Letter pubs.acs.org/NanoLett
Surface Magnetism of Cobalt Nanoislands Controlled by Atomic Hydrogen Jewook Park,† Changwon Park, Mina Yoon, and An-Ping Li* Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *
ABSTRACT: Controlling the spin states of the surface and interface is key to spintronic applications of magnetic materials. Here, we report the evolution of surface magnetism of Co nanoislands on Cu(111) upon hydrogen adsorption and desorption with the hope of realizing reversible control of spindependent tunneling. Spin-polarized scanning tunneling microscopy reveals three types of hydrogen-induced surface superstructures, 1H-(2 × 2), 2H-(2 × 2), and 6H-(3 × 3), with increasing H coverage. The prominent magnetic surface states of Co, while being preserved at low H coverage, become suppressed as the H coverage level increases, which can then be recovered by H desorption. First-principles calculations reveal the origin of the observed magnetic surface states by capturing the asymmetry between the spin-polarized surface states and identify the role of hydrogen in controlling the magnetic states. Our study offers new insights into the chemical control of magnetism in low-dimensional systems. KEYWORDS: Spin-polarized scanning tunneling microscopy, nanomagnetism, chemisorbed hydrogen, surface reconstruction, surface magnetism, adsorption and desorption
C
Pt(111) can be controlled through hydrogenation. A key electronic feature determining the surface magnetism of Co is a minority d-like orbital resonant surface. However, it is not clear how the magnetic properties of the individual Co atoms12,13 evolve into those of the crystalline Co surface or how the magnetic properties of the surface state will respond to chemical adsorption. The surface of nanoscale triangular Co islands deposited on Cu(111), hereafter referred to as Co islands, was proposed as a model system to study structural, electrical, and magnetic properties of the magnetic materials with nanoscale confinement.14,15 Both spin-averaged and spin-polarized (SP) scanning tunneling microscopy and spectroscopy (STM/STS) were used to explore the Co islands extensively in previous reports.13−19 Interestingly, dissociative adsorption of hydrogen molecules (H2) was reported on Co accompanied by the long-range ordering of hydrogen atoms on Co islands.20 The STM/STS studies on these surfaces so far were limited to spinaveraged,20−24 and the associated spin configurations remain to be explored. In this Letter, we demonstrate that adsorbed hydrogens can reduce the surface magnetization of Co islands on Cu(111), and this is accompanied by the suppression of magnetic surface states. Three different types of H-induced surface superstructures, 1H-(2 × 2), 2H-(2 × 2), and 6H-(3 × 3), on Co
obalt is an interesting material for both spintronic applications and fundamental physics research. Stability and controllability of magnetism at the surface and interface are crucial in applications ranging from magnetic recording media1 and spin valves2 to magnetic multilayers with magnetoresistance3 and hard drive sensors. The ferromagnetism of bulk cobalt comes from the exchange splitting of partially occupied d-electrons, which leads to a relative energy shift of the majority and the minority spin bands. Furthermore, the magnetic properties of Co surface states can be tailored by controlling the chemical environment.4−6 Indeed, nonmagnetic impurities at the interface of Co/Al2O3/Co tunnel junctions were found to largely suppress the tunneling spin polarization,7 atom adsorption, or reactive molecule exposure reported to be capable of altering the spin of Co atoms,5 and direct adsorption of oxygen or sulfur on Fe nanoislands was shown to also affect spin-polarized scattering.8,9 In particular, hydrogen is expected to change the magnetization of the Co surface layer through the hybridization of the H 1s level with the Co 3d electrons.10,11 Because the majority spin 3d band is completely occupied in Co, the addition of 3d electrons through hybridization will increase the number of minority spin states. As a result, a net magnetic moment of the surface atoms, defined as the difference between the occupation number of majority spin and minority spin electrons, is reduced with hydrogen chemisorption.11 Despite the broad interest and general expectations, the chemical control of the surface magnetism of Co remains to be demonstrated. Dubout et al.5 have recently demonstrated that the spin of individual Co adatoms on © XXXX American Chemical Society
