Influence of the magnetization damping on dynamic hysteresis loops in single domain particles

This article reports on the influence of the magnetization damping on dynamic hysteresis loops in single-domain particles with uniaxial anisotropy. The approach is based on the Neel-Brown theory and the hierarchy of differential recurrence relations, which follow from averaging over the realizations of the stochastic Landau-Lifshitz equation. A new method of solution is proposed, where the resulting system of differential equations is solved directly using optimized algorithms to explore its sparsity. All parameters involved in uniaxial systems are treated in detail, with particular attention given to the frequency dependence. It is shown that in the ferromagnetic resonance region, novel phenomena are observed for even moderately low values of the damping. The hysteresis loops assume remarkably unusual shapes, which are also followed by a pronounced reduction of their heights. Also demonstrated is that these features remain for randomly oriented ensembles and, moreover, are approximately independent of temperature and particle size. (C) 2012 American Institute of Physics. [doi:10.1063/1.3684629]

The association between low alcohol use and traffic risk behaviors among Brazilian college students

Although there are a large number of studies focused on binge drinking and traffic risk behaviors (TRB), little is known regarding low levels of alcohol consumption and its association to TRB. The aim of this cross-sectional study is to examine the association of low to moderate alcohol intake pattern and TRB in college students in Brazil. 7037 students from a National representative sample were selected under rigorous inclusion criteria. All study participants voluntarily fulfilled a structured, anonymous, and self-questionnaire regarding alcohol and drug use, social-demographic data, and TRB. Alcohol was assessed according to the average number of alcoholic units consumed on standard occasions over the past 12 months. The associations between alcohol intake and TRB were summarized with odds ratio and their confidence interval obtained from logistic regression. Compared with abstainers students who consumed only one alcohol unit had the risk of being a passenger in a car driven by a drunk driver increased by almost four times, students who reported using five or more units were increased by almost five times the risk of being involved in a car crash. Compared with students who consumed one alcohol unit, the risk of driving under the influence of alcohol increased four times in students using three alcohol units. Age group, use of illicit drugs, employment status, gender, and marital status significantly influenced occurrence of TRB among college students. Our study highlights the potential detrimental effects of low and moderate pattern of alcohol consumption and its relation to riding with an intoxicated driver and other TRB. These data suggest that targeted interventions should be implemented in order to prevent negative consequences due to alcohol use in this population. (C) 2012 Elsevier Inc. All rights reserved,

RISK FOR INJURIES AMONG MOTORCYCLISTS INVOLVED IN TRAFFIC INCIDENTS

The objective of this study was to identify, among motorcyclists involved in traffic incidents, the factors associated with risk of injuries. In 2004, in the city of Maringa-PR, it was determined that there were a total of 2,362 motorcyclists involved in traffic incidents, according to records from the local Military Police. Multivariate analysis was applied to identify the factors associated with the presence of injury. A significantly higher probability of injury was observed among motorcyclists involved in collisions (odds Ratio = 11.19) and falls (odds Ratio = 3.81); the estimated odds ratio for females was close to four, and those involved in incidents including up to two vehicles were 2.63 times more likely to have injuries. Women involved in motorcycle falls and collisions with up to two vehicles stood out as a high-risk group for injuries.

Subdivision of Hysteresis Loss in Mn-Zn Ferrite Toroidal Cores

The hysteresis loss subdivision method proved to be a strong tool to help in the analysis of different energy dissipation mechanisms along the quasi-static hysteresis loop measured on electrical steels. This paper used the samemethod to discuss the mechanisms involving the energy loss dissipation in Mn-Zn ferrite toroidal cores. The samples, sintered under controlled atmosphere in industrial conditions, were measured under triangular waveform excitation at very low frequency (5 mHz) and peak flux densities varying from 0.05 T to 0.45 T. The results show a different behavior between the low inductions hysteresis loss (WLI) and the high induction hysteresis loss (WHI) which proves the existence of different energy dissipation mechanisms affecting these loss components.

Magnetic hyperthermia investigation of cobalt ferrite nanoparticles: Comparison between experiment, linear response theory, and dynamic hysteresis simulations

Considerable effort has been made in recent years to optimize materials properties for magnetic hyperthermia applications. However, due to the complexity of the problem, several aspects pertaining to the combined influence of the different parameters involved still remain unclear. In this paper, we discuss in detail the role of the magnetic anisotropy on the specific absorption rate of cobalt-ferrite nanoparticles with diameters ranging from 3 to 14 nm. The structural characterization was carried out using x-ray diffraction and Rietveld analysis and all relevant magnetic parameters were extracted from vibrating sample magnetometry. Hyperthermia investigations were performed at 500 kHz with a sinusoidal magnetic field amplitude of up to 68 Oe. The specific absorption rate was investigated as a function of the coercive field, saturation magnetization, particle size, and magnetic anisotropy. The experimental results were also compared with theoretical predictions from the linear response theory and dynamic hysteresis simulations, where exceptional agreement was found in both cases. Our results show that the specific absorption rate has a narrow and pronounced maxima for intermediate anisotropy values. This not only highlights the importance of this parameter but also shows that in order to obtain optimum efficiency in hyperthermia applications, it is necessary to carefully tailor the materials properties during the synthesis process. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729271]

Dynamic symmetry loss of high-frequency hysteresis loops in single-domain particles with uniaxial anisotropy

Understanding how magnetic materials respond to rapidly varying magnetic fields, as in dynamic hysteresis loops, constitutes a complex and physically interesting problem. But in order to accomplish a thorough investigation, one must necessarily consider the effects of thermal fluctuations. Albeit being present in all real systems, these are seldom included in numerical studies. The notable exceptions are the Ising systems, which have been extensively studied in the past, but describe only one of the many mechanisms of magnetization reversal known to occur. In this paper we employ the Stochastic Landau-Lifshitz formalism to study high-frequency hysteresis loops of single-domain particles with uniaxial anisotropy at an arbitrary temperature. We show that in certain conditions the magnetic response may become predominantly out-of-phase and the loops may undergo a dynamic symmetry loss. This is found to be a direct consequence of the competing responses due to the thermal fluctuations and the gyroscopic motion of the magnetization. We have also found the magnetic behavior to be exceedingly sensitive to temperature variations, not only within the superparamagnetic-ferromagnetic transition range usually considered, but specially at even lower temperatures, where the bulk of interesting phenomena is seen to take place. (C) 2011 Elsevier B.V. All rights reserved.

Longitudinal dynamic hysteresis in single-domain particles

We present results for longitudinal dynamic hysteresis in single domain particles with uniaxial anisotropy. The combined influence of temperature, field-sweeping frequency, and field amplitude is discussed in detail. A novel and efficient numerical method is proposed, based on the direct solution of the infinite hierarchy of differential recurrence relations obtained from averaging over the stochastic realizations of the magnetic Langevin equation. (C) 2012 American Institute of Physics. [doi:10.1063/1.3676416]