Biomedical Engineering Reference
In-Depth Information
for developing magnetoelectronic devices. h ese include novel spintronic
devices such as tunnel magneto resistance (TMR) sensors and spin valves
with electric i eld tunable functions. A typical TMR device consists of two
layers of ferromagnetic materials separated by a thin tunnel barrier (~2 nm)
made of a multiferroic thin i lm [22]. In such a device, spin transport across
the barrier can be electrically tuned. If the antiferromagnetic spin orienta-
tions in the multiferroic pinning layer can be electrically tuned, then magne-
toresistance of the device can be controlled by the applied electric i eld [23].
One can also explore multiple state memory elements, where data are stored
both in the electric and the magnetic polarizations.
13.1.4 Previous Work Done on Multiferroic BiFeO
3
In magnetic perovskite-structure oxides and related materials, multiferroism
is most commonly achieved by making use of the stereo chemical activity of
the lone pair on the large (A-site) cation to provide the ferroelectricity, while
keeping the small B-site cation magnetic. h is is the mechanism for ferro-
electricity in Bi-based magnetic ferroelectrics. h ere is a scarcity of multi-
ferroic materials in nature because the conditions for being simultaneously
ferroelectric and ferromagnetic are dii cult to achieve [24-27]. However,
the magnetoelectric (ME) ef ect is restricted to insulators and is typically
too small to be used in applications. Although pure multiferroics are less
in nature due to their simultaneous ferroelectric and ferromagnetic behav-
ior, there are still some materials which are quite promising. h ese materials
are BiFeO
3
(BFO), BiCrO
3
and BiMnO
3
, etc. Among these, BiFeO
3
(BFO) is
known to be the only material that exhibits a rhombohedrally distorted ferro-
electric perovskite structure with space group R3c or C
3
v6
and shows G-type
antiferromagnetic ordering up to 370 C. h is compound (BFO) shows dis-
placement type ferroelectric ordering (T
C
~ 830 C) and canted spin aligned
antiferromagnetic ordering (T
N
~ 370 C) simultaneously [28-30]. h e theo-
retical investigation of the magnetoelectric nature in BiFeO
3
was given by
Ravindran
et al.
and others [31-33]. Although the structure and properties
of BiFeO
3
have been extensively studied since i rst discovered in the 1960s,
several obstacles still need to be overcome for practical applications.
An important aspect that emerges upon examination of the properties
of single-crystal BiFeO
3
is that it has a spontaneous polarization that is sig-
nii cantly smaller than the expected value for a ferroelectric with a high
T
C
.
Recently, enhancement of spontaneous polarization and related properties
have been reported for heteroepitaxially constrained thin i lms of BiFeO
3
,
and more recent theoretical calculations also correctly predicated the pres-
ence of large polarization in this material [18, 34-36].
