The Magnetic Behaviour of the :111:-ORIENTED Iron Whisker.

The magnetic behaviour of {111}-oriented iron whiskers is studied as a function of magnetic field applied along the whisker length and as a function of temperature. Based on ac susceptibility measurements and observed Bitter patterns in low applied fields, a domain structure is proposed for this orientation of whisker. This is compared with observations made of {100} and the rare {110}-oriented iron whiskers. The technique of growing iron whiskers is described. Theories of whisker growth are discussed. The role of carbon in iron whisker growth and its detection in small concentrations is considered. The approach to saturation in the {111} direction for an iron whisker with its long axis in that direction is studied by ac susceptibility measurements. The data at room temperature is analyzed to give the magnetization, intrinsic susceptibility, and demagnetizing field at each of 15 cross-sections along the length for applied fields from 10 to (TURN) 1000 oe. It is concluded that mean field anisotropy theory does not account for the results. The approach to saturation in the {111} direction may represent the behaviour of the 3-state Potts model. The magnetic response of a {111}-oriented iron whisker is measured up to and through the Curie temperature. The anisotropy constant is found from the response in the range of fields between that necessary to bring the magnetization in the central cross-section of the whisker to M(,s)/SQRT.(3(' )and that to reach M(,s). We extract an anisotropy field H(,k) proportional to K(,1)/M(,s) and analyze its dependence on M(,s) and on temperature. The results indicate that K(,1)/M(,s) (TURN) M(,s)('n) with n = 3.11 (+OR-) 0.05 for the temperature range 0.0005 < (T(,c) - T)/T(,c) < 0.002 with n increasing at lower temperatures. To our knowledge this is the first observation of the power law behaviour of the anisotropy in a cubic ferromagnet just below the Curie temperature. At lower temperatures the analysis is complicated by the field dependence of the demagnetizing field. The anisotropy field H(,k) is extracted from the critical applied field which just saturates the centre of a {111} whisker at each temperature. This is accomplished by calculating the demagnetizing field using a method of modelling the sources of the demagnetizing field on the surface of a cylinder. The sensitivity of the method permits extraction of K(,1) values over five orders of magnitude. The results are compared with those of others from torque curve and FMR measurements. For the purposes of the above analysis, the temperature dependence of the spontaneous magnetization M(,s) is measured from room temperature to the Curie temperature. This is derived from the departure field which just saturates the centre of a {100} iron whisker. Minor corrections for the anisotropy are made. The results agree well with the measurements of others. A fit function to the data is given which can be used as the calibration curve of an iron whisker magnetic thermometer.