Terahertz Faraday effect in single molecule magnets

Faraday rotation in the terahertz frequency range was observed in molecular magnetic systems. The effect is strongest near the magnetic resonance of the single molecule magnet ${\mathrm{Mn}}_{12}\mathrm{Ac}$ at $\nu =300\mathrm{GHz}$, where the Faraday rotation exceeds $150°∕\mathrm{mm}$. Below the magnetization blocking temperature, the effect was observed in the magnetized state of the sample in zero field. Surprisingly, it could even be detected in nonmagnetized states, applying small magnetic fields $(H⩽1\mathrm{T})$, that do not create net magnetization of the sample. All observations were quantitatively explained without fit procedures using known ${\mathrm{Mn}}_{12}\mathrm{Ac}$ spin-Hamiltonian parameters.

Figure 1

Transmission spectra recorded at $T=1.77\mathrm{K}$ and $H=\pm 1\mathrm{T}$ on a single-crystal mosaic of ${\mathrm{Mn}}_{12}\mathrm{Ac}$, using right-hand circularly (RHC) and left-hand circularly (LHC) polarized radiation. The results for the positive-field-cooled sample $(H=+1\mathrm{T})$ are shown in the top panel and for the negative-field-cooled sample in the bottom panel.

Figure 2

Zero-field transmission spectra recorded using linearly polarized light on a single-crystal mosaic of ${\mathrm{Mn}}_{12}\mathrm{Ac}$ in nonmagnetized (open circles, ZFC) and magnetized (closed circles, FC) states at zero external magnetic field and a temperature of $T=1.77\mathrm{K}$, and at various analyzer angles, as indicated. The lines are theoretical calculations (see text). The middle panel shows a scheme of the experimental setup.

Figure 3

Transmission spectra recorded with linearly polarized light on a nonmagnetized single-crystal mosaic of ${\mathrm{Mn}}_{12}\mathrm{Ac}$ at $T=1.77\mathrm{K}$ and at various applied fields. The lines are theoretical calculations (see text). The inset shows the $\mid \pm 10⟩$ and $\mid \pm 9⟩$ levels in a longitudinal magnetic field; the arrows indicate the transitions belonging to the resonance lines observed in the spectrum.

Figure 4

Frequency dependent Faraday rotation (top panel) and polarization ellipticity (bottom panel) determined from the experimental data (symbols) together with the calculated behavior (lines). The middle part shows the calculated polarization of the radiation at various frequencies, where the $x$ axis corresponds to $\alpha =0°$.