Conference-ACSIN-12 & ICSPM 21-Scanning Tunneling Microscopy Study of an Altered Fe 3 O 4 ( 001 ) Thin Films Surface by Hydrogen Adsorption

We report on changes of surface structures induced by hydrogen adsorption on a Jahn-Teller distorted magnetite (Fe3O4)(001) surface and priorities of hydrogen-adsorbed sites by means of in-situ scanning tunneling microscopy. The experiments have revealed that surface Fe cations relax toward the bulk-terminated positions by OH species formed between adsorbed-hydrogen and surface ON anion (The labeled “N”in ON denotes that the O-O distance in the surface is compressed compared with bulk). Moreover, two types of surface ON sites were found to be almost equivalent for hydrogen adsorption. [DOI: 10.1380/ejssnt.2014.26]


I. INTRODUCTION
Fe 3 O 4 , known as the oldest magnet, has been attracting much attention for its potential application to spintronics devices because of its electrical and magnetic properties such as half-metallicity [1,2] and a high Curie temperature of 858 K. Due to these fascinating properties, Fe 3 O 4 (001) films have been expected to incorporate into multi-layered samples, as necessary for the practical device application.However, magnetic tunnel junction devices using Fe 3 O 4 (001) films have never demonstrated predicted performance [3,4].The reason is closely concerned with significant reduction of surface electron spin polarization compared with bulk.The electronic and electron spin states for surface Fe cations differ from ones for bulk Fe cations since there is no Fe atoms to be bonded above the surface Fe cations.In fact, theoretical and experimental electron spin polarization measurements of a clean Fe 3 O 4 (001) surface have shown much lower values than expected [5][6][7][8][9][10][11][12].
Recently, the studies on modification of the surface electronic and electron spin states by introducing adsorbates on it [9][10][11][12][13][14][15][16][17][18] have been attracted attention, because the method offers us a way to enhance the surface electron spin polarization.It is also applicable to improve unexpected properties of a clean Fe 3 O 4 (001) surface.In particular, hydrogen has been extensively studied as such an adsorbate from theoretical and experimental point of views [9][10][11][12][13][14]17].Their studies showed that hydrogen adsorbates on a clean Fe 3 O 4 (001) surface enhance the surface electron spin polarization at the Fermi level from experimental values of less than −5% in a clean Fe 3 O 4 (001) surface [9,10,12] to theoretically almost −100% [10][11][12][13] or experimentally at least −50% at room temperature [9,10,12].Even so, the detailed mechanism for its enhancement and the reason why the experimental values are much lower than the theoretical ones remain at issue.Furthermore, there have been few experimental reports on atomic-scale structures, local electronic and electron spin states of a hydrogen-adsorbed Fe 3 O 4 (001) surface.Thus, experimental revealing of their details has important implications for elucidation of the issues.
Several surface termination models have been proposed in a Fe 3 O 4 (001) surface [19][20][21][22][23]. Bulk Fe 3 O 4 has a cubic inverse spinel structure with a lattice constant of 0.8397 nm [24].In the [001] direction, A-layers of tetrahedral iron cations (Fe A ) and B-layers containing both oxygen anions and octahedral iron cations (Fe B ) are alternately stacked, as shown in Figure 1(a).The distances between A-A or B-B layers and between A-B layers are 0.21 and 0.105 nm, respectively.The distance between two Fe A cations within same rows of A-layers is 0.6 nm.On the other hand, the distances between two Fe B cations within same rows and between two Fe B rows of B-layers are 0.3 and 0.6 nm, respectively.The surface termination in a Fe 3 O 4 (001) surface is either A-layer or B-layer and the coexistence of both A-layer termination and B-layer termination has been ruled out through step-height measurements in all the studies [21].Early studies on the surface terminations have reported that the stable A-layer termination is a half-filled A-layer termination consisting of one Fe A per unit cell [19,20] and the stable B-layer termination is a termination either with one oxygen vacancy [19,21] or hydroxyl group [19] per unit cell.However, theoretical calculations based on ab initio thermodynamics [22] and a combination of density functional theory (DFT) calculations and low energy electron diffraction (LEED) analyses [23] performed by Pentcheva et al. have demonstrated that a Jahn-Teller distorted B-layer termination is the most stable surface termination of a clean Fe 3 O 4 (001) surface over a broad range of oxygen pressures.The characteristics are in-plane relaxations perpendicular to atomic rows for surface oxygen anions and Fe B cations, as shown in Fig. 1(b).Hereinafter, two inequivalent surface oxygen anions generated by Jahn-Teller distortion are referred to as O N and O W , respectively, in imitation of the previous report [14].The labeled "N"(narrow) in O N and "W"(wide) in O W denote that the O-O distance in the surface is compressed and expanded compared with bulk, respectively.
In this paper, atomic geometries of a hydrogenadsorbed Fe 3 O 4 (001) films surface are characterized using scanning tunneling microscopy (STM).We experimentally demonstrate that atomic positions of surface Fe B  [22,23].The labeled "N"and "W"denote that the O-O distance in the surface is compressed and expanded compared with bulk, respectively.cations relax toward the ones of bulk Fe B cations following hydrogen adsorption.We also investigate adsorption-site dependences.

