Ferromagnetic Resonance of Iron Whisker Crystals

Abstract

The ferromagnetic resonance absorption characteristics of iron single crystals (grown by K. W. DeBlois of this laboratory) have been examined between −196°C and 850°C. The crystals are of filamentary growth habit and are commonly known as whiskers. The selected crystals have 〈100〉 growth directions, and their square cross sections are bounded by {100} crystal planes. They are typically 10 μ on a side, but sizes ranging from 2 to 40 μ have been examined. Resonance absorption is observed using standard techniques with the dc magnetic field parallel to the length of the whisker. The microwave energy penetrates only a small fraction of the volume due to eddy current limitation. Two resonance modes may be observed. One is driven by the uniform microwave magnetic field, the other by its curl component. The curl mode is equivalent to resonance in a flat plane of infinite extent and no edge corrections are required. The uniform mode subject to an additional field arising from the uncompensated magnetic pole distributions at the specimen edges. After identification, the uniform mode is uninteresting because of distortions that probably result from the inhomogeneities of the edge demagnetizing field. The curl mode on the other hand exhibits a line shape and width that, in some cases, may be evaluated from first principles. The line width in these cases is accounted for almost entirely by exchange and conductivity effects, no phenomenological damping mechanism being required. Because other crystals of iron exhibit much broader resonance line widths, these results suggest that crystal perfection, particularly surfaces in the case of metals, may be the determining factor of ferromagnetic resonance line widths in bulk crystals. The data yield for the spectroscopic splitting factor, g, a temperature and frequency independent value of 2.05±0.01. The exchange stiffness parameter, A, determined in a self-consistent way with this splitting factor, is 25±5×10−7 erg/cm at 20°C.