A nonlinear susceptibilities (the third derivative of a magnetization
mS by a magnetic field
h ) of the
S=1/2 ferromagnetic Heisenberg chain and the classical Heisenberg chain are calculated at low temperatures
T. In both chains the nonlinear susceptibilities diverge as
T−6 and a linear susceptibilities diverge as
T−2. The arbitrary spin
S Heisenberg ferromagnet
[ H=∑i=1N{−JSiSi+1−(h/S)Siz} (J>0), ] has a scaling relation between
mS, h and
T: mS(T,h)=F(S2Jh/T2). The scaling function
F(x)=(2
x/3)-(44
x3/135) + O(
x5) is common to all values of spin
S.We discuss some properties of an integrable and conformally invariant Heisenberg chain with alternating spins S and S' (S'>S) in the presence of a magnetic field. The zero-field magnetic susceptibility is computed and turns out to be only dependent on the spin S. For small enough magnetic field, the low-lying excitations are related to the deformed SU(2) parafermionic field theory at levels 2S and 2(S'-S). Generalizations to these results for other mixed systems are commented on.