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Marcelo Knobel

Bio: Marcelo Knobel is an academic researcher from State University of Campinas. The author has contributed to research in topics: Magnetization & Amorphous solid. The author has an hindex of 44, co-authored 354 publications receiving 8243 citations. Previous affiliations of Marcelo Knobel include Federico Santa María Technical University & Polytechnic University of Turin.


Papers
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Journal ArticleDOI
TL;DR: An overview on magnetic of nanostructured magnetic materials is presented, with particular emphasis on the basic features displayed by granular nanomagnetic solids.
Abstract: An overview on magnetic of nanostructured magnetic materials is presented, with particular emphasis on the basic features displayed by granular nanomagnetic solids. Besides a review of the basic concepts and experimental techniques, the role of structural disorder (mainly the distribution of grain sizes), interparticle magnetic interactions and surface effects are also discussed with some detail. Recent results, models and trends on the area are also discussed.

449 citations

Journal ArticleDOI
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

Journal ArticleDOI
TL;DR: The giant magneto-impedance effect (GMI) as mentioned in this paper is a phenomenon that changes the complex impedance of soft magnetic materials upon the application of an external magnetic field, which is strongly dependent on the frequency of the applied current and the magnetic anisotropies present in the material, among other factors.

268 citations

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TL;DR: In this paper, a systematic study of granular films by means of transmission electron microscopy (TEM), dc and ac initial magnetic susceptibility, and thermoremanent magnetization (TRM) is presented.
Abstract: A systematic study of $\mathrm{Co}({\mathrm{SiO}}_{2})$ granular films by means of transmission electron microscopy (TEM), dc and ac initial magnetic susceptibility, and thermoremanent magnetization (TRM) is presented. The experimental results are compared with simulations of zero-field-cooled (ZFC) and field-cooled (FC) magnetization and TRM curves obtained using a simple model of noninteracting nanoparticles. The simulated ZFC/FC curves, using the actual parameters obtained from the TEM images, show a different behavior than the experimental magnetic data. The effect of the dipolar interaction among particles introduces a self-averaging effect over a correlation length \ensuremath{\Lambda}, which results in a larger average ``magnetic'' size of the apparent particles together with a narrower size distribution. The analysis of the ZFC/FC curves in the framework of independent ``particle clusters'' of volume ${\ensuremath{\Lambda}}^{3},$ involving about 25 real particles, explains very well the observed difference between the experimental data for the median blocking temperature $〈{T}_{B}〉$ and their distribution width with respect to the ones expected from the structural observations by TEM. The experimental TRM curves also differ from those obtained from the theoretical model, starting to decrease at a lower temperature than expected from the model, also indicating the strong influence of dipole-dipole interactions.

226 citations

Journal ArticleDOI
TL;DR: In this article, spherical magnetic nanoparticles with narrow size distribution and organic capping were diluted in paraffin with different concentrations to verify the role of dipolar interactions on the macroscopic magnetic behavior.
Abstract: Spherical magnetic nanoparticles with narrow size distribution and organic capping were diluted in paraffin with different concentrations to verify the role of dipolar interactions on the macroscopic magnetic behavior. Increasing concentration of magnetic nanoparticles leads to higher blocking temperatures. The experimental data were analyzed by means of a recently proposed model that takes into account magnetic interactions of dipolar origin, and an excellent agreement was found. Considering the magnetic interaction among particles it was possible to obtain the real magnetic moment and estimate structural parameters that are consistent with the ones obtained by small angle x-ray scattering and transmission electron microscopy.

195 citations


Cited by
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01 Sep 1955
TL;DR: In this paper, the authors restrict their attention to the ferrites and a few other closely related materials, which are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present.
Abstract: In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

2,659 citations

Journal ArticleDOI
TL;DR: The phenomenology of exchange bias and related effects in nanostructures is reviewed in this paper, where the main applications of exchange biased nanostructure are summarized and the implications of the nanometer dimensions on some of the existing exchange bias theories are briefly discussed.

1,721 citations

Posted Content
TL;DR: The two-step solution-phase reactions to form hybrid materials of Mn(3)O(4) nanoparticles on reduced graphene oxide (RGO) sheets for lithium ion battery applications should offer a new technique for the design and synthesis of battery electrodes based on highly insulating materials.
Abstract: We developed two-step solution-phase reactions to form hybrid materials of Mn3O4 nanoparticles on reduced graphene oxide (RGO) sheets for lithium ion battery applications. Mn3O4 nanoparticles grown selectively on RGO sheets over free particle growth in solution allowed for the electrically insulating Mn3O4 nanoparticles wired up to a current collector through the underlying conducting graphene network. The Mn3O4 nanoparticles formed on RGO show a high specific capacity up to ~900mAh/g near its theoretical capacity with good rate capability and cycling stability, owing to the intimate interactions between the graphene substrates and the Mn3O4 nanoparticles grown atop. The Mn3O4/RGO hybrid could be a promising candidate material for high-capacity, low-cost, and environmentally friendly anode for lithium ion batteries. Our growth-on-graphene approach should offer a new technique for design and synthesis of battery electrodes based on highly insulating materials.

1,587 citations

Journal ArticleDOI
TL;DR: In this paper, the authors summarize the recent developments in the synthesis, structural characterization, properties, and applications of amorphous and nanocrystalline soft magnetic materials, including: kinetics and thermodynamics, structure, microstructure, and intrinsic and extrinsic magnetic properties.

1,453 citations