Bernard Doudin - US grants

9874657
Doudin
The aim of this CAREER project is to investigate the electrical properties of nanometer-sized ferromagnetic islands separated by insulating barriers from larger ferromagnetic contacts--an extension of the single-electron device, where electrons can be added individually to a small conducting island ("Coulomb island"). The magnetoresistance observed in tunneling magnetic systems is attributed to the spin polarization of the electrons in the ferromagnetic electrodes. Changing the magnetic orientation results in a drastic variation of the junction conductance, permitting the magnetic island to play the role of a gate in a single-electron transistor device. Control of current flow will be monitored through the magnetic orientation of the island, without the necessity of an electrical gate connection. A reduction in size compared to semiconductor devices can be expected because of the larger carrier density of metals. Ultra-small devices will be fabricated by electrochemical template synthesis. Previous accomplishments on individual junctions of 0.01 mm2 area, with a 1 nm thick insulator separating two ferromagnetic half-wires will be the starting point of these studies. Specific goals of the project are: to understand the influence of impurities in the insulator, revealed by electron blockade effects and strong fluctuations of the recorded voltage; to synthesize junctions two orders of magnitude smaller; to make double junction systems, embedding an island of volume smaller than 100 nm3; to investigate systems where the island or the electron source(sink) is not ferromagnetic; and, to gain access into the spin relaxation time in the ferromagnetic island.
The interdisciplinary nature of the project will be the basis for a new undergraduate laboratory course introducing students to the science of electronics. The research topics will serve as a motivating application for students at all levels. The laboratory topics will be directly derived from the research, and will introduce electrical measurements on mesoscopic systems and techniques of small signal detection. A multi-disciplinary approach will be gained by introducing undergraduate and graduate students in physics to the technique of electrochemistry. The synthesis/processing methods employed in the research can be learned by undergraduate students in a relatively short time. Electrical connections to the samples can be initially preformed without special precautions, and suitable devices can be studied immediately which provide good examples for low signal measurements, for the study of noise properties, and for understanding the differences between high and low impedance circuits.
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The project addresses fundamental research issues in a topical area of materials science having high technological relevance. The research will contribute basic materials science, physics, and engineering knowledge at a fundamental level to important aspects of electronic materials and advanced devices/circuits. The scope of the project will expose students to challenges in materials synthesis, processing, and characterization. An important feature of the project is the strong emphasis on education, and on the integration of research and education.
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9975909
Sellmyer
This grant will provide partial support for the development of a new cluster-deposition facility for the synthesis of a variety of magnetic nanostructures. The facility will be capable of producing nearly monodispersed clusters with mean sizes ranging from 200 to 15,000 atoms, and to deposit these clusters onto a substrate alone or in combination with different elements or alloys. The resulting films are expected to have nanostructures of different types including homogeneous amorphous or crystalline alloys or compounds, multilayers, and mesoscopic nanocomposites. The sizes of the clusters and their concentration can be varied over wide ranges, and the matrix materials of the nanocomposites can be chosen independently. The clusters themselves may have properties significantly different from bulk materials of the same composition. This allows excellent control of the properties of the films through those of the clusters and their interactions.

The potential impact of these magnetic nanostructures on materials research and magnetic
technologies is significant. The production of independent or weakly interacting superparamagnetic particles is possible, thus paving the way for basic studies including spin-glass-like phase transitions. Novel nanoparticulate, high-anisotropy and high-coercivity films will be made with high potential to increase the areal density of magnetic-recording media by a factor of ten. New types of structures for the study of tunnel magnetoresistance and Coulomb blockade will be possible. There is an excellent chance that model exchange-coupled hard-soft permanent magnets will be achievable with world-record energy products.

The new cluster-deposition system will simultaneously benefit the research and education of a large group of graduate and undergraduate students, as well as several postdoctoral research associates. The University of Nebraska has a large and diverse experimental and theoretical group in nanostructured magnetic materials that heretofore have been prepared by other techniques. The combination of this group and expertise in cluster-deposition instrumentation can be expected to significantly advance the field of nanomagnetism and its applications.
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This is an ideal "integration of research and education" enterprise. It is expected it will have significant

9980705
Doudin

This three-year award for U.S.-France collaboration involves Bernard Doudin and David Sellmyer from the University of Nebraska and Wolfgang Wernsdorfer of the Louis Neel Magnet Laboratory in Grenoble, France. The investigators propose to investigate the magnetic properties of individual ferromagnets with control morphology in the sub-100 nanometer dimension range using ultra-sensitive magnetic measurements at low temperatures. Experiments will be performed on magnetic needles where the shape controls the magnetization reversal mechanism. Slow-dynamic measurements will also be performed. The objective is to characterize the barrier energy and the mechanism of magnetization hopping over this barrier. The Nebraska group brings to this collaboration unique expertise in nanoscale magnetism, hard ferromagnetism, synthesis, and magnetic recording media. This is complemented by the theoretical expertise of the French group on quantum effects in magnetization reversal. It also provides access to a unique magnetometer developed by the French investigator, which uses micro SQUIDS to measure the magnetization reversal of magnets.

This award represents the U.S. side of a joint proposal to the NSF and the French National Center for Scientific Research (CNRS). NSF will cover travel funds and living expenses for the U.S. investigator. The CNRS will support the visits of the French researchers to the United States. The collaboration will advance understanding of quantum mechanical properties of magnets and the approach for tailoring sample properties for application in magnetic recording media.

