Viscous Remanent Magnetization
Encyclopedia
Viscous remanent magnetization (Abbreviated VRM), also known as viscous magnetization, is remanence
that is acquired by ferromagnetic materials by sitting in a magnetic field
for some time. The natural remanent magnetization
of an igneous rock
can be altered by this process. This is generally an unwanted component and some form of stepwise demagnetization must be used to remove it.
. In the simplest case, there is a single characteristic time called the thermal relaxation time. If the starting magnetization is and the magnetization in equilibrium
is , the magnetization after a time is
If is a natural remanent magnetization
(NRM) acquired in the Earth's field at one time, and is the equilibrium magnetization in a different field at some later time, the change in remanence is the VRM.
The above equation is applicable to the simplest single domain
magnets, those described by the Stoner–Wohlfarth model. The relaxation time depends on factors such as the size of the magnet and its magnetic anisotropy
. Rocks have collections of magnetic minerals with varying size and anisotropy, and therefore a broad spectrum of relaxation times. The magnetization tends to have a logarithmic dependence on time, so the rate of change is often represented by a viscosity coefficient
(NRM) in a rock, acquired when the rock was originally formed. Viscous remanent magnetization is regarded as noise. Any component of the NRM that is in the direction of the present Earth's field is suspect because it may have been acquired since the last geomagnetic reversal
. Fortunately, VRM is often removed by the first steps in a stepwise thermal demagnetization of the NRM.
VRM may also be acquired in the laboratory while measuring the NRM. To avoid this, paleomagnetists make their measurements in a magnetically shielded environment. Often the magnetometer
is housed in a room with walls made of mu-metal
.
Remanence
Remanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...
that is acquired by ferromagnetic materials by sitting in a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
for some time. The natural remanent magnetization
Natural Remanent Magnetization
Natural remanent magnetization is the permanent magnetism of a rock or sediment. In some forms, it can preserve a record of the Earth's field and the tectonic movement of the rock over millions of years...
of an igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
can be altered by this process. This is generally an unwanted component and some form of stepwise demagnetization must be used to remove it.
Origin of VRM
Viscous remanent magnetization is the result of jumps between magnetic states driven by thermal fluctuationsThermal fluctuations
In statistical mechanics, thermal fluctuations are random deviations of a system from its equilibrium. All thermal fluctuations become larger and more frequent as the temperature increases, and likewise they disappear altogether as temperature approaches absolute zero.Thermal fluctuations are a...
. In the simplest case, there is a single characteristic time called the thermal relaxation time. If the starting magnetization is and the magnetization in equilibrium
Equilibrium
Equilibrium is the condition of a system in which competing influences are balanced. The word may refer to:-Biology:* Equilibrioception, the sense of a balance present in human beings and other animals...
is , the magnetization after a time is
If is a natural remanent magnetization
Natural Remanent Magnetization
Natural remanent magnetization is the permanent magnetism of a rock or sediment. In some forms, it can preserve a record of the Earth's field and the tectonic movement of the rock over millions of years...
(NRM) acquired in the Earth's field at one time, and is the equilibrium magnetization in a different field at some later time, the change in remanence is the VRM.
The above equation is applicable to the simplest single domain
Single domain (magnetic)
Single domain, in magnetism, refers to the state of a ferromagnet in which the magnetization does not vary across the magnet. A magnetic particle that stays in a single domain state for all magnetic fields is called a single domain particle . Such particles are very small...
magnets, those described by the Stoner–Wohlfarth model. The relaxation time depends on factors such as the size of the magnet and its magnetic anisotropy
Magnetic anisotropy
Magnetic anisotropy is the direction dependence of a material's magnetic properties. In the absence of an applied magnetic field, a magnetically isotropic material has no preferential direction for its magnetic moment while a magnetically anisotropic material will align its moment with one of the...
. Rocks have collections of magnetic minerals with varying size and anisotropy, and therefore a broad spectrum of relaxation times. The magnetization tends to have a logarithmic dependence on time, so the rate of change is often represented by a viscosity coefficient
Significance for paleomagnetism
Paleomagnetists are interested in the primary natural remanent magnetizationNatural Remanent Magnetization
Natural remanent magnetization is the permanent magnetism of a rock or sediment. In some forms, it can preserve a record of the Earth's field and the tectonic movement of the rock over millions of years...
(NRM) in a rock, acquired when the rock was originally formed. Viscous remanent magnetization is regarded as noise. Any component of the NRM that is in the direction of the present Earth's field is suspect because it may have been acquired since the last geomagnetic reversal
Geomagnetic reversal
A geomagnetic reversal is a change in the Earth's magnetic field such that the positions of magnetic north and magnetic south are interchanged. The Earth's field has alternated between periods of normal polarity, in which the direction of the field was the same as the present direction, and reverse...
. Fortunately, VRM is often removed by the first steps in a stepwise thermal demagnetization of the NRM.
VRM may also be acquired in the laboratory while measuring the NRM. To avoid this, paleomagnetists make their measurements in a magnetically shielded environment. Often the magnetometer
Magnetometer
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature...
is housed in a room with walls made of mu-metal
Mu-metal
Mu-metal is a nickel-iron alloy that is notable for its high magnetic permeability. The high permeability makes mu-metal very effective at screening static or low-frequency magnetic fields, which cannot be attenuated by other methods. The name came from the Greek letter mu which represents...
.