SYNTHESIS AND CHARACTERIZATION OF NICKEL FERRITE-BARIUM TITANATE CERAMIC COMPOSITES

Magnetoelectric composites were synthesized from piezoelectric BaTiO3 and ferrimagnetic NiFe2O4. These two phases are mixed, mechanically milled and heat treated. A characterization is made by means of electronic microscopy and x-ray diffraction. The results of the magnetic, electric, ferroelectric and piezoelectrical response of two different compositions are obtained. The composites are superparamagnetic in all the measured temperature range. It was observe a magnetic change induced by a structural transition of the ferroelectric phase. The composites behaves as an acceptable mechanical resonator when the content of the ferroelectric phase is equal or greater than 60 wt.%

Time-dependent magnetization in co-precipitated cobalt ferrite

A study of the magnetic aftereffect in co-precipitated cobalt ferrite is presented.Measurements of the magnetic viscosity S were performed at room temperature along the demagnetization curve for different applied fields Hap over a wide range of fields (0 kOe<Hap<- 7 kOe).The interrelation function n ¼ ðMrev/ðMirr)Hi between the DCD reversible Mrev and irreversible Mirr magnetization components was determined as well. The experimental results for Sn (Hi) where Hi is the internal field, showed a broad distribution with a maximum at Hi = 2.7 kOe. However, the irreversible susceptibility Xirr displays a maximum at Hc = 0.75 kOe, the coercivity of the material. The experimental behavior of n and the non-proportionality between Sn and Xiirr suggest that the magnetic viscosity in this material is principally supplied by events of nucleation of inverse domains and the depinning of domain walls. When the main mechanism of reversal magnetization changes to rotation of magnetic moments for all the grains, the magnetic viscosity decreases

Magnetization reversal in co-precipitated cobalt ferrite

A study about the magnetic viscosity and magnetization reversal in coprecipitated cobalt ferrite was carried out. Measurements of direct current demagnetization reversible Mrev and irreversible Mirr magnetization as well as magnetic viscosity Sn were performed at room temperature along the demagnetization curve for different applied fields Hap(0 > Hap> - 7 kOe). From these data Mrev(Mirr)Hi curves were built. The experimental results show a minimum in the Mrev(Mirr)Hi curves and a nonproportionality between Sn and Xirr; suggesting two different contributions to the reversal magnetization during the demagnetization process

Influence of magnetic water treatment on the calcium carbonate phase formation and the electrochemical corrosion behavior of carbon steel M.E.

A study of the influence of magnetic water treatment on the calcium carbonate phase formation and the electrochemical corrosion behavior of carbon steel was carried out. The water was exposed to a magnetic field of 1 T with a speed of flow through the magnet of 0.77 m/s (1.56 ml/s). The observations demonstrated that the calcite is the main form of calcium carbonate, and became aragonite by action of the magnetic field. The aragonite content, expressed in wt.% with respect to the total calcium carbonate content, grew constantly with respect to the water without magnetic treatment up to 30 h of incubation time (18 wt.% of aragonite for water without magnetic treatment and 38 wt.% for water with an incubation time of 30 h). For incubation times greater than 30 h and up to 200 h the aragonite contain fluctuated about 35 wt.%. The electrochemical corrosion behavior was evaluated using polarization curves (ASTM G5-94). The polarization curves showed that the rate of corrosion increase with the magnetic water treatment

Annealing effect on microstructure and coercivity of YCo5 nanoparticles obtained by mechanical milling

Nanocrystalline YCo5 powders with high coercivity were prepared by mechanical milling and subsequent heat treatment at 820 °C for different annealing times, ta = 2.5, 3.0, 3.5 and 4.5 min, obtaining average crystallite sizes of 17, 19, 32 and 39 nm., respectively. The coercivity values were determined from the hysteresis loops measured at maxima fields of Hm = 5 and 20 T. The highest coercivity was obtained for the sample exhibiting 19 nm, where at room temperature and Hm = 5 T, the coercivity value is of 9.0 kOe. At 77 K and Hm = 5 T, the coercivity increase to 11.8 kOe and for Hm = 20 T, a higher value such as 13.1 kOe was found. The Ms/Mr ratio is enhanced to 0.62 indicating the occurrence of exchange interaction among nanocrystalline magnets

Study of titanomagnetite sands from Costa Rica

In Cahuita town, Limon province, at the Atlantic coast of Costa Rica there are titanomagnetite sands. The chemical, structural and magnetic characterization of the titanomagnetite sands can clarify aspects of their geological formation; as well as suggest applications from them. The magnetic sand is composed mainly of titanomagnetite and hematite, with very small quartz impurities quantities, pyroxene, etc. The refinement of X-rays diffraction patterns by the method of Rietveld indicates an 86.03% in weight of titanomagnetite and a 13.97% in weight of hematite. The particles size of this magnetic sand have an interval of 75–300m and a specific surface area of 1.18m2/g. The specific magnetization of the saturation of the sand is 55.8 emu/g and the intrinsic coercivity is 39.6 Oe. In this work a characterization of the titanomagnetite sands is presented the chemical composition was determined by absorption spectroscopy. The present crystalline phases were determined by X-ray diffraction. The morphology and composition of the particles were determined by scanning electron microscopy together with microanalytical techniques. The classification of the particles by shape and color were carried out using optical microscopy. The magnetic properties and the hyperfine structure were measured by vibrating sample magnetometry and Mössbauer spectroscopy

High magnetic field facilities in Latin America

The EC supported a network (under the Framework 5 ALFA Programme) designated HIFIELD (Project number II0147FI) and entitled: “Measurement methods involving high magnetic fields for advanced and novel materials”. As a result, high field facilities were initiated, constructed or extended at the following laboratories in Latin America: Univ. Cordoba (Argentina), CES, Merida (Venezuela), CIMAV, Chihuahua (Mexico), Univ. Federal de Rio de Janeiro (Brazil)

Reversible processes in magnetization reversal of co-precipitated cobalt ferrite

A study of the magnetization reversal process in a co-precipitated cobalt ferrite material at 300K was carried out. The evolution of the reversible Mrev and irreversibleMirr magnetization components were determined by measuring sets of recoil curves from different points on the initial magnetization curve and demagnetization curve. From these data, MrevðMirrÞHi curves were built, being Hi the internal field of the sample. The interrelation function Z ¼ ðMrev=MirrÞHi was determined as well. The results suggest that domain wall movement subject to pinning is the dominant mechanism for the reversal magnetization process in this material. r 2002 Elsevier Science B.V. All rights reserved

In situ comparison of the critical current density in YBa2Cu3O7− thin films measured by the screening technique under two criteria

In this investigation we report the determination of the critical current density Jc flowing in a YBa2Cu3O7−S superconducting thin film. Estimation of Jc was carried out by an inductive technique, the so-called screening technique, in which both the imaginary part of the fundamental harmonic of susceptibility Xi′ 1 and third harmonic voltage V3 criteria were considered for the determination of the full penetration field. In order to verify the reliability of this technique under two criteria, we investigated the homogeneity of Jc via transport measurements conducted on four microbridges patterned in the same film. Based on the transport method we found that both techniques yield similar results in the determination of Jc for temperatures close to critical temperature Tc. However, at temperatures relatively far from Tc, the V3 criterion showed a better agreement with the transport data. Furthermore, using both criteria, we propose a methodology to estimate in situ the coil factor associated with the V3 criterion, avoiding in this way the need of implementing an additional technique