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5 Magnetic Scanning Probe Techniques

5 Magnetic Scanning Probe Techniques

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Microscopy – Nanoscopy

2.5 Magnetic Scanning Probe Techniques

The magnetic properties of solids, which arise due to the spin alignment of electrons,

can be studied by scanning probe microscopy such as magnetic force microscopy

(MFM; see [2.20, 2.64]) or, with atomic resolution, by spin-polarized scanning tunneling microscopy (SP-STM; see [2.10]). In addition to the brief survey given here,

a more detailed discussion of magnetic nano-imaging will be given in Sect. 8.1.

2.5.1 Magnetic Force Microscopy (MFM)

The MFM non-contact imaging mode is sensitive to the magnetostatic dipole–dipole

interaction between tip and sample. If a ferromagnetic tip approaches a magnetic

sample surface within a distance of typically 10–50 nm, the tip interacts magnetically with the stray field emanating from the sample [2.65]. The long-range

magnetic dipole interaction is usually probed by using the a. c. detection technique

and gradients rather than the magnetic dipole forces are measured.

For a magnetic domain in the tip, that is small compared to the extent of the

sample stray field Bs , the tip can be considered as a point dipole with the magnetic

moment m yielding [2.66] a force

F(dipole) = ∇ (m · Bs ) = (m · ∇) Bs .

Therefore, in the point-dipole limit, the MFM images are closely related to the

spatial distribution of the magnetic stray field gradient rather than the stray field.

Since magnetic forces Fmag may be either attractive or repulsive, problems with

the feedback loop stability may emerge. Therefore, an attractive electrostatic force

Fel via a bias voltage of 1–10 V between tip and specimen is applied. A separation

of topography and magnetic structure has been demonstrated by imaging a discretetrack magnetic recording sample with the magnetic structure of bits (see Fig. 2.23).

Fig. 2.23 Comparison of the different length scales involved in magnetic data storage. (a) and (b)

show magnetic force microscopy (MFM) images of bit tracks of a magnetic tape whereas (c) shows

a nanoscale magnetic medium where the imaging has been performed by spin-polarized tunneling

microscopy (SP-STM) which is directly sensitive to the local magnetization rather than to the

magnetic stray field sensed by MFM. (Reprinted with permission from [2.10]. © 2000 Wiley-VCH)


Magnetic Scanning Probe Techniques


Care should be taken for not destroying the magnetic surface structure by the

interaction of the magnetic tip (Ni or Ni-coated W) with the magnetic surface [2.64].

A requirement for high spatial resolution in MFM is to operate the probe tip close

to the sample surfaces. Standard experimental resolution is typically of the order of

50–100 nm with a theoretical MFM resolution limit of about 5–10 nm (see [2.20]).

2.5.2 Spin-Polarized Scanning Tunneling Microscopy (SP-STM)

For distinguishing electrons with different spin alignments in magnetic solids by

STM techniques, the probe tip can be coated with a thin film of magnetic material

which exhibits a strong difference in the number of states with different spin orientations [2.67]. This leads to a difference in tunneling conductance for parallel or

antiparallel alignment of the spins in the tip and the substrate. Due to the atomic

resolution capabilities of the STM, spin structures down to the atomic level can be

studied as demonstrated for ferromagnetic [2.68] or antiferromagnetic [2.69] specimens (see Fig. 2.24). In addition, novel magnetic devices for ultradense data storage

Fig. 2.24 Schematic of imaging a 2D antiferromagnet by means of spin-polarized scanning tunneling microscopy (SP-STM) making use of a magnetically coated probe tip. Depending on the

relative spin orientation in the tip and in the substrate, the tunneling current differs and therefore

enables the direct visualization of the antiferromagnetic structure on an atomic level. A single

atomic layer of manganese on a tungsten substrate has been imaged by making use of an ironcoated probe tip at 16 K [2.69, 2.10]. The green and red spheres with the arrows symbolize the

manganese atoms with opposite directions of the magnetic moments. The black and the white

stripes in the center of the figure are experimental STM images (see [2.69]). (Reprinted with

permission from [2.10]. © 2000 Wiley-VCH)



Microscopy – Nanoscopy

may be studied and controlled by the direct detection of the local magnetization in

the near-field regime (Fig. 2.23c) instead of probing the long-range magnetic stray

field (Fig. 2.23a, b).

