Electric Clock for NanoMagnet Logic Circuits - Field-Coupled Nanocomputing: Paradigms, Progress, and Perspectives - page 97

measurements of the piezoelectric properties with an atomic force microscope.
Ferroelectricity in nanometer-thick layers [ 97 ] has also found evidence for copoly-
mer ferroelectrics. Regarding the lateral dimension, interesting progress has been
done in the achievement of nanometric ferroelectric islands obtained, for exam-
ple, by ion milling or through self-assembled nanostructures [ 98 ]. Nevertheless
these dimensions are still relatively large compared with the theoretical val-
ues discussed above. A promising route could be the study of 1D nanostruc-
ture: ferroelectric BaTiO3 nanorods with diameters as small as 5 nm have been
achieved [ 99 ].

3.2 High Magnetostriction Materials

Magnetostrictive materials are broadly defined as materials that undergo a
change in shape due to change in the magnetization state of the material. In
principle, all ferromagnetic materials exhibit a change in shape resulting from
magnetization change. In most common materials, nickel, iron, and cobalt, the
change in length is on the order of 10 parts per million. This type of magnetostric-
tion has been termed Joule magnetostriction after James P. Joules discovery in
the 1850s. The relatively small change in shape of these materials limited their
use in engineering.
Following the advances in permanent magnet research, novel magnetostric-
tive compounds containing iron, terbium and dysprosium have become available
in commercial quantities. Rare earth- iron giant magnetostrictive material dis-
covered by Clarke [ 100 ] feature magnetostrain that are two order of magnitude
higher than nickel ((in the range of 500-1000 ppm), as shown in Table 4 .How-
ever, to exploit such elastic strains a linear behavior vs applied field is required.
Application for highly magnetostrictive materials are miniature magnetostric-
tive actuators and motors [ 101 ], magnetostrictive linear actuators [ 102 ], sonar,
valve actuation and active vibration control.
More recently an emerging class of magnetostrictive materials iron-gallium
alloys (Galfenol) [ 103 ] has been discovered that exhibit moderate magnetostric-
tion (350 10 6 ) at low magnetic fields (8 kA/m) along with low magnetic ×

Table 4. Comparison between the magnetostriction coecient of common mgnetic
and high magnetostrictive materials.

Material ʻ S (ppm)

Fe 14

Ni 33

Co 50

Ni 80 Fe 20 27

DyFe 2 650

TbFe 2 2630

Tb 0 . 6 Dy 0 . 7 Fe 1 . 9 2400