L.C. Sampaio

Dipolar magnetic interactions among magnetic microwires

Abstract This paper presents a brief review of experiments and simulations on dipole–dipole interacting magnetic microwires that form a linear chain. In particular, recent studies on (a) amorphous Fe 77.5 Si 7.5 B 15 and (b) granular Cu 45 Ni 25 Co 29 Mn 1 microwires are discussed. Hysteresis loops were performed at room temperature for an array of N microwires ( N =1–5 for (a) and N =1–100 for (b)) and compared with Monte Carlo simulations. In the case of amorphous soft magnetic materials (a), clear steps and plateaux on the demagnetization are visible, each step corresponding to the magnetization reversal of an individual wire. In the case of CuNiCoMn microwires (b), that exhibit a granular structure and a nearly superparamagnetic behavior when isolated, it was found that the coercivity abruptly changes from 20 to 700xa0Oe for N around 10. Novel Monte Carlo simulations are presented, providing useful insights about the effect of long-range interactions that account for the interesting observed magnetic behaviors.

Magnetic properties of the pseudo-binary intermetallic compounds: Hf(Fe1−xAlx)2

Abstract From magnetization measurements in polycrystalline Hf(Fe 1− x Al x ) 2 samples, it was observed that the magnetization is not saturated even at high fields (70 kOe). It is suggested that in Hf(Fe 1− x Al x ) 2 , a high anisotropy energy and consequently narrow domain walls exist. Magnetization vs. temperature curves present a different behaviour if the sample is cooled with field (FC) or without (ZFC) field. This difference is applied field dependent and is a maximum in the range 0.45 x T C vs. x ), two different slopes were found and T C vanishes at x =0.63, approximately. To compare the Hf(Fe 1− x Al x ) 2 and Zr(Fe 1− x Al x ) 2 results, we performed the same measurements on a Zr (Fe 0.68 Al 0.32 ) 2 sample and found a similar irreversibility of ZFC magnetization which has not been detected in the literature.

X-ray magnetic circular dichroism in Fe/NiO thin films

Abstract The interface of thin films of Fe/NiO (1xa00xa00)-oriented single crystals was studied by X-ray magnetic circular dichroism (XMCD). Thin layers of Fe were removed by “in situ” sputtering, followed by XMCD measurements, resulting in a depth profile. A decrease from the bulk value in the value of the Fe moment at the interface of Fe with the antiferromagnetic NiO(1xa00xa00) was observed.

Coercive field and domain wall properties in (DyχY1-χ) Fe2 and (ErχY1-χFe2 intermetallic compounds

Isothermal d.c. magnetization measurements were performed for (RχY1-χ)Fe2 (Rue5fcDy, Er, 0<χ<1) at several temperatures. The observed pronounced decay of the coercive field Hc with temperature was fitted with an exponential curve at low temperatures for concentrations near 30%. The coercive field Hc and the propagation field Hp showed maximum values in the concentration range 30%–40% for Dy or Er. These results can be explained by intrinsic domain wall pinning. Time relaxation effects of the remanent magnetization were detected and attributed to domain wall surface deformations.

Tailoring coercivity in an array of glass-coated microwires

Abstract The influence of long-range dipole–dipole interaction on the coercivity of an array of glass-coated CuNiCo microwires is investigated by means of magnetic hysteresis measurements and Monte Carlo simulations. The microwires exhibit a granular structure consisting of Co grains in a Cu-rich matrix. Hysteresis loops for an array of N microwires (1

89Y nuclear-magnetic-resonance measurements in (DyxY1-x)Fe2 compounds

In a previous work, 89Y measurements at 4.2 K in (RxY1−x)Fe2 compounds, with R=Tb and Ho and 0.05≤x≤0.20, have shown broad nuclear‐magnetic‐resonance (NMR) spectra, due to transferred hyperfine interactions. 89Y NMR signals could not be observed with R=Dy in the same composition range. In the present work, compounds were prepared with Dy in the concentration range 0.005≤x≤0.02 to investigate the origin of this anomaly. Compared to the pure YFe2 compound, the observed NMR spectra are broadened and show a shift towards higher frequencies. With increasing Dy concentration, the NMR signals fall very rapidly. Magnetization measurements at 4.2 K were performed in samples with Tb, Ho, and Dy; the results indicate that the behavior of the Dy compounds is qualitatively different in the low‐magnetic‐field region of the curves, requiring a minimum applied field to start increasing the magnetization. The NMR data and the magnetization results of the Dy compounds can both be explained as arising from a much lower mobi...

Intrinsic domain wall pinning in Hf(Fe0.5Al0.5)2

Abstract The magnetization versus temperature curve for Hf(Fe 0.5 Al 0.5 ) 2 in low applied field presents a maximum and also an irreversibility up to near the Curie temperature. For temperatures lower than approximately 7 K, a critical field (propagation field) exists in the virgin magnetization curves. Even in high fields (70 kOe) the magnetization values are not saturated, suggesting high anisotropy energy and consequently narrow domain walls. An exponential temperature decrease of the coercive field is observed. These phenomena can be explained by intrinsic domain wall pinning. The time effect in the demagnetization region of the hysteresis loop was measured and it is related to the wall deformation (kink creation). A brief comparison with other systems is made.

Effects of the distribution of energy barriers on the magnetic relaxation of Ba-ferrite small particles at low temperatures

Abstract We have made magnetic viscosity measurements S ( H , T ) in Ba-ferrite small particles. Both volume π ( V ) and switching field ν ( H 0 ) distributions were taken into account to understand the behaviour observed at low temperatures, S ( H , T ) α (T) 1 2 . We have found a scaling function for the relaxation measurements on field and temperature.

Determination of prediction intervals for a future number of failures: a statistical and Monte Carlo approach

In this work, we present a new procedure, called sub-sampling, to obtain data concerning time of failure in trials without replacement, (NRT). With this data it is possible to determine the prediction interval (PI) for the future number of failures. We also present an alternative way to evaluate the coverage probability of the prediction interval (PI). The results presented show that the method proposed is reliable and can be useful for the statistical analyses of quality control of processes.

Magnetic behavior of the Hf(Fe1−xAlx)2 system (abstract)

Magnetic properties of pseudobinary compounds Hf(Fe1−xAlx)2 have been investigated as a function of temperature and concentration in the range 0≤x≤0.60. The present work is motivated to understand the role of Hf in the system, compared to the systems Y(Fe1−xAlx)2 and Zr(Fe1−xAlx)2, where freezing phenomena characteristic of a cluster‐glass regime can be observed in the Y case in the concentration range 0.1≤x≤0.35.1 The magnetization measurements are made as a function of temperature and static magnetic applied field. The system shows a mixture of a C14 (hexagonal) and a C15 (cubic) Laves phase structures from x=0 up to x<0.15; for x≥0.15, only the C14 hexagonal phase is present. The Curie temperature TC decreases with Al concentration and tends to TC=0 about x=0.60. We have observed an irreversible behavior of the magnetization curves as a function of temperature when the samples are cooled with (FC) and without (ZFC) an external field, up to temperatures near TC. In the low‐temperature range the magnetiz...