energetically unfavorably large charge transfer and thenumber of occupied states on the Fe atom therefore remainsalmost unchanged.20 The result of the charge conservationand a new, less attractive potential is a shift of mainly theFIG. 2. Paramagnetic Fe d-projected density of states ~DOS! forbulk Fe ~top!, 2 ML of Fe embedded in bulk V, 1 ML of Fe em-bedded in bulk V, 1% Fe dissolved in bulk V, and bulk V ~bottom!.FIG. 3. Paramagnetic Fe d-projected density of states ~DOS! forbulk Fe ~top!, 1 ML of Fe embedded in bulk Cr, 1% Fe dissolved inbulk Cr, and bulk Cr ~bottom!.68 55BRIEF REPORTSantibonding peak and the pseudogap towards higher ener-gies. This is essentially what is seen in Figs. 2 and 3.In summary, we have shown that the magnetic moment ofFe in a V neighborhood approaches zero with increasingnumber of V neighbors while for Fe in a Cr surrounding themagnetic moment decreases linearly to a value of 1.85mB .This change in magnetic behavior, which is in agreementwith experiments, is a consequence of the formation of apseudogap in the bcc state density. This picture should alsoapply to the other Vb and VIb metals.We are grateful to the Swedish Natural Science ResearchCouncil for financial support. Also the support from theSwedish Materials Consortium No. 9 is gratefully acknowl-edged. The Center for Atomic-scale Materials Physics issponsored by the Danish National Research Foundation.*Present address: Massachusetts Institute of Technology, Cam-bridge, MA 02139.1B.T. Matthias, M. Peter, H.J. Williams, A.M. Clogston, E. Coren-zwit, and R.C. Sherwood, Phys. Rev. Lett. 5, 542 ~1960!.2A.M. Clogston, B.T. Matthias, M. Peter, H.J. Williams, E. Coren-zwit, and R.C. Sherwood, Phys. Rev. 125, 541 ~1962!.3D. Riegel, L. Bu¨ermann, K.D. Gross, M. Luszik-Bhadra, and S.N.Mishra, Phys. Rev. Lett. 62, 1989 ~1989!.4P. Lang, B. Drittler, R. Zeller, and P.H. Dederichs, J. Phys. Con-dens. Matter 4, 911 ~1992!.5M. Akai, H. Akai, and J. Kanamori, J. Phys. Soc. Jpn. 54, 4257~1985!.6V.I. Anisimov, V.P. Antropov, A.I. Liechtenstein, V.A. Gubanov,and A.V. Postnikov, Phys. Rev. B 37, 5598 ~1988!.7B. Drittler, N. Stefanou, S. Blu¨gel, R. Zeller, and P.H. Dederichs,Phys. Rev. B 40, 8203 ~1989!.8D.D. Johnson, F.J. Pinski, and J.B. Staunton, J. Appl. Phys. 61,3715 ~1987!.9E.G. Moroni and T. Jarlborg, Phys. Rev. B 47, 3255 ~1993!.10S. Mirbt, O. Eriksson, H.L. Skriver, and B. Johansson, Phys. Rev.B 52, 15 070 ~1995!.11H.L. Skriver and N.M. Rosengaard, Phys. Rev. B 43, 9538~1991!.12O.K. Andersen, Phys. Rev. B 12, 3060 ~1975!.13H.L. Skriver, The LMTO Method ~Springer-Verlag, Berlin,1984!.14O.K. Andersen and O. Jepsen, Phys. Rev. Lett. 53, 2571~1984!.15O.K. Andersen, O. Jepsen, and D. Glo¨tzel, in Highlights ofCondensed-Matter Theory, edited by F. Bassani, F. Fumi, andM.P. Tosi ~North-Holland, New York, 1985!.16O.K. Andersen, Z. Pawlowska, and O. Jepsen, Phys. Rev. B 34,5253 ~1986!.17S.H. Vosko, L. Wilk, and M. Nusair, Can. J. Phys. 58, 1200~1980!.18 I.A. Abrikosov and H.L. Skriver, Phys. Rev. B 47, 16 532 ~1993!.19 J. Friedel, Nuovo Cimento 10, Suppl. No. 4, 287 ~1958!.20 In the actual calculations the valence charge within the Fe atomicsphere varied between 8.3 and 8.0 in the case of a V host,depending on the number of foreign Fe neighbors. For the Crhost this variation was between 8.2 and 8.0.55 69BRIEF REPORTS

Magnetic properties of Fe embedded in V and Cr: Thin films and dilute alloys

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Magnetic properties of Fe embedded in V and Cr: Thin films and dilute alloys

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