Magnetic dynamics in magnetic multilayers and spintronic devices
Date
2021
Authors
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Publisher
University of Delaware
Abstract
A critical issue in spintronic devices is how to efficiently switch the magnetization of a nanomagnet. The switching is typically occurred at ferromagnetic resonant frequency. One of the very desired features is to enhance the resonant frequency so as to increase the switching speed. In this thesis. we use optical mode in coupled FM bilayers to significantly enhance the resonance frequency because the ferromagnetic exchange coupling is much stronger compared with other effective field introduced by, for example, strain or anisotropy. we theoretically and experimentally study the resonant frequency in Co90Fe10/Ta/(Ni80Fe20)1−xCux trilayer. we also validate the theoretical model with experimental results and use the theoretical model to further design heterostructures with desired properties such as enhanced intensity in OM. All of our experimental results are fully explained by our model which also allows us to identify the coupling mechanism as Néel “orange-peel” coupling and extract the interlayer coupling strength. In current magnetic random access memories (MRAMs), the magnetization is switched via spin transfer torques (STT) or spin orbit torques (SOTs). In this thesis, we use a time-resolved (TR) Magneto-optic Kerr effect (TRMOKE) to investigate the SOT-induced magnetization dynamics in Py/Pt and Ta/CoFeB heterostructures. We have answered three questions as we set to explore: (1) the field-like torque determines initial oscillation magnitude and the damping-like torque determines the final steady position; (2) both the effect field of damping-like toque hDL and field-like torque hFL can be extracted from the TRMOKE spectrum, and (3) the ratio hDL/ hFL measured from dynamics are the same as those measured at low frequency. Finally, we also explore the way of using voltage to switch the magnetization in order to significantly reduce the energy consumption. We employed an antiferromagnetic Cr2O3 film with net a net magnetization at the film surface. The surface magnetization can be isothermally switched via applied electric field (voltage), enabling “writing” function in the memory cell. We solved various challenges in developing the final device including (1) exchange coupling between the surface magnetization and the magnetization in a ferromagnetic heterostructure of Co/Pd multilayer, (2) the exchange coupling between Co/Pd and CoFeB which has perpendicular magnetization and serves as a bottom electrode of magnetic tunnel junction for readout. (3) the leakage issue of Cr2O3, (4) the contact to middle layer that is less than 5 nm thick, and (5) device layout and detailed fabrication steps. Although we did not have time to fully demonstrate the devices, all key steps have been well resolved.
Description
Keywords
Magnetic dynamics, Magnetic multilayers, Spintronic devices