Richard Craig – EngD Systems Thinking in Cyberdefence

Here is an EMSA conference paper on some work from my BEng final year project.

S. I. Sandaccia[1], R. Craig, D. P. Makhnovskiya[1], and L. V. Paninaa[1]
[1]School of Computing, Communications and Electronics, University of Plymouth, Drake Circus, Plymouth, PL4 8AA, United Kingdom.

Co-rich amorphous magnetic wires exhibit very large change in high frequency impedance, when subjected to a dc axial magnetic field ex H .[1] This magneto-impedance (MI) effect is originated from the skin effect in conjunction with the transverse magnetisation processes, which can be very sensitive to ex H for certain magnetic configurations. Applying a stress or a torque also can cause the change in the wire transverse magnetisation and very large variations in impedance [2]. This paper investigates the effect of applied tensile stress on MI in glass-coated Co68.5Mn6.5Si10B15 amorphous wires at frequencies 0.5−3 GHz and discusses the application of this effect for wireless stress/pressure sensors and stress-tuneable microwave devices.

The complex-valued impedance is found by measuring S11-parameter (reflection) by means of the Hewlett-Packard 8753E Vector Network Analyser (VNA) with a specially designed microstripe cell to minimise the post calibration mismatches. The wire is loaded with a weight in its centre. The cell is placed into the Helmholtz coil producing a dc magnetic field. Figure 1 shows the plots of impedance vs. applied stress for two frequencies 500 MHz and 2.5 GHz with ex H as a parameter. A large stress effect on MI appears only in the presence of the field and is in the range of 100% and 60% per 180 MPa at 500 GHz and 2.5 GHz, respectively, for ex H =3 Oe. These experimental results are in very good agreement with theory. Co-based amorphous wires have a circumferential anisotropy originated by coupling between negative magnetostriction and dominant tensile axial stress. Then, the applied tensile stress alone will not cause the change in the direction of the equilibrium magnetisation, and as a result, will not produce noticeable changes in the impedance at high frequencies. Applying the field ex H of the order of the anisotropy field saturates the wire in the axial direction. The tensile stress which enlarges the circumferential anisotropy in the case of negative magnetostriction acts in opposite way and rotates the magnetisation back to the circular direction. This process is accompanied by large changes in the high frequency wire impedance, which has been experimentally proven in our previous works on microwave MI.

Tensile stress at GHz frequencies

References:
[1] D. P. Makhnovskiy, L. V. Panina, and D. J. Mapps, Phys. Rev. B 63, 144424 (2001).
[2] L.P. Shen, T. Uchiyama, K Mohri, and K. Bushida, IEEE Trans. Magn. 33, 3355 (1997).
[3] S. Sandacci, D. P. Makhnovskiy, L. V. Panina, J. Magn. Magn. Mater., accepted (2004).