The first-Order phase transition in $\beta$-MnO$_{2}$ studied by $^{55}$Mn Nuclear Magnetic Resonance

$\beta$-MnO$_{2}$ has a conductivity due to negative charge carriers generated from slight off-stoichiometry of oxygens. We obtained 55Mn antiferromagnetic nuclear magnetic resonance (NMR) signal for $\beta$-MnO$_{2}$ at low temperature for the first time. The resonance frequency of the main peak centered at 254 MHz is nearly unchanged from 4.2 K to 130 K though it is known that the magnetic phase changes near 94 K from the antiferromagnetic state to the paramagnetic state. This suggests that the magnetic phase transition is a first order transition and the mixed phases coexist both below and above the phase transition temperature. Side peaks were found at 161 MHz, 181 MHz and 297 MHz. Through annealing $\beta$-MnO$_ {2}$, which generates oxygen defects, we investigated whether these peaks originate from manganese ions with their valence other than 4+. However, the NMR spectra of the annealed $\beta$- MnO$_{2}$ showed no difference in side peaks.