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.