Formation of Mn3O4„001… on MnO(001): Surface and interface structural stability

Veronika Bayer and Raimund PodlouckyInstitut für Physikalische Chemie and Center for Computational Materials Science, Universität Wien, Sensengasse 8/7, A-1090 Wien,

Austria

Cesare Franchini*Department of Computational Materials Physics and Institut für Physikalische Chemie, Universität Wien, Sensengasse 8, A-1090 Wien,

Austria

Francesco Allegretti,† Bo Xu, Georg Parteder, Michael G. Ramsey, Svetlozar Surnev, and Falko P. NetzerInstitut für Experimentalphysik, Karl-Franzens-Universität Graz, Universitätsplatz 5, Graz A-8010, Austria

�Received 29 June 2007; published 24 October 2007�

X-ray absorption and photoemission spectroscopies, high-resolution electron energy loss spectroscopy, spotprofile analysis low energy electron diffraction, and density functional theory calculations are employed tostudy the growth of �001� oriented Mn3O4 surfaces on a Pd�100�-supported MnO�001� substrate, with theHausmannite planar lattice constants aligned along the �110� direction of the underlying MnO�001� support.We show that despite the rather large lattice mismatch, abrupt interfaces may exist between rocksalt MnO andHausmannite. We argue that this process is facilitated by the relatively low computed strain energy and wepropose realistic models for the interface. An atop site registry between the Mn�O� atoms of the oxygen richMn3O4 termination and the MnO�001� O�Mn� atoms underneath is found to be the energetically most favorableconfiguration. The significant planar expansion is accompanied by a large compression of the Mn3O4 verticallattice constant, yielding structural distortion of the O-Mn-O octahedral axis. Spot profile analysis low energyelectron diffraction experiments show that the conversion reaction proceeds easily in both directions, thusindicating the reversible redox character of the transition.

DOI: 10.1103/PhysRevB.76.165428 PACS number�s�: 68.35.�p, 68.47.Gh, 61.14.�x, 79.60.�i

I. INTRODUCTION

Hausmannite Mn3O4 is a good prototype of spinel com-pounds displaying a Jahn-Teller distortion. It is widely usedas a catalyst for several processes1,2 and has gained consid-erable attention for its exotic canted ferrimagneticstructure.3,4 More recently, attempts have been made to pre-pare Mn3O4 magnetic nanorods5 and nanoparticles.6 Despitethe number of experimental and theoretical investigations onthe bulk phase of Mn3O4,3,4,7–9 there is a dearth of studiesdealing with the surface properties of this system. Like otherspinel-like structured compounds,10 Mn3O4 terminations be-long to the intriguing class of polar surfaces, formally char-acterized by an uncompensated electrostatic potential givingrise to a dipole moment perpendicular to the surface.11 How-ever, it has been noted12 that this formal instability is anartificial consequence of the oversimplistic ionic model andcan be removed by a number of mechanisms involvingchanges localized near the surface, such as charge redistribu-tions, reconstructions, and the interaction with foreignatoms.13

In contrast to the formal unrealistic instability, it is re-ported that the cleavage of Mn3O4 leads distinctly to �001�oriented surfaces.14 With the progress of thin-film technol-ogy, epitaxial growth of Mn3O4�001� has been achieved onMgO�100� as driven by the favorable matching between theMgO and Mn3O4 lattice constants.15 Formation of Mn3O4films �110� oriented on SrTiO3�110� has been alsoreported.16 Circumstantial evidence indicates thatMn3O4-like surfaces can be obtained by mild oxidation ofsingle crystal MnO�001�.17 In general, however, extensiveapproaches to the structural stabilization of Mn3O4 surfacesare missing in the literature.

In this context, the preparation and characterization of amagnetic metal oxide epitaxial film have become very im-portant. Though these systems can be investigated by stan-dard surface science techniques, the preparation of definedoxide surfaces is difficult and critically depends on prepara-tion conditions such as oxidation temperature and oxygenpartial pressure. This renders the interpretation of the experi-mental results not easy. To improve and reinforce under-standing, it turned out that the combination of experimentalinvestigations together with state-of-the-art computationalmethods provides a powerful tool capable of disclosing thepeculiar traits of surface science problems. Density func-tional theory �DFT�-based calculations on this class of mate-rials have been extensively performed for magnetite �Fe3O4�for both the �001� �Refs. 18 and 19� and �111� �Refs. 20 and21� terminations as well as for the formation of polar inter-faces assembled with Fe3O4 films.10 Still, the application ofcomputational techniques on Hausmannite surfaces is absent.This lack of knowledge is a boost for new work aiming toenrich our understanding of magnetic metal oxide perovskitefilms.

In our previous studies, we have focused on the epitaxialstabilization and the ground state properties of low indexneutral and polar MnO surfaces.22–24 Most importantly forthe present purposes, we demonstrated that well-orderedMnO�001� and MnO�111� films can be epitaxially grown ona Pd�100� substrate under suitable preparation conditions.24

Here, we show that Pd�100�-supported MnO�001� films canbe easily converted into Mn3O4�001� surfaces by high tem-perature oxidation at intermediate oxygen pressure. The evo-lution of the x-ray absorption and photoemission spectra forthis transition indicates a clear separation between the two

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phases, giving rise to a sharp interface involving no conver-sion into other intermediate chemical species. In combina-tion with this experimental characterization, we also report afirst principles analysis of the structural stability of the lowindex Mn3O4 surfaces, which provides energetic argumentsfor the higher stability of the �001� orientation. In addition,inspired by our experimental findings, we propose a struc-tural model for the Mn3O4�001� /MnO�001� interface. Wedemonstrate that, despite the rather large lattice mismatch of�9%, MnO�001� is a favorable support for the formation ofMn3O4�001� /MnO�001� interfaces thanks to the relativelylow strain energy required to stretch the planar Mn3O4 latticeconstant.