Received: September 28, 2016 Revised: November 21, 2016 Published: December 1, 2016 A
DOI: 10.1021/acs.nanolett.6b04062 Nano Lett. XXXX, XXX, XXX−XXX
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lattice as shown in Figure 1b. With the introduction of hydrogen by controlling the partial pressure and exposure time of H2 in UHV chamber, we have subsequently observed hydrogen adsorption patterns of 1H-(2 × 2), 2H-(2 × 2), and 6H-(3 × 3) (Figure 1c−e). The H-coverage (θH) of the 1H-(2 × 2) pattern is 0.25 ML, namely, one H atom per 2 × 2 Co unit cell and 0.5 and 0.67 ML for 2H-(2 × 2) and 6H-(3 × 3), respectively. The Co islands with the highest H coverage of 0.67 ML are the dominant surface after an extensive period of exposure to residual H2 gas in the UHV, as captured in timelapse STM images (Figure S1). Interestingly, two different types of (2 × 2) superstructure, 1H-(2 × 2) and 2H-(2 × 2), are observed, and they show distinctively different spindependent surface states as explained below. Spin-polarized differential conductance spectra (SP-dI/dV) probe the energy-resolved local density of states (LDOS) under the STM tip position and are thus ideal for investigating spindependent surface states. We first carried out the SP-STM study on pristine Co islands. Figure 2a shows a dI/dV distribution map superimposed on the corresponding threedimensional topography of Co islands at V = −0.5 V. The islands with magnetization parallel (P) and antiparallel (AP) to the tip magnetization direction are labeled as white and red, respectively. At V = −0.5 V, the contrast of dI/dV between two magnetization directions is most prominent in our measurements.15 The contrast varies and can even be inverted with the change of the bias voltage because the contributions to LDOS from different spin states are bias dependent (Figure S2). Previous SP-STM studies have shown a spin-dependent contrast in dI/dV spectroscopy at around −0.3 V and attributed it to the minority spin surface state of Co.13,17 We find that this −0.3 V peak is not obvious in the SP-dI/dV spectroscopy at 130 K (Figure 2d,g) but becomes more pronounced at 38 K (inset). The spin-polarized tunneling spectroscopy remains stable during the STM/STS measurements at 130 K as shown in Figure 2a and d and Figure S2c and d. Note, this temperature is a little bit higher than usually used for spin-polarized STM measurement on Co islands on Cu(111).13,17 Apparently, superparamagnetic transitions of the Co islands do not happen at this measurement temperature. Actually, the blocking temperature of superparamagnetic transitions of the Co islands can be estimated from the coercive magnetic field of the similar Co islands on Cu(111). Following the results of Ouazi et al.,27 the transition temperature can be determined from the condition that tmeas = tN, where tmeas is the time of one spectroscopy scan, and tN is the Ń eel relaxation time, which is defined by tN = t0 exp(E/kT), where E is the energy barrier of spin flip. By using t0 = 10−10 s and tmeas = 100 s, E is estimated to be 0.5 eV for 4000 atom-sized Co islands. On the basis of these parameters, the superparamagnetic transition temperature is estimated as 226 K, which is higher than our measurement temperatures. For the 1H-(2 × 2) Co surface, the spin-polarized states are still obvious in the dI/dV map acquired at V = −0.5 V, as shown in Figure 2b. Similar to the pristine case, a higher tunneling conductance is observed when the islands have an AP magnetization than a P magnetization with respect to the tip at −0.5 V with the bias-dependent dI/dV curves shown in Figure 2d for an unfaulted island. Besides the spin contrast, F and U islands have slightly different electronic properties, e.g., a stacking-sensitive dz2-like surface state peak appears at −0.35 V for fcc and at −0.28 V for hcp-stacked Co islands.17 Thus, the
