Spin transfer torques in the heavy metal/ferromagnet/oxide hetero-structure Ta/CoFeB/MgO
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Abstract
Spin-orbit interaction based spin transfer torque promises ultra-efficient magnetization switching used for modern information storage devices based on emergent quasi-particles such as domain walls and skyrmions. Recently, spin structure dynamics, materials, and systems with tailored spin-orbit torques are being developed.
A method, which allows to detect the acting torques in a given system as a function of the magnetization direction is the torque magnetometry method, which is based on a higher harmonics analysis of the anomalous Hall effect.
So far it has not been shown that the spin-orbit torques measured with this torque magnetometry method for homogeneous spin-textures are consistent with results of effective collective torques acting on more complex spin-textures such, as domain walls and skyrmions.
In this thesis, we address this issue by measuring both the torques for homogeneous spin-textures with the torque magnetometry method, as well as the effective collective torques for domain walls within domain wall depinning experiments, on the same sample.
We show that the effective fields acting on magnetic domain walls that govern the efficiency of their dynamics require a sophisticated analysis taking into account the full angular dependence of the torques.
Using a one-dimensional model, we compare the spin-orbit torque efficiencies by depinning measurements and spin torque magnetometry.
We show that the effective fields can be accurately determined with both methods individually, but find good agreement only when first, one takes into account a geometrical conversion factor of π/2 and second, one neglects, that the measurement results for the domain wall depinning show the maximum torque efficiency in a direction not expected from the simple spin-orbit torque model.
However, our method allows us now to rapidly screen materials by using the fast torque magnetometry method and to predict the resulting quasi-particle dynamics. Finally, we discuss the validity of this approach and give recommendations on how to eliminate given uncertainties.