The paper is organized as follows. In the next section, weintroduce the description of the experimental techniquesalong with the complementary computational DFT-based ap-proach employed. The results are discussed in Sec. III andcorresponding subsections. The main relevant conclusionsare drawn in Sec. IV.

II. TECHNICAL ASPECTS

A. Experimental procedure

To prepare manganese oxide nanolayers with Mn3O4 sto-ichiometry, the following procedure has been adopted. MnOfilms with typical thickness of 20 ML �monolayers� and�001� growth direction were initially deposited on a cleanPd�100� surface following our established procedure,24

which involves reactive evaporation of Mn metal in oxygenatmosphere at moderate pressure �5�10−7 mbar� and subse-quent annealing in vacuum up to 770 K. The Mn depositionrate was monitored by a quartz crystal microbalance: anevaporation rate of 1 ML min−1 was usually employed, with1 ML containing 1.32�1015 Mn atoms/cm2 as referred tothe Pd�100� surface density. The well-ordered MnO�001�films were subsequently postoxidized at higher O2 pressure�5�10−6–2�10−5 mbar�, keeping the substrate at 770 K fortypically 20–30 min and then allowing it to cool down toroom temperature before closing the oxygen flux.

The stoichiometry of the resulting Mn3O4 films and theirelectronic and phonon structures were elucidated by high-resolution x-ray photoemission spectroscopy �HR-XPS� andx-ray absorption spectroscopy �XAS� and high-resolutionelectron energy loss spectroscopy �HREELS�, respectively.Spot profile analysis low energy electron diffraction �SPA-LEED� was employed to monitor the structural transitionupon oxidation of MnO�001� and to investigate the long-range order quality and the surface symmetry.

The HR-XPS experiments were carried out at the beam-line I311 at the Swedish Synchrotron Radiation FacilityMAX II in Lund, recording photoemission spectra from theMn 2p, O 1s, Mn 3s core levels, and from the valence band,with photon energies of 750–800, 620, 180, and 120 eV,respectively; a total resolution in the range between 300 and100 meV has been used. The XAS spectra were measured atroom temperature on the high energy branch of the APEbeamline at the synchrotron radiation facility ELETTRA inTrieste and also at the beamline I311 at MAX II. The

HREELS measurements were performed using an ErEELSspectrometer described elsewhere.25 In particular, theHREELS spectra were taken with a primary energy of5.5 eV in specular reflection geometry �in=�out=60°, and atypical resolution of 5 meV was achieved, as measured at thefull width at half maximum of the reflected primary peak.Finally, the electron diffraction experiments were performedin a UHV dedicated chamber, equipped with a commercialSPA-LEED apparatus manufactured by Omicron, as reportedpreviously.26 The transfer width of the SPA-LEED instru-ment was �1000 Å, as determined with a Si�111� sample.

B. Computational details

For the present calculations, the Vienna ab-initio simula-tion package �VASP� program27,28 within the projector-augmented-wave method29,30 has been adopted. Our calcula-tions were carried out within the generalized gradient spindensity approximation �SGGA� to the DFT in the Perdew-Burke-Ernzerhof �PBE� parametrization scheme.31 We em-ployed the optimized PBE lattice constants �listed in Table I�and the most favorable theoretical ferrimagnetic orderings, ascomputed in Refs. 9 and 32. To verify the quality of theoptimized PBE structure, we have also carried out SGGA+U calculations within the approach of Duradev et al.33 forthe most favorable Mn3O4 surface andMn3O4�001� /MnO�001� interface. We adopted the value U−J=5.0 eV.

As regards the geometrical setup, the Mn3O4 surfaces andthe Mn3O4�001� /MnO�001� interface have been modeled byrepeated slabs separated by a vacuum region of 10–15 Å.The three bulklike bottommost layers were kept fixed, exceptwhen otherwise stated. To relax the remaining atomic posi-tions �the topmost 5–12 layers, depending on the model�, weused the interatomic forces calculated through the Hellmann-Feynmann theorem and the geometry was optimized until thechange in the total energy was smaller than 10−3 eV betweentwo consecutive ionic configurations. Most calculations wereperformed using a 4�4�1 Monkhorst-Pack k�-point gridand a plane wave energy cutoff of about 280 eV. As will bediscussed later, to investigate the energetics, various termi-nations and interface stackings were used.

To evaluate the relative stability of the different surfacemodels, we made use of the generalized surface energy �,

� = �Eslab − nO12EO2

− nMnE�Mn�/A , �1�

where Eslab is the DFT total energy of the given Mn3O4model containing nO and nMn oxygen and manganese atoms,

TABLE I. Computed lattice constants of MnO and Mn3O4 alongwith the experimental values taken from Refs. 34 and 35,respectively.

Bulka

��c

��

MnO PBE 4.369

Expt. 4.44

Mn3O4 PBE 5.844 9.498

Expt. 5.76 9.46

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respectively. EO2and E�Mn

are the O and Mn referenceenergies,22 whereas A is the surface of the two-dimensional�2D� unit cell.

