Imaging of Spin Dynamics in Closure Domain and Vortex Structures

Time-resolved Kerr microscopy is used to study the excitations of individual micron- scale ferromagnetic thin film elements in their remnant state. Thin (18 nm) square elements with edge dimensions between 1 and 10 $\mu$m form closure domain structures with 90 degree Neel walls between domains. We identify two classes of excitations in these systems. The first corresponds to precession of the magnetization about the local demagnetizing field in each quadrant, while the second excitation is localized in the domain walls. Two modes are also identified in ferromagnetic disks with thicknesses of 60 nm and diameters from 2 $\mu$m down to 500 nm. The equilibrium state of each disk is a vortex with a singularity at the center. As in the squares, the higher frequency mode is due to precession about the internal field, but in this case the lower frequency mode corresponds to gyrotropic motion of the entire vortex. These results demonstrate clearly the existence of well-defined excitations in inhomogeneously magnetized microstructures.Comment: PDF File (Figures at reduced resolution

High bandwidth electron tunneling trasnducer using frequency downmixing readout of nanomechanical motion

Electron tunneling transduction based on quantum tunneling is very sensitive to the change of the distance from the probing tip apex to the sample surface and can be used as displacement transducer to detect the miniscule displacement of NEMS devices. However a limitation in electron tunneling transduction is the low detection bandwidth due to readout circuit frequency rolloff at a few 10's kHz. Here a novel electron tunneling transduction utilizing frequency downmixing directly in the tunneling junction overcomes the limitation of the detection bandwidth [1]. With this technique the high frequency vibration modes of doubly-clamped beams are measured, well above the RC rolloff of the STM measuring circuits. \ua9 2011 IEEE.Peer reviewed: YesNRC publication: Ye

Stiction-free fabrication of lithographic nanostructures on resist-supported nanomechanical resonators

The authors report a highly flexible process for nanostructure lithography to incorporate specific functions in micro- and nanomechanical devices. The unique step involves electron beam patterning on top of released, resist-supported, surface micromachined structures, hence avoiding hydrofluoric acid etching of sensitive materials during the device release. The authors demonstrate the process by creating large arrays of nanomechanical torque magnetometers on silicon-on-insulator substrates. The fabricated devices show a thermomechanical noise-limited magnetic moment sensitivity in the range of 5 7 106 \u3bcB at room temperature and can be utilized to study both magnetostatics and dynamics in nanomagnets across a wide temperature range. The fabrication process can be generalized for the deposition and patterning of a wide range of materials on micro-/nanomechanical resonators. \ua9 2013 Crown.Peer reviewed: YesNRC publication: Ye

Nanomechanical torsional resonator torque magnetometry (invited)

Micromechanical resonators are very useful for detection of magnetic torque. We have developed nanoscale torsional resonators fabricated within silicon nitride membranes, as a platform for magnetometry of nanoscale magnetic elements. We describe the rotational magnetic hysteresis of a 10 nm thick film deposited on a resonator, and a study of magnetic hysteresis in a single, 1 m diameter permalloy disk. The torsional resonator is patterned using a dual beam scanning electron/focused ion system. For the 1 m diameter disk, it is found to be possible to tune the conditions such that an apparent magnetic supercooling of vortex nucleation is observed, as would be suggested by the modified Landau theory of the C- to vortex-state switch as a first-order phase transition. Complementary transmission electron and Lorentz microscopy of the same structures have also been performed. \ua9 2011 American Institute of Physics.Peer reviewed: YesNRC publication: Ye