Spin-transfer torques in perpendicular magnetic anisotropy ultrathin CoFeB layers

Spin-transfer torques in ultra-thin magnetic films

Ultra-thin films (~nm) have recently been integrated into state-of-the-art magnetic tunnel junctions in order to realise spin-transfer-torque driven magnetic random access memories (MRAM). These films possess perpendicular anisotropy due to their high surface to volume ratio and induced magnetic anisotropy in the presence of adjacent layers.

In contrast to the usual rare-earth based multilayers (e.g. Co/Pt, Co/Pd), MgO and other oxides have been shown to induce perpendicular magnetic anisotropy in thin films. Both the understanding and control of such films is crucial for their exploitation in spin-transfer driven devices.

Complimentary techniques of extraordinary Hall effect, spin-torque ferromagnetic resonance, standard ferromagnetic resonance as well as standard magnetometry are used to determine the magnetic properties of both extended films and free standing nanopillar structures.

We calculated and measured spin-transfer torques in nanopillars including various degrees of perpendicular anysotropy for the free layer. Both in-plane and perpendicular anisotropy have to be considered, given that shape distortions can occur during patterning. The experimental and theoretcal results are found to be in good agreement.

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2. "Tunnel magnetoresistance angular and bias dependence enabling tuneable wireless communication", E. Kowalska, A. Fukushima,V. Sluka, C. Fowley, A. Kakay, Y. Aleksandrov, J. Lindner, J. Fassbender, S. Yuasa, A. Deac, Scientific Reports  9, Article number: 9541 (2019).

3. "Zero-field dynamics stabilized by in-plane shape anisotropy in MgO-based spin-torque oscillators", E. Kowalska, A. Kakay, V. Sluka, C. Fowley, J. Lindner, J. Fassbender, A. Deac, J. Appl. Phys.125, 083902 (2019).