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ENGRI_1110_Lect17_09_posted

ENGRI_1110_Lect17_09_posted - Origin Of Magnetic Properties...

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Origin Of Magnetic Properties Origin Of Magnetic Properties www.aacg.bham.ac.uk/.../magnetic_recording.htm Spin Moment Orbital Moment
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Ferromagnetic Fe, Ni Antiferromagnetic Cr, Mn Exchange Interaction: acts to align neighboring atoms Parallel: Ferromagnetic (not exactly what bulk magnets would do)  Antiparallel: Antiferromagnetic
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Relationship of B and H Relationship of B and H
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Ferromagnetic materials when Ferromagnetic materials when formed into useful shapes do not formed into useful shapes do not start off permanently magnetized start off permanently magnetized Why???
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Curie Temperature Curie Temperature Material Curie T (K) Fe 1043 Co 1388 Ni 627 Fe 3 O 4 858 CrO 2 386 www.geol.binghamton.edu At C urie  te m pe rature ,  the rm al e ne rg y  pre ve nts  alig nm e nt o f  m ag ne tic  m o m e nts  in  fe rro m ag ne ts Entro py e ffe c t
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Magnetic materials lower their energy by dividing into multiple domains Domains are costly – energy associated with their formation
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Domain Walls Domain Walls The thickness of magnetic walls is a balance between: Exchange energy which acts to keep its moment parallel to its neighbors (domain wall thick) Anisotropy energy which acts to keep all moments parallel to particular crystal direction (domain wall thin)
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Domain Walls Domain Walls www.aacg.bham.ac.uk/.../magnetic_recording.htm
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Magnetization Magnetization H = 0 H > 0 M H M H Domain Wall
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Magnetization Magnetization H >> 0 H >>> 0 M H M H
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Magnetic Hysteresis Magnetic Hysteresis Once a magnetic material is saturated, decreasing H again  does not return M (or B) to the same position.
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