Physics

Dr. Samridh Jaiswal
University of Mainz, Germany

Abstract :

The need to produce low cost, high density magnetic data storage devices requires a core understanding of the magnetic spin textures and their underlying mechanisms. Ferromagnets (FM) sandwiched between Heavy Metal (HM) and oxide thin films have shown novel spin textures such as spin spirals and skyrmions arising from an interfacial Dzyaloshinskii-Moriya interaction (DMI) [1, 2]. This DMI manifests due to the high spin-orbit coupling of the HM and the structural inversion asymmetry at the HM/FM interface [3]. In this work, we study the DMI and the skyrmion motion in continuous thin films of perpendicularly magnetised W/CoFeB/MgO. The DMI leads to an asymmetric expansion of magnetic bubble domains in the presence of an in-plane applied field [4]. Full field Kerr microscopy was used to observe magnetic bubble domains and on application of an in-plane field the asymmetric expansion of each domain wall was measured. The degree of the asymmetric expansion of the bubble domain was used to estimate the value for DMI energy which was found to be 0.68±0.05 mJ/m2 for a single repetition of the stack. Scanning Transmission X-ray Microscopy (STXM), using synchrotron X-ray facilities, was used to image the magnetic spin texture in such films grown on SiN substrates with multiple stack repetitions. We observe stable room temperature magnetic skyrmions in patterned nanowires and study their reproducible current induced motion. Additionally, magnetic stripe domains in the nanowires and their expansion due to the applied magnetic field were used to calculate the DMI energy value, which was found to be 0.73±0.5 mJ/m2 . We demonstrate two separate methods used to determine the DMI in the same multi-layered stack. Furthermore, a variation in magnetic properties by tuning the growth conditions of the W seed layer is explained which leads to a reduced magnetic pinning and stable room temperature skyrmions in such material structures. [1] S. Woo et al., Nature Mater. 15, 501 (2016) [2] C. Moreau-Luchaire et al., Nature Nanotech. 11, 444 (2016) [3] A. Thiaville et al., Eur. Phys. Lett. 100, 57002 (2012) [4] A. Hrabec et al., Phys. Rev. B 90, 020402(R) (2014)