PHYSICAL REVIEW B 67, 214431 ~2003!

Two-dimensional magnetically split Fermi surface ofc„2Ã2…MnCu ÕCu„001…

F. Schiller,1 S. V. Halilov,2 and C. Laubschat1

1Institut fur Oberflachenphysik und Mikrostrukturphysik, TU Dresden, D-01062 Dresden, Germany2Center for Computational Materials Science, Naval Research Laboratory, Washington, DC 20375

and DMSE, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA~Received 21 January 2003; published 20 June 2003!

In the c(232)MnCu/Cu~001! surface compound two occupied bands of different spin character are ob-

served close to theM c(232) point of the surface Brillouin zone by comparison of photoemission experimentand theory. Their Fermi edge crossing result in two separate Fermi surface sheets: A holelike Fermi surfaceappears in a band gap close to theLp(131) point of Cu~001! and is interpreted as the majority spin componentof an exchange split Shockley state. The other one is starlike shaped and is built from the minority spin statelocated at the manganese surface sites as concluded from layer-Korringa Kohn Rostoker calculations.

DOI: 10.1103/PhysRevB.67.214431 PACS number~s!: 73.20.At, 75.70.Rf, 71.18.1y

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A challenging aim of current research in magnetism isoccurrence of spin-dependent electron transport throughfaces and interfaces.1 Magnetic surface compounds are inteesting in this aspect for technological applications sincemagnetic phenomena themselves are restricted to the omost atomic surface layer. The quality of magnetic surfacompounds and interfaces often plays a decisive role fohigh level of spin injection in associated devices. Exampare Co/Cu multilayers designed as spin valve,2 or similarmultilayer systems used for the purposes of magneticdom access memory units.3 Main requirement for these materials are the presence of flat atomically abrupt interfacharacterized by strong and well-defined changes in spinlarization. Additionally, in order to contribute to transpoexchange split bands should cross the Fermi energyEF form-ing a spin-polarized density of states~DOS! at EF . Verypromising candidates in this respect are two-dimensio~2D! surface compounds such asc(232)MnCu/Cu~001!.4

Manganese and copper do not form bulk compounds butbe grown artificially in the form of sandwich structures.5,6

Deposition of half a monolayer~ML ! of manganese ontoCu~001! substrate leads to the formation of an orderedc(232) superstructure where the face-centered Cu atomsreplaced by manganese forming a checkerboard structu7,8

as sketched in Fig. 1~a!. Similar manganese basedc(232)surface compounds have been found on the fcc~001! surfacesof the 3d elements Ni,7 Co,9 and Fe~Ref. 10! as well as forthe noble metals Ag~Ref. 11! and Au.12

In c(232)MnCu/Cu~001! manganese atoms are in a higspin ground state that stabilizes the structure and shiftsmanganese atoms out of the surface plane by 0.3 Å4,13

Theoretical investigations predict a ferromagnetic groustate4,14 with local Mn magnetic moments between 3.75mBand 4.09mB .14–17The relatively large variation in the valuereflects the sensitivity of the magnetic moment on the bance between on-site and intersite interactions that are diently handled by the individual theoretical approaches.cent experimental investigations gave evidence for lorange magnetic order below 50 K~Ref. 18! for the MnCusurface compound. Calculations predict an exchange sting of the main Mn 3d band between 2.7 and 4.4 eV,14,15

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with an almost completely occupied majority band aempty states of the minority spin. Inverse photoemiss~PE! experiments found the center of gravity of the Mn 3dderived minority spin bands 1.8 eV above the Ferlevel,15,19 while the spin majority part was observed by Pbetween 3.0 and 3.7 eV below the Fermi energy.15,20While afull separation of the spin-split bands was concluded frthese results, recent studies found parts of the spin minobands to be occupied.17,21

The aim of the present work is to study possible Fermlevel crossings of bands with different spin polarizationthe system. To this end results of spin-integrated angresolved photoemission experiments are compared wthose of spin-resolved layer Korringa-Kohn-Rostok~LKKR ! calculations. Two occupied states of different sp

FIG. 1. ~Color online! Structure ~a! and SBZ ~b! of thec(232)MnCu/Cu~001! system. ~c! Manganese induced changeto photoemission spectra after deposition of 0.5 ML Mn onCu~001! forming the c(232) MnCu surface compound. Thspectra were taken with a photon energy of 21.2 eV along

G2Xp(131) direction.

©2003 The American Physical Society31-1

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F. SCHILLER, S. V. HALILOV, AND C. LAUBSCHAT PHYSICAL REVIEW B67, 214431 ~2003!

character are found atEF close to theM c(232) point of thec(232) surface Brillouin zone of MnCu. One of them forma holelike Fermi surface close to theM c(232) point and isderived from the majority spin component of an exchansplit Shockley state of Cu~001!. The other one is located athe Mn surface sites and reveals minority spin characFrom our calculations a local magnetic moment of 3.83mBfor the Mn atoms and an exchange splitting of 3.5 eV formain Mn 3d-derived bands are predicted.

The photoemission experiments were performed at rotemperature with a two-dimensional high-resolution angresolved hemispherical analyzer~SCIENTA 200!. Thesamples were illuminated by a high-intensity He dischalamb ~Gammadata VUV 5000! with monochromatized photons of hn521.2 eV ~HeIa) and 23.1 eV~HeIb). Energyand angular resolutions were set to 30 meV and 0.3°, restively. The sample was mounted on a six-axis manipulathat provides three independent sample rotations~polar angleu, azimuthal anglew, and tilt anglea). Thec(232)MnCusurface compound was grown on a Cu~001! substrate thatwas previously cleaned by soft argon sputtering~ion energy400 eV! for about 10 min followed by sample annealing~5min, 800 K! to achieve surface ordering. This proceduresulted in a sharpp(131) low energy electron diffraction~LEED! pattern with low background intensity. Cleanlinewas checked by observing theXp(131) and M p(131) surfacestates and the absence of the typical oxygen derived sigat about 6 eV binding energy in the PE spectra. The Feenergy positionEF was determined by the inflection point othe always visible Fermi edge atkW points far away fromFermi level crossings of Cu derived bands~e.g., ki50) toavoid line-shape problems. Thec(232)MnCu surface com-pound was grown by depositing half a monolayer Mn onthe Cu~001! surface at room temperature. Manganese wevaporated from a tantalum crucible. During the depositthe base pressure of 631029 Pa rose into the 1028 Pa range.The deposition rate was determined by the inspection ofLEED pattern using the fact that the bestc(232) pattern isobtained for 0.5 ML Mn.7 No traces of contamination werobserved in PE spectra after deposition. In earlier photoemsion experiments the binding energy range between 2 aeV, where the Cu~001! 3d bands lie, was found to be nearunaffected by Mn.6,20 Mn-induced changes occur around tXp(131)5M c(232) point of the surface Brillouin zone~SBZ!as shown in the photoemission spectra of Fig. 1~c!.22 Figures2~a! and 2~b! show the experimentally observed dispersioin a density plot as derived from the photoemission enedistribution curves~EDC! at a photon energy of 21.2 eV~HeIa) along GXp(131) for clean Cu~001! and for thec(232)MnCu/Cu~001! system, respectively. A color scale is usedindicate the PE intensity. For the pure Cu~001! surface strongemissions from a bulk band are observed~labeleda) thatapproach the Fermi energy without crossing forming the wknown ‘‘neck of the dog bone’’ shaped structure of the Fersurface cut@compare it with Fig. 3~a!#. Two other featuresare worth to note: First there exists an additional weak emsion ~labeledU) dispersing also towards the Fermi leveThis feature was predicted but not observed in previ

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studies.23 The second feature is the Cu-derived Shockley sface state around theXp(131) point ~labeledb) ~Refs. 24,25!that indicates a well ordered surface. Lying close aboveFermi level this state becomes partly occupied due to therexcitation.26 Previously this state was reported to be belothe Fermi level.24 A possible explanation for this discrepancis the presence of impurities below the detection limit lihydrogen that may change the work function and affthereby the position of the Shockley state.30 Another possi-bility lies in the different methods applied to the determintion of the Fermi level positions. In earlier photoemissiexperiments the high kinetic energy PE onset was uwhich results in higher binding energy positions than tmethod used in this study.

Mn deposition of half a monolayer results in additionstates as seen in the spectral density plot derived from theEDC’s shown in Fig. 2~b!. For comparison the Cu~001! banddispersions from Fig. 2~a! are shown as dotted lines. Thformer Shockley state (b) of Cu is now shifted towardshigher binding energies. The magnitude of this shift isaccordance with the change of work function.17 In addition,there appears a second stateg at higher binding energy thareveals weaker dispersion.

