Crystal structure, magnetic susceptibility, and EPR study of bis-( β -alaninium) tetrachlorocuprate(II): Spin-diffusion effects in a two-dimensional square planar ferromagnet with anisotropic and antisymmetric exchange

The EPR spectra of the title compound revealed an unusual linewidth anisotropy. The anisotropy showed behavior characteristic of relaxation effects due both to exchange anisotropy and to spin diffusion. This is the first ferromagnetic layer salt of the ${(R\mathrm{N}{\mathrm{H}}_{3})}_{2}(\mathrm{Cu}{\mathrm{Cl}}_{4})$ type which has exhibited spin-diffusion effects. For this reason, the crystal structure of the salt was determined and the single-crystal magnetic behavior characterized pursuant to the analysis of the EPR linewidth anisotropy. The crystal structure of the salt ($\ensuremath{\beta}$${\ensuremath{-}\mathrm{a}\mathrm{l}\mathrm{a}\ensuremath{-}\mathrm{H})}_{2}$Cu${\mathrm{Cl}}_{4}$ (where $\ensuremath{\beta}$-ala-H is the $\ensuremath{\beta}$-alaninium cation, $^{+}\mathrm{N}$${\mathrm{H}}_{3}$C${\mathrm{H}}_{2}$C${\mathrm{H}}_{2}$COOH) has been determined by x-ray-diffraction techniques. The salt is monoclinic, space group $\frac{I2}{c}$, with $a=7.407(2)$ \AA{}, $b=7.734(1)$ \AA{}, $c=24.236$ \AA{}, and $\ensuremath{\beta}=91.82(1)\ifmmode^\circ\else\textdegree\fi{}$. The salt forms the typical two-dimensional Cu-Cl networks sandwiched between layers of organic cations. The planar Cu${\mathrm{Cl}}_{4}^{2\ensuremath{-}}$ ions (Cu\char22{}Cl avg=2.290 \AA{}) assumes a typical 4+2 coordination geometry by bridging to adjacent ions with interionic Cu\char22{}Cl distances of 3.077 \AA{}. The carboxylic acid groups of $\ensuremath{\beta}$-ala-H ions from adjacent layers form hydrogen-bonded dimers, providing stability to the lattice. The magnetic susceptibilities of powder and single-crystal samples of ($\ensuremath{\beta}$${\ensuremath{-}\mathrm{a}\mathrm{l}\mathrm{a}\ensuremath{-}\mathrm{H})}_{2}$Cu${\mathrm{Cl}}_{4}$ were measured between 2 and 150 K on a PAR vibrating sample magnetometer. The data showed the presence of strong ferromagnetic coupling and were analyzed in terms of a quadratic layer ferromagnet. The intralayer exchange constant is $\frac{J}{k}=13.8$ K, the smallest reported to date for an ${(R\mathrm{N}{\mathrm{H}}_{3})}_{2}\mathrm{Cu}{\mathrm{Cl}}_{4}$ salt. Single-crystal data are consistent with ferromagnetic coupling between layers. The salt behaves as an $\mathrm{XY}$ magnet in which the axis perpendicular to the layer (${c}^{*}$) is the hard axis of magnetization. The out-of-plane anisotropy ${H}_{a}^{\mathrm{out}}$ and the in-plane anisotropy ${H}_{a}^{\mathrm{in}}$ are estimated to be 850 and 150 Oe, respectively. The EPR spectra of the salt were recorded at 9 and 35 GHz over the temperature range 77-300 K. The spectral parameters were independent of frequency, and the linewidth remained Lorentzian at all temperatures and orientations. The components of the $g$ tensor were found to be ${g}_{a}=2.171$, ${g}_{b}=2.151$, and ${g}_{c}=2.045$. The linewidth anisotropy was analyzed in terms of two processes: short-time processes dominated by spin anisotropy and longer-time spin-diffusion effects. The spin-diffusion term gives rise to a ${(3{cos}^{2}\ensuremath{\theta}\ensuremath{-}1)}^{2}$ angular dependence characteristic of two-dimensional systems. The remaining contributions to the linewidths were analyzed in terms of the anisotropic exchange ($D$) and the antisymmetric exchange ($\stackrel{\ensuremath{\rightarrow}}{\mathrm{d}}$), and yielded $\frac{{d}_{x}}{k}=0.15$ K, $\frac{{d}_{y}}{k}=0.12$ K, and $\frac{{({{d}_{z}}^{2}+2.5{D}^{2})}^{\frac{1}{2}}}{k}=0.15$ K. The existence of the spin-diffusive effects is attributed to the combination of small interlayer perturbations and small spin anisotropy.