Magnon Condensates in Spin-Transfer Torque Nanocontacts*

In ferromagnets with uniaxial magnetic anisotropy there is an attractive interaction between spin-wave excitations or magnons. This can lead to the formation of a magnon condensate, predicted in the late 1970s---also known as a magnetic droplet soliton [1]. Only recently has it been possible to realize this state experimentally by creating a non-equilibrium magnon population using spin-transfer torques from a spin-polarized current. Experiments are conducted using a nanocontact to a thin film with perpendicular magnetic anisotropy [3-6]. DC and high frequency transport measurements demonstrate that magnetic droplet solitons exhibit a strong hysteretic response to field and current, showing the existence of bistable states: droplet and non-droplet states [4]. We also present the first direct observation of droplet solitons using scanning transmission x-ray microscopy (SXTM) [5]. Element resolved x-ray magnetic circular dichroism images show an abrupt onset of magnetic solitons at a threshold current, as predicted by theory [2]. The amplitude of the excitation, however, is far less than predicted. A possible origin of this discrepancy is a resonant drift instability, whereby the droplet periodically moves out of the contact region, annihilates and renucleates in the nanocontact [6,7]. Our recent measurements of the time scale for droplet generation and annihilation with pulsed currents show that annihilation takes several ns but the generation time is much longer, $\sim$ 100 ns. These recent results will be presented along with a prespective on future experiments with magnon condensates.\\ \\ 1. A. Ivanov and A. M. Kosevich JETP {\bf 45}, 1050 (1978) \\ 2. M. A. Hoefer, T. J. Silva, and M. W. Keller, PRB {\bf 82}, 054432 (2010)\\ 3. M. Mohseni {\it et al.}, Science {\bf 339}, 1295 (2013)\\ 4. F. Macia, D. Backes and A. D. Kent, Nat. Nano. {\bf 9}, 992 (2014)\\ 5. D. Backes, F. Macia, S. Bonetti, R. Kukreja, H. Ohldag and A. D. Kent, PRL {\bf 115}, 127205 (2015)\\ 6. S. Lendnez, N. Statuto, D. Backes A. D. Kent and F. Macia, PRB {\bf 92}, 174426 (2015)\\ 7. P. Wills, E. Iacocca, and M. A. Hoefer, PRB {\bf 93}, 144408 (2016)\\