Tuning metamaterials by using amorphous magnetic microwires

In this work, we demonstrate theoretically and experimentally the possibility of tuning the electromagnetic properties of metamaterials with magnetic fields by incorporating amorphous magnetic microwires. The large permeability of these wires at microwave frequencies allows tuning the resonance of the metamaterial by using magnetic fields of the order of tens of Oe. We describe here the physical basis of the interaction between a prototypical magnetic metamaterial with magnetic microwires and electromagnetic waves plus providing detailed calculations and experimental results for the case of an array of Split Ring Resonators with Co-based microwires

Coercivity and its thermal dependence in microsized magnetic particles: Influence of grain boundaries

Fe_(73.5)Si_(13.5)B_9Nb_3Cu_1 powder particles have been obtained by gas atomization. Magnetization curves and coercivity were studied for particles ranging in size up to 1000μ. The overall magnetic behavior of such material is consequence of compositional heterogeneity of the microstructure as a whole. Anomalous temperature variation of coercivity (H_c) (i.e., a decrease in H_c with decreasing temperature) together with a decrease of saturation magnetization has been observed for less than 25 μm size. The origin of this behavior has been ascribed to metastable FeCu and FeNbSi phases in combination with an Fe-rich one. Making magnetic powders with coercive fields of the order of mOe remains a challenge for researchers. Our experiment has allowed us, at low temperature, achieving a coercive field of 9 Oe, much lower than those observed so far in this type of materials. This behaviour has been related with a FeCu phase present on grain boundaries

Gas diffusion electrodes on the electrosynthesis of controllable iron oxide nanoparticles

The electrosynthesis of iron oxide nanoparticles offers a green route, with significant energy and environmental advantages. Yet, this is mostly restricted by the oxygen solubility in the electrolyte. Gas-diffusion electrodes (GDEs) can be used to overcome that limitation, but so far they not been explored for nanoparticle synthesis. Here, we develop a fast, environmentally-friendly, room temperature electrosynthesis route for iron oxide nanocrystals, which we term gas-diffusion electrocrystallization (GDEx). A GDE is used to generate oxidants and hydroxide in-situ, enabling the oxidative synthesis of a single iron salt (e.g., FeCl_2) into nanoparticles. Oxygen is reduced to reactive oxygen species, triggering the controlled oxidation of Fe^(2+) to Fe^(3+), forming Fe_(3-x)O_(4-x) (0 <= x <= 1). The stoichiometry and lattice parameter of the resulting oxides can be controlled and predictively modelled, resulting in highly-defective, strain-heavy nanoparticles. The size of the nanocrystals can be tuned from 5 nm to 20 nm, with a large saturation magnetization range (23 to 73 A m^2 kg^(-1)), as well as minimal coercivity (similar to 1 kA m^(-1)). Using only air, NaCl, and FeCl_2, a biocompatible approach is achieved, besides a remarkable level of control over key parameters, with a view on minimizing the addition of chemicals for enhanced production and applications

Colossal heating efficiency via eddy currents in amorphous microwires with nearly zero magnetostriction

It is well stablished that heating efficiency of magnetic nanoparticles under radiofrequency fields is due to the hysteresis power losses. In the case of microwires (MWs), it is not clear at all since they undergo non-coherent reversal mechanisms that decrease the coercive field and, consequently, the heating efficiency should be much smaller than the nanoparticles. However, colossal heating efficiency has been observed in MWs with values ranging from 1000 to 2800W/g, depending on length and number of microwires, at field as low as H = 36 Oe at f = 625 kHz. It is inferred that this colossal heating is due to the Joule effect originated by the eddy currents induced by the induction field B = M + chi H parallel to longitudinal axis. This effect is observed in MWs with nearly zero magnetostrictive constant as Fe_ (2.25)Co_(72.75)Si_(10)B_(15) of 30 mu m magnetic diameter and 5 mm length, a length for which the inner core domain of the MWs becomes axial. This colossal heating is reached with only 24 W of power supplied making these MWs very promising for inductive heating applications at a very low energy cost

Scattering of microwaves by a passive array antenna based on amorphous ferromagnetic microwires for wireless sensors with biomedical applications

Co-based amorphous microwires presenting the giant magnetoimpedance effect are proposed as sensing elements for high sensitivity biosensors. In this work we report an experimental method for contactless detection of stress, temperature, and liquid concentration with application in medical sensors using the giant magnetoimpedance effect on microwires in the GHz range. The method is based on the scattering of electromagnetic microwaves by FeCoSiB amorphous metallic microwires. A modulation of the scattering parameter is achieved by applying a magnetic bias field that tunes the magnetic permeability of the ferromagnetic microwires. We demonstrate that the OFF/ON switching of the bias activates or cancels the amorphous ferromagnetic microwires (AFMW) antenna behavior. We show the advantages of measuring the performing time dependent frequency sweeps. In this case, the AC-bias modulation of the scattering coefficient versus frequency may be clearly appreciated. Furthermore, this modulation is enhanced by using arrays of microwires with an increasing number of individual microwires according to the antenna radiation theory. Transmission spectra show significant changes in the range of 3 dB for a relatively weak magnetic field of 15 Oe. A demonstration of the possibilities of the method for biomedical applications is shown by means of wireless temperature detector from 0 to 100 degrees C

Spin transition nanoparticles made electrochemically

Materials displaying novel magnetic ground states signify the most exciting prospects for nanoscopic devices for nanoelectronics and spintronics. Spin transition materials, e.g., spin liquids and spin glasses, are at the forefront of this pursuit; but the few synthesis routes available do not produce them at the nanoscale. Thus, it remains an open question if and how their spin transition nature persists at such small dimensions. Here we demonstrate a new route to synthesize nanoparticles of spin transition materials, gas-diffusion electrocrystallization (GDEx), wherein the reactive precipitation of soluble metal ions with the products of the oxygen reduction reaction (ORR), i.e., in situ produced H_2O_2, OH^-, drives their formation at the electrochemical interface. Using mixtures of Cu^(2+) and Zn^(2+) as the metal precursors, we form spin transition materials of the herbertsmithite family-heralded as the first experimental material known to exhibit the properties of a quantum spin liquid (QSL). Single-crystal nanoparticles of similar to 10-16 nm were produced by GDEx, with variable Cu/Zn stoichiometry at the interlayer sites of Zn_xCu_(4-x)(OH)_6Cl_2. For x = 1 (herbertsmithite) the GDEx nanoparticles demonstrated a quasi-QSL behavior, whereas for x = 0.3 (0.3 < x < 1 for paratacamite) and x = 0 (clinoatacamite) a spin-glass behavior was evidenced. Finally, our discovery not only confirms redox reactions as the driving force to produce spin transition nanoparticles, but also proves a simple way to switch between these magnetic ground states within an electrochemical system, paving the way to further explore its reversibility and overarching implications

Boosting the tunable microwave scattering signature of sensing array platforms consisting of amorphous ferromagnetic Fe_2.25Co_72.75Si_10B_15 microwires and its amplification by intercalating cu microwires

The following work addresses new configurations of sensing array platforms that are composed of Co-based amorphous ferromagnetic microwires (MWs) to obtain an enhanced modulation of the microwave scattering effects through the application of low strength DC or AC magnetic fields. An amorphous MW is an ultrasoft ferromagnetic material (coercivity similar to 0.2 Oe) with a circumferential magnetic anisotropy that provides a high surface sensitivity when it is subjected to an external magnetic field. Firstly, microwave scattering experiments are performed as a function of the length and number of MWs placed parallel to each other forming an array. Subsequently, three array configurations are designed, achieving high S_21 scattering coefficients up to about -50 dB. The influence of DC and AC magnetic fields on S_21 has been analyzed in frequency and time domains representation, respectively. In addition, the MWs sensing array has been overlapped by polymeric surfaces and the variations of their micrometric thicknesses also cause strong changes in the S_21 amplitude with displacements in the frequency that are associated to the maximum scattering behavior. Finally, a new concept for amplifying microwave scattering is provided by intercalating Cu MWs into the linear Co-based arrays. The designed mixed system that is composed by Co-based and Cu MWs exhibits a higher S_21 coefficient when compared to a single Co-based MW system because of higher electrical conductivity of Cu. However, the ability to modulate the resulting electromagnetic scattering is conferred by the giant magneto-impedance (GMI) effects coming from properties of the ultrasoft amorphous MWs. The mixed array platform covers a wide range of sensor applications, demonstrating the feasibility of tuning the S_21 amplitude over a wide scattering range by applying AC or DC magnetic fields and tuning the resonant frequency position according to the polymeric slab thickness

Magnetic phase diagram of nanostructured zinc ferrite as a function of inversion degree delta

Magnetic properties of spinel zinc ferrites are strongly linked to the synthesis method and the processing route since they control the microstructure of the resulting material. In this work, ZnFe_2O_4 nanoparticles were synthesized by the mechanochemical reaction of stoichiometric ZnO and alpha-Fe2O3, and single-phase ZnFe_2O_4 was obtained after 150 h of milling. The as-milled samples, with a high inversion degree, were subjected to different thermal annealings up to 600 ºC to control the inversion degree and, consequently, the magnetic properties. The as-milled samples, with a crystallite size of 11 nm and inversion degree delta = 0.57, showed ferrimagnetic behavior even above room temperature, as shown by Rietveld refinements of the X-ray diffraction pattern and superconducting quantum interference device magnetometry. The successive thermal treatments at 300, 400, 500, and 600 degrees C decrease delta from 0.15 to 0.18, affecting the magnetic properties. A magnetic phase diagram as a function of delta can be inferred from the results: for delta 0.5, a new antiferromagnetic order appeared due to the overpopulation of nonmagnetic Zn on octahedral sites that leads to equally distributed magnetic cations in octahedral and tetrahedral sites

Novel one-pot sol-gel synthesis route of Fe_3C/few-layered graphene core/shell nanoparticles embedded in a carbon matrix

Fe_3C/few-layered graphene core/shell nanoparticles embedded in a carbon matrix are synthesized by a novel two-step surfactant sol-gel strategy, where the processes of hydrolysis, polycondensation and drying take place in a one-pot. The present approach is based on the combined action of oleic acid and oleylamine, which act sterically on the precursor micelles when a densification temperature is performed in a reducing atmosphere. The structural and magnetic evolution of the formed compounds is investigated, ranging from iron oxides such as Fe_3O_4 and FeO, to the formation of pure Fe_3C/C samples from 700 degrees C onwards. Interestingly, Fe_3C nanoparticles with a size of similar to 20 nm crystallize immersed in the carbon matrix and the surrounding environment forms an oriented encapsulation built by few-layered graphene. The nanostructures show a saturation magnetization of similar to 43 emu/g and a moderate coercivity of similar to 500 Oe. Thereby, an innovative chemical route to produce single phase Fe_3C nanoparticles is described, and an effective method of few-layered graphene passivation is proposed, yielding a product with a high magnetic response and high chemical stability against environmental corrosion

Correction: A feasible pathway to stabilize monoclinic and tetragonal phase coexistence in barium titanate-based ceramics

Depto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu