Magnetic Materials I. A. Review Magnetic Moment - μ I Direction is determined by the ______________ _____________ ____________ B. Spinning Electrons L The magnetic moment for a spinning electron is called the Bohr magnetron: μ B 9.27 10 24 J/T II. Orbiting Electrons And Magnetic Moments In the early part of the 20th century, Neils Bohr proposed a model of the atom in which electrons traveled in circular orbits around the nucleus due to the Coulomb force. Using the Bohr model as shown below, we have v R Electron Nucleus I dq dt μ From Physics I, we know that the magnitude of the angular momentum of the electron is given by: L Thus, we have μ According to Bohr's model, the electron is only allowed to exist in orbits where the magnitude of its angular momentum is given by Ln n h 2π where h is Plank's constant and is equal to 6.63x10-34 Js n is a positive integer Substituting Bohr's angular momentum hypothesis into our previous equation, we have n h e μ nμB 2 m 2 π Note: It is an interesting fact that Bohr's model correctly predicted the magnetic moment due to orbital angular momentum and spin angular momentum. When Bohr proposed the model, the concept of electron spin had not been invented. Furthermore, Bohr realized that according to classical physics his model should not be stable since an accelerating charge should emit energy and spiral into the nucleus. In modern physics, we learn that the electron does not travel in circular orbits. In fact, the entire concept of a particle trajectory is invalid at the atomic level! III. Groups Of Electrons And Magnetism We now consider a string of electrons. For an atom, electrons begin filling the various energy levels (shells). For a complete shell, the magnetic moment due to spin and orbital angular momentum vectors for the individual electrons add to zero. However, the outer (valence) electrons may produce a net magnetic moment since the shell isn't filled!! Consider the following string of electrons shown below μ Total From our picture, we see that if an atom has an odd number of outer electrons then it will have some magnetism due to spin. IV. Ferromagnetism In some materials like iron and nickel, the atom's magnetic moment is sufficiently strong to cause the individual atoms to interact so as to form larger regions of magnetism called domains. Each domain acts as a single larger magnet. A. Initially - No External Magnetic Field B. Apply An External Magnetic Field B ext Domains _______________ when an external magnetic field is applied. Thus, the magnetic field _________________________ . C. Remove External Magnetic Field The domains ___________________ _____________________ even after the magnetic field is removed. D. Heat Or Bang The Magnet The domains are _________ ___________________ _________________.