Spin polarized scanning tunneling microscopy

Spin-polarized scanning tunneling microscopy (SP-STM) is a specialized application of scanning tunneling microscopy (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. Spin-polarized scanning tunneling microscopy (SP-STM) is a specialized application of scanning tunneling microscopy (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. If the tip is magnetized, electrons with spins matching the tip's magnetization will have a higher chance of tunneling. This is essentially the effect of tunnel magnetoresistance and the tip/surface essentially acts as a spin valve. Since a scan using only a magnetized tip cannot distinguish between current changes due to magnetization or space separation, multi-domain structures and/or topographical information from another source (frequently conventional STM) must be utilized. This makes possible magnetic imaging down to the atomic scale, for example, in antiferromagnetic system. Topographical and magnetic information can be simultaneously obtained if the tip´s magnetization is modulated at a high frequency (20–30 kHz) using a small coil wound around the tip. The tip´s magnetization thus flips too fast for the STM feedback loop to respond to and topographical information is obtained intact. The high frequency signal is separated using a lock-in amplifier and this signal provides the magnetic information about the surface. In standard scanning tunneling microscopy (STM), the tunneling probability of electrons between the probe tip and the sample strongly depends on the distance between them, as it decays exponentially as the separation increases. In spin-polarized STM (SP-STM) the tunneling current also depends on the spin-orientation of the tip and the sample. The local density of states (LDOS) of the magnetic tip and the sample is different for different spin orientations, and tunneling can occur only between the states with parallel spin (ignoring spin flip processes). When the spin of sample and the tip are parallel there are many available states to which the electrons can tunnel, thus resulting in a large tunneling current. 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 G = d I / d U {displaystyle G=mathrm {d} I/mathrm {d} U} , which for small bias is given by In the more general case, with finite bias voltage U { extstyle U} , the expression for the tunneling current at tip location r { extstyle mathbf {r} } becomes 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. In order to prevent oxidization the tip preparation usually has to be done in ultra-high vacuum (UHV). There are three main ways to obtain probe tip suitable for SP-STM measurements: SP-STM can be operated in one of three modes: constant current, and spectroscopic mode which are similar to standard STM operation modes but with spin-resolution, or modulated tip magnetization mode which is unique to SP-STM measurements. In constant current mode, the tip-sample separation is kept constant by an electric feedback loop. The measured tunneling current I {displaystyle I} consists of spin-averaged and spin-dependent components ( I = I 0 + I d {displaystyle I=I_{0}+I_{mathrm {d} }} ) which can be decomposed from the data. 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 I d {displaystyle I_{mathrm {d} }} . Thus SP-STM is an excellent method to observe magnetic structure rather than atomic structure of the sample. The downside is that it is difficult to study larger than atomic scales in constant-current mode as the topographical features of the surface may interfere with the magnetic features making data analysis very difficult. The second mode of operation is spin-resolved spectroscopic mode which measures local differential tunneling conductance d I / d U {displaystyle mathrm {d} I/mathrm {d} U} as a function of bias voltage U {displaystyle U} and spatial coordinates of the tip. Spectroscopic mode can be used under constant-current conditions in which the sample-tip separation varies resulting in superposition of topographic and electronic information which can then be separated. If spectroscopic mode is used with constant tip-sample separation, the measured d I / d U {displaystyle mathrm {d} I/mathrm {d} U} is directly related to the spin-resolved LDOS of the sample whereas the measured tunneling current I {displaystyle I} is proportional to the energy-integrated spin-polarized LDOS. By combining the spectroscopic mode with constant-current mode, it is possible to obtain both topographic and spin-resolved surface data.