Pressure Effect on the 3-D Magnetic Ordering of a Quasi-1-D Enantiopure Molecular Magnet Vladimir Laukhin,* ,† Benjamı ´n Martı ´nez, ‡ Josep Fontcuberta, ‡ David B. Amabilino, § Maria Minguet, § and Jaume Veciana § Institucio ´ Catalana de Recerca i Estudis AVanc ¸ ats (ICREA) and Institut de Cie ` ncia de Materials de Barcelona (ICMAB-CSIC) Campus UniVersitari 08193-Bellaterra, Spain, and Department of Magnetic and Superconducting Materials and Department of Molecular Nanoscience and Organic Materials at Institut de Cie ` ncia de Materials de Barcelona (ICMAB-CSIC) Campus UniVersitari 08193-Bellaterra, Spain ReceiVed: April 22, 2004; In Final Form: September 16, 2004 Magnetic measurements of the quasi 1-D molecular magnet formed by the coordination polymer of purely organic R-nitronyl nitroxide radical and manganese(II) ions, namely [(R)-3MLNN-Mn(hfac) 2 ], have been carried out under pressure up to 10 kbar at low temperatures. The results obtained show that the temperature of 3-D ferromagnetic ordering of the 1-D ferrimagnetic chains in this molecular magnet increases substantially under pressure. Analysis of the experimental data obtained, as well as a review of the ferromagnetic ordering temperature with respect to interchain coupling distances in closely related compounds, gives one grounds to suggest that the interchain magnetic coupling in the 3-D ferromagnetic ordering for this series of coordination complexes cannot be only dipolar (as often inferred). Rather, the interaction must be stronger, and in all likelihood, it is mediated mainly by exchange interactions. Introduction Interest in the properties of molecular magnetic materials has resulted in dramatic progress in this area over the last couple of decades. 1-4 Essentially all of the common magnetic phe- nomena associated with conventional transition metals and rare- earth-based magnets can be found in molecular-based materials. However, a major advantage of molecule-based magnets is their production by means of controllable molecular chemistry, which allows fine-tuning of structures and magnetic behaviors. 5,6 Thus, it is possible to modulate structural parameters in order to elucidate the important factors governing otherwise complex behavior. Quasi-one-dimensional (1-D) metallo-organic mag- netic systems are a prominent example of this ability to vary properties over broad range. 7,8 Many of these quasi-1-D complexes order magnetically in three dimensions (3-D) at low temperatures, and it has been argued that this results from ferromagnetic interactions between the 1-D spin chains. 9-12 However, the mechanistic rational for this ordering remains, still, somewhat enigmatic. 13 Understanding the nature of the magnetic interactions in molecular systems constitutes a fun- damental step in the development of new molecular magnets with higher magnetic ordering temperatures (T C ). A prime example of the 3-D magnetic ordering phenomenon is that of the coordination polymers formed by R-nitronyl nitroxides and manganese(II) bis(hexafluoroacetylacetonate), [Mn(hfac) 2 ]. In these materials, the antiferromagnetically coupled spin 1 / 2 of the radical and spin 5 / 2 of the metal ion lead to 1-D ferrimagnetic chains with a remnant spin of 2 per repeat unit. 14 The interchain interactions leading to bulk ordering were originally thought to arise from dipolar interactions because of the relatively large magnetic moment at low temperature, as well as the lack of obvious interchain close contacts and superexchange pathway. However, calculations of dipolar inter- actions in combination with EPR and SQUID measurements showed that the dipolar interaction alone cannot explain the type of magnetic order exhibited by compounds of this series. 15 Instead, single-ion anisotropies were appealed to as a possible origin of the ordering phenomenon, suggesting strong enough nearest-neighbor-exchange interactions. 15 Hence, the problem of the ordering phenomena in this family of compounds seems somewhat more profound than might be assumed. Being able to describe the nature of this interaction would constitute a major step forward in the understanding of these materials and the design of new molecular magnets. Recently, 16 we reported the preparation, crystal structure, and optical and magnetic properties of a new coordination compound comprised of one of the purely organic chiral R-nitronyl nitroxides, 17,18 namely (R)-3MLNN, and manganese(II) bis(h- exafluoroacetylacetonate), [Mn(hfac) 2 ]. This crystalline enan- tiopure chiral complex, (R)-3MLNN-Mn(hfac) 2 (1) has homo- chirality as well as a ferromagnetic phase transition at T C ) 3 K. Below T C , at fields larger than around 10 kOe, the magnetization * Corresponding Author: Fax: +34-93-580-5729. Telephone: +34-93- 580-1853. E-mail: vladimir@icmab.es. † ICREA and ICMAB-CSIC. ‡ Department of Magnetic and Superconducting Materials at ICMAB- CSIC. § Department of Molecular Nanoscience and Organic Materials at ICMAB-CSIC. 18441 J. Phys. Chem. B 2004, 108, 18441-18445 10.1021/jp048237v CCC: $27.50 © 2004 American Chemical Society Published on Web 11/09/2004