On the magnetic anisotropy of lanthanide-containing metallomesogens

A general theoretical model of the origin of the magnetic anisotropy in par amagnetic metal-containing liquid crystals is developed. General relations between the molecular magnetic anisotropy of mesogenic lanthanide complexes and the macroscopic magnetic anisotropy of these liquid crystals in the me sophase are obtained. The net magnetic anisotropy of a real metallomesogen is shown to be the result of a complex interplay between the molecular magn etic anisotropy, orientation of the long molecular axis, and disorder effec ts. The sign of the magnetic anisotropy Delta chi depends not only on the a nisotropy of the tensor of molecular magnetic susceptibility, but also on t he orientation of the long molecular axis of rodlike lanthanide complexes w ith respect to the principal magnetic axes of the molecular tensor of magne tic susceptibility. The influence of micro- and macroscopic disorder in rea l liquid crystals is discussed. Numerical parametric calculations were used to rationalize the variation of the magnitude and sign of the magnetic ani sotropy in a series of isostructural lanthanide-containing metallomesogens. Experimental magnetic susceptibility and magnetic anisotropy of a series o f [Ln(LH)(3)(NO3)(3)] compounds (LH is a Schiff base) are well reproduced b y calculations based on the present model. Limitations of the Bleaney theor y of magnetic anisotropy are analyzed. (C) 2000 American Institute of Physi cs. [S0021-9606(00)71146-9].