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Science 31 August 2007:
Vol. 317. no. 5842, pp. 1199 - 1203
DOI: 10.1126/science.1146110
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Reports
Large Magnetic Anisotropy of a Single Atomic Spin Embedded in a Surface Molecular Network
Cyrus F. Hirjibehedin1,
Chiung-Yuan Lin1,2,
Alexander F. Otte1,3,
Markus Ternes1,4,
Christopher P. Lutz1,
Barbara A. Jones1 and
Andreas J. Heinrich1
1 IBM Research Division, Almaden Research Center, San Jose, CA 95120, USA.
2 Center for Probing the Nanoscale, Stanford University, Stanford, CA 94309, USA.
3 Kamerlingh Onnes Laboratorium, Universiteit Leiden, 2300 RA Leiden, Netherlands.
4 Institut de Physique des Nanostructures, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.
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Fig. 1. Fe atoms on CuN. (A) (Left) Processed (37) constant-current topograph (10 mV, 0.5 nA) of two adjacent CuN islands with a single adsorbed Fe atom. The topograph is negative-curvature (high-pass) filtered to enhance contrast, with lattice positions of Cu (yellow dots) and N (green dots) atoms overlayed. The light vertical features on the left side of the image are formed by the absence of single rows of N atoms from the CuN surface. The topographic peak of the Fe atom (blue cross) shows its binding site: on top of a Cu site with two N atoms as horizontal neighbors. (Right) Cross section of the unfiltered topograph along the dashed line indicated in the left panel. (B) The charge density for a CuN surface on Cu(100) calculated with the DFT methods described in the text along the N (left) and hollow (right) directions. The scale for the magnitude of the charge density is shown at the bottom in units of e/a03. Solid yellow and green circles with gray edges label the centers of the Cu and N atoms, respectively. The numbers inside the circles indicate the net charge on selected atoms in units of e (25). (C) Same as (B) with an Fe atom (blue) adsorbed on the CuN on top of a surface Cu site.
[View Larger Version of this Image (96K GIF file)]
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Fig. 2. Conductance spectra of Fe atoms on CuN. (A) Spectra taken with the STM tip positioned above an Fe atom at T = 0.5 K and B = 0 to 7 T oriented in the N direction. The spectra were acquired at a nominal junction impedance of 10 megohm (10 mV, 1 nA) and were not sensitive to junction impedance. Successive spectra are vertically offset by 0.023 nA/mV for clarity. Red, green, and blue upward arrows indicate the positions of the first, second, and third excitations, respectively, at B = 0 T as calculated by Eq. 1 with the fit parameters described in the text. Downward arrows show the same excitations at B = 7 T with the magnetic field oriented along the z axis of Eq. 1. (B) Same as (A) with B oriented along the hollow direction for the spectra; this direction corresponds to the x axis of Eq. 1. Also included are magenta downward arrows indicating the calculated position of the fourth transition at B = 7 T. (C) Energies for the first (red triangles), second (green circles), and third (blue triangles) steps observed in the spectra acquired with B along the N direction, including those shown in (A). Solid lines indicate excitation energies calculated by Eq. 1 with the magnetic field oriented along the z axis. (D) Step energies for the spectra acquired with B along the hollow direction, which corresponds to the x axis of Eq. 1, including those shown in (B). (E) Relative step heights for the first (red), second (green), and third (blue) excitations as a function of B along the N direction. The individual step heights are normalized by the sum of the three step heights at each value of B. The fourth excitation is not included because its intensities are negligible in this range of B. Solid lines denote normalized transition intensities calculated using Eq. 2 with the fit parameters discussed in the text. (F) Simulated spectra, as described in the text, with B along the hollow direction and an effective temperature of 0.8 K. Arrows are the same as in (B). These spectra are scaled by an overall constant and offset to match those shown in (B). a.u., arbitrary units.
[View Larger Version of this Image (39K GIF file)]
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Fig. 3. Conductance spectra and structure of Mn atoms on CuN. (A) Spectra (black) taken with the tip positioned above a Mn atom at T = 0.5 K, with the magnetic field oriented out-of-plane. All spectra were acquired at a nominal junction impedance of 10 megohm (10 mV, 1 nA) andare offset by 0.025 nA/mV for clarity. Red lines represent simulated spectra with an effective temperature of 0.7 K. The simulated inelastic spectra are scaled by an overall constant and offset to match the observed spectra. (B) Step energy of two different Mn atoms, indicated by circles and triangles, with B oriented out-of-plane. Colored lines show the possible transitions energies calculated using Eq. 1 with the fit parameters listed in the text for B along the z direction. Because the anisotropy parameters are substantially smaller than those for Fe, level crossings complicate the assignment of the spin excitations at small magnetic fields. (C) Charge density for a Mn atom adsorbed on a CuN surface on Cu(100) calculated by DFT along the N and hollow directions. Solid yellow, green, and blue circles with gray edges label the centers of the Cu, N, and Mn atoms, respectively. The charge-density scale is the same as that shown in Fig. 1.
[View Larger Version of this Image (17K GIF file)]
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Fig. 4. Calculated net–spin-density distribution for Fe on CuN. Contours (purple) of constant net spin density (0.01 e/a 30), as calculated by DFT for an Fe atom adsorbed on a Cu site on a CuN surface, are shown (25). Only the Fe atom and the atoms in the CuN surface layer are shown for clarity. Small yellow, green, and blue balls indicate the positions of the Cu, N, and Fe atoms, respectively, in the surface layer.
[View Larger Version of this Image (34K GIF file)]
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