Nanophysics Group Seminars

The challenge to modern magnetic microscopies is provide both spatial resolution in the nanometer regime, a time resolution on a ps to fs scale and elemental specificity which allows to study novel multicomponent and multifunctional magnetic nanostructures and their ultrafast spin dynamics which is of both fundamental and technological interest.

Magnetic soft X-ray microscopy combines X-ray magnetic circular dichroism (X-MCD) as element specific magnetic contrast mechanism with high spatial and temporal resolution. Fresnel zone plates used as X-ray optical elements provide a spatial resolution down to currently <15~nm [1] which approaches fundamental magnetic length scales such as the grain size [2] and magnetic exchange lengths. Images can be recorded in external magnetic fields giving access to study magnetization reversal phenomena on the nanoscale. Utilizing the inherent time structure of current synchrotron sources fast magnetization dynamics with 70ps time resolution, imited by the lengths of the electron bunches, can be performed with a stroboscopic pump-probe scheme [3]. Recent achievements of magnetic soft X-ray microscopy are presented by selected examples on magnetic multilayers and nanostructured systems where both classical Oersted fields as well as spin torque phenomena are used to manipulate the magnetisation [4]. Future perspectives of magnetic soft X-ray microscopy aiming for <10nm spatial and fs time resolution will be discussed.

This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE-AC02-05-CH11231.

Magnetic domain structure in a nanogranular CoCrPt thin film imaged with 15nm spatial resolution X-ray optics.. TEM analysis yields an average grain size of 20nm [2].

Magnetic field sensors utilizing the Extraordinary Magnetoresistance effect (EMR) have been proposed for application in future magnetic recording applications. For over a decade, Giant Magnetoresistance (GMR) sensor technology has scaled straightforwardly and has been used by the industry as the sensor of choice. Recent demonstrations show it may be extendable to at least an areal density of 230 Gbit/in2. However, as critical dimensions decrease below 50 nm, thermal magnetic-noise and spin torque become increasingly difficult sources of noise and instability to avoid in GMR (and related devices with a magnetic sense layer). EMR devices are metal semiconductor heterostructures comprised of a high mobility semiconductor in parallel and in contact with a low resistance metallic shunt. A magnetic field applied perpendicular to the wafer plane changes electrical potentials within the device by selectively steering the current between the semiconductor and the shunt. Although this phenomenon is similar to the Hall Effect, modeling and experiments have suggested that the sensitivity is larger than Hall sensors. Importantly, no ferromagnetic materials are incorporated in EMR, eliminating magnetic noise sources present in sensors with magnetic sense layers.

• Describe the physics behind EMR,
• Show modeling results predicting magnetoresistance behavior,
• Compare those predictions with results from devices we have fabricated,
• Describe measurements of basic materials parameters obtained from simpler devices we have also fabricated,
• Explore some of what we can learn from low temperature measurements.

EMR results from the combination of the Hall Effect (potential induced along the perimeter of a device by current flowing in a magnetic field) and Corbino Effect (where current flows at an angle with respect to the electric field at the boundary of a metal equipotential). The resulting device is measured in a non-local 4-point measurement much as a Hall device. Finite Element Modeling (FEM) of such a structure shows that the magnetoresistance is optimized by high mobility with low semiconductor-metal contact resistance, with an optimized geometry and with the voltage probe leads located to either side of the drain lead and close to it. We have built such devices with dimensions of a few microns down to 50nm, measured them in magnetic fields as high a 9T, and find they have substantial signal. Using Hall bars and crosses we have measured transport properties of our devices, including the mobility and carrier concentration of our 2DEG quantum well vs temperature. Significantly, we have measured the bias dependence of the carrier concentration, which shows impact ionization effects, and the drift velocity, which saturates in our smallest devices at nearly the theoretical limit of about 1E8 cm/s. At low temperatures we measure Quantum Hall Effect plateaus, Shubnikov deHaas conductance oscillations, negative bend resistance and edge skipping orbits to help us characterize our devices.

Hard disk drive magnetic recording has consistently provided more capacity and faster access to data for more than 40 years. The astounding 50 million times increase in areal density of recorded bits and 10 thousand times increase in data rate are the result of regular transitions from one technology to the next. When a technology incorporated in HDD can no longer deliver the required performance as density increases, scientists and engineers have found an alternative technology to replace it. Chief examples are the transitions in read back transducers: first from inductive horseshoe magnet to thin film inductive readers, then to the anisotropic magnetoresistance (AMR) reader, followed by giant magnetoresistance (GMR) readers which have been the standard for 10 years. The industry is poised to transition to the reader using magnetic tunnel junction (MTJ) technology.

Today, in order to continue providing increased performance in HDDs, innovation is required in read back sensors, where fundamental limits appear to offer substantial challenges. This talk will focus on the requirements for read back sensors in the range of 100 Gbit/in2 to 2 Tbit/in2. Constraints imposed by both the recording system and scaling to smaller size will be described. While large signal is essential, of equal importance is low noise, so the common noise sources (Johnson, shot, magnetic, and spin torque) will be reviewed next. Then a variety of future sensor candidates that have been proposed, including tunneling magnetoresistance, current perpendicular to the plane GMR, magnetic transistor, and extraordinary magnetoresistance will be discussed in light of their specific signal, noise and scaling characteristics. In conclusion, an outlook for future sensor research will be presented.

Two techniques that allow synthesis of nanoparticles with photolithographic techniques are presented. The first part of the talk focuses on photolithography of II-VI quantum dots. In our QDPL (quantum dot photolithography) technique, the solvent of a porous matrix is exchanged with an aqueous solution of precursors. The precursors are dissociated and react to form nanoparticles in exposed regions. Our technique is extremely versatile; for example, a wide variety of radiation types ranging from infrared light to ultraviolet to X-rays can be used to fabricate patterned composites. The mean size of the nanoparticles is controlled by varying exposure time, precursor concentration, or, more conveniently, by adding a capping agent to the parent solution. The quantum yield of the composites is increased to up to about 30% by photocorroding defects on the surfaces of the quantum dots. Perspective applications of the technique to fabricate quantum-dot based photonic devices are discussed, and preliminary results are presented.

The second technique focuses on electrically conducting polymers. A technological need exists which requires synthesis of nanofibers of conducting polymers. A technique will be presented to fabricate polymer nanofibers which is much simpler than existing techniques and, more importantly, can be integrated with photolithographic techniques. We will then show how our two techniques can be combined to produce composites of PEDOT nanofibers and quantum dots for solar cell applications.