Griffiths phase, metal-insulator transition, and magnetoresistance of doped manganites

A phenomenological model is developed for systematic study of the universal features in metal-insulator transition and magnetoresistivity of mixed-phase manganites. The approach is based on utilization of some hypothesis appropriate to the Preisach picture of the magnetization process for half-metallic ferromagnets and an assumption that in doped manganites a Griffiths-type phase exists just above the magnetic-ordering temperature. Within the model, the system is considered as a random three-dimensional resistor network where a self-consistent formation of paths with metal and polaron types of conductivity is not only due to magnetic field variation but also due to temperature changes, as well. Both mechanisms of intrinsic percolation transition are considered on one basis. The theory is able to replicate the basic regularities found experimentally for doped manganites resistivity dependence on temperature and magnetic field without the need for empirical input from the magnetoresistive data. Within the approach a natural basis has arisen for a qualitative classification of magnetoresistive materials into those, such as La0.7Sr0.3MnO3, showing modest magnetoresistivity, and those, such as La0.7Ca0.3MnO3, showing large magnetoresistivity

Including effects of microstructure and anisotropy in theoretical models describing hysteresis of ferromagnetic materials

Two recent theoretical hysteresis models (Jiles-Atherton model and energetic model) are examined with respect to their capability to describe the dependence of the magnetization on magnetic field, microstructure, and anisotropy. It is shown that the classical Rayleigh law for the behavior of magnetization at low fields and the Stoner-Wohlfarth theory of domain magnetization rotation in noninteracting magnetic single domain particles can be considered as limiting cases of a more general theoretical treatment of hysteresis in ferromagnetism

Multifunctional induction coil sensor for evaluation of carbon content in carbon steel

Carbon steel has proven to be an important structural and functional material that plays an irreplaceable role in the worldwide economy. The influence of carbon on the mechanical and magnetic properties of the steel is well understood. Thus, the precise knowledge of the amount of carbon content in steel is crucial. Magnetic Barkhausen noise (MBN), magnetic hysteresis loop (MHL) and impedance measurements are reliable tools to assess carbon content. In this work, a multifunctional induction coil sensor used for MBN, MHL and impedance measurements is designed and optimised. A multifunctional measurement system using the optimised induction coil is employed to measure MBN, MHL and impedance signals. The parabolic dependence of the maximum value of MBN envelope on carbon content in steel is theoretically analysed and experimentally verified. Coercive field and remanence from MHL measurements as well as the maximum impedance value are found to be proportional to carbon content and their dependence is explained with analytical simulations

Quantitative evaluation of the effect of temperature on magnetic Barkhausen noise

The effect of temperature on magnetic Barkhausen noise (MBN) can be divided into two types: the direct effect of temperature itself and the indirect effect of thermally induced stress. The theoretical model is proposed in this paper for describing these effects of temperature on the MBN signal. For the case considering the direct effect of temperature only, the analytical model allows the prediction of the effect of temperature on MBN profile, and based on the model, a simple linear calibration curve is presented to evaluate the effect of temperature on MBN amplitude quantitatively. While for the case where the indirect effect of thermal stress is taken into account in addition to the direct effect, the proposed theoretical model allows the deduction of parabolic function for quantitative evaluation of the combined effect on MBN. Both effects of temperature on MBN, i.e., the direct only and the combined one, have been studied experimentally on 0.5mm thickness non-oriented (NO) electrical steel and the adhesive structure of NO steel and ceramic glass, respectively. The reciprocal of the measured MBN peak amplitude (1/MBNp) in the first case shows a linear function of temperature, which agrees with the proposed linear calibration curve. While in the experiments considering the combined effects, 1/MBNp shows parabolic dependence on temperature, which is further simplified as a piecewise function for the practical applications

Phenomenological modelling of first order phase transitions in magnetic systems

First order phase transitions may occur in several magnetic systems, with two structural phases having different magnetic properties each and a structural transition between them. Here, a novel physics based phenomenological model of such systems is proposed, in which magnetization is represented by the volumetric amounts of ferromagnetism (described by extended Jiles-Atherton theory) and paramagnetism (described by the Curie-Weiss law) in respective phases. An identification procedure to extract material parameters from experimental data is proposed. The proposed phenomenological approach was successfully applied to magnetocaloric Gd5(Six Ge 1−x)4 system and also has the potential to describe the behavior of Griffiths phase magnetic systems

Relationship between hysteretic behavior of magnetization and magnetoresistance in half-metallic ferromagnets

