Sample Fabrication and Characterization
3.1 Magnetic Imaging with XMCD-PEEM
Magnetic imaging is a key technique when studying magnetic structures. For this purpose a number of different techniques exist with each one having its own spe-cific advantages and disadvantages. While scanning techniques like magnetic force microscopy (MFM) or spin-polarized scanning electron microscopy (SEMPA) of-fer high spatial resolution down to 5 nm [KKK09], the acquisition time is larger compared to full field techniques. Some techniques require the sample to be mea-sured in transmission like Lorentz Microscopy and Scanning Transmission X-ray Microscopy (STXM). The samples need to be patterned on membranes which com-plicates the fabrication. Membranes also have a lower heat conductivity and thus Joule heating might become a problem in transport measurements.
A good overview over the different magnetic imaging techniques is found in [Kuc06]. The technique used in this work is Xray Magnetic Circular Dichroism -Photo Emission Electron Microscopy (XMCD-PEEM), which is a powerful combi-nation of PEEM imaging with the XMCD effect, that yields the magnetic contrast.
The technique and its requirements are discussed in this section.
3.1.1 Synchrotron and Undulator Radiation
XMCD-PEEM requires special polarized monochromatic light only available at synchrotron facilities. In this section, the basics of this synchrotron radiation are introduced.
Every charged particle that is accelerated induces a dipole radiation. This effect is significantly enhanced at relativistic velocities, when the total energy of
33
34 Experimental Techniques
the particle E is much larger than its rest mass energy (γ = E/m0c2 ≫ 1). In synchrotron storage rings electrons are accelerated close to the velocity of light and magnetic fields of so-called bending magnets are used to keep them on a circular orbit. These magnetic fields change the electron direction and therefore accelerate the electrons which causes the particle to emit radiation. The main propagation direction of the photons is inside a small cone tangential to the electron trajectory and the angle of the cone is decreased by a factor of γ compared to the normal dipole radiation. The intensity of the radiation also depends on γ and as a side effect of this radiation an electron circulating the synchrotron with energyE loses per turn the energy of [Eri06]:
∆E[keV] = 88.6E4[GeV]
ρ[m] , (3.1)
withρbeing the synchrotron radius. This energy must be delivered to the elec-tron beam to guarantee a stable operation. Modern synchroelec-tron sources operate at electron energies of a few GeV to obtain very intense radiation. The radiation emitted by a bending magnet has a continuous energy spectrum ranging from the infrared to the hard x-ray. Modern, third generation synchrotron are character-ized by their use of insertion devices instead of bending magnets as light sources.
Such undulators and wigglers are periodic magnetic arrays creating an alternating magnetic field. A simple schematic of such an insertion device is shown in Fig. 3.1.
The periodicity results in a small sinusoidal motion of the bypassing electrons in a plane perpendicular to the magnetic field. The emitted light is linearly polarized in that plane of the electron motion. The radiation is emitted in narrow cones for each period (see Fig. 3.1), whose divergence is again inversely proportional to γ.
If the electron deflection is very strong the characteristics of the radiation become similar to the sum of a series of bending magnets. These insertion devices are
Figure 3.1: A planar undulator, producing linear polarized light.
3.1 Magnetic Imaging with XMCD-PEEM 35
called wigglers. At smaller deflections the different radiation cones can overlap and photons of a certain energy range form different radiation cones can interfere.
Thus the emitted energy spectrum is not continuous but peaks at a well defined wavelength λ, depending on the period of the magnetic array (λu), the energy of the electrons (E), and the magnetic field (B). These insertion devicess are called undulators. Due to the resonance behavior, the brilliance (photon flux in a certain energy range per unit solid angle) of an undulator is orders of magnitude larger than for a bending magnet (or wiggler) even though the total photon flux for the bending magnet might be higher than for the undulator.
The way the electrons oscillate inside the insertion device determines the polar-ization of the emitted photons. Sinusoidal oscillation in one plane causes linearly polarized light [Fig. 3.1]. More complex arrangements of the magnets can result in a helical motion of the electrons and create circularly polarized light. By changing the relative phase of the insertion device magnets a single insertion device can produce linear horizontal, linear vertical, circular or elliptical polarization. More information on synchrotron physics can be found in Ref. [Hof04]
Typically only one wavelength (with a certain bandwidth) is required for the experiment, this means that the biggest part of the emission must be blocked.
This is done by a monochromator (a grating, typically 1000 lines per mm, used under grazing incidence). By turning the magnetic field B of the insertion device (most commonly by changing the gap between the magnets) the insertion device is also tuned to this desired photon energy for maximum intensity. By refocusing the X-ray optics, the beam is directed and focused onto the experiment. Typically, two spherical mirrors are used that are mounted perpendicular to each other. The refocusing elements and the monochromator are part of the beamline that directs the X-rays from the synchrotron to the experiment.
