Geology Reference
In-Depth Information
2.4
Fine Particle Magnetism
2.4.1
Hysteresis
Ferromagnetism can be best described with a hysteresis plot that shows the
magnetic behavior of a collection of small ferromagnetic particles while a
field is being applied (Figure 2.3). There are four important parameters that
can be garnered from a hysteresis plot that describe the characteristics of the
ferromagnetic grains in the sample: the coercivity (H c or B c ), the coercivity
of remanence (indicated by either H cr or B cr ), the saturation magnetization
( J sat or M s ), and the saturation remanence (either J rs or M rs ). This confusing
array of letters comes from the fact that H represents magnetic field and B
represents magnetic induction. B is the combination of the magnetic field
and the magnetization induced in matter (Table 2.1). Since the magnetiza-
tion induced in air is essentially zero, B and H are virtually have the same
value in air, where most hysteresis measurements are made, and B and H are
used interchangeably. M stands for magnetic moment, and J for magnetiza-
tion, or magnetic moment normalized by either mass or volume, so they
both represent the magnetism induced in the sample by application of a
magnetic field. For the purposes of this topic, we'll stick to B and J, but the
(a)
3e-5
Inner shelf
-3e-5
Figure 2.3 Example
hysteresis loops for samples
from the Cretaceous Cupido
Formation from Mexico (see
Chapter 6). The constricted
“waist” in the top loop
probably comes from a
mixture of different size
ferromagnetic grains. Source:
Hinnov, Kodama, Anastasio,
Elrick & Latta 2013. Copyright
2013 by the Geological Society
of London.
(b)
3e-5
Middle shelf
-3e-5
-1.0T
Applied ￿eld (T)
-1.0T
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