Spin-polarized scanning tunneling microscopy
Spin-polarized scanning tunneling microscopy (SP-STM) is a type of scanning tunneling microscope (STM) that can provide detailed information of magnetic phenomena on the single-atom scale additional to the atomic topography gained with STM.
SP-STM opened a novel approach to static and dynamic magnetic processes as precise investigations of domain walls in ferromagnetic and antiferromagnetic systems, as well as thermal and current-induced switching of nanomagnetic particles.
An extremely sharp tip coated with a thin layer of magnetic material is moved systematically over a sample.
A voltage is applied between the tip and the sample allowing electrons to tunnel between the two, resulting in a current.
In the absence of magnetic phenomena, the strength of this current is indicative for local electronic properties.
The tip's magnetization thus flips too fast for the STM feedback loop to respond to and topographical information is obtained intact.
In spin-polarized STM (SP-STM) the tunneling current also depends on the spin-orientation of the tip and the sample.
On the other hand, if the spins are antiparallel most of the available states are already filled and the tunneling current will be significantly smaller.
With SP -STM it is then possible to probe the spin dependent local density of states of magnetic samples by measuring the tunneling conductance
is the tunneling matrix element which describes the transitions between the spin dependent states of the tip and the sample,
), this expression reduces to the Tersoff and Hamann model for standard STM tunneling conductance.
[2] The most critical component in the SP-STM setup is the probe tip which has to be atomically sharp to offer spatial resolution down to atomic level, have large enough spin polarization to provide sufficient signal to noise ratio, but at the same time have small enough stray magnetic field to enable nondestructive magnetic probing of the sample, and finally the spin orientation at the tip apex has to be controlled in order to determine which spin orientation of the sample is imaged.
Tunneling current is primarily dominated by the smallest non-zero reciprocal lattice vector, which means that as magnetic superstructures usually have the longest real space periodicities (and thus the shortest reciprocal space periodicities), bring the largest contribution to the spin-dependent tunneling current
is directly related to the spin-resolved LDOS of the sample whereas the measured tunneling current
Since spin-polarized LDOS can change not only magnitude but also sign as a function of energy, the measured tunneling current can vanish even if there is finite magnetization in the sample.
Thus bias dependence of the spin-polarized tunneling current in modulated magnetization mode has to be studied as well.
Only ferromagnetic tips are suitable for modulated magnetization mode meaning that their stray fields might make nondestructive imaging impossible.
[10] The spin polarized scanning tunneling microscope is a versatile instrument which has gained tremendous attention due to its enhanced surface sensitivity and lateral resolution up to atomic scale, and can be used as an important tool to study ferromagnetic materials, such as dysprosium (Dy), quasi-2D thin films, nano islands and quasi-1D nanowire that have high magnetic anisotropy, etc.
In a study carried out by L. Berbil-Bautista et al.,[11] the magnetic domain wall or Néel wall of the width 2-5 nm present in these materials is observed by bringing the chromium (Cr)-coated tungsten tip close to the Dy layer.
The magnetic contrast is enhanced due to the presence of electronic states that are not occupied in the cluster of Dy atoms present on the apex of the tip.
[11] The formation of 360° domain walls in ferromagnetic films plays an important role in making magnetic random access memory devices.
In a study carried out by A. Kubetzka et al.,[12] the SP-STM was used to measure the evolution of 360° domain wall profiles of two atomic layer iron nanowires by varying the external magnetic field between 550-800 mT.
[12] The quantum interference phenomena has been observed in cobalt islands deposited on copper(111) substrate.
Spin polarized-STM has been used to investigate the electronic structure of triangular Cobalt islands deposited on copper(111).
This study shows that the substrate and islands exhibit their individual standing wave patterns and this can be used to find the spin polarized material.
Non-magnetic impurities, such as oxygen on magnetic surface (iron double layer on tungsten (W) substrate) causes formation of spin polarized waves.
This study shows that anisotropic scattering states can be observed around individual oxygen atoms adsorbed on iron double layer.
[14] Similarly, existence of 2D anti-ferromagnetism at the interface of manganese (Mn) and W(110) has been observed using SP-STM technique.
[15] Another way to obtain the magnetization distribution is to have the tip provide a strong stream of spin polarized electrons.
[16] One limitation of this method is that the most effective source of spin polarized electrons is obtained by having the incident laser light shine directly opposite of the tip, i.e. through the sample itself.
