L06 - GLE 594: An introduction to applied geophysics...

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1 GLE 594: An introduction to applied geophysics Magnetic Methods Fall 2004 Reading Today : 75-86 Next Lecture : 86 - 111. Instruments and Surveying
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2 Induced magnetization (J I ) and magnetic susceptibility • A magnetizable body acquires magnetization when H field is applied – Disappears when field is removed – Field ‘induces’ magnetization in material • The induced magnetization is parallel and proportional to H: J I = κ H (due to the earth: J I = κ F/ µ o ) k = susceptibility k = µ r -1 – Dimensionless, however, k SI =4 π k cgs Cause of magnetic susceptibility • At the atomic level, materials have a net magnetic moment due to: – Rotation of electrons in various shells around nucleus – The spin of the electrons – Number of electrons in each shell – That is, it is a quantum effect • All of above result that each atomic nucleus can be though of as a small magnetic dipole with its own moment
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3 • Diamagnetic • All electron shells are full, thus there is no net moment. • In the presence of an external field, the net moment opposes the external field, i.e., slightly negative susceptibility. • Paramagnetic • Materials contain unpaired electrons in incomplete electron shells. • However magnetic moment of each atom is uncoupled from others so they all behave independently. • Results in weakly magnetic materials, i.e. small susceptibility Classifications of magnetic materials • Ferromagnetic • Materials contain unpaired electrons in incomplete electron shells • Magnetic moment of each atom is coupled to others in surrounding ‘domain” such they all become parallel. • Caused by overlapping electron orbits • Gives rise to a spontaneous magnetization even in absence of an external field • Magnets are ferromagnetic • Examples: Cobalt, iron and nickel Classifications of magnetic materials
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4 • Anti-ferromagnetic • Almost identical to ferromagnetic except that the moments of neighboring sublattices are aligned opposite to each other and cancel out • Thus no net magnetization is measured • Example: Hematite •F e r r i magnetic • Sublattices exhibit ferromagnetically but then couple antiferromagnetically between each other
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Concept of hysteresis • Complex relationship between B and H that occurs in ferromagnetic materials. B flattens off with increasing H at ‘saturation’ – When H is decreased, B does not follow same curve – Will have ‘remanent’ B value at zero H Remanent magnetization (RM) • Permanent magnetization of rock installed during its formation ( J R ). • Ferromagnetic materials
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This note was uploaded on 05/09/2010 for the course EARTH SCIE APPLIED GE taught by Professor Es during the Spring '09 term at IIT Bombay.

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L06 - GLE 594: An introduction to applied geophysics...

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