Magnetism and Magnetic Materials
Professors Gavin Lawes, Boris E. Nadgorny, Ratna Naik, and Prem Vaishnava
Our research program in magnetic materials focuses on the fabrication and property characterization of magnetic thin films and the development of these materials for device and sensor applications. Of particular interest are studies of domain structures in single-layered ferromagnetic thin films, multilayer structures exhibiting giant magnetoresistance, spin-polarized magnetic tunnel junctions, and hybrid semiconductor/ferromagnetic devices. Characterizations are primarily focused on transport (magnetoresistance and I-V characteristics) and magnetic (SQUID magnetometry, vibrating sample magnetometry, ac susceptometry, and ferromagnetic resonance) measurements between liquid helium and room temperature. Additionally magnetic force microscopy is utilized to determine the local magnetic structures of sub micron-size features and a point conductance technique based on the Andreev reflection of electrons has been developed for measuring the spin polarization of ferromagnetic materials. Magnetic films and devices are fabricated by molecular beam epitaxy (MBE), electron-beam, and sputter deposition systems.This program also makes extensive use of the facilities and research collaborations associated with the interdisciplinary Smart Sensor and Integrated Microsystems program.
Specific research projects include:
Spin Polarization Mapping (Nadgorny): This research focuses on the implementation of nanoscale spin polarization measurements utilizing Micro Electro Mechanical Systems (MEMS) positioning technology in combination with electroforming of three-dimensional (3D) microstructures. A simple new technique of electrodeposited (ED) photoresists makes possible conformal coatings of highly structured surfaces and, when combined with electroplating, can form new types of advanced 3D metal microstructures. By developing an array of micro-tips integrated with through-wafer interconnects into a single testing chip, individually addressable m x n arrays of microscopic superconducting tips on a testing chip can be fabricated. Then this testing chip will be applied to thin films and planar magnetic nanostructures to map the spin polarization over a larger cross-section and will allow non-destructive device evaluation at any stage of the device fabrication. Furthermore, since both electrodeposition and electroplating processes have the ability of forming complex topographies, this technique has the potential to develop other 3D metallic microstructures for other potential applications.
Magnetic Nanoparticles (Lawes, Naik): This research project focuses on synthesizing and characterizing the properties of magnetic oxide nanoparticles, which typically range from 3 nm to 15 nm in diameter. In order to investigate how the magnetism of materials changes on the nanoscale, extensive studies of the magnetic properties of these systems are correlated with detailed measurements on their structure. One particularly intriguing problem focuses on understanding the role of interparticle interactions in determining the magnetic properties of these materials. Magnetic nanoparticles may also be important for biomedical applications. One system under extensive investigation at Wayne State is gamma-Fe_2O_3 nanoparticles in an alginate matrix, which is being studied for applications in targeted drug delivery, as an MRI contrast agent, and for hyperthermic treatments of malignant tumors.
Multiferroics and Magnetodielectrics (Lawes): Materials exhibiting simultaneous magnetic and ferroelectric order (multiferroics) are particularly interesting systems in which to investigate spin-charge coupling. Recently, in certain classes of multiferroic materials, it has been established that ferroelectricity is induced by the magnetic structure. These systems show very strong magnetoelectric coupling, to the extent that in some cases the spontaneous polarization can be switched by an external magnetic field. Understanding the properties of these multiferroics is crucial for incorporating these materials into novel magnetoelectric devices. Beyond single-phase multiferroics, various nanocomposite materials can also show substantial magnetodielectric effects. Magnetic nanoparticles embedded in an insulating matrix have been observed to exhibit a magnetization dependent dielectric constant. These composite materials may offer more versatility for device design than the single-phase multiferroic materials.