Effects of quantum mechanics on the deflagration threshold in the molecular magnet ${\text{Mn}}_{12}$ acetate

We report experimental studies of the stability of a ${\text{Mn}}_{12}\text{-Ac}$ crystal against magnetic avalanches as a function of the magnitude and direction of the magnetic field, as well as a function of temperature. Strong evidence for quantum effects associated with this phenomenon is seen in the $\left(H_z,H_x\right)$ metastability diagram. The data provide further support to the theory of magnetic deflagration.

Figure 1

(Color online) Three-dimensional plot of the energy barrier (in kelvins) as a function of longitudinal and transverse components of the magnetic field (in teslas). Insets show a set of horizontal cross sections of the plot. (a) The energy barrier for a classical spin vector with energy given by Eq. (1); the $T=0$ cross section is the Stoner-Wolfarth astroid. (b) The effective energy barrier for a quantum spin described by the Hamiltonian of Eq. (1); the right lower panel shows cross sections for energies $U=35$, $U=40$, $U=45$, and $U=50\text{K}$.

Figure 2

(Color online) Area of stability against ignition of avalanches (straight cyan curve) and against slow relaxation (dashed black curve). Red circles show points where avalanches should occur when sweeping the field back and forth at a given angle ${\theta }_i$ within the first quadrant. The angle ${\theta }_c$ denotes to the crossing point between the slow relaxation and avalanche stability areas.

Figure 3

(Color online) (a) Angle dependence of the metastability area measured through the occurrence of avalanches. Black triangles are for $T_0=1.8\text{K}$ while blue squares are for $T_0=2.2\text{K}$. Nonsolid symbols are images of symmetrically measured points. (b) Temperature data for field sweeps at different angles; the three curves correspond to $24°$, $39°$, and $44°$, respectively, at $T=2.2\text{K}$.