Daniel R. Cornejo

Reversible and irreversible magnetization in hybrid magnets

Magnetization reversal was studied in hybrid magnets consisting of ferrite and ribbons of the Magnequench alloy MQP-Q (Nd2Fe14B+α-Fe). Recent studies of demagnetization processes in permanent magnets have used two different methods for determining the reversible magnetization, often arriving at conflicting results. The reversible Mrev and irreversible Mirr magnetizations were determined here by the two different procedures: (i) In the dc demagnetization method, the demagnetization remanence is measured for reverse magnetic fields after saturation in the forward direction. Assuming the change in magnetization along the recoil curve is reversible, the remanent magnetization is identified with the irreversible magnetization Mirr. (ii) Alternatively, the intrinsic reversible susceptibility χrevi was measured by defining it as the mean slope of a small minor loop performed after determining the time dependence of M. The reversible susceptibility was then integrated to obtain Mrev. Although the two procedures f...

Alginate/magnetite hybrid beads for magnetically stimulated release of dopamine.

Hybrid beads composed of magnetite nanoparticles (MNP) and alginate (Alg) were synthesized and coded as Alg-MNP. They were incubated in dopamine (DOPA) solution (5 g/L), at pH 7.4 and 8 °C, during 12 h, promoting the DOPA loaded magnetic beads, coded as Alg-MNP/DOPA. The release of DOPA was further evaluated in the absence and the presence of external magnetic field (EMF) of 0.4 T. The products Alg-MNP and Alg-MNP/DOPA were characterized by scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), Fourier transform infrared vibrational spectroscopy (FTIR), UV spectrophotometry, thermogravimetric analyses (TGA), inductively coupled plasma atomic emission spectroscopy (ICP-AES) analyses and superconducting quantum interference device (SQUID) magnetometer. The magnetic and chemical properties of Alg-MNP beads were not affected by DOPA loading. The incorporation of DOPA into the beads depended on the pH and on the negative charge density. At pH 7.4 38% of DOPA were loaded into Alg-MNP beads, whereas at pH 2 or using neat Alg beads (lower charge density than Alg-MNP) the loading efficiency decreased to one third or less. In the absence of EMF, 24% of the loaded DOPA was released from Alg-MNP at pH 7.4 over a period of 26 h. The released amount increased to 33% under the stimulus of EMF. A model was proposed to explain the loading efficiency of charged drugs, as DOPA, into hybrid beads and the role played by EMF on delivery systems, where drug and matrix are oppositely charged. The results suggest that the alginate combined with magnetite nanoparticles is a promising system for release of DOPA in the presence of EMF.

Magnetic properties and underlayer thickness in SmCo/Cr films

SmCo films with a Cr underlayer have been investigated as potentially attractive candidates for high density recording media. Magnetron sputtering was used here to produce Cr/SmCox/Cr films on Si (100) substrates. The magnetic films were deposited at a substrate temperature of 350 °C and an Ar pressure of 5 mTorr. Cr underlayers were deposited both at 25 °C and at 350 °C and exhibited different textures. Layer thicknesses were evaluated using Rutherford backscattering spectroscopy, while SmCo grain size and underlayer texture were determined from a Rietveld analysis of x-ray diffraction data. The magnetic properties were measured with a vibrating sample magnetometer. The resulting films had in-plane coercivities in the range 4–8 kOe. For a SmCo layer with a thickness of 200 nm, the Cr underlayer thickness was varied in the range 75–300 nm to study its effect on intergranular coupling in the films. For thermally demagnetized samples, both magnetizing and demagnetizing remanence curves were measured and use...

Moving Preisach model analysis of nanocrystalline SmFeCo

Nanocrystalline SmFeCo, prepared by mechanically alloying the elementary powders and subsequent annealing (600 °C/15 min), showed remanence enhancement (η=Ir/Is=0.67) and Hc=18.6 kOe. Starting from the ac-demagnetized state, minor hysteresis loops were determined for magnetic fields up to 70 kOe for numerical calculation of the Preisach function p(hc,hu), where hc and hu are the elementary loop coercive field and interaction field. The numerically determined irreversible part of p(hc,hu) had a form indicating that it could be factored: pirr(hc,hu)=f(hc)g(hu), where f(hc) and g(hu) are log–normal and Gaussian distributions. An analytic expression for p(hc,hu)=f(hc)g(hu)+prev was fitted to experimental hysteresis curves. The analytic expression for p(hc,hu) was used with the moving Preisach model to calculate minor loops and Henkel plots. Agreement between theory and experiment was very good.

