Read Back Signals in Magnetic Recording - Research Group Fidler

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Read Head Design In presence of shields with high permeability the free layer sees an additional mirror current, which must also be considered, especially for small gap distances. These mirror currents reduce the current field as shown in Figure 5.4. 5.1.6 Crystalline Anisotropy Similar to the shape anisotropy the crystalline anisotropy can be used for biasing. For example a field cooling treatment of a material can generate an intrinsic anisotropy. Nevertheless this bias scheme is not often used in GMR sensors, because the free layer should be able to rotate freely and the pinned layer is normally biased by exchange anisotropy. 5.2 Shielding To avoid influence of external fields and neighboring bit transitions, shields with high permeability enclose the GMR element. Shields normally consist of Permalloy (Ni80Fe20). The relative permeability of Permalloy is in the order of 4 10 . The gap distance between both shields is limited to 60 nm for CIP spin valves and 20 nm for CPP spin valves (see Section 2.8.3). In order to have an improvement of the read back signal the gap distance should not exceed the bit length. 5.3 Thermal Stability The sense current leads to a heating of the GMR element. The properties of the GMR sensor are temperature dependent. For instance the exchange bias field decreases with temperature (see Figure 5.2). Moreover the noise increases. To achieve thermal stability a constant sensing current is applied instead of a constant voltage. Then the change in resistance is determined by measuring the change in voltage. The heat production of the current limits the current density. The heat dissipation is especially a problem for GMR sensors working in CIP mode, because for the CPP mode the shields are used as leads. Therefore the electric conductance between GMR sensor and shields is larger, which normally also leads to a larger thermal conductance due to the Wiedemann-Franz Law. 64
FEM Simulations 6 FEM SimulationsEquation Section (Next) 6.1 Model The model of the read head used in the following calculations is very simplified, because of a shortage of detailed information. Despite cooperation with hard disk manufacturers, their information was very restricted. Nevertheless we were able to simulate the qualitative behavior and to prove the functionality of our code. The whole read head model is separated into two parts: A conductor model and a magnetic model. 6.1.1 Magnetic Model A simple GMR read head basically exists of five parts, which define the magnetic behavior: The free layer, the pinned layer, the hard bias, the shields, and finally the antiferromagnet for fixing the pinned layer. The magnetic net moment of an antiferromagnetic material is zero. Apart from exchange coupling the antiferromagnet does not contribute to the magnetic behavior. Therefore the antiferromagnetic part can be neglected, if the exchange coupling is taken into account by adding the exchange bias field to the effective field within the pinned layer. The geometry of the FEM model for the calculations is shown in Figure 6.1. The free layer and the pinned layer dimensions are h GMR = 80 nm in height and w GMR = 100 nm in lateral FL PL direction. Their thicknesses are t = 5 nm and t = 2 nm , and their spontaneous FL polarizations are J = 1.2 T , which is the average of a composite free layer (NiFe/CoFe) and s J = 1.885 T (CoFe) respectively. Both layers are assumed to have no anisotropy and are PL s Cu separated from each other by a t = 2.7 nm thin nonmagnetic Cu layer. The two hard bias magnets have uniaxial anisotropy in lateral direction HB 5 3 K 1 = 2.5⋅ 10 J/m and a spontaneous HB polarization of J = 1T. Their lateral distance from the GMR layer structure is 10 nm. The s exchange bias field is always set to H ex = 0.05 T , unless otherwise noted. This corresponds 65
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DIPLOMARBEIT Read Back Signals in M
Page 3 and 4:
Abstract This thesis concentrates o
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Contents 3.2.4 Signal of Written Bi
Page 7 and 8:
Introduction 1 IntroductionEquation
Page 9 and 10:
Introduction In 1955, before the RA
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2 BasicsEquation Section (Next) 2.1
Page 13 and 14: Basics curl H = 0 . (2.15) So H is
Page 15 and 16: Basics This is justified, because t
Page 17 and 18: Basics Here J ij represents the exc
Page 19 and 20: s Basics ∂J α ∂J =−γ J× He
Page 21 and 22: Basics for spin down electrons. An
Page 23 and 24: Basics Figure 2.4: Schematic model
Page 25 and 26: Basics channels in the parallel and
Page 27 and 28: Basics Nowadays there are again eff
Page 29 and 30: Analytical Calculations 3 Analytica
Page 31 and 32: Analytical Calculations developing
Page 33 and 34: Analytical Calculations t C 8 π =
Page 35 and 36: µ 0 Hsig [T] 0.030 0.025 0.020 0.0
Page 37 and 38: Analytical Calculations n n ⎛HH,
Page 39 and 40: z Analytical Calculations Figure 3.
Page 41 and 42: 4 Numerical MethodsEquation Numeric
Page 43 and 44: Numerical Methods ϕ ( x ) =δ . (4
Page 45 and 46: Numerical Methods current flowing o
Page 47 and 48: Numerical Methods ∫ ∫ ∫ si =
Page 49 and 50: Numerical Methods Figure 4.2: The l
Page 51 and 52: Numerical Methods j =−σgrad Φ.
Page 53 and 54: Numerical Methods within the volume
Page 55 and 56: Numerical Methods Integration of Am
Page 57 and 58: 4.5 Time integration Numerical Meth
Page 59 and 60: 5 Read Head DesignEquation 5.1 Bias
Page 61 and 62: Read Head Design The exchange bias
Page 63: Read Head Design free layer (see Fi
Page 67 and 68: FEM Simulations Magnetic Material J
Page 69 and 70: 6.2 Results FEM Simulations In the
Page 71 and 72: Output [V] 0.108 0.107 0.106 0.105
Page 73 and 74: FEM Simulations has a greater influ
Page 75 and 76: Output Voltage [V] 0.1074 0.1072 0.
Page 77 and 78: 7 OutlookEquation Section (Next) Ou
Page 79 and 80: Appendix A In case we have the func
Page 81 and 82: Appendix B Moreover the second inte
Page 83 and 84: List of Figures List of Figures Fig
Page 85 and 86: List of Figures Figure 6.1: The mag
Page 87 and 88: Bibliography [11] H. Katayama, M. H
Page 89 and 90: Bibliography [38] W. F. Brown Jr.,
Page 91: Lebenslauf Otmar Ertl Reinprechtsdo
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Read Back Signals in Magnetic Recording - Research Group Fidler

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