Low-Temperature Nuclear Magnetic Resonance Studies ofEuB6

We report the results of ${}^{153}\mathrm{Eu}$ and ${}^{11}\mathrm{B}$ nuclear magnetic resonance (NMR) studies of ${\mathrm{EuB}}_6$ at temperatures above 0.04 K and in magnetic fields between 0 and 7 T. We have observed a surprising evolution of the ${}^{153}\mathrm{Eu}$ NMR spectrum at very low temperatures, which we interpret as evidence for a complex electronic ground state of ${\mathrm{EuB}}_6$, involving the coexistence of two magnetically very similar, but electronically inequivalent, phases. The low temperature ${}^{11}\mathrm{B}$ NMR spectra reveal two inequivalent boron sites with internal fields of $-0.02\mathrm{}$ and $-0.35\mathrm{}\mathrm{T}$.