We report our advances in nuclear magnetic resonance force microscopy
(NMRFM) in three areas: 1) MEMS microfabrication studies of single-crystal-silicon mechanical oscillators using double-sided processing; 2) micromagnetometry, anisotropy, and dissipation studies of individual permalloy micromagnets on oscillators; and 3) mechanical-oscillator detection of NMR in the magnet-on-oscillator scanning
mode. In the first area, we report details of our back-etch microfabrication process, and characterize oscillator resonant frequency, quality factor, and spring constant by measuring the noise
spectral density of oscillator motion. In the second studies, we report changes in the resonant frequency and quality factor for each of four modes of our oscillators for two shapes and sizes of permalloy thin-film (~30 and 180~nm) micromagnets; a simple, quantitative model is used to describe both low-field softening and high-field stiffening. Finally, we report scanning-mode NMR force detection of an ammonium-sulfate single-crystal interface and a polymethyl-methyl-acrylate thin film at room temperature. These latter studies use 2-μm-radius permalloy magnets on silicon oscillators to image the NMR response from resonant volumes as small as 3 μm<sup>3</sup>. These NMRFM studies are the first reported that attain sub-micron resonant-slice resolution at room temperature.
Single-crystal silicon triple-torsional micro-oscillators have been fabricated, characterized, and modeled primarily for use in a magnetic resonance force microscope. These structures exploit a high-Q triple-torsional mode of oscillation while providing added stability. Fabrication involves lithography, reactive ion etch, and a final KOH wet-etch, with the final oscillator material being single-crystal boron-doped silicon. Typical oscillators were 250 nm thick and 10 - 200 microns in lateral dimensions. Finite element modeling provided the sequence and structure of the ten lowest-frequency modes and indicated that the upper torsional mode best isolates the motion from losses to the base. The oscillators were excited piezoelectrically and the resulting frequency-dependent motion was detected with fiber-optic interferometry, with a 0.002 nm/Hz<SUP>1/2</SUP> resolution. Phase-sensitive motion detection at various points on the oscillator facilitated the assignment of the principle modes. Magnetic excitation was also investigated in order to best excite the torsional resonances. Cobalt micromagnets with moments below 10<SUP>-12</SUP> J/T were electron-beam deposited onto oscillators, and the magnetic forces were measured. MRFM, the primary intended application of these novel structures, is discussed; in particular, an overview is given of an experiment which uses a double-torsional micro-oscillator for the force detection of nuclear magnetic resonance. All topics discussed in this work are being combined in order to achieve a NMRFM single-sweep sensitivity as low as 10<SUP>-16</SUP> N/Hz<SUP>1/2</SUP> at room temperature.
We report laser-ablation studies of highly-oriented thin films of the electron-doped infinite-layer copper-oxide compounds Sr<SUB>1- x</SUB>La<SUB>x</SUB>CuO<SUB>2</SUB>. The primary synthesis variables were substrate or buffer layer material, temperature, laser fluence, target- substrate distance, and oxygen pressure. The films were characterized by x-ray diffraction, atomic force microscopy (AFM), Rutherford back-scattering (RBS), and electrical resistivity. Films were deposited on strontium titanate (001) or on buffer layers of T'-phase copper oxides, Ln<SUB>2</SUB>CuO<SUB>4</SUB> (Ln equals Pr, Sm) on SrTiO<SUB>3</SUB> (001). The in-plane lattice constants of such T'-phase materials (a equals 0.391 - 0.396 nm) could provide a structure more amenable to electron doping than strontium titanate (a equals 0.390 nm). Extremely flat buffer layers were obtained from stoichiometric targets of Sm<SUB>2</SUB>CuO<SUB>4</SUB> and Pr<SUB>2</SUB>CuO<SUB>4</SUB>. However, ablation from stoichiometric infinite-layer targets onto buffer layers resulted in mixtures of infinite-layer and chain/ladder phases. Non-stoichiometric deposition was confirmed by RBS analysis. We thus utilized non-stoichiometric targets to obtain single-phased infinite-layer films. The x-ray rocking curves of highly-oriented epitaxial infinite-layer films exhibited full- widths at half maximum as narrow as 0.05 degrees. Infinite-layer films grown on T'-phase buffer layers exhibited lattice constants closer to those of the bulk superconductor than films grown directly on SrTiO<SUB>3</SUB>.
