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Phys Rev Lett. 2015 Aug 28;115(9):097802. Epub 2015 Aug 28.

Ferromagnetic Switching of Knotted Vector Fields in Liquid Crystal Colloids.

Zhang Q1,2, Ackerman PJ1,3, Liu Q1, Smalyukh II1,2,3,4,5.

Author information

1
Department of Physics, University of Colorado, Boulder, Colorado 80309, USA.
2
Materials Science and Engineering Program, University of Colorado, Boulder, Colorado 80309, USA.
3
Department of Electrical, Computer and Energy Engineering, University of Colorado, Boulder, Colorado 80309, USA.
4
Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309, USA.
5
Renewable and Sustainable Energy Institute, National Renewable Energy Laboratory and University of Colorado, Boulder, Colorado 80309, USA.

Abstract

We experimentally realize polydomain and monodomain chiral ferromagnetic liquid crystal colloids that exhibit solitonic and knotted vector field configurations. Formed by dispersions of ferromagnetic nanoplatelets in chiral nematic liquid crystals, these colloidal ferromagnets exhibit spontaneous long-range alignment of magnetic dipole moments of individual platelets, giving rise to a continuum of the magnetization field M(r). Competing effects of surface confinement and chirality prompt spontaneous formation and enable the optical generation of localized twisted solitonic structures with double-twist tubes and torus knots of M(r), which exhibit a strong sensitivity to the direction of weak magnetic fields ∼1  mT. Numerical modeling, implemented through free energy minimization to arrive at a field-dependent three-dimensional M(r), shows a good agreement with experiments and provides insights into the torus knot topology of observed field configurations and the corresponding physical underpinnings.

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