III. RESULTS AND DISCUSSION

We begin the presentation of our results by providing ex-perimental evidence for the formation of Mn3O4�001�-likesurfaces on a MnO�001� substrate. In the subsequent part�Sec. III B�, we show that the Mn3O4�001� oxygen rich ter-mination is the most favorable Mn3O4 surface among the lowindex models selected. Finally, to gain more insight into theexperimentally observed Mn3O4�001� /MnO�001� interface,we focus our attention on the search of theoretically stablemodels for this interfacial phase.

A. Growth of Mn3O4„001… by oxidation of MnO(001):Experimental evidence

In this section, we demonstrate that a Mn3O4 phase withwell-defined stoichiometry and �001� surface symmetry canbe grown by oxidation of Pd-supported MnO�001�. As wehave shown,24 20 ML thick MnO films display a bulklikebehavior in terms of both their spectral fingerprint and in-plane lattice constant �a=3.14±0.03 Å�. In particular, in Fig.1�a� �bottom�, the Mn 2p3/2 photoemission spectrum exhibitsthe characteristic doublet structure attributed to local andnonlocal screening effects and the charge transfer satellite

�S� at about 6 eV higher binding energy, which are observedfor bulk crystals, while in the Mn 3s spectrum �Fig. 1�b��, theexchange splitting amounts to 6.0 eV, a value entirely com-patible with the monoxide stoichiometry. After 20 min expo-sure to molecular oxygen at elevated pressure �2�10−5 mbar� and high sample temperature �770 K�, theMn 2p3/2 photoemission spectrum is markedly changed: Themain line appears distinctly broadened with its center ofmass shifted by �0.8 eV to higher binding energy, and thesatellite S is completely quenched. These changes are con-sistent with the presence of higher manganese oxidationstates, as reported in the literature.17,36 Accordingly, the split-ting of the Mn 3s core level decreases to 5.4 eV �Fig. 1�b��:as the Mn 3s splitting originates from the exchange couplingbetween the 3s hole created after the photoemission and the3d electrons, it is expected to be the highest for MnO, due tothe 3d5 high spin configuration of the Mn2+ ion, and to de-crease for higher Mn oxidation states. However, since themeasured experimental values7,36–38 of the exchange splittingfall typically in the range 5.3–5.5 eV for Mn3O4,5.2–5.7 eV for Mn2O3, and 4.5–4.9 eV for MnO2, the5.4 eV value that we have reproducibly observed is compat-ible with both Mn2O3 and Mn3O4 and does not allow anunambiguous assignment of the stoichiometry of the newphase.

The effect of the oxidation on the valence band photo-emission spectra is shown in Fig. 1�c�. In the region between2 and 7 eV below the Fermi level, whose features are mainlyascribed to the Mn 3d multiplet structure in the ligand field

645650 640 635

630

0 50 100 200150

635 640 645 655 660 665650

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Loss Energy (meV)

Binding Energy (eV)

MnO(001)

MnO(001)

MnO(001)

MnO

MnO(001)

95 85 7590 80

hν=180 eVMn 3s

HREELS

hν=750 eV3/2Mn 2p ∆E =5.4 eVs

∆E =6.0 eVs

oxidized phase:

oxidized phase:

(b)

(d)

(e)

(a)

0510202530 15

Binding Energy (eV)

hν=120 eVvalence band (c)

L2

L3

Mn O3 4

Mn O3 4

Mn L -edge XAS2,3

Mn O3 4

Mn O3 4

oxidized phase:

Mn O3 4

48

65

130

x2

x2 67

83

150 166

FIG. 1. �Color online� Comparison between 20 ML MnO�001� layers on Pd�100� and the Mn3O4 phase prepared by oxidation ofMnO�001� at the conditions described in the text. �a� Mn 2p3/2 core level spectra excited by a photon energy of 750 eV. The peak indicatedby S corresponds to the charge transfer satellite of MnO. �b� Mn 3s core level spectra excited by a photon energy of 180 eV. Thecorresponding exchange spin splitting is indicated. �c� Valence band spectra excited by a photon energy of 120 eV. �d� Mn L2,3 edge XASspectra in total yield mode. �e� HREELS spectra. The energies of the main energy losses are reported.

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overlapping with O 2p states,39 a clear redistribution of in-tensity is observed, together with the appearance of a strongshoulder at 7.2 eV. The O 2s photoemission line at about22 eV appears also changed in shape and intensity. Impor-tantly, the structure at 10.5 eV—initially attributed to defectstructures or impurity phases40 and afterward recognized asan intrinsic part of the MnO spectrum39—is completely re-moved after oxidation. In fact, it has not been observed in theXPS valence band of Mn3O4.39

The unambiguous assignment of the stoichiometry of theMn oxide phase after oxidation is provided by the x-ray ab-sorption at the Mn L2,3 edge. The XAS spectra reported inFig. 1�d� were recorded at the APE beamline at ELETTRA intotal electron yield mode by measuring the sample drain cur-rent with the photon beam normal to the sample surface andan energy resolution of approximately 0.15 eV. The L3 andL2 white lines are shown after normalization to the incidentphoton beam and after subtraction of a linear background. Asis evident, the characteristic fingerprint of MnO �Fig. 1�d�,bottom� is completely lost upon oxidation �Fig. 1�d�, top�,and the intensity ratio of the L3 to L2 lines is reduced from�3.8 to �2.6. Since the I�L3� / I�L2� ratio can be related tothe occupancy of the 3d orbitals on the Mn ion,41 the overallchanges are again consistent with the �partial� presence ofMn ions with 3d4 configuration. In particular, the profile of

the L3 peak is considerably broadened and exhibits a tripletfine structure which is a distinctive signature of Mn3O4.42

