Alex de Lozanne

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.

Electronic transport properties of (001)/(110) oriented La2/3MnO3−δ thin films

We report the unusual transport properties found in La2/3MnO3−δ thin films on Al2O3 (1102). Powder x-ray diffraction shows that the film has a mixture of perpendicular (110) and (001) crystal orientations. Unlike epitaxial or polycrystalline La1−xMnO3−δ samples, in which the peak resistance temperature Tp shifts toward a higher temperature under the influence of magnetic field, the Tp of this particular film remains almost the same even in fields up to 5 T. The film becomes insulating at a low temperature Tm(∼45 K), but the trend is reversed by the applied magnetic field.

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. Microsc. Res. Tech., 2006.

High-field magnetic force microscopy as susceptibility imaging

We describe an extension of variable-temperature magnetic force microscopy (MFM) that allows spatial discrimination between the different states that exist in magnetically phase-separated materials. Some manganites exhibit a micrometer-scale separation of phases that are either ferromagnetic, paramagnetic, or antiferromagnetic. In an applied field large enough to saturate the ferromagnetic phase, any MFM contrast arising from the variation of the magnetization (domain walls, domains of differing orientation) is eliminated, while the nonferromagnetic phases are magnetized according to their susceptibilities. The different phases can then be discerned by their respective contrast levels in the MFM images.

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.

Micromagnetic study of single-domain FePt nanocrystals overcoated with silica

Chemically-synthesized FePt nanocrystals must be annealed at a high temperature (>550 °C) to induce the hard ferromagnetic L 10 phase. Unfortunately, the organic stabilizer covering these nanocrystals degrades at these temperatures and the nanocrystals sinter, resulting in the loss of control over nanocrystal size and separation in the film. We have developed a silica overcoating strategy to prevent nanocrystal sintering. In this study, 6 nm diameter FePt nanocrystals were coated with 17 nm thick shells of silica using an inverse micelle process. Magnetization measurements of the annealed FePt@SiO2 nanocrystals indicate ferromagnetism with a high coercivity at room temperature. Magnetic force microscopy (MFM) results show that the film composed of nanocrystals behaves as a dipole after magnetization by an 8 T external field. The individual nanocrystals are modelled as single-domain particles with random crystallographic orientations. We propose that the interparticle magnetic dipole interaction is weaker than the magnetocrystalline energy in the remanent state, leading to an unusual material with no magnetic anisotropy and no domains. Films of these nanoparticles are promising candidates for magnetic media with a data storage density of ~Tb/in2.

Compact variable-temperature magnetic force microscope with optical access and lateral cantilever positioning

We describe a compact design for a variable-temperature magnetic force microscope that incorporates a novel mechanical device for the lateral positioning of a piezoresistive cantilever under the guidance of an external optical microscope. The small size of the instrument makes it possible to perform low-temperature experiments by inserting the probe directly into a liquid-helium storage Dewar or into any open or closed liquid-nitrogen container. Besides convenience, this also means that the cycle time for exchanging tips and∕or samples can be as short as 4 h, including warm-up and cooldown. The probe is long enough to reach the middle of an 8 T superconducting magnet. We present the details of this design and show some results.

Localized measurement of penetration depth for a high Tc superconductor single crystal using a magnetic force microscope

Abstract We have measured the temperature dependence of the penetration depth, λ ( T ), down to 6.7 K in a YBCO single crystal using a homemade, frequency-modulated magnetic force microscope. It employs a piezo-resistive cantilever oscillating at its resonant frequency with a magnetic tip located on the free end. The magnetic field generated from the tip induces a shielding current in the sample that in turn interacts with the magnetic tip (magnetic levitation force). We first measured the force gradient (by measuring the change in cantilevers effective resonant frequency) between the tip and the sample as a function of their separation, a : F ′ = F ′ ( a , T ) at different temperatures. A model in which λ ( T ) is an adjustable parameter is then exploited to fit the data. The λ ( T ) thus obtained grows with T in terms of λ (0)(1+ T 2 / T c 2 ) for T / T c ⩽0.786 and λ (0)/(1− T 2 / T c 2 ) 1/2 for T / T c ⩾0.786, which is in good agreement with macroscopic measurements.

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.

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%.