Very Large Magnetoresistance
and Field Sensing Characteristics of Electrodeposited Single-Crystal Bismuth
Thin Films
F. Y. Yang, Kai Liu, Kimin Hong, D. H. Reich, P. C. Searson.
and C. L. Chien
Bismuth (Bi) is a semimetallic element
with unusual electronic properties due to its highly
anisotropic Fermi surface, low carrier
concentrations, and small carrier effective
masses. The small carrier effective masses and the very long
mean free path mean that bulk single crystals of Bi exhibit very large
MR effects. Despite this, fabrication of high quality Bi thin films, a
requirement for both the scientific study and technological application
of its unusual transport properties, has been difficult up to now.
Here we report on a fabrication and
processing method for achieving high quality single-crystalline
Bi thin films that exhibit very large MR effects. We have employed
electrodeposition in conjunction with a suitable post-deposition annealing.
We have succeeded in fabricating trigonal-axis oriented single-crystalline
Bi thin films that exhibit MR ratios of as much as 3,800
(or 380,000%) at low temperatures and 2.5
(or 250%) at room temperature with a non-hysteretic and quasi-linear field
dependence. The high quality of the Bi films is further illustrated by
the observation of clean Shubnikov-de Haas
(S-dH) oscillations. We also demonstrate a simple hybrid structure
with a large MR effect of 30 % at 200 Oe and a field sensitivity of 0.2
%/Oe at room temperature. These electrodeposited Bi thin films are new
media suitable for studying the unusual transport properties of Bi and
the development of field-sensing devices.
I. X-ray diffraction and High resolution
TEM picture of the Bi film
The Bi thin films are electrodeposited
from aqueous solutions of Bi(NO3)3
5H2O.
After suitable processing by annealing, the Bi films become single-crystalline,
with the trigonal-axis orientated perpendicular to the film plane as shown
in Fig. 1a, which exhibits only the (003), (006), and (009) peaks, demonstrating
that the Bi film is exclusively trigonal-axis oriented. Pole-figure measurements
of the (116) peaks show the expected 6-fold symmetry, as seen from Fig.
1b. The six spots in Fig. 1b shows there are six (116) reciprocal vectors
surrounding the trigonal axis at a tilt angle of ~ 41°
. A F -scan about
the Bi (116) peaks at the expected tilt angle 41°
is shown in Fig. 1c, where six peaks separated by 60° are observed.
High resolution transmission electron micrograph (TEM) also shows single-crystal
nature of the Bi thin films (Figure 2). The six-fold symmetry is clearly
seen. These x-ray diffraction results and TEM results conclusively demonstrate
that the as-deposited polycrystalline Bi thin films, after suitable annealing,
become single-crystalline trigonal-axis oriented Bi films.
Figure 1: X-ray diffraction patterns
of a 5-m m single-crystalline
Bi film: (a) q-2q
scan shows that the film is trigonal-axis oriented, (b) pole-figure scan
of the (116) peaks exhibits the expected 6-fold symmetry, where the radial
direction is the tilt angle y
(between the normal direction of the film and the diffraction plane, (0°
to 90° ) and the
angular direction is the rotation angle F
(0° to 360°
). (c) F scan at 41û-tilt
of the (116) diffraction shows the six equally spaced peaks.
Figure 2: High resolution TEM picture of single-crystalline Bi film
along trigonal axis.
II. Magnetoresistance of Bi films
The MR has been measured in three geometries: perpendicular
(P), longitudinal (L), and transverse (T), where the magnetic field H,
up to 5 T, is applied respectively perpendicular to the film plane, parallel
to the current, and in the film plane but perpendicular to the current.
The field dependence of the resistivity and MR ratio for polycrystalline
and single-crystalline 20 µm Bi films at 5 K is shown in Figure 3
for the three measuring geometries. The perpendicular MR is always the
largest and the longitudinal MR the smallest. The anisotropy is due to
the orientation of the cyclotron orbits in the measuring geometry. It is
noted that the MR for the Bi films, especially the single-crystalline films,
is enormous. The MR ratio for the 20-µm polycrystalline Bi film at
5 K is about 25, whereas the single-crystalline film show an MR ratio of
3,800 !
Figure 3: Magnetoresistance results
of a 20-µm electrodeposited Bi film: (a) as-deposited polycrystalline
film at 5 K in the perpendicular (P), transverse (T) and longitudinal (L)
geometries, (b) as-deposited film at 300 K in the P-geometry, (c) single-crystalline
film at 5 K in the P, T, and L geometries, and (d) single-crystalline film
at 300 K in the P-geometry.
III. Shubnikov-de Haas oscillations.
For the single-crystalline Bi films, there are additional
oscillations superimposed on the magnetoresistance in all three geometries.
These are the Shubnikov-de Haas (S-dH) oscillations observable only for
high quality single-crystal samples. Figure 4 shows the S-dH oscillations
of a 10 µm film at 0.06 K up to H = 9 T. The S-dH oscillations can
be more clearly shown by subtracting the MR data at 4 K from that at 0.06
K. More than eight S-dH oscillations are observed to be periodic in 1/H
as shown in Fig. 4(b).
Figure 4: (a) Field dependence of
perpendicular magnetoresistance R of a 10-µm Bi film at 10 K, 4 K
and 0.06 K and the Shubnikov-de Haas (S-dH) oscillations shown by R(0.06
K) — R(4 K). (b) The S-dH oscillations are shown to be periodic in 1/H.
IV. Fild sensing using Bi film.
Also of interest, particularly for technological
applications, is the MR effect at room temperature. As expected, the MR
effect at room temperature is much smaller because of a much smaller mean
free path, but nevertheless, a very large MR still remains. The MR ratios
at room temperature of both polycrystalline and single-crystalline Bi films
are about 2 to 3, roughly the same for all the films within the thickness
range of 1 — 20 µm as shown in Figure 3. It should be noted that
the MR ratio of 2.5 (or 250 %) for the Bi films at 300 K is much larger
than the largest GMR observed at any temperature. The non-hysteretic MR
of the Bi films, which increases quasi-linearly with field, can potentially
be used for wide-range field and current sensors.
Using suitable hybrid structures, the MR effect in
Bi can also be utilized to detect smaller magnetic fields. To illustrate
the feasibility of such hybrid structures, one example is shown in Fig.
5. Utilizing the strong response to small magnetic fields of a soft magnetic
material such as Fe, a large MR effect can be observed in a Bi sensing
element placed near the edge of the soft magnetic material, where a large
local magnetic field exists. As shown in Fig. 5a and 5b, at an external
field of 200 Oe, a hybrid sensor involving a Bi film and small Fe slabs
registers a MR effect of 6000 % at 5 K. At room temperature, it shows an
effect of 30 % at 200 Oe and a field sensitivity of about 0.2 %/Oe over
a wide range of fields from 50 to 200 Oe. With more accurately defined
flux concentrators, it should be possible to dramatically improve the sensitivity
of such hybrid structures.
Figure 5: Enhanced low field
sensitivity at (a) T = 5K and (b) room temperature from a hybrid structure
of Bi placed in Fe flux concentrator.
Finally, the electrodeposition method with which
Bi films are made is intrinsically low-cost and can be readily scaled up.
All the Bi films studied were deposited on gold underlayers with lateral
dimensions defined by shadow masks or lithography, which are amenable to
mass production. Together with the very large MR effect realized, Bi thin
films offer another medium for exploring not only the intricate transport
properties of semimetals, but for technological applications in field and
current sensing as well.
Last Modified 2/7/99