1. STM Conference Talk: Dirk Sander
June 14th, 2013

2. Dirk Sander in a Single Slide
Dirk Sander has spent the last decade studying the magnetic properties of dimensionally reduced systems and their dependence on electronic structure, crystalline structure and morphology.

3. Background
Group leader in Spin Polarized STM in Magnetic Field and Stress Measurements at the Max-Planck-Institut für Mikrostrukturphysik Experimental Department 1 in Halle, Germany working under Prof. J. Kirschner

4. Background
Group leader in Spin Polarized STM in Magnetic Field and Stress Measurements at the Max-Planck-Institut für Mikrostrukturphysik Experimental Department 1 in Halle, Germany working under Prof. J. Kirschner

5. SP-STM Capabilities
Omicron Cryogenic Scanning Probe Microscope with Janis LHe cooled cryostat. The cryostat cools as low as 7 K, and it also contains superconducting magnets for STM measurements in fields of up to 8 T.
The group recently commissioned a sub-K STM with a vector magnet to study: “field-dependent spectroscopic properties of individual nanostructures at temperatures as low as 0.3 K”. This instrument is to focus on investigations of the interplay between superconductivity and ferromagnetism at the nanoscale.
Spectra of the differential conductance of 80 ML of Pb on Cu(111) with a Nb tip at 0.35 K.

6. Non-Exhaustive Publication History
Studying the magnetic properties of dimensionally reduced systems and their dependence on electronic structure, crystalline structure and morphology.
W. Pan, D. Sander, M. Lin, & J. Kirschner, PRB, 68, (2003)
D. Sander, Zhen Tian and J. Kirschner, Sensors, 8, (2008)
D. Sander, Z. Tian, & J. Kirschner, J. Phys.: Condens. Matter (2009)
Z. Tian, D. Sander, & J. Kirschner, PRB, 79, (2009)
Z. Tian, D. Sander, et al., PRB 81, (2010)
H. Oka, P. A. Ignatiev, S. Wedekind, G. Rodary, L. Niebergall, V. S. Stepanyuk, D. Sander, J. Kirschner Science 327, 843 (2010)
Phark et al., ACS Nano, 5, (2011)
H. Oka, K. Tao, S. Wedekind, G. Rodary, V. S. Stepanyuk, D. Sander, and J. Kirschner, PRL 107, (2011)
Phark et al., PRB, (2012)

7. Non-Exhaustive Publication History
Studying the magnetic properties of dimensionally reduced systems and their dependence on electronic structure, crystalline structure and morphology.
W. Pan, D. Sander, M. Lin, & J. Kirschner, PRB, 68, (2003)
D. Sander, Zhen Tian and J. Kirschner, Sensors, 8, (2008)
D. Sander, Z. Tian, & J. Kirschner, J. Phys.: Condens. Matter (2009)
Z. Tian, D. Sander, & J. Kirschner, PRB, 79, (2009)
Z. Tian, D. Sander, et al., PRB 81, (2010)
H. Oka, P. A. Ignatiev, S. Wedekind, G. Rodary, L. Niebergall, V. S. Stepanyuk, D. Sander, J. Kirschner Science 327, 843 (2010)
Phark et al., ACS Nano, 5, (2011)
H. Oka, K. Tao, S. Wedekind, G. Rodary, V. S. Stepanyuk, D. Sander, and J. Kirschner, PRL 107, (2011)
Phark et al., PRB, (2012)

8. Science 327, 843 (2010)
Performed SP-STM experiments at 8 K of Co islands on Cu(111) looking for spin dependent quantum interference within the islands. To detect magnetic contrast they used Cr-Co-W tips in an external magnetic field of 4 T.

9. Science 327, 843 (2010) If eT ||eS then: The theory:
To look at the spatial distribution of the spin polarization they measured the spatially resolved map of the differential tunneling conductance asymmetry AdI/dV:
If eT ||eS then:
Wiesendanger Rev. Mod. Phys. 81, (2009)

10. Science 327, 843 (2010)

11. Science 327, 843 (2010)
How do they know whether the magnetization of tip and sample is parallel or antiparallel?

12. Science 327, 843 (2010)
How do they know whether the magnetization of tip and sample is parallel or antiparallel?
By sweeping the external field.

13. Science 327, 843 (2010)
To understand why the majority polarization occupies the center of the island and the minority lives in edge states they modeled the Co nanoislands using DFT in the KKR Green’s function method.

14. Spin polarization map from DFT
Science 327, 843 (2010)
Spin polarization map from DFT
AdI/dV
The dI/dV asymmetry map of the Co island can be interpreted as:
qualitatively shows a spatial distribution of the spin polarization on the Co island at a certain energy
The modulation pattern in the dI/dV asymmetry originates from the LDOS of the majority spin, which is due to the electron confinement of the free electron-like s-p surface state
The spatial modulation is therefore coming from spin dependent quantum interference!

15. Science 327, 843 (2010)

16. Science 327, 843 (2010)
All you need to get published in Science is a great experiment combined with an equally great colour scheme!

17. Science 327, 843 (2010) to PRL 107, (2011)
What else can spin dependent quantum interference be used for?
Tunneling between magnetic electrodes depends on their relative magnetization direction, this effect is known as tunneling magnetoresistance (TMR) and is used in all current hard drive readheads.
Taking the same experimental set-up as in the Science paper Sander’s group set out to measure the spatial modulation of the TMR in the tip-Co island tunneling junction

18. PRL 107, (2011)

19. PRL 107, (2011)
Maximum positive TMR ratio of 50%

20. PRL 107, (2011)
“The modulation pattern found in the TMR ratio map is similar to that of a dI/dV asymmetry map which qualitatively corresponds to spin polarization.”

21. PRL 107, (2011)

22. Summary and Conclusions
Dirk Sander’s group at MPI is one of the world’s preeminent leaders in spin polarized STM
they have been the first to observe nanoscale magnetic phenomenon such as spin dependent quantum interference and ~1 nm spatial variation in the TMR
With a new high field sub-K STM I would expect to see them again in PRL and Science in the not-too-distant future

23. Summary and Conclusions
Dirk Sander’s group at MPI is one of the world’s pre-eminent leaders in spin polarized STM
they have been the first to observe nanoscale magnetic phenomenon such as spin dependent quantum interference and ~1 nm spatial variation in the TMR
With a new high field sub-K STM I would expect to see them again in PRL and Science in the not-too-distant future
Thanks for listening!