Atomic force microscopy (AFM) probes the surface of a sample with a sharp tip, a couple of microns long and often less than 100Å in diameter. The resolution of AFM is usually 100 times better than the optical diffraction limit.
The tip is located at the free end of a cantilever that is 100 to 200 μm long. The tip will bend or reflect when it is scanned through the sample surface. There is an incident light injected to the cantilever that will be reflected. We can detect the reflected light angle to determine the sample surface topography indirectly, as Figure 3.6 shows. Several forces typically contribute to the deflection of an AFM cantilever. An inter-atomic force known as van der Waals’ is most commonly associated with AFM.
Compare to other microscopy, such as optical microscope and an electronic microscope, the major advantage of AFM is that it has no lens and beam irradiation.
Therefore, the resolution will not be limited by the diffraction limit.
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Figure 3.6 Block diagram of atomic force microscope using beam deflection detection.
As the cantilever is displaced via its interaction with the surface, so too will the reflection of the laser beam be displaced on the surface of the photodiode. (By wiki)
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All of the CIP resistivity measurements in our study were performed in the Quantum Design Inc. physical property measurement system (PPMS). The PPMS can provide an experimental environment in the temperature range of 1.9 to 400 K and magnetic field of 9T by low noise bi-polar power supply. In the DC Resistivity option, the current range is from 5 nA to 5 mA and voltage sensitivity is 20 nV. Moreover, the option of AC Transport measurement system (ACT) contains a precision current source and voltage detector providing four different types of automated, electrical transport measurements: AC resistivity, five-wire Hall effect, I-V curve, and critical current.
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3.6 Schematic illustration of PPMS
The sample is first put on the Horizontal & Vertical Sample Rotators, and the rotators is inserted in the sample chamber. The sample chamber can be pumped to 15 torr to avoid thermal drift effect. The sample chamber is put in the vacuum flask which is equipped with liquid He to lower the temperature. Next to the sample, there is a thermal circuit to raise the temperature. The thermal circuit is controlled by the PID circuit.
Figure 3.7 the Horizontal & Vertical Sample Rotators (from PPMS Horizontal rotator manual)
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The magnet current can control the field induced by the superconducting magnet coil.
The current flow into the superconducting magnet coil is controlled by a well-designed persistent switch. As Figure 3.8 shows, the left one is superconducting magnet coil, the middle is the heater, and the right one is power supply.
Figure 3.8 Schematic illustration of persistent switch
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Chapter 4
In 2011, Oepon’s group constructed Pt/Co/Pt trilayer structures and discovered that when the current passed through the sample, which was parallel to the interface, the resistivity had an abnormal result when the field was rotate in the plane perpendicular to the current direction. That is the resistivity of field perpendicular to the plane (Perpendicular MR, PMR) was larger than the one with field parallel the plane (Transverse MR). This result defied the Geometric Size Effect (GSE) in the single ferromagnetic thin film. As Figure 4.1 shows, they changed the thickness of Co in the Pt/Co/Pt structure and discovered that the MR ratio increased with the increasing Co thickness when the Co thickness was thinner than 7nm. But when the Co thickness was larger than 7nm, the MR ratio decreased with the trend of 1/tco. They considered this particular magnetoresistance phenomenon to be unrelated to the inner part of Co layer.
Therefore, this phenomenon was due to the Pt/Co interface contribution. They called this phenomenon anisotropic interface magnetoresistance (AIMR). [10]
Figure 4.1 MR ratio versus the thickness of Co [10].
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In the last decade, there have seen growing interest on research in spintronics such as Spin Hall Effect (SHE)[11][12], spin Seeback Effect (SSE)[14] and spin pumping Effect [13]. Take Spin Hall Effect for example, SHE can convert a charge current in a nonmagnetic metal, which has strong spin-orbit coupling (SOC), into a pure spin current in the transverse direction. Although this spin current cannot easily be detected by usual electrical measurement method, it can be detected by some pure spin detectors.
When passes through materials with strong SOC, the spin current converts into charge current resulting in charge accumulation in the transverse direction and a voltage drop, which can be detected easily. Pt is the most popular pure spin detector due to its large SOC.
Pt/YIG (yttrium iron garnet, Y3Fe5O12) bilayers have been widely investigated due to the essential roles mentioned above. In 2012, C. L. Chien’s group measured the magnetoresistance in the Pt/YIG structure. They found that Pt/YIG exhibited a new type of MR with unique characteristics that were very different from those of other known MR phenomena, as shown in Figure 4.2. Pt/YIG shows a pronounced MR (Longitudinal MR equal to Perpendicular MR, both were larger than Transverse MR).
As we know, Pt is a normal metal with no ferromagnetic characteristics, and YIG is an insulator. Therefore Pt and YIG should not show MR phenomenon individually.
