  The implementation and control of room temperature ferromagnetism (RTFM) by
adding magnetic atoms to a semiconductor's lattice has been one of the most
important problems in solid state state physics in the last decade. Herein we
report for the first time, to our knowledge, on the mechanism that allows RTFM
to be tuned by the inclusion of \emph{non-magnetic} aluminum in nickel ferrite.
This material, NiFe$_{2-x}$Al$_x$O$_4$ (x=0, 0.5, 1.5), has already shown much
promise for magnetic semiconductor technologies, and we are able to add to its
versatility technological viability with our results. The site occupancies and
valencies of Fe atoms (Fe$^{3+}$ T$_d$, Fe$^{2+}$ O$_h$, and Fe$^{3+}$ O$_h$)
can be methodically controlled by including aluminum. Using the fact that
aluminum strongly prefers a 3+ octahedral environment, we can selectively fill
iron sites with aluminum atoms, and hence specifically tune the magnetic
contributions for each of the iron sites, and therefore the bulk material as
well. Interestingly, the influence of the aluminum is weak on the electronic
structure (supplemental material), allowing one to retain the desirable
electronic properties while achieving desirable magnetic properties.