II. EXPERIMENTAL
The experiments were performed in an Omicron ultrahigh-vacuum (UHV) system consisting of a preparation chamber for Fe 3 O 4 (001) films growth and an analysis chamber for STM and x-ray photoelectron spectroscopy (XPS) measurements.The base pressure of both chambers was 5.0×10 −11 mbar.The epitaxial Fe 3 O 4 (001) films were prepared on mechanically polished MgO(001) singlecrystal substrates by the deposition of Fe at 523 K in the presence of oxygen [25,26].The substrates were cleaned in situ by heating at 523 K for at least 16 hours and then annealed at 1073 K for 1 hour in oxygen atmosphere.Fe was evaporated from high-purity Fe rods heated by electron bombardment.The growth rate was 0.9 ML/min and the films thickness was 20 nm.During the films formation, the oxygen pressure was set in the range from 7.0 × 10 −7 to 1.0 × 10 −6 mbar.To obtain an atomically flat surface, the grown films were post-annealed at 523 K for 30 min in the same oxygen atmosphere.The surface cleanliness and stoichiometry were confirmed by XPS measurements.
STM measurements were performed at room temperature with electrochemically etched W tips.A bias voltage was applied to the sample with respect to the grounded tip.All STM images shown in this paper were taken in constant-current mode and acquired with a setpoint of 0.3 nA at a bias voltage of +1.2 V.

