Built-in Magnetic Field Research Articles

Spin Manipulation of Heterogeneous Molecular Electrocatalysts by an Integrated Magnetic Field for Efficient Oxygen Redox Reactions.

Understanding the spin-dependent activity of nitrogen-coordinated single metal atom (M-N-C) electrocatalysts for oxygen reduction and evolution reactions (ORR and OER) remains challenging due to the lack of structure-defined catalysts and effective spin manipulation tools. Herein, both challenges using a magnetic field integrated heterogeneous molecular electrocatalyst prepared by anchoring cobalt phthalocyanine (CoPc) deposited carbon black on polymer-protected magnet nanoparticles, are addressed. The built-in magnetic field can shift the Co center from low- to high-spin (HS) state without atomic structure modification, affording one-order higher turnover frequency, a 50% increased H2O2 selectivity for ORR, and a ≈4000% magnetocurrent enhancement for OER. This catalyst can significantly minimize magnet usage, enabling safe and continuous production of a pure H2O2 solution for 100 h from a 100 cm2 electrolyzer. The new strategy demonstrated here also applies to other metal phthalocyanine-based catalysts, offering a universal platform for studying spin-related electrochemical processes.

Insight into (Electro)magnetic Interactions within Facet-Engineered BaFe12O19/TiO2 Magnetic Photocatalysts.

A series of facet-engineered TiO2/BaFe12O19 composites were synthesized through hydrothermal growth of both phases and subsequent deposition of the different, faceted TiO2 nanoparticles onto BaFe12O19 microplates. The well-defined geometry of the composite and uniaxial magnetic anisotropy of the ferrite allowed alternate interfaces between both phases and fixed the orientation between the TiO2 crystal structure and the remanent magnetic field within BaFe12O19. The morphology and crystal structure of the composites were confirmed by a combination of scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses together