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Nonlinear Magnetization Dynamics in Nanoscale Confined Ferromagnets


The nonlinear magnetization dynamics in zero-dimensional magnets have received increasing attention in recent years due to their potential applications in high-density data storage and spintronic devices. It also shows parallels to AMO physics, nonlinear optics, quantum information systems, and even some astrophysical phenomena. Magnon scattering processes, which constitute a major dissipation channel in nanomagnets, play a major role in determining the energy efficiency of spintronic applications.

In this work, first I report an experimental observation of different kinds of magnon- magnon interactions affecting the magnetic damping within the nanomagnet. I focus on predominant nonlinear magnon scattering processes in nanomagnets, including degenerate and non-degenerate three magnon scattering and degenerate four magnon scattering. To do the experiment, I implement magnetic tunnel junction (MTJ) nanodevices, consisting of a free layer and a synthetic antiferromagnet at their core. It is shown these nonlinear processes can redefine and invert the nanomagnet’s response to spin torques. A theory is established to explain this counter-intuitive effect, demonstrating the damping parameter of a spin wave stops being frequency-independent and becomes a resonant function of the excitation frequency.

Controlling magnon processes and thus magnetic damping is the key to improving the performance of future computer systems. In this regard, I propose an approach for controlling magnon scattering by a nanoscale dipole switch. I demonstrate an experimental proof-of-concept in MTJ nanodevices. By triggering the spin-flop transition in the synthetic antiferromagnet and utilizing its dipole field, a three-magnon process in the free-layer is toggled. The switching of the synthetic antiferromagnet allows for controllably tuning the strength of the magnon interaction by at least one order of magnitude, leading to two distinct dissipative states.

Additionally, I study the magnetic switching of a nanoscale synthetic antiferromagnet (SAF) with a perpendicular order parameter incorporated in a MTJ structure. I drive the MTJ with microwave pulses and observe spin-flip switching of the SAF by resonant ex- citation of an individual magnon mode of the SAF. The magnon spectrum of the nanoscale SAF is discrete. Evaluation of the switching probability distributions shows that various modes from this spectrum can facilitate the switching. Moreover, it suggests a substantial contribution of the thermal spin-torque.

Finally, I investigate the electric noise in MTJs whose understanding is a prerequisite for employing them in next-generation applications. I observe random telegraph noise, which is frequently found in MTJs but not yet fully understood. By varying the temperature in the range of 80–300 K for an MTJ in the parallel state, we encounter anomalous device resistance (steps) attributed to magneto-structural phase transitions of iron oxide clusters at the CoFeB/MgO interface. At temperatures of these anomalies, telegraph noise shows a significant increase. This correlation suggests that the oxide clusters have a significant impact on MTJ noise characteristics.

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