Research ArticleCRYSTAL STRUCTURE

Field-controlled structures in ferromagnetic cholesteric liquid crystals

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Science Advances  06 Oct 2017:
Vol. 3, no. 10, e1701336
DOI: 10.1126/sciadv.1701336
  • Fig. 1 Ferromagnetic CLC in homeotropic LC cells.

    (A) Schematic of director orientation in bulk CLC. (B) Schematic of magnetic nanoplatelets in a nematic LC. Magnetic orientation of the platelets is denoted by red (N) and blue (S) colors. Schematic of ferromagnetic CLC in a homeotropic cell. Red arrows represent the orientation of M, and yellow cylinders represent the orientation of n. During the filling of the LC cell, parallel electric and magnetic fields were applied so that the director and the magnetization were uniformly oriented and the sample was magnetically monodomain. When B is perpendicular to E, pTIC structure appears. (C) CLC in a wedge cell with perpendicular (homeotropic) anchoring. (D) Fingerprint structure of ferromagnetic CLC in the case of d/P = 0.94. In the dark regions, which separate the fingers, the director and the magnetization are uniformly oriented. (E) Fingerprint structure (left), structure immediately after the unwinding (middle), and structure 15 min later (right) in the case of d/P = 1.74.

  • Fig. 2 POM images of structures that evolve from pTIC when either B or U is slowly reduced (0.16 mT/0.7 s and 10 mV/1.5 s).

    (A) d/P = 0.94 and B is reduced. (B) d/P = 1.74 and U is reduced at constant B. (C) d/P = 1.74 and B is reduced at constant U. (D) Blue dots denote experimentally obtained values of Ut(B) and Bt(U), where the pTIC becomes unstable, and red dots denote theoretical prediction of Bt(U) obtained by stability analysis. In cases where U > 1.56 V, the direct transition from the pTIC to the fingerprint configuration was not observed; however, nucleation of cholesteric fingers was present.

  • Fig. 3 Reversal of magnetic field in the case of d/P = 1.74.

    (A and B) The POM images show the homogeneous magneto-optic response. Schematics below the images show the calculated corresponding director and magnetic field configurations. Every configuration is represented from two side views: (A) U > Ut(B) and (B) UUt(B). Here, two resulting TICs were observed: the pTIC2 (the second image and the region outside the lines in the third image) and the reversed pTIC (within the lines in the third image). (C) Evolution of structures from pTIC2 outside and reversed pTIC within the lines after B is turned off. (D) Response of the sample on the reversal of B at magnitudes around Bt and U = 0. The third image shows the stable structure that evolves continuously from monodomain pTIC2 at B = 0.

  • Fig. 4 POM images and cross section in the xz plane of calculated structures with 1D periodicity.

    (A) d/P = 0.94 (structure that evolves from pTIC when external fields are switched off). (B) d/P = 1.74 (transient structure that evolves from pTIC at constant U = 1.5 when B is switched off. In the POM image, evolution of fingers in different stages can be observed, whereas the simulation shows the last stage of evolution.) (C) d/P = 1.74 (structure that evolves from pTIC2 when external fields are switched off). The direction of out-of-plane orientation of the magnetization is shown in some typical regions by gray circles surrounding “x” or the dot. The yellow lines on POM images denote the regions that correspond to the accompanying calculated structures.

  • Fig. 5 Schematic of 2D periodic instability, which causes diamond pattern in the case of d/P = 1.74 and U = 0.

    The structure evolves continuously from pTIC by two critical fluctuations. The maximal amplitude of the first fluctuation is located closer to the top, and that of the second fluctuation is located closer to the bottom of the layer. Schematics show the structure at the instability. Gray arrows behind the first and third schematics show the calculated wave vectors of the fluctuation, which is dominant in the corresponding region. The yellow rectangle on the POM image denotes the region that corresponds to the three in-plane cross sections.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/10/e1701336/DC1

    fig. S1. Inhomogeneous nucleation of cholesteric fingers in the case of d/P = 0.94 in the absence of external fields.

    fig. S2. Comparison of the response of pTIC and pTIC2 to the electric field.

    fig. S3. Phase diagrams showing stability of pTIC versus pTIC2 as obtained from 1D model.

    fig. S4. POM images of structures that evolve from pTIC in the case of constant B and d/P = 1.74.

    fig. S5. The eight smallest nontrivial eigenvalues of fluctuations as a function of external magnetic field showing the destabilization of the pTIC structure.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Inhomogeneous nucleation of cholesteric fingers in the case of d/P = 0.94 in the absence of external fields.
    • fig. S2. Comparison of the response of pTIC and pTIC2 to the electric field.
    • fig. S3. Phase diagrams showing stability of pTIC versus pTIC2 as obtained from 1D model.
    • fig. S4. POM images of structures that evolve from pTIC in the case of constant B and d/P = 1.74.
    • fig. S5. The eight smallest nontrivial eigenvalues of fluctuations as a function of external magnetic field showing the destabilization of the pTIC structure.

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