Physical Properties of Nano-scale Magnets

A variety of experiments are aimed at exploring the classical and quantum mechanical interactions in mesocopic magnetic structures. Our research exploits advances in miniaturization and highly sensitive magnetometry to probe a variety of new physical systems, where two distinct experimental efforts investigate phenomena scaling the classical to the quantum regimes. The magnetic properties of arrays of STM-fabricated ferromagnetic particles are studied as a function of their dimension using a new high sensitivity 2DHG Hall magnetometer. In contrast to the expectations of classical magnetism, the results reveal surprising magnetic instability of the small structures. At even smaller length scales and lower temperatures, magnetic particles have been proposed as systems where quantum mechanics may produce measurable manifestations on the macroscopic level. Low temperature measurements of the magnetic noise and susceptibility in a variety of artificially-engineered proteins are performed using a combination of superconducting integrated circuits to form a sensitive miniature dc SQUID-based microsusceptometer. The results are compared to theoretical predictions for macroscopic quantum coherence in small antiferromagnetic systems.

Submicron GaMn Ferromagnets

Submicron ferromagnets are fabricated in GaAs through manganese ion implantation and subsequent rapid thermal annealing. the leftmost image (1) seen below is a room-temperature atomic force microscope image of these precipitates, which are GaMn rich and crystalline. The corresponding magnetic-force microscope image below center (2) demonstrates that many of the precipitates are ferromagnetic.

The rightmost image (3) in the figure above is a plan-view transmission electron microscopy (TEM) image of these precipitates. These measurements reveal that the precipitates form at the sample surface and that average diameters can be varied from 100 nm to 400 nm by changing implantation doses and annealing conditions. SQUID measurements show that after annealing the implanted semiconductor films are ferromagnetic well above 300 K, with coercive fields ranging from 1000-5000 Gauss.

STM Magnets

This work was originally started several years ago in our group at IBM in order to produce nanometer-scale ferromagnets for ultra-low temperature studies of magnetic quantum tunneling. The present STM research is an active collaborative effort with Professor Stephan von Molnar at Florida State University and Dr. Andrew Kent. The growth of the magnets is shown in the following picture. A metallorganic precursor, in this case iron pentacarbonyl, is introduced in the UHV chamber. When a tunneling current is produced between the tip of the STM and the substrate, the metallorganic gas is dissociated and iron deposited locally. The iron may be deposited on either the substrate or the tip depending on the sign of the voltage bias. The aspect ratio of the particles may be controlled by maintaining the tunneling current as the tip is withdrawn from the substrate. It is possible to grow magnets on the substrate with aspect ratio almost as high as 10 and on the tip as high as 100.

Atomic Force Microscope (AFM) image of an array of single domain Fe magnets grown by STM deposition on top of a 2DEG Hall magnetometer. The magnets are approximately 40 nm in diameter.

Magnetic Force Microscope (MFM) image of the array after it was thermally randomized. The magnetic field from each ferromagnet is imaged and seen to be aligned along it's major axis, either pointing "up" (white) or "down" (black).

A magnetic field of 200 Gauss is applied to align (almost) all the magnets.

Artificially-Engineered Ferritin Proteins

In mammals, ferritin is used to store iron in the body in the form of an iron oxide bound in a spherical protein (apoferritin). The diagram below shows the assembly of the apoferrtin. First, a dimer (a) is formed burying the largest hydrophobic surface. Then the other hydrophobic surfaces (the shaded ends of the dimers) are buried to form the icosatetramer protein (d). Iron in the form of Fe(II) can enter through the open channels and is then oxidized on catalytic sites on the inside surface of the protein. The growth of the crystal is limited by the apoferritin shell, allowing for a maximum of 4500 Fe(III) ions in natural ferritin in the form of ferrihydrite, an antiferromagnetic mineral. In collaboration with Professor Steve Mann at the University of Bath, our work has focussed on the magnetic properties of artificial ferritins where the ferrihydrite core is removed and replaced with either ferrihydrite or other minerals such as magnetite or maghemite, which are ferrimagnetic. Through this technique of biomineralization, the size of the particles may also be controlled from 100 to 3000 ions, allowing for systematic studies of magnetism in nanometer scale particles.