Investigation of the Magnetic Vortex State for Spin-Valve Sensors

Wurft T. Investigation of the Magnetic Vortex State for Spin-Valve Sensors. Bielefeld: Universität Bielefeld; 2018.Many automotive applications, such as wheel speed and current sensing, require magnetic sensors with a large signal range, a negligible small hysteresis and a large linear operating range. All these requirements can be met when using a Tunneling Magnetoresistance (TMR) spin-valve sensor with a free layer (FL) that operates in the vortex state. The magnetic vortex has been intensively studied in the last two decades due to its special static and dynamic properties and promising applications. In the present work, the static properties of the magnetic vortex, especially its critical fields - nucleation field Hn and annihilation field Han - are investigated. These fields are the key parameters regarding its implementation in a sensor. The investigated TMR spin-valve vortex sensor concept has a large potential for commercial use, for example, as speed or current sensor. Geometrical factors such as diameter d (0.8 − 4.1 μm) and thickness t (10 – 50 nm) and the influence of material (Co90Fe10, Co60Fe20B20, and Ni81Fe19) in circular, disk-shaped FL elements are investigated experimentally and compared to micro-magnetic simulations. The initially expected universal scaling of Hn with the aspect ratio d t is only observed if exceeding a certain FL thickness. It is shown for a certain diameter of 1.1 μm that only above t = 35 nm the stray field energy of the saturated disk drastically increases with thickness, following a linear trend as a function of t. This effect is linked to a significant increase of the out-of-plane magnetization at the edge of the disk with increasing thickness below t = 35 nm. Phase diagrams of magnetic states - as a function of t and d – are extracted from micro-magnetic simulations and give information about the occurrence of different pre-vortex states. The results are consistent with experimentally observed phase transitions and allow the conclusion that for a Co60Fe20B20 thickness of 20 nm the S-state delays vortex nucleation. The formation of the double vortex (DV) state is only observed for t = 35 and 50 nm as the formation probability increases with increasing t. The DV is causing a delay of the single vortex nucleation, like the S-state. It is shown that the magneto-crystalline anisotropy of Co90Fe10 is not only causing a delay of vortex nucleation but can also lead to a drastic increase of the stability of the DV as well as to the formation of hysteretic states with even more than two vortex cores. Moreover, it is demonstrated that the critical fields are also sensitive to the properties of the edge of the FL: a magnetically disturbed edge and a sloped edge lead to a drastic reduction of Han and edge roughness may favor or avoid the formation of intermediate states which affects Hn and its distribution. In addition, a reduced change in TMR signal during vortex annihilation is observed for Co90Fe10. This effect can be explained by the introduction of an electrically inactive area at the edge of the FL. Extrinsic factors such as temperature and magnetic bias fields are studied in terms of how they shift Hn and Han. At elevated temperatures the Hn shift (ΔHn) is always smaller than the temperature-induced reduction of the saturation magnetization (Ms). For t = 20 nm, thermally assisted energy barrier jumps even become dominant. Thus, on average vortex nucleation is observed earlier if the temperature is raised from 35 to 150 °C. Furthermore, individual elements show in principle negative as well as positive ΔHn values, depending on whether or not energy barriers delay vortex nucleation at 35 °C. Temperature-induced Han shifts (ΔHan) are on average always negative but individual Co90Fe10 elements also show positive values which are expected to be linked to the direction of rotation of the vortex state. For t = 20 nm, the average ΔHan is almost exclusively caused by the reduction of Ms. For t = 35 and 50 nm, the influence of thermally assisted energy barrier jumps on Hn increases. Surprisingly, additionally applied in-plane bias fields, perpendicular to the field of the hysteresis loop, can facilitate vortex nucleation significantly. Three different explanations are found, depending on t and the choice of material: (1) reduction of the magneto-crystalline coercivity, (2) reduction of the configurational stability, or (3) an increase of the number of possible vortex nucleation sites. For t = 20 nm Co90Fe10, an average ΔHn of almost 14 % is observed when applying a bias field of Hy = 80 Oe. For such a bias field, no positive average shifts are observed for Co60Fe20B20 but individual elements show both positive and negative shifts