Unconventional $S=2$ alternating chain realized by a metal-radical hybrid-spin approach

We demonstrate an advanced spin-system design using a hybrid spin consisting of a strongly coupled metal ion and verdazyl radical. Ab initio calculation, magnetization, and ESR measurements evidenced the first realization of an $S=2$ ferromagnetic-antiferromagnetic alternating chain with Ising anisotropy in a Mn-verdazyl complex $\left[{\mathrm{Mn}\left(\mathrm{hfac}\right)}_2\right]·(o-\mathrm{Py}-\mathrm{V})$ [hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate; o-Py-V = 3-(2-pyridyl)-1,5-diphenylverdazyl]. Furthermore, we find an anomalous change in magnetization at 1/4 of the saturation value, which is probably a manifestation of the quantum nature of the system.

Figure 1

(a) Molecular structure of [Mn(hfac)${}_2]·$($o$-Py-V). (b) Schematic view of intramolecular interaction between spins on ${\mathrm{Mn}}^{2+}$ and verdazyl radical and resulting spin states. (c) Crystal structure forming an alternating chain along the $c$ axis and (d) the corresponding spin model. Hydrogen atoms are omitted for clarity.

Figure 2

Temperature dependence of magnetic susceptibility ($\chi =M/H$) of [Mn(hfac)${}_2]·$($o$-Py-V) at 0.01 and 0.05 T for $H//b$. The upper and lower insets show the temperature dependence of $\chi T$ for high- and low-temperature regions, respectively. The solid line represents the calculated results for the $S_{\mathrm{Mn}}-S_{\mathrm{V}}$ dimer coupled by $J_{\mathrm{intra}}$.

Figure 3

Specific heat of [Mn(hfac)${}_2]·$($o$-Py-V) at (a) 0 and 0.5 T for $H//b$ and (b) 0.05 T for the field parallel to the three crystallographic axes. The values for $H//a$ and $H//c$ have been shifted up by 2.0 and 5.0 J/mol K, respectively.

Figure 4

Magnetization curve of [Mn(hfac)${}_2]·$($o$-Py-V) at 0.08 K for (a) $H//b$ and (b) $H//c$. The vertical dashed line shows the field where $dM/dH$ for $H//c$ exhibits the nontrivial peak. The solid lines with symbols represent the results calculated using the MC method.

Figure 5

(a) Temperature dependence and (b)–(d) frequency dependence of ESR absorption spectra of [Mn(hfac)${}_2]·$($o$-Py-V) at 94.3 GHz and 1.5 K for (a) and (b) $H//a$, (c) $H//b$, and (d) $H//c$. The arrows indicate the resonance fields. Each inset shows the calculated energy branch.

Figure 6

Frequency-field diagram of the resonance fields at 1.5 K for $H//a, H//b$, and $H//c$. The solid lines indicate the calculated paramagnetic resonance modes of the hybrid spin $S$ = 2 with on-site biaxial anisotropy, which correspond to the transitions indicated by the arrows in the energy branches.