Observation of electronic inhomogeneity and charge density waves in a bilayer La(2-2x)Sr(1+2x)Mn2O7 single crystal.

We employed a scanning tunneling microscope to image the (001) surface topography and local density of states (LDOS) in La(2-2x)Sr(1+2x)Mn(2)O(7) (x=0.32, LSMO) single crystals below the Curie temperature (T(C)≈120 K). The LDOS maps revealed a stripelike modulation propagating along the tetragonal a axis with a wavelength of about 16 Å, which is indicative of a charge density wave (CDW). The observed CDW in the x=0.32 sample is far from the Fermi surface nesting instability as compared with the data of angle resolved photoemission spectroscopy in an x=0.40 sample. The stripe model developed previously for cuprates can explain the observed CDW in our LSMO sample, indicating that competing interactions between localized and itinerant phases are the origin of the spatial modulations present intrinsically in cuprates and manganites.

Compact scanning tunneling microscope for spin polarization measurements.

We present a design for a scanning tunneling microscope that operates in ultrahigh vacuum down to liquid helium temperatures in magnetic fields up to 8 T. The main design philosophy is to keep everything compact in order to minimize the consumption of cryogens for initial cool-down and for extended operation. In order to achieve this, new ideas were implemented in the design of the microscope body, dewars, vacuum chamber, manipulators, support frame, and vibration isolation. After a brief description of these designs, the results of initial tests are presented.

Simple electronics for inertial and Pan-type piezoelectric positioners used in scanning probe microscopes.

We describe a new type of circuit designed for driving piezoelectric positioners that rely on the stick-slip phenomenon. The circuit can be used for inertial positioners that have only one piezoelectric element (or multiple elements that are moved simultaneously) or for designs using a sequential movement of independent piezoelectric elements. A relay switches the piezoelectric elements between a high voltage source and ground, thus creating a fast voltage step followed by a slow ramp produced by the exponential discharging of the piezoelectric elements through a series resistor. A timing cascade is generated by having each relay power the next relay in the sequence. This design is simple and inexpensive. While it was developed for scanning probe microscopes, it may be useful for any piezoelectric motor based on a fast jump followed by a slow relaxation.

Printed magnetic FePt nanocrystal films.

Patterned monolayers and multilayers of FePt nanocrystals were printed onto substrates by first assembling nanocrystals on a Langmuir-Blodgett (LB) trough and then lifting them onto prepatterned polydimethylsiloxane (PDMS) stamps, followed by transfer printing onto the substrate. Patterned features, including micrometer-size circles, lines, and squares, could be printed using this approach. The magnetic properties of the printed nanocrystal films were also measured using magnetic force microscopy (MFM). Room-temperature MFM could detect a remanent (permanent) magnetization from multilayer (>3 nanocrystals thick) films of chemically ordered L1(0) FePt nanocrystals.

Robust Ohmic contact junctions between metallic tips and multiwalled carbon nanotubes for scanned probe microscopy.

We demonstrate a simple method that uses a scanning electron microscope for making a reliable low resistance contact between a single multiwalled carbon nanotube and a metallic tungsten probe tip or a Si cantilever. This method consists of using electron beam induced decomposition of background gases and voltage pulses to remove contaminants. The electrical quality of the contact is monitored in situ by measuring the current flow at constant bias or by observing the decay of current fluctuations. The quality of the contacts is confirmed via current-voltage spectroscopy. This method produces very stable, low resistance, mechanically robust contacts with high success rates approaching 100%.

Compact variable-temperature scanning force microscope.

A compact design for a cryogenic variable-temperature scanning force microscope using a fiber-optic interferometer to measure cantilever deflection is presented. The tip-sample coarse approach and the lateral tip positioning are performed by piezoelectric positioners in situ. The microscope has been operated at temperatures between 6 and 300 K. It is designed to fit into an 8 T superconducting magnet with the field applied in the out-of-plane direction. The results of scanning in various modes are demonstrated, showing contrast based on magnetic field gradients or surface potentials.

Magnetic imaging of a supercooling glass transition in a weakly disordered ferromagnet.

Spin glasses are founded in the frustration and randomness of microscopic magnetic interactions. They are non-ergodic systems where replica symmetry is broken. Although magnetic glassy behaviour has been observed in many colossal magnetoresistive manganites, there is no consensus that they are spin glasses. Here, an intriguing glass transition in (La,Pr,Ca)MnO3 is imaged using a variable-temperature magnetic force microscope. In contrast to the speculated spin-glass picture, our results show that the observed static magnetic configuration seen below the glass-transition temperature arises from the cooperative freezing of the first-order antiferromagnetic (charge ordered) to ferromagnetic transition. Our data also suggest that accommodation strain is important in the kinetics of the phase transition. This cooperative freezing idea has been applied to structural glasses including window glasses and supercooled liquids, and may be applicable across many systems to any first-order phase transition occurring on a complex free-energy landscape.

Application of magnetic force microscopy in nanomaterials characterization.

This review describes the basic technical aspects of magnetic force microscopy and how this technique has been applied to the study of colossal magnetoresistance materials, superconductors, and patterned magnetic materials. Recently, current distribution in a patterned aluminum strip has been measured by magnetic force microscopy, opening the possibility of measuring currents in buried interconnects in integrated circuits.

Wavy and rough cracks in silicon.

Single-crystal silicon exhibits a strong preference to cleave along a few certain planes, but in experiments we observe wavy cracks with almost no evidence of a preferred fracture direction. Furthermore, we find that the fracture surface is an anisotropic and self-affine fractal over five decades in length scale in the direction of the crack with a roughness exponent of 0.78. In our experiments a 1-4 cm wide strip of single-crystal silicon is heated to 378 degrees C and lowered into a 20 degrees C water bath at speeds of 0.2-5 cm/s. The thermal gradient produces cracks that, depending on the speed, are straight, wavy with amplitude 0.1-0.5 cm and wavelength 0.3-1 cm, or multibranched. The transition from one mode of fracture to another is discontinuous and hysteretic.

Direct observation of percolation in a manganite thin film.

Upon cooling, the isolated ferromagnetic domains in thin films of La0.33Pr0.34Ca0.33MnO3 start to grow and merge at the metal-insulator transition temperature TP1, leading to a steep drop in resistivity, and continue to grow far below TP1. In contrast, upon warming, the ferromagnetic domain size remains unchanged until near the transition temperature. The jump in the resistivity results from the decrease in the average magnetization. The ferromagnetic domains almost disappear at a temperature TP2 higher than TP1, showing a local magnetic hysteresis in agreement with the resistivity hysteresis. Even well above TP2, some ferromagnetic domains with higher transition temperatures are observed, indicating magnetic inhomogeneity. These results may shed more light on the origin of the magnetoresistance in these materials.