Download Spin-polarized Scanning Tunnelling Microscopy

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Figure 1: (a) Working principle of a scanning tunnelling microscope. (b) Topographic image of
single Fe atoms on a copper surface. (c) Same as (b) but with magnetic contrast superimposed.
Spin-polarized Scanning Tunnelling Microscopy
There are several techniques that can be used to make magnetic domains visible,
such as the Bitter method (using a fine iron powder) or the Kerr effect (using the
rotation of the light’s polarization in a reflection experiment). These work fine since
the typical domain size is not very small (many µm). However, recent experimental
progress has made it possible to observe magnetic properties on a truly atomic
scale using a scanning tunnelling microscope (STM) with a magnetically polarized
tip.
STM is based on the quantum mechanical tunnelling effect. In contrast to a classical
particle, a quantum mechanical particle cannot be trapped completely by a finite
potential step when its energy is lower than the height of the step. What actually
happens is that the wave function of the particle leaks out and the decay away from
the step is exponential. This decay of the wave function from a solid into vacuum
is used in the STM to map the structure of a solid on the atomic scale.
Fig. 1(a) shows the working principle for an STM. It consists of an atomically sharp
tip very close to the sample. The tip can be moved with high precision using three
mutually orthogonal piezoelectric transducers, i.e. rods made from a piezoelectric
material with a length that is controlled by an applied high voltage. A small voltage
is applied between the tip and the sample and the tip is then approached to the
surface until the decaying wave functions from the sample states and the tip states
overlap in vacuum and a current starts to flow. This current is typically in the nA
order of magnitude and the tip-sample distance is a few Ångströms. The sign of
the applied voltage determines if electrons tunnel out of the tip into the sample or
vice versa.
The instrument can now be used to generate an image of the surface. This is done
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by moving the tip in the x and y directions while using a feed-back loop in order
to hold the current constant. In order to achieve such a constant current, the tip
has to be moved in the z direction to follow the atomic contours on the surface.
This means that if the z motion is mapped as a function of x and y, one obtains
an atomic scale image. Such an image of the surface topography as shown in Fig.
1(b) for single Fe atoms adsorbed on a copper surface.
The techniques works only because the tunnelling current decays so dramatically
with the distance from the sample. In most realistic situations, the tip is not very
sharp on the atomic scale, as shown in the magnifications of Fig. 1(a). If, however,
one atom sticks out further than the others, the distance dependence of the current
will guarantee that most of the tunnelling current flows through this atom. This is
also the key to making the technique magnetically sensitive: If the atom on the very
apex of the tip is magnetized in one particular direction, the electrons emitted from
this atom will be slightly spin polarized. This, in turn, influences the tunnelling
into a surface with magnetically polarized atoms: the tunnelling will be slightly
more favourable into a magnetic domain or even a single atom with the same spin
polarization as the tip than to one with the opposite polarization.
While the tunnelling current is already small, the influence of the spin polarization
on the current is much smaller still so that detecting current differences due to spin
polarization is very difficult. Moreover, it is not easy to disentangle the magnetic
signal from the structural signal. If, for instance, the microscope’s entire scanning
region has only one magnetization, it is not possible to detect this.
Despite of these difficulties, spin-polarized STM has made enormous progress and
it is now even possible to observe the magnetization of single atoms. An example
is shown in Fig. 1(c). The image shows the same situation as in Fig. 1(b) of
Fe atoms on a copper surface. These atoms carry a magnetic moment and they
even couple to each other. The colour coding in the image corresponds to the
magnetization of the atoms and shows that the coupling between atoms in the
chain in the front is anti-ferromagnetic, for instance. For these particular images
the magnetic information has not been extracted from the total tunnelling current
but spectroscopically for a particular bias voltage range where the magnetic contrast
is largest. This has then be superimposed on the image in Fig. 1(b).
The images are courtesy of Alexander Ako Khajetoorians and for more details on
this experiment see A. Khajetoorians et al., Science 332, 6033 (2011). For more
information on spin-polarized STM, see R. Wiesendanger, Rev. Mod. Phys. 81,
1495 (2009).
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Online note to accompany the book “Solid State Physics - An Introduction”, Wiley VCH. Copyright
(C) 2014 by Philip Hofmann.
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