Biomedical Engineering Reference
In-Depth Information
electron microscopy techniques. In an unprecedented experiment, the
scientists used a scanning tunneling microscope that was built inside an
electron microscope to apply an electric stimulus to the sample while
observing its response at the atomic scale.
Using this technique, they obtained, for the first time, direct evidence that
a small electric stimulus can distort the shape of the crystal lattice and also
cause changes in the way charges travel through the lattice. The lattice
distortions accompanied the charge carrier as it moved through the lattice,
producing a particle-like excitation called a polaron. Polarons can be
pictured as a charge carrier surrounded by a 'cloth' of the accompanying
lattice vibrations. This research showed polarons melting and reordering -
that is, undergoing a transition from solid to liquid to solid again - in
response to the applied current. This has been identified as the key
mechanism for CMR. The technique has also allowed scientists to study
polaron behavior. Static long-range ordering of polarons forms a polaron
solid, which represents a new type of charge and orbital ordered state. The
related lattice distortions connect this phenomenon to colossal resistance
effects, and suggest ways of modifying charge density and electronic
interactions at the vicinity of electric interfaces and electrodes.
Colossal resistance effects could result in miniaturization of electric
circuits that operate at lower power. Research on CMR has had a direct
impact on the development of new electronic and spintronic devices (devices
that use a combination of electron spin and charge). Such devices include
new forms of 'non-volatile' computer memory, i.e. memory that can retain
stored information even when not powered, such as resistive random access
memory (RRAM).
As we have noted, magnetoresistance [70-74] in materials is of enormous
technological importance, particularly in the design of magnetic memory
systems, since these materials can be used as read heads for hard disks,
magnetic storage and sensing devices. The effectiveness of these materials is
directly related to the percentage change of resistance in an external
magnetic field. Magnetoresistance is defined as:
rð
H
;
T
Þrð
0
;
T
Þ
MR
ð
T
Þ¼
½
2
:
2
rð
0
;
T
Þ
where
(0, T) are the resistivity at a given temperature T in the
presence and absence of magnetic field H, respectively.
Perovskite manganites RE
1
X
A
X
MnO
3
(RE=trivalent rare earth ele-
ment such as La, Pr, Sm, Gd, etc. and A=divalent alkaline earth ion such
as Ca, Sr, Ba, etc.) have attracted considerable interest in recent years
because of their CMR behavior. In the perovskite structure, (RE, A)
elements occupy the A-site position (corner of a cube) and manganese
ρ
(H, T) and
ρ
