3.5 Magneto-Optical Kerr Effect (MOKE)
Magneto-optics effect is a phenomenon similar to optic activity. When a linear polarized electro-magnetic wave (EM wave, light) pass through a material which is with self-magnetism of induced magnetism, The propagation speed of right and left circular palorized light will be different, that a linear polarized EM wave which consists of right and left circular palorized EM wave will change its polarization angle, this is called Faraday effect, discovered by Faraday in 1845.
In 1877, Kerr found a result similar to Faraday’s. If a linear polarized light is incident into a ferromagnetic sample, since of the different reflection coefficients of right and left circular polarization components, the reflection beam will become el-liptical polarized. This phenomenon is called magneto-optical Kerr effect. The angle between the primary axis of the elliptical polarization and the linear polarization is called Kerr rotation, and the ellipticity of the elliptical polarization is called Kerr elliptical.
Let r+eiθ+ and r−eiθ− stand for the reflection coefficients of right and left circular polarization, respectively. The Kerr rotation and Kerr ellipticity can be illustrated as ϕk = −θ+−θ2 + and εk = ba = rr+−r−
++r− , respectively. Both of them are proven to be proportional to the magnetization of sample. Thus by measuring ϕk and εk with cyclic applied magnetic field, we can get thehysteresis loop. In general, there are three types of MOKEmeasurement. Each of them has different geometry of the magnetization and the light path, In the polar Kerr effect, the magnetization lies in the plane of incidenceand is perpendicular to the surface. In the longitudinal Kerr effect, the magnetization lies in the plane of incidence and is parallel to the surface. In the transverse geometry, the magnetization is perpendicular to the plane of incidence and on the surface.
The angle between the primary axis of the elliptical polarization and the linear polarization is called Kerr rotation, and the ellipticity of the elliptical polarization is called Kerr elliptical, as shown in Fig. 3.13.
In magnetic ultrathin films, the Kerr signal is so small that the noise may result
3.5. Magneto-Optical Kerr Effect (MOKE) 29
Figure 3.13: Schematic illustration of magneto optical Kerr effect. After reflected from the ferromagnetic sample, the linear polarized laser beam becomes elliptical polarized.
Figure 3.14: Different geometry for MOKE measurement.
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3.5. Magneto-Optical Kerr Effect (MOKE) 30
Figure 3.15: Schematic illustration of AC MOKE.
in significant effect. Therefore, Practically in experiment here, a modulator is added on the laser sourcer and modulated signal can be taken by lock-in technique with a larger ratio of signal to noise. The schematic illustration is shown in Fig. 3.16.
Figure 3.16: Schematic display of a DC-MOKE loop in C207, NTNU
3.5. Magneto-Optical Kerr Effect (MOKE) 31
Figure 3.17: The switch of controlling the monopolar DC-power supply for inverse current
Figure 3.18: Schematic display of various magnetic field direction switch.
The production of magnetic field are consist of four eletrical magnets, with the max field of 1500 Oe each and 4300 Oe combinatively.
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3.5. Magneto-Optical Kerr Effect (MOKE) 32
Figure 3.19: In-plane measurement with associated perpendicular field by the MOKE in C207, Department of physics, NTNU. The magnetic field intensity of in-plane file is stable; but due to the sample or the light spotposition, the inten-sity of perpendicular field in in-plane measurement has a range of±70%. The max perpendicular field in in-plane peasurement is about10%.
Chapter 4
Experiment and results
All the experiments are under UHV environment, usually less than 8.0 × 10−10torr.
Before preparing sample, we fill the cold trap with liquid nitrogen, that would help TSP working better. Sometimes, we isolate main chamber to get the base pressure reach 3.0 × 10−10 torr to preserve sample from pollution in a time.
4.1 Si (111)7 × 7 obseavation
The silicon(111) slab as a substrate we used is n-type high doped, which can easily be heated by low voltage due to its small resistance about 4 Ω. When the silicon substrate was sent into the UHV chamber, it must be heated at 600 oC for more than 6 hours to completely get rid of the oxidation.