3.1.2 Photo Emission Electron Microscopy
In Photo Emission Electron Microscopy (PEEM) electrons from the sample sur-face are excited by incoming photons (in contrast for instance to Low Energy Electron Microscopy (LEEM) where electrons are used). First experiments were done as early as 1933 [Brü33, Poh34]. With the availability of synchrotron radia-tion, PEEM became an important technique for spectroscopic material studies. A schematic setup of an PEEM fabricated by ELMITEC is shown in Fig. 3.2. The sample is illuminated by an X-ray beam under an angle of16◦ with respect to the sample surface. The emitted electrons from the sample surface are accelerated by
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a voltage applied between the sample and the objective lens of typically 20 kV. As a result of the high X-ray energies, electrons with a broad range of kinetic energies are emitted. To obtain better resolution, it is important to limit the chromatic aberration and therefore the kinetic energy of the electrons. This is achieved by an energy analyzer in combination with a small exit slit that selects the desired electron energy. The electron beam is then directed onto an electron micro channel plate where it is amplified and finally converted into visible light by a phosphorous screen, that is imaged by a CCD camera.
The photon absorption and electron emission process is schematically shown in Fig. 3.3. The X-ray photon is absorbed by a core level electron that is lifted to the Fermi level leaving a hole at the core level (a). An electron from the Fermi level relaxes back to the free core level state and the energy is transferred to a high
ener-Figure 3.2: Schematic setup of a PEEM. By magnetic lenses an enlarged image of the sample is projected onto the micro channel plate (MCP). An energy analyzer is used to minimize the chromatic aberration (image courtesy of F. Nolting).
3.1 Magnetic Imaging with XMCD-PEEM 37
Figure 3.3: Photon absorption and electron emission process: (a) A photon is absorbed by a2pcore electron that is resonantly excited to the Fermi level. (b) An electron from the Fermi surface relaxes back to the free core level creating a high energetic Auger electron.
(c) The Auger electron has enough energy to induce a cascade of secondary electrons that are lifted above the vacuum energy and eventually are emitted from the material.
getic Auger electron as indicated in Fig 3.3 (b). This electron thermalizes through electron-electron scattering and creates a cascade of secondary electrons with a broad energy spectrum. These electrons have enough energy to leave the material [Fig 3.3 (c)] and finally are detected by the PEEM. So not photo electrons as the name PEEM would suggest but actually secondary electrons are imaged [Kuc06].
Currently efforts are made to further increase the PEEM resolution by using specially aberration corrected electron optics. In theory, the resolution can be as low as 0.5 nm [FWU+97, SGF+02] and recently a resolution below 2 nm was demonstrated [Elm].
3.1.3 Magnetic Circular Dichroism
First predictions of the dependence of the X-ray absorption cross section on the photon polarity, range back to the sixties and seventies [BS65, ES75]. However it took more than twenty years till these first calculations were successfully proven by experiment. G. Schütz and co-workers were the first to measure an X-ray magnetic circular dichroism (XMCD) effect in iron [SWW+87, SFM+89]. Thole et al. soon realized that XMCD spectra can reveal important information about the orbital and spin moment and they derived the sum rules [TCS+92, CKTA93, dL98]
that allow for a separation of the density of states, orbital and spin moment from the XMCD spectra. The following paragraphs are dedicated to the origin of the XMCD effect in ferromagnets.
A photon with energy ~ω can be absorbed by an electron and according to Fermi’s golden rule, the transition probability Tif from the initial core level state
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iwith energyǫi to the final statef with energyǫi can be written as:
Tif ∝ hf|Hint|iiδ(ǫf −ǫi−~ω)ρ(ǫf), (3.2) withρ(ǫf)being the density of free states of the final state andHthe Hamiltonian of the photon-electron interaction: Hint = e/mep·A[NR09]. The analysis of the equation leads to a number of dipole selection rules for transitions between electronic states of the form |n, l, s, msi (n: main quantum number, l: orbital quantum number, s= 1/2: electron spin,ms=±1/2: spin orientation):
• ∆l=±1,
• ∆ml=−1,0,+1,
• ∆s= 0,
• ∆ms= 0.
Thus a 2p core level electron can be excited into free 3d states at the Fermi level, since ∆l = 1. According to the selection rules the spin quantum numbers are not affected by the transition and the transition probability therefore does not depend on the photon polarization.
However in normal materials the 2p levels are not degenerate but split into the2p1/2 and the2p3/2 level, due to the spin-orbit coupling. The coupling of spin and orbital moment now yields a polarization dependent transition probability for the transitions 2p1/2 → 3dand 2p3/2 → 3d. The corresponding peaks in the absorption spectrum are called L3 andL2 edges.
The matrix element hf|Hint|ii can be calculated using the Clebsch-Gordon Coefficients. A basic quantum mechanic calculation [SV01] yields the spin polar-ization values for theL2/3 edges as they are listed in Table 3.1. Here a spin-orbit splitting of the 3d band was neglected and an identical density of states for the two spin orientations at the Fermi level was assumed. Without spin-orbit splitting both transitions would be equally exited and since the number of 2p3/2 electrons
L2 L3
↑ ↓ ↑ ↓
left 25% 75% 62.5% 37.5%
right 75% 25% 37.5% 62.5%
Table 3.1: Spin polarization values for theL2 andL3 edge when excited by left or right polarized light (from [SV01]).
