Magnetic Tunnel Junctions (MTJs) are multi-layered devices that use tunnel magnetoresistance (TMR) to detect changes in magnetic field. 1. MTJs with specific characteristics are relatively complex to fabricate due to the high dependence of their magnetic properties, especially perpendicular magnetic anisotropy (PMA), on individual film thicknesses and interfaces. Post-growth modification tools, such as ion irradiation 2, are perfect when the properties of such structures are to be modified. Ion irradiation is a versatile tool that causes structural changes in the material’s lattice by forming point defects, that in turn cause changes in the magnetic properties, which can be controlled by tuning the fluence, energy, and ion current density of the ion beam 3.
In this study, we use 30 keV argon and neon ions to modify the magnetic properties of a MTJ half stack Sub-SiO2/ Ta (5)/ MgO (1.5)/ Co40Fe40B20 (1.6)/ W (0.5)/ Ta (2) by irradiating with fluences between 1×1013 ions/cm2 and 1x1015 ions/cm2. The low fluence ion irradiation (<1014 ions/cm2) changed the anisotropy of magnetic layers from out-of-plane to in-plane, and even achieve a canted anisotropy. The calculated displacement per atom (DPA) in CoFeB using the TRIM 4 simulation tool ranges between 0.13 and 32.75 for fluences 1×1013 ats/cm2 and 1x1015 ions/cm2. We found that an irradiation of even 1013 ions/cm2 fluence may cause a reduction in effective anisotropy by around 53%. We envisage that low fluence irradiation (<1014 ions/cm2) limits significant intermixing of layers which avoids the formation of a dead layer in the stack and degradation of TMR performance. These findings are important for fabricating better magnetic sensors with lower signal-to-noise ratios, MRAMs with voltage tunability, tuning properties of spin-orbit-torque based devices, etc. Detailed experimental results obtained from Magneto Optical Kerr Effect Spectroscopy (MOKE) and superconducting quantum interference device (SQUID) supported by DYN-TRIM simulations will be presented in the conference.
References:
1 C. Zheng, K. Zhu, S.C. de Freitas, J.Y. Chang, J.E. Davies, P. Eames, P.P. Freitas, O. Kazakova, C.G. Kim, C.W. Leung, S.H. Liou, A. Ognev, S.N. Piramanayagam, P. Ripka, A. Samardak, K.H. Shin, S.Y. Tong, M.J. Tung, S.X. Wang, S. Xue, X. Yin, P.W.T. Pong, Magnetoresistive Sensor Development Roadmap (Non-Recording Applications), IEEE Transactions on Magnetics. 55 (2019) 1–30.
2 J. V. Kennedy, W.J. Trompetter, P.P. Murmu, J. Leveneur, P. Gupta, H. Fiedler, F. Fang, J. Futter, C. Purcell, Evolution of Rutherford’s ion beam science to applied research activities at GNS Science, J R Soc N Z. (2021) 1–18.
3 A. Mahendra, P. Gupta, S. Granville, J. Kennedy, Tailoring of magnetic anisotropy by ion irradiation for magnetic tunnel junction sensors, Journal of Alloys and Compounds. 910 (2022) 164902.
4 J.F. Ziegler, M.D. Ziegler, J.P. Biersack, SRIM - The stopping and range of ions in matter (2010), Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms. 268 (2010) 1818–1823.