Effect of AC gradient field on magnetic measurements with an alternating gradient magnetometer

The effect of the AC gradient field in an alternating gradient magnetometer (AGM) on measurements of the field dependence of magnetic viscosity and ir reversible susceptibility has been examined both theoretically and experime ntally. Two effects are predicted: (1) The AC field produces a change in th e shape of the curves by an amount which is proportional to the product of the second derivative of the curve with respect to H and the square of the effective AC field over the sample. (2) The AC field induces magnetization reversal because of the nonlinear dependence of the relaxation rate on the applied field. For symmetrically shaped curves, the first effect leads to a reduction in the peak and a broadening of the curve. The second effect cau ses the curves to be shifted towards lower values of H by an amount that de pends on the size of the AC field relative to the anisotropy field. Measure ments have been made on samples with coercive fields ranging from 64 to 120 0 Oe which are in semi-quantitative agreement with the calculations. The re sults provide guidance about how to minimize these effects when using the A GM. (C) 1999 Elsevier Science B.V. All rights reserved.