Magnetic skyrmions are topological spin textures with great potential for spintronics applications (1). The appearance of skyrmions requires a careful balance between the magnetic anisotropy and the Dzyaloshinskii-Moriya interaction, and being able to tune these values with a voltage is an important tool for material optimisation for skyrmion devices.
In this work we have explored generation of both volatile and non-volatile skyrmions in MgO/Mn2CoAl/Pd thin films using different ionic liquid gating voltage sequences and magneto-optical Kerr effect (MOKE) microscopy. With a negative gate voltage of -2.5 V, we are able to generate non-volatile skyrmions ~ 1 μm diameter in films with perpendicular magnetic anisotropy Keff ~ 1 x 104 Jm-3. The same result is achieved in higher Keff films by ‘training’ or repeatedly cycling the gate voltage, achieving a giant voltage tunability of magnetic anisotropy of 109.8 mT V-1. Interestingly, volatile skyrmions can also be generated, by first applying a large negative ‘trigger’ voltage, and then skyrmions appear at a positive gate voltage. Using X-ray photoelectron spectroscopy, we explain the generation of skyrmions in terms of magneto-ionic and electrostatic effects (2). Our results show the potential of ionic liquid gating for achieving large anisotropy changes to reversibly or irreversibly generate skyrmions from materials with a range of starting anisotropy values.
(1) X. Zhang, Y. Zhou, K. M. Song, T.-E. Park, J. Xia, M. Ezawa, X. Liu, W. Zhao, G. Zhao, and S. Woo, J. Phys. Cond. Mat. 32, 143001 (2020).
(2) Y. Zhang, G. Dubuis, C. Doyle, T. Butler, and S. Granville, Phys. Rev. Appl. 16, 014030 (2021).