“…The polycrystalline sample was synthesized by the solid-state reaction method using Co 3 O 4 (99.9985%) and Al 2 O 3 (99.995%) from Alfa Aesar as the starting materials, the details of which can be found in Ref. [191]. Nuclear magnetic resonance (NMR) measurements were conducted by probing the 27 Al nuclei (I = 5/2, γ/2π = 11.09375 MHz/T, Q = 0.149 barns) and the 59 Co nuclei (I = 7/2, γ/2π = 10.03 MHz/T, Q = 0.4 barns) by using a phase-coherent spin-echo pulse spectrometer.…”