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Unformatted text preview: 1 Magnetic Properties • Flashlight without batteries magnet moving back and forth solenoid capacitor 2 Tiny Magnets for Data Storage • Currently, we store ~ 100 Gb/in 2 • Each bit occupies 80 nm " 80 nm • Need strong magnets – Improved read-back signal – Thermal stability 3 Tiny Magnets for Data Storage • Energy required to ¡ip the magnet (magnetic domain) = KV , where V is the volume and K = anisotropy constant • The domain may spontaneously ¡ip due to thermal excitation (superparamagnetic effect) • Require KV > 65 k B T to ensure thermal stability 4 De¢nitions and Units • Magnetic ¢eld H (ampere/meter, or oersted) – 1 amp/m = 4 ! " 10-3 oe – Can be produced by a current loop or solenoid – Earth’s magnetic ¢eld in mid-lattitudes ~ 0.3-0.5 oe – A solenoid with n turns per meter and current I produces a magnetic ¢eld inside the solenoid equal to nI – A single current loop with radius R and current I produces a magnetic ¢eld equal to I/2 ! R at the center of the loop 5 De¢nitions and Units • Magnetic moment – In old days, it was believed that magnetism is due to the existence of magnetic charges, analogous to electricity – Strength of a given magnet is given by the magnetic moment m , de¢ned as magnetic charge " distance between the magnetic charges. – Now, we know that magnetism is due to unpaired spins and orbital motion of electrons) – Each electron spin acts as a microscopic magnet with a magnetic moment = 9.27 " 10-24 amp-m 2 = μ B (Bohr magneton) 6 De¢nitions and Units • Magnetization – The strength of an actual magnet is measured by the magnetic moment per unit volume ---> magnetization M – Unit: ampere/meter, same as H 7 Defnitions and Units • Flux Density or Magnetic Induction B – Consider a piece o¡ unmagnetized iron – Electron spins organized into 6 equivalent domains <100> with zero net magnetization – Apply feld along the  direction magnetic field 8 Defnitions and Units • Flux Density or Magnetic Induction B – Visualize the magnetic feld inside the material as ¢ux lines – Flux lines become more dense, one set due to the applied feld, another set due to the alignment o¡ spins...
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This note was uploaded on 11/21/2011 for the course MAT SCI 201 taught by Professor Matsci during the Spring '10 term at Northwestern.
- Spring '10