Geology Reference
In-Depth Information
Ely 26
10
5
Figure 2.4 FORC
distribution diagram
from a Lake Ely sediment
sample showing a 10-40 mT
coercivity range for the
ferromagnetic magnetofossils
in the sediment. The spread
on the H u axis suggests some
magnetic interactions
between the particles.
0
-5
-10
0
20
40 60
Hc (mT)
80
100
reader is warned that M and H, meaning essentially the same parameters,
can show up in different publications. The ratios of the four hysteresis
parameters indicate something about the domain state (see Section 2.4.3) of
the magnetic grains and hence something about their magnetic grain size.
A more sophisticated use of hysteresis behavior is first-order reversal curve
(FORC) distribution diagrams (Figure 2.4), in which multiple hysteresis
loops are acquired for a sample over a great range of field strengths. Analysis
of FORC diagrams indicates not only domain state and hence magnetic
particle grain size but also whether there are magnetic interactions between
the magnetic grains in a sample. Tauxe (2010) provides a more detailed
explanation of FORC diagrams.
The main point about the hysteresis loop is it indicates that ferromag-
netic particles have irreversible magnetic behavior. As the applied field
(B) is increased, the magnetization (J) of the sample increases in the same
direction. If the field is only increased to small values, similar to that of
the Earth's field (~50 μT), when the field is turned off, the magnetization
of the sample also returns to zero. This is the only reversible behavior for
a ferromagnetic substance and is denoted as ferromagnetic susceptibility.
It is much greater in magnitude than paramagnetic susceptibility, but the
macroscopic behavior is similar. If the field is increased beyond these low
values, the magnetic behavior is no longer reversible and when the field
is turned off, the sample retains a magnetization called an isothermal
remanent magnetization (IRM) . If the field is then increased and the
magnetization levels out and no longer increases, the magnetization is
saturated and denoted by J sat . J sat is a direct measure of the total number of
magnetic grains in a sample, but can only be measured while the field is
applied. In most paleomagnetic laboratories the magnetometers used can
only measure the sample when the field is turned off (and, in fact, brought
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