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Seminar - Christian Hahn

Monday 23rd September 2019 at 11h00, Christian Hahn (Physikalisch-Technische Bundesanstalt, Braunschweig, Germany) will give a seminar on "Spin Transfer Torque Driven Magnetization Dynamics – Understanding Timescales"

The interaction between spin current and magnetization has been intensely researched because of its ability to probe magnetization (particularly in nanostructures) and excite dynamical phenomena. Two decades ago it was postulated how a spin-polarized current would provide angular momentum to local magnetic moments and allow for the control and even total cancellation of spin-wave damping via spin transfer torque (STT) [1,2]. Here, I will focus on two phenomena: the process of magnetization switching in perpendicular magnetic tunnel junctions (pMTJs) [3] and magnetic droplet solitons created under nano-contacts to magnetic thin films with perpendicular anisotropy [4].
The first part will illustrate how time resolved measurements of the pMTJ’s device resistance, during spin current induced switching, allows for characterization of different reversal mechanisms. For this purpose, we extract the characteristic times associated with switching onset and duration from individual time-traces. Device size dependent variations in the switching duration are consistent with a reversed domain nucleation and propagation at low current amplitudes [5].
For the second part, I describe spin torque driven nano-oscillators (STNOs), which generate ac-oscillations of tens of GHz. Here, above a threshold-current, the STT driven magnons form a particle-like and nanoscale spin-wave excitation, the droplet soliton. In view of their potential use for magnon computing, we study the critical timescales over which these objects can be generated or will be dissipated [6]. In a low-temperature (4 K) study [7] we show there are multiple and, under certain conditions, combinations of droplet modes, each mode with a distinct high-frequency spin precession, which can be influenced by varying the applied fields and currents.

[1] J. Slonczewski, J. Magn. Magn. Mater. 159, L1–L7 (1996)
[2] L. Berger, Phys. Rev. B 54, 9353–9358 (1996)
[3] A.D. Kent and D.C. Worledge, Nature Nanotechnology 10,187 (2015)
[4] Hoefer et al., Physical Review B 82,054432 (2010)
[5] C. Hahn et al., PRB 94, 214432 (2016)
[6] J. Hang, et al., Scientific reports 8 (1), 6847 (2018)
[7] N. Statuto, et al., Physical Review B 99 (17), 174436 (2019)


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