Denis Rancourt
Rancourt D.G. (1987)
In: Legrand A.P., Flandrois S. (eds) Chemical Physics of Intercalation. NATO ASI Series (Series B: Physics), vol 172. Springer, Boston, MA. https://doi.org/10.1007/978-1-4757-9649-0_4
Abstract
The intrinsic magnetism of two-dimensional (2D) lattices is of fundamental interest and has been the subject of intense theoretical studies. These studies have motivated much of the experimental work on layered and intercalated compounds. The idea of an “effective dimensionality” which can be varied from three to two as the inter-layer or inter-intercalate separation is increased has been an additional incentive. The latter notion is mainly the product of intuitive curiosity since theory has been of little help in guiding our expectation concerning such intermediate dimensionalities. That is, surprizingly little is known about local moments on quasi-2D lattices [which are essentially three-dimensional (3D) lattices with symmetries lower than cubic] compared to exactly 2D lattices. Nonetheless, one might expect to observe an h-T phase diagram such as that represented in Fig. 1 where h is the inter-layer or inter-intercalate separation and T is the absolute temperature.
Fig. 1
Hypothetical h-T phase diagram where h is the inter-layer distance
For example, h could be the c-axis of a tetragonal system in which the spins are on the corners of the square end faces. This hypothetical phase diagram exhibits three regions: a high temperature paramagnetic (P) phase, a phase where some form of 2D order is stabilized and a 3D ordered phase. Particular examples of the meaning and somewhat limited usefulness of such a phase diagram are given below. It is of more consequence, however, to admit that the relation between structure and magnetism is of a far greater variety (and subtlety) than that which can be expressed by the single parameter h. This view is stressed and illustrated by many examples throughout the present paper.