Ezio, Randy, Majid, Sunjae and Johan publish in PRL - "Confined dissipative droplet solitons in spin-valve nanowires with perpendicular magnetic anisotropy"
This analytical and numerical work builds on our previous experimental demonstration of magnetic droplet solitons [Science 339, 1295 (2013)] where we confirmed the predictions by Ivanov and Kosevich from over 35 years ago [Zh. Eksp. Teor. Fiz. 72, 2000 (1977)] and the more recent predictions by Hoefer, Silva and Keller [Phys. Rev. B 82, 054432 (2010)].
Droplets form naturally in systems with attractive forces. Water droplets are e.g. formed from the attractive polar interactions between water molecules. In ultrathin magnetic films the so-called perpendicular magnetic anisotropy (PMA) can create an attraction between spin wave excitations (magnons) and if a sufficient number of magnons are present in a region (analogous to a sufficient number of water molecules) they can condense into a magnetic droplet soliton.
Much like water on extended surfaces, droplets are easily formed in extended PMA layers. However, the nucleation in a narrow channel is not easily predicted. In this paper, we show by numerical simulations that magnetic droplets can indeed be excited in nanowires and in doing so can acquire two novel forms: an edge and a quasi-one-dimensional (Q1D) nanowire mode. The edge mode arises as the physical boundaries attract the droplet the same way as a water drop would be attracted by a neighboring polar surface, e.g. a piece of glass. On the other hand, the Q1D exists in very narrow wires, where both boundaries attract the droplet. As a result, the Q1D acquires an additional property known as chirality which is a well-defined magnetic state that makes it especially resistant to disturbances. Due to these characteristics, such novel droplet modes can be identified by their intrinsically different frequency. Our simulations suggest means to experimentally prepare and observe these modes for their detailed physical study and in the prospect of nanoscopic applications for data storage.
E. Iacocca, R.K. Dumas, L. Bookman, S.M. Mohseni, S. Chung, M. Hoefer, and J. Åkerman, Confined dissipative droplet solitons in spin-valve nanowires with perpendicular magnetic anisotropy, Phys. Rev. Lett. 112, 047201 (2014).
A truly unique experience! It was a great honor and pleasure to be invited to EINC 2013 and speak about our recent results on Magnetic Droplet Solitons. The Ester Island is a fascinating place with a dramatic and exciting history which can be highly recommended. Although it's essentially on the opposite side of the planet, the travel is surprisingly manageable, with e.g. Air France flying directly from Paris to Santiago and then Lan flying to Easter Island. I want to particular highlight the evening plenary talks given by Nobel Laureates Dan Shechtman (Nobel Prize 2011) and Claude Cohen-Tannoudji (Nobel Prize 1997), who gave very inspiring presentations on “Quasi- Periodic Materials – Crystal Redifined”, and “Manipulating Atoms with Photons and Photons with Atoms”.
This YouTube clip is from SSF's (The Swedish Foundation for Strategic Research) YouTube Channel where they show an interview of Johan Åkerman and Sohrab Redjai Sani recently aired on TV4 Science (Swedish: TV4 Vetenskap). Half of the clip is in Swedish but if you jump to about 3:35 grad student Sohrab explains some of the lithography steps in English.
Drops are natural objects in systems with attractive forces. Water drops are e.g. formed from the attractive polar interaction between the H atoms in one H2O molecule and the O atom in another. In ultrathin magnetic films the so-called perpendicular magnetic anisotropy (PMA) can create an attraction between spin wave excitations (magnons) and if a sufficient number of magnons are present in a region (analogous to a sufficient number of water molecules) they can condense into a magnon drop where all spins in the drop precess in-phase on a single magnon frequency.
Magnon drops were predicted theoretically by Ivanov and Kosevich over 35 years ago [Zh. Eksp. Teor. Fiz. 72, 2000 (1977)]. More recently, Hoefer, Silva and Keller demonstrated analytically and numerically that nano-contact spin torque oscillators (NC-STOs) with PMA free layers should be able to nucleate and sustain the dissipative (= lossy and actively driven) analogue of magnon drops; they called this new potential soliton object a magnetic droplet [Phys. Rev. B 82, 054432 (2010)].
In this work, we present the first experimental demonstration of magnetic droplets. Our devices consist of NC-STOs with a Co fixed layer and a [Co/Ni] multilayer free layer. The creation of a magnetic droplet is experimentally observed as a dramatic 10 GHz drop in the measured microwave frequency, accompanied with a sharp increase in the device resistance. The droplet displays a wide range of additional magnetodynamic phenomena, experimentally observed as a number of sidebands at different frequencies. Our work both brings closure to a long-standing theoretical prediction and provides the nanomagnetic and spintronic community with a novel dynamic nanomagnetic object, which joins the magnetic domain wall and magnetic vortex with similar potential for rich science.
S. M. Mohseni, S. R. Sani, J. Persson, T. N. Anh Nguyen, S. Chung, Ye. Pogoryelov, P. K. Muduli, E. Iacocca, A. Eklund, R. K. Dumas, S. Bonetti, A. Deac, M. A. Hoefer, and J. Åkerman, Spin Torque–Generated Magnetic Droplet Solitons, Science 339, 1295 (2013).