There is a critical need for a technique capable of non-invasive high resolution imaging of single copies of delicate biomolecules and as-fabricated semiconductor and spintronics devices. Magnetic resonance force microscopy (MRFM) is a non-invasive, three-dimensional imaging technique that employs attonewton-sensitivity cantilevers to mechanically detect electron spin resonance [1] and nuclear magnetic resonance [2]. The recent demonstration of 4 nm resolution imaging of a virus using MRFM establishes that the technique can achieve single-particle imaging with resolution competitive with cryo-electron microscopy [2]. The sample-on-cantilever geometry used in the experiment of Ref. 2, however, requires small, robust samples and is inapplicable to as-fabricated devices. We propose to image semiconductor devices by instead affixing to the cantilever the submicron magnetic particle required to achieve high spin sensitivity and spatial resolution. To minimize surface dissipation and achieve high signal to noise, the magnet must overhang the leading edge of the cantilever [3]. We recently demonstrated an approach to fabricating cantilevers with such integrated overhanging nanomagnets that achieves high yield [4]. Moreover, the novel tip fabrication method enabled the prototyping of new tip designs in less than sixteen hours of processing time [4], compared to the more than two weeks of processing time required for the best previous method [3].

 

Here we report harnessing this rapid prototyping technique to fabricate and characterize nickel and cobalt-iron-boron (CoFeB) nanorods. All nanomagnets are defined using electron beam lithography. The nickel nanorods are evaporated followed by liftoff, whereas the CoFeB nanorods are deposited by conformal sputtering and patterned by ion milling. The magnetic properties of the nanomagnets are determined using frequency-shift cantilever magnetometry and superconducting quantum interference device measurements. The elemental composition – paying particular attention to the extent of surface damage – is determined by scanning transmission electron spectroscopy and electron energy loss spectroscopy. We will detail work to develop a protocol for improved encasement of nanorods overhanging the cantilever leading edge to protect against damage, as well as our progress in implementing the nanomagnet-tipped cantilevers in MRFM experiments to rapidly detect single electron spins.

 

[1] EW Moore et al., Proc. Natl. Acad. Sci. 106(52), 22251 (2009).

[2] C Degen et al., Proc. Natl. Acad. Sci. 106(5), 1313 (2009).

[3] SA Hickman et al., ACS Nano 4(12), 7141 (2010).

[4] JG Longenecker et al., J. Vac. Sci. Technol. B, in press.