Dc and ac magnetic properties of the two-dimensional molecular-based ferrimagnetic materials A2M2[Cu(opba)]3nsolv [A+=cation, MII=MnII or CoII, opba=ortho-phenylenebis(oxamato) and solv=solvent molecule

Abstract

This paper is devoted to a thorough study of the magnetic properties, both in the dc and ac modes, of two-dimensional ferrimagnetic materials. The general formula of the compounds, abbreviated as A 2 M 2 Cu 3 , is A 2 M 2 [Cu(opba)] 3 nsolv where A + stands for a counter cation (radical cation, alkali-metal or tetraalkylammonium), opba is ortho-phenylenebis(oxamato), M II is Mn II or Co II , and solv is a solvent molecule (Me 2 SO or H 2 O). The dc magnetic susceptibility data for all compounds down to ca. 50 K are characteristic of two-dimensional ferrimagnets. In the A 2 Mn 2 Cu 3 series, three classes of compounds have been distinguished. Class I has a unique representative, A 2 Mn 2 Cu 3 , where A + is the radical cation 2-(1-methylpyridinium-4-yl)-4,4,5,5-tetramethylimadozolin-1-oxyl-3-oxide, the structure of which has already been solved. This compound is a magnet with T c =22.5 K. Class II corresponds to compounds with large cations (tetraethyl- and n-tetrabutylammonium). They also behave as magnets with T c around 15 K. Class III corresponds to compounds with small cations (alkali-metal ions and tetramethylammonium). They behave as metamagnets with a long-range antiferromagnetic ordering in zero field around 15 K, and a field-induced ferromagnetic state. The critical fields are of the order of 0.15 kOe. All the A 2 Co 2 Cu 3 compounds are magnets with T c around 30 K. Furthermore, the cobalt derivatives show a very strong coercivity, with coercive fields of several kOe at 5 K. They also display a pronounced magnetic after-effect in the ordered phase. In the course of this work several peculiar features have been observed. In particular, the A 2 Mn 2 Cu 3 compounds have been found to present weak but significant negative out-of-phase ac magnetic signals at temperatures just above T c . All the observed phenomena are discussed and, in particular, a mechanism for the long-range ordering in these two-dimensional compounds has been proposed.