(0001) are identified, and their magnetic properties are examined by combining SP-STM and first-principles calculations. The spin-dependent contrast of differential conductance, identified on pristine Co, can survive on 1H-(2 × 2) with low H coverage but becomes suppressed in 2H-(2 × 2) and 6H-(3 × 3) as the H dosing level increases. The surface magnetism can be reversibly controlled by the H adsorption and desorption process. We find that the exchange splitting of partially occupied d-electrons, which makes Co ferromagnetic, also leads to spin-dependent surface states. The spin contrast disappears if sufficient hydrogen atoms suppress the spillout of surface states. A clean Cu(111) single crystal was prepared by sputtering and postannealing treatment in an ultrahigh vacuum condition (UHV, 250 nA] or high bias [(b) V = ± 1.8 V] can remove H atoms and recover spindependent surface states.20 Figure 3a shows the SP-dI/dV map acquired after the high-current treatment, where the spin contrast is largely recovered. Figure 3b shows the dI/dV spectra before and after removing H atoms with high-bias treatment, corresponding to the 2H-(2 × 2) and the pristine Co surfaces. Remarkably, the spin minority peak around −0.3 V is clearly
STS spectra of Co islands can be categorized into four types if considering stacking types (U or F) and the relative orientations between tip and island magnetization (P or AP). To discriminate between the structure-related electronic contrast and the magnetization-dependent spin contrast, we statistically analyzed STS spectra acquired on different Co islands (see the Methods). Figure 2e shows the categorized dI/dV spectra of the 1H-(2 × 2) surface analyzed over 42 islands at 38 K, where solid (dotted) line represents U (F) stacking order and red (blue) color corresponds to P (AP) configuration. The dI/dV spectra for the U and F islands are only slightly different, but the difference for the P and AP configurations is much more pronounced, meaning the SP-dI/dV is mostly dominated by the relative magnetic configurations between the tip and the islands. Note, no discernible difference in hydrogen adsorption is seen for the U and F islands. The magnetic asymmetry, defined by A = (P − AP)/(P + AP),19 is shown in Figure 2f for the 1H-(2 × 2) surface, where the solid and dotted curves correspond to the U and F islands measured at 38 K, respectively. Again, the difference between the F and U islands at varying bias voltages is small, but the magnetic asymmetry is largest at −0.5 V. The magnetic asymmetry remains appreciable at 130 K despite the thermal broadening effects (dashed line in Figure 2f). In addition, by comparing pristine Co islands (Figure 2d inset) and 1H-(2 × 2) (Figure 2e), we find the LDOS peak of minority spin states is shifted to low voltage by ∼0.2 V on the 1H-(2 × 2) surface as compared to that of the pristine Co surface. Unlike the 1H-(2 × 2) Co surface, the 2H-(2 × 2) Co surface with a higher H-coverage does not exhibit a dI/dV contrast between the P and AP islands, as shown in Figure 2c. The tunneling conductance of the 2H-(2 × 2) Co surface is highly suppressed and becomes even lower than that of the Cu(111) surface. Note that the Cu(111) surface structure remains intact in the hydrogen adsorption process due to a high dissociation barrier.28,29 The suppressed tunneling conductance
Figure 3. Spin-polarized STM image and spectra on the 2H-(2 × 2) surface after H desorption. (a) dI/dV map (40 × 40 nm2, V = −0.5 V, I = 1 nA, T = 130 K) after high bias sweeping. (b) dI/dV spectra before (green) and after (black) H desorption in an STM tip treatment with a series of bias sweep at ±1.8 V. (inset) Zoomed-in dI/ dV curve after H desorption. Black arrow marks the minority spin state. C
DOI: 10.1021/acs.nanolett.6b04062 Nano Lett. XXXX, XXX, XXX−XXX
