- Post Doctoral
MIT Unit Affiliation:
- Electrical Engineering & Computer Science
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Date PhD Completed:
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The manufacturing of nanocontact-based spin-torque oscillators (NC-STOs) has opened the door for spintronic devices to play a part as active microwave elements. The NC-STO has the capability of converting a direct current into a microwave signal, and vice versa, by utilizing the spin transfer torque (STT) in ferromagnetic multilayer systems. However, the high-frequency operation of NC-STOs typically requires high magnetic fields and the microwave power they generate is rather limited. As a result, NC-STOs are not yet commercially used, and they require improvements in both material systems and device geometries before they can find actual use in microwave applications.
In order to improve and advance this technology, NC-STOs are required with both different nanocontact (NC) sizes and geometries, and using differ- ent stacks of magnetic materials. This dissertation presents experimental in- vestigations into the manufacturing of such devices using different fabrication techniques and a number of different magnetic material stacks. Currently, the fabrication of NC-STOs is limited to advanced laboratories, because NC fabri- cation requires high-resolution lithography tools. In the present work, we have developed an alternative method of fabrication, which does not require such tools and has the capability of fabricating NC-STOs having one to hundreds of NCs in a variety of sizes, possibly down to 20 nm. Devices fabricated with this method have shown mutual synchronization of three parallel-connected NCs, and pairwise synchronization in devices with four and five NCs.
Furthermore, the present work demonstrates low-field operation (down to 0.02 Tesla) of NC-STOs at a record high frequency of 12 GHz. This was achieved by implementing multilayers with a perpendicular magnetic anisotropy (PMA) material in the free layer of the NC-STO. In addition, the fabricated devices revealed an unexpected dynamic regime under large external applied field (above 0.4 Tesla). The new dynamic regime was found to be due to an entirely novel nanomagnetic dynamic object â a so-called magnetic droplet soli- ton, predicted theoretically in 1977 but not experimentally observed until now. Detailed experiments and micromagnetic simulations show that the droplet has very rich dynamics.
Finally, spin-torque-induced transverse spin wave instabilities have been studied. A NC-STO with a material stack consisting of a single ferromag- netic metal sandwiched between two non-ferromagnetic metals was fabricated. Prior to this work, evidence of spin wave instabilities was reported as resis- tance switching in nanopillar- and mechanical point contact based STOs. In the present work, the fabricated NC-STOs showed actual microwave signals up to 3 GHz under zero applied field with strong current hysteresis. All the fabricated NC-STOs open up new means of studying STT in different environ- ments, in order to resolve their current drawbacks for industrial applications.
5 Recent Papers:
"Direct observation of magnetization dynamics generated by nano-contact spin- torque vortex oscillators" ; P. Keatley, S. R. Sani,S. M. Mohseni, P. Dürrenfeld, T. Loughran, J. Åkerman, and R. Hickenv Phys. Rev. B 94, 060402(R) – 2016. Link
"Super-harmonic injection locking of nano-contact spin-torque vortex oscillators"; P. S. Keatley, S. R. Sani, G. Hrkac, S. M. Mohseni, P. Dürrenfeld, J. ̊Akerman, and R. Hickenv arXiv:1604.04462. Link
"Magnetic droplet nucleation boundary in orthogonal spin-torquenano-oscillators" ; Sunjae Chung, Anders Eklund, Ezio Iacocca, Seyed Majid Mohseni, Sohrab R. Sani, Lake Bookman, Mark A. Hoefer, Randy K. Dumas, J. Åkerman, Nature Communications, 7, 11209, doi:10.1038/ncomms11209, (2016). Link
"Spin-wave-beam driven synchronization of nanocontact spin-torque oscillators." ; A. Houshang, E. Iacocca, P. Dürrenfeld, S. R. Sani , J. ̊Akerman and R. K. Dumas, Nature Nanotechnology, 11, 280286, (2016). Link
"Holographic Magnetic Imaging of Single-Layer Nanocontact Spin-Transfer Oscillators" ; Erick O. Burgos Parra, Nick Bukin ; Maxime Dupraz ; Guillaume Beutier ; Sohrab Redjai Sani ; Horia Popescu ; Stuart A. Cavill ; J. Åkerman ; Nicolas Jaouen ; Paul S. Keatley ; Robert J. Hicken ; Gerrit van der Laan ; Feodor Y. Ogrin, IEEE Transactions on Magnetics , Volume:52 , Issue: 7, Article 6500304, (2016). Link