Transcript Magnetic Domains & Remanence Acquisition

Magnetic Domains & Remanence Acquisition
Earths Magnetic Field
Water
Slurry
Rock
……..how rocks get magnetized
Magnetic Remanence
When the magnetization of a body produces an external
field (i.e. it possess a remanence), it has magnetostatic
energy or an energy of self demagnetisation. [Atomic
magnetic moments are dipoles and can most simply be
modelled as pairs of magnetic charges. In a given
magnetic particle the magnetic charges of adjacent atoms
cancel internally within the particle but produce a
magnetic charge distribution at the surface of the
particle. For a uniformly magnetized spherical particle
one hemisphere has a positive charge and the other a
negative charge. This charge distribution gives rise to an
energy known as the magnetostatic energy.]
Magnetic Remanence
(From McElhinny & McFadden, 2000)
Rock Magnetism: Magnetostatic Energy
For a uniformly magnetized grain the magnetostatic
energy is proportional to the square of the magnetization
and the magnetostatic energy becomes extremely high for
ferromagnetic materials with a high magnetization. To
reduce this magnetostatic energy magnetic domains form
within grains, which reduce the size of regions of uniform
charge thereby reducing the surface magnetization.
Rock Magnetism: Domains
Subdivision of a ferromagnetic grain into domains.
(After Dunlop & Özdemir, 1997)
Rock Magnetism: Domains
Grains with two or more magnetic domains are termed
multi-domain. Internal to each domain the magnetization
is equal to the saturation magnetization, but because
charges of opposite sign are adjacent the net
magnetization for the grain is less than the saturation
magnetization. The region separating domains is known as
the domain wall. Domain walls themselves possess a finite
energy that is related to energy exchange between
adjacent atoms and is proportional to the area of the wall.
(After Butler, 1992)
Rock Magnetism: Domains
With increasing grain size, and increased surface area,
more magnetic domains are formed to counteract the
increase in magnetostatic energy due to the increase in
the surface area. Conversely, as grains get smaller the
number of domains decreases until the energy involved in
erecting a domain wall becomes larger than the decrease
in magnetostatic energy resulting from dividing the grain
into two domains. Grains with only one magnetic domain
are termed single-domain. The grain diameter below
which grains are single-domain is dependent on the grain
shape and the magnetization. Grains with a low
magnetization possess low magnetostatic energy and
hence little incentive to form magnetic domains.
Rock Magnetism: Domains
Single-domain, two-domain
and multi-domain grain
configurations (top) and the
likely number of domains
versus grain diameter for
magnetite (bottom). The
black bars represent the
range of grain sizes sizes for
which that number of
domains is the lowest energy
domain state.
(After Moon & Merrill, 1985;
& Van der Voo, 1990)
Rock Magnetism: Domains
Domains arise due to a balance of energies within a grain.
The major magnetic energies are; the exchange energy,
Eex; the magnetostatic or demagnetising energy, Ed; and
the anisotropy energy, Eanis.
The total magnetic energy, Etot is given by:
Etot = Eex + Ed + Eanis
The Domain State of a magnetic grain is controlled by
minimising these energies and thus is controlled by grain
size, shape and mineralogy; magnetic field, temperature,
stress and crystal defects as well as other more minor
factors.
Rock Magnetism: Domains
Exchange energy (Eex) is the energy associated with
coupling through the interaction of the electrons of
adjacent atoms. Eex is minimised by the parallel
(ferromagnetic) or antiparallel
(antiferromagnetic/ferrimagnetic) alignment of adjacent
magnetic moments.
Eex = -2JeSi•Sj
Where Si•and Sj are the spin vectors for adjacent atoms
and Je is the exchange integral. (Where Je > 0, energy is
minimised by spins being parallel (ferromagnetism).
Where Je < 0, energy is minimised by spins being
antiparallel (antiferromagnetism, ferrimagnetism).)