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monolayer Co. As the number of layers increases, the minority and majority surface states evolve in a qualitatively different fashion. Although the surface band does not qualitatively change in the majority spin channel with the addition of Co layer, the surface band (with positive dispersion) and d band (with negative dispersion) of the minority spin channel begin to overlap and repel each other. As a result, the downward shifted band becomes a minority spin surface band and gradually changes its orbital character from p (red) to d (blue) from Γ to K as shown the right panel of Figure 4b. The distinctive surface states between the majority and minority spins are manifested in LDOS (Figure 4c). In the majority spin channel, the onset of the surface states is responsible for the increase in LDOS from −0.2 eV. A strong peak around −0.4 eV in the minority spin channel, which is the most prominent feature in the spin-asymmetric tunneling conductance, originates from the p-d hybridized surface states. The strong peak is caused by deviation of the surface band from a paraboloid and the inclusion of orbital character. We now compare the calculated electronic structures of pristine Co with superstructures of 1H-(2 × 2) and 2H-(2 × 2) on the Co surface. For comparison, the band structures of the pristine Co trilayer are folded to be compatible with 2 × 2 supercell. Panels a−c in Figure 5 show their band structures, emphasizing the surface-localized states with the size of data points proportional to the amplitude of wave functions at 3 Å above the topmost Co atoms and different colors for majority (red) and minority (blue) spin states. The surface states are greatly reduced as H atoms are introduced on the surface. Therefore, the size of data points is doubled for the 1H-(2 × 2) and 2H-(2 × 2) superstructures for visualization. The surface states of the 1H-(2 × 2) superstructure are very similar to those of pristine Co, and the minority spin peaks are at −0.4 eV both for the pristine Co and the 1H-(2 × 2) (Figure 5d and e). Thus, the experimentally observed 0.2 eV peak shift (Figure 2e) for the 1H-(2 × 2) surface does not come from the change of electronic structures. Panels g−i in Figure 5 show the electron density isosurfaces of the surface states of pristine and reconstructed structures at the Γ-point. They clearly visualize the uniformly distributed 2D surface states of the pristine Co layer and the slight and local suppression at the H sites for the 1H-(2 × 2) structure. As the H coverage further increases in 2H-(2 × 2), the surface states change significantly, and the Coderived surface state becomes the H−Co bonded state located at approximately −1.5 eV at the Γ-points (Figure 5c, right panel). The high H coverage suppresses the strong spin asymmetry in LDOS of 2H-(2 × 2) (Figure 5f). We further evaluated the effect of hydrogen adsorption on the total magnetic moments of Co islands. First-principles calculations show that the net magnetic reductions are exclusively from the localized Co d orbitals by 0.04, 0.05, and 0.12 μB per surface Co atom for 1H-(2 × 2), 2H-(2 × 2), and 6H-(3 × 3), respectively, as compared to the pristine Co surface, although such local magnetic moment changes can not be detected by spindependent tunneling current measurements. In conclusion, we demonstrated the chemical control of surface magnetism through H adsorption and desorption on Co nanoislands on Cu(111). We found three different types of surface structures, 1H-(2 × 2), 2H-(2 × 2), and 6H-(3 × 3), with different H coverages. The magnetic surface states on pristine Co survive at low H coverage (0.25 ML) structure but become suppressed as the H coverage increases (0.5 ML). After H desorption in an STM tip treatment process, the magnetic
recovered after H desorption as shown in the zoomed-in spectrum (inset). To understand the physical mechanism of the spin contrast (magnetic asymmetry) of Co islands and the effects of hydrogen adsorption on their magnetic properties, we performed density functional theory (DFT) calculations based on atomic configurations as shown in Figure 1b−e. Figure 4a shows the spin-polarized band structures of pristine,
Figure 4. Calculated electronic structures of pristine Co on Cu(111). (a) Majority (left) and minority (right) electronic band structures. Wave functions projected on Co s, p, and d orbitals are colored in green, red, and blue, respectively. (b) Magnified view of the squared area in (a), where the size of data points is proportional to the lateral averages of squared wave functions at 3 Å above the topmost Co atom. (c) LDOS at 3 Å above the topmost Co atom with majority (upper) and minority (lower) spin channels.
