Analyzing the enforcement of a high-spin ground state for a metallacrown single-molecule magnet

We have studied element-selective magnetic properties of the hetero- and homometallic metallacrowns $\mathrm{Cu}{\left(\mathrm{II}\right)}_2[12-{\mathrm{MC}}_{YN\left(Shi\right)}-4]$ ($Y=\text{Cu}$, Fe, in short ${\mathrm{CuCu}}_4$ and ${\mathrm{CuFe}}_4$). These metallacrowns comprise four Fe or Cu ions surrounding a central Cu ion. Using x-ray magnetic circular dichroism we have probed local symmetries, electronic configuration, orbital and spin magnetic moments of the magnetic ions. The ratio between the Cu and Fe moment of $-0.11$ is independent of temperature in the range of 15 K to 90 K. The Cu moment shows antiparallel to the Fe moment. For ${\mathrm{CuCu}}_4$ we confirm the predicted $S=1/2$ ground state. The comparison of the spectral dependence indicates a localization of the magnetic moment at the central Cu ion. The orbital to spin moment ratio is 0.03 for the Fe ion while a larger ratio of 0.12 was measured for the Cu ion in both compounds. The experimental results are compared to a mean field model and to an isotropic Heisenberg model.

Figure 1

Molecular structure of the ${\mathrm{CuFe}}_4$ 12-MC-4 complex (left); color scheme: Cu(II)(light blue), Fe(III) (gold), O (red), N (blue), C (black). Square magnetic model of the ${\mathrm{CuFe}}_4$ 12-MC-4 complex (right).

Figure 2

X-ray absorption and XMCD spectra measured at the Fe (a) and Cu $L_{3,2}$ (b) edges for the ${\mathrm{CuFe}}_4$ 12-MC-4 metallacrown. The step function subtracted for the sum rule analysis is indicated as a dotted line. The external field is $\pm 7$ Tesla and the temperature 22 K.

Figure 3

X-ray absorption and XMCD spectra (full lines) simulated at the Fe (a) and Cu ${\mathrm{L}}_{3,2}$ (b) edges. using CTM4XAS with the crystal field parameters $10Dq=1.6$ eV (0.7 eV) and $Ds=0.1$ eV (0.0 eV) for Fe (Cu).

Figure 4

Total magnetic moments for ${\mathrm{CuCu}}_4$ (circles) and ${\mathrm{CuFe}}_4$ (squares) as determined by the sum rule analysis for a field of 7 T, considering four moments in the scaffold and one center moment, as a function temperature. XMCD results are compared with SQUID magnetometry. SQUID data were taken from Refs. [24, 33] (full and dashed lines) for the same applied field.

Figure 5

Element-selective total magnetic moments as determined by the sum rule analysis versus temperature (dots and squares). Full and dashed lines indicate the temperature dependent Fe and Cu moments calculated by the mean field model.

Figure 6

Ratio of Cu and Fe moments as a function of temperature (circles). The ratio is compared with a constant spin ratio (CSR) (full blue line) and with the mean field model (MFM) (dashed red line).

Figure 7

X-ray absorption (a) and XMCD spectra (b) at the Cu $L_{3,2}$ edges for the ${\mathrm{CuCu}}_4$ 12-MC-4 metallacrown (full lines). The external field is $\pm 7$ Tesla and the temperature 20 K. For direct comparison the corresponding Cu spectra as measured for ${\mathrm{CuFe}}_4$ are shown, too (dotted lines), with reversed sign of the XMCD spectrum. All spectra are normalized to the maximum value.