A simple magneto - optical Kerr effect should have light source, two polarizers, electromagnetic coil and photodiode detector. The light source is solid-state laser for 4.5mW and the wavelength is 670 nm. The light of laser is polarized by first linear polarizer, and become elliptically polarized light after reflected from sample magnetized. Afterword, the second linear polarizer can be used for checking the change of elliptically polarization along with the switched magnetic field.
Fig.16. The magneto - optical Kerr effect system. At the top is no core MOKE for YIG measuring.
The polarizing axis of polarizer is close to the semi-minor axis of elliptical polarization.
When the elliptical polarization is twisted due to the magnetization of sample changed by magnetic field, the projection of elliptical polarization on polarizer will be changed.
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Fig. 17 The elliptical polarization switched by magnetic field. The rad line is the angular of second polarizer.
In Fig. 17, there has two parabolas representing the light intensity about semi-minor axis of elliptical polarization. They will exhibit in turn with the switched magnetic field.
The X-axis is the angle between the semi-minor axis of elliptical polarization and polarizing axis of polarizer. If the polarizer is fixed at positive angle side, the light intensity in positive magnetic field is stronger than the light intensity in negative magnetic field. The light intensity changing represent the high and low side of magnetic hysteresis loop. When the polarizer is turned to the negative angle side, the hysteresis loop will become upside-down. In special cases, there will be nothing when the polarizer angle is near the cross of two parabolas. Two elliptical polarization have same projection on polarizer and the light intensity will not be changed by magnetic field. So we should avoid this when we doing MOKE measurement.
Fig. 18. The schematic diagram of polar, longitudinal and transversal MOKE
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MOKE can be categorize by the magnetic field direction with respect to the plane of incidence and thereflecting surface. The polar MOKE is the magnetic field perpendicular to the reflecting surface of sample, so someone will call it as perpendicular MOKE. When doing polar MOKE, we usually make the incident light much close to the normal incidence for getting better polar Kerr signal. The longitudinal MOKE is the magnetic field parallel to the incident plane and sample surface. Same as the polar MOKE, we also make the incident light much close to the sample surface for getting better longitudinal Kerr signal. If someone mention in-plane MOKE, that means the longitudinal MOKE. The transverse MOKE is the magnetic field parallel to the sample surface but perpendicular to the incident plane. In transverse MOKE, we may get a strange magnetic hysteresis loop if the easy axis of sample is parallel to the magnetic field but perpendicular to the incident plane. Only when the magnetic moment is switching by magnetic field, the magnetic moment will be parallel to the incident plane. So we will get to peak in transverse MOKE hysteresis loop at where the coercivity is in longitudinal MOKE hysteresis loop.
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450μm × 450μm). The low temperature system can measure a sample 1cm × 1 cm large at 90K, and also can be used to expose sample to hydrogen. So we can observe the domain motion change after hydrogen absorption on our CoPd alloy sample.
Fig.19. The Kerr microscope system.
There has two direction of magnetic field, in plane and perpendicular, can be used. At in plane mode, we can rotate the angular of sample and magnetic field. There also have three kinds of light mode can be chosen, so we can measure polar, longitudinal, transverse MOKE without moving sample. That is convenient to us for measuring the easy axis of Fe/Pd/Fe multilayers on MgO(001).
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Fig. 20. The different mode of MOKE in the Kerr microscope.
Fig. 20 shows the different mode of MOKE in the Kerr microscope. There have 8 LED lights in Kerr microscope. With the combination of LED lights, we can use different mode of MOKE in our experiment. The plus sign means two direction of LED light will open at same time. The horizontal vector of light will be offset and we can get the pure polar Kerr signal. The minus sign means two direction of LED light will alternately open. The vertical vector of light will be offset and we can get the pure longitudinal or transversal Kerr signal.
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and oxygen ion when the material is evaporated by laser. In order to make sure that the bonding of molecular we deposit on sample is same as the target, the oxygen is filling the chamber as 8 × 10−2 to 1 × 10−1 𝑚𝑏𝑎𝑟.
Fig. 21. The pulsed laser deposition system in Prof. Lo’ s lab.
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3.5 Photoluminescence (PL)
Photoluminescence (PL) is that any matter absorbs photons and then emit light. The electron in matter will excite to higher energy level after absorption of photons, then fall to original level. If there has energy band gap in the matter, light emission will happen in the relaxation process. The relaxation processes are various and the emission light may excite electrons to be re-radiated. Observation of photoluminescence at a certain energy can used to resolve the energy band gap of matter. So it is an appropriate instrument to observe the quality of semiconductor thin film.
Fig. 22. The Photoluminescence system in Prof. Lo’ s lab.
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3 × 10−9 𝑚𝑏𝑎𝑟. The ZnO layer was deposited at room temperature or 600 K with oxygen pressure of 8 × 10−2 𝑚𝑏𝑎𝑟 by PLD system. The energy of laser was 350mJ per second. The top and bottom of Au were used as electrodes when applying voltage.
The parameter of E-beam heat evaporator was 1000V, 40mA for Au and 85mA for Fe.
Each layer was deposited with different mask shown in Fig. 23.
Fig. 23 There are the masks and holders for fixing the Al2O3 substrate when depositing each layer.
Number 4 was the bottom of the holder. Number 3 was at middle to fix substrate. Number 1 and 2 were at top as mask for depositing the top and bottom of Au. The mask for depositing ZnO was a baffle which is not in the picture.
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Fig. 24. The process of depositing Au/Fe/ZnO/Au multiple layers on Al2O3 substrate.
First was Au at bottom as an electrode. Depositing Two strips of Au was for one more chance of measure because too much current will damage the sample. The second step is depositing ZnO at PLD chamber. We were used to make the area of ZnO is two thirds on substrate for ensuring the top Fe isolated from bottom Au. The final was depositing Fe and Au layer. Au was used as the protective layer to Isolate the Fe layer and air. The area of top layer was less than one third of substrate to prevent the contact with bottom Au. The size of the junction area is 1mm×1.5mm (The area each layer was overlap).
Fig. 25. Side view of Au/Fe/ZnO/Au multiple layers on Al2O3 substrate.[33]
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Fig. 26. PL spectra of Fe/ZnO heterostructures measured before and after applying a current to induce the coercivity enhancement. [33]
Fig. 26 showed the photoluminescence(PL) spectra of Fe/ZnO heterostructures. Four emission peaks identified from the spectra were 2.65, 3.00, 3.20, and 3.35 eV. They represented oxygen-defect related, donor-acceptor pair, bound exciton, and free exciton emission lines. [8-10] The PLintensity was reduced after current annealing. The emission peak of free exciton emission was relatively reduced and the free exciton emission peak was increased. The above observations could be represented that oxygen in ZnO was lost or captured by Fe at the interface during annealing.
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