The Mn3O4 stoichiometry of the oxidized film is also con-firmed by the HREELS phonon spectrum �Fig. 1�e��. Whilethe MnO�001� film is characterized by a Fuchs-Kliewer pho-non loss peak at �65 meV �Ref. 24� and a much weakerstructure at �48 meV �the peak at 130 meV is a double lossof the 65 meV peak�, the oxidized film is dominated by anew phonon loss at 83 meV, analogous to the sharp vibra-tional peak at 650–654 cm−1 ��81 meV� that has been de-tected by Raman spectroscopy from mineral and syntheticHausmannite Mn3O4 crystals.43,44 This phonon value is alsoin good agreement with data on Mn3O4 nanorods45 and withsimilar vibration modes reported for Co3O4�110� �84.6 meV,Ref. 46� and Fe3O4 �83 meV, Ref. 47�. This phonon loss hasbeen assigned to the A1g mode,44,47 which corresponds to themetal-O breathing vibration of the divalent metal ions intetrahedral coordination and is characteristic of all spinel-likestructures.47,48

By means of SPA-LEED, we now address the issue of thesurface symmetry and growth direction of the Mn3O4 film. InFig. 2, we show the drastic change in the LEED pattern uponoxidation: the sharp �1�1� square pattern characteristic ofwell-ordered MnO�001� develops into a weaker �2�2�-likepattern with broader spots, which preserves the square sym-

[110]

[110]

100%

100%

0%

0%

-100% 100%0%-100%

-100%

(a)

[110]

[110]

(b)

K (% MnO(001) BZ)

100500-50-100

SPA-LEED scan along [110]

MnO(001)

3 4Mn O (001)

(c)

Intensity(arb.units)

FIG. 2. �Color online� 2D SPA-LEED pattern �a� of 20 ML MnO�001� layers on Pd�100� and �b� of the Mn3O4�001� phase prepared byoxidation of MnO�001� at the conditions described in the text. Both diffraction patterns were recorded with an electron energy of 125 eV.�c� Corresponding 1D SPA-LEED profiles centered on the �00� reflex and recorded along the �110� substrate direction at Ep=125 eV.

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metry of the unit cell but signals a larger periodicity in thereal space. However, this periodicity does not correspond toan exact �2�2� pattern: a line scan along the �110� symme-try direction of the �001� substrate �Fig. 2�c�� shows that theapparent �2 spots are the first order spots of Mn3O4, whichare located at �53±1�% of the MnO�001� Brillouin zone andin real space describe a square lattice with a unit cell param-eter of 5.9±0.1 Å. From the geometry of the bulk Hausman-nite spinel structure, we therefore infer that the Mn3O4 filmis �001� oriented, with the a and b sides of the unit cellaligned along the �110� directions of the MnO�001� substrate.Note that even with the large ±0.1 Å experimental error�larger than the typical uncertainty of SPA-LEED measure-ments, due to the poor definition and k-space position of thefirst order diffraction spots in the present case�, our datademonstrate that the supported Mn3O4 phase grows strainedrelative to the bulk phase �5.76 Å�. This is likely to originatefrom the 9% lattice mismatch with the �2 supercell of theunderlying MnO�001� and from the relatively low energycost required to expand the lattice, which we will discusslater on. The rather streaky LEED pattern and the consider-able increase of the �00� spot width upon oxidation also sug-gest a reduction of the long-range order probably accompa-

nied by a change of morphology in the oxide phase. In fact,dramatic morphological changes have been observed inatomic force microscopy data, which will be publishedelsewhere:49 these data show that while the MnO�001� sur-face consists of atomically flat terraces with lateral dimen-sions of up to 500 Å �Ref. 24�, the Mn3O4 phase develops inform of elongated crystallites with the main axis orientedalong the �110� symmetry directions. This change of mor-phology and the reduction of surface order may be related tothe lattice strain relief during the epitaxial growth.

To obtain further insight into the MnO�001�→Mn3O4�001� transition, we investigated the evolution ofthe process as a function of sequential exposures ofMnO�001� to oxygen at 770 K. The results are illustrated inFig. 3. The x-ray absorption and photoemission experimentswere performed at the Max II synchrotron in Lund, with theMn L2,3 edge and O K edge spectra in Figs. 3�a� and 3�b�recorded at normal incidence in partial electron yield mode�Auger yield spectra recorded simultaneously did not showappreciable differences in the evolution�. The reported expo-sure values give merely an approximate estimate of the realoxygen exposure, as they only refer to the exposure at 770 Kin 5�10−6 mbar O2 �in fact, a lower oxygen flux was main-

30 25 20 15 10 5 0

Binding Energy (eV)

valence bandhν=120 eV

(d)

548544540536532528

Photon Energy (eV)

b 50 La MnO(001)

b 50 La MnO(001)

d subtraction: c-a

d subtraction: c-a

c 250 L

c 250 L

O K-edge XAS (a)

656652648644640636

Photon Energy (eV)

Mn L -edge XAS2,3

Mn 2p

hν=800 eV

(c)

660 655 650 645 640 635

Binding Energy (eV)

hν=800 eV

e 750 L

e 750 L

e 750 L

e 750 Lf 1500 L

f 1500 L

f 1500 L

f 1500 L

(b)

c 250 Lc 250 L

b 50 L

b 50 L

a MnO(001)

a MnO(001)

