F. Y. Yang

Giant positive magnetoresistance of Bi nanowire arrays in high magnetic fields

We have studied the magnetoresistance of electrodeposited Bi wires with diameters between 200 nm and 2 μm in magnetic fields up to B=55 T. In zero field, the resistance increases with decreasing temperature, indicating that the mean free path is strongly influenced by the nanowire geometry. The high-field magnetoresistance shows strong dependence on field orientation; typically 200% for B parallel to the wires, and 600%–800% for B perpendicular to the wires. The perpendicular magnetoresistance is well described by a modified two-current model which suggests that the high-field response of the arrays is fairly insensitive to the wire diameter, and is dominated by bulk properties of Bi.

Very large magnetoresistance in electrodeposited single-crystal Bi thin films (invited)

Single-crystal bismuth thin films, fabricated by electrodeposition and suitable annealing, exhibit very large magnetoresistance of 400 000% at 5 K and 300% at 300 K, as well as pronounced Shubnikov–de Haas oscillations. A hybrid structure demonstrates the potential for field sensing with sensitivities of 35%/Oe at 5 K and 0.2%/Oe at 300 K.

Large magnetoresistance and finite-size effect in electrodeposited bismuth lines

Finite-size effects in Bi lines of cross section 3 μm×w, where w=8–120 μm, have been studied. The lines were made by optical lithography followed by electrodeposition. At T=5 K, the magnetoresistance (MR) decreases monotonically with decreasing line width of the Bi films, from 44 000% at w=120 μm to 4000% at w=8 μm in a field of 50 kOe. The decrease of the MR with decreasing linewidth is due to a decrease of the size-limited effective-mean-free path. At low temperatures, both the resistivity and the magnetoresistivity show a strong dependence on w, whereas at room temperature, they are independent of w. The MR at room temperature remains at about 230% for linewidths w=8–120 μm at 50 kOe. This demonstrates that microstructured Bi lines can be made for field-sensing applications without compromising their MR characteristics.

Magnetoresistance anisotropy of a Bi antidot array

We have measured the magnetoresistance of a Bi antidot array film. The magnetoresistance exhibits a pronounced angular dependence on the direction of the magnetic field, e.g., the longitudinal relative magnetoresistance at 5 K displays an angular variation between 1.8 and 3.4 in a field of 4.9 T. The effect is the result of the interference between cigar-shaped regions of strong current distortions around the voids in the Bi film. The measurements can be understood and quantitatively reproduced by theoretical calculations.

Exchange coupling between Cr and ferromagnetic thin films

Exchange bias has been observed in Ni81Fe19/Cr bilayers. The exchange bias field for Cr thickness tCr larger than 62 A increases with tCr and levels off at tCr=150 A, whereas coercivity increases without saturation. The blocking temperature increases with the Cr thickness. Due to the presence of commensurate spin density waves in the thicker but strained Cr layers, blocking temperature as much as 425 K has been observed. Exchange bias has also been observed in Co/Cr bilayers.