The S-ratio is commonly used in rock magnetic and enviromagnetic studies to quantify the relative abundances of minerals with high and low remanent coercivities. As most frequently defined (e.g. by Evans and Heller, 2003), it is calculated as −IRM−300mT / SIRM, where SIRM is the saturation remanence magnetization, and IRM−300mT is the remaining magnetization after applying a 300mT reverse field to a saturated sample. In my experience, the usual procedure for determining the S-ratio on a discrete sample is:
Apply a saturating field with a pulse magnetizer (or, at least, apply the strongest field possible with the equipment).
Measure the saturation magnetization in a magnetometer.
Apply the 300mT reverse field with the pulse magnetizer.
Measure the new magnetization in the magnetometer.
On a large set of samples, things can be made quicker by working in batches and using an automated sample handling system, but it's still necessary to make two trips to the pulse magnetizer and two trips to the magnetometer. It struck me recently that, with a cryomagnetometer capable of measuring all three axes of magnetization simultaneously, a faster and more convenient procedure should be possible:
Apply the saturating field along (say) the x axis of the sample.
Apply the 300mT field along an orthogonal axis -- the y axis, say.
Measure the total magnetization vector in a 3-axis magnetometer.
Calculate the SIRM as the sum of the magnetizations along the x and y axes, and IRM−300mT as the magnetization along the y axis.
In fact, a similar (but far more involved) technique is already commonly applied: the triaxial IRM and thermal demagnetization protocol of Lowrie (1990), which involves applying decreasing IRMs along three orthogonal axes. In Lowrie's technique, however, there is no mention of calculating the SIRM as the sum of the orthogonal components.
Is anyone already using this technique? Are there any obvious pitfalls I've missed? I am tempted to try it the next time I need to determine an S-ratio, after a pilot study to confirm that the SIRMs are additive in the way I assume.
Evans, M. E. and Heller, F. (2003). Environmental magnetism: principles and applications of enviromagnetics. San Diego: Academic Press.
Lowrie, W. (1990). Identification of ferromagnetic minerals in a rock by coercivity and unblocking temperature properties. Geophysical Research Letters, 17(2), 159–162.