Intensity(arb.units)

Intensity(arb.units)

FIG. 3. �Color online� Modification of thespectral signature of a 20 ML thick MnO�001�film under sequential exposure to molecular oxy-gen at 770 K and 5�10−6 mbar: �a� O K edgeXAS, �b� Mn L2,3 edge XAS, �c� Mn 2p core lev-els excited by a photon energy of 800 eV, and �d�valence band photoemission spectra excited by aphoton energy of 120 eV. The total exposure foreach oxidation step is given in langmuir units.The curves labeled a refer to the bare MnO�001�;the curves labeled b, c, e, and f correspond to 50,250, 750, and 1500 L exposures, respectively;curves d in �a� and �b� are magnified differencespectra obtained by subtracting the MnO spectra�appropriately attenuated� from curves c.

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tained during the cooling down to prevent surface reduction�.After 50 L �1 L=10−6 Torr s� exposure, only minor modifi-cations are observed in the XAS spectra and in the Mn 2pand valence band photoemission, with a marginal reductionin intensity of some of the main spectral features. The mostnotable change is the small intensity increase in the pre-edgeregion of the O K edge spectra of MnO �Fig. 3�a�, curves aand b�, signaling the incorporation of oxygen into the sur-face, as also corroborated by a corresponding increase in theO 1s core level photoemission �not shown�. Although imagesacquired with a standard LEED apparatus do not show anyclear extra spot superimposed onto the MnO�001� pattern,accurate measurements of SPA-LEED line profiles �notshown� indicate that a superstructure with square unit cell ofside 6.0±0.15 Å is locally present already at the early stagesof oxidation. Both the pre-edge feature, analogous to thatobserved in the oxidation of iron oxides,50 and the SPA-LEED data therefore suggest that the formation ofMn3O4�001� proceeds from the very beginning byoxidation—at least locally—of the topmost surface layers.This picture is reinforced by additional exposure to 200 L,which produces more drastic spectroscopic changes �curveslabeled c in Fig. 3�. In particular, the profile of the Mn 2p3/2line loses the characteristic doublet structure of MnO andshifts to higher binding energy, while in the Mn 3d region ofthe valence band, redistribution of intensity occurs, and inthe O K edge spectrum, the pre-edge feature mentionedabove increases further. Although the shapes of the Mn L2,3edge and O K edge spectra still bear strong resemblance withthe corresponding MnO profiles, the difference spectra ob-tained by subtracting the MnO spectra �properly weighted�from the original data �curves d in Figs. 3�a� and 3�b�� showunequivocally the characteristic fingerprint of Mn3O4 �Refs.41 and 42� �for comparison, see also curves e and f in Figs.3�a� and 3�b� obtained after further exposures�. As the 2Dpattern recorded with conventional LEED at this stage is stilldominated by the MnO�001� diffraction spots, which appearonly slightly attenuated in intensity, the evidence presentedabove indicates that the two Mn oxide phases coexist in thesystem, with the underlying MnO�001� retaining its well-ordered structure.

The exposure to a total amount of 750 L �curves e in Fig.3� marks the transition to the Mn3O4-like film, at least withinthe probing depth of the different experimental techniques,with the LEED pattern now exhibiting the first order spots ofMn3O4�001� and the photoemission and absorption spectrabearing no memory of the initial MnO�001� profiles. Furtherexposure �curves f in Fig. 3� produces only marginal changesin the spectral signature, although the complete conversionof the film to Mn3O4 is probably not yet accomplished, as itis seen by comparison of Figs. 2 and 3. In fact, the Mn 3ssplitting corresponding to the f curves in Fig. 3 is 5.6 eV,slightly higher than that of pure Mn3O4.

Interestingly, the exposure-dependent evolution of the va-lence band spectra reported after normalization to the highenergy tail shows clearly that the curves cross each other at anumber of common points �indicated by arrows in Fig. 3�d��,no matter what the oxygen exposure.51 The occurrence ofsuch isosbestic points in photoemission has been treatedthoroughly by Anderson and Nyberg,52 who demonstrated

that the crossing at common points is not accidental but in-stead related to the presence of a single adsorbate phaseoverlying the substrate. More generally, in chemistry, isos-bestic points �for example, in a set of absorption spectra� areused as evidence that a chemical reaction involving conver-sion of a chemical species into another proceeds without theformation of intermediate products. In the present case,therefore, the occurrence of isosbestic points in the valenceband spectra also points toward the direct and full conversionof MnO�001� into Mn3O4�001� within the probing depth ofthe photoemission process. Moreover, SPA-LEED experi-ments carried out in parallel showed that the conversion re-action proceeds easily in both directions, such that annealingin UHV of Mn3O4�001� at 900–950 K leads to a pure andwell-ordered MnO�001� film, which in turn can be readilyreoxidized to the original Mn3O4�001�. A rationale for thereversible character of the transition may be recognized inthe presence of common structural aspects, leading naturallyto the conversion of MnO�001� into Mn3O4�001� and viceversa through the formation of a sharp and ordered interfacebetween the two phases. These aspects will be investigatedtheoretically in Sec. III C.

