Magnetization dynamics of single domain nanomagnets

When a ferromagnet is made sufficiently small: down to nanometer scale, it will lose its complicated multi-domain structure and become a single domain magnet. A single domain magnet has a uniform magnetization vector whose magnitude is constant and direction is determined by the applied field and by its internal forces. By shape anisotropy of the magnet, for example, an ellipsoidal magnet, it can be forced to have only two preferred magnetization directions. This small size and binary magnetization states property makes a single domain nanomagnet (SDNM) perfectly preferable for high density data storage applications. The areal density can potentially be increased up to 1Tbit/in2 if an individual single domain magnet is used for storage of one bit. Characterization and thorough understanding of the dynamic properties of single domain nanomagnets will provide invaluable information for high-speed device operation such as data storage media and spintronic devices.;We present an experimental investigation of the switching dynamics of individual nanomagnets and nanomagnet arrays with both high spatial resolution (< 100nm) and high temporal resolution (100fs). First of all, we developed a Kerr cavity enhancement technique, with which we were able to detect the static magnetization switching of a single nanomagnet down to 50nm. Thus, we are expected to be able to gain sufficient spatial resolution in the far-field without resorting to the much more complicated near-field approach. A geometric model was developed to understand our experimental data, and later optimized for further spatial sensitivity improvement. Moreover, we built a two-color time-resolved MOKE (Magneto-optic Kerr Effect) setup based on the conventional pump-probe technique, and studied the picosecond dynamic properties of individual nanomagnet and nanomagnet arrays as a function of magnet dimension, aspect ratio and bias magnetic field. Spatial sensitivity to nanomagnet diameters as small as 100nm was achieved by use of cavity enhancement of the magneto-optic Kerr effect (CE-MOKE). This is the first time that the magnetization dynamics of single single-domain nanomagnets were observed. The coherent Ferromagnetic Resonance (FMR) mode from individual cylindrical nickel magnets of various diameters was separated from other spin wave modes through fast Fourier transform (FFT). We studied the bias field dependence and magnet size dependent of the FMR mode frequency and its damping. Object-Oriented Micro Magnetic Framework (OOMMF) software was used to simulate the 3D magnetic characteristics of individual nanomagnets. The simulation results show that the micromagnetic structure of measured cylindrical magnets undergoes a transition from multi-domain to an in-plane vortex state at 1 mum, and another transition at 200nm from vortex state to fully single domain.