  Magnetic materials with the spinel structure (A$^{2+}$B$^{3+}_2$O$^4$) form
the core of numerous magnetic devices, but ZnFe$_2$O$_4$ constitutes a peculiar
example where the nature of the magnetism is still unresolved. Susceptibility
measurements revealed a cusp around $T_c=13\;\mathrm{K}$ resembling an
antiferromagnetic transition, despite the positive Curie-Weiss temperature
determined to be $\Theta_{CW}=102.8(1)\;\mathrm{K}$. Bifurcation of
field-cooled and zero-field-cooled data below $T_c$ in conjunction with a
frequency dependence of the peak position and a non-zero imaginary component
below $T_c$ shows it is in fact associated with a spin-glass transition. Highly
structured magnetic diffuse neutron scattering from single crystals develops
between $50\;\mathrm{K}$ and $25\;\mathrm{K}$ revealing the presence of
magnetic disorder which is correlated in nature. Here, the 3D-m$\Delta$PDF
method is used to visualize the local magnetic ordering preferences, and
ferromagnetic nearest-neighbor and antiferromagnetic third nearest-neighbor
correlations are shown to be dominant. Their temperature dependence is
extraordinary with some flipping in sign, and a strongly varying correlation
length. The correlations can be explained by orbital interaction mechanisms for
the magnetic pathways, and a preferred spin cluster. Our study demonstrates the
power of the 3D-m$\Delta$PDF method in visualizing complex quantum phenomena
thereby providing a way to obtain an atomic scale understanding of magnetic
frustration.