This is a joint award from the Major Research Instrumentation program and the NSF/EPSCoR program to the University of Nebraska (UN). The award supports the acquisition of a focussed ion beam (FIB) workstation for processing of single crystals and nanometer size materials in order to investigate their magnetic and magneto-optic properties. FIB is an ultra-high precision tool for etching and writing with 10-nanometer resolution. The workstation will complement existing facility at UN and will be used in a number of currently funded projects at the institution. It provides new capability for the study of devices at the nano scale with a high probability for discovery of new phenomena. The instrument will benefit the education and training of a large number of graduate and undergraduate students as well as postdocs at the University of Nebraska.
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This is a joint award from the Major Research Instrumentation program and the NSF/EPSCoR program to the University of Nebraska (UN). The award supports the acquisition of a focussed ion beam (FIB) workstation for processing of single crystals and nanometer size materials in order to investigate their magnetic and magneto-optic properties. FIB is an ultra-high precision tool for etching and writing with 10-nanometer resolution. The workstation will complement existing facility at UN and will be used in a number of currently funded projects at the institution. It provides new capability for the study of devices at the nano scale with a high probability for discovery of new phenomena. The instrument will benefit the education and training of a large number of graduate and undergraduate students as well as postdocs at the University of Nebraska.

Intellectual Merit: This research involves making and investigating high-resistivity B5C boron carbide as a major, new candidate for the dielectric barrier material in tunnel magnetoresistive devices. Based on their unique experience of making and using B5C in a variety of semiconductor applications, with chosen bandgaps from 0.9 to 2.7 eV, the investigators will use this material to help understand the fundamental properties of magnet/barrier interfaces that are highly important in magnetoelectronics and, specifically, spin electronics (spintronics) - one of the most promising future paths in electronics.

The research team will use their abilities in spin-resolved photoemission to provide information on the surface electronic states and, since the dielectric film comprises such light atoms, to gain access to the important buried magnet/boron carbide interface. High-resolution electron microscopy and spectroscopy techniques will provide structural, dielectric, and bonding information on the B5C polytypes. Investigations of photoconductivity under applied magnetic fields will be used to gain better understanding of the influence of local impurity states in the barrier on magnetoresistance properties. Control of the orientation and polarization of the incident light will help identify these states and provide initial data for correlation with photoemission. Magnetic impurity atoms will be incorporated during dielectric growth in order to provide additional control of barrier states and so of magnetoresistance. Since initial results on chromium oxide junctions showed it is possible to modify the magnetic state of an island in the junctions by changing the polarity of the applied electric current, magnetic inclusions will also be incorporated in the barrier in order to test new ideas of spin transfer in non-oxide magnetic junctions.

Broad Impacts: This project will train graduate and undergraduate students, particularly from underrepresented groups, in sophisticated experimental techniques and provide them with opportunities to communicate their results to peers, scientists and engineers in top journals and at major professional and industrial meetings. The research will allow the team to improve its capabilities for making boron carbide-based materials and devices, which have many applications, including neutron detection as two of the team recently demonstrated. Making magnetic sensors, or particle detectors, has become increasingly important for ensuring the security of our nation, and will continue to motivate the students on the team, and their peers. Opportunities will be sought to encourage understanding of the results, and their societal benefits, by people in business and industry, including through technology transfer, and by the general public, legislators and school students.

The research will provide access to fundamental understanding of the physics of magnet/barrier interfaces and to practical technology for applying this understanding to spintronics. The additional device properties accessible by incorporating metallic inclusions in the barrier are expected to spur new magnetic memory applications.

Most importantly, the research on boron carbide dielectric barriers is expected to provide a means of avoiding the serious reduction in the ferromagnet surface states caused by oxide dielectric barriers and therefore of avoiding a major problem for the development of spintronics. The research may therefore provide the basis for a key enabling technology for spintronics.

Professors Jody G. Redepenning and Bernard Doudin of the University of Nebraska-Lincoln are supported by the Analytical and Surface Chemistry Program in the Division of Chemistry to develop chemically modified nano-electrodes to be used in magnetoelectronic devices, such as permanent memories, reconfigurable logics, and fast electronic components. To accomplish this goal, the investigators plan to explore two types of barriers and to reduce the size of the junctions. One will be comprised of silicon oxide produced by repetitive hydrolysis/condensation of tetramethylorthosilicate on the surfaces of the electrodes. A second class of junctions will be produced by irreversible polymerization of appropriate metal complexes onto the surfaces in the junction. The idea is to passivate the surfaces of magnetoresistance tunnel junctions while introducing thin dielectric materials with controllable properties. The goal is to understand the elusive properties of barriers and how resonant tunneling effects occur.
The discovery of giant magneto-resistance led to commercial devices such as hard-disk read heads, magnetic field sensors and magnetic memory chips. Tunnel magneto-resistance devices are another example of development from laboratory proof of principle to industrial applications within a remarkably short time. Future generations of magnetic sensors and memory elements are likely to be be constructed in the nano-scale. Understanding magnetoresistance properties at the nanoscale will help build the knowledge base required for future device miniaturization. This research will involve graduate students, undergraduate students, and high-school teachers, giving them complementary and interdisciplinary education in nanotechnology. The interdisciplinary nature of this project will create new synergies between chemists and physicists.