B. Stability of low index Mn3O4 surfaces

The slab models utilized for the considered low index�001�, �110�, and �100� surfaces of Mn3O4 are depicted inFig. 4. The �001� orientation of the Mn3O4 tetragonal spinelstructure consists of layers possessing a square a�a 2D unitcell. The complete bulklike stacking is formed by eight con-secutive layers, i.e., the local atom arrangement on the ithlayer is reestablished in the �i+8�th layer. There are twopossible as-cleaved terminations for this orientation: either asurface completely covered with manganese �labeled Mn-t inFig. 4�a�� or a mixed oxygen and manganese surface withstoichiometry Mn2O4 �labeled Mn2O4-t�. Similar to the �001�case, the �110� orientation is also composed of three-dimensional units formed by eight consecutive layers andcan be either Mn-t or Mn4O8-t terminated. Its 2D unit cell,depicted in Fig. 4�b�, has a rectangular a�2�c geometry.Finally, the �100� surface sketched in Fig. 4�c� shows a four-layer periodicity with an a�c 2D unit cell and exhibitsmixed O/Mn terminations with either Mn4O4 �Mn4O4-t� orMn2O4-t stoichiometry.

Overall, we have computed the relative surface energy ofsix different models. As mentioned above, we adopted a fer-rimagnetic ordering for the spins, schematically sketched inFig. 4�a�. The corresponding energies are listed in Table II.The �001� orientation is found to be the most favorable fol-lowed by the �110� and the �100� surfaces, the latter beingthe least stable one. Mn-t and Mn4O4-t models are signifi-cantly less stable than oxygen rich terminations for all orien-tations. In particular, Mn2O4-t Mn3O4�001� displays the low-est surface energy, 20–60 meV/Å more stable than the othermodels considered. The higher stability of the �001� surfacewith respect to the other low index orientations is in line withthe experimentally observed formation of a single-crystalline, transparent, and uniform Mn3O4�001� film on aMgO�100� substrate.14 The above theoretical conclusions

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corroborate our experimental analysis on the growth of well-defined Mn3O4 films with a �001� orientation by oxidation ofMnO�001� discussed in the previous section.

Surface induced structural modifications of Mn3O4�001�are localized within the topmost two layers and can be de-scribed in terms of the buckling of the outermost species andthe changes in the interlayer distance with respect to the bulkgeometry. A schematic view of the Mn2O4-t Mn3O4�001�termination is depicted in Fig. 4�a�, in the panel labeled by�S-1�. The square unit cell contains two Mn atoms in the�±1/4 ,1 /2� as-frozen positions sandwiched by two O rowsshifted by �±1/4�a along the �100� direction with respect tothe Mn row. In the optimized structure, we observe a squeez-ing of the inter-row distance ��row� by �0.4 Å and a corru-gation ��corr� of �0.2 Å along the O rows due to an oppositevertical displacement of adjacent oxygen atoms. The uncor-rugated Mn chain is vertically accommodated exactly in the

middle between the buckled oxygens. As for the interlayerdistances �dij�, we find that d12 experiences a huge contrac-tion of 26% with respect to the bulk value, partially compen-sated by an appreciable expansion of d23 �9%�. In the thirdlayer and below, the bulklike geometry is re-established. Tocheck our results, we have also performed PBE+U calcula-tions using an on-site Coulomb repulsion U−J=5.0 eV be-tween the majority and minority partially filled Mn d states.Though the PBE+U Mn3O4 optimized lattice constant�5.936 Å� is significantly larger than the PBE one �5.844 Å�,the resulting relaxed geometries are almost identical. In fact,within PBE+U, we obtain �row=0.1 Å, �corr=0.22 Å, d12=−24%, and d23=9%.

C. Density functional theory structure of theMn3O4„001… /MnO„001… interface

In this final subsection, we will focus on modeling theinterface between MnO and Mn3O4, in order to providecomplementary information to the experimental findings de-scribed above. MnO has a distorted rocksalt structure with alattice constant of 4.44 Å and two atoms per unit cell; Haus-mannite has the distorted spinel structure with a lattice con-stant of 5.76 Å and 14 atoms per primitive cell. In the mon-oxide, the Mn2+ cations occupy all octahedral sites, whereasin the Mn3O4 spinel, there are two different kinds of Mnions, Mn2+ and Mn3+, which populate tetrahedral and octa-hedral sites, respectively. Both Hausmannite and MnO havesquare symmetry along the �001� direction and display arather large lattice mismatch of 9% along the MnO �110�direction. Despite this unfavorable matching of the latticeconstant, we have shown above that Mn3O4 films �001� ori-ented can be grown on the MnO�001� substrate with the�100�Mn3O4

direction aligned along the �110�MnO. A similartransition has also been observed in CoO�100�, whichevolves smoothly to Co3O4�100� �Refs. 53 and 54� underoxidation conditions. The lattice mismatch between rocksaltCoO �aCoO=4.26 � and spinel Co3O4�aCo3O4

=8.084 �,though smaller than that between MnO and Mn3O4, is alsosignificantly large �5.4%�.

In order to find theoretical arguments providing clarifica-tion of the experimentally observed substrate-induced expan-sion of the Mn3O4 lattice constant, we have computed thestrain energy required to enlarge the Mn3O4 PBE lattice con-stant �5.844 � to twice the MnO in-plane PBE value�6.18 �. In terms of pressure, this would correspond to acomputed lateral strain of roughly 20 GPa. By allowing thec /a ratio to relax, we found that the strained structure expe-riences a strong shrinking of the c /a by �16%. As a conse-quence, the resulting relaxed volume is only 2% smaller than

Mn 2O4−t Mn O −t

4 8 Mn 2O4−t

Mn O44−t

S−7

S−6

S−5

S−4

S−3

S−2

S−1

S

a

c

a) (001)

Mn−t

b) (110)

Mn−t

c) (100)

2

c

a

a

a

FIG. 4. �Color online� Schematic layer-by-layer view of the lowindex Mn3O4 surfaces: �a� �001�, �b� �110�, and �c� �100�. Black andgray �red� circles indicate manganese and oxygen atoms, respec-tively. S denotes the surface layer, S-1 the first subsurface layer, andso on. For the most stable �001� surface, the ferrimagnetic spinconfiguration adopted is also shown. The dimensions of the 2Dplanar unit cell employed for each orientation are given in the bot-tom layer in terms of the bulk lattice parameters a and c.

