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).