Cu2+complex ofL-isoleucine: A five-coordinate two-dimensional ferromagnet

Magnetic susceptibility and ESR measurements on the ${\mathrm{Cu}}^{2+}$ complex of the amino acid $L$-isoleucine (${\mathrm{C}}_{12}$ ${\mathrm{H}}_{24}$Cu${\mathrm{N}}_2$ ${\mathrm{O}}_4$·${\mathrm{H}}_2$O) are reported. Between 4.2 and 0.24 K, the magnetic behavior is best described as due to a two-dimensional ferromagnetic Heisenberg interaction with $\frac{J_F}{k}=0.120$ K. A transition to a magnetically ordered state takes place at $T_c=0.117$ K. Well below $T_c$ a magnetic field of approximately 140 Oe applied in the $\mathrm{ac}$ plane causes a magnetic phase transition. A zero-temperature Hamiltonian is proposed to explain the behavior in applied field. In the model, spins in each layer are coupled by the ferromagnetic exchange and by an antisymmetric term of the form $\overrightarrow{\mathrm{D}}·{\overrightarrow{\mathrm{S}}}_1\times {\overrightarrow{\mathrm{S}}}_2$ with $\frac{D}{k}=0.012\mathrm{}$ K. An antiferromagnetic exchange between the layers $\frac{J_{\mathrm{AF}}}{k}=0.02\mathrm{}$ K produces the three-dimensional ordering.