2.6 Progress in Electron Microscopy

The transmission electron microscope, which was invented in the early thirties of

the last century [2.11] (Nobel prize 1985 for Ernst Ruska) has since played a most

important role in basic research, engineering, and medicine. Recent development

in aberration correction, however, accelerated atomic resolution routines providing

fundamental insight into nanotechnology. In addition, cryoelectron tomography

is an important technique for the 3D observation in cell biology and electron

tomography is also making progress in materials science.

2.6.1 Aberration-Corrected Electron Microscopy

One of the highest obstacles in improving the resolution of an electron microscope

has always been the blurring of the image caused by lens aberrations, notably the

spherical aberration. Rays passing through a spherically aberrated lens at a high

angle to the optical axis are brought to a focus closer to the lens than the rays passing

at a small angle to the optical axis (Fig. 2.25). These incorrectly focused high-angle


rays produce a smearing in the image. The point resolution is given by ∼ Cs λ3/4

[2.70] where λ is the wave length and Cs the coefficient of spherical aberration. The

point resolution limit can, therefore, be improved by reducing λ or by reducing Cs .

In the past, by increasing the voltage of the electron beam to 1250 keV (electron

wave length λ = 0.74 nm) a resolution of 0.10 nm could be obtained [2.71]. Highvoltage electron microscopy, though, is inappropriate for many specimens because

of radiation damage.

Aberration correction by reducing or eliminating Cs is a recent route to improving microscope performance. Additional multipole lenses, such as hexapoles [2.72]

as aberration correctors, can yield a negative Cs to match the positive Cs of the

microscope objective lens so that the point resolution can be improved.

Two basically different technical variants of transmission electron microscopes

(TEMs) are used [2.12]. In conventional TEM, the specimen is illuminated by a

near-parallel bundle of electrons, and the image is formed by a sequence of lenses

equivalent to the lenses used in a light microscope. In scanning TEM (STEM), a

fine probe is formed by focusing the incident electrons and is then scanned across

the specimen. The transmitted electrons are registered by detectors and the resulting

signal is displayed on a video screen. In conventional TEM, the aberrations of the

objective lens determine the image quality, whereas in STEM, the aberrations of the

probe-forming lens are of relevance for the quality.

The basis of the quantum mechanical interaction of the incident electron wave

field with the atomic potentials, which is the basis of TEM imaging, is contained


Progress in Electron Microscopy




Fig. 2.25 (a) Schematic of spherical aberration of a converging lens. The scattering power

increases with increasing angle (with respect to the optical axis) at which the electrons enter the

lens. As a result, the electrons passing the lens at its periphery are focused at a distance in front of

the image that is defined by low-angle beams. The image of a point P of the specimen is broadened into a “point-spread disk” of radius R (red). (b) Aberration correction. Spherical aberration is

compensated by combining the converging lens with a suitable diverging lens. In electron optics,

diverging lenses are realized by combinations of multipole lenses. (Reprinted with permission from

[2.12]. © 2008 AAAS)

in the exit plane wave function (EPWF). This can be derived by focus-variation

techniques (see [2.12]) where EPWF is calculated by fitting model functions to a

series of images taken under different defocus values. Then the EPWF is calculated

for a model structure of the sample which is iteratively improved to obtain the best

fit between the calculated and experimental EPWFs. The result is generically not an

image in the conventional sense but a computer model of the structure that gives the

atomic species and coordinates.

Theoretically, the optimum imaging contrast is achieved by combining a small

defocus with a small negative value of the spherical-aberration parameter Cs

(see [2.12]). Under these negative spherical-aberration imaging (NCSI) conditions,

atoms appear bright on a dark background. The strong contrast achieved by NCSI,

which makes few-electron atoms such as oxygen, nitrogen, or even boron visible, is

considered to be one of the major advances of spherical-aberration correction (see

[2.12]). In a

3 {111} twin boundary of BaTiO3 (Fig. 2.26), all the atom species

including oxygen (arrows) can be identified. By evaluating the local intensity signal,

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