Read Back Signals in Magnetic Recording - Research Group Fidler

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Read Head Design On the other hand the signal field decays with distance from the recording layer (see Section 3.4) and therefore a higher effective (average) signal field is reached for smaller sensor heights. Thus a compromise must be found. Usual width-to-height ratios are between 1.2 and 1.5. 5.1.2 Hard Bias To align the magnetization of the free layer hard magnets are used (see Figure 2.4). Usually the GMR element is placed between two hard magnets, whose static magnetic field effects the perpendicular alignment in respect to the pinned layer. The permanent magnetic fields also influence the magnetization of the pinned layer. Therefore the biasing field for the pinned layer has to overcome the hard bias field. 5.1.3 Exchange Bias Exchange bias is based on the exchange anisotropy, which was discovered by Meiklejohn and Bean 50 years ago [32]. They investigated small Co particles coated with its antiferromagnetic oxide CoO. A field cooling process, i.e. heating up above the Néel temperature and cooling under applying a magnetic field, leads to a shift of the hysteresis loop, which is shown in Figure 5.1. The exchange bias effect can be characterized by the displacement of the hysteresis loop, denoted by H ex . Figure 5.1: The shift of the hysteresis loop after a field cooling treatment of a ferromagnetic/antiferromagnetic system [19] and the temperature dependence of the exchange bias field for NiFe(4-8nm)/Cu(2.2nm)/CoFe(2nm)/IrMn(15nm) spin valves with different free layer thicknesses [33]. 60
Read Head Design The exchange bias field depends on the thicknesses of the antiferromagnet and the ferromagnetic layer. If the antiferromagnet is thicker than the ferromagnetic layer the exchange bias field decreases with the thickness of the ferromagnet t p like H J 1 t p − [34]. ex ex = . (5.3) µ 0 Ms, ptp Here J ex is the exchange bias interaction parameter [J/m 2 ]. This kind of dependence leads to the conclusion, that the exchange anisotropy is an interface effect. The maximum exchange bias field, which can be achieved for usual layer thicknesses, is about 0.05 T [20]. The exchange bias field decreases linearly with temperature [33]. Figure 5.1 shows the temperature dependence of the exchange bias field for NiFe(4-8nm)/Cu(2.2nm)/CoFe(2nm)/ IrMn(15nm) spin valves with different free layer thicknesses. The exchange bias effect is used to align the magnetization of the pinned layer perpendicular to the recording layer. The advantage of this bias scheme is that the free layer is not influenced by the antiferromagnet. Unfortunately the stray field of the pinned layer causes an unwanted torsion of the free layer’s magnetization, which leads to a displacement of the equilibrium state towards the antiparallel state. The result is an asymmetric sensor curve. 5.1.4 Synthetic Antiferromagnet To reduce the asymmetry of the transfer curve of the spin valve, a stronger exchange bias is required. The limitation of the exchange bias field leads to an alternative spin valve design [35]. Instead of the direct coupling of the antiferromagnet with the pinned layer, an additional ferromagnetic layer is introduced which is separated by a very thin (0.7-0.9 nm) nonmagnetic layer (mostly Ru) as shown in Figure 5.2. This layer structure is also called a synthetic antiferromagnet, because the pinned layer is antiferromagnetically coupled with the additional ferromagnetic layer by interlayer exchange coupling (IEC). 61
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DIPLOMARBEIT Read Back Signals in M
Page 3 and 4:
Abstract This thesis concentrates o
Page 5 and 6:
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
Page 11 and 12: 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: 5 Read Head DesignEquation 5.1 Bias
Page 63 and 64: Read Head Design free layer (see Fi
Page 65 and 66: FEM Simulations 6 FEM SimulationsEq
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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