A generalized Preisach description of hysteretic magnetotransport properties of half-metallic ferromagnets (HMFs) is proposed. Assuming that the system consists of an assembly of elementary bistable hysterons distributed in energy levels and with a range of possible energy barriers, the connection between irreversible magnetic and transport properties of HMFs is found. Within this model, both the magnetization hysteresis and resistivity hysteresis of HMFs can be described by a few simple assumptions common to description of hysteresis phenomena leading to some fundamental relationship

Effect of argon sputtering on XPS depth-profiling results of Si/Nb/Si

Ultrathin Si/Nb/Si trilayer is an excellent example of a system for which dimensionality effects, together with other factors like type of a substrate material and growth method, influence strongly its superconducting properties. This study offers some important insights into experimental investigation of density of states of such a system with the aim to identify an electronic structure of the interface as a function of niobium layer thickness. For that, two Si/Nb/Si trilayers with 9.5 and 1.3 nm thick niobium layer buried in amorphous silicon were studied using high-resolution (HR) XPS depth-profile techniques. Strong influence of sputtering was observed, which resulted in severe intermixture of Si and Nb atoms. Nevertheless, a sharp top interface and metallic phase of niobium were detected for the thicker layer sample. On the contrary, a Nb-rich mixed alloy at top interface was observed for the thinner layer sample

Remote angular displacement sensor based on Faraday effect: experiment and modelling

Recently, we have reported a magneto-optic angular displacement (MOAD) sensor where both the incident and reflected laser beams pass through a magneto-optic (MO) film. In this letter, we report a modified MOAD sensor where only a reflected laser beam passes through the MO film. With the modified configuration, the modified MOAD sensor is a truly remote sensor such that the MO film can be located close to the detector and far from the sample. Furthermore, the modified sensor system can measure angular displacements with an improved resolution of 1×10−3 deg, which is ten times better than that previously reported

Physical, electrical and magnetic properties of nano-sized Co-Cr substituted magnesium ferrites

Co-Cr substituted magnesium ferrite nanomaterials (Mg1−xCoxCrxFe2−xO4 with x = 0.0−0.5) have been prepared by the polyethylene glycol assisted micro emulsion method. X-ray diffraction analysis confirms the single-phase cubic close-packed lattice formation of synthesized materials. Hysteresis loops are measured up to field of 4 MA/m and high field region of these loops are modeled using the Law of Approach to saturation to calculate the magnetocrystalline anisotropy constant. The saturation magnetization of the samples increases initially from 148 kA/m for x = 0.0 to 299 kA/m (x = 0.3) and then decreases to 187 kA/m (x = 0.5). Curie temperature for this series is found to be in the range of 618-766 K. Room temperature resistivity increases gradually from 7.5 × 108 Ω cm (x = 0.0) to 3.47 × 109 Ω cm (x = 0.5). Additionally, dielectric measurements are carried out at room temperature in a frequency range of 100 Hz to 3 MHz. With improvement in the values of the above-mentioned properties, the synthesized materials could be suitable for potential application in some magnetic and microwave devices

An investigation of various controller designs for multi-link robotic system (Robogymnast)

An approach to controlling the three-link Robogymnast robotic gymnast and assessing stability is proposed and examined. In the study, a conventionally configured linear quadratic regulator is applied and compared with a fuzzy logic linear quadratic regulator hybrid approach for stabilising the Robogymnast. The Robogymnast is designed to replicate the movement of a human as they hang with both hands holding the high bar and then work to wing up into a handstand, still gripping the bar. The system, therefore has a securely attached link between the hand element and the ‘high bar’, which is mounted on ball bearings and can rotate freely. Moreover, in the study, a mathematical model for the system is linearised, investigating the means of determining the state space in the system by applying Lagrange’s equation. The fuzzy logic linear quadratic regulator controller is used to identify how far the system responses stabilise when it is implemented. This paper investigates factors affecting the control of swing-up in the underactuated three-link Robogymnast. Moreover, a system simulation using MATLAB Simulink is conducted to show the impact of factors including overshoot, rising, and settling time. The principal objective of the study lies in investigating how a linear quadratic regulator or fuzzy logic controller with a linear quadratic regulator (FLQR) can be applied to the Robogymnast, and to assess system behaviour under five scenarios, namely the original value, this value plus or minus ±25%, and plus or minus ±50%. In order to further assess the performance of the controllers used, a comparison is made between the outcomes found here and findings in the recent literature with fuzzy linear quadratic regulator controllers