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.