Rock Magnetism: Domains
Magnetostatic energy (Ed) also called the internal field
energy arises from interaction of a crystals’ magnetization
with itself. This energy is dependent on the geometry of
the crystal. It is large for equant grains and small for
elongate grains. An effect of this energy is shape
anisotropy; it is easier for magnetisation to lie along the
long axis of an elongate crystal. Shape anisotropy is thus
uniaxial with two "easy" directions of magnetisation.
Ed = ½ 0NVM2
Where N is the demagnetising factor, V is the volume and
M is the magnetisation, and 0 is the permeability of free
space.
Rock Magnetism: Domains
The anisotropy energy (Eanis), the total anisotropy energy,
is the sum of the magnetocrystalline, magnetostrictive and
magnetoelastic anisotropies.
The total anisotropic energy Eanis is thus given by:
Eanis = Ek + Estric + Eme
Rock Magnetism: Domains
The magnetocrystalline energy (Ek) arises from the
interaction of the permanent magnetic moment with the
anisotropic crystalline electric field. This simple means that
atomic dipole moments align more easily along certain
crystallographic axes than others. In magnetite, the [111]
axes are the preferred orientation of magnetisation in the
absence of an external field. This gives a cubic crystalline
anisotropy and therefore 8 "easy" orientations of
magnetisation. Shape anisotropy is a much stronger effect
than crystalline anisotropy and dominates over it for
elongations greater than a few percent.
Rock Magnetism: Domains
Magnetization of a single
crystal of magnetite along
different crystallographic
axes. [111] is the
magnetocrystalline easy
axis, while [100] is the
magnetocrystalline hard
axis.
(After Nagata, 1961)
Rock Magnetism: Domains
The magnetostrictive energy (Estric) arises from the fact that
when a crystal is magnetised it changes shape. This
introduces the possibility of magnetically induced mechanical
stress in grains where the whole volume is not magnetised in
the same direction.
Rock Magnetism: Domains
The magnetoelastic energy (Eme) results from the effects
of stress on a crystal, which alters the direction of
spontaneous magnetisation. The origins of stress may be
external (macrostress) or internal, (microstress), due to
crystal imperfections e.g. dislocations, inclusions etc.
Microstress is important when considering intra-crystal
magnetic processes and remanence.
Rock Magnetism: Domains
The total anisotropic energy Eanis is thus given by:
Eanis = Ek + Estric + Eme
For magnetically uniaxial grains Eanis = KVsin2
Where K is the anisotropy constant, V is the grain volume
and  is the angle between the direction of magnetization
and the easy axis.
Rock Magnetism: Domains
Whether a grains will subdivide into two or more magnetic
domains is strongly influenced by the size of the grain and
at the critical size the energy of a single domain grain will
be the same as the energy of the 2 domain state + the
energy required to erect a domain wall between the two
domains.
ESD = E2D + EW
ESD = ½ 0NVM2
For a sphere N is 4/3 and the volume = d3/6 where d is
the diameter.
Rock Magnetism: Domains
The energy of the wall (EW) = wLW where w is the wall
energy per unit area, L is the length of the wall and W is
the width of the wall.
Combining the above equations gives a critical diameter d
such that:
D = 4w/0NSDM2
If we substitute values for magnetite the critical diameter
comes out to be about 0.04m. This is actually less than
the wall width for the domain walls in magnetite (about
0.2m).
Rock Magnetism: Single domain grains
The theory of the magnetization of an assemblage of
single-domain particles is essentially due to the work of
Néel (1955). The grain-size below which particles are
single-domain is termed the single-domain threshold grain
size (d0). This size is dependent on the grain shape and
saturation magnetization. For haematite, which has a low
saturation magnetization, this grain diameter is circa.
15m, so a large proportion of haematite encountered in
rocks is single-domain. Magnetite has a much higher
saturation magnetization, and for cubic magnetite the
critical diameter for single domain behaviour is
approximately 0.05m. Elongate magnetite particles may
still be single-domain up to 1m. Single-domain particles
can be very efficient carriers of remanent magnetization.
Rock Magnetism: Single domain grains
Magnetite at 290°K
(After Butler & Banerjee, 1975)
Rock Magnetism: Single domain grains
During demagnetization the net magnetic moment of a
single-domain grain cannot be reduced by internal
cancellation of domain movements through domain wall
movement. Instead, magnetic moments can only be made
to change direction or rotated toward the applied field.
However, there are resistances to the rotation of the
magnetization, the dominant ones being shape anisotropy
and magnetocrystalline anisotropy.
Rock Magnetism: Single domain grains
Shape anisotropy is related, as the name suggests, to the
shape of the grains. Highly elongate grains have a much
lower magnetostatic energy when magnetized along their
length rather than perpendicular to their length. This is
because the percentage of surface area covered by
magnetic charges is small when the magnetization lies
along the long axis. Magnetization perpendicular to the
long axis produces a substantial surface charge. The
magnetic charge distribution produces a field internal to
the grain, called the internal demagnetizing field, which
opposes the magnetization of the grain. Therefore the
internal demagnetizing field perpendicular to the long axis
will be greater than that along the long axis.
Rock Magnetism: Single domain grains
The difference in magnetization along and perpendicular
to the long axis gives rise to a difference in magnetostatic
energy, which represents a barrier to rotation of the
magnetization through the perpendicular direction. To
force the magnetization to rotate through this barrier an
external magnetizing field is required, which is known as
the microscopic coercive force. For needle-shaped singledomain magnetite, at room temperature, this field
reaches a maximum of approximately 300mT, though in
naturally occurring magnetite this extreme shape
anisotropy is rarely encountered. More usually coercivities
of magnetite lie within the range 30-70mT. The coercivity
of haematite is at least 0.1T and usually higher.
Physics of Magnetism: Hysteresis
J
JS
JR
HC
H
Hysteresis
Loop
When a ferromagnet is subjected to a
cyclic change in the external field the
magnetisation is not directly
proportional to the applied field by
there is a lag in the magnetisation,
which is known as hysteresis. H is the
applied field, J is the induced
magnetization. Js is the saturation
magnetization, Jr is the saturation
remanence and Hc is the coercivity.
The various hysteresis properties are
not solely intrinsic properties but are
dependent on grain size, domain
state, stresses and temperature.
Because hysteresis parameters are
dependent on grain size, they are
useful for magnetic grain sizing of
natural samples.
Physics of Magnetism: Hysteresis
(After Butler, 1992)
Natural Remanent Magnetization (NRM)
The natural remanent magnetization (NRM) of a rock is the
magnetization present in a rock prior to laboratory
treatment and depends on the geomagnetic field and
geological processes that have operated on the rock during
and after its formation. NRM can typically contain a
number of components of magnetization of differing ages.
The NRM component acquired during the formation of a
rock is usually referred to as primary NRM and those
acquired subsequent to rock formation as secondary NRMs.
Secondary NRMs can often mask or sometimes completely
obscure the primary NRM. Therefore understanding the
modes of acquisition of NRM by a rock is of critical
importance in interpreting the significance of measured
components of NRM.
Natural Remanent Magnetization (NRM)
The principal forms of NRM are:
1)
2)
3)
4)
Thermoremanent magnetization
Chemical remanent magnetization
Detrital remanent magnetization
Viscous remanent magnetization.
NRM: Thermoremanent Magnetization (TRM)
A thermoremanent magnetization (TRM) is the NRM
produced in a rock when cooling it from above the Curie
temperature in the presence of a magnetic field. However
this magnetization will, in time, reach magnetic
equilibrium with the surrounding field. A measure of this
time is the relaxation time (t). Relaxation time is given in
the equation:
vBC J S
1
t  exp(
)
C
2kT
where C is a frequency factor, v is the volume of the grain,
Bc is the grain's coercivity, Js is the spontaneous
magnetization, k is Boltzmans constant and T is the
absolute temperature.
NRM: Thermoremanent Magnetization (TRM)
(After Van der Voo, 1990)
NRM: Thermoremanent Magnetization (TRM)
1
vBC JS
t  exp(
)
C
2kT
(After Butler, 1992)
NRM: Thermoremanent Magnetization (TRM)
One of the most important features of this equation is that
the relaxation time is strongly dependent on the absolute
temperature and directly related to the coercivity. At the
Curie temperature the mineral will have a short relaxation
time and will rapidly align with the applied field.
