Alastair Fraser

Quantitative Magneto-Mechanical Detection and Control of the Barkhausen Effect

Controlling Magnetic Noise Ferromagnetic materials contain a number of magnetic domains, with individual domains switching stochastically as the field strength is increased. As magnetic memory elements shrink in size, it is important to understand, and ultimately control, this magnetic noise. Using a magnetic vortex core integrated with a nanomechanical torsion balance, Burgess et al. (p. 1051, published online 17 January) created a two-dimensional map of the magnetic potential within the sample with nanoscale resolution. Moreover, introducing geometric defects (dimples) in the sample allowed the magnetization to be stabilized. A magnetic vortex core is used to probe nanoscale changes in magnetization. Quantitative characterization of intrinsic and artificial defects in ferromagnetic structures is critical to future magnetic storage based on vortices or domain walls moving through nanostructured devices. Using torsional magnetometry, we observe finite size modifications to the Barkhausen effect in the limiting case of a single vortex core interacting with individual pointlike pinning sites in a magnetic thin film. The Barkhausen effect in this limit becomes a quantitative two-dimensional nanoscale probe of local energetics in the film. Tailoring the pinning potential using single-point focused ion beam implantation demonstrates control of the effect and points the way to integrated magneto-mechanical devices incorporating quantum pinning effects.

Time-domain control of ultrahigh-frequency nanomechanical systems.

Nanoelectromechanical systems could have applications in fields as diverse as ultrasensitive mass detection and mechanical computation, and can also be used to explore fundamental phenomena such as quantized heat conductance and quantum-limited displacement. Most nanomechanical studies to date have been performed in the frequency domain. However, applications in computation and information storage will require transient excitation and high-speed time-domain operation of nanomechanical systems. Here we show a time-resolved optical approach to the transduction of ultrahigh-frequency nanoelectromechanical systems, and demonstrate that coherent control of nanomechanical oscillation is possible through appropriate pulse programming. A series of cantilevers with resonant frequencies ranging from less than 10 MHz to over 1 GHz are characterized using the same pulse parameters.

Bulk focused ion beam fabrication with three-dimensional shape control of nanoelectromechanical systems

Although focused ion beam (FIB) milling has previously been used for fabrication of compliant nanostructures and devices, few instances of FIB nanomachining of such devices out of bulk materials have been reported. We use FIB to fabricate nanoelectromechanical systems (NEMS) devices out of bulk materials. Ion impingement from multiple directions allows sculpting with considerable three-dimensional control of device shape, including tapering and notching. Finite-element modeling of device frequencies agrees with optical interferometric measurements, including for the effect of a localized notch. We envision that bulk FIB fabrication will be useful for NEMS prototyping, milling of tough-to-machine materials and generalized nanostructure fabrication with three-dimensional shape control.

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.