  We propose the realization of linear crystals of cold ions which contain
different atomic species for investigating quantum phase transitions and
frustration effects in spin system beyond the commonly discussed case of
$s=1/2$. Mutual spin-spin interactions between ions can be tailored via the
Zeeman effect by applying oscillating magnetic fields with strong gradients.
Further, collective vibrational modes in the mixed ion crystal can be used to
enhance and to vary the strength of spin-spin interactions and even to switch
those forces from a ferro- to an antiferromagnetic character. We consider the
behavior of the effective spin-spin couplings in an ion crystal of spin-1/2
ions doped with high magnetic moment ions with spin S=3. We analyze the ground
state phase diagram and find regions with different spin orders including
ferrimagnetic states. In the most simple non-trivial example we deal with a
linear $\{$Ca$^+$, Mn$^+$, Ca$^+\}$ crystal with spins of $\{1/2,3,1/2}$. To
show the feasibility with current state-of-the-art experiments, we discuss how
quantum phases might be detected using a collective Stern-Gerlach effect of the
ion crystal and high resolution spectroscopy. Here, the state-dependent
laser-induced fluorescence of the indicator spin-1/2 ion, of species
$^{40}$Ca$^+$, reveals also the spin state of the simulator spin-3 ions,
$^{50}$Mn$^+$ as this does not possess suitable levels for optical excitation
and detection.
