University of California Berkeley MSE 200A Fall 2017 Hard Magnets Magnetic

University of california berkeley mse 200a fall 2017

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University of California, Berkeley MSE 200A Fall, 2017 Hard Magnets: Magnetic Memory M s M r H c domain motion orientation Magnetic elements on disc, tape or surface Isolated, individual particles; field orientation records information Magnetic elements: Hard for good memory Not too hard, for erasure and re-write Media characteristics Generally magnetic oxides for shape and chemical stability Size less than minimum domain size for hardness Perpendicular recording difficult, but provides high density parallel recording perpendicular recording
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J.W. Morris, Jr. University of California, Berkeley MSE 200A Fall, 2017 Soft Magnets Soft magnet materials Small hysteresis loop Low energy loss per cycle Optimized for cyclic machinery Generators, transformers Motors Read-write heads Electromagnetic shielding Materials requirements Magnetic isotropy Low energy required to rotate moment Homogenous, defect-free microstructure Large grain size Large-grained Fe-Si transformer steel Amorphous metallic glasses Electrical insulation minimizes electrical losses Ferrites (Fe 2 O 3 , LiFe 2 O 3 ) M s M r H c
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J.W. Morris, Jr. University of California, Berkeley MSE 200A Fall, 2017 Piezomagnetism (Magnetostriction) Magnetic field elastic strain Magnetic mechanical Can reach very high frequency Small energy requirements Piezomagnetic transducers High frequency oscillators Sound recording High quality speakers Materials Ni Ni-Fe ( invar ) TbDyFe ( terfenol ) 36Ni-Fe Fe Ni (dL/L)x10 6 0.2 0.4 0.6 30 20 10 0 -10 -20 -30 B(tesla)
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J.W. Morris, Jr. University of California, Berkeley MSE 200A Fall, 2017 Superconductivity Superconductivity = loss of electrical resistance Superconductors are not just good conductors Electrons are fermions , obey Pauli Exclusion Because of exclusion, all metals have resistance Electron is excited to conduct Loses energy on collision, returns to paired state For superconductivity, must turn electrons into bosons Electrons are paired into carriers with integral spin Integral spin boson All carriers may be in the same ground state Applications Conductors : high field magnets, storage devices, transmission Junctions : Josephson junctions used in detectors (SQUIDS)
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J.W. Morris, Jr. University of California, Berkeley MSE 200A Fall, 2017 Superconductivity Curious scientific history Discovery: Onnes and his graduate students Accidental discovery by refrigeration specialists Basic theory: “Cooper pairs” The curious case of Chemistry Professor Ogg In Ogg's theory it was his intent That the current keep going, once sent To save himself trouble He put them in double And instead of stopping, they went - G Gamow
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J.W. Morris, Jr.
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  • Fall '08
  • Staff
  • Magnetism, Magnetic Field, University of California, J.W. Morris

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