A study of cantilever-free instrumentation for nanoscale magnetic measurements.

摘要

The evolution of the Atomic Force Microscope (AFM) into the Magnetic Force Microscope (MFM) and Magnetic Resonance Force Microscope (MRFM) has had a substantial impact on the characterization of nanoscale phenomena. Detection of 10-17 Newtons per root Hertz has occurred with use of an ultra-sensitive cantilever along with optical interferometry methods within these geometries. The sensitivity of these platforms is dependent on the characteristics of the cantilever, where increased length and a low Young's modulus increase the force sensitivity (meters/newtons). Using IC fabrication techniques, the realization of generating cantilevers with this sensitivity is feasible, but stress compensation layers are required to prevent the free end from curling. Aside from the difficultly in fabrication, the cantilever based approach has one fixed spring constant yielding a finite detectable magnetic force range. An alternative approach incorporating the magnetic levitation of a magnet with an integrated reflector, known as the birdie, has been investigated.;The goals of the cantilever-free instrumentation are two fold: (1) To replace the traditional cantilever with a magnetically levitated birdie (which will be scaled down to investigate nanoscale phenomena) through the creation of a virtual cantilever; (2) Investigate the detectable magnetic force range (tunability) of the virtual cantilever. The first 1-D milli-levitation platform has been fabricated and its preliminary characterization has been performed, showing a minimum detectable force in the nano-Newton range with a 10X tunability in spring constant. This high degree of force sensitivity and tunability confirms the design and enables the use for magnetic sample investigation. To further increase the utility of the cantilever-free approach, the birdie has been magnetically levitated in 3D by control circuitry that has been developed and characterized. The magnetic behavior of the custom designed X, Y and Z coil sets have been measured and compared to finite element analysis (FEA). Experimental data sets were used to fit coefficients in a magnetic field expansion. The magnetic field predicted by the magnetic field expansion using the experimental coefficients were in good agreement to FEA results. For the control of the birdie in 3-D, a tracking system has been designed that determines the position of the birdie and generates a proportional response depending on the error signal. Marginal control of the birdie within the bore of the X, Y and Z coil sets has been shown. The cantilever-free platform was able to sense forces at the nano-Newton level. This was demonstrated by observing the deflection of the birdie in response to known magnetic fields generated by a test coil located on the top of the coil set. Demonstration of the approach to detect external magnetic fields brought on by a test coil has been accomplished and the nano-Newton force sensitivity verified through deflection of the sensitive element (birdie). The cantilever-free instrumentation in which the ultra-sensitive cantilever is replaced with a magnetically levitated birdie, has been demonstrated with 10X tunability in spring constant and nano-Newton force sensitivity.