The development of new magnetic materials has attracted attention of researchers of different areas. In the last decades, a distinguished class of materials emerged in magnetism, in which the magnetic moment is delocalized over molecules. By varying the synthetic conditions it is possible to obtain a large variety of structures and properties using the same starting molecules. These materials have a great scientific appeal due to the possibility of presenting not only magnetic, but also optical or electrical transport properties. In this review we will present an overview of some molecular magnetic compounds, in particular molecular nanomagnets.

Neste trabalho é descrito o estudo das propriedades magnéticas de um novo complexo cobre(II)-NIT, onde NIT representa um radical do tipo nitronil nitróxido. O precursor (NBu4)4[Cu2(bopba)] (onde NBu4+ é o cátion tetrabutilamônio e bopba é o bis-o-fenilenobis(oxamato)) teve a sua estrutura cristalina determinada por difração de raios X e também foi caracterizado magneticamente. Através da complexação deste precursor com o radical catiônico nitronil nitróxido, um novo complexo foi obtido, o (pEtRad)4[Cu2(bopba)] (onde pEtRad+ é o radical catiônico para-2-(4-piridil)-4,4,5,5-tetrametilimidazolina-1-oxil-3 -óxido). Os estudos magnéticos sugerem que neste complexo existe uma competição entre interações ferromagnéticas e antiferromagnéticas. A interação ferromagnética pode ser explicada com base na ortogonalidade dos orbitais magnéticos envolvidos.

In this work we describe the structure and the magnetic properties of a new copper(II)-NIT complex, where NIT stands for nitronyl nitroxide radical. This was achieved using (NBu4)4[Cu2(bopba)] as a precursor (where NBu4+ is the tetrabutylammonium cation and bopba is bis-o-phenylenebis(oxamate)), whose crystal structure was resolved and its magnetics properties studied. From the reaction of this precursor with a cationic nitronyl nitroxide radical, a new compound was prepared and magnetically characterized, namely (pEtRad)4[Cu2(bopba)] (where pEtRad+ stands for the cationic para-2-(4-pyridil)-4,4,5,5-tetramethylimidazoline-1 -oxyl-3-oxide radical). The magnetic properties suggest that in this six-spin complex there is a competition between short range ferromagnetic and antiferromagnetic interactions. The ferromagnetic interaction can be explained by the orthogonality of the involved magnetic orbitals.

O complexo [Fe2III(BPClNOL)2(SO4)] 3 foi sintetizado pela reação do pré-ligante N-(2-hidroxibenzil)-N-(2-metilpiridil)[(3-cloro)(2-hidroxi)]propilamina (H2BPClNOL) com FeSO4.7H2O. O composto oxidado apresenta absorção máxima em 519 nm (épsilon = 1,40X10³ dm³ mol-1 cm-1) e a espécie de valência mista absorve em 439 nm (épsilon = 9,8X10² dm³ mol-1 cm-1). O processo de redução da espécie oxidada ocorre em -0,843 V vs. Fc/Fc+. A espectroscopia Mössbauer confirmou que o composto 3 possui átomos de Fe3+, spin alto. Medidas magnéticas mostram que a unidade binuclear apresenta acoplamento antiferromagnético, J/kB = - 15 K (J =-10,4 cm-1). O complexo [Fe2III(BPClNOL)2(SO4)] 3 é o terceiro integrante de uma família de compostos binucleares de ferro sintetizados com o ligante H2BPClNOL ([Fe2III(BPClNOL)2 (OAc)]+ 1, [Fe2III(BPClNOL)2(H2O) 2]2+ 2). A comparação das propriedades físico-químicas dos complexos indica que o número de grupos pontes, assim como a natureza dos mesmos, apresentam significativas influências sobre o espectro eletrônico e as propriedades eletroquímicas dos referidos complexos.

Complex [Fe2III(BPClNOL)2(SO4)] 3 was synthesized through the reaction of the pro-ligand N-(2-hydroxybenzyl)-N-(2-pyridylmethyl)[(3-chloro)(2-hydroxy)]propylamine (H2BPClNOL) and FeSO4.7H2O. The maximum absorption for the oxidized compound was at 519 nm (epsilon = 1.40X10³ dm³ mol-1 cm-1) while that of the mixed-valence compound was at 439 nm (epsilon = 9.8X10² dm³ mol-1 cm-1). The redox process to transform the oxidized compound to the mixed-valence form occurs at E1/2 = -0.843 V vs. Fc/Fc+. The Mössbauer spectrum confirms the presence of FeIII high spin ions and the magnetic studies reveal a weak antiferromagnetically coupled dimer, J/kB = - 15 K ( J = -10.4 cm-1). Complex 3 is the third of a family of dinuclear iron compounds synthesized with the H2BPClNOL ligand ([Fe2III(BPClNOL)2 (OAc)]+ 1, [Fe2III(BPClNOL)2(H2O) 2]2+ 2). Analysis of the physicochemical properties of these complexes allowed us to infer that the number and kind of bridging groups have a significant influence on their electrochemical and UV-Vis properties.

Cobalt ferrite composite thin films, CoFe2O4/SiO2, were prepared by sol-gel process, using tetraethylorthosilicate (TEOS) as a precursor for silica, and metallic nitrates as precursors for ferrite. The obtained dip-coated films thermally treated at 550, 750, and 950 ºC were transparent, homogeneous and adherent. Their magnetic properties were measured from 2.5 K to 300 K using a SQUID magnetometer. Superparamagnetic behaviour was observed at room temperature and below a blocking temperature appears with coercivity values increasing with annealing temperature.