Multifrequency ESR in ${\text{ET}}_2\text{MnCu}{\left[\text{N}{\left(\text{CN}\right)}_2\right]}_4$: A radical cation salt with quasi-two-dimensional magnetic layers in a three-dimensional polymeric structure

The radical cation salt, ${\text{ET}}_2\text{MnCu}{\left[\text{N}{\left(\text{CN}\right)}_2\right]}_4$, $\left[\text{ET}=\text{bis}\left(\text{ethylenedithio}\right)\text{tetrathiafulvalene}\right]$ with an unusual three-dimensional anionic polymeric network is studied by x-ray diffraction, static susceptibility measurements, and electron spin resonance (ESR) at frequencies between 9 and 420 GHz. The magnetic properties are determined by the alternating two-dimensional layers of the ${\text{Mn}}^{2+}$ ions of the network and the partially charged ET molecules. At ambient temperature the overlap between ${\text{Mn}}^{2+}$ ions and ET molecules is weak and an exchange integral $\left|J_{\text{Mn}-\text{ET}}\right|\approx 4\cdot {10}^{-2}\text{K}$ is estimated from their resolved ESR lines. At lower temperatures, ${\text{ET}}_2\text{MnCu}{\left[\text{N}{\left(\text{CN}\right)}_2\right]}_4$ is not a simple system of weakly interacting paramagnetic ions in spite of the isotropic, Curie-like static susceptibility. There are first-order phase transitions at 292 K and in the range of 120–180 K. One of the lattice constants shows anomalous temperature dependence below 292 K. Anisotropic ESR shifts appear below 150 K, which we explain by demagnetizing fields of the platelike crystals and an exchange-narrowed fine structure. The latter contributes significantly to the shift when the populations of Zeeman levels are altered in high magnetic fields at low temperatures. We estimated the exchange coupling between ${\text{Mn}}^{2+}$ ions within a layer, $J_{\text{Mn-Mn}}\approx -48\text{K}$ and determined the fine structure parameters below 150 K, showing a distortion in the plane of the ${\text{Mn}}^{2+}$ ions.

Figure 1

Layers of ET molecules interpenetrated by the 3D polymeric anion network. Projection of the structure on the $\left(a,c\right)$ plane.

Figure 2

Octahedrally coordinated manganese ions connected by dicyanamid (dca) ligands in the $\left(b,c\right)$ plane. Magnetic properties arise mostly from the manganese ions.

Figure 3

(Color online) Temperature dependence of the central $\text{C}=\text{C}$ bond lengths in the ET radical cations. A, B, C, and D denote crystallographically independent ET radical cations. Dotted lines mark the boundaries between regions corresponding to neutral, $+1/2$ and $+1$ oxidation states of the molecule, in order of increasing bond length.

Figure 4

Magnetic moment vs magnetic field along the $b$ axis at several temperatures. Solid lines are sums of $S=5/2$ and $S=1/2$ Brillouin functions corresponding to free ${\text{Mn}}^{2+}$ ions and ET molecules, respectively. The amplitude of the function was fitted to the 20 K data and there are no further fitting parameters in the curves.

Figure 5

(Color online) Upper curve: ESR spectrum in the $a^{\ast }$ direction at 420 GHz and 300 K. Resolved lines correspond to the ${\text{Mn}}^{2+}$ ions and the ET molecules. The small sharp signal at 14.96 T is the $g$-factor reference. Lower curve: simulation of the measured spectrum.

Figure 6

(Color online) Shift of the ESR line with respect to the ${\text{KC}}_{60}$ reference in the $a^{\ast }$ direction at 300 K as a function of frequency, corrected for dipole fields, $\Delta B_a=\Delta B_a^{\text{meas}}-\Delta B_{\text{Ma}}$ (see Sec. 4C1). The Mn and ET lines shift nonlinearly (solid squares and circles, respectively) but the average of the two resonances (triangles) is linear with frequency (solid line). The lines are guides to the eye.

Figure 7

(Color online) Shifts of the Mn and the ET ESR lines with respect to the ${\text{KC}}_{60}$ reference as a function of temperature on cooling at 420 GHz in the $a^{\ast }$ direction. The phase transition at 291 K is marked by a jump in the shift of the ET line. There is another broad transition between 180 and 120 K where the ET line shifts rapidly toward the Mn line.

Figure 8

(Color online) Frequency dependence of the line width along the $b$ and $c$ directions at 260 K. The line width shows a quadratic frequency dependence above 75 GHz.

Figure 9

(Color online) Temperature dependence of the resonance field (a) at 9 GHz and (b) at 210 GHz in the three principal directions. A large anisotropy is observed below 150 K. In panel (b) the calculated ESR shifts due to dipole-dipole interaction, $\Delta B_{\text{Ma}}\left(T\right),\Delta B_{\text{Mb}}\left(T\right)$ and $\Delta B_{\text{Mc}}\left(T\right)$, are indicated by dotted, dashed, and solid lines, respectively.

Figure 10

(Color online) (a) Shift of the ESR for $B\parallel b$ at 210 GHz. Vertical solid line (ref.): ${\text{KC}}_{60}$ reference; dashed line (ZFS): shift calculated from exchange-narrowed zero-field splitting; dash dotted line $\left(\Delta B_{\text{Mb}}\right)$: shift from dipolar fields; solid line $\left(\text{ZFS}+\Delta B_{\text{Mb}}\right)$: sum of the shifts from unresolved ZFS and dipolar fields. (b) Simulation of the spectrum of an isolated spin with ZFS at two temperatures. The intensities of the transitions depend on the population of the Zeeman levels and change with temperature. Dashed and dotted lines indicate the intensity-weighted average of the resonance field at 300 and 2 K, respectively. (ZFS parameters: $b_{20}=-340\text{MHz}$, $b_{22}=1057\text{MHz}$. Relative amplitudes of the spectra are arbitrary.)