Direct visualization of magnetic correlations in frustrated spinel ZnFe${}_2$O${}_4$

Abstract

Magnetic materials with the spinel structure (A${}^{2+}$B${}_2^{3+}$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.