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Nanomagnet Applications in Multiferroic Heterostructures

  • Author(s): Li, Xu
  • Advisor(s): Lynch, Christopher S.
  • et al.
Abstract

Over the past decade there have been considerable research efforts directed at controlling spin dynamics in nanomagnets. Spin dynamics generally covers two regimes, one is large amplitude motion associated with equilibrium magnetization state change with storage media applications that use parallel and antiparallel magnetization configurations to represent “0” and “1” states of memory bits. The other type is small amplitude oscillation, wherein spin precesses around its equilibrium state within a small cone angle. This behavior is generally used to design radio frequency(RF) devices. One of the approaches to controlling spin dynamics in nanomagnets is to use strain-mediated multiferroic heterostructures that consist of a nanomagnet deposited on a piezoelectric substrate wherein the magnetic state in the nanomagnet can be altered by electric field induced piezo-strain in piezoelectric substrates. Three innovative applications of nanomagnets via this piezo-driven method for both types of spin dynamics are presented in this dissertation: perpendicular magnetic tunnel junctions (pMTJ) in magnetoresistive random access memory(MRAM), surface acoustic wave(SAW) driven spin wave resonance (SWR) devices for RF transducers design, and multiferroic multiband tunable filters. Finite element analysis (FEA) simulation results in this dissertation demonstrated a 180-degree magnetization switch in the free layer of a pMTJ in response to an in-plane effective magnetic field induced by strain. Experimental work on thin film characterization is discussed that provides PMA material candidates for future pMTJ designs. A longer distance travelling spin wave (up to 1200um) in a SAW driven SWR device was obtained through magnetoelastic coupling. Three factors that affect energy transmission rate in this device have been investigated, which are film size, SAW wave vector k and spin pumping effect. Furthermore, a new type of E-field tuning multiband filter based on multiferroic heterostructure was proposed in this dissertation, which exhibits multiple absorption peaks and a large E-field tunability resulting from the acoustic and optical modes of spin interaction in magnetic layered structures. This work provides valuable insights into spin dynamics in nanomagnets. It opens up a new design space for next generation spintronic devices.

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