Spin-pair tunneling in Mn 3 single-molecule magnet

We report spin-pair tunneling observed in a ${\mathrm{Mn}}_{3}$ single-molecule magnet, which is a crystal with a two-dimensional network of identical exchange coupling. We observe a series of extra quantum tunnelings by the ac susceptibility measurements, and demonstrate that these are mainly thermally assisted tunnelings of a pair of two spins from the same initial state to the same final state simultaneously. The resonant field of spin-pair tunneling can be expressed as ${H}_{z}=lD/g{\ensuremath{\mu}}_{0}{\ensuremath{\mu}}_{B}+({n}_{\ensuremath{\downarrow}}\ensuremath{-}{n}_{\ensuremath{\uparrow}})JS/2g{\ensuremath{\mu}}_{0}{\ensuremath{\mu}}_{B}$, and the splitting interval ($|J|S/g{\ensuremath{\mu}}_{0}{\ensuremath{\mu}}_{B}$) is half that of the single-spin tunneling ($2|J|S/g{\ensuremath{\mu}}_{0}{\ensuremath{\mu}}_{B}$), which is analogous to the relationship between the magnetic flux quantum in superconductors ($h/2e$) and common metals ($h/e$).