Neil E. Jenkins

Force-gradient detected nuclear magnetic resonance

We have detected nuclear magnetic resonance in GaAs by selectively inverting 71Ga spins to create a local force gradient which shifts the mechanical resonance frequency of a nearby magnet tipped microcantilever. Employing a low spring constant cantilever (60 μN/m) at 4.4 K and 7 T, we demonstrate a magnetic moment sensitivity of 7.5×10−21 J/T, equal to the Curie Law magnetization from 7.1×108 71Ga spins. The method obviates the need to spin-lock sample magnetization during detection, allowing the signal to be collected for a time approaching the full spin-lattice relaxation time.

Batch fabrication and characterization of ultrasensitive cantilevers with submicron magnetic tips

We have batch fabricated ultrasensitive silicon cantilevers with integrated submicron magnetic tips and have characterized both their mechanical and magnetic properties. Cantilevers with spring constants as small as 10−5 N/m were fabricated, with quality factors in the range of 2.5–3.5×104 and a force sensitivity as good as 64×10−18 N Hz−1/2 at room temperature in vacuum. Cantilever spring constants were measured by observing thermomechanical position fluctuations with a fiber optic interferometer, while resonance frequencies and quality factors were inferred from cantilever ring down transients. Polycrystalline nickel tips as small as 1.2 μm×0.4 μm×0.2 μm were fabricated on the cantilevers by electron beam lithography, thermal evaporation, and lift-off. Tip magnetic moments were inferred from the shift of the cantilever frequency versus magnetic field and show a 0.60±0.12 T saturation magnetization, indicating that less than 28 nm of oxide forms on the tips during processing. Force sensitivity was demons...

Thermomagnetic fluctuations and hysteresis loops of magnetic cantilevers for magnetic resonance force microscopy

We have used frequency-shift cantilever magnetometry to study individual nickel magnets patterned at the end of ultra-sensitive silicon cantilevers for use in magnetic resonance force microscopy (MRFM). We present a procedure for inferring a magnets full hysteresis curve from the response of cantilever resonance frequency versus magnetic field. Hysteresis loops and small-angle fluctuations were determined at 4.2 K with an applied magnetic field up to 6 T for magnets covering a range of dimensions and aspect ratios. Compared to magnetic materials with higher anisotropy, we find that nickel is preferable for MRFM experiments on nuclear spins at high magnetic fields.