Small-angle light scattering study of quasi-two-dimensional structures of magnetic particles

We studied quasi-two-dimensional magnetic domain structures using a small-angle light scattering technique. The structures are formed when magnetic particles in an aqueous suspension between two parallel glass plates are assembled and oriented in a magnetic field H perpendicular to the plates. Three different structure regions can be identified, and they are bounded by two critical field lines H_c1 (φ) and H_c2(φ). Below H_c1 (φ) the system consists of single particles; between H_c1 (φ) and H_C2 (φ), the particles assemble into elongated disordered domains; and above H_c2 (φ), the domains develop a strong in-plane correlation. In the strong field limit H H_c2, the structure factor of the magnetic fluid resembles that of a two-dimensional liquid. However, long lasting metastability and hysteresis observed in the experiment suggest that the system is essentially locked in a glassy state.