3.1 Magnetic Imaging with XMCD-PEEM 39
Figure 3.4: (a) permalloy XAS curves for different polarizations. The energy range covers the L2 and3 iron edge. (b) XMCD-PEEM image of a 4 x 2µmpermalloy structure.
is twice the number of 2p1/2 electrons the combined spin polarization for the two transitions cancels out.
Any imbalance of the density of states for the two spin channels creates an imbalance in the absorption intensity for the two X-ray polarities. The polarized X-rays and the asymmetric spin dependent density of states at the Fermi level both act as spin filters for the electrons. If the relative orientation of these two filters is changed either by changing the magnetization or the photon chirality the total electron yield changes.
The increasing occupancy of the 3d electron band when going from Fe to Ni results in a decrease in the number of free states. Thus the transition probability goes down and the peak intensity is reduced from Fe to Ni accordingly. Further-more the spin momentum of iron is higher than that of nickel, thus for permalloy the iron L-edge gives a stronger contrast than the Ni L-edge although more Ni is contained in permalloy (Fe20Ni80). In Fig. 3.4 (a) the total electron yield as a function of photon energy is shown for a permalloy sample. The two peaks correspond to the L3 and L2 iron edge. The background stems from excitations into s and p-like states that show no resonant behavior and do not depend on the magnetization. Oxidation of the material results in a slight shift of the core levels that creates a so-called oxygen shoulder for theL3 peak at about 709 eV [CPJ+95].
The three top curves correspond to regions with magnetization pointing par-allel (blue), perpendicular (green) and anti-parpar-allel (red) to the direction of the incoming X-rays. The corresponding image is shown on Fig. 3.4 (b). The magnetic
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component of the signal can be separated when calculating the normalized sum of the electron yield for the two polarizations Y±, which gives the pure magnetic XMCD-signal as it is shown by the bottom black curve in Fig. 3.4 (a):
XM CD= Y+−Y−
Y++Y−. (3.3)
From the fact that the spin-orbit interaction is reversed for the two transitions (l+s for L3 edge andl−sfor the L2 edge) it is obvious that the XMCD signal also has opposite sign for the two peaks. Furthermore the oxidation shoulder does not contribute to the magnetic XMCD signal.
3.1.4 X-ray Magnetic Circular Dichroism PEEM
The combination of the XMCD effect and direct PEEM imaging was pioneered by Stöhr et al.. They directly imaged the magnetic domains in a ferromag-net [SWS+93]. Taking two images of the same structure with reversed photon helicity and subtracting them yields the pure magnetic contrast, whereas the sum of the two images contains the topographic information. XMCD-PEEM magnetic imaging technique has a number of decisive advantages:
• it is non-intrusive,
• it is fast (direct imaging, unlike scanning techniques),
• it has good spatial resolution of about 20 nm (can be reduced down to 2 nm, when special aberration corrected electron optics are used [FWU+97]),
• it is element specific by setting the X-ray energy to the element specific resonance peak.
However, its main disadvantage is that it requires special polarized X-rays that are only available at synchrotron sources.
In this work, commercial ELMITEC PEEMs were used, that are permanently installed at the end stations of the different synchrotron sources. The results pre-sented here were obtained during beam times at BESSY II in Berlin (www.bessy.de), at the SLS in Villigen/Switzerland (sls.web.psi.ch), ELETTRA in Triest/Italy (www.elettra.trieste.it) and Diamond in Oxford/ England (www.diamond.ac.uk).
Fig. 3.5 shows the UHV system and the PEEM installed at the Swiss Light Source.
The sample is illuminated by the X-rays under an angle of 16◦. Since the XMCD signal is proportional to the scalar product of magnetization and light polarization
3.1 Magnetic Imaging with XMCD-PEEM 41
Figure 3.5: Image of the UHV system of the SIM beamline at the SLS.
the in-plane contrast is about1/tan 16◦≈3.5times stronger than the out-of-plane sensitivity. Thus samples with in-plane magnetization give stronger signal and are easier to image.
At the right hand side of Fig. 3.5, the HV-rack is visible that contains the power supplies for the magnetic lenses. The preparation chamber where samples can be cleaned by Ar-sputtering is located at the left hand side.
An XMCD-PEEM image of a magnetic structure is shown Fig. 3.4(b). Do-mains with magnetization pointing in the direction of the incoming X-rays are bright, whereas domains with anti-parallel alignment appear dark. Domains with perpendicular orientation yield no magnetic contribution to the XMCD signal like the non-magnetic substrate.
A more detailed image of the microscope is shown in Fig. 3.6. The electrons can either be excited by the X-rays or by an UV-lamp. In addition, the microscope offers the possibility to switch into the LEEM mode where low energy electrons are used. The sample stage can be rotated by 360◦. Taking a second XMCD-image of the90◦ rotated sample makes it possible measure the second in-plane magneti-zation component and hence to reveal the complete in-plane magnetimagneti-zation.
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Figure 3.6: The PEEM at the SIM beamline (SLS) shown in more detail.