In order to analyze the experimentally observed featuPE spectra have been calculated on the basis of a LKformalism~see Refs. 17,27 for a detailed description!. Basedon a multiple-scattering technique, the method treats dynics of the photo-excited electrons as propagating througcomplex potential, which simulates the bulk and the surfaon the same footing~‘‘one-step’’ PE model!. Appropriateboundary conditions at the surface are transmission reflecfor PE and total reflection for the layer resolved densitystates~LDOS!. While the real part of the potential is treatewithin the local spin-density approximation to densitfunctional theory, the imaginary part of the self-energysponsible for finite lifetime of quasiparticles, is introducefrom the best matching to experimental broadening. Forphotocurrent calculations, the energy dependence ofimaginary potential was assumed to vary according0.05(E2EF) eV for hole excitations, i.e., for states witenergy belowEF , and -1 eV for the electron excitations, i.eunoccupied states. An energy independent imaginary potial of -0.1 eV was used for the LDOS calculations atenergies in order to prevent from missing surface staThere is a variety of deficiencies which mitigate the trational three-step model from a plausible interpretation ofPE results. Within this method, the initial and final statesthe PE process are assumed to be Bloch states with annite lifetime. As a result, the method is not capable to dscribe transitions into evanescent bandgap states, thespectra from surface states are out of scope as well.so-called one-step model being free of these difficultieschosen as a conceptual framework for the PE calculatioThe method is a multiple-scattering or dynamic approaclosely linked to low energy electron diffraction~LEED!theory. The great advantage from LEED theory clearly cosists in the possiblity of treating all scattering events succsively by starting with the single ion core scattering aending up with the multiple scattering within a layer an

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TWO-DIMENSIONAL MAGNETICALLY SPLIT FERMI . . . PHYSICAL REVIEW B 67, 214431 ~2003!

between the layers of the truncated crystal. Particularly,final states within the one-step model are treated as tireversed LEED states~see Ref. 27, and references therei!,instead of conventional Bloch states used in the three-model. Thus, the effects of self-energy corrections for eltrons and holes are treated dynamically on an equal foot

The calculated PE spectra of thec(232)MnCu/Cu~001!system reveal a strong exchange splitting of the Cu-deriShockley state, from which the majority spin componentoccupied while the minority spin part lies above the Ferenergy~labeledb2). This is illustrated in Fig. 2~c!, where thecalculated LKKR PE spectra are given in a density plot us

FIG. 2. ~Color! Experimentally measured band dispersion in t

G2Xp(131) direction in Cu~001! ~a! and c(232)MnCu ~b! in adensity plot (hn521.2 eV! using the indicated color scale as compared to the result of LKKR PE calculations forc(232)MnCu ~c!.~d! the corresponding calculated LDOS at the Mn surface site althe two white lines indicated in~c!.

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the same color scale representation as the experimentalFigure 2~d! shows the spin-resolved atomic LDOS at the Msurface sites for two polar angles of the electron emissiouindicated by white lines in the photocurrent calculation. Tenergy positions of peaks in the calculated PE spectraindicated by dashed lines in the LDOS. Note that some vintense and sharp maxima of the LDOS do not contributethe PE signal due to symmetry selection rules and crosection effects. In the PE calculations three bands with pbolic dispersions are observed around theXp(131) point. Two

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FIG. 3. ~Color! Experimental Fermi surface cuts of~a! Cu~001!and ~b! c(232)MnCu/Cu~001! taken at hn521.2 eV, ~c! and ~d!the Shockley state region of Cu~001! andc(232)MnCu taken at aphoton energy of 23.1 eV,~e! and~f! the region of the Mn-inducedminority spin state of Cu~001! andc(232)MnCu, respectively.~g!and ~h! show Fermi surface cuts of Cu~001! and the surface compound as obtained from LKKR calculations.

