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Encyclopedia of Physical Science and Technology EN014A-653 July 28, 2001 20:55

Rare Earth Elements and Materials 19

antilock brakes, dashboard computers, etc.), and house-
hold electronics (dishwashers, washing machines, air con-
ditioners, etc.). Permanent magnets are also used as timing
motors in industrial robots, military and space technology,
and clocks and watches.
Among the commercially important families of perma-
nent magnets, two contain rare earth elements, namely, the
samarium–cobalt and neodymium–iron–boron. As com- FIGURE 14 Molecular structures of two commonly used lan-
1
thanide shifts reagents in H nuclear magnetic resonance (NMR)
pared with the non-rare-earth-containing permanent mag-
spectoscopy.
nets, both rare earth-based permanent magnets have much
higher-energy product (the figure-of-merit used to com-
pare permanent magnets, which is a value proportional to
accessible coordination sites so that the molecule being
the amount of stored magnetic energy per unit volume of
analyzed can form a bond via a heteroatom to the para-
magnet).
magnetic center. The use of lanthanide shift reagents in
The samarium–cobalt magnets were discovered in the 1 H NMR can in certain cases dramatically simplifies the
1960s. Its performance made it possible to obtain intense interpretation of the spectra and in other cases aids the
magnetic energy in low volume, and subsequently, minia- assessment of the composition of complex mixtures.
turization utilized in timing motors, or, even more spectac-
ularly, in audiovisuals where the use of the Sm–Co mag-
3. Contrast-Enhancing Agents for Magnetic
net, for example, made it possible to refine the miniature Resonance Imaging
earphones used with Walkman s.
But the most significant discovery in this area came in Whereas a quarter century ago activity was focused on
the 1980s with the discovery of neodymium–iron–boron the use of complexes of Eu, Pr, and Yb as chemical shift
(Nd 2 Fe 14 B) permanent magnets. This family of magnets reagents for NMR spectroscopy, the current interest cen-
has shown the fastest development of any permanent mag- ters on the in vivo application of related paramagnetic
net yet discovered, and currently constitutes over 25% of gadolinium complexes as commercially important con-
the total worldwide market. The major driving force for trast agents in magnetic resonance imaging (MRI). This
the growth in the use of these magnets has been the signif- diagnostic imaging technique relies upon the detection
icant size, weight, and performance advantage that they of the spatially localized proton NMR signals of water
can provide over other magnets. These materials are now present in living body fluids. The water signal intensity is
a key design feature in a wide range of high technology, dependent upon a number of factors particularly the values
high growth applications, notably spindle and stepper mo- of the water proton relaxation time, which decreases sub-
tors for the computer peripheral and consumer electronics stantially when the water oxygen is close to a highly para-
industry. Virtually all of the hard disk drives manufactured magnetic substance such as a rare earth complex. These
worldwide employ spindle motors using Nd 2 Fe 14 B mag- paramagnetic species are not themselves directly imaged
nets. Other major applications include motors for floppy but rather enhance contrast indirectly by affecting the nu-
disk drives, printers, fax machines, cameras, camcorders, clear magnetic relaxation times of the water protons in
and VCR’s. Driven by these and other developing appli- surrounding tissues, providing enhanced imaging contrast
cations, the market for bonded neodymium–iron–boron is between normal and diseased tissues.
expected to continue growing. Paramagnetic “contrast” agents are now used routinely
in clinical practice to enhance the signal intensity obtained
3+
in an MRI image. Attention is focused on the Gd ion
2. NMR Shift Reagents
which couples a large magnetic moment (S = 7/2) with
Since most complexes of trivalent RE are paramagnetic, a long electron-spin relaxation time, two properties that
they do not usually give useful NMR spectra. The reso- allow efficient nuclear-spin relaxation. The contrast agents
nances are at unusual chemical shifts and are often very are administered as stable complexes to avoid toxicity of
broad. Nevertheless, certain complexes called lanthanide thefreemetalions.Theclinicallyapprovedcontrastagents
shift reagents are used to induce chemical shift changes are highly stable and water-soluble complexes of Gd 3+
selectively in other molecules via through space mag- with the ligands in Fig. 15 or their derivatives.
netic interactions. These complexes comprise a lanthanide The MRI technique uses no invasive radiation such as
ion surrounded by three β-diketonate ligands, such as in X-rays, and to date no negative side effects have been
Fig. 14. The ligands are chosen such that the complexes demonstrated on the species under normal examination
are soluble in nonpolar solvents but still have reasonably conditions. With the ever growth of MRI procedures,