5.1.1 Weiss Molecular Field Theory We start by identifying the order parameter of the magnet which distinguishes the ordered (magnetic) from the disordered (nonmagnetic) phase. This effect is explained in classical theory by the presence of a molecular field within the ferromagnetic material, which was first postulated by Weiss … centred tetragonal structure which is ferromagnetic, and the ferromagnetism is retained on tempering. This was later re ned into a theory According the Weiss’ hypothesis, the latter one can be thought to be proportional to the very same magnetization: namely, B ∗ = B + B mol = B + λM z, (1.13) where λ is the phenomenological Weiss’ dimensionless constant. The basis of the Weiss molecular theory of ferromagnetism is that below the Curie temperature, a ferromagnet is composed of small, spontaneously magnetized regions called domains and the total magnetic moment of the ma-terial is the vector sum of the magnetic moments of the individual domains. Ferromagnetism Ferromagnetism is only possible when atoms are arranged in a lattice and the atomic magnetic moments can interact to align parallel to each other. The classical Weiss / Heisenberg theory of ferromagnetism, taught in the universities and presented in many textbooks (e. g.,[1-4]), deals basically with a special case of the collinear (parallel and antiparallel) spin arrangement. Weiss assumed that this field is proportional to the magnetization, i.e. Key: The Exchange Interaction Other materials: Cobalt (Co), Tc=1404 K Nickel (Ni), Tc= 631K Weiss theory is a good phenomenological theory of magnetism, But does not explain source of large Weiss field. Successes of the Weiss Molecular Field model i) it gives a good account of, # The spontaneous magnetisation of the feromagnet, # The temperature variation of magnetisation, M (T)/M(0) # The magnetic susceptibility above T C - called the Curie_Weiss Law! BE =λM (2) where λ is the Weiss constant. The Weiss’ mean field theory of the ferromagnetism … For describ-ing a paramagnetic to ferromagnetic transition, the obvious choice is the (local) magnetization m= hs ii: (5.1) We now focus on a single spin, s Heisenberg and Dirac showed later that ferromagnetism is a quantum mechanical effect that fundamentally arises from Coulomb (electric) interaction. local magnetic field (Weiss effective field) at the site of each dipole with a constant g •Consider a collection of N identical atoms per unit volume, with total angular momentum J, and use QM treatment of atomic paramagnetism sat J Ferromagnetism – Molecular field theory … The Curie-Weiss law can be derived using arguments proposed by Weiss. Magnetic domains (proposed by Weiss 1906) ... From W. Wernsdorfer’s pdf The first single molecule magnet (1980): Mn 12-acetate (s=10) Title: Weiss theory of ferromagnetism Author: CamScanner Subject: Weiss theory of ferromagnetism 3.1.2 Weiss domain theory Weiss (1906, 1907) postulated that atoms in ferromagnetic materials had permanent magnetic moments which were aligned parallel to one another over extensive regions of a sample. The logic behind the theory in … In the ferromagnetic materials the moments are magnetized spontaneously, which implies the presence of an internal field to produce this magnetization. € T C = µ 0N g 2J(J+1)µ B 2 3k γ …

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