I. Introduction

Magnetic nanostructures, such as multilayers and granular solids with metallic constituents, have attracted a great deal of attention, due to the realization of such new phenomenon as Giant Magnetoresistance and Interlayer Exchange Coupling.  The constituent materials in these nanostructures include transition metals, alloys, and nobel metal elements.

Arrays of nanowires are a new type of nanostructures that exhibit quasi-1D characteristics.  Metallic nanowires and multilayered nanowires have been successfully fabricated before.  In this work, we report the study of semimetallic bismuth (Bi) nanowires where we have observed large positive magnetoresistance (MR) and strong finite-size effects.

Special attributes of Bi:

Long carrier mean free path (~ 1 mm)
Large Fermi wavelength (~ 400 Å)

Ideal for studying both classical & quantum finite-size effects

   II. Fabrication and Characterization

Electrochemical deposition of nanowires into porous polycarbonate membranes.

X-ray & electron diffraction:    polycrystalline rhombohedral structure
SEM:    Cylindrical shape of the wires formed by the pores of the membrane
TEM:    Large elongated grains along the wire direction
 
 

Fig. 1. (a) Top-view SEM image of 400 nm Bi nanowires with the supporting polycarbonate membrane partially removed. (b) Dark-field TEM image of a single 200 nm Bi  nanowire.  The inset shows the electron diffraction pattern taken from one of the grains.
 
 

  III. Results and Discussions

    1. Resistance Enhancement

400 nm Bi nanowires: 1 x 10⁶ wires/mm²
R _{Single wire} > 1000 W vs. 10² W using bulk resistivity

Nanowire dimensions << carrier mean free path
 

    2. Temperature Dependence of Transport Properties

Fig. 2. Temperature dependence of the resistance of the 400 nm Bi nanowires in zero field, and 50 kOe field applied perpendicular and parallel to the wires.

H=0, R(5K)/R(293K) ~ 1.5, non-exponential

Negative temperature coefficient of resistance (TCR) vs. positive TCR in bulk Bi

Carrier concentration ---- negative TCR
Carrier mobility ---- positive TCR, suppressed by structural imperfections and finiste-size effects

H sufficiently large ---- resistance maximum

    3. Resistance maximum

Fig. 3. Temperature dependence of resistance (normalized to the value at 5K) of the 400-nm Bi wires showing the resistance maximum for various values of (a) transverse and (b) longitudinal magnetic field.

The resistance maximum depends on

Magnetic field strength
Magnetic field orientation
Nanowire diameter

Proposed mechanism:

Impeded phon scattering processes at low temperature by magnetic and size quantization.

    4. Magnetic field dependence of MR

Fig. 4. Transverse (H^) and longitudinal (H//) MR of 400-nm Bi nanowires at (a) 300 K and (b) 32 K.

MR characteristics:

Positive
Large magnitude
Quadratic at low fields, linear at high fields
Non-hysteretic

Origin of MR in Bi

Ordinary magnetoresistance, w_ct ~ 1/n
        carrier concentration n: orders of magnitude smaller than common metals

Compensated metal:    Hall field cannot balance out the Lorentz force
 
 

References

1. Kai Liu, C. L. Chien, P. C. Searson, and Kui Yu-Zhang, Appl. Phys. Lett. 73, 1436 (1998). (Full article, PDF file)
2. Kai Liu, C. L. Chien, and P. C. Searson, Phys. Rev. B (Rapid Communications) 58, 14681 (1998). (Full article, PDF file)
3. Kai Liu, C. L. Chien, P. C. Searson, and Kui Yu-Zhang, IEEE Trans. Magn. 34, 1093 (1998).

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Contact Us

Kai Liu:              kliu@pha.jhu.edu
C. L. Chien:       clc@pha.jhu.edu
P. C. Searson:     searson@jhu.edu
Kui Yu-Zhang:     kui@univ-mlv.fr

Last modified 2/7/99 by Kai Liu