title: High-gradient nanomagnets on cantilevers for sensitive detection of nuclear magnetic resonance.
abstract: Detection of magnetic resonance as a force between a magnetic tip and nuclear spins has previously been shown to enable sub-10 nm resolution 1H imaging. Maximizing the spin force in such a magnetic resonance force microscopy (MRFM) experiment demands a high field gradient. In order to study a wide range of samples, it is equally desirable to locate the magnetic tip on the force sensor. Here we report the development of attonewton-sensitivity cantilevers with high-gradient cobalt nanomagnet tips. The damage layer thickness and saturation magnetization of the magnetic material were characterized by X-ray photoelectron spectroscopy and superconducting quantum interference device magnetometry. The coercive field and saturation magnetization of an individual tip were quantified in situ using frequency-shift cantilever magnetometry. Measurements of cantilever dissipation versus magnetic field and tip–sample separation were conducted. MRFM signals from protons in a polystyrene film were studied versus rf irradiation frequency and tip–sample separation, and from this data the tip field and tip-field gradient were evaluated. Magnetic tip performance was assessed by numerically modeling the frequency dependence of the magnetic resonance signal. We observed a tip-field gradient ∂B(z)(tip)/∂z estimated to be between 4.4 and 5.4 MT m(–1), which is comparable to the gradient used in recent 4 nm resolution 1H imaging experiments and larger by nearly an order of magnitude than the gradient achieved in prior magnet-on-cantilever MRFM experiments.
authors.affiliation: Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States.
authors.forename: Jonilyn G
authors.initials: JG
authors.lastname: Longenecker
authors.forename: H J
authors.initials: HJ
authors.lastname: Mamin
authors.forename: Alexander W
authors.initials: AW
authors.lastname: Senko
authors.forename: Lei
authors.initials: L
authors.lastname: Chen
authors.forename: Charles T
authors.initials: CT
authors.lastname: Rettner
authors.forename: Daniel
authors.initials: D
authors.lastname: Rugar
authors.forename: John A
authors.initials: JA
authors.lastname: Marohn
mesh_terms: D004867:Equipment Design; D019544:Equipment Failure Analysis; D009682:Magnetic Resonance Spectroscopy / Q000295:instrumentation*; D059346:Magnets; D055617:Micro-Electrical-Mechanical Systems / Q000295:instrumentation*; D036103:Nanotechnology / Q000295:instrumentation*; D014159:Transducers