Field-induced quantum magnetism in the verdazyl-based charge-transfer salt $\left[o\text{−}\mathrm{MePy}\text{−}\mathrm{V}\text{−}{\left(p\text{−}\mathrm{Br}\right)}_2\right]{\mathrm{FeCl}}_4$

We successfully synthesized a verdazyl-based charge-transfer salt $\left[o\text{−}\mathrm{MePy}\text{−}\mathrm{V}\text{−}{\left(p\text{−}\mathrm{Br}\right)}_2\right]{\mathrm{FeCl}}_4$, which has $S_{\mathrm{V}}=1/2$ on the radical $o\text{−}\mathrm{MePy}\text{−}\mathrm{V}\text{−}{\left(p\text{−}\mathrm{Br}\right)}_2$ and $S_{\mathrm{Fe}}=5/2$ on the ${\mathrm{FeCl}}_4$ anion. Ab initio molecular orbital calculations indicate the formation of an $S_{\mathrm{V}}=1/2$ honeycomb lattice composed of three types of exchange interaction with two types of inequivalent sites. Further, the $S_{\mathrm{V}}=1/2$ at one site is sandwiched by $S_{\mathrm{Fe}}=5/2$ spins through antiferromagnetic (AF) interactions. The magnetic properties indicate that the dominant AF interactions between the $S_{\mathrm{V}}=1/2$ spins form a gapped singlet state, and the remaining $S_{\mathrm{Fe}}=5/2$ spins cause an AF order. The magnetization curve exhibits a linear increase up to approximately 7 T, and an unconventional 5/6 magnetization plateau appears between 7 and 40 T. We discuss the differences between the effective interactions associated with the magnetic properties of the present compound and ($o$-MePy-V)${\mathrm{FeCl}}_4$. We explain the low-field linear magnetization curve through a mean-field approximation of an $S_{\mathrm{Fe}}=5/2$ spin model. At higher field regions, the 5/6 magnetization plateau and subsequent nonlinear increase are reproduced by the $S_{\mathrm{V}}=1/2$ AF dimer, in which a particular internal field is applied to one of the spin sites. The ESR signals in the low-temperature and low-field regime are explained by conventional two-sublattice AF resonance modes with easy-axis anisotropy. These results demonstrate that exchange interactions between $S_{\mathrm{V}}=1/2$ and $S_{\mathrm{Fe}}=5/2$ spins in $\left[o\text{−}\mathrm{MePy}\text{−}\mathrm{V}\text{−}{\left(p\text{−}\mathrm{Br}\right)}_2\right]{\mathrm{FeCl}}_4$ realize unconventional magnetic properties with low-field classical behavior and field-induced quantum behavior.

Figure 1

(a) Molecular structure of $\left[o\text{−}\mathrm{MePy}\text{−}\mathrm{V}\text{−}{\left(p\text{−}\mathrm{Br}\right)}_2\right]{\mathrm{FeCl}}_4$. Molecular pairs associated with exchange interactions (b) $J_1$, (c) $J_2$, (d) $J_3$, and (e) $J_4$ and $J_5$. Hydrogen atoms are omitted for clarity. The dashed lines indicate C-C and N-Cl short contacts. Crystal structure in the (f) $ab$ and (g) $bc$ planes. The lines represent $J_1$, $J_2$, and $J_3$ forming the honeycomb lattice.

Figure 2

Two-dimensional spin model composed of $J_i$ ($i=1-5$) with $S_{\mathrm{V}}=1/2$ and $S_{\mathrm{Fe}}=5/2$ in the $ab$ plane. $J_1$, $J_2$, and $J_3$ form the honeycomb lattice of $S_{\mathrm{V}}$, and $S_{\mathrm{V}}$ on the site shown by the gray ball is sandwiched by $S_{\mathrm{Fe}}=5/2$ spins through $J_4$ and $J_5$, yielding the particular internal field.

Figure 3

Temperature dependence of magnetic susceptibility ($\chi =M/H$) of $\left[o\text{−}\mathrm{MePy}\text{−}\mathrm{V}\text{−}{\left(p\text{−}\mathrm{Br}\right)}_2\right]{\mathrm{FeCl}}_4$ at 0.1 and 1.0 T. The inset shows the temperature dependence of $\chi T$.

Figure 4

Specific heat $C_{\mathrm{p}}$ of $\left[o\text{−}\mathrm{MePy}\text{−}\mathrm{V}\text{−}{\left(p\text{−}\mathrm{Br}\right)}_2\right]{\mathrm{FeCl}}_4$ at zero field. The upper and lower insets show the low-temperature part of $C_{\mathrm{p}}/T$ and the evaluated entropy, respectively. The solid line shows the $T$-linear fit below approximately 0.8 K.

Figure 5

(a) Specific heat $C_{\mathrm{p}}$ of $\left[o\text{−}\mathrm{MePy}\text{−}\mathrm{V}\text{−}{\left(p\text{−}\mathrm{Br}\right)}_2\right]{\mathrm{FeCl}}_4$ in the low-temperature regions at various magnetic fields. The arrows indicate the phase transition temperatures. For clarity, the values for 0, 1.0, 2.0, 3.0, 4.0, 5.0, 5.5, and 6.0 T have been shifted up by 36, 31, 26, 21, 16, 13, 9.0, and 4.7 J ${\mathrm{mol}}^{-1}\mathrm{K}$, respectively. (b) Magnetic field vs temperature phase diagram showing AF order of $S_{\mathrm{Fe}}=5/2$ spins. The square and circles indicate the phase boundaries determined from the magnetic susceptibility and the specific heat, respectively

Figure 6

Magnetization curve $M$ of $\left[o\text{−}\mathrm{MePy}\text{−}\mathrm{V}\text{−}{\left(p\text{−}\mathrm{Br}\right)}_2\right]{\mathrm{FeCl}}_4$ at 1.5 K. The solid and dashed lines represent the calculated result for the $S_{\mathrm{V}}=1/2$ AF dimer at the experimental temperature and for the $S_{\mathrm{Fe}}=5/2$ spin model using the mean-field approximation at zero temperature, respectively. The inset shows $dM/dH$ at 1.8 K, and the arrow indicates a sharp peak associated with the spin-flop transition at $H_{\mathrm{SF}}$.

Figure 7

Frequency dependence of ESR absorption spectra of $\left[o\text{−}\mathrm{MePy}\text{−}\mathrm{V}\text{−}{\left(p\text{−}\mathrm{Br}\right)}_2\right]{\mathrm{FeCl}}_4$ at 1.7 K for (a) directly detected high frequencies and (b) low frequencies measured by cylindrical high-sensitivity cavities. The arrows indicate resonance signals.

Figure 8

Frequency-field plot of the ESR fields at 1.7 K. The lines indicate the calculated AF resonance modes for $H\parallel z$ and $H\perp z$. The discontinuous changes of the lines correspond to the spin-flop transition at $H_{SF}$.

Figure 9

One of the ${\mathrm{FeCl}}_4$ layers of $\left[o\text{−}\mathrm{MePy}\text{−}\mathrm{V}\text{−}{\left(p\text{−}\mathrm{Br}\right)}_2\right]{\mathrm{FeCl}}_4$ in the $ab$ plane. The dashed lines indicate Cl-Cl short contacts. The corresponding distances for the other ${\mathrm{FeCl}}_4$ layer are in parentheses.