Geology Reference
In-Depth Information
as close to zero as possible). In this case, if the sample has reached
saturation, the saturation remanence is obtained (J
rs
) when the field is
reduced to zero. For a collection of small,
single domain (SD)
grains, the
ratio of J
rs
to J
sat
is 0.5, if the magnetizations of the grains are randomly
oriented in space. J
rs
is also called the
saturation isothermal remanent
magnetization (SIRM)
.
In order to decrease a sample's magnetization to zero, i.e., to totally demag-
netize a sample, the field must be applied in the opposite direction. When
the sample's magnetization is zero, with the field still turned on, the value of
the field needed to maintain zero magnetization is called the coercivity of
the sample (B
c
). The coercivity is a measure of the “hardness” of the sample's
magnetization, or how much the sample needs to be coerced with an applied
field to force its magnetization down to zero. If, however, the field is then
turned off, the magnetization will return to some positive value. In order to
have the sample stay totally demagnetized when the field is turned off, the
field must be increased in the opposite direction to the coercivity of rema-
nence value (B
cr
) that is greater than the coercivity (B
c
). The sample actually
becomes magnetized in the opposite direction while the field is applied, but
when the field is turned off, the sample's magnetization goes to zero and is
completely demagnetized.
Often the hysteresis parameters are plotted as ratios and are used to easily
summarize the magnetic characteristics of the sample with one point on a
plot. This is called the Day plot (Day et al. 1977) in which J
rs
/J
sat
is plotted as
function of B
cr
/B
c
(Figure 2.5). Points in the upper left hand corner of the plot
(J
rs
/J
sat
~ 0.5 and B
cr
/B
c
~ 0.5 indicate SD magnetic behavior while points in
the lower right (J
rs
/J
sat
~ 0.05, B
cr
/B
c
~ >5) indicate a collection of grains with
many magnetic domains. Unfortunately, in reality, many samples plot in the
central region of the plot indicating either a third kind of magnetic behavior:
pseudo single domain (PSD)
(grains that have multiple domains but behave
like they have just one domain) or a mixture of SD and
multidomain (MD)
grains. Because the Day plot isn't too useful in distinguishing magnetically
between samples, it has fallen out of favor for describing the magnetization of
samples. A modification of the Day plot in which J
rs
/J
sat
is plotted as a function
of B
c
is also used to evaluate magnetic particle size (Figure 2.5). FORC dia-
grams are currently considered one of the best ways to describe a sample's
ferromagnetic behavior.
Before starting a rock magnetic cyclostratigraphic study, it is important
to identify the magnetic minerals in the rocks so the appropriate rock
magnetic parameter can be chosen to construct the time series. A critical
determination is whether the magnetic mineralogy of the rocks is domi-
nated by magnetite or hematite. One of the best ways to make the determi-
nation is to conduct an IRM acquisition experiment. In this experiment, a
sample is exposed to higher and higher DC magnetic fields, usually in an
impulse magnetizer that creates a brief, but strong, magnetic field by dis-
charging a capacitor through a coil that surrounds the sample. After
exposure to the field, the sample is measured. The field is increased until the
