Atomic Force Microscopy Probe with Integrated Loop and Shielded Leads for Micromagnetic Sensing
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J18.9.1
Atomic Force Microscopy Probe with Integrated Loop and Shielded Leads for Micromagnetic Sensing D.P. Lagally1, A. Karbassi2, Y. Wang2, C.A. Paulson2, and D. W. van der Weide2, 1 Materials Science Program, 2Department of Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, WI 53706
INTRODUCTION The effort to produce an instrument that can achieve high spatial resolution, nondestructive, surface and sub-surface imaging for a variety of materials comes with many challenges. One approach, magnetic resonance-force microscopy (MRFM), lies at the nexus of two sensitive technologies: magnetic force microscopy (MFM) and magnetic resonance imaging (MRI). MFM uses a magnetic tip in a standard atomic force microscope (AFM) to obtain magnetic information about a surface. A difference in the magnetic moments of surface atoms in different regions on the surface varies the cantilever resonance. MRI, on the other hand, uses the spin states of magnetically biased atoms to differentiate between chemical species. In the presence of a magnetic field, an energy splitting is induced between electrons’ up and down spin states in the sample. This energy splitting defines a unique Larmor frequency defined by hω=mB,
(1)
where ω is the resonance frequency, m is the magnetic moment of a single spin, B is the magnetic field, and h is Planck's constant. In MRFM, the energy to flip the spins is provided by a microcoil, which produces a magnetic field at radio frequencies (RF, less than 3 x 109 Hz). When the frequency of the RF microcoil matches the Larmor frequency, the spin system absorbs RF energy, and magnetic forces change. Magnetic-resonance behavior requires a sensitive detection instrument, as the interaction force between a small magnet and an electron spin is estimated to be in the attonewton level (10-18 N). MFM provides this sensitive detection with the magnetic cantilever. The interaction between the magnetic field gradient produced by a magnetic tip and the magnetic moment in the sample causes the cantilever vibration to be slightly altered. The advantage provided by magnetic resonance force microscopy (MRFM) is the ability to glean information not otherwise available from other scanned-probe techniques. The ability to set the slice of sample that is undergoing resonance allows non-destructive examination at high sensitivity and depth resolution. Applications of this tool might include achieving a better understanding of the chemical interactions of proteins, examination of integrated circuits, and analysis of quantum dots. Ideally, one could now perform magnetic-resonance imaging on individual cells the same way doctors perform MRI on the human body. The standard MRFM setup uses a small magnetic nanoparticle glued to a cantilever as the source of the local magnetic field. There is an alternative way to implement MRFM: The BiotSavart law dictates that a current-carrying loop will produce a magnetic field. This field is strongest in the center of the loop and decreases as 1/r2, with r as the axial
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