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• W2287810411 abstract "Today, smaller and smaller electron and nuclear magnetic resonance structuresare extensively studied both from an applied and from a fundamental point of view.The powerful tool of magnetic resonance imaging (MRI) has demonstrated thatit is possible to visualize subsurface three dimensional structures with micrometerresolution [1] containing 1012 nuclear spins; nuclear magnetic resonance (NMR) spectroscopyhas the capacity to determine the three dimensional structure of biologicalmacromolecules [2]. Owing to the larger gyromagnetic ratio of electrons as comparedto paramagnetic nuclei, electron spin resonance (ESR) has pushed detectionsensitivity to 107 spins [3]. Finally, a single electron spin [4] has been detected bymagnetic resonance force microscopy (MRFM), employing a device which combinestwo sensing technologies, namely magnetic resonance imaging (MRI) and atomicforce microscopy (AFM). The ultimate goal of MRFM is to map the interior of amaterial sample, such as a complicated semiconductor structure or a bio-molecule,at atomic scale resolution.The idea of introducing MRFM to improve the detection sensitivity down to asingle spin and thus to resolve atoms of proteins [5],[6] was originally proposed in1992. Ten years later, Rugar and co-workers reported the detection of a single electronspin resonance in a silica substrate with paramagnetic defects, using a magneticresonance force microscope [4] with a lateral resolution of 25 nm in one dimension.To achieve this single spin detection, the magnetic resonance force microscopy usesa soft cantilever with a tiny hard magnetic tip material. The inhomogeneity fieldBinhom generated from the magnetic tip is superimposed with the homogenous magneticfield B0 which polarizes the sample. For a radio frequency field the resonancecondition is fulfilled in the region where !1 = (B0 + Bgrad) and where is thegyromagnetic ratio of electron or proton. Consequently, the next foreseeable step isto detect a single nuclear spin. In fact, the correspondence between ESR and NMRis very close, and much of the basic theory of ESR is directly applicable to NMR.ESR requires an unpaired electron whereas NMR requires an unpaired nuclear spinfor detection. Furthermore, an external static magnetic field is necessary in bothESR and NMR detection. The major difference between the two techniques is dueto the gyromagnetic ratio of the proton and electron. ESR entails the higher electrongyromagnetic ratio, as compared to the nuclear gyromagnetic ratio involved inNMR and the sensitivity of EPR is correspondingly higher (approximately a factorof 1000).The force generated by a single spin is in the attonewton range. Thus, noncommercial, soft single crystalline silicon cantilevers with a high quality factor andminimized spring constants have to be used for detecting a single spin. Measurements are performed at liquid helium temperature where thermal noise is reducedby a factor of 10. The UHV condition makes for a very stable environment reducingthe oxidation of the sample and of the cantilever. In our low temperature forcemicroscope force sensitivities on the order of 10−18 N/pHz at 10 K are obtainedwithout any external static field [7]. A force sensitivity in the order of 9x10−18N/pHz should be reached at 4 K in a static magnetic field of 100 mT.In this work we design, build and assemble the entire UHV machine workingat a pressure of <10−10 mbar and at helium boiling temperature starting from theexisting microscope and the Janis cryostat. This work took about one year producinghundreds of schemes and designs. The entire cryogenic machine plan is detailed inthe appendix. For detailed subsystem schemes please refer to the scheme library inthe appendices.The extreme high sensitivity of 10−18 N/pHz that the magnetic force resonancemicroscope should reach, requires the study of interaction phenomena. The smallspring constant for high force sensitivity makes it necessary to have the cantileverperpendicular to the sample surface. Otherwise, the cantilever will stick electrostaticallyto the sample surface. This vertical configuration introduces new designparameters involving the cantilever’s approach to the sample. In fact the cantileveris subject not only on the lateral force gradient but also to a vertical force. The verticalattractive force as a uniform force will cause an increase in the frequency similarto the uniform gravitational force that causes a pendulum to have a frequency thatis proportional to gravity.The tip-sample interaction dissipation is then measured by the Q factor changeas a function of the distance. The dissipation is caused mainly by the electrostaticcharge fluctuation. The fluctuation of charge stored on a capacitance C induces thenoise denoted as ”KTC”. The noise of the fluctuation charge is on the order ofobserved charge fluctuations of single-electron transistors. This shows a probablycommon origin of the charge fluctuation.A severe loss in force sensitivity and a frequency shift are observed while exposingthe cantilever with a magnetic tip to a homogenous magnetic field. The micrometersized magnetic particles generate a magnetic field of 500 Gauss and magneticfield gradients (dB/dz>> 1x105 T/m). To minimize the damping losses of thecantilevers with ferromagnetic particles various magnetic materials (e.g. Sm2Co17,SmCo5, Nd2Fe14B, and Pr2Fe14B) with different grain materials and domain sizesare investigated. The lowest magnetic dissipation is observed with SmCo5 tips havinga higher anisotropy constant. A correlation between frequency of oscillation andmagnetic field hysteresis is then measured. A detection sensitivity in the order of10−18N/pHz is reached at 100 mT. This sensitivity should be enough for measuringless than 100 electron spins.Finally, a home-built spectrometer is compared with a home-built magnetic resonanceforce microscope with the sample mounted on the cantilever. At room temperatureand at 50 mT the magnetic resonance force microscope has a sensitivityimprovement of a factor of more than 100000. This suggests the huge potential ofthis instrument for biological and chemical sample analysis.This work is part of ultimate limits of measurement of module IX of the NationalCenter of Competence in Research in Nanoscience (NCCR). The NCCR is the national Swiss research projects in nano technologies with the leading house in Basel.The main goal of this submodule is to ultimately perform single spin experimentsat low temperature and in ultra high vacuum (UHV). Achieving this goal requiresmechanical force sensors to be improved and all relevant forces to be understood.The channels of energy dissipation should be determined in order to improve thedetection sensitivity." @default.
• W2287810411 created "2016-06-24" @default.
• W2287810411 creator A5004854842 @default.
• W2287810411 date "2005-01-01" @default.
• W2287810411 modified "2023-09-27" @default.
• W2287810411 title "Magnetic resonance force microscopy : interaction forces and channels of energy dissipation" @default.
• W2287810411 doi "https://doi.org/10.5451/unibas-003610290" @default.
• W2287810411 hasPublicationYear "2005" @default.
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