TABLE II. PBE surface energy � �meV/Å2� for the Mn3O4 �001�, �110�, and �100� surfaces for thedifferent terminations considered.

�001� �110� �100�

Mn2O4-t Mn-t Mn4O8-t Mn-t Mn2O4-t Mn4O4-t

0 17 37 50 61 68

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the unstrained Mn3O4 optimized volume. By comparing thetotal energies of the unstrained �aMn3O4

� and strained�aMnO�Mn3O4 lattices, we finally obtain a rather small strainenergy of 21 meV/atom. Therefore, it is reasonable to expectthat �i� Mn3O4�001� films will be fairly well adapted to a�2�2� MnO�001� substrate along the �110� direction and �ii�the formation of a Mn3O4�001� overlayer may be accompa-nied by a substantial vertical shrinking, likely to induce sig-nificant compression of the interlayer distances, as comparedto the optimized bulk value.

Based on the two distinct Mn3O4�001� terminations pre-sented in Fig. 4�a� and considering that MnO�001� possessesa unique planar unit containing an equal number of manga-nese and oxygen atoms, two models of the interface emergenaturally: �1� interface A is constructed by placing the Mn-tMn3O4�001� surface on the MnO�001� substrate and �2� in-terface B is initiated with the MnO2-t termination. Variationscan be constructed by changing the stacking of the interfacialMn3O4�001� slice. Overall, we have created four modelsshown in Fig. 5: submodels A1 and B1 correspond to an atopsite registry of the Mn3O4�001� interfacial manganese atomson the MnO�001� oxygen atoms underneath, whereas sub-models A2 and B2 refer to a bridge site MnMn3O4

-OMnO junc-tion. The stability of all four interface models has been in-vestigated within a bulk setup using a unit cell containingfive layers of both MnO and Mn3O4. By fixing the latticeconstant to the MnO value, the corresponding enthalpies offormation were evaluated. The values obtained are listed inTable III. The results indicate that interface B1 has the lowestenergy and represents by far the most favorable model.

Guided by the above structural and energetic predictions,we are able to model a computationally consistent slab forthe experimentally examined Mn3O4�001� /MnO�001� inter-face. It should be emphasized that although we were able toprobe the soundness of different structures, including models

that might not exist experimentally, caution is required in thecomparison with the experimental situation. To unambigu-ously ascertain the validity of the proposed model, furtherexperimental information would be necessary, such as high-resolution transmission electron microscopy images of theinterfacial region and quantitative estimation of the structuralrelaxations by LEED. Keeping this in mind, we constructed aMn3O4�001� /MnO�001� slab built by merging a five layerthick MnO�001� substrate with nine layers of Mn3O4�001�,the latter corresponding to a complete tetragonal Mn3O4 unitcell.

By employing a �2�2� planar unit cell possessing theMnO lattice constant and preserving the bulk internal geom-etry in the bottom three MnO layers, we performed a fullstructural optimization. The resulting optimized structure ispresented in Table IV and the corresponding charge densityplot is shown in Fig. 6.

In Fig. 6�a�, we plot the charge density layer by layerfrom the surface �S� down to the interfacial region �I1 and

Mn O3 4

Mn O −t/top2 4

(001)

74 meV82 meV 0 meV 38 meV

Mn−t/bridge Mn O −t/bridge2 4Mn−t/top

A1 A2 B1 B2

MnO (001)

FIG. 5. �Color online� Model structure for theMn3O4�001� /MnO�001� interface in the �001�plane and calculated relative stability. Black andgray �red� circles indicate manganese and oxygenatoms, respectively �see text for details�.

TABLE III. Formal relative enthalpy of formation Ef �meV/atom� for the model interfaces A1, A2, B1, and B2 sketched in Fig.5. The enthalpy of formation of the most stable model �B1� is takenas energy reference �EB1=0 eV�.

Mn2O4-t Mn-t

A1 A2 B1 B2

Ef 82 74 0 38

interface

MnO

Mn O3 4

[100]

[001

][0

10]

[001

]

[010]

S−5S−6 S−1S−2S−4 S−3D

A ......

....

S

A B C D A B C D

S−7

S−7

S−3S−1

[100]

I1I2S−6S−4S−2S

A D

S−5

c)b)

a)I1 I2

FIG. 6. �Color online� Charge density plots for theMn3O4�001� /MnO�001� interface: �a� layer-by-layer decompositionof the charge density along the �001� direction. The dashed lines A,B, C and D �only A and D labels are given in the figure� are 2Dprojections of the �001� planes displayed in panels �b� and �c� show-ing vertical cuts of the charge density along the �100� and �010�directions, respectively. Dark gray �red� and light gray �light blue�circles indicate the position of oxygen and manganese atoms, re-spectively. The arrows highlight the most significant internal struc-tural relaxations. The light gray �yellow� line marks the separationbetween the Mn3O4 and MnO components, whereas the dark �blue�line indicates the unrelaxed starting position of the topmostMn3O4�001� layer.

