We have studied the (210) surface of ammonium perchlorate (NH4C1O4) with an atomic force microscope (AFM) in air. Large scale images show monomolecular steps that are often aligned with major crystalline directions. Surface modification is clearly observed with forces as low as 30 nN, but is usually limited to the bottom and the sides of the field of view, where the tip scans for a long time and where it turns around, respectively. At the molecular scale we have observed structures with substantial order, with minimum lateral sizes of 3 Å. The identification of these features is difficult due to the large unit cell of this compound (40 atoms per unit cell), and the lack of perfect order in the images. We use the SARCH/LATUSE program developed by Van Hove and Hermann to analyze this structure. Ammonium perchlorate is hygroscopic; it is therefore possible that surface modification is enhanced by the moisture present in air. Experiments in a dry atmosphere have confirmed this explanation. We have therefore designed and built a UHV (ultra-high vacuum) AFM for these studies. The first images from this instrument are presented here.

Using a multi-walled carbon nanotube (MW-CNT) tip, we have observed the honeycomb lattice of highly oriented pyrolytic graphite (HOPG) via scanning tunneling microscopy (STM). This observation was made after acquiring typical STM images of HOPG, i.e., showing the triangular lattice. We consider this change is due to atomic reconfiguration at the apex of the MW-CNT tip induced by continuous STM scanning. The atomic-level sharpening of CNT tips will be useful to image samples with small lattice constants or to obtain orbital information from samples with an orbital ordering such as manganites.

We present studies of the magnetic domain structure of UMn$_2$Ge$_2$ single crystals using a home-built low temperature magnetic force microscope. The material has two distinct magnetic ordering temperatures, originating from the Mn and U moments. At room temperature, where the Mn moments dominate, there are flower-like domain patterns similar to those observed in uniaxial ferromagnets. After exposing the sample to a one-tesla magnetic field near 40 K, the evolution of the magnetic domains are imaged through zero-field warming up to 200 K. Near the ordering temperature of the uranium moments a clear change in the domain wall motion is observed. The domain size analysis of the flower-like pattern reveals that the domain structure is consistent with a model of branching domains.

We present studies of the magnetic domain structure of UMn$_2$Ge$_2$ single crystals using a home-built low temperature magnetic force microscope. The material has two distinct magnetic ordering temperatures, originating from the Mn and U moments. At room temperature, where the Mn moments dominate, there are flower-like domain patterns similar to those observed in uniaxial ferromagnets. After exposing the sample to a one-tesla magnetic field near 40 K, the evolution of the magnetic domains are imaged through zero-field warming up to 200 K. Near the ordering temperature of the uranium moments a clear change in the domain wall motion is observed. The domain size analysis of the flower-like pattern reveals that the domain structure is consistent with a model of branching domains.

The growth of Co-substituted BaTiO3 (BTO) films on Ge(001) substrates by molecular beam epitaxy is demonstrated. Energy-dispersive x-ray spectroscopy and transmission electron microscopy images confirm the uniform Co distribution. However, no evidence of magnetic ordering is observed in samples grown for Co concentrations between 2% and 40%. Piezoresponse force microscopy measurements show that a 5% Co-substituted BTO sample exhibits ferroelectric behavior. First-principles calculations indicate that while Co atoms couple ferromagnetically in the absence of oxygen vacancies, the occurrence of oxygen vacancies leads to locally antiferromagnetically coupled complexes with relatively strong spin coupling. The presence of a significant amount of oxygen vacancies is suggested by x-ray photoelectron spectroscopy measurements.

The authors present a simple design for a magnetically coupled manipulator capable of grabbing small samples in an ultrahigh vacuum environment. The opening and closing of the sample grabber is accomplished by the relative rotation of independent rings of magnets. This manipulator provides high values of axial force (30 lb) and azimuthal torque (23 in. lb).

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.

A carbon nanotube (CNT) tip was fabricated at the apex of an etched tungsten wire by chemical vapor deposition and used for scanning tunneling microscopy. The honeycomb lattice of graphite in the STM images was resolved with a CNT tip at T=79 K. The superior spatial resolution originating from the p orbitals of a CNT is responsible for the image of the honeycomb lattice of graphite in the STM images. The CNT tips are useful to image samples whose lattice constants are small and to get orbital information in samples with orbital ordering due to their superior spatial resolution with the sharp p orbitals.

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.

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.

Submitted for the MAR11 Meeting of The American Physical Society Low Temperature Scanning Tunneling Microscope for Spin Polarization Measurements1 SEONG HEON KIM, RYAN JAEHNE, LEUJEN CHEN, ALEX DE LOZANNE, Department of Physics, University of Texas at Austin — We describe a new design for a 4K scanning tunneling microscope (STM) with an 8 tesla superconducting magnet to be used for spin polarized measurements. The novel aspects include a compact design for the chamber and the STM, the use of a secondary STM for in-situ tip characterization, and new ideas for vibration isolation. We developed a new STM head unit with 1 inch diameter and 3.2 inch length. This microscope is small enough to be installed within the small space available in the 2 inch diameter bore of our superconducting magnet. To achieve this small size, we modified the typical Pan-type z-approach walker. We also developed new simple and inexpensive electronics to control any stick-slip walker. 1Supported by NSF DMR-0923231 Seong Heon Kim Department of Physics, University of Texas at Austin Date submitted: 19 Nov 2010 Electronic form version 1.4