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F. SCHILLER, S. V. HALILOV, AND C. LAUBSCHAT PHYSICAL REVIEW B67, 214431 ~2003!

of them represent the spin-split Shockley state discusabove~labeledb andb2). A third state,g, crosses the Fermenergy at 0.9 and 1.6 Å21. This state reveals minority spicharacter and is localized at the Mn surface sites. Experimtally it is observed only for high-qualityc(232) surfaces atcoverages very close to half a monolayer Mn.17 While in thepresent experiment this state is masked at low emisangles~low kW values! by the intense Cu-derived bulk emissions, it may be observed there by using higher photonergies, e.g.,hn532 eV.17

While band structure investigations along the high symetry directions give a good insight into the band dispsions and chemical bonding, another very important aspfor the characterization of the electronic structure isFermi surface that describes the distribution of Fermi-lecrossings inkW space. Figure 3 shows the photoemissiontensity distribution atEF ~Fermi surface cut! as a function ofthe electron wave-vector component parallel to the surfki ,x,y . Figures 3~a! and 3~b! compare Fermi surface cuts foCu~001! and c(232)MnCu/Cu~001! as measured at a photon energy of 21.2 eV. Experimentally only a 90° sector wmeasured and the full Fermi surface cut was reconstruapplying symmetry operations. As in the band dispersithe main copper derived structures are preserved upondeposition. Additionally, some new features appear inmarkedkW region close to theXp(131) point that are furtheremphasized in Figs. 3~c!–3~f!. The Shockley state regions oCu~001! andc(232)MnCu/Cu~001! presented in Figs. 3~c!and 3~d! were taken at a photon energy of 23.1 eV~He Ib).This photon energy was chosen by two reasons:~i! the vis-ible band gap around the CuXp(131) point is largest becausthe transitions take place very close to theLp(131) point ofthe three-dimensional Cu Brillouin zone~necks of the Fermisurface! and ~ii ! the intensity of the Shockley state is largthan at 21.2 eV photon energy.25 For pure Cu~001! theShockley state only touches the Fermi energy, as was alreshown in Fig. 2~a!. From the ellipsoidal form in the Fermsurface cut it becomes evident that the dispersion relatiodistinct for theGXp(131) and XM p(131) high symmetry di-rections indicating different effective masses. This is evmore pronounced for the respective Shockley state ofc(232)MnCu surface compound. As obtained both by eperiment and LKKR calculation the Fermi surface builtthe majority spin component of the Shockley state revealselliptical shape around theXp(131)-point @Figs. 3~d! and3~h!#. A simple least squares fit of the experimental datathe surface compound results in an effective electron mas

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m* 5(0.1360.01)Me along the GXp(131) direction whereMe denotes the free electron mass. The effective elecmass in XM p(131) direction, however, amounts tom*5(0.2960.01)Me . The second state,g, that crosses theFermi energy at higherkW i values in Fig. 2~b!, is seen in moredetail in Fig. 3~f! while Fig. 3~e! shows the same region opure Cu~001!. Apart from the ‘‘dog bone’’ structure of theCu~001! Fermi surface, there already exist additional featuin the Fermi surface cut of pure Cu at higher emission angoutside the high symmetry directions. Such features moriginate from indirect transitions28 and are also observed ithe Fermi surface cuts obtained from our LKKR PE calcutions, which are shown in Figs. 3~g! and 3~h!. In the Fermisurface cut of thec(232)MnCu surface compound thesfeatures are preserved but with lower intensity. Additionathere appear Mn-induced structures marked by solid linAccording to our band structure investigations this partthe Fermi surface is derived from the minority spin band tis localized at the Mn surface sites. Translational displaments of the observed minority spin Fermi surface featuby reciprocal lattice vectors into all parts of thec(232)SBZ results in a starlike Fermi surface as indicated inlower part of Fig. 3~f! by solid lines. In the first SBZ thisminority spin Fermi surface has only low intensity. Selectirules may be responsible for quenching the PE emissionthis region.29

In summary, both our conventional PE experimenas well as Fermi surface measurementsc(232)MnCu/Cu~001! are in excellent agreement with results of spin-resolved LKKR calculations, emphasizing thigh quality of the synthesized 2D magnetic compoundcombination of Fermi surface mapping and LKKR analyhas been found to be a powerful tool to investigate surfmagnetism without applying explicit spin-resolved expemental methods. In contrast to previous findings, statesboth spin characters are partially occupied in this surfcompound but well separated from each other both in eneandkW space. The Fermi surface built from the majority sppart of the Shockley state inc(232)MnCu/Cu~001! is ellip-tic around theXp(131) point. Furthermore, a Mn-induced mnority spin band of thec(232)MnCu/Cu~001! surface com-pound was observed and the respective Fermi surfacefound to be star-like shaped around theM c(232)5Xp(131)point.

The work was supported by the Deutsche Forschungsmeinschaft, Contract No. LA 655/6-3.

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