In the earlier time, our silicon slab were from Unisoku company, with it’s size of 2×12 × 0.5mm3 and resistance about4.6 Ω at room temperature. Later, we use the silicon slab from prof. Kuo Chien-Cheng’s laboratory in National Sun Yat-Sen university, with its size of 2×9 × 0.5mm3 and resistance about12 Ω at room tem-perature. Although both these silicon slab are all high doped, but we cannot apply the same current to degas or flash, since the resistance is different, after overheating and breaking few silicon slabs, we can catch the temperature by observing the color.
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4.1. Si (111)7 × 7 obseavation 34
Figure 4.1: The process of preparation Si(111)7 × 7 of (a)Si(111)from Unisoku com-pany and (b)from prof. Kuo Chien-Cheng’s laboratory in National Sun Yat-Sen university.
The last flash process is the very key to obtain large Si(111)7 × 7 terrace or narrow terrace. If the current finally reduced rapidly from 1000 oC, or said, fast cooling down, there will be narrow terrace on the Silicon surface; on the contrary, a larger terraces is possibly available as the current is reduced very slowly. The pressure is higher than 1.0 × 10−8 torr only during the early degassing process, the maximum pressure of other flash process is under 2.0 ×10−9 torr. No matter which process we choosed, there is always 7 × 7 structure on silicon(111) surface. The morphology analysis in main chamber is equipped with a STM(RT-STM, Unisoku).
4.1. Si (111)7 × 7 obseavation 35
Figure 4.2: The STM image first obtained in our laboratory.
Figure 4.3: A comparison with reference to calibrate [59]
In figure 4.2, we can see a terrace with the width about 30nm and its recon-struction of Si (111)7 × 7. There are few defects, this is becuase the cooling process August 14, 2009
4.1. Si (111)7 × 7 obseavation 36 is still not slow enough. In figure 4.3, we drag a line and have a lineprofile to see the high between two terraces, and compare with a reference, the high defference in these two detail illustraton is about 5 %. Then in figure 4.4 we can calibrate the size of our STM image with a theoretical model of Si (111)7 × 7.
Figure 4.4: A scale comparison for calibration. (a) Our STM image, (b) a Si (111)7×
7 model.
And we can see the faulted half and un-faulted half unit cell of Si (111)7 × 7 in larger scale. Another special structure of Si (111)7 × 7, the fualted and unfualted half unit cell can also be distinguished in our STM image showed in figure 4.5. Usu-ally, to see the ualted and unfualted half unit cell image by STM, a 100nm × 100nm scale (or larger, 200nm × 200nm) is better for us in our system.
Figure 4.5: (a)The triangles indicate the fualted half and un-faulted half unit cell and (b)the shcemic picture from Omicro company.
4.2. Highly ordered pyrolytic graphite (HOPG) 37
4.2 Highly ordered pyrolytic graphite (HOPG)
We use a commercial graphite to calibrate our STM image. Figure 4.6 shows two STM HOPG images we took with the model comparision at the right side, the discussion of varios STM images of HOPG was mentioned in the previous section in chapter 1. Figure 4.7 is the geometrical, or the long-width scale, comparison between our STM image and reference model, taking a rectangle block is helpful to obsreve, the width of our STM image is shorter than that of reference’s, the rea-son is our sample is slightly on a slant, not perfectly paralell to the STM scanning surface, thus we have this result. And figure 4.8 shows a lineprofile and a reference comparison.
Figure 4.6: (a)and (c) are STM images of the same graphite with different scanning condition. (b)and (d)are their schematic illustraton respectively [60].
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4.2. Highly ordered pyrolytic graphite (HOPG) 38
Figure 4.7: A magnitude check [60].
Figure 4.8: The heigh calibration [60]. (a) our STM image, (b) from reference and (c) the lineprofile in our STM image from (a).