Internal friction in nickel near the Curie temperature

Abstract

The PUCOT has been used at 40 kHz to measure the internal friction in polycrystalline Ni over the temperature range 600–670 K for several values of applied magnetic field. The curves of the amplitude independent internal friction as a function of temperature showed maximum-minimum combinations near T c in agreement with related results of other authors. These anomalies near T c were analyzed as peaks or dips in damping. Above a field of 50 mT there was saturation of the peak height and depth of minimum. Both the peaks and dips showed thermal hysteresis suggesting that a nucleation and growth mechanism occurs near T c. It is possible that the peak or dip is associated with diffusion involving matrix atoms and clusters of atoms of the major impurities (C, O or Si). The position of the minima led to an acceptable estimate for T c of 631 K. From amplitude dependence studies the estimates for the minor loop length of the ultrasonically vibrating dislocation line (55–230 nm) were found to be less than the magnetic domain sizes (∼ 10 −5 m) in Ni confirming that the pinning of dislocations takes place on a much finer scale than the domain size. Some of the internal friction data agreed well with the predictions from the G-L theory (17) and support the damping mechanism due to pinned dislocations. Other results did not agree with the G-L model making further studies with high-purity nickel single crystals a worthwhile follow-up to this study.