Showing papers by "Miguel A. Novak published in 2011"

Determination of the relevant magnetic interactions in low-dimensional molecular materials: the fundamental role of single crystal high frequency EPR.

Abstract: A new one-dimensional copper(II) complex with formula [Cu(hfac)2(N3TEMPO)]n (hfac = hexafluoroacetylacetonate and N3TEMPO = 4-azido-2,2,6,6-tetramethylpiperidine-1-oxyl) has been synthesized and investigated by X-ray crystallography, magnetometry and multifrequency single crystal EPR. The system crystallizes in the P space group with two non equivalent copper(II) ions in the unit cell, the two nitroxide radicals being coordinated to Cu(1) in axial positions. The copper(II) ions are bridged by N3TEMPO radicals resulting in a zig-zag chain structure. The magnetic susceptibility data were at first satisfactorily modeled assuming an alternating spin chain along the monodimensional covalent skeleton, with a ferromagnetic interaction between Cu(1) and the nitroxide moieties and a weaker antiferromagnetic interaction between these and Cu(2) (J1 = −13.8 cm−1, J2 = +2.4 cm−1). However, single crystal EPR studies performed at the X- and W-band clearly demonstrate that the observed magnetic monodimensional character of the complex is actually due to the intermolecular contacts involving N3TEMPO ligands. This prompted us to fit the magnetic data using a consistent model, pointing out the fundamental role of single crystal EPR data in defining a correct model to describe the magnetic properties of molecular low dimensional systems.

Design of 1D and 2D molecule-based magnets with the ligand 4,5-dimethyl-1,2-phenylenebis(oxamato).

Abstract: Three new bimetallic oxamato-based magnets with the proligand 4,5-dimethyl-1,2-phenylenebis(oxamato) (dmopba) were synthesized using water or dimethylsulfoxide (DMSO) as solvents. Single crystal X-ray diffraction provided structures for two of them: [MnCu(dmopba)(H2O)3]n·4nH2O (1) and [MnCu(dmopba)(DMSO)3]n·nDMSO (2). The crystalline structures for both 1 and 2 consist of linearly ordered oxamato-bridged MnIICuII bimetallic chains. The magnetic characterization revealed a typical behaviour of ferrimagnetic chains for 1 and 2. Least-squares fits of the experimental magnetic data performed in the 300–20 K temperature range led to JMnCu = −27.9 cm−1, gCu = 2.09 and gMn = 1.98 for 1 and JMnCu = −30.5 cm−1, gCu = 2.09 and gMn = 2.02 for 2 (H = −JMnCu∑SMn,i(SCu,i + SCu,i−1)). The two-dimensional ferrimagnetic system [Me4N]2n{Co2[Cu(dmopba)]3}n·4nDMSO·nH2O (3) was prepared by reaction of CoII ions and an excess of [Cu(dmopba)]2− in DMSO. The study of the temperature dependence of the magnetic susceptibility as well as the temperature and field dependences of the magnetization revealed a cluster glass-like behaviour for 3.

Dynamic study of the internal magnetic order of Mn 3 O 4 nanoparticles

Abstract: The dynamic magnetic properties of Mn3O4 nanoparticles with mean diameter 〈ϕ〉 = 15 nm have been investigated by frequency and dc-field (HDC) dependence of the in phase (χ′) and out of phase (χ″) ac-susceptibility. The studies were performed in non-interacting and interacting systems of Mn3O4 nanoparticles diluted in a polymer with concentrations 1.5 and 17.6%. The ac-susceptibility of the non-interacting system, measured with HDC = 0 Oe, presents only one maximum located at TC = 42 K associated to the paramagnetic (PM)–ferrimagnetic (FiM) transition. In contrast, the susceptibility of the interacting system shows two anomalies. One frequency independent peak associated to TC and a second low temperature maximum was observed in χ″, located at TP. The position of the TP maximum shifts to higher temperature when the frequency increases. The relation between the relaxation time and TP was well described by the Vogel–Fulcher law. When the susceptibility was measured with HDC = 20 kOe, the PM–FiM transition was observed in both systems. Remarkably, in the non-interacting system, the low temperature anomaly is evidenced by the magnetic field. This anomaly is present as a well-defined maximum, which shifts to higher temperature when the frequency increases.