Because Pt is a material which is close to the Stoner Criterion, it will acquire induced magnetization by YIG layer and causes the MR effect.
Pt is a well-known material with strong spin orbital coupling (SOC) and has usually been used as a spin current detector. In the emerging spintronic research, the relation between magnetic proximity effect (MPE) and pure spin current detector must be examined carefully.
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Figure 4.2 The scan direction of the sample and the Hybrid MR [?].
E. Saitoh’s group from Japan proposed a different aspect to explain the abnormal magnetoresistance in the Pt/YIG structure. They published Spin Hall magnetoresistance theory in Physic Review Letter in 2013. In their theory, the charge current pass through the Pt layer will be converted into a pure spin current in the transverse direction. With spin-angular-momentum exchange between magnetization M in YIG and conduction-electron spin polarization in Pt, Spin-flip scattering is activated when the spin direction of spin current and M are not collinear. Some part of the spin current is then absorbed by the magnetization as a spin-transfer torque even at an interface to a magnetic insulator and the spin current reflection is suppressed. This absorption is maximized if M is perpendicular to the spin direction and zero if parallel. When the absorption is zero, the reflected spin current will be converted into charge current due to the inverse Spin Hall effect (ISHE), which enhances the conductivity. Conductivity enhancement due to SHE and ISHE is expected to be maximized (minimized) when M is in transverse (longitudinal and perpendicular) direction. The Pt film resistance is therefore affected by the magnetization direction in YIG, giving rise to the spin Hall magnetoresistance (SMR)[15].
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Figure 4.3 Schematic for SMR [15].
To further investigate the physical origin of hybrid magnetoresistance, C. L. Chien’s group did two measurements on the altered YIG surface and use SiO2
(7% Fe) to replace the YIG in 2014[26]. The altered YIG surface called the YIG
BB greatly reduces the spin-mixing conductance at the Pt/YIG interface, thus, blocking the spin current transmission. InSiO
2(7% Fe) sample, the concentration of the Fe is too low to be ferromagnetic, but it still contains Fe composition so that the SiO
2(7% Fe) still can simulate the MPE but without spin current.
Interesting results shown in Figure 4.4, Pt(3nm)/SiO2(7% Fe) exhibits very similar MR behavior to that of Pt/YIGBB at all fields. However, the Pt(3nm)/SiO2(7% Fe) and Pt/YIGBB show different results to that of conventional Pt/YIG at small field.
The Pt(3nm)/SiO2(7% Fe) and Pt/YIGBB show the same angular dependence as that of Pt(3nm)/YIG. All of them exhibit (cos 𝜃)2 angular dependence in the ϕxy scan and θyz
scan with saturation field. Hence, they claim that in the Pt/YIG sample, the SMR influences the results at small field while the MPE dominates at high field.
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Figure 4.4 the MR and AHE results of Pt(3nm)/SiO2(7% Fe) and Pt/YIGBB. [26]
Au is a material used as a spin current detector with a relatively long spin diffusion length. Besides, Au is far from the stoner criteria so that it couldn’t have magnetization induced by the YIG. Therefore, if the Au is inserted into the Pt/YIG, the MPE will diminish but the spin Hall magnetoresistance should survive. C. L. Chien’s group did the experiment as mention above. Interesting results were obtained as shown in Figure 4.5.
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Figure 4.5 Field dependence of the (a) Hall resistance RH at different temperatures, (b) R∥ and RT for Pt3 nm/Au2 nm/YIG. Insert of (a) shows the temperature dependence of the anomalous Hall resistance RAHE for Pt3 nm/Au2 nm/YIG. Field dependence of the (c) Hall resistance RH at different temperatures, (d) R∥ and RT for Pt3 nm/Au6 nm/YIG. [26]
In the Pt(3nm)/Au(2nm)/YIG sample, the AHE results still show the negative AHE
signals which means that the MPE still exists in this sample. The AHE signal is
increasing with decreasing temperature which agrees with the MPE model. Although
the AHE signals are smaller than that of Pt(3nm)/YIG, the inserted Au layer is still too
thin to eliminate the MPE. As a result, the MR ratio of Pt(3nm)/Au(2nm)/YIG increases
with increasing field similar to the Pt(3nm)/YIG results. In the Pt(3nm)/Au(6nm)/YIG,
the MPE is obstructed, shows only ordinary Hall effect signals but without AHE. The
MR ratio of Pt(3nm)/Au(6nm)/YIG decreases with increasing field. This is the intrinsic
spin current related MR without the MPE.
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Chapter 5
Sample parameters
The samples were deposited by sputter deposition, the sample geometry is like a key which is defined by the e-beam lithography. The samples were measured by the standard four probe measurement. We can measure the Hall voltage by using the electrode VH and Vb at transverse direction.
Figure 5.1.1 sample geometry
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