the unfaulted Co trilayer island on Cu(111) surface (see the Methods for details). Differences in the electronic structures of the faulted structure are negligible; thus, we only focus on one of the configurations (Figure S5). Here, we highlight the orbital characteristics of Co, and the Cu-dominated states (with more than 70% of states from Cu) are grayed out for visual clarity. The overall band structure of the Co trilayer essentially preserves the key characteristics of the bulk Co band, that is, dispersive s bands crossing flat d bands. Because of the strong exchange interaction between Co d electrons, the minority spin d band shifts ∼2 eV higher in energy. The surface states dominating the tunneling current are shown in Figure 4b. For the surface states to be distinguished from other states, the squared amplitudes of wave functions were calculated at 3 Å above the topmost Co atoms and represented by the size of data points proportionally. Their orbital characters are also shown by the same color code in Figure 4a. It appears nontrivial that the majority and minority surface bands have inversed dispersions and different orbital characters. To understand the origin of this asymmetry, we investigated the evolution of the Co surface states by increasing the number of layers from monolayer and bilayer to trilayer. It turns out that both majority and minority surface states are degenerated in the D
DOI: 10.1021/acs.nanolett.6b04062 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 5. Calculated electronic structures of Co islands on Cu (111) with different amounts of H coverage. Majority (lefts) and minority (rights) spin bands of (a) pristine Co trilayer (2 × 2 supercell), (b) 1H-(2 × 2), and (c) 2H-(2 × 2) superstructures on Co trilayers. Each point in the band is plotted with the size proportional to the lateral averages of squared wave functions at 3 Å above the topmost Co atoms. Point sizes are doubled for 1H-(2 × 2) and 2H-(2 × 2). LDOS at 3 Å above the topmost Co atom of (a−c) are shown in (d−f), and spatial distributions of surface states at the Γ-point [denoted as black arrows in (a−c)] are in (g−i), respectively. Blue and white balls represent Co and H atoms, respectively, and yellow indicates electron density isosurfaces of 9 × 10−4/bohr3.
islands. All the data was taken with Ni tip except Figure 2d and Figures S2 and S3, which are acquired using a Cr-coated W tip. SP-STM/STM. SP-STM/STS was performed using a variable-temperature STM (Omicron) with Nanonis (SPECS) controller and magnetic tip in UHV chamber. We used constant current mode for topography images and obtained dI/ dV maps with a lock-in technique with AC modulation (1 kHz, 10−30 mV). All STM images and spectroscopy data were analyzed and processed with WSxM software. Both Cr-coated W tip32 and electrochemically etched nickel tip33 were used for SP-STM. Both tips displayed spin-contrast on pristine Co islands. Statistical Analysis. For the statistical analysis, the grid dI/ dV spectroscopy data were first acquired on Co islands (see the Figure S3). The point spectra were then averaged for each of the islands with the exception of points in the rim area where polarization orientations can be different.17 Finally, the dI/dV spectroscopy data from different islands were categorized into four types according to the stacking orders and magnetization alignments. We intentionally selected large islands (side length islands >12 nm) to minimize size-dependent mesoscopic relaxations in the Co islands.18 Controlling Hydrogen Coverage. The hydrogen molecules are the most abundant residues in the UHV chamber according to measurements with a residual gas analyzer (SRS RGA-100). In a UHV with a total pressure ∼6 × 10−11 Torr and hydrogen partial pressure of ∼6 × 10−10 Torr, we found that Co islands stayed clean up to 8 h, as confirmed by atomic resolution STM images and dI/dV spectroscopy (Figure S2). For hydrogen dosing, ultrahigh purity hydrogen gas was introduced into the chamber to reach a total base pressure of 2.4 × 10−10 Torr. 1H-(2 × 2), 2H-(2 × 2), and 6H-(3 × 3)
surface states can largely recover, thus allowing the reversible control of the spin-dependent tunneling conductance. The intra-atomic exchange splitting, which makes Co ferromagnetic, also endows spin dependence to surface states by d-p hybridization. This is manifested in a spin contrast in differential conductance, but the contrast is suppressed if sufficient hydrogen atoms passivate the spin-dependent surface states.