Micro-oscillators of different designs and dimensions have been fabricated for use in a nuclear magnetic resonance force microscope. The various designs include double and triple torsional oscillators which have high Q's at room temperature (approximately equals 10,000) when operating at the upper cantilever and upper torsional resonances. Depending on design and dimensions, the resonance frequencies vary from tens to hundreds of kHz. Typical dimensions of the designs are (200 X 150) micrometers <SUP>2</SUP> X 200 nm thick. To fabricate these devices, microelectric fabrication techniques were employed. Si (100) wafers were patterned, etched, and boron-implanted at a dose of 4.2 X 10<SUP>16</SUP> cm<SUP>-2</SUP> and an energy of 134 keV. A post-implant anneal was then performed at 1000 degree(s)C, followed by a KOH wet-etch which leaves the free-standing boron-doped oscillators. Depending on the doping level, anneal, and etch parameters, the thickness of the oscillators varies from 100 - 400 nm. In order to optimize the design and fabrication process, resonance frequencies and Q's have been characterized using fiber-optic interferometry. For example, the upper cantilever resonance of one design has been found to have a minimum detectable force of 1.5 X 10<SUP>-16</SUP> N/(root)Hz at room temperature.
Superconductor-insulator-normal metal (SIN) and superconductor-insulator-superconductor (SIS) tunnel junctions provide important information on pairing state symmetry and mechanism. Measurements of such junctions on high T<SUB>c</SUB> superconductors (HTS) are reported using mechanical point contacts, which generally display the optimum characteristics that can be obtained from HTS native-surface tunnel barriers. New tunneling data on the infinite-layer cuprate, Sr<SUB>1-x</SUB>Nd<SUB>x</SUB>CuO<SUB>2</SUB> are reported which show a remarkable similarity to another electron-doped cuprate, Nd<SUB>1.85</SUB>Ce<SUB>0.85</SUB>CuO<SUB>4</SUB>. In particular, there is a strong, asymmetric linear background conductance that is indicative of inelastic tunneling from a continuum of states. A discussion is given of the anomalous 'dip' feature found in the tunneling and photoemission data on BSCCO 2212. It is shown that a similar feature is found in many cuprate junctions and that this dip scales with the gap energy over a wide range. New data on the single-layer, tetragonal cuprate, Tl<SUB>2</SUB>Ba<SUB>2</SUB>CuO<SUB>6</SUB> (Tl2201) are presented and discussed in light of recent published results on the similar compound HgBa<SUB>2</SUB>CuO<SUB>4</SUB> (Hg1201). The Hg1201 data display a low, flat sub-gap tunneling conductance which is consistent with a BCS density of states whereas the Tl2201 data display a cusp-like feature at zero bias which is more consistent with d<SUB>x</SUB>2<SUB>-y</SUB>2 symmetry.
The undoped phases of the copper-oxide materials are antiferromagnetic insulators, with a gap of 1.5 - 2 eV. Infrared spectroscopy of these compounds reveals weak absorption, possibly of magnetic origin, in this gap. When the materials are doped, oscillator strength is removed from the charge transfer band. This oscillator strength moves to low frequency, to become midinfrared and free carrier absorption. A systematic study of the electron-doped Nd<SUB>2- x</SUB>Ce<SUB>x</SUB>CuO<SUB>4-y</SUB> system reveals that the growth of low-frequency oscillator strength with doping concentration x is twice as rapid as in the case of hole-doped materials, such as La<SUB>2-x</SUB>Sr<SUB>x</SUB>CuO<SUB>4</SUB>. This behavior is in accord with electronic structure models based on the 3-band Hubbard model and inconsistent with one-band behavior. However, an anomaly occurs for samples which are doped to the critical concentration for superconductivity; these have a greater than expected free-carrier concentration and weaker charge-transfer bands.
We review our recent results on the surface structure and spectroscopy of the chain layer of YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-x</SUB> obtained with a low temperature scanning tunneling microscope. Foremost is the discovery of a long wavelength (approximately 1.3 nm) modulation of the electronic density along the CuO chains, which has now been confirmed by neutron diffraction to also exist in the bulk. Spectroscopically, we observe an energy gap (20 - 25 meV) which disappears near oxygen vacancies. We also give experimental details not previously published.