Layer-by-Layer Self-Assembly: Toward Magnetic Films with Tunable Anisotropy

Highly anisotropic magnetic films are possible to fabricate by control of the coupling between individual magnetic particles. Selective control over coupling in the horizontal and vertical directions are of both fundamental and practical interest. Here we show such control in the multiple layer-by-layer (LBL) self-assembly of layered double hydroxide (LDH) nanosheets (x = 1, 2, 3 and 4) with thicknesses of 5-15 nm, co-assembled with 3-aminopropyl-trimethoxysilane (APTS) modified spherical Fe3O4 nanoparticles (APTS-Fe3O4 NPs) on quartz substrates. The electrostatic charge density on the LDH sheets, controlled by the Mg/Al composition ratio, affects the NP packing in a single horizontal layer, while the thickness of the LDH sheets controls magnetic coupling between layers. The tunable magnetic properties (coercivity H-c, saturation magnetization M-s, anisotropy, and blocking temperatures) are measured as a function of these parameters. The maximum saturation magnetizations M-s, 36.3 and 25.1 emu.g(-1) in the perpendicular and parallel direction, respectively, are found for the sample of x = 3 = Mg/Al ratio in the LDH layer, and 15 nm LDH layer thickness. This work provides a general method to adjust the anisotropy of magnetic films based on directional control of coupling of magnetic nanoparticles between and across, LDH nanosheets. We outline how higher anisotropy and even finer control could be achieved by pH and composition control over the electrostatic charge of the assembly components.