Field-Induced Magnetoelastic Instabilities in Antiferromagnetic Molecular Wheels

The magnetic torque of the antiferromagnetic molecular wheel ${\mathrm{CsFe}}_8$ was studied down to 50 mK and up to 28 T. Below about 0.5 K phase transitions were observed at the field-induced level crossings (LCs). Intermolecular magnetic interactions are very weak excluding field-induced magnetic ordering. A magnetoelastic coupling was considered. A generic model shows that the wheel structure is unconditionally unstable at the LCs, and the predicted torque curves explain the essential features of the data well.

Figure 1

(a) Torque vs field, $\tau (B)$, for a single crystal of $\mathbf{2}$ at 55, 100, 200, 300, and 400 mK (black, red, blue, green, magenta) ($\phi =-3.3°$). The right inset shows $d\tau /dB$ at the first LC, arrows indicate the onset of anomalies. The left inset shows the structure of ${\mathrm{CsFe}}_8$. (b) $\tau (B)$ for a single crystal of $\mathbf{1}$ at 60 and 700 mK (black, red) ($\phi =93.6°$). The right inset shows the data as $d\tau /dB$. The left inset shows a calculated energy spectrum as function of field neglecting magnetic anisotropy ($J=-20.6\mathrm{K}$, arrows mark the first two LCs, the involved levels are labeled by $S,M$).

Figure 2

(a) $d\tau /dB$ at the first LC for a single crystal of $\mathbf{1}$ for several temperatures from 55 mK to 1 K ($\phi \approx 93°$). (b) $B\mathrm{\text{−}}T$ phase diagram as derived from the data shown in panel (a) (lines are guides to the eyes). The solid symbols indicate the critical fields, the open symbols the half-maximum fields for $T>T_c$. The dashed line indicates the field of the $S=0\to S=1$ LC. The dash-dotted lines indicate the half-maximum fields as expected for thermal broadening.

Figure 3

Field dependence of (a) the order parameter ${\Delta }_0$, (b) the torque contribution $\delta {\tau }_1$, and (c) the torque contribution $\delta {\tau }_2$ around a LC at $b_0$ [the plots were normalized to ${\alpha }^2/(2k)$, $\partial b_0/\partial \phi$, and $-\partial {\Delta }_0/\partial \phi$, respectively].