Strong interplay between magnetic and structural properties in the spin-$\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$ chain molecular compound $\text{D-}{\mathrm{F}}_5\text{PNN}$

We present elastic and inelastic neutron scattering results on spin-1/2 chain antiferromagnet $\text{D-}{\mathrm{F}}_5\text{PNN}$. We show that a structural transition occurs in zero field below $T_c\sim 710\text{mK}$ from $C2/c$ space group (uniform chains) to $Pc$ (bond-alternating chains). When applying a magnetic field at 50 mK, the space group gets back to $C2/c$ above $H_{c1}\sim 1\text{T}$. From the dispersion curves of the magnetic excitations measured at $T=50\text{mK}$, we determine the values of the exchange parameters at 0 and 9 T, which evidence that the magnetic field makes the chains evolve from alternating to uniform. This change in behavior can be correlated unambiguously with the field-induced structural transition. These results evidence “mirror” field- and temperature-induced magnetocrystalline transitions.

Figure 1

(Color online) RT structure of $\text{D-}{\mathrm{F}}_5\text{PNN}$. (a) Projection of a single molecule in the plane of the ${\text{ONCNO}}^{-}$ group (in the dashed ellipse) on which the unpaired electron is delocalized. (b) Projection along the $b$ axis, evidencing the magnetic chains running along $a+c$ (dashed lines). For sake of clarity, the deuterium atoms are not represented.

Figure 2

(Color online) (a) Temperature and (b) field dependence of the $10\overline{10}$ integrated intensity, measured at $H=0$ for increasing temperature and at $T=50\text{mK}$ for increasing and decreasing field, respectively. Inset: rocking curves on the $10\overline{10}$ reflection for various increasing field values.

Figure 3

(Color online) Dispersion curves of the magnetic excitations at $H=0$ and 9 T for $H\parallel b$, measured along the chain direction (closed symbols). The solid lines are fit to the data (see text). Inset: background-subtracted constant-$Q$ energy scans measured at $H=0$ (empty symbols) and fitted by one or two Gaussian function(s) (solid lines).

Figure 4

(Color online) Schematic view of the spin density (dark spots), in the ${\text{ONCNO}}^{-}$ group (ellipses), along the chains. (a) Uniform chains at $T>T_c$; (b) dimerized chains at $T<T_c$ (spin-Peierls scenario); (c) spin-density ordering in dimerized chains at $T<T_c$.