FORC Utility

First order reversal curve (FORC) measurements provide unprecedented insight into the magnetic properties of materials, information that is not possible to obtain from a hysteresis loop alone. FORC curves help in identifying the distribution of switching and interaction fields, and in distinguishing between multiple phases in composite or hybrid materials containing more than one phase.

A FORC is measured by saturating a sample in a field H_sat, decreasing the field to a reversal field H_a, then sweeping the field back to H_sat in a series of regular field steps H_b. This process is repeated for many values of H_a, yielding a series of FORCs. The measured magnetization at each step as a function of H_a and H_b gives M(H_a, H_b), which is then plotted as a function of H_a and H_b in field space.

A FORC measurement

This FORC data acquisition utility provides a convenient way for 7400 and 7400-S Series VSM users to set up and run FORC measurements. Resulting data sets can be imported into popular FORC analysis software packages like FORCinel and VARIFORC.

The user then runs the measurement using the IDEAS software, after which the utility converts the results into a file that can be imported into FORCinel and other similar analysis packages.

FORC Analysis—R.J. Harrison, J.M. Feinberg, FORCinel: An improved algorithm for calculating first-order reversal curve distributions using locally weighted regression smoothing, Geochemistry, Geophysics, Geosystems. 9 11 (2008).

Software like FORCinel is used to view the FORC curves shown in the first diagram, and to derive FORC distribution diagrams such as the one shown above. The FORC distribution ρ(H_a, H_b) is the mixed second derivative, i.e., ρ(H_a, H_b) = -∂² M(H_a, H_b)/ ∂H_a∂H_b. Analysis packages will output graphical 2D or 3D FORC diagrams, contour plots of ρ(H_a, H_b) with the axis rotated by changing coordinates from (H_a, H_b) to H_c = (H_b - H_a)/2 and H_u = (H_b + H_a)/2. These diagrams represent the distributions of interaction fields (H_u) and of switching fields (H_c).