Spin Valve Systems for Angle Sensor Applications - tuprints

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30 2 Theory MR (%) 8 7 6 5 4 3 2 1 H saf H c3 H ex H eb 0 -800 -600 -400 -200 0 200 400 600 800 µ 0 H(mT) FL RL PL Figure 2.25: MR curve measured from a spin valve incorporating a SAF. For µ0H>-600 mT, the magnetization of each layer are parallel due to the high applied field as shown in Figure 2.25. The MR is at a minimum at this point. As the applied field decreases to intermediate field values, the magnetization in the reference layer goes antiparallel to the free and pinned layer. The pinned layer remains fixed due to exchange bias effect and the antiferromagnetic interlayer coupling forces the reference layer into the antiparallel position. The MR rises to a maximum until it levels off. The point at which the plateau region in the MR curve ends is referred to as the antiferromagnetic interlayer coupling field value Hsaf of the spin valve (see Figure 2.25). After the field direction switches to positive field direction, the magnetization of the free layer reverses while the magnetization of the reference and pinned layers remains fixed (see Figure 2.25). The MR drops again to minimum while the free and reference magnetizations are parallel to one another. The pinned layer remains fixed by the exchange bias effect. At intermediate field values, the MR begins to rise as seen in Figure 2.25. The Heb of the spin valve has been reached at this point and the pinned layer magnetization begins to follow the direction of the applied field. The reference and pinned layers go through a “scissoring” motion at intermediate fields due to this rotation of the pinned layer while the reference layer will rotate in the opposite direction of the pinned layer due to the antiferromagnetic interlayer coupling across the Ru spacer layer (JSAF> Keb). As the applied field increase to higher values the antiferromagnetic interlayer coupling is overcome by the applied field and the magnetization of the free, reference, and pinned layers are aligned in the direction of the applied field. The MR again drops to a minimum at this point. Other important magnetic parameters of a spin valve with SAF can be determined from the major loop and minor loop of a MR Curve. The coercivity of the free layer, Hc1, and the interlayer coupling He are determined the same as with a simple spin valve (see Figure 2.13.) It is important note that He is measure of the residual interlayer coupling between the free and reference layers across the Cu spacer layer, not pinned and free layer as in the case of the simple spin valve. The hysteresis measured in the major loop of the MR curve is the coercivity of the reference layer, Hc3, and not of the pinned layer (see Figure 2.25). The coercivity of the pinned layer, Hc2, can now only be determined from a magnetization measurement as shown in Figure 2.26. The majority of the spin dependent resistance changes occur between the reference and free layer. The coercivity of the pinned layer cannot be directly measured from the MR curve due to this fact.
2.8 GMR 360° Angle Sensor 31 Magnetisation (emu) FL RL PL H eb H c2 H saf -800 -600 -400 -200 0 200 400 600 800 µ 0 H (mT) Figure 2.26: Magnetization curve measured from a spin valve incorporating an SAF. 2.8 GMR 360° Angle Sensor 2.8.1 Design of GMR 360° Angle Sensor Metals typically have a large temperature coefficient resistance, e.g. on the order of 0.1%/K, due to the increase in lattice vibration with the rise in temperature. A wheatstone bridge configuration is used to eliminate the temperature dependence of metallic thin films such as those in a spin valve system and to remove the signal offset typical of single resistance element. Automotive sensors must function in a temperature range from –40°C to 150°C and therefore the resistance changes due to temperature must be taken into consideration. A wheatstone bridge has four identical elements, R1, R2, R3 and R4 as shown in Figure 2.27. VB is the applied bias voltage and Vout is the output signal from the Wheatstone bridge. The use of four elements with identical resistances compensates for any resistance changes induced by a temperature change. The Vout of a Wheatstone bridge can be described as follows: ( )( ) ⎟⎟ ⎛ R ⎞ 1 ⋅R 4 − R 2 ⋅R 3 V = ⎜ out Vb (2.23) ⎝ R1 + R 2 R 3 + R 4 ⎠ when R1= R2= R3= R4, then Vout is zero. A change in a Vout, in the ideal case, can therefore only be due to resistance changes induced from effects other than temperature changes The Wheatstone bridge circuit of a spin valve angle sensor has one important difference to that of a previously described Wheatstone bridge circuit. Each adjacent element in the Wheatstone bridge has the same resistance, but the bias directions in two elements are 180° to the two other elements as shown in Figure 2.27. As a result, this bridge has a bipolar output with the full signal amplitude of a single element.
Page 1: Spin Valve Systems for Angle Sensor
Page 4 and 5: II 4.1.2 Measurement Setup ........
Page 7 and 8: 1Introduction Research on the Giant
Page 9: The most challenging of the researc
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Page 14 and 15: 8 MR (%) 20 15 10 5 0 -80 -60 -40 -
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Page 24 and 25: 18 2 Theory a) b) uniaxial K f K eb
Page 26 and 27: 20 2 Theory a) M b) b) c) fdf Bias
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Page 32 and 33: 26 2 Theory origin of exchange bias
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Page 38 and 39: 32 2 Theory 0 360 0 360 R1 R2 Vb Vo
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Page 42 and 43: 36 3 Experimental Methods gas intak
Page 44 and 45: 3.1 Sputter Deposition 39 a) Figure
Page 46 and 47: 3.2 Excimer-Laser 3.2.1 Theory and
Page 48 and 49: 42 3 Experimental Methods refill th
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Page 52 and 53: 46 4 Characterization Methods 1 2 3
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Page 58 and 59: 52 4 Characterization Methods Quali
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Page 62 and 63: 56 5 Stoner-Wohlfarth Model: Spin V
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Page 66 and 67: 60 6 Results and Discussion behavio
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Page 78 and 79: 72 6 Results and Discussion MR (%)
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80 6 Results and Discussion θ eb -
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82 6 Results and Discussion 6.1.7 S
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84 6 Results and Discussion MR (%)
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86 6 Results and Discussion as the
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88 6 Results and Discussion Intensi
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90 6 Results and Discussion layers
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106 6 Results and Discussion 6.4 Io
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108 6 Results and Discussion 6.4.2
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110 6 Results and Discussion valve
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116 6 Results and Discussion 15% la
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130 6 Results and Discussion The va
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132 6 Results and Discussion H ex (
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140 6 Results and Discussion antife
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7Demonstrator of a 360° GMR Angle
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7.2 Demonstrator in Operation 145 a
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148 8 Summary of Conclusions 4. Det
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150 8 Summary of Conclusions The sa
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152 9 Zusammenfassung und Ausblick
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154 9 Zusammenfassung und Ausblick
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156 10 Appendices Table 10.3: Magne
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158 10 Appendices pinned layer (p.
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160 11 Citations and D. Hilton. Lon
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162 11 Citations [Haa99] G. Haas, M
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164 11 Citations 3874-3879 (1988).
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166 11 Citations sputtered permallo
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ACKNOWLEDGMENTS I wish to express m
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List of Publications 1. A. Johnson,
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