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METHODS
Preparation of Co Islands. A clean Cu(111) single crystal (Purity = 99.9999%; orientation accuracy < 0.1°) was prepared by several cycles of Ar+ sputtering and postannealing at 850 K. The atomically resolved STM images confirmed the surface quality of the Cu(111) at room temperature (Figure S2). The Co islands were prepared using thermal deposition at room temperature (Co coverage ≈ 0.5−0.7 ML; growth rate < 0.1 ML/min). After the Co deposition, the sample was immediately transferred to the precooled STM stage (within 2 min; TSTM = 38 or 130 K) to prevent intermixing of Cu and Co atoms. Magnetic Tip Preparation. We used both Cr-coated W tip32 and electrochemically etched nickel tip33 as spin-polarized probes. The electrochemically etched W tip was first cleaned by e-beam heating under UHV conditions and then coated with high purity (5N) Cr using an e-beam evaporator. The Cr film thickness is less than 100 ML for out-of-plane spin sensitivity.34 The Ni tip was prepared by electrochemical etching Ni wire using KCl solution, which was then rinsed with distilled water and ammonium hydroxide to remove residual salts. After being loaded in the UHV chamber, the Ni tip was briefly annealed at ∼400 °C. Both tips displayed similar spin-sensitivity to the Co E
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(2) Chopra, H. D.; Sullivan, M. R.; Armstrong, J. N.; Hua, S. Z. The quantum spin-valve in cobalt atomic point contacts. Nat. Mater. 2005, 4, 832−837. (3) Parkin, S. S. P.; Bhadra, R.; Roche, K. P. Oscillatory magnetic exchange coupling through thin copper layers. Phys. Rev. Lett. 1991, 66, 2152−2155. (4) Liu, L.; Yang, K.; Jiang, Y.; Song, B.; Xiao, W.; Li, L.; Zhou, H.; Wang, Y.; Du, S.; Ouyang, M.; Hofer, W. A.; Castro Neto, A. H.; Gao, H.-J. Reversible Single Spin Control of Individual Magnetic Molecule by Hydrogen Atom Adsorption. Sci. Rep. 2013, 3, 1210. (5) Dubout, Q.; Donati, F.; Wäckerlin, C.; Calleja, F.; Etzkorn, M.; Lehnert, A.; Claude, L.; Gambardella, P.; Brune, H. Controlling the Spin of Co Atoms on Pt(111) by Hydrogen Adsorption. Phys. Rev. Lett. 2015, 114, 106807. (6) Hermanns, C. F.; Bernien, M.; Krüger, A.; Walter, W.; Chang, Y.M.; Weschke, E.; Kuch, W. Huge magnetically coupled orbital moments of Co porphyrin molecules and their control by CO adsorption. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 104420. (7) LeClair, P.; Swagten, H. J. M.; Kohlhepp, J. T.; van de Veerdonk, R. J. M.; de Jonge, W. J. M. Apparent Spin Polarization Decay in CuDusted Co/Al2O3/Co Tunnel Junctions. Phys. Rev. Lett. 2000, 84, 2933−2936. (8) von Bergmann, K.; Bode, M.; Kubetzka, A.; Heide, M.; Blügel, S.; Wiesendanger, R. Spin-Polarized Electron Scattering at Single Oxygen Adsorbates on a Magnetic Surface. Phys. Rev. Lett. 2004, 92, 046801. (9) Berbil-Bautista, L.; Krause, S.; Hän ke, T.; Bode, M.; Wiesendanger, R. Spin-polarized scanning tunneling microscopy through an adsorbate layer: Sulfur-covered Fe/W(1 1 0). Surf. Sci. 2006, 600, L20−L24. (10) Abeledo, C. R.; Selwood, P. W. Chemisorption of Hydrogen on Cobalt. J. Chem. Phys. 1962, 37, 2709−2712. (11) Mankey, G. J.; Kief, M. T.; Huang, F.; Willis, R. F. Hydrogen chemisorption on ferromagnetic thin film surfaces. J. Vac. Sci. Technol., A 1993, 11, 2034−2039. (12) Barral, M. A.; Weissmann, M.; Llois, A. M. Characterization of the surface states of Co(0001), Co(111), and ultrathin films of Co on Cu(111). Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 125433. (13) Diekhöner, L.; Schneider, M. A.; Baranov, A. N.; Stepanyuk, V. S.; Bruno, P.; Kern, K. Surface States of Cobalt Nanoislands on Cu(111). Phys. Rev. Lett. 2003, 90, 236801. (14) Oka, H.; Brovko, O. O.; Corbetta, M.; Stepanyuk, V. S.; Sander, D.; Kirschner, J. Spin-polarized quantum confinement in nanostructures: Scanning tunneling microscopy. Rev. Mod. Phys. 2014, 86, 1127−1168. (15) Pietzsch, O.; Kubetzka, A.; Bode, M.; Wiesendanger, R. SpinPolarized Scanning Tunneling Spectroscopy of Nanoscale Cobalt Islands on Cu(111). Phys. Rev. Lett. 2004, 92, 057202. (16) Gambardella, P.; Rusponi, S.; Veronese, M.; Dhesi, S. S.; Grazioli, C.; Dallmeyer, A.; Cabria, I.; Zeller, R.; Dederichs, P. H.; Kern, K.; Carbone, C.; Brune, H. Giant Magnetic Anisotropy of Single Cobalt Atoms and Nanoparticles. Science 2003, 300, 1130−1133. (17) Pietzsch, O.; Okatov, S.; Kubetzka, A.; Bode, M.; Heinze, S.; Lichtenstein, A.; Wiesendanger, R. Spin-Resolved Electronic Structure of Nanoscale Cobalt Islands on Cu(111). Phys. Rev. Lett. 2006, 96, 237203. (18) Rastei, M. V.; Heinrich, B.; Limot, L.; Ignatiev, P. A.; Stepanyuk, V. S.; Bruno, P.; Bucher, J. P. Size-Dependent Surface States of Strained Cobalt Nanoislands on Cu(111). Phys. Rev. Lett. 2007, 99, 246102. (19) Wiesendanger, R. Spin mapping at the nanoscale and atomic scale. Rev. Mod. Phys. 2009, 81, 1495−1550. (20) Lewis, E. A.; Le, D.; Murphy, C. J.; Jewell, A. D.; Mattera, M. F. G.; Liriano, M. L.; Rahman, T. S.; Sykes, E. C. H. Dissociative Hydrogen Adsorption on Close-Packed Cobalt Nanoparticle Surfaces. J. Phys. Chem. C 2012, 116, 25868−25873. (21) Lewis, E. A.; Le, D.; Jewell, A. D.; Murphy, C. J.; Rahman, T. S.; Sykes, E. C. H. Visualization of Compression and Spillover in a
surfaces were observed with 29, 121, and 162 L of hydrogen exposure, respectively (Figure S3d). Remarkably, the apparent hydrogen exposure is higher than that reported previously,20,24 although the resulting surface superstructures are similar. First-Principles Theoretical Calculations. The density functional theory calculations employed the Perdew−Burke− Ernzerhof (PBE) exchange-correlation functional and the projector augmented wave method for ionic potentials as implemented in the Vienna Ab Initio Simulation Package (VASP). Fifteen layers of Cu slabs were used to mimic the Cu substrates, and 20 Å of vacuum were used to avoid spurious interslab interactions. The energy cutoff of plane wave basis was 400 eV, and 18 × 18 × 1 k-points including Γ-point were sampled for converged charge densities. All atoms were fully relaxed with a force criterion of 0.02 eV/Å. On the basis of the STM studies, a pristine Co island is 2 ML high from the Cu(111) surface and possibly has an additional Co layer buried under the surface.35 For clarity, we calculated the band structures of both the bilayer and the trilayer Co films and found that the presence of an additional layer does not change much the dispersion and energies of the surface state and resulting magnetic asymmetry (see Figure S4).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b04062. STM images of Co islands with 6H-(3 × 3) superstructures, STM images and bias-dependent SP-dI/dV map of pristine Cu(111) and Co islands, simultaneously acquired SP-STM image and dI/dV maps on a Co island, comparison of electronic band structures of Co bilayer and trilayer on Cu(111), and stacking-dependent band structures of the Co trilayer on Cu(111) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
