D. C. Fortin

Nanotorsional resonator torque magnetometry

Magnetic torque is used to actuate nanotorsional resonators, which are fabricated by focused-ion-beam milling of permalloy coated silicon nitride membranes. Optical interferometry is used to measure the mechanical response of two torsion modes at resonance, which is proportional to the magnetization vector of the nanomagnetic volume. By varying the bias magnetic field, the magnetic behavior can be measured with excellent sensitivity (≈108μB) for single magnetic elements.

Thermo-mechanical sensitivity calibration of nanotorsional magnetometers

We report on the fabrication of sensitive nanotorsional resonators, which can be utilized as magnetometers for investigating the magnetization dynamics in small magnetic elements. The thermo-mechanical noise is calibrated with the resonator displacement in order to determine the ultimate mechanical torque sensitivity of the magnetometer.

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 μm diameter permalloy disk. The torsional resonator is patterned using a dual beam scanning electron/focused ion system. For the 1 μm 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.

Super-rolloff electron tunneling transduction of nanomechanical motion using frequency downmixing

A downmixed transduction technique is demonstrated which eliminates the high-frequency cutoff problem in traditional electron tunneling instrumentation. We measure the ∼1 MHz vibrational modes of a micromechanical beam two orders of magnitude above the electronic bandwidth of our readout circuitry with no fundamental limitations anticipated up to microwave frequencies. The displacement sensitivity of 40 fm/Hz1/2 demonstrates the viability of this technique as a sensitive displacement transducer for high-frequency nanoelectromechanical systems. Backaction from the tunneling tip on the device induces resonance frequency shifts of order 1%.

Nanoscale Structure, Dynamics, and Aging Behavior of Metallic Glass Thin Films

Scanning tunnelling microscopy observations resolve the structure and dynamics of metallic glass Cu100−xHfx films and demonstrate scanning tunnelling microscopy control of aging at a metallic glass surface. Surface clusters exhibit heterogeneous hopping dynamics. Low Hf concentration films feature an aged surface of larger, slower clusters. Argon ion-sputtering destroys the aged configuration, yielding a surface in constant fluctuation. Scanning tunnelling microscopy can locally restore the relaxed state, allowing for nanoscale lithographic definition of aged sections.

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 10s 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.