59 747 752 758 tc c eg euo tc c 694k

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Unformatted text preview: cal ordering remains. T >> T ⇒ tanh x ≈ x C 1 gµ B Beff 1 (7.50) M (T ) = gµ B natoms tanh 2 2 kT 4kTC Beff = BA + B = M +B 22 natoms g µ B ↑ (7.57 ) B: applied field ⇒ 7-29 2 g 2 µ B n atoms 1 M (T ) = B T − TC 4k (7.56) ⇒ χ= C T − TC with 2 µ 0 µ B g J J ( J + 1) C= n atoms 3k (7.57) (7.58) Currie constant for Currie-Weiss law with J = S =1/2 θ = TC (7.59) (7.47), (7.52), (7.58) ⇒ TC = Cγ E.g., EuO: γ = TC / C = 69.4K / 4.68k = 15 BMF = µ0γ 1.930×10 6 A / m M0 12(3) s = 36T >> experimental fields 7-30 • External fields hardly affect magnetization of ferromagnets well below Tc. • BMF >> dipole field of neigbouring atoms Order of magnitude: µ0 µ B ≈ 0.04T 4πa 3 a: lattice parameter BMF no magnetic field ! (due to quantummechanical exchange interaction) ⇒ BMF does not appear in Maxwell eqs. Mean field approximation describes exchange interaction between localized electrons. Good model for: - lanthanides (partially filled 4f-shells) -...
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This note was uploaded on 01/19/2010 for the course MATERIALS M504 taught by Professor Adelung during the Spring '02 term at Uni Kiel.

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