Miguel A. Novak

Bio: Miguel A. Novak is an academic researcher from Federal University of Rio de Janeiro. The author has contributed to research in topics: Magnetization & Magnetic susceptibility. The author has an hindex of 34, co-authored 118 publications receiving 8148 citations. Previous affiliations of Miguel A. Novak include Centre national de la recherche scientifique.

Magnetic bistability in a metal-ion cluster

Abstract: MAGNETIC materials of mesoscopic dimensions (a few to many thousands of atoms) may exhibit novel and useful properties such as giant magnetostriction, magnetoresistivity and magnetocaloric effects1–4. Such materials also allow one to study the transition from molecular to bulk-like magnetic behaviour. One approach for preparing mesoscopic magnetic materials is to fragment bulk ferromagnets; a more controllable method is to take a 'bottom-up' approach, using chemistry to grow well defined clusters of metal ions5,6. Lis7 has described a twelve-ion manganese cluster in which eight of the Mn ions are in the +3 oxidation state (spin S=2) and four are in the +4 state (S=3/2). These ions are magnetically coupled to give an S=10 ground state8, giving rise to unusual magnetic relaxation properties8,9. Here we report that the magnetization of the Mn12 cluster is highly anisotropic and that the magnetization relaxation time becomes very long below a temperature of 4 K, giving rise to pronounced hysteresis. This behaviour is not, however, strictly analogous to that of a bulk ferromagnet, in which magnetization hysteresis results from the motion of domain walls. In principle, a bistable magnetic unit of this sort could act as a data storage device.

A ferromagnetic transition at 1.48 K in an organic nitroxide

Abstract: IN one of the pioneering studies of magnetism at the microscopic level, Heisenberg1 concluded that ferromagnetism could not exist in compounds consisting only of light elements. A few well defined2, purely organic materials (consisting only of the elements carbon, hydrogen, oxygen and nitrogen) have subsequently been found3–5 to exhibit evidence of ferromagnetic interactions at low tem-peratures, with a small number of these showing a transition to a true ferromagnetic state. For example, the p-nitrophenyl nitronyl nitroxide radical has a Curie (transition) temperature of 0.60 K (ref. 6), and a possible ferromagnetic transition has been identified7,8 in a C60-based ionic complex. More recently, it was reported9 that the magnetization and magnetic susceptibility behaviour of a crystalline nitroxide biradical, N,N′-dioxy-l,3,5,7-tetramethyl-2,6-diazaadamantane, are indicative of ferromagnetic interactions. Here we report the existence of a ferromagnetic transition in this system, with a Curie temperature of 1.48 ± 0.02 K. As yet, this is the highest transition temperature found for a purely organic, non-ionic material.

Granular Cu-Co alloys as interacting superparamagnets

Abstract: The anhysteretic magnetization of the granular metallic alloy ${\mathrm{Cu}}_{90}{\mathrm{Co}}_{10}$ is experimentally studied over a wide temperature range (2--700 K). The measurements definitely exclude that this alloy is a simple superparamagnet, even in the high-temperature limit, although some features of granular systems [such as the typical Langevin-like form of the anhysteretic magnetization curves $M(H)]$ are often taken as evidence of superparamagnetism. A phenomenological theory is proposed, explicitly considering that particle moments interact through long-ranged dipolar random forces, whose effect is pictured in terms of a temperature ${T}^{*},$ adding to the actual temperature T in the denominator of the Langevin function argument. This simple formula explains all features of the experimental $M(H)$ curves. The theory indicates that the actual magnetic moments on interacting Co particles are systematically larger than those obtained fitting the magnetic data to a conventional Langevin function. The ${\mathrm{Cu}}_{90}{\mathrm{Co}}_{10}$ granular alloy is therefore identified as an ``interacting superparamagnet'' ISP. The ISP regime appears as separating the high-temperature, conventional superparamagnetic phase from the low-temperature, blocked-particle regime. In this way, a magnetic-regime diagram can be drawn for each granular system. The competition between single-particle and collective blocking mechanisms is briefly analyzed. The proposed interpretation is thought to be applicable to other fine particle systems; its main features and intrinsic limits are discussed.

The Chemistry and Applications of Metal-Organic Frameworks

Abstract: Crystalline metal-organic frameworks (MOFs) are formed by reticular synthesis, which creates strong bonds between inorganic and organic units. Careful selection of MOF constituents can yield crystals of ultrahigh porosity and high thermal and chemical stability. These characteristics allow the interior of MOFs to be chemically altered for use in gas separation, gas storage, and catalysis, among other applications. The precision commonly exercised in their chemical modification and the ability to expand their metrics without changing the underlying topology have not been achieved with other solids. MOFs whose chemical composition and shape of building units can be multiply varied within a particular structure already exist and may lead to materials that offer a synergistic combination of properties.

Engineering coordination polymers towards applications

Abstract: The development in the field of coordination polymers or metal-organic coordination networks, MOCNs (metal-organic frameworks, MOFs) is assessed in terms of property investigations in the areas of catalysis, chirality, conductivity, luminescence, magnetism, spin-transition (spin-crossover), non-linear optics (NLO) and porosity or zeolitic behavior upon which potential applications could be based.

Electronics using hybrid-molecular and mono-molecular devices

Abstract: The semiconductor industry has seen a remarkable miniaturization trend, driven by many scientific and technological innovations. But if this trend is to continue, and provide ever faster and cheaper computers, the size of microelectronic circuit components will soon need to reach the scale of atoms or molecules—a goal that will require conceptually new device structures. The idea that a few molecules, or even a single molecule, could be embedded between electrodes and perform the basic functions of digital electronics—rectification, amplification and storage—was first put forward in the mid-1970s. The concept is now realized for individual components, but the economic fabrication of complete circuits at the molecular level remains challenging because of the difficulty of connecting molecules to one another. A possible solution to this problem is ‘mono-molecular’ electronics, in which a single molecule will integrate the elementary functions and interconnections required for computation.

Recent advances in the liquid-phase syntheses of inorganic nanoparticles.

Abstract: The development of novel materials is a fundamental focal point of chemical research; and this interest is mandated by advancements in all areas of industry and technology. A good example of the synergism between scientific discovery and technological development is the electronics industry, where discoveries of new semiconducting materials resulted in the evolution from vacuum tubes to diodes and transistors, and eventually to miniature chips. The progression of this technology led to the development * To whom correspondence should be addressed. B.L.C.: (504) 2801385 (phone); (504) 280-3185 (fax); bcushing@uno.edu (e-mail). C.J.O.: (504)280-6846(phone);(504)280-3185(fax);coconnor@uno.edu (e-mail). 3893 Chem. Rev. 2004, 104, 3893−3946