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.[1]
Viscous remanent magnetization is the result of jumps between magnetic states driven by thermal fluctuations. In the simplest case, there is a single characteristic time τ called the thermal relaxation time. If the starting magnetization is M0 and the magnetization in equilibrium is Meq, the magnetization after a time t is
If M0 is a natural remanent magnetization (NRM) acquired in the Earth's field at one time, and Meq is the equilibrium magnetization in a different field at some later time, the change in remanence M(t) -M0 is the VRM.[1]
The above equation is applicable to the simplest single domain magnets, those described by the Stoner–Wohlfarth model. The relaxation time τ 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[1]
Paleomagnetists are interested in the primarynatural remanent magnetization (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. VRM is often removed by the first steps in a stepwise thermal demagnetization of the NRM. [1]
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.[2]
. Paleomagnetism laboratory, Department of Earth, Planetary and Atmospheric Sciences, MIT. Archived from the original on 2012-02-07. Retrieved 16 September 2011.
December 20, 2023
viscous, remanent, magnetization, abbreviated, also, known, viscous, magnetization, remanence, that, acquired, ferromagnetic, materials, sitting, magnetic, field, some, time, natural, remanent, magnetization, igneous, rock, altered, this, process, this, genera. 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 1 Contents 1 Origin 2 Significance for paleomagnetism 3 See also 4 Notes 5 ReferencesOrigin editSee also Superparamagnetism Viscous remanent magnetization is the result of jumps between magnetic states driven by thermal fluctuations In the simplest case there is a single characteristic time t called the thermal relaxation time If the starting magnetization is M0 and the magnetization in equilibrium is Meq the magnetization after a time t is M t M 0 M eq M 0 exp t t displaystyle M t M 0 left M text eq M 0 right exp t tau nbsp If M0 is a natural remanent magnetization NRM acquired in the Earth s field at one time and Meq is the equilibrium magnetization in a different field at some later time the change in remanence M t M0 is the VRM 1 The above equation is applicable to the simplest single domain magnets those described by the Stoner Wohlfarth model The relaxation time t 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 1 S M log t displaystyle S frac partial M partial log t nbsp Significance for paleomagnetism editMain article Paleomagnetism Paleomagnetists are interested in the primary natural remanent magnetization 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 VRM is often removed by the first steps in a stepwise thermal demagnetization of the NRM 1 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 2 See also editRock magnetismNotes edit a b c d Dunlop amp Ozdemir 1997 Chapter 10 MIT 2011References editDunlop David J Ozdemir Ozden 1997 Rock Magnetism Fundamentals and Frontiers Cambridge Univ Press ISBN 978 0 521 32514 1 Paleomagnetic instruments Paleomagnetism laboratory Department of Earth Planetary and Atmospheric Sciences MIT Archived from the original on 2012 02 07 Retrieved 16 September 2011 Retrieved from https en wikipedia org w index php title Viscous remanent magnetization amp oldid 1088078766, wikipedia, wiki, book, books, library,
