ABSTRACT
The quantity tf(x) may be defined as the total spin in a
small volume around the point x divided by that small
volume, i . e . , a “local spin density '1 or "local order
parameter. M (The choice of the size of that small volume
is an important subject and w il l be discussed in chapter II. )
The spin configuration changes as a result of thermal ag ita-
tion. At thermal equilibrium the spin configuration follows
a probability distribution dictated by the laws of statistical
16 INTROD UCTION
mechanics.