The anisotropy splitting in the ground state of the single-molecule magnet ${\mathrm{Mn}}_4{\mathrm{O}}_3\mathrm{Br}{\left(\mathrm{O}\mathrm{Ac}\right)}_3{\left(\mathrm{dbm}\right)}_3$ is studied by inelastic neutron scattering as a function of hydrostatic pressure. This allows a tuning of the anisotropy and thus the energy barrier for slow magnetization relaxation at low temperatures. The value of the negative axial anisotropy parameter $D_{\text{cluster}}$ changes from $-0.0627\left(1\right)\mathrm{meV}$ at ambient to $-0.0603\left(3\right)\mathrm{meV}$ at $12\mathrm{kbar}$ pressure, and in the same pressure range the height of the energy barrier between up and down spins is reduced from $1.260\left(5\right)\mathrm{meV}\text{to}1.213\left(9\right)\mathrm{meV}$. Since the $\mathrm{Mn}\mathrm{Br}$ bond is significantly softer and thus more compressible than the $\mathrm{Mn}\mathrm{O}$ bonds, pressure induces a tilt of the single ion ${\mathrm{Mn}}^{3+}$ anisotropy axes, resulting in the net reduction of the axial cluster anisotropy.

• Received 9 September 2004

DOI:https://doi.org/10.1103/PhysRevB.70.172413