No.19, October 2005

Feature Article: Magnetism and Spintronics at Nanometer and Atomiclength Scales
The author of this article Dr. Arthur R. Smith, an associate professor of physics at Ohio University. His Ph.D. is from the University of Texas. Arthur's research areas are experimental semiconductor physics and thin films. He was the recipient of the Presidential Early Career Award for Scientists and Engineers in October 2000.

In the year 2006, the electronics and magnetic data storage industries have reached levels of performance far beyond what may have been imagined in the 1930's and 40's when the computer was invented. The triumph of modern computation technology has largely been due to the invention of the transistor by Bardeen, Brattain and Shockley in 1947. The transistor allows both computer operations as well as random access memory (RAM) data storage, working much more efficiently than the vacuum tube predecessor. Permanent data storage also became a key problem for computation, resulting in the first use of magnetic tape storage to record computer data in 1951 and the first commercial magnetic hard disk – the IBM 350 RAMAC disk drive – in 1956 which stored a total of 5 Megabyte of information on fifty 24-inch platters.1.,2 Compare that to today's hard disks which can easily exceed 100 GigaByte in a palm-sized package, and one can appreciate the advances of the magnetic data storage industry.
Current research, however, is looking at ways to push these advances to new heights. One of the key issues is how densely information can be packed. To this end, it is of great interest to explore how small a single magnetic storage bit can be. Currently a typical high density hard disk stores 1 bit on an area of order 100 nanometer × 100 nanometer. This is just within the length defined to be "nanoscale." However, magnetic information in nature can vary on much smaller length scales. For example, in an antiferromagnet, the magnetic orientation ("bit") alternates up and down with a period of about 0.4 nanometers. Therefore, theoretically it should be possible to vastly increase the density of data storage.
One of the primary focuses on my research at Ohio University in Clippinger Lab 151 is the investigation of magnetic surfaces using a new technique called spin-polarized scanning tunneling microscopy (SP-STM), in which it is possible to image the magnetic material surfaces with magnetic resolution down to sub-nanometer scale (see Fig. 1). This method uses a magnetic-coated STM tip – the needle-shaped probe used to scan the surface. A typical magnetic coating would be a 1-2 nm thick layer of elemental iron. Our first publication on this topic (2002) revealed the alternation of the magnetic vector direction of the surface of antiferromagnetic manganese nitride (see Fig. 2). The magnetic repeat period of this surface was only about 1.2 nm.
We have more recently been investigating the magnetic structure using voltage-dependent SP-STM, in which we observe how the magnetic image of the surface depends on the applied tip-sample voltage. Our experimental information is then compared with theory predictions from collaborators at Case Western Reserve University and the Max Planck Institute in Düsseldorf, Germany. We have found that first–principles theory simulations agree very well with the experimental voltage-dependence, and we have now proven that the SP-STM voltage-dependence is that of a sample surface under study, not the probing tip. We find as a result that SP-STM is a powerful and reliable method for studying the nanomagnetism of surfaces.
I am now applying the SP-STM technique in Lab 151 to a variety of new spin-polarized materials. This is due to another of my primary interests – namely the new research area known as spintronics. Spintronics research is focused towards developing science and technology utilizing the dual-valued quantum electron property of spin (either or ) for novel device/computation functionality, instead of (or in addition to) the single-valued quantum electron property of charge (value = e). Toward this direction, we are using molecular beam epitaxy thin film growth techniques in Lab 151 to fabricate magnetic-doped semiconductor layers whose charge carriers (electrons or holes) can be spin-polarized, which means that the number of electrons having spin- is greater than the number with spin- (or vise versa). Such materials are at the frontier of research which is aimed at ultimately developing computation speed/performance in selected applications, which could potentially go well beyond what is possible with the fastest computers in existence today. To achieve that, however, these spintronic devices need to become nano-sized. Thus, nano-sized, spin-polarized objects become of great interest.
Nanomagnetism and nanospintronics research at Ohio University received a boost in 2003 with a four-year, $1.14 million grant that I and colleagues (Professors Hla, Sandler and Ulloa) received from the National Science Foundation to explore artificial, nano-sized spin structures. This research is currently making great progress and involves not only faculty but also a group of approximately 10-15 graduate and undergraduate students and postdocs. At the same time, I am currently constructing a new magnetics/spintronics laboratory (major project of my recent sabbatical), which will include a hybrid molecular/LASER beam epitaxy thin film growth machine combined with a superconducting magnet, variable-low-temperature STM. This new facility will allow spin-polarized imaging and spectroscopy over a wide range of temperature (4 - 300 K) and in applied magnetic fields (up to 4.5 Tesla). The funding for this new equipment was obtained in 2005 through an Office of Naval Research DURIP grant for $426 K. The new facility will be housed in a custom-renovated lab space (ARB 102). The new "Lab 102" is expected to be completed in August 2006, and the equipment setup and construction should be operational by January 2007.
(1) http://en.wikipedia.org/wiki/Magnetic_tape
(2) http://en.wikipedia.org/wiki/Magnetic_storage

 
Dept. of Physics & Astronomy, Clippinger Lab 251B, Athens, OH 45701
Tel: 740-593-1718 Fax: 740-593-0433 Email:physics@ohio.edu