Showing papers by "Miguel A. Novak published in 2001"
1,045 citations
TL;DR: In this article, 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.
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.
312 citations
TL;DR: In this article, the slow dynamics of the magnetization in the paramagnetic phase, predicted by Glauber for 1d Ising ferromagnets, has been observed with ac susceptibility and SQUID magnetometry measurements in a molecular chain comprising alternating Co{2+} spins and organic radical spins strongly antiferromagneticically coupled.
Abstract: The slow dynamics (10^-6 s - 10^4 s) of the magnetization in the paramagnetic phase, predicted by Glauber for 1d Ising ferromagnets, has been observed with ac susceptibility and SQUID magnetometry measurements in a molecular chain comprising alternating Co{2+} spins and organic radical spins strongly antiferromagnetically coupled. An Arrhenius behavior with activation energy Delta=152 K has been observed for ten decades of relaxation time and found to be consistent with the Glauber model. We have extended this model to take into account the ferrimagnetic nature of the chain as well as its helicoidal structure.
75 citations
TL;DR: In this article, a simple model describing the amount of iron incorporation in the polymeric template is proposed, and the magnetite-based composite was investigated by atomic absorption, transmission electron microscopy, Mo 1ssbauer spectroscopy, X-ray diffraction, and magnetization data.
Abstract: Preparation of size-controllable magnetite (Fe3O4) nanoparticles by alkaline oxidation of ferrous ion adsorbed in sulfonated mesoporous styrenedivinylbenzene copolymer is described. It was observed that the magnetite nanoparticle size increases by ion-charging the sulfonated polymeric template with ferrous aqueous solution at increasing iron concentration. A simple model describing the amount of iron incorporation in the polymeric template is proposed. The magnetite-based composite was investigated by atomic absorption, transmission electron microscopy, Mo 1ssbauer spectroscopy, X-ray diffraction, and magnetization data.
64 citations
TL;DR: In this article, three bimetallic compounds have been synthesized, the formulae of which are [3-MeRad] 2 [Cu(opba)]·4H 2 O (1), [3MeRad + ]·6DMSO·8H2 O (2), and [3 -MeRad 2 [Mn 2 {Cu (opba)} 3 ]·3DMS O(2.5H 2 ).
16 citations
TL;DR: In this paper, specific heat measurements at very low temperatures of the single molecule magnets Mn12ac and Fe8 were presented, and the experimental results were explained considering a distribution of internal fields of dipolar and hyperfine origin as well as quantum tunneling of the magnetization.
12 citations
TL;DR: In this paper, atomic force microscopy (AFM) and magnetization measurements were used to unfold the particle-size polydispersity profile of a magnetite-based magnetic fluid sample.
9 citations
TL;DR: The magnetization curves of Co 10 Cu 90 as-cast and annealed melt-spun ribbons were measured for 4.2 < T < 600 K as discussed by the authors, showing that the narrow hysteresis found in these systems present an unusual temperature dependence of the coercive field H c (T), with a decrease in the low T region due to superparamagnetic unblocking followed by an increase in T and a maximum around room temperature.
5 citations
TL;DR: In this article, preliminary high-eld pulse magnetization measurements were done in the new IFW Dresden facility on powder MnAc quantum nanomagnets, at different sweep rates.
Abstract: We present preliminary high-"eld pulse magnetization measurements done in the new IFW Dresden facility on powder Mn Ac quantum nanomagnets. Measurements were done between 1.9 and 4.2 K; at di!erent sweep rates; as well as in static "elds in a SQUID magnetometer for comparison.The pulsed "eld technique opens new possibilities to study quantum tunneling in nanomagnets. 2001 Elsevier Science B.V. All rights reserved.
2 citations
TL;DR: The bulk magnetic properties of pure organic ferromagnet Dupeyredioxyl (N, N, N ′-dioxy-1,3,5,7-tetramethyl-2,6-diazaadamantane, C 12 N 2 O 2 H 20 ) are presented in this paper.
1 citations