A linear polarization laser beam was used for MOKE measurement. The linearly polarized light is consists of the left-hand circularly polarized light and right-hand circularly polarized light component. The linearly polarized light would change in both phase and reflected intensity after reflecting from a magnetized surface. The different propagating velocity of left-hand
circu-Figure 3.9: The apparatus of MOKE[27].
larly polarized light and right-hand circularly polarized light results in the phase difference, and the reflected intensity changes due to the different ab-sorption of left-hand circularly polarized light and right-hand circularly po-larized light. Hence, the linearly popo-larized light transforms into elliptically polarized light, and the effect is called Magneto-Optical Kerr Effect.
The angle between original linear polarized light and the long axis of elliptically polarized light is called Kerr rotation angle (θk), the ellipticity of the elliptically polarized light is called Kerr elliptically(εk). In general description, θk and εk are very small ( 1◦), and they are proportional to magnetization of the magnetic material (M).
Magneto-optical Kerr effect can be divided into three types, which are depending on the direction of the magnetization vector (M) with respect to the plane of incidence and the sample surface, as shown in Fig. 3.11.
When the magnetization vector is perpendicular to the reflection surface and parallel to the plane of incidence, the effect is called the polar Kerr ef-fect(Polar MOKE). To simplify the analysis, near normal incidence is usually employed when doing experiments in the polar geometry.
Figure 3.10: The illustration of MOKE[27].
In the longitudinal MOKE(In-Plane MOKE), the magnetization vector is parallel to both the reflection surface and the plane of incidence. The lon-gitudinal setup involves light reflected at an angle from the reflection surface and not normal to it, as above in the polar MOKE case. In the same manner, linearly polarized light incident on the surface becomes elliptically polarized, with the change in polarization directly proportional to the component of magnetization that is parallel to the reflection surface and parallel to the plane of incidence.
When the magnetization is perpendicular to the plane of incidence and parallel to the surface, it is said to be in the transverse configuration(Transverse MOKE). In this case, the incident light is also not normal to the reflection surface but instead of measuring the polarity of the light after reflection, the reflectivity is measured. This change in reflectivity is proportional to the component of magnetization that is perpendicular to the plane of incidence and parallel to the surface.
Figure 3.11: Longitudinal (In-Plane), Polar (Perpendicular) and Transverse MOKE[27].
Figure 3.12: The experimental setup of MOKE[27].
Chapter 4 Results
4.1 O-3x3/W(111)
The sample preparation and investigation were performed in an ultrahigh vacuum chamber with a base pres sure 3x10−10torr. The Co and Ni films were deposited at room temperature (RT) by e-beam heated thermal evaporation.
The magnetic properties of the n ML Co, Ni thin films on O-3x3/W(111) were detected in air using polar and longitudinal magneto-optical Kerr effect (MOKE) at room temperature, after covering a protective Pd layer.
Figure 4.1: Preparation conditions and procedures of O-3x3/W(111). Left and right axes are O2 pressure and annealing temperature, respectively.
In previous reports, oxygen on W(111) formed different faceted structures with different annealing temperature. In our experiment, we find a 3x3 struc-ture for oxygen on W(111). How could we form the 3x3 strucstruc-ture? Usually, we cleaned W(111) with 2000 K annealing under O2 atmosphere, in order to get ride of carbon contamination. In this case, the procedure of sample preparation as shown in Fig.4.1. The first step, we cleaned W(111) by cyclic 1500 K annealing under 1x10−6 torr O2 for removing carbon. The second step, after pumping out the oxygen and recovering the base pressure lower than 5x10−10 torr, a well order 3x3 reconstructed surface was formed on the W(111) surface by 1700 K post annealing. Both the two steps of annealing procedure are essential for the 3x3 reconstruction.
Figure 4.2: Low energy electron diffraction (LEED) patterns of W(111) after (a) the first step annealing under 10−6 torr Oxygen and (b) the second step annealing in an ultrahigh vacuum with a base pressure better than 5x10−10 torr. The inset in (a) exhibits the magnified image of the LEED spot, indi-cated by the rectangle.
Why the two steps of annealing procedure are necessary for forming the 3x3 reconstruction? Fig. 4.2(a) shows the LEED patterns taken after the 1st annealing of W(111) under 1x10−6 torr O2 with various annealing tem-perature. For the 1200 K annealing, as exhibited in the magnified LEED
spot image of Fig. 4.2(a), the triangular shape of diffraction spot was ob-served, indicating the formation of faceted surface, which was composed of both planar (111) surface and (112)-3-sided pyramids. From Fig. 4.2(a), we know that the LEED image has no significant change with the increase of annealing temperature. It also pointed out that there was no any 3x3 re-construction without the post annealing. Fig. 4.2(b) is LEED image of the second step annealing in an ultrahigh vacuum with a base pressure better than 5x10−10 torr. It shows that the post annealing lead to the evolution of surface structure. We can clearly see that six-fold satellites around the 1x1 spots appears with post annealing between 1000 and 1600 K. While gradually increase the annealing temperature to 1200-1400 K, the 3x3 reconstruction was more clear to observe. It means that 1200-1400 K is the most suitable annealing temperature for the formation of O-3x3/W(111) surface.
Figure 4.3: Auger electron spectrum measured from the reconstructed O-3x3/W(111) surface. The intensity ratio of O(510 eV) to W(170 eV) is around 1/3. (b) LEED patterns of O-3x3/W(111) and clean W(111).
We used AES to investigate the surface chemical composition of the O-3x3/W(111) structure. Fig. 4.3(a) shows the AES spectrum taken from the O-3x3/W(111) surface. From Fig. 4.3(a), there are several peaks exist in the AES spectrum. The peak 170 eV and 353 eV belong to W(111).
The other peak 510 eV is oxygen. Besides of the W(170 eV) and W353eV peaks, only the O(510 eV) peak is observable. The possible contamination of CO can be excluded, since the C(272 eV) peak is indiscernible. Thus, the 3x3 reconstruction caused by oxygen effect can be demonstrated. In each O-3x3/W(111) preparations, the ratio of O(510 eV) /W(170 eV) is always close to the value of 1/3. It indicates that the required oxygen quantity and the covering rate for this 3x3-reconstruction is quite stable and nearly invariant in the repeated preparations. Fig. 4.3(b) shows more clearly the LEED patterns of O-3x3/W(111) and clean 1x1-W(111) surfaces.