LADDER ISING SPIN CONFIGURATIONS .2. MAGNETIC-PROPERTIES

In this paper we consider ladders with two, three, and four coupled Is ing spin chains, characterized by interchain and intrachain couplings, to study in detail their magnetic behaviour for different ratios of t he interaction constants and different values of magnetic field, by us ing a transfer matrix method and a complex algorithm, realizing the di agonalization and the differentiation. We have proposed a combined alg orithm to realize also the generation of the elements of the transfer matrix for more complicated ladder configurations. The problem of Isin g ladders is straightforward with a crossover between a 2D high-temper ature region and a 1D low-temperature region when the 2D correlation l ength equals the width. To emphasize the fact that there is no finite magnetization in absence of the magnetic field, we study also analytic ally this case for the ladder with two coupled Ising chains. In presen ce of the magnetic field, for very weak magnetic fields (of the order 10(-8) in reduced units) the magnetization goes from the value one to the value zero in a narrow interval of nonzero temperatures, where als o the susceptibility exhibits a very high peak. This narrow interval, increasing the number of chains, is displaced towards higher temperatu res. Also, in the case of interchain antiferromagnetic couplings, the formation of interchain spin pairs becomes clear.