Changbae Hyun

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.

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.

Focused ion beam deposition of Co71Cr17Pt12 and Ni80Fe20 on tips for magnetic force microscopy

We demonstrate that a focused ion beam can deposit magnetic coatings on cantilevers used for atomic force microscopy, thereby producing a sensor for magnetic force microscopy. This technique is highly versatile, allowing the convenient deposition of complex or expensive materials, such as Co71Cr17Pt12. A second material chosen for this demonstration was the permalloy (Ni80Fe20). We show magnetic images acquired with these cantilevers to illustrate their excellent properties and the differences between coatings. In principle, multilayer coatings could be easily made with this technique.

Sintering Effect of Annealed FePt Nanocrystal Films Observed by Magnetic Force Microscopy

Chemically synthesized FePt nanocrystals can exhibit room temperature ferromagnetism after being annealed at temperatures above 500degC. In thick films composed of FePt nanocrystals, the coercivity can be quite large. However, the coercivity of thin films has been found to decrease significantly with decreasing thickness, to the point that ferromagnetism at room temperature is lost. We studied 12 to 55 nm thick films by using magnetic force microscopy (MFM) under external applied fields. We made smooth films by spin casting 4-nm-diameter FePt nanocrystals and annealing them at 605degC-630degC. Thin FePt films showed lower coercivity than thick films. To help interpret the MFM images, we obtained complementary magnetic and structural data by superconducting quantum interference device (SQUID) magnetometry, transmission electron microscopy (TEM), and X-ray diffraction. We conclude that the magnetic properties of these films are strongly affected by nanocrystal aggregation that occurs during annealing