III. RESULTS AND DISCUSSION
Figure 2(a) shows an overview STM image of the asgrown Fe 3 O 4 (001) films surface.Some bright protrusions can be seen within atomic rows in the large terrace.Although they are also observed in the proximity of surface defects such as steps edges, this paper discusses bright protrusions other than those.Previous STM studies of a freshly prepared Fe 3 O 4 (001) surface have also observed them [5,8,21,27,28].As a result of XPS measurements of the surface, we have observed no peaks derived from impurities such as CO and no peak shifts for O and Fe (data not shown).In addition, a increase of the bright protrusions with increasing time of keeping the samples in UHV was observed.Therefore, they should not be attributed to surface impurities and surface intrinsic defects.The red oval in Fig. 2(a) contains no bright protrusions, indicating that surface Fe B cations contained in the region are all equivalent.On the other hand, the green oval contains some bright protrusions.These results indicate that the latter contains some altered Fe B cations whose electronic states are distinct from unaltered Fe B cations contained in the former.Hence, such bright protrusions observed in STM images of the Fe 3 O 4 (001) surface are referred to as "altered Fe B "and otherwise "unaltered Fe B "(unaltered Fe B are Fe B cations in a Jahn-Teller distorted B-layer surface).Figure 2(b) shows a high-resolution STM image of the area containing some altered Fe B cations.It can be clearly seen that two neighboring altered Fe B within atomic rows are bright protrusions, i.e., electronic states for the two Fe B cations are modified in pairs with respect to unaltered Fe B cations.This result is in agreement with Parkinson et al.'s STM results of atomic-hydrogen [13] or dissociated-water [14] adsorbed Fe 3 O 4 (001) surfaces.They concluded that surface hydroxyl groups are generated by hydrogen bonding to surface O N sites and electronic states for two surface Fe B cations adjacent to them are modified in pairs, as shown in Fig. 2(b) [14].So far, however, the detailed mechanism remains unknown.STM experiments have demonstrated that Fe B rows of a clean Fe 3 O 4 (001) surface show Jahn-Teller distorted wave-like structures [5,8,22,23,29].However, STM measurements of a hydrogen-adsorbed Fe 3 O 4 (001) surface have revealed that the configuration of surface atomic rows turns to straight upon hydrogen adsorption [13].Our result (Fig. 3(a)) corresponds to the previous reports, suggesting that hydrogen adsorbates on the surface induce generation of altered Fe B cations.Besides, recent Mulakaluri et al.'s theoretical calculations have demonstrated that not clean Fe 3 O 4 (001) B-layer terminations but hydrogen-adsorbed Fe 3 O 4 (001) surface terminations are most stabilized in the pressure ranges accessible in UHV experiments [17].In general, metal oxides surface tends to promote dissociative adsorption of water by the presence of both cations and anions on their surface because the cation sites act as Lewis acids, attracting the lone pairs of the water molecule and the surface oxygen acts a Brønsted base site, attracting the protons of the water molecule [15,16].From this property of metal oxides surface and good agreements between our results and previous reports, we concluded that hydrogen adsorption on the surface by dissociation of UHV residual gases alters the surface atomic geometries and electronic states.As mentioned above, in fact, we have observed a increase of bright protrusions originated from the dissociative adsorption with the increased keeping time in UHV.In the following, we proceed with our discussions determining that the bright protrusions are altered Fe B cations adjacent to hydroxyl groups generated by hydrogen bonding to surface O N sites, as shown in Fig. 2(b).corresponds to the distance between two Fe B cations along the direction in the bulk, indicating that atomic positions of altered Fe B cations are consistent with ones of bulk Fe B cations.Furthermore, it demonstrates that the configuration of altered Fe B rows is straight similar to the bulk Fe B rows, because atomic displacements induced by hydroxyl groups are expected to occur at two surface Fe B cations adjacent to them.This STM result showing adsorbedhydrogen induced changes of surface atomic geometries agrees well with the previous DFT results [13], which have shown that the surface atoms relax back to bulkterminated positions following hydrogen adsorption.A clean Fe 3 O 4 (001) surface indicating Jahn-Teller distorted wave-like structures is the ( √ 2 × √ 2)R45 • reconstructed surface as shown in Fig. 3(a), however LEED observations and DFT studies of a hydrogen-adsorbed Fe 3 O 4 (001) surface have revealed that the surface exhibits (1 × 1) symmetry same as bulk [9,[11][12][13].
We have often observed an interesting phenomenon during scanning the surface such that the bright protrusions jump into a neighboring row as shown in the bottom part of Fig. 3(a).The STM image discontinuously changes at the line position indicated by the outside black arrow.In the STM image, the tip scanned the surface from the top to the bottom and the left to the right.This sort of switching phenomenon of bright protrusions has also been reported by the previous STM measurement of a hydrogen-adsorbed Fe 3 O 4 (001) surface [13].The hydrogen atom adsorbed on an O N site tends to hop into the neighboring O N site.In our STM image, the same things were happened as indicated by the atomic arrangement model shown in Figure 3(a).This hopping phenomenon did not depend upon the tip scanning direction and was frequently observed.We could identify more than one hopped site in nearly every measurement (wide area scan: 40×40 nm 2 to 50×50 nm 2 ).These facts indicate that hydrogen should be bonded to an O N site not an O W site, and hops into a neighboring O N site rather than a neighboring O W site.
Recent DFT studies of Fe 3 O 4 (001) surfaces with one adsorbed-hydrogen atom per unit cell showed that the neighboring O N anions are not equivalent for hydrogen adsorption [17].They indicate that adsorption energies for these O N sites are different by 0.05 eV.To distinguish these two types of O N sites we label them as O N1 and O N2 .In the surface atomic arrangement model depicted in Fig. 4(a), these oxygen sites within [110] atomic rows are marked as "N1"and "N2", respectively.Since the DFT calculations indicate that one of these sites is preferable for hydrogen to be adsorbed, we have expected that observed STM images should reflect this difference.As seen in the high-resolution STM image shown in Fig. 4(a), the hydrogen-adsorbed O N1 and O N2 sites are both identified.Therefore, we investigated the numbers of hydrogenadsorbed sites for their O N anions using larger-scale STM images (40×40 nm 2 to 50×50 nm 2 ) such as Fig. 4(b).Checking more than 1000 bright protrusions within [110] atomic rows observed in the STM images of single sample surface, the histogram for the emergence at their O N sites was obtained.The histogram shown in Fig. 4(c) suggests that there is no obvious difference for the emergence between these sites.We also performed the same investigations concerning two types of O N sites within [1][2][3][4][5][6][7][8][9][10] atomic rows.Although each of these O N sites is equivalent to either O N1 site or O N2 site, the correspondence is unclear due to antiphase domain boundaries introduced in the films.Therefore, we label them as O N3 and O N4 (not O N1 and O N2 ).In the surface atomic arrangement model depicted in the inset of Fig. 4(b), these oxygen sites are marked as "N3"and "N4", respectively.The histogram for the emergence at their O N sites shows the similar results to the former.These results show no clear adsorption-site dependences between O N sites.Thus, we revealed that two types of O N sites of a Jahn-Teller distorted Fe 3 O 4 (001) surface are almost equivalent for it.Although statistical analyses of adsorbed-hydrogen hopping frequency are not yet sufficient, there seems no clear difference in the hopping between N1 → N2 and N2 → N1 or N3 → N4 and N4 → N3.The hydrogen coverage on our Fe 3 O 4 (001) films surface is estimated to be 0.12 ML, i.e., 12% of the O N sites are occupied.This paper shows the priorities of hydrogen-adsorbed sites at low hydrogen coverage, however there have also been no reports on relations between hydrogen coverage and adsorptionsite dependence.Moreover, our results obtained at room temperature are somewhat different from the previous theoretical report [17].Further STM investigations of Fe 3 O 4 (001) surfaces with varying hydrogen coverages and at varying temperatures are required to give newer experimental insights into a hydrogen-adsorbed Fe 3 O 4 (001) surface.

