Don Eigler IBM Fellow. Spin Excitation Spectroscopy : A Tool Set For Nano-Scale Spin Systems

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1 Don Eigler IBM Fellow Spin Excitation Spectroscopy : A Tool Set For Nano-Scale Spin Systems NSF Grantees Conference, Arlington, VA. December 6, 2010

2 A Challenge Build a Spin-Only Nano-Scale Digital Computer Question: How can one can engineer computational functionality into a nanometer-scale system of spins? Prerequisite: A tool for measuring spin properties at the atomic scale.

3 An STM is a powerful tool, but conventional STM topographs tell nothing about spin

4 Spin Excitation Spectroscopy Part I : Energetics

5 Spin Excitation Spectroscopy A variant of inelastic electron tunneling spectroscopy R. C. Jaklevic, J. J. Lambe, Phys. Rev. Lett. 17, 1179 (1966) H E gμ B H H A Non-Magnetic Tip An Atom with a Magnetic Moment di/dv σ e σ e + σ ie A Non-Magnetic Surface -gμ B H 0 gμ B H ev A.J. Heinrich, J.A. Gupta, C.P. Lutz and D.M. Eigler, Science 306, 466 (2004) Κ Β Τ g H B

6 Scaled di/dv The Spin Excitation Signal From a Single Manganese Atom B=0.0 T B=2.8 T B=4.2 T B=5.6 T B=7.0 T Voltage [mv] A.J. Heinrich, J.A. Gupta, C.P. Lutz and D.M. Eigler, Science 306, 466 (2004)

7 Site-Specific Measurement of Single-Atom g-value 0.8 Mn atom 1, g=2.01±0.03 (near island edge) Mn atom 2, g=1.88±0.02 (middle of island ) D [mev] B [T] Mn on Al 2 O 3 island on NiAl A.J. Heinrich, J.A. Gupta, C.P. Lutz and D.M. Eigler, Science 306, 466 (2004)

8 di/dv [a.u.] Energy [mev] Measurement of Single-Atom Anisotropy Energy B = 0T 3.5T 7.0T D = D 0 + g B B g = 2 D 0 = 0.18meV 0.2 B perpendicular B parallel Voltage [mv] B [T] Magneto-crystalline anisotropy: H = gµ B S B + DS z 2 Easy axis perpendicular to surface Mn Monomer Anisotropy Energy = 0.18 mev C. Hirjibehedin, C-Y Lin, A.F. Otte, M. Ternes, C.P. Lutz, B.A. Jones and A.J. Heinrich, Science 317, 1199(2007)

9 di/dv [a.u.] Measurement of Exchange Coupling Between Atoms Mn Dimer on CuN on Cu (100) 1.5 B=7T 1.0 B=0T Voltage [mv] J = 6mV C. Hirjibehedin, C.P. Lutz and A.J. Heinrich, Science 312, 1021(2006)

10 di/dv [A.U.] Field Dependence of Mn Dimer Spectrum Voltage [mv] T 4T T Voltage [mv] B [T] Dimer step at ~6mV splits into three distinct steps

11 Energy Determination of Ground State Spin Configuration For S=0 (singlet) the first excited state is S=1 (triplet) Singlet has no low-lying excitations 1,+1> 1,-0> 1,-1> 0,0> Magnetic Field The Ground State of the Mn Dimer is a Spin Singlet. The Mn Atoms are Antiferromagnetically Coupled C. Hirjibehedin, C.P. Lutz and A.J. Heinrich, Science 312, 1021(2006)

12 Spin Excitation Spectroscopy Part II : Dynamics

13 di/dv ( S) High Magneto-Crystalline Anisotropy Fe-Cu Complex 0.16 Fe-Cu Mn mV 0.08 B=1.5T +15.9mV Voltage (mv) Very large easy-axis magneto-crystalline anisotropy Spectra show saturation behavior when tunnel current is large Question: Could it have a long spin relaxation time, T 1?

14 Pump-Probe Technique to Measure Spin Relaxation Time, T 1 1) Pump: Use a super-threshold voltage pulse to create a spin excitation 2) Probe: Use a sub-threshold voltage pulse to sense the spin orientation at a variably delayed time following the pump pulse

15 Average Conductance Single Atom Spin Dynamics Pump-Probe Measurement of Electron Spin Relaxation Time, Magneto-Resistive Tunneling Conductance T 1 variable delay B B Threshold for spin excitation Pump Pulse Probe Pulse T 1 Ground State Spin Configuration High elastic conductance Excited State Spin Configuration Low elastic conductance Delay

16 probe current (pa) Probe current (pa) Single-Atom Pump-Probe Measurement of T Mn atom FeCu complex FeCu Mn -18 exponential fit Delay delay time ( s) T 1 = 180 ± 12 ns S. Loth, M. Etzkorn, C.P. Lutz, D.M. Eigler and A.J. Heinrich, Science, 329, 1628 (2010)

17 spin lifetime (ns) Magnetic Field and Site Dependence of T magnetic field (T) Ability to Measure Site Dependent Variations in T 1

18 Spin Contrast Imaging Magnetic Resonance Imaging Contrast From Spatial Variations in Nuclear T1

19 A Comparison Between Magnetic Resonance & Spin Excitation Spectroscopy : NMR & ESR Spin Excitation Spectroscopy g-value T 2 Not yet T 1 Spectral Resolution Excellent Poor Spatial Resolution Poor Excellent Field Homogeneity Issue No Problem Sample Heterogeneity Issue No Problem

20 What We Can Learn From Spin Excitation Spectroscopy 10Mn 1nm Energetics Atom Specific g-value Magnitude Anisotropy Energy Symmetry of Anisotropy Field Orientation of Anisotropy Field Magnitude and Sign of Exchange Energy Ground State Spin Configuration Excited State Spin Configuration Dynamics Atom Specific Relaxation Time, T 1 Magnetic Field Dependence of T 1 Correlation of Spin Properties with Local Structural, Electronic, Chemical & Mechanical Properties

21 Cast of Characters Andreas Heinrich IBM Team Leader Chris Lutz IBM Resident Genius Bruce Melior IBM Technical Support Jay Gupta Ohio State Postdoc Cyrus Hirjibehedin UCL Postdoc Markus Ternes MPI Stuttgart Postdocs Sebastian Loth TBD Postdoc Sander Otte TU Delft Grad Student Markus Etzkorn EPFL Visitor Barbara Jones IBM Theory C-Y Lin Taiwan Theory Postdoc Work partially funded by the Office of Naval Research