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I2�. I1 and I2 represent the topmost MnO layer and the firstMn3O4 layer, respectively. The dashed lines drawn in I1 andI2 indicate the �100� and �010� cuts corresponding to thevertical slices shown in panels �b� and �c�, whereas the fulllines indicate the interface region and the starting position ofthe unrelaxed surface. We first notice that in line with thebulk interface calculation, a huge vertical contraction of 20%of the Mn3O4 region is obtained, which compensates for theplanar strain. As a consequence of the latter strain, the dis-tance between adjacent surface oxygen rows increases by0.7 Å, in contrast with the contraction observed in the un-strained Mn3O4�001� surface. In terms of interlayer dis-tances, we found that the relaxations are well localizedwithin the first three layers and do not affect the rest of theslab �see Table IV�. By referring the changes of the interlayerdistances to the bulk interlayer spacing corresponding to theoptimized strained c /a ratio of 1.36, we observe a significantcompression of the first interlayer distance �d12=−29.5% �,which is partially canceled out by the expansion of d23�+20.0% �. Deep down in the Mn3O4 part of the slab, thesituation remains practically unchanged, including the dis-tance dI2 between I2 and �S-7�. At the interface, the distancedI between the contact Mn3O4 and MnO layers is found to beslightly larger than the MnO�001� interlayer distance. Theformation of the interface leads to an upward shift of themanganese atoms placed in the MnO layer, thus inducing arelatively small buckling of 0.2 Å between these atoms andthe interfacial MnO oxygens. It is worth noting that the bareMnO�001� surface exhibits a larger displacement of the top-most manganese atoms and an almost doubled surfacebuckling.23 The rest of the MnO slab remains unaffected andreproduces the bulklike behavior. In line with theMn3O4�001� surface, also in this case, PBE+U provides aquantitative almost identical picture. We can therefore con-clude that the growth of strained Mn3O4�001� films on aMnO �001� oriented substrate inhibits the structural relax-ations observed in the clean MnO�001� surface. Finally, be-cause of the large vertical compression experienced in theMn3O4�001� side of the slab, there is a general in-plane re-arrangement of the atoms. As shown by the light gray �yel-low� arrows in Figs. 6�b� and 6�c�, the displacements mainlyinvolve the movement of oxygens along the �010� direction,which leads to an important distortion of the octahedralO-Mn-O chain.

IV. CONCLUSIONS

We have presented a combined experimental and DFT-based study of the growth of Mn3O4�001� films on a

MnO�001� substrate through the formation of a distinctMn3O4�001� /MnO�001� interface. X-ray absorption andphotoemission demonstrate that a Pd�100�-supportedMnO�001� substrate can be readily oxidized to produceMn3O4-like surfaces. Significant changes of the Mn 2p3/2

core level �increasing binding energy� and Mn 3s states �de-crease of the exchange splitting� are observed along with theappearance of the characteristic Mn3O4 phonon loss at83 meV. Accordingly, the Mn L2,3 edge and O K edge XASprofiles assume the spectral shape of Hausmannite. SPA-LEED measurements reveal the �001� surface symmetry andpoint to a 2.5% expansion of the in-plane lattice constant.

DFT calculations indicate that �i� the Mn2O4 terminatedMn3O4�001� is the most favorable termination among thelow index surfaces and that �ii� the expansion of the Mn3O4

planar lattice constant to the �2 MnO value requires a ratherlow strain energy of 22 meV/atom. This provides an ener-getic rationale for the easy epitaxy observed, despite thelarge lattice mismatch of 9%. In fact, at variance with theexperimentally observed formation of Co3O4 on CoO�100�,where the oxidation process stops at the limiting thickness of�5 Å �Ref. 53�, Mn3O4 nanolayers are formed with no lim-iting thickness within the probing depth of the experimentalsetup. Different possible structural models for theMn3O4�001� /MnO�001� interface have been tested. The cal-culations show that a well-defined junction betweenMnO�001� and Mn3O4�001� is possible and provides a de-tailed atomic picture of the interfacial registry. The preferredmodel interface begins with a Mn2O4 terminated Mn3O4

layer accommodated onto the underlying MnO�001� sub-strate, with the Mn3O4 Mn atoms placed in atop site posi-tions with respect to the MnO oxygens underneath. Furtherexperiments will be necessary to confirm the validity of theproposed picture.

ACKNOWLEDGMENTS

This work has been supported by the Austrian ScienceFunds FWF within the Joint Research Program S90 and theScience College W4 and by the 6th Framework Programmeof the European Community �GSOMEN�. Technical assis-tance of the staff at the Beamline I311 �MAX II, Lund� andat the Beamline APE �ELETTRA, Trieste� is gratefully ac-knowledged.

TABLE IV. Calculated interlayer distances �dij� for the Mn3O4�001� /MnO�001� optimized structure, given both in Å �first row� and inpercent with respect to the strained interlayer spacing �second row�. dI1 and dI2 indicate the distance between the first two MnO�001� andMn3O4�001� layers at the interface, whereas dI is the interfacial distance between the MnO�001� and Mn3O4�001� contact planes.

d12 d23 d34 d45 d56 d67 d78 dI2 dI dI1

0.74 1.26 0.95 0.95 1.02 1.00 1.10 1.02 2.19 2.27

−29.5 +20.0 −0.1 −0.1 −0.1 +0.1 0.0 0.0

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