Hybrid composites of xanthan and magnetic nanoparticles for cellular uptake

We describe a fast and simple method to prepare composite films of magnetite nanoparticles and xanthan networks. The particles are distributed close to hybrid film surface, generating a coercivity of 27 ± 2 Oe at 300 K. The proliferation of fibroblast cells on the hybrid composites was successful, particularly when an external magnetic field was applied.

Application of the Preisach model to nanocrystalline magnets

Experimental results are presented for several exchange-coupled nanocrystalline alloys of composition Sm 18 (Fe,Co) 82 with di!erent Fe contents. Minor loops, Henkel plots, and recoil curves are shown to be in excellent agreement with calculations performed using the moving Preisach model (MPM). Experimental reversible and irreversible magnetization components are found to be in excellent agreement with MPM simulations. ( 1999 Elsevier Science B.V. All rights reserved.

Neuronal adhesion, proliferation and differentiation of embryonic stem cells on hybrid scaffolds made of xanthan and magnetite nanoparticles

Hybrid scaffolds made of xanthan and magnetite nanoparticles (XCA/mag) were prepared by dipping xanthan membranes (XCA) into dispersions of magnetic nanoparticles for different periods of time. The resulting hybrid scaffolds presented magnetization values ranging from 0.25 emu g(-1) to 1.80 emu g(-1) at 70 kOe and corresponding iron contents ranging from 0.25% to 2.3%, respectively. They were applied as matrices for in vitro embryoid body adhesion and neuronal differentiation of embryonic stem cells; for comparison, neat XCA and commercial plastic plates were also used. Adhesion rates were more pronounced when cells were seeded on XCA/mag than on neat XCA or plastic dishes; however, proliferation levels were independent from those of the scaffold type. Embryonic stem cells showed similar differentiation rates on XCA/mag scaffolds with magnetization of 0.25 and 0.60 emu g(-1), but did not survive on scaffolds with 1.80 emu g(-1). Differentiation rates, expressed as the number of neurons obtained on the chosen scaffolds, were the largest on neat XCA, which has a high density of negative charge, and were smallest on the commercial plastic dishes. The local magnetic field inherent of magnetite particles present on the surface of XCA/mag facilitates synapse formation, because synaptophysin expression and electrical transmission were increased when compared to the other scaffolds used. We conclude that XCA/mag and XCA hydrogels are scaffolds with distinguishable performance for adhesion and differentiation of ESCs into neurons.

Preisach analysis of reversible magnetization in SmCo/Fe films

SmCo(50 nm)/Fe(y nm) bilayers (y=0, 2.5, 5, 20) were prepared by magnetron sputtering on Si substrates. The films showed coercivities in the range 1.5–14 kOe as well as remanence enhancement, characteristic of exchange-coupled systems. Henkel plots for these bilayers showed that the demagnetizing interactions increase as the thickness of the Fe layer increases. For each film, a set of minor loops was measured in order to determine the Preisach distributions. With suitable analytic forms for these distributions, we obtained a description of the experimental data for the major loops, the minor loops and recoil curves. The comparison with experiment is excellent. From the moving Preisach model we have determined the evolution of the reversible and irreversible magnetization components during the magnetization and demagnetization processes. The most important source of Mrev is the reversible rotation of the magnetization in the grains of the SmCo layer. The soft Fe layer, when well coupled to the SmCo layer, ...

AC susceptibility as a tool to probe the dipolar interaction in magnetic nanoparticles

Abstract The dipolar interaction is known to substantially affect the properties of magnetic nanoparticles. This is particularly important when the particles are kept in a fluid suspension or packed within nano-carriers. In addition to its usual long-range nature, in these cases the dipolar interaction may also induce the formation of clusters of particles, thereby strongly modifying their magnetic anisotropies. In this paper we show how AC susceptibility may be used to obtain information regarding the influence of the dipolar interaction in a sample. We develop a model which includes both aspects of the dipolar interaction and may be fitted directly to the susceptibility data. The usual long-range nature of the interaction is implemented using a mean-field approximation, whereas the particle-particle aggregation is modeled using a distribution of anisotropy constants. The model is then applied to two samples studied at different concentrations. One consists of spherical magnetite nanoparticles dispersed in oil and the other of cubic magnetite nanoparticles embedded on polymeric nanospheres. We also introduce a simple technique to address the presence of the dipolar interaction in a given sample, based on the height of the AC susceptibility peaks for different driving frequencies.

Magnetic behavior of 10 nm-magnetite particles diluted in lyotropic liquid crystals

A magnetic study of 10nm magnetite nanoparticles diluted in lyotropic liquid crystal and common liquids was carried out. In the liquid crystal the ZFC-FC curves showed a clear irreversible behavior, and it was possible to distinguish the nematic from the isotropic phase since the magnetization followed the dependence of the nematic order parameter with the temperature. This behavior could be mimicked by Monte Carlo simulation.