Nanomagnet Dynamics with Magnetostatic and Magnetoelastic Interelement Coupling
Densely packed nanomagnet arrays are intensely investigated as the basis of spintronic devices as well as for understanding the fundamental physics of the magnetic spins in confined structures. As nanomagnet devices reduce their dimensions to the nanometer scale, their behavior is critically modified by numerous factors such as the finite size, shape, interfacial effects, fabrication defects, and interelement coupling. Among them, this thesis specifically addresses two factors controlling the interelement coupling.
First, we discuss dipolar coupling between nanomagnets with a novel dynamic separation approach. By probing densely packed arrays of nickel elliptic disks and distinguishing signals in the frequency domain, individual subgroups of nanomagnets are characterized beyond the diffraction-limited spatial resolution. The technique is applied to nanomagnet arrays consisting of nickel elliptic disks with different orientations. Supplemented with micromagnetic simulations, the effect of the dipolar coupling on a specific nanomagnet subgroup is identified. The second part investigates the magnetization dynamics magnetoelastically coupled with surface acoustic waves (SAWs). In nanopatterned periodic arrays, magnetization precession and SAWs are simultaneously excited by a pump laser pulse. We show for the first time that the magnetization response is indeed coupled to the SAWs in nanomagnet arrays and the spin wave spectra are distinctly altered from the unperturbed ones, showing pinning and enhancement of the magnetization precession at the SAW frequencies. Taking the magnetoelastic effect into account, a newly developed simulation procedure demonstrates excellent reproduction of the measurements. Extension modules for OOMMF micromagnetic simulation framework have been developed to offer general magnetoelastic modeling capability and are now publically available. Utilizing these experimental and modeling techniques, we present a novel experimental method for characterizing the damping parameter of nanostructured magnets. The linewidth of the pinning of the magnetization precession is directly connected with the Gilbert magnetization damping parameter and is utilized as an accurate measure for its experimental estimation, avoiding usual issues associated with the time-domain analysis. The new method enables accurate characterization of the damping parameter of nanopatterned magnets, governing important spintronic device characteristics such as threshold current density in spin transfer torque magnetic random access memories and transition jitter in heat-assisted magnetic recording.