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In an atom, the magnetic field is due to the coupled spin and orbital magnetic moments associated with the motion of electrons. The spin magnetic moment is due to the precession of the electrons about their own axes where as the orbital magnetic moment is due to the motion of electrons around the nucleus. The resultant combination of the spin and orbital magnetic moments of the constituent atoms of a material gives rise to the observed magnetic properties. Figure 1: The "Right Hand Rule" for an induced magnetic field
Historically, magnetism has been recognized for thousands of years. An account, that is probably apochryphal, tells of a shepherd called Magnes in Crete who around 900 B.C discovered the naturally occurring magnet lodestone (a form of the the spinel magnetite, Fe
3O 4) in a region later named Magnesia. Supposedly while he was walking over a deposit, the lodestone pulled the nails out of his sandals and the metal tip from his staff. The Classical Theory of Magnetism
The classical theory of magnetism was well developed before quantum mechanics.
Lenz's Law (1834), states that when a substance is placed within a magnetic field, H, the field within the substance, B, differs from H by the induced field, 4πI, which is proportional to the intensity of magnetization, I. Figure 4: Spin crossover for \(Fe(phen)_2(NCS)_2\)
At the higher temperature the ground state is
5T 2g while at low temperatures it changes to 1A 1g. The changeover is found at about 174K.In solution studies, it is possible to calculate the heat of conversion from the one isomer to the other.