Spin Splitting of $s$ and $p$ States in Single Atoms and Magnetic Coupling in Dimers on a Surface

H. J. Lee, W. Ho, and M. Persson

Phys. Rev. Lett. 92, 186802 – Published 6 May 2004

Abstract

Electronic states of magnetic atoms (Mn, Fe, and Co) and artificially assembled dimers ( ${Mn}_{2}$ , ${Fe}_{2}$ , and ${Co}_{2}$ ) on a NiAl(110) surface were probed by scanning tunneling spectroscopy at 17 K. Resonance peaks characteristic of each adsorbed species were observed in the unoccupied density of states. Comparison of the measured spectra with calculations by density functional theory revealed spin splitting in the unoccupied states with $s$ and $p$ characters for the single magnetic adatoms and addimers. The magnitude of the resonance splitting for the adatoms increased with the calculated values of magnetic moments. The resonance structures for the addimers exhibited signatures of their internal magnetic coupling.

Received 25 September 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.186802

Authors & Affiliations

H. J. Lee^* and W. Ho^†

Department of Physics and Astronomy and Department of Chemistry, University of California, Irvine, California 92697-4575, USA

M. Persson

Department of Applied Physics, Chalmers/Göteborg University, S-41296 Göteborg, Sweden

^*On leave of absence from Department of Physics, Cornell University, Ithaca, New York 14853, USA.
^†Corresponding author: wilsonho@uci.edu

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Vol. 92, Iss. 18 — 7 May 2004

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Figure 1
$d I / d V$ spectra for bare NiAl surface, Mn, Fe, and Co adatoms and ${Mn}_{2}$ , ${Fe}_{2}$ , and ${Co}_{2}$ addimers. The adsorption site of adatoms on the NiAl(110) surface is also shown. All spectra were obtained with the same W tip. Tunneling gap was set at $I = 1 nA$ and $V_{sample} = 3 V$ before turning off the feedback and ramping the dc sample bias from 0 to 3 V in 37.5 mV steps and 300 ms dwell time per step. An ac modulation of $10 {mV}_{rms}$ at 200 Hz was added to the dc sample bias for lock-in detection. Each spectrum is an average of 10 scans. For the adatoms, the spectra were recorded above each adatom, and for the addimers, above the center of each addimer. The spectra are plotted on the same scale and are offset for clarity. The dotted lines denote the zero in $d I / d V$ .Reuse & Permissions
Figure 2
(a) Calculated spin-polarized, partial density of states (DOS) of the adatoms. For clarity, the $s$ , $p$ , and $d$ (divided by a factor of 20) partial waves are offset from each other. The majority and minority spin DOS are given by dashed and dotted lines, respectively. (b) Calculated local DOS (LDOS) for the adatoms and the NiAl(110) surface. The LDOS for majority spin [ $ρ_{↑} (ε)$ ], minority spin [ $ρ_{↓} (ε)$ ], and spin-isotropic sum [ $\bar{ρ} (ε) = ρ_{↑} (ε) + ρ_{↓} (ε)$ ] are given by the dashed, dotted, and solid lines, respectively, and are obtained above the adatom at a distance of 7 Å from the NiAl plane. Note that the structures in the occupied DOS were found to be largely suppressed in the LDOS and are not shown here. The short vertical markers indicate the positions of the experimentally observed peaks in Fig. 1.Reuse & Permissions
Figure 3
Schematic of the adatom-adatom interactions between unoccupied resonance states of the adatoms in the addimer: (a) nonmagnetic, (b) ferromagnetic, and (c) antiferromagnetic states of the addimer. The arrows are spin labels for the unoccupied electronic states.Reuse & Permissions
Figure 4
Calculated LDOS for the ferromagnetic (FM) ground state and the antiferromagnetic (AFM) state of ${Mn}_{2}$ , ${Fe}_{2}$ , and ${Co}_{2}$ addimers. The LDOS for $ρ_{↑} (ε)$ , $ρ_{↓} (ε)$ , and $\bar{ρ} (ε)$ are given by the dashed, dotted, and solid lines, respectively, and are obtained above one of the adatoms in the addimer at a distance of 7 Å from the NiAl plane. In the case of the AFM state, majority and minority spins are defined with respect to the adatom under the tip. The short vertical markers indicate the positions of the experimentally observed peaks in Fig. 1.Reuse & Permissions

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