Langmuir-Blodgett films of known layered solids: Preparation and structural properties of octadecylphosphonate bilayers with divalent metals and characterization of a magnetic Langmuir-Blodgett film

Langmuir-Blodgett (LB) films of a series of divalent metal octadecylphosphonates have been prepared and characterized. The films are each shown to be LB analogs of known solid-state metal phosphonates possessing 2-dimensional ionic-covalent metal phosphonate layers. The metal phosphonate layers crystallize during the LB deposition process. Films were characterized with XPS, X-ray diffraction, ellipsometry, attenuated total reflectance FTIR, and, in the case of the manganese film, SQUID magnetometry. Octadecylphosphonate films with Mn²⁺ Mg²⁺, and Cd²⁺ form with the stoichiometry M(O₃PC₁₇H₃₇)·H₂O and have metal phosphonate bonding consistent with the analogous M(O₃PR)·H₂O layered solids. The Ca²⁺ film forms as Ca(HO₃PC₁₈H₃₇)₂, which is also a known solid-state phase. Magnetic measurements reveal that the manganese oetadecylphosphonate film undergoes a magnetic ordering transition at 13.5 K resulting in a 'weak ferromagnet'. The behavior is similar to that of the known layered solid- state manganese alkylphosphonates which are also 'weak ferromagnets'. The magnetic ordering is antiferromagnetic where incomplete cancellation of the magnetic sublattices, due to low site symmetry, results in a spontaneous magnetization. A spin-flop transition is observed at 2.5 T in magnetization vs applied field measurements of the ordered state. The film also exhibits magnetic memory, with a small remnant magnetization and a coercive field of 20 mT at 2 K. The results demonstrate that magnetic ordering phenomena can be incorporated into LB films and that LB film methods can be used to prepare monolayer and multilayer films of known solid-state materials.