Reverse Micelle Synthesis of Co−Al LDHs: Control of Particle Size and Magnetic Properties

Nanoplatelet Co−Al layered double hydroxides (LDHs) {Co 1.8 Al 1.1 (OH) 6 (DDS) x · y H 2 O (DDS = dodecyl sulfate; x ≈ 0.9; y = 2−6)} have been prepared using a homogeneous precipitation approach in water pools formed by a water-in-oil reverse microemulsion. The size of the LDH particles is controlled by adjusting the water to surfactant ratio (ω). The particle sizes vary from about 60 × 60 × 13 nm 3 to 160 × 160 × 40 nm 3 with increasing ω. This contrasts with platelets of Co 1.89 Al(OH) 6 (DDS) 0.8 ·4.3H 2 O prepared by conventional homogeneous precipitation which had dimensions about 2−3 μm wide and 0.5−1 μm thick. All the Co−Al LDH samples were paramagnetic above 50 K. The molar magnetic susceptibilities of each sample were very similar and they could all be fitted to the Curie−Weiss law with μ eff ca. 5.2 μ B per Co 2+ and θ ca. 6 K. Below 50 K, we observed a rapid rise in χ M T indicative of cooperative ferromagnetic interactions. At very low temperature the LDH nanoplatelets show magnetic behavior which is different to Co 1.8 Al 1.1 (OH) 6 (DDS) x · y H 2 O made by conventional homogeneous precipitation. The nanoplatelets show bifurcation in their field-cooled (FC) and zero-field-cooled (ZFC) magnetizations and also a frequency dependence of the in-phase (χ′) and out-of-phase (χ′′) ac susceptibilities, which is dependent on the size of the nanoplatelets.