On the magnetic anisotropy of lanthanide-containing metallomesogens

Abstract

A general theoretical model of the origin of the magnetic anisotropy in paramagnetic 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 mesophase are obtained. The net magnetic anisotropy of a real metallomesogen is shown to be the result of a complex interplay between the molecular magnetic anisotropy, orientation of the long molecular axis, and disorder effects. The sign of the magnetic anisotropy ⌬ depends not only on the anisotropy of the tensor of molecular magnetic susceptibility, but also on the orientation of the long molecular axis of rodlike lanthanide complexes with respect to the principal magnetic axes of the molecular tensor of magnetic susceptibility. The influence of microand macroscopic disorder in real liquid crystals is discussed. Numerical parametric calculations were used to rationalize the variation of the magnitude and sign of the magnetic anisotropy in a series of isostructural lanthanide-containing metallomesogens. Experimental magnetic susceptibility and magnetic anisotropy of a series of ͓Ln͑LH͒ 3 ͑NO 3 ͒ 3 ͔ compounds ͑LH is a Schiff base͒ are well reproduced by calculations based on the present model. Limitations of the Bleaney theory of magnetic anisotropy are analyzed.

Magnetic Properties of Liquid Crystals

“…The great interest in and potential for lanthanide-containing liquid crystals is their high magnetic anisotropy [30]. This has been measured for a number of materials and is found to be two or three orders of magnitude higher than for other paramagnetic liquid crystals.…”

Probing the Magnetic Anisotropy of Lanthanide‐Containing Metallomesogens by Luminescence Spectroscopy

“…The magnetic anisotropy has been calculated with the crystal-field parameters as the input in an algorithms described in ref. [7]. …”

“…Because the magnetic susceptibility is isotropic in cubic symmetry, and because the second rank crystal field parameters vanish in a cubic environment too (the fourth and six rank parameters being restricted in fixed ratios), a strong dependence of the value of the magnetic anisotropy on the value of the rank two crystal-field parameters is expected and this is proved by detailed calculations. [7] Taking into account this information and the fact that no major structural changes have been observed yet for lanthanide-containing metallomesogens over the lanthanide series, [5c] a method for a fast estimation of the second rank crystal-field parameters can also give an indication about the magnetic anisotropy expected for a given compound.…”

“…[12] The crystalfield parameters were used to calculate the maximum value of the magnetic anisotropy in these compounds. [7] The actual observed magnetic anisotropy is smaller, because of the order parameters of the liquid-crystalline phase and because the axis of maximum susceptibility does not coincide with the molecular long axis. [7] The calculated values can be considered as an upper limit.…”

“…[7] It was shown how the magnetic anisotropy can be calculated with the aid of a set of B k q crystal-field parameters, even in cases of a low symmetry. Such crystal-field parameters are often used for the interpretation of optical data, but it is much less known that these are also very useful for the calculation of magnetic data, especially for the simulation of the temperature dependence of the magnetic susceptibility.…”