  Changes in magnetic critical behaviour of quenched structurally-disordered
magnets are usually exemplified in experiments and in MC simulations by diluted
systems consisting of magnetic and non-magnetic components. By our study we aim
to show, that similar effects can be observed not only for diluted magnets with
non-magnetic impurities, but may be implemented, e.g., by presence of two (and
more) chemically different magnetic components as well. To this end, we
consider a model of the structurally-disordered quenched magnet where all
lattice sites are occupied by Ising-like spins of different length $L$. In such
random spin length Ising model the length $L$ of each spin is a random variable
governed by the distribution function $p(L)$. We show that this model belongs
to the universality class of the site-diluted Ising model. This proves that
both models are described by the same values of asymptotic critical exponents.
However, their effective critical behaviour differs. As a case study we
consider a quenched mixture of two different magnets, with values of elementary
magnetic moments $L_1=1$ and $L_2=s$, and of concentration $c$ and $1-c$,
correspondingly. We apply field-theoretical renormalization group approach to
analyze the renormalization group flow for different initial conditions,
triggered by $s$ and $c$, and to calculate effective critical exponents further
away from the fixed points of the renormalization group transformation. We show
how the effective exponents are governed by difference in properties of the
magnetic components.
