Any system may be characterized by its response to external stimuli. For
example, in electronics the proverbial “black box” is characterized by its
measured output voltage when an input current is applied. This transfer
impedance, as it is called, provides all the information necessary to understand
the operation of the black box. If we know what is in the black box – for
example, the detailed arrangement of resistors, diodes, etc. – then we can
predict, through analysis, what the transfer impedance will be.
Similarly, a system of charges and currents, such as a crystal, may be
characterized by a response function. In this text we shall be concerned
mainly with the response of such a system to a magnetic field. In this case
the “output” is the magnetization and the response function is the magnetic
susceptibility. A complete analysis of the magnetic susceptibility is virtually
impossible since the system consists of about 1021 particles. Therefore we usually
look to a measured susceptibility for clues to the important mechanisms
active in the system and then use these to analyze the system. In order to
carry out such a program, we must know what possible mechanisms exist and
what effect they have on the susceptibility.