In order for the sensor to work the pinned multilayer should remain pinned at all times, during mounting and its lifetime. Although GMR and TMR-sensors are al-ready produced and applied in hard disk technology, applying them in the automotive industry requires different properties of the GMR/TMR-sensors. The sensors have to work properly at high external fields and high temperatures. The stability of the SAF and EB is the main subject of investigation in this thesis.
Exchange))
Figure 1.3: A regular GMR-based sensor (left) and a GMR-SAF sensor (right).
In the regular sensor, current passes through two FM layers (green and blue) and a spacer layer (yellow). The bottom F-layer is pinned by EB, whereas the top layer is free to rotate. In the GMR-SAF sensor, an additional FM layer and spacer layer are used and are configured such that this additional layer is antiferromagnetically coupled to the other FM layer. This results in no net magnetic moment for the pinned multilayer.
This thesis can be divided in two parts. First, the magnetic characteristics and thermal stability of the SAF are investigated. Later, the AF layer is added and the magnetic properties and field stability of stacks containing both EB and an SAF are investigated.
Chapter 2 will start by describing the theory of EB and synthetic antiferromagnets seperately, after which a system containing both EB and an SAF will be discussed.
Chapter 3 covers the experimental setups used to create and investigate both the SAF stacks and the EB-SAF multilayers. All multilayers are grown using a sputter system, and for the magnetic characterization a Magneto-optical Kerr effect (MOKE) setup and a vibrating sample measurement - superconducting quantum interference device (VSM-SQUID) are used. In order to set the exchang bias, an Argon oven as well as a build-in annealing module in the VSM-SQUID setup are used. Chapter 4 covers experimental results that are obtained during research. First, a growth study on different spacer materials is presented. Next, the thermal stability of an SAF-system is investigated. Finally, a study on the magnetic properties and field stability of an EB-SAF system is presented. Chapter 5 presents the conclusions of this study and Chapter 6 provides an outlook and recommendations for future research on EB-SAF systems.
Theory
This chapter provides the theoretical background necessary to understand the exper-imental results that will be described in Chapter 4. In the first section the Stoner-Wohlfarth model is introduced which can often be used to model the magnetic behavior of magnetic thin films in external fields. In section 2.2 the Meiklejohn-Bean model and the Fulcomer-Charap model describing the EB effect is introduced. The EB effect is used in the spin-valve systems to pin the magnetization of an FM layer in a pref-ered direction. The stability of this pinning effect is the one of the main subjects of this thesis. The stability of the EB effect is discussed in section 2.3. Next, the theory of interlayer exchange coupling is presented which is used to create the SAF based on Ruderman-Kittel-Kasuya-Yosida (RKKY)-interaction. The RKKY-interaction and a simplystic model are introduced to translate the RKKY-interaction to a system of FM layers seperated by a spacer layer. Bruno introduced a model to qualitively describe the RKKY-coupling in magnetic multilayers[5][6]. His model is briefly intro-duced in section 2.4. Finally, EB and the RKKY-coupling are combined to create an EB-SAF coupled stack. The magnetic properties of this stack are described in section 2.6.
2.1 Stoner-Wohlfarth Model
When describing the reversal of magnetization in magnetic thin films, the Stoner-Wohlfarth model is often used, which is a macrospin model based on the assumption of a uniform response of the magnetization when an external magnetic field is ap-plied. In the Stoner-Wohlfarth model, the magnetization of the thin film will rotate uniformly with the applied external field. Typical magnetic loops as described within the Stoner-Wohlfarth model are shown in figure 2.1 and 2.2.
6
H) +2KF/μ0MF) D2KF/μ0MF)
+MF)
DMF)
H) easy)axis)
M)
M)
Figure 2.1: Hysteresis loop for the Stoner-Wohlfarth model for a field applied along the hard axis. The magnetization slowly rotates towards the applied field direction, untill it completely aligns and saturation is reached.
M)
Hc) H) H)
Hard)axis) easy)axis)
M)
M)
+2KF/μ0MF) D2KF/μ0MF)
+MF)
DMF)
Figure 2.2: Hysteresis loop when the field is applied along the easy axis. A minimum applied field is needed to switch the magnetization direction. When the magnetization is switched and the field is applied in the opposite direction, again a minimum field is required to switch the magnetization back. This results in magnetic behavior that depends on the field sweep direction.
Hard)axis)
Easy)axis) θ)
M) H)
β) KF) x)
y)
Figure 2.3: Schematic of the vectors and angles for the magnetization M, the anisotropy KF and the applied field H as defined for the Stoner-Wohlfarth model.
Figure adapted from [4].
The total magnetic energy per unit area of a magnetic thin film with a uniaxial magnetocrystalline anisotropy in an applied field along the applied field direction is given by
E = −µ0HtFMFcos(θ − β) + KtFsin2(β) (2.1) With H the applied field, MF the saturation magnetization, K the magnetocrys-talline anisotropy of the layer and tF the thickness of the FM layer. The first term in equation (2.1) is the Zeeman energy contribution describing the effect of the applied field on the magnetization, and the second term is the magnetocrystalline anisotropy term. The angles θ and β are defined as the angles between the applied field and the magnetization respectively with the direction of the anistropy of the layer, as depicted in figure 2.3.
The stable configuration of the magnetization corresponds to an energy mini-mization. Equation (2.1) is minimized with respect to β resulting in
−µ0HtFMFsin(θ − β) + KFtFsin(2β) = 0 (2.2)
µ0HtFMFcos(β − θ) + 2KFtFcos(2β) > 0 (2.3)
β = 0. When applying a field along the hard axis (θ = ±π/2), the magnetization rotates gradually towards the field direction, untill it aligns completely and saturation is reached. A hysteresis loop for a field applied along the hard axis is shown in figure 2.1. When applying the magnetic field along the easy axis (θ = −π) for small fields the magnetization remains aligned along β = 0 since the system is in a local energy minimum. Applying a magnetic field changes the energy landscape and in order to remove the local energy minimum state, a minimum field is required. When this field is applied, the magnetization switches and the system reaches the global energy minimum state (β = −π). When the magnetic field is then reversed (θ = 0), the global energy minimum changes to β = 0, and the system needs a minimum applied field again to switch to this global energy minimum. This results in magnetic loops that depend on the field sweep direction and an example is shown in figure 2.2.
The minimum field to switch the magnetization is the coercive field Hc that can be obtained from (2.2) and (2.3) and is equal to
Hc = ±2KF
µ0MF (2.4)