An-Ping Li: 0000-0003-4400-7493 Present Address †
J.P.: Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), Pohang 790-784, Korea Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. A portion of the work (J.P.) is supported by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. DOE. Computing resources were provided by the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
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REFERENCES
(1) Richter, H. J.; Harkness, S. D. Media for Magnetic Recording Beyond 100 Gbit/in.2. MRS Bull. 2006, 31, 384−388. F
DOI: 10.1021/acs.nanolett.6b04062 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters Coadsorbed System: Syngas on Cobalt Nanoparticles. ACS Nano 2013, 7, 4384−4392. (22) Lewis, E. A.; Marcinkowski, M. D.; Murphy, C. J.; Liriano, M. L.; Sykes, E. C. H. Hydrogen Dissociation, Spillover, and Desorption from Cu-Supported Co Nanoparticles. J. Phys. Chem. Lett. 2014, 5, 3380−3385. (23) Sicot, M.; Kurnosikov, O.; Adam, O. A. O.; Swagten, H. J. M.; Koopmans, B. STM-induced desorption of hydrogen from Co nanoislands. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 035417. (24) Sicot, M.; Kurnosikov, O.; Swagten, H. J. M.; Koopmans, B. Hydrogen superstructures on Co nanoislands and Cu(1 1 1). Surf. Sci. 2008, 602, 3667−3673. (25) Rabe, A.; Memmel, N.; Steltenpohl, A.; Fauster, T. RoomTemperature Instability of Co/Cu(111). Phys. Rev. Lett. 1994, 73, 2728−2731. (26) Vázquez de Parga, A. L.; García-Vidal, F. J.; Miranda, R. Detecting Electronic States at Stacking Faults in Magnetic Thin Films by Tunneling Spectroscopy. Phys. Rev. Lett. 2000, 85, 4365−4368. (27) Ouazi, S.; Wedekind, S.; Rodary, G.; Oka, H.; Sander, D.; Kirschner, J. Magnetization Reversal of Individual Co Nanoislands. Phys. Rev. Lett. 2012, 108, 107206. (28) Anger, G.; Winkler, A.; Rendulic, K. D. Adsorption and desorption kinetics in the systems H2/Cu(111), H2/Cu(110) and H2/Cu(100). Surf. Sci. 1989, 220, 1−17. (29) Gupta, J. A.; Lutz, C. P.; Heinrich, A. J.; Eigler, D. M. Strongly coverage-dependent excitations of adsorbed molecular hydrogen. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 115416. (30) Qin, S.; Kim, T.-H.; Zhang, Y.; Ouyang, W.; Weitering, H. H.; Shih, C.-K.; Baddorf, A. P.; Wu, R.; Li, A.-P. Correlating Electronic Transport to Atomic Structures in Self-Assembled Quantum Wires. Nano Lett. 2012, 12, 938−942. (31) Qin, S.; Hellstrom, S.; Bao, Z.; Boyanov, B.; Li, A.-P. Contacting nanowires and nanotubes with atomic precision for electronic transport. Appl. Phys. Lett. 2012, 100, 103103. (32) Kubetzka, A.; Bode, M.; Pietzsch, O.; Wiesendanger, R. SpinPolarized Scanning Tunneling Microscopy with Antiferromagnetic Probe Tips. Phys. Rev. Lett. 2002, 88, 057201. (33) Cavallini, M.; Biscarini, F. Electrochemically etched nickel tips for spin polarized scanning tunneling microscopy. Rev. Sci. Instrum. 2000, 71, 4457−4460. (34) Bode, M. Spin-polarized scanning tunnelling microscopy. Rep. Prog. Phys. 2003, 66, 523. (35) Pedersen, M. Ø.; Bönicke, I. A.; Laegsgaard, E.; Stensgaard, I.; Ruban, A.; Nørskov, J. K.; Besenbacher, F. Growth of Co on Cu(111): subsurface growth of trilayer Co islands. Surf. Sci. 1997, 387, 86−101.
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DOI: 10.1021/acs.nanolett.6b04062 Nano Lett. XXXX, XXX, XXX−XXX
