Controllable magnetic properties of layered copper hydroxides, Cu2(OH)3X (X=carboxylates)

Physical and chemical properties of organic/inorganic hybrid nanocomposites are attractive, because they are potential materials yielding new functionality, such as a cooperative phenomenon, multi-functional property and so on. Layered copper hydroxides, Cu2(OH)3X (X=carboxylate anions), are two dimensional magnetic materials whose structures consist of alternating stacking of the magnetic copper hydroxides layer and the carboxylate layer. In this paper we report the preparations, structures and magnetic properties of various copper hydroxides. In the series of Cu2(OH)3(n-CmH2m+1COO), the carboxylate anions form an interdigitated monolayer in the m=0 and 1 materials, while those do a bilayer in the m=6–11 materials. The intermediate materials (m=2–5) exhibit both mono- and bi-layer structures, which are governed by the condition at their preparations. The temperature dependence of the paramagnetic susceptibilities of the m=0 and 1 materials suggests an intralayer ferromagnetic interaction, while that of the m=6–11 materials does an antiferromagnetic interaction. Further, the longer alkyl chain materials show a divergence of the ac magnetic susceptibility at 22 K, indicating a ferromagnetic order. It is found that the materials show a drastic change in magnetism, depending on the molecular orientation of the carboxylate anions. The Cu K-edge EXAFS spectroscopy at 20 K indicates a characteristic difference in local structure between the shorter and the longer alkyl chain materials, they are large enough to explain the drastic magnetic change. In this report, we also show controllable magnetic properties of Cu2(OH)3(8-((p-(phenylazo)phenyl)oxy)octanoate), derived from cooperation between the inorganic layer carrying magnetism and the organic layer playing a role of sensor. A reversible structural transformation occurs as a result of soaking in hot methanol and acetonitrile. The X-ray diffraction patterns indicate that the organic layer in the material exhibits an interdigitated monolayer structure in hot methanol, while it does a membrane-like bilayer structure in acetonitrile. The monolayer phase is paramagnetic down to 3 K, while the bilayer phase shows a ferromagnetic order below 10.8 K. The reversible packing change of the organic guest molecules results in the drastic modification in the magnetic properties of the inorganic layer. The material in the bilayer phase exhibits a trans-to-cis isomerization reaction of the azobenzene moiety by UV light illumination, although there is little change of the magnetic properties after the photoreaction. The photoreaction is irreversible under irradiation of VIS light.