Magnetic field controlled composite paramagnetic-diamagnetic colloidal phases.
We report on differently ordered colloidal phases of a mixture of paramagnetic and diamagnetic colloids subject to a quickly varying time dependent magnetic field. Effectively paramagnetic and effectively diamagnetic colloids are created from paramagnetic and nonmagnetic colloids immersed into a thin film of aqueous ferrofluid. The time-averaged dipole interaction between induced dipoles can be characterized by a uniaxial external precession angle and a biaxial eccentricity characterizing the anisotropy of the external field modulation. The variation of both control parameters causes a sequence of transitions between differently correlated orientation order between the paramagnetic and diamagnetic colloids. We observe the formation of bonds between paramagnets and diamagnets along one or two directions with a staggered order of the magnetic moments. Bonds between similar particles with uniform order of the magnetic moments form along directions orthogonal to bonds between different particles along the staggered directions. When the external precession angle passes the magic angle, the particle order rearranges and staggered directions with bonds between different particles change into uniformly ordered directions with bonds between similar particles and vice versa. The transition in order occurs in two steps with a biaxial phase intervening between the two uniaxial ordering phases.
Institut für Experimentalphysik, Universität Bayreuth, 95440 Bayreuth, Germany.
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Fischer