Magnetomechanical effects under torsional strain in iron, cobalt and nickel

The change of magnetization as a function of applied torque has been invest igated for cylindrical rods of 99.9% pure iron, cobalt and nickel. The rela tionship between the magnetomechanical effect and magneto strict ion in the se materials has also been investigated. Torque was applied with amplitude up to +/-5 N in with no associated bending moment. The magnetic field near the sample surface was measured using a Hall effect sensor at the center of the sample. The results showed a linear dependence of this field on torque once the transient effects of the first few stress cycles had been overcom e. The derivative of the surface circumferential magnetic field with respec t to torque for a nickel rod was 19A N-1 m(-2). Cobalt and iron, on the oth er hand, showed smaller sensitivity to torsional stress, with changes less than 1 A N-1 m(-2). The difference in behavior is due to the higher ratio o f magnetostriction to anisotropy in nickel. Magnetostriction measurements w ere made and it was found that the piezomagnetic coefficient d, that is the slope of the magnetostriction curve d lambda /dH, at low magnetic field is an important figure of merit in determining sensitivity of magnetization t o torque. It is shown that the sensitivity of magnetic induction to stress dB/d sigma is dependent on the ratio of magnetostriction to anisotropy. (C) 2001 Published by Elsevier Science B.V.