A novel case of magnetic relaxation in the 99Ru Mössbauer spectrum of Na3RuO4

The magnetic properties of Na₃RuO₄ have been studied using ⁹⁹Ru Mössbauer spectroscopy. Magnetic hyperfine splitting is seen at temperatures below 30°K. The value of the flux density at 4.2°K (58.58 T) is compatible with a saturation field at an $\text{S =}\text{3}\text{2}\text{Ru}{}^{\mathrm{V}}$ ion, and at intermediate temperatures approximately follows an $\text{S =}\text{3}\text{2}$ Brillouin function. The published interpretation of magnetic susceptibility data in terms of tetranuclear intracluster antiferromagnetism is shown to be incorrect, and it is concluded that Na₃RuO₄ shows antiferromagnetic three-dimensional long-range order with a Néel temperature of T_N = 30 ± 1°K. Between 25 and 30°K motional narrowing of the spectrum is seen, which is probably due to a slow spin-spin relaxation within the crystal-field levels of the ⁴A_2g ground state of the Ru^V ion. This is the first observation of relaxation effects in a ⁹⁹Ru spectrum.