IV. CONCLUSION
Atomic structures of a hydrogen-adsorbed Fe 3 O 4 (001) surface have been revealed by means of STM measurements.
The measurements showed that surface Fe B cations relax toward the bulk-terminated positions following hydrogen adsorption.We also investigated adsorption-site dependences for hydrogen to be adsorbed, and revealed that there is no obvious dependences between two types of O N sites of a Jahn-Teller distorted Fe 3 O 4 (001) surface.

FIG. 1 :
FIG. 1: (a) Cubic inverse spinel structure of Fe3O4.Tetrahedral iron cations (FeA) in A-layers, octahedral iron cations (FeB) and oxygen anions in B-layers are indicated by purple, red and gray circles.(b) Top view of a Jahn-Teller distorted B-layer termination proposed by Pentcheva et al.[22,23].The labeled "N"and "W"denote that the O-O distance in the surface is compressed and expanded compared with bulk, respectively.

Figure 3 (FIG. 2 :
FIG.2:(a) Overview STM image of the as-grown Fe3O4(001) film surface.The red and green ovals show the regions containing unaltered and some altered FeB, respectively.(b) High-resolution STM image of the area containing some altered FeB and the projected atomic arrangement model[14].The red, green, gray and blue circles show unaltered FeB, altered FeB, oxygen and hydrogen.The labeled "N"and "W"correspond to the ones shown in Fig.1(b).

FIG. 3 :
FIG. 3: (a) High-resolution STM image of the area containing both unaltered and altered FeB, and the projected atomic arrangement models.The two yellow squares show ( √ 2 × √ 2)R45 • symmetry in a Jahn-Teller distorted Fe3O4(001) surface and (1 × 1) symmetry in a hydrogen-adsorbed Fe3O4(001) surface, respectively.The white wavy and straight dashed lines show the configurations of unaltered and altered FeB rows, respectively.The black arrow outside the STM image shows the line position of discontinuity for bright protrusions.The inside black arrow shows the direction of hydrogen hopping.(b) Cross-sectional line profile taken along the white solid line shown in (a).The color coding of the ions corresponds to the one in Fig. 2(b).

Figure 3 (FIG. 4 :
Figure 3(b) shows a cross-sectional line profile taken along the white solid line shown in Fig.3(a).It shows not only the distances between two unaltered Fe B along the perpendicular direction to surface atomic rows but also the distances between two altered Fe B along the direction.The former distances consist of two different values, 0.56 nm and 0.64 nm.Wavy atomic rows observed in STM images of a Jahn-Teller distorted Fe 3 O 4 (001) surface are attributed to such in-plane relaxations perpendicular to atomic rows direction for surface Fe B cations.On the other hand, the latter distances are 0.60 nm.This value