Experimental evidence of a collinear antiferromagnetic ordering in the frustrated CoAl₂O₄ spinel

Nuclear magnetic resonance (NMR), neutron diffraction (ND), x-ray diffraction, magnetic susceptibility χ, and specific-heat measurements on the frustrated A-site spinel CoAl₂O₄ compound reveal a collinear antiferromagnetic ordering below T_N = 9.8(2) K. A high-quality powder sample characterized by x-ray diffraction that indicates a relatively low Co-Al inversion parameter x=0.057(20) in (Co_1-xAl _x)[Al_2-xCo_x]O₄, shows a broad maximum around 15 K in χ(T) and a sharp peak at T_N in heat capacity C_p. The average-ordered magnetic moment of Co2⁺ (S=32) ions at the A site is estimated to be 2.4(1) μ_B from NMR and 1.9(5) μ_B from ND which are smaller than the expected value of 3 μ_B for S=32 and g = 2. Antiferromagnetic spin fluctuations and correlations in the paramagnetic state are revealed from the χ, NMR, and ND measurements, which are due to spin frustration and site inversion effects in the system. The ND data also show short-range dynamic magnetic ordering that persists to a temperature that is almost twice T_N.