The plateau-non-plateau transition in the antiferromagnetic-(ferromagnetic)
(n) polymerized S = 1/2 XXZ chains under the magnetic field is investigated
. The universality class of this transition belongs to the Brezinskii-Koste
rlitz-Thouless (BKT) type. The critical points are determined by level spec
troscopy analysis of the numerical diagonalization data for 4 less than or
equal to p less than or equal to 13 where p(equivalent to n + 1) is the siz
e of a unit cell. It is found that the critical strength of ferromagnetic c
oupling decreases with p for small p but increases for large enough p. It i
s also found that the plateau for large p is wide enough for moderate value
s of exchange coupling so that it should be easily observed experimentally.
This is in contrast to the plateaus for p = 3 chains which are narrow for
a wide range of exchange coupling even away from the critical point.