Lorena Gonzalez-Legarreta

Today, there is an increasing demand for progress in the magnetoimpedance (MI) response of cost-effective soft-magnetic materials for use in high-performance sensing devices. In view of this, we investigate here the field-annealing effects on soft-magnetic properties, magnetoimpedance response, and field sensitivity in the nanocrystalline (Fe₃Ni)₈₁Nb₇B₁₂ alloy ribbons. We observe that within the low-frequency regime, between 2 and 5 MHz, the zero-field-annealed (ZFA) ribbons exhibit the highest magnetoimpedance values. By magneto-optical Kerr effect measurements, we show that this result stems from the formation of irregular transversally patched magnetic domains after annealing, which in turn explains the induced transverse anisotropy necessary to increase the GMI response. In addition, we discuss this increment in terms of skin effect. Moreover, we report that the highest sensitivity of ca. 189%/(kA/m)—15%/Oe—is achieved for the field-annealed samples whose magnetic field was applied transverse to the ribbon axis. These findings are of practical importance to develop and refine highly sensitive magnetic sensors.

Amorphous magnetic microwires can be suitable for a variety of technological applications due to their excellent magnetic softness and giant magnetoimpedance (GMI) effect. Several approaches for optimization of soft magnetic properties and GMI effect of magnetic microwires covered with an insulating, flexible, and biocompatible glass coating with tunable magnetic properties are overviewed. The high GMI effect and soft magnetic properties, achieved even in as-prepared Co-rich microwires with a vanishing magnetostriction coefficient, can be further improved by appropriate heat treatment (including stress-annealing and Joule heating). Although as-prepared Fe-rich amorphous microwires exhibit low GMI ratio and rectangular hysteresis loops, stress-annealing, Joule heating, and combined stress-annealed followed by conventional furnace annealing can substantially improve the GMI effect (by more than an order of magnitude).

There is a pressing demand to improve the performance of cost-effective soft magnetic materials for use in high performance sensors and devices. Giant Magneto-impedance effect (GMI), or fast single domain wall (DW) propagation can be observed in properly processed magnetic microwires. In this paper we have identified the routes to obtain microwires with unique combination of magnetic properties allowing observation of fast and single DW propagation and GMI effect in the same microwire. By modifying the annealing conditions, we have found the appropriate regimes allowing achievement of the highest GMI ratio and the fastest DW dynamics. The observed experimental results are discussed considering the radial distribution of magnetic anisotropy and the correlation of GMI effect, and DW dynamics with bulk and surface magnetization processes. Studies of both Fe- and Co-rich microwires, using the magneto-optical Kerr effect, MOKE, provide information on the magnetic structure in the outer shell of microwires. We have demonstrated the existence of the spiral helical structure in both studied microwires. At the same time, torsion mechanical stresses induce helical bistability in the same microwires, which allow us to consider these microwires as materials suitable for sensors based on the large Barkhausen jump.

The influence of magnetic anisotropy, post-processing conditions, and defects on the domain wall (DW) dynamics of amorphous and nanocrystalline Fe-, Ni-, and Co-rich microwires with spontaneous and annealing-induced magnetic bistability has been thoroughly analyzed, with an emphasis placed on the influence of magnetoelastic, induced and magnetocrystalline anisotropies. Minimizing magnetoelastic anisotropy, either by the selection of a chemical composition with a low magnetostriction coefficient or by heat treatment, is an appropriate route for DW dynamics optimization in magnetic microwires. Stress-annealing allows further improvement of DW velocity and hence is a promising method for optimization of DW dynamics in magnetic microwires. The origin of current-driven DW propagation in annealing-induced magnetic bistability is attributed to magnetostatic interaction of outer domain shell with transverse magnetization orientation and inner axially magnetized core. The beneficial influence of the stress-annealing on DW dynamics has been explained considering that it allows increasing of the volume of outer domain shell with transverse magnetization orientation at the expense of decreasing the radius of inner axially magnetized core. Such transverse magnetic anisotropy can similarly affect the DW dynamics as the applied transverse magnetic field and hence is beneficial for DW dynamics optimization. Stress-annealing allows designing the magnetic anisotropy distribution more favorable for the DW dynamics improvement. Results on DW dynamics in various families of nanocrystalline microwires are provided. The role of saturation magnetization on DW mobility improvement is discussed. The DW shape, its correlation with the magnetic anisotropy constant and the microwire diameter, as well as manipulation of the DW shape by induced magnetic anisotropy are discussed. The engineering of DW propagation through local stress-annealing and DW collision is demonstrated.

Understanding the biological effects triggered by nanomaterials is crucial, not only in nanomedicine but also in toxicology. The dose–response relation is relevant in biological tests due to its use for determining appropriate dosages for drugs and toxicity limits. Carbon nanotubes can trigger numerous unusual biological effects, many of which could have unique applications in biotechnology and medicine. However, their resuspension in saline solutions and the accurate determination of their concentration after dispersion in biological media are major handicaps to identify the magnitude of the response of organisms as a function of this exposure. This difficulty has led to inconsistent results and misinterpretations of their in vivo behavior, limiting their potential use in nanomedicine. The lack of a suitable protocol that allows comparing different studies of the content of carbon nanotubes and their adequate resuspension in culture cell media gives rise to this study. Here, we describe a methodology to functionalize, resuspend and determine the carbon nanotube concentration in biocompatible media based on UV-Vis spectroscopy. This method allows us to accurately estimate the concentration of these resuspended carbon nanotubes, after removing bundles and micrometric aggregates, which can be used as a calibration standard, for dosage-dependent studies in biological systems. This method can also be extended to any other nanomaterial to properly quantify the actual concentration.

The influence of post-processing conditions on the magnetic properties of amorphous and nanocrystalline microwires has been thoroughly analyzed, paying attention to the influence of magnetoelastic, induced and magnetocrystalline anisotropies on the hysteresis loops of Fe-, Ni-, and Co-rich microwires. We showed that magnetic properties of glass-coated microwires can be tuned by the selection of appropriate chemical composition and geometry in as-prepared state or further considerably modified by appropriate post-processing, which consists of either annealing or glass-coated removal. Furthermore, stress-annealing or Joule heating can further effectively modify the magnetic properties of amorphous magnetic microwires owing to induced magnetic anisotropy. Devitrification of microwires can be useful for either magnetic softening or magnetic hardening of the microwires. Depending on the chemical composition of the metallic nucleus and on structural features (grain size, precipitating phases), nanocrystalline microwires can exhibit either soft magnetic properties or semi-hard magnetic properties. We demonstrated that the microwires with coercivities from 1 A/m to 40 kA/m can be prepared.

Amorphous soft magnetic microwires have attracted much attention in the area of sensor applications due to their excellent properties. In this work, we study the influence of annealing treatments (stress and conventional) in the giant magnetoimpedance (GMI) response and the field sensitivity of the soft magnetic Co_69.2Fe_3.6Ni₁B_12.5Si₁₁Mo_1.5C_1.2 glass-coated microwires. Here we report a remarkable and simultaneous enhancement of GMI effect and field sensitivity. The highest sensitivity of 104%/Oe and the GMI response of 234% were achieved for 300 °C stress-annealed samples at 472 and 236 MPa, respectively. Additionally, we found that stress-annealed microwires exhibit a frequency dependence on maximal GMI response and field sensitivity. These findings are obtained by fine-tuning their magnetoeslastic anisotropies through stress-annealing treatments of as-prepared microwires at the proper temperature and axial applied stress upon annealing. We hope that the results presented here widen the scope of investigations for the future design of soft magnetic materials for sensor purposes.