Robert Lee White - US grants

This project explores the origin of magnetic anisotropy in ultrathin iron films deposited on single crystal tungsten substrates using ultra-high vacuum sputtering techniques. The films will be characterized in-situ during deposition and subsequent crystallization using grazing incidence X-ray scattering to monitor the crystallization process and to monitor lattice accommodation and the number and orientation of crystal dislocations generated. Magnetic anisotropy measurements will be made and correlated with microscopic structural features. A second part of the project is devoted to incorporation and study of the effects of rare earth atoms at the iron-tungsten interface. Rare earth atoms at the interface in dilute amounts have the potential to produce large magnetic anisotropy, and this effect will be studied. The dominating importance of interfacial effects in films which are only a few atomic layers thick can in principle be used to tune the magnetic anisotropy and magnetic moment of a given sample to a desired value. Since the mean free path of electrons in these multilayer media may be substantially longer than the layer thicknesses, remarkable electron transport properties, such as the giant magneto resistance effect have been observed. Many of the properties of multilayer ultrathin films are of interest for technological application, especially for data storage either magnetic or magneto optic.

9710395 White Lithographically patterned thin film magnetic media offer the potential for extending the areal bit density of magnetic data storage media perhaps two orders of magnitude while preserving satisfactory signal-to-noise properties. Present magnetic media are formed of a continuous featureless thin film comprised of many very small independently acting single-domain grains. The size, shape, and position of the data bit is determined by the magnetic fields from the write head and each bit contains thousands of grains. As data densities get higher, and bit size smaller, the granularity of the medium has become troublesome, giving rise to unacceptable "media noise". In principle the noise could be reduced by decreasing grain size, but grains much smaller than are presently in use are unstable against thermally activated magnetization reversal. In this study, the magnetic structure and recording potential of lithographically patterned nanoparticle arrays will be studied. In such a medium, each nanoparticle is a single domain and a single data bit. The initial studies will be on epitaxial cobalt thin films, where the crystalline uniaxial anisotropy should allow the synthesis of single-domain bar shaped islands magnetized in plane and also parallel to the short axis of the bar. Such a nanoparticle array is compatible with present read-write technology. The single-domain character of isolated individual nanoparticles will be carefully examined using magnetic force microscopy and Lorentz microscopy. The stability of the magnetization pattern in an array will be determined, and the signal-to-noise characteristics of an array measured. To escape the restriction to a Cartesian or hexagonal geometry inherent in the epitaxial films, synthesis will be pursued of appropriately magnetized bar-shaped nanoparticles whose orientation on the substrate can be arbitrary (radial or circumferential on a disk, for instance). The required uniaxial anisotropy will be achieved using the anisotro pic strain relief in a bar-shaped nanoparticle, coupled with the magnetostriction of the material. The amorphous SmFe and SmCoFe family of highly magnetostrictive films will be explored for this purpose. Again, single domain character, array stability, and signal/noise characteristics will be evaluated. ***

9810185 White Exchange anisotropy is the name given to the vector exchange interaction between an antiferromagnet and a ferromagnet. If the antiferromagnet is appropriately biased, this interaction produces a shift of the hysteresis loop away from the usual position of symmetry about H=O. The magnitude of the shift is known as the exchange field, He. The interaction also produces a broadening of the hysteresis loop, an increase in the coercive field, Hc. Exchange anisotropy was discovered some 40 years ago, but remained a scientific curiosity until it was realized in the 1980's that this effect could be useful for pinning the direction of magnetic thin films, important for the magnetoresistive sensors and spin valves important today in magnetic data storage systems. Since then there has been an explosion in the number of scientists and engineers working on exchange anisotropy, but mostly from a very applied point of view. The result is that there is a lot of empirical information on exchange anisotropy, but still very little fundamental understanding of die phenomenon. For exchange anisotropy to exist there must be a magnetic polarization of the antiferromagnet at the antiferromagnet/ferromagnet interface, and magnetic anisotropy in the antiferromagnet. There are two present hypotheses about the origins of the interfacial polarization of the antiferromagnet. One invokes unbalanced spins at the interface, and the other a canting of the antiferromagnetic spin structure at the interface. The unbalanced spin picture implies a dependence of the polarization upon the domain structure of the antiferromagnet. Experimental evidence exists supporting both hypotheses. It may in fact be the case that both occur but under different circumstances. In order to understand the phenomenon of exchange anisotropy it is necessary to be able to observe (1) the interfacial polarization of the antiferromagnet, (2) the domain structure in the antiferromagnet, and (3) the magnetic anisotropy of the anti ferromagnet. Unfortunately, until recently, only the third parameter has been directly measurable. They have realized that X-ray magnetic dichroism can be used to measure directly the other "hidden" parameters. X-ray magnetic circular dichroism, which is element-specific and has monolayer sensitivity, can be used to measure directly the interfacial polarization. X-ray magnetic linear dichroism determines the axis of an ordered system but not the net magnetization. It can therefore measure the spin system orientation in both antiferromagnets and ferromagnetism. They propose to use the linear dichroism to observe both antiferromagnetic domain structure and the relative orientation of the ferro- and antiferro-magnetic films. The antiferromagnet they propose to use initially in their studies is cubic NiO because it has a very simple crystal structure and a known simple spin structure. They have developed a technique for MBE deposition of NiO and of the magnetic metals needed for their samples. The MBE technique has both fine control on the film thickness and produces high quality epitaxial films.

Objective
The objective of this research is to develop a new biochip technology based on spin valve sensor arrays and magnetic nanoparticles, which may eliminate the need for sample amplification, and make rapid pathogen detection possible. The approach is based on two technical components: (1) a DNA fragment detector with a sensitivity of ~100 femto-molar that is at least 10-fold better than the present technology, and (2) a sample preparation system allowing rapid and efficient concentration of the DNA (or protein) samples. Both components utilize monodisperse magnetic nanoparticles (nanotags) with a mean diameter ranging from 10 to 100 nm to label pathogen targets.
Intellectual Merit
The intellectual merit of the project is that the magnetic biochip allows greater specificity and sensitivity for pathogen detection and quantitation. In particular, the intrinsic multiplexing capability of magnetic DNA chip will create what economists call a ?disruptive? technology. Because it is cost-effective and easy to use, the magnetic DNA assay can define a new standard of healthcare for infectious diseases.
Broader Impact
The broader impact extends well beyond magnetics, spintronics, nanotechnology, and biology. It will eventually make it possible to detect, and in some cases treat, diseases such as cystic fibrosis, cervical cancer, sickle-cell anemia, and diabetes. The same magnetic chip can be adapted for use in genomics, forensics, biodefense, and cancer detection. The fundamental knowledge of magnetic biosensors and nanoparticles would have wide-ranging implications for micromagnetic and nanomagnetic devices in microelectronics as well as medicine. The research will attract and train undergraduates and under-represented minorities.