Structure and magnetic properties of the single-molecule magnet $\left[{\mathrm{Mn}}_{11}\mathrm{Cr}{\mathrm{O}}_{12}{\left({\mathrm{O}}_2\mathrm{C}\mathrm{C}{\mathrm{H}}_3\right)}_{16}{\left({\mathrm{H}}_2\mathrm{O}\right)}_4\right]\bullet 2\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}\bullet 4{\mathrm{H}}_2\mathrm{O}$: Magnetization manipulation and dipolar-biased tunneling in a ${\mathrm{Mn}}_{11}\mathrm{Cr}∕{\mathrm{Mn}}_{12}$ mixed crystal

The structural and magnetic properties of the single-molecule magnet $\left[{\mathrm{Mn}}_{11}\mathrm{Cr}{\mathrm{O}}_{12}{\left({\mathrm{O}}_2\mathrm{C}\mathrm{C}{\mathrm{H}}_3\right)}_{16}{\left({\mathrm{H}}_2\mathrm{O}\right)}_4\right]\bullet 2\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}\bullet 4{\mathrm{H}}_2\mathrm{O}$ $\left({\mathrm{Mn}}_{11}\mathrm{Cr}\right)$ are studied through the analysis of a ${\mathrm{Mn}}_{11}\mathrm{Cr}∕{\mathrm{Mn}}_{12}$ $(\approx 1:1)$ mixed crystal, where ${\mathrm{Mn}}_{12}$ is $\left[{\mathrm{Mn}}_{12}{\mathrm{O}}_{12}{\left({\mathrm{O}}_2{\mathrm{C}\mathrm{C}\mathrm{H}}_3\right)}_{16}{\left({\mathrm{H}}_2\mathrm{O}\right)}_4\right]\bullet 2\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}\bullet 4{\mathrm{H}}_2\mathrm{O}$. X-ray absorption spectra reveal that the Cr ion in ${\mathrm{Mn}}_{11}\mathrm{Cr}$ is in the $+3$ valence state and occupies a specific ${\mathrm{Mn}}^{3+}$ site in the ${\mathrm{Mn}}_{12}$ skeleton. High-frequency electron paramagnetic resonance (EPR) spectra are well explained by assuming that ${\mathrm{Mn}}_{11}\mathrm{Cr}$ is in a ground spin-state of $S=19∕2$ with nearly the same EPR parameter set as for ${\mathrm{Mn}}_{12}$. The lower spin quantum number results in lower barrier height $\left(56.8\mathrm{K}\right)$ compared to ${\mathrm{Mn}}_{12}$. The magnetization curves indicate a coercive field of $0.95\mathrm{T}$ for ${\mathrm{Mn}}_{11}\mathrm{Cr}$ at $1.8\mathrm{K}$, nearly half that for ${\mathrm{Mn}}_{12}$. Quantum tunneling of magnetization (QTM) in ${\mathrm{Mn}}_{11}\mathrm{Cr}$ is observed below the blocking temperature $T_B$, with the same field interval as for ${\mathrm{Mn}}_{12}$. The magnetization of ${\mathrm{Mn}}_{11}\mathrm{Cr}$ and ${\mathrm{Mn}}_{12}$ in the mixed crystal can be independently manipulated by utilizing the difference between their coercive fields. The resonance fields of QTM in ${\mathrm{Mn}}_{11}\mathrm{Cr}$ are significantly affected by the magnetization direction of ${\mathrm{Mn}}_{12}$, suggesting the effect of dipolar-biased tunneling.

Figure 1

(a) The Cr (solid curve) and Mn (broken curve) $K$-edge EXAFS functions $k^3\chi \left(k\right)$ of ${\mathrm{Mn}}_{11}\mathrm{Cr}*$, together with the theoretical simulation results using FEFF8. The theoretical curves were obtained by assuming that Cr occupies site a (dotted curve), b (broken curve), or c (solid curve). (b) Fourier transforms of Fig. 1a. The molecular structure is given in the inset, where sites a, b, and c are shown.

Figure 2

High-frequency EPR $\left(381.5\mathrm{GHz}\right)$ spectra for ${\mathrm{Mn}}_{11}\mathrm{Cr}*$ (solid curves) and ${\mathrm{Mn}}_{12}$ (broken curves) at various temperatures.

Figure 3

The resonance points for ${\mathrm{Mn}}_{11}\mathrm{Cr}*$ at the eight frequencies. The closed and open circles indicate the positions of the odd-number and even-number absorptions, respectively. The solid and broken lines are theoretical ones for the $S=10$ and $S=19∕2$ spin species, respectively, with the same parameter set (see the text).

Figure 4

Temperature dependence of ac magnetic susceptibilities for polycrystalline samples of (a) ${\mathrm{Mn}}_{12}$ and (b) ${\mathrm{Mn}}_{11}\mathrm{Cr}*$ at various frequencies. The dependence for ${\mathrm{Mn}}_{11}\mathrm{Cr}*$ $\left(10\mathrm{Hz}\right)$ can be decomposed into two contributions (c).

Figure 5

Arrhenius plots of relaxation times $\tau$ obtained from the frequency dependence of the ac magnetic susceptibility. Open circles and squares denote the relaxations of ${\mathrm{Mn}}_{12}$ and ${\mathrm{Mn}}_{11}\mathrm{Cr}$ in ${\mathrm{Mn}}_{11}\mathrm{Cr}*$, respectively. Closed circles represent the results for pure ${\mathrm{Mn}}_{12}$.

Figure 6

Magnetization curves for ${\mathrm{Mn}}_{12}$ (a) and ${\mathrm{Mn}}_{11}\mathrm{Cr}*$ (b) at $1.8\mathrm{K}$ at the sweeping rate $0.17\mathrm{mT}∕\mathrm{s}$. The lower parts show the plots of $\mathrm{d}M∕\mathrm{d}B$.

Figure 7

Magnetization curves for ${\mathrm{Mn}}_{11}\mathrm{Cr}*$ at $1.8\mathrm{K}$ showing the four stable states. Solid and broken arrows depict the directions of magnetization of ${\mathrm{Mn}}_{12}$ and ${\mathrm{Mn}}_{11}\mathrm{Cr}$, respectively.

Figure 8

(a) The full magnetization (broken curve) and minor loop measurement results (solid curve) for ${\mathrm{Mn}}_{11}\mathrm{Cr}*$ at $1.8\mathrm{K}$. (b) Plots of $\mathrm{d}M∕\mathrm{d}B$ in the range $0–1.2\mathrm{T}$.

Figure 9

Angular dependence of magnetization curves for (a) ${\mathrm{Mn}}_{11}\mathrm{Cr}*$ at $1.8\mathrm{K}$ and (b) resonant fields for QTM. Solid curves represent the theoretical ones given by Eq. (4).