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Unformatted text preview: Paramagnetic and ferromagnetic resonance imaging with a tip-on- cantilever magnetic resonance force microscope K. Wago, a) D. Botkin, C. S. Yannoni, and D. Rugar IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120 ~ Received 20 October 1997; accepted for publication 20 March 1998 ! A magnetic resonance force microscope with a ‘‘tip-on-cantilever’’ configuration was used to compare imaging characteristics of paramagnetic and ferromagnetic samples. Three-dimensional electron paramagnetic resonance ~ EPR ! imaging of diphenylpicrylhydrazil ~ DPPH ! particles was accomplished by scanning the sample in two dimensions while stepping an external field. The EPR force map showed broad response reflecting the size and shape of the sample, allowing a three-dimensional real-space magnetization image to be successfully reconstructed. In contrast to the EPR case, ferromagnetic resonance imaging of a micron-scale yttrium iron garnet sample showed no significant line broadening despite the strong field gradient ( ; 10 G/ m m). Two-dimensional force maps revealed spatial dependence of magnetostatic and magnetoelastic modes. © 1998 American Institute of Physics. @ S0003-6951 ~ 98 ! 01621-0 # Magnetic resonance force microscopy 1,2 ~ MRFM ! has previously been performed with the sample mounted on a micromechanical cantilever positioned close to a millimeter- size field gradient source. 3–11 In this letter, we describe an improved microscope configuration, where the gradient source consists of a magnetic tip mounted directly on the cantilever. This allows a much wider variety of samples to be investigated and greatly improves the convenience of the technique. We have used this improved microscope to com- pare and contrast two- and three-dimensional imaging char- acteristics of paramagnetic and ferromagnetic samples. The experimental apparatus, schematically shown in Fig. 1, was based on a previously described low temperature magnetic resonance force detection apparatus. 10,11 The gra- dient source-cantilever combination consists of a 55- m m- radius nickel sphere 12 glued with epoxy onto a commercial single-crystal-silicon cantilever 13 having dimensions 450 m m 3 40 m m 3 2 m m ~ spring constant k . 0.1 N/m ! . The mechanical resonance frequency of the cantilever with the tip affixed was 1.1 kHz. The motion of the cantilever was mea- sured by a fiber-optic interferometer. Samples were placed on top of a microstripline resonator that generated the 12 GHz microwave field B 1 ~ 0.1–1 G ! used to induce magnetic resonance. The polarizing field B was a combination of the homogeneous field B ext applied in the z direction by an ex- ternal superconducting magnet and the inhomogeneous field B tip from the magnetic tip. B ext also served to magnetize the magnetic tip to its saturation magnetization. The sample and resonator were mounted on a 15-cm-long piezoelectric tube scanner that had a scan range of approximately 100 m...
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