Sep. 25, 1998 REHOVOT, Israel--September 24, 1998--Weizmann Institute scientists have created a new class of magnetic materials made of clusters of inorganic molecules. These molecular magnets, described in the September 24 issue of Nature, display an unusual combination of properties that open up new research possibilities and may lead to a broad range of future applications in the microelectronics industry.
The molecules of nickel dichloride that make up the new magnets are much smaller in size than the metal-organic compounds used previously to create most molecular magnets.
Another distinguishing feature of the new molecules is their shape. Some have a cage-like structure that resembles fullerenes (the soccer-ball like molecules named after the architect R. Buckminster Fuller), while others are shaped like tubes, called nanotubes.
The method by which the nickel dichloride clusters have been created is also unusual. Rather than producing chunks of magnetic material, the scientists built the magnetic molecules from individual atoms. The molecules then self-assemble into a spherical layer one molecule thick. This method of creating a magnetic material, known as the "bottom-up" approach, gives scientists precise control over the size and structure of the magnet's molecules and the number of their layers. This, in turn, allows them to tailor the material to specific needs.
"The bottom-up approach gives us enormous flexibility," says research leader Prof. Reshef Tenne of the Weizmann Institute of Science's Materials and Interfaces Department. "It's like constructing a building from individual bricks as opposed to moving around the walls within a prefabricated house." Tenne conducted this research with graduate student Yaron Rosenfeld Hacohen, in collaboration with Dr. Enrique Grunbaum of Weizmann and Drs. Jeremy Sloan and John Hutchison of the University of Oxford.
The study of fullerenes and nanotubes in inorganic materials was pioneered by Tenne and his Weizmann Institute colleagues in the early 1990s, creating a new avenue of research in materials science. The production of the nickel dichloride molecular magnets further expands this field, by introducing to it a totally new family of compounds.
Potential Future Uses
Molecular magnets are being developed because they represent the ultimate in miniaturization for the microelectronics industry, which is looking for ways to create smaller and smaller devices. In particular, such magnets are intended to allow as much computer memory as possible to be packed into a limited space.
Hard-disk memory is usually built up of a multitude of magnetic switches, in which a change between the "on" and "off" positions is performed by altering the switch's magnetic polarity. An ideal magnetic switch must be operated by a relatively weak magnetic force, so that its polarity can be altered with relative ease. Yet at the same time it must be sufficiently stable so as to preserve its polarity long-term.
Numerous molecular switches are currently being developed, but scientists run into a problem when they place them next to one another. Magnetic forces work over a relatively long range, so when the minuscule magnets are packaged tightly, interference results. Thus when the polarity of one of these magnets is switched, the orientation of neighboring magnets changes as well. Such interference makes it impossible to store information reliably over a long period of time.
The new nickel-dichloride structures created by Tenne and his team promise to offer a solution to this problem. They are expected to be much less influenced by the magnetic fields of their neighbors and to be relatively "indifferent" to other environmental influences, such as temperature. Their seamless structure also suggests they should not be sensitive to "hostile" chemical effects of the environment, such as oxidation.
In addition, because these structures contain no impurities and because their spatial structure is well defined, their magnetic properties can be defined in a precise manner according to predetermined needs.
Apart from switches for computer memory, Tenne's molecular magnets in the form of nanotubes may have a multitude of other industrial applications. Thanks to their small size, they may be used for extremely fine "etching" of information onto magnetic disks, the process known as lithography, and for "reading" this information. Such "reading" is performed, for example, during the quality control of computer chips, and a nickel dichloride nanostructure could do this at a far greater resolution than any existing device.
Since the magnets are new and are expected to display intriguing magnetic behaviors which have not yet been fully investigated, they may find other, unexpected applications in the future.
Additional uses may arise from the fact that the nickel-dichloride magnetic materials -- unlike most other molecular magnets -- are semiconductors. This means that they can be used to create switches operated by an electrical current, and not only a magnetic field. Furthermore, they can also be operated optically because they selectively absorb light of certain wavelength. This combination of properties makes the new molecular magnets extremely versatile.
Tenne's team is now developing methods for synthesizing large quantities of nickel dichloride magnetic materials in order to study their magnetic properties in greater detail and pave the way for industrial testing.
This research was funded by Israel's Ministry of Science and the Israel Science Foundation.
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