Pulsed-field studies of the magnetization reversal in molecular nanomagnets

We report experimental studies of crystals of ${\mathrm{Mn}}_{12}$ molecular magnetic clusters in pulsed magnetic fields with sweep rates up to $4\times {10}^3\mathrm{T}∕\mathrm{s}$. The steps in the magnetization curve are observed at fields that are shifted with respect to the resonant field values. The shift systematically increases as the rate of the field sweep goes up. These data are consistent with the theory of the collective dipolar relaxation in molecular magnets.

Figure 1

Field dependence of the sweep rate.

Figure 2

Field dependence of $dM∕dB$ for different sweep rates at $B=1.3\mathrm{T}$.

Figure 3

Field dependence of the third resonance peak for a fixed sweep rate $(dB∕dt=1.1\mathrm{kT}∕\mathrm{s})$ and for different temperatures $T=0.723\mathrm{K}$ and $T=1.074\mathrm{K}$.

Figure 4

A pair of tunnel-split levels ${\epsilon }^{+}∕{\epsilon }^{-}$ versus the energy bias $W={\epsilon }_m-{\epsilon }_{m^{\prime }}=g\mid m-m^{\prime }\mid {\mu }_B(H\left(t\right)-H_R)$. $H_R$ denotes the (third) resonance field. The total magnetization reversal occurs after crossing the resonance $H\left(t\right)>H_R$ via superradiant magnetic dipolar transitions between the levels $m$ and $m^{\prime }$, with unperturbed energies ${\epsilon }_m$ and ${\epsilon }_{m^{\prime }}$, respectively.

Figure 5

Plot of $\sqrt{r}(d{M}_z∕d\delta H)$ versus $\delta H∕\sqrt{r}$ with $\delta H=H\left(t\right)-H_R$ at ${\mu }_0{H}_R=1.34\mathrm{T}$ for all curves at different sweep rates from $1\text{to}3\mathrm{kT}∕\mathrm{s}$.