The temperature at which a particular mineral acquires its
magnetization is known as its blocking temperature (Tb). As
a mineral phase can have a blocking temperature below the
Curie point, albeit with a longer relaxation time, a TRM can
be acquired over a range of blocking temperatures that are
distributed from the Curie point down. As the temperature
decreases through the Tb of an individual grain, the grain
experiences a large increase in its relaxation time,
effectively ‘freezing in’ the magnetization relative to
geological or experimental time scales.
NRM: Thermoremanent Magnetization (TRM)
As the rocks cool they record
the direction of the Earths
magnetic field
Ascending Magma
In the case of igneous rocks
the magnetization is
acquired as the rock cools
through the Curie
temperature of the
particular magnetic mineral
(at temperatures above the
Curie Temperature magnetic
minerals lose their magnetic
properties). As the mineral
cools through the Curie
Temperature it retains a
record of the direction and
strength of the Earth’s
magnetic field.
NRM: Thermoremanent Magnetization (TRM)
If the acquisition of a
magnetization through a
range of Tbs is regarded
as a stepwise process,
the TRM acquired over a
particular temperature
interval is termed
Partial TRM or PTRM.
The sum of all PTRMs
should give the total
TRM (Thellier, 1951).
(After Van der Voo, 1990)
NRM: Thermoremanent Magnetization (TRM)
A theoretical model for the acquisition of TRM in single
domain ferromagnetic grains was given by Néel (1955). This
model adequately explained the acquisition of TRM in cases
where the assemblage of grains has a uniaxial anisotropy
and grains are of equal size with a single Tb. In practice
rocks would be expected to have a random distribution of
isotropic axes and a variety of grain sizes with
corresponding variations in Tb.
NRM: Chemical Remanent Magnetization (CRM)
Chemical remanent magnetization (CRM) is produced by
chemical reactions involving ferromagnetic minerals
including precipitation of a new magnetic mineral phase and
alteration of pre-existing minerals (both ferromagnetic and
non-magnetic). The new ferromagnetic mineral ‘locks in’ a
record of the earth's magnetic field direction at the time of
its formation. In contrast with TRM, where grains acquire a
magnetization at a constant volume and decreasing
temperature, CRMs are acquired at a constant temperature
with changes in volume.
1
vBC JS
t  exp(
)
C
2kT
NRM: Chemical Remanent Magnetization (CRM)
During chemical formation of a ferromagnetic mineral grains
grow from a zero initial volume. Newly nucleated particles
are small, have short relaxation times and are superparamagnetic. During the growth of the grains they become
ferromagnetic and relaxation times increase dramatically.
The diameter at which the grains change from being superparamagnetic to ferromagnetic is known as the blocking
diameter (0.02m for haematite. As grains pass through the
blocking diameter they record the applied magnetic field,
and continued grain growth can produce a remanent
magnetization that is stable over geological time.
NRM: Chemical Remanent Magnetization (CRM)
1
vBC JS
t  exp(
)
C
2kT
(After Butler, 1992)
NRM: Chemical Remanent Magnetization (CRM)
A number of factors govern the rate of CRM acquisition.
Chemically immature sediments (those with an abundance of
low-oxidation-state minerals) experience more rapid
oxidation than chemically mature sediments and therefore
acquire most of their CRM quickly. Therefore the chemical
maturity of the sediment is a possible tool in recognising
whether a sediment acquired its CRM rapidly or over a longer
period of time. Secondly, the grain size of the sediment is of
importance, given that fine-grained sediments have a larger
surface to volume ratio and are likely to undergo more rapid
chemical changes than coarser sediments. Finally the plaeoclimate and depositional environment also play a role. An
oxygenating depositional environment is much more likely to
result in rapid oxidation and warm moist paleo-climates tend
to prolong the magnetization process in red beds.
NRM: Detrital Remanent Magnetization (DRM)
Detrital remanent magnetization (DRM) is produced by the
alignment of small magnetized particles during the
deposition and lithification of sediments. The acquisition of
DRM is a complicated process given the large number of
processes involved during the formation of sedimentary
rocks. There is a large variety of initial mineralogies, many
minerals not being in equilibrium with each other or their
depositional environment, and sediments are subject to
large variety of post-depositional processes, such as
bioturbation, prior to lithification.
NRM: Detrital Remanent Magnetization (DRM)
Earths Magnetic Field
Water
Slurry
Rock
(Modified
after Cox &
Hart, 1986)
In sediments magnetic minerals make up a tiny proportion of the rock (<0.1%). As the
magnetic grains sink through the water column they align themselves with the
ambient magnetic field (in this case the Earth’s magnetic field). When the sediment
accumulates and solidifies into a rock the magnetic grains become ‘locked in’ and
thus preserve a record of the magnetic field at the time of formation of the rock.
NRM: Detrital Remanent Magnetization (DRM)
The classic model of DRM acquisition was proposed by
Collinson (1965) which dealt only with the aligning effect of
the applied magnetic field on a particle at the sedimentwater interface. This yielded a characteristic alignment
time of 1 second, implying rapid and complete alignment of
ferromagnetic particles with the geomagnetic field.
However this model does not hold for natural cases or
laboratory experiments as a number of other important
considerations were not taken into account.
NRM: Detrital Remanent Magnetization (DRM)
Firstly, magnetic grains and especially inequidimensional
ones interact with each other, and the final alignment of the
grains is therefore a compromise between alignment with
the magnetic field and adjacent magnetic particles.
Secondly, laboratory experiments indicate that the
inclination of DRM tends to be consistently shallower than
the applied field. A simple explanation for this is that grains
tend to be magnetized along the long axis of the particle
due to shape anisotropy. These long axes are subject to
gravitational torque which tends to rotate them toward the
horizontal.
NRM: Detrital Remanent Magnetization (DRM)
There are a number of problems with this explanation as
measurements of the inclination error in natural sediments
tend to be less than those in laboratory experiments,
indicating that there are other factors to be taken into
account, notably the possibility of further post-depositional
DRM (pDRM). PDRM has been attributed to Brownian motion,
where magnetized particles are reoriented by the Brownian
motion of the surrounding water. PDRM has been shown to
be free of the inclination problem of DRM and it seems
likely that DRMs in natural sediments are DRMs with some
portion of later pDRM.
NRM: Detrital Remanent Magnetization (DRM)
Experimental production
of PDRM in the laboratory
plotted versus the
inclination of the applied
field.
(After Kent, 1973)
Most sediments, and particularly red beds, are thought to
carry CRM, both DRM and pDRM being subject to CRM
overprinting early in the diagenetic history of the rock.
NRM: Viscous Remanent Magnetization (VRM)
Viscous remanent magnetization (VRM) is the magnetization
acquired during exposure to weak magnetic fields. It is
proportional to the intensity of the ambient field and
proportional to the logarithm of the time of exposure to the
field. VRM at a given temperature is given by;
VRM  S log t
where t is the time of exposure to the field and S is the
viscosity coefficient. The viscosity coefficient (S) has been
shown to be proportional to temperature. Because of the
logarithmic growth of VRM with time, viscous
magnetizations tend to be dominated by recent magnetic
fields and generally rocks with a high proportion of VRM
tend to have NRM aligned with the present geomagnetic
field.
NRM: Isothermal Remanent Magnetization (IRM)
Isothermal remanent magnetization (IRM) is acquired in the
presence of a direct field at a constant temperature. IRM
curves or hysteresis loops are often used in laboratory
experiments to identify magnetic carriers in rocks. A
demagnetized sample is subjected to an applied magnetic
field (H) and the induced magnetization (Ji) is then
measured. The induced magnetization per unit volume (J) is
plotted against H, where H is increased in a series of steps
up to maximum and then reversed and increased to a
maximum in the reversed direction. The resultant hysteresis
loop is characteristic of the remanence carrier(s) in the
rock. The maximum induced magnetization is known as the
saturation magnetization (Js) and depends linearly on the
concentration of the ferromagnetic mineral involved.
Physics of Magnetism: Hysteresis
J
JS
JR
HC
H
Hysteresis
Loop
When a ferromagnet is subjected to a
cyclic change in the external field the
magnetisation is not directly
proportional to the applied field by
there is a lag in the magnetisation,
which is known as hysteresis. H is the
applied field, J is the induced
magnetization. Js is the saturation
magnetization, Jr is the saturation
remanence and Hc is the coercivity.
The various hysteresis properties are
not solely intrinsic properties but are
dependent on grain size, domain
state, stresses and temperature.
Because hysteresis parameters are
dependent on grain size, they are
useful for magnetic grain sizing of
natural samples.
NRM: Isothermal Remanent Magnetization (IRM)
The applied field (H) required to achieve saturation
(>700mT for haematite, 300mT for magnetite) can be used
as an indication of the identity and domain states of the
magnetic carriers. The reversed field required to reduce Js
to zero is known as the coercivity of remanence (Hcr).
Typical values of Hcr for magnetite are 20-80mT and >300mT
for haematite, though higher values are not unknown,
indicating hard magnetic components.
NRM: Isothermal Remanent Magnetization (IRM)
In nature occurrences of IRM
tend to be restricted to
outcrops that have been
subjected to lightning strikes.
Electrical currents of lightning
can exceed 104-105 amperes
and induce magnetic fields of
up 10mT within 1m of the
strike. These can generally be
recognised by abnormally high
NRM intensities and in some
instances by a high scatter in
the NRM directions.
www.gsfc.nasa.gov/
NRM: Isothermal Remanent Magnetization (IRM)
The current travels radially
from the point of impact, and
the distance it travels depends
on the conductivity of the
rock, and whether it is wet.
The resulting magnetic
directions are usually highly
scattered.
NRM: AF Demagnetization
(After Van der Voo, 1990)
Alternating field (AF)
demagnetization is achieved
by the cycling of a magnetized
rock sample through hysteresis
loops with decreasing
amplitude in a zero Dc field.
The magnetic moment of
grains with a coercivity less
than the peak field applied is
thereby nullified. The process
is repeated for successively
higher fields until the NRM is
effectively demagnetized or
the maximum peak field is
attained.
NRM: Thermal Demagnetization
Progressive thermal demagnetization is achieved by
stepwise heating to the maximum unblocking temperature.
Samples are then cooled in a field-free chamber which
allows for random orientation of particles or domain
moments. The magnetization of the rock sample is
measured at room temperature between each heating
cycle. The lower blocking temperature components are
progressively removed leaving those of high thermal
stability. Treatments are typically in 20-100°C steps though
this depends on the nature of the NRM. Treatments are
usually concentrated over temperature intervals where a
large proportion of magnetic grains un-block, (i.e. a
thermally discrete spectrum or where detailed analysis is
required.
NRM: Thermal Demagnetization
(After Van der Voo, 1990)
NRM: Thermal Demagnetization
(After Butler, 1992)
NRM: Thermal Demagnetization
(After Butler, 1992)
NRM: Thermal Demagnetization
Normalized Intensity
1.0
0°
200°
400°
Temperature (°C)
600°
NRM: Thermal Demagnetization
One drawback with the technique is that the heating cycle
can produce mineralogical alteration within the rock and
for the magnetic minerals these alteration products
include; production of Maghemite from Magnetite at 150250°C, Hematite from Maghemite at 350-450°C, Hematite
from Magnetite at >500°C and the reduction of Hematite to
Magnetite at >550°C. Alterations such as these will change
the magnetic characteristics of the sample and it becomes
particularly important that field-cancellation in the furnace
be as complete as possible to avoid the acquisition of
spurious TRM during cooling.
Magnetism in Oxides
IlmenoHaematite
series
TitanoMagnetite
series
NRM: Thermal Demagnetization
Up, W
NRM
1.0
520oC
530oC
540oC
550oC
500mA/m
N
2
GP35
(Hornblendite)
4
6
x100 oC
8