Magnetoelastic interactions in iron whiskers

Abstract

The effect of stress on the nucleation fields of iron whiskers was investigated. The whiskers were mounted between a movable rod and a stationary rod. Tension was applied to the whiskers by displacing the tops of a pair of E-shaped leaf springs relative to the centers of the springs to which the movable rod was bonded. The nucleation field I was taken to be the applied magnetic field necessary to initiate the reversal of the magnetization of a whisker. The nucleation fields were determined using an assembly of three small coils. The cross-sectional areas of the whiskers were estimated by four methods. Sixty-nine whiskers were tested. The nucleation field at zero stress was measured for 62 of these, an only the breaking stress for the other 7 whiskers was determined. The nucleation fields of 13 whiskers were measured as a function of stress. For 5 of these thirteen whiskers, a small section was found for which the nucleation fields at zero stress approached 565 oersteds-- the value predicted for an ideal crystal. These sections were assumed to be near perfect in crystalline structure. Applying tension to the whiskers apparently caused imperfections to be propagated through, the whiskers. When the imperfections approached the near perfect part of the whisker, a deterioration of the nucleation field was observed. Before this effect became active, the nucleation fields increased linearly with tension. The average slope of the straight lines which represented this data was found 2 to be 2.94 +- .32 oe/kg/mm^2 . These results agree with micromagnetics theory which predicts that the nucleation fields of a perfect crystal should be a linear function of stress with a slope equal to 3 lambda_100/M_s, where lambda_100 is the saturation magnetostriction constant in the [100] direction and M_s is the saturation magnetization. If M_s is taken as 1714 emu/cc, then the value of lambda_100 determined from the data accumulated in this experiment is (17.5 +- 1.9) x 10^-6. This value is within the range of values reported in the literature. The nucleation field was also measured as a function of stress for eight imperfect crystals. For these crystals, it was found that the nucleation field was a linear function of stress with a slope which tended to depend upon the degree of crystalline perfection (as indicated by the nucleation field at zero stress). The behavior of the nucleation fields of the whiskers tested in this experiment was comparable to that reported by other investigators. The breaking stresses of the whiskers tested were found to be a factor of five lower than those reported by S. S. Brenner.