Spin-Polarized Scanning Tunneling Microscopy
SP-STM is a general-purpose technique for imaging spin structure of surfaces with the ultimate in spatial resolution. SP-STM combines 2 things: 1. inherent spatial resolution of STM (lateral 0.2 Angstrom, vertical 0.01 Angstrom) 2. sensitivity to surface magnetization vector
These 2 features make an unprecedented technique:
Figure 1. Schematic illustration of the SP-STM method. The tunneling current and thus the height depends on the relative orientations between tip and sample spins.
Some important points about SP-STM: 1. Whereas normal STM is sensitive to the “normal” non-magnetic local density of states of the sample surface, SP-STM is sensitive to the magnetic local density of states of the sample surface 2. SP-STM is sensitive to the spin density of the surface in the vicinity of the Fermi level, within the energy window defined by the applied bias voltage 3. Since the total magnetization of any point on a surface is related to the occupied spin density integrated over all energy, SP-STM is not sensitive to the total magnetic moment of the sample surface Rather, it is only sensitive to the partial magnetic moment near the Fermi level 4. SP-STM is not sensitive to the magnetic field (it is not a fieldsensitive technique) 5. If either the tip or sample magnetic local density of states is equal to zero, Then there will be no spin contrast 6. Or, if tip and sample magnetization vectors are perpendicular, spin contrast will be zero.
It is possible to be sensitive to different types of sample magnetization Either in-plane Or out-of-plane
To be sensitive to in-plane vs. out-of-plane sample magnetization, it is important to be able to control the magnetization vector direction of the tip
Wiesendanger group in Hamburg, Germany have pioneered this capability and have reported several recipes for producing tips having different spin polarization directions
As an example:
- A. Kubetzka, M. Bode, O. Pietzsch, and R. Wiesendanger, “SpinPolarized Scanning Tunneling Microscopy with Antiferromagnetic Probe Tips,” Phys. Rev. Lett. 88(5), 057201 (2002).
Equations governing SP-STM
Assume that the tip and sample have magnetic polarization magnitudes Pt and Ps Assume the angle between the tip and sample magnetic polarizations is θ Then the tunneling current is going to go according to:
This equation can be written in terms of the local densities of states of the sample and tip as:
Atomic-Scale Spin-Polarized Scanning Tunneling Microscopy and Atomic Force Microscopy: a Review, Arthur R. Smith, Journal of Scanning Probe Microscopy 1(1), 3 (2006).
In the above equation,
Vs is the applied sample bias voltage nt is the total LDOS of the tip (spin majority + spin minority) ns is the normal LDOS of the sample (spin majority + spin minority) mt is the magnetic LDOS of the tip (spin majority – spin minority) ms is the magnetic LDOS of the sample (spin majority – spin minority) We can write these quantities as:
nt = nt,> + nt,< ns = ns,> + ns,< mt = mt,> − mt,< ms = ms,> − ms,