Size dependence of magnetic switching in perpendicularly magnetized MgO/Co/Pt pillars close to the spin reorientation transition

We investigated the magnetic switching of MgO/Co/Pt pillars with perpendicular magnetic anisotropy, for lateral pillar sizes from 30 nm to 2 µm and for Co layer thicknesses between 1.8 and 2.6 nm. For all Co thicknesses, the coercivity strongly increases upon decreasing the pillar size. A comparison with micromagnetic simulations shows that the change in coercivity is determined by size-dependent demagnetizing effects. Our results show that small pillars with perpendicular magnetization and a tunable coercivity can be fabricated from continuous layers with in-plane magnetization.

Oxide/ferromagnetic metal (FM)/Normal metal (Pt,Ta) trilayered structures where the FM layer has a perpendicular magnetic anisotropy (PMA) [1] are important building blocks for the fabrication of perpendicular magnetic tunnel junctions (MTJ) [2]. Although the magnetic properties and magnetization switching in continuous layers and nanostructures with perpendicular magnetization have been widely studied, most of these studies have been performed on materials with strong PMA like CoPt and FePt L10 ordered alloys, where the bulk magneto-crystalline anisotropy causes the PMA. The strong PMA in these materials allows good stability against thermal agitation when the lateral sizes are reduced and makes them promising for high density data storage, but their electronic transport properties make them less adapted for use in MTJ. For this purpose, direct contact between the magnetic layer and an oxide is needed, which explains the large increase in the research on perpendicular MTJ after the discovery of perpendicular interfacial magnetic anisotropy at certain interfaces between a ferromagnetic layer and an oxide [1-3]. In this case, the interfacial PMA is relatively small and the magnetization orientation depends on the competition between the interfacial PMA and the demagnetizing energy, which favors in-plane magnetization. This competition depends both on the FM layer thickness and the lateral size of the MTJ element.

We have performed a systematic investigation of field-induced magnetization switching in MgO/Co/Pt pillars with Co thicknesses between 1.8 and 2.6 nm, and lateral sizes between 30 nm and 2 µm. This Co thickness range is around the spin reorientation transition, where in the continuous layers the preferred orientation of the magnetization changes from out-of-plane to in-plane when the Co thickness increases. For each Co thickness, we observe a strong increase of the coercivity upon decreasing the pillar size. The results are shown in Fig. 1, where the coercivity is plotted as a function of pillar size for different thicknesses of the Co layer. The measurements were performed using a focussed magneto-optical Kerr effect setup.

In Fig. 1, we also plot the perpendicular demagnetizing factor NZ [4], for a 2.47 nm thick Co layer. The experimental trend closely follows the trend of NZ, indicating that the increase of the coercivity upon decreasing size is mainly due to size dependent demagnetizing effects. This is confirmed by micromagnetic simulations using the experimental parameters for magnetization and anisotropy, showing the same trend. They also show that for the larger pillars the magnetization reversal starts in the center of the pillars, where the perpendicular demagnetizing field is the largest.

For more details, please read our recent publication : Applied Physics Letters 104, 012404 (2014)

[1] S. Monso, B. Rodmacq, S. Auffret, G. Casali, F. Fettar, B. Gilles, B. Dieny, and P. Boyer, Appl. Phys. Lett. 80, 4157 (2002). [2] S. Ikeda, K. Miura, H. Yamamoto, K. Mizunuma, H. Gan, M. Endo, S. Kanai, J. Hayakawa, F. Matsukura, and H. Ohno, Nature Mater. 9, 721 (2010). [3] B. Rodmacq, S. Auffret, B. Dieny, S. Monso, and P. Boyer, J. Appl. Phys. 93, 7513 (2003). [4] A. Aharoni, J. Appl. Phys. 83, 3432 (1998).

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