Theory of Magnetism in Noncrystalline Solids
This chapter presents the theoretical aspects on rapidly growing area of magnetism. The question of whether or not ferromagnetism may exist in noncrystalline substances was theoretically studied first by A. I. Gubanov. As ferromagnetism is caused by exchange interaction between neighboring atoms, there is no need to have a rigorous periodicity in the distribution of magnetic atoms. According to the accepted Kondo theory, with increasing impurity concentration, the minimum is suppressed and finally washed out as magnetic ordering sets in, since magnetic ordering of the local spins destroys the freedom of spins to flip. The chapter refers the Simpson's theory to the case of an amorphous "random" antiferromagnet instead of a "true" amorphous antiferromagnet in which some partial correlation among the spin orientations of near neighbors may exist. The existence of antiferromagnetism is structurally restricted in amorphous solids. A variety of amorphous ferromagnetic materials have been reported to have a resistivity minimum well below the magnetic ordering temperature.