Magnetic materials have long been interesting from both scientific and technological points of view. From magnetic storage to compasses for navigation, the magnetic properties of materials continue to drive the development of new technologies. One technologically relevant magnetic phenomenon is magnetoresistance (MR), in which the electrical transport properties are strongly affected by applied magnetic fields. Most magnetoresistive materials exhibit diminished electrical resistivity as the applied magnetic field strength increases.

In some materials, MR behavior is intrinsically related to the details of the structure and electron interactions within the crystal. Other materials may not display MR behavior alone, but can be used to prepare composite materials or devices that do exhibit a magnetoresistive response. In the Stacy group, we are interested in these latter cases of induced magnetoresistance.

Magnetoresistive Nanowires and Nanowire Arrays

Within the past decade, one-dimensional nanowires and nanowire arrays have captured the interests of many groups in a wide variety of fields, mainly due to their unusual properties and potential for integration into and miniaturization of current technologies. Template-assisted electrodeposition provides a convenient, cost-effective route toward the fabrication of nanowire arrays. This project extends our previous studies of template-assisted electrodeposition of nanowire arrays to largely unstudied magnetoresistive materials.

Current theories of magnetoresistive behavior vary widely, and many are still not completely understood. The reduction of magnetoresistive systems to 1-D should provide an experimental testing ground for predictions of pre-existing models and possibly give rise to new models altogether. Moreover, MR measurements of nanowire arrays will not only provide new insights into the mechanisms of magnetoresistance, but also carry a high potential for revealing new and unexpected discoveries in the field of magnetoresistivity. In addition to measurements of nanowire arrays, a method for making electrical contact to single nanowires during electrochemical growth has recently been adopted by our group to make measurements on single wires in an effort to separate collective array effects from the instrinsic properties of our nanowires.

Non-Stoichiometric Silver Chalcogenide Nanowires and Nanowire Arrays

Non-stoichiometric Ag chalcogenides (Ag₂X, X = S, Se, Te) represent a non-magnetic system that has high potential for engineering applications due to its unique MR properties (e.g., 200 % MR at room temperature and 55 kOe, large linear field range, and no saturation at fields up to 55 T). It has been suggested that a mechanism by Lorentz force scattering is present in the silver chalcogenide system, where Ag metal inclusions dispersed throughout the compound act as highly conductive inhomogeneities. Our work involves the electrodeposition of silver selenide nanowires, allowing precise control over composition, defects, and the microstructure on the nanoscale; thus enabling more controlled studies toward the elucidation of the MR mechanisms involving nanoscale inhomogeneities.

Magnetite Nanowires and Nanowire Arrays

Geometric MR in non-magnetic semiconductors depends strongly on the boundary conditions to current flow and mobility of carriers, and therefore implies the possibility of optimization of extrinsic MR effects through control of shape, size and dimensionality. Promising studies on Fe₃O₄ have already shown that giant extrinsic MR (500 %) at room temperature is possible, although the exact mechanism of the MR is still an ongoing debate. Nanowire studies of Fe₃O₄ are also underway to better understand and predict the MR effects in highly spin-polarized materials such as magnetite. We are involved with the synthesis and subsequent characterization of magnetite nanowires to better understand and predict the MR effects in highly spin-polarized materials such as magnetite.

Spin-Polarized Chalcogenides and Magnetic Tunnel Junctions

The promise of spin-based electronics is predicated on the ability to control transport of electron spin. The nature of magnetism at the boundaries of spin-polarized materials is one outstanding issue in this field, and is one that may be critical in the development and deployment of spin polarized devices in memory applications. A basic understanding of spin polarized devices has been obtained from junctions comprising metallic, ferromagnetic electrodes separated by an amorphous binary oxide layer, as shown in the device schematic at left. However, detailed understanding of spin-polarized tunneling depends on the fabrication of isostructural, chemically tunable junction devices with nanoscale control over the material interfaces.

Chalcogenide spinels offer a wide array of magnetic and electrical properties within an isostructural family of compounds. As part of a Nanoscale Interdisciplinary Research Team (NIRT), we are investigating the spin polarization of a series of sulfide and selenide spinels, including copper selenochromite (CuCr₂Se₄) and its bromine-doped analogues. While these materials are not intrinsically magnetoresistive, the trilayer thin film devices (tunnel junctions) should display MR behavior depending on the relative magnetic polarization of the ferromagnetic layers. In partnership with our NIRT collaborators, such thin films and junction devices are being fabricated by pulsed laser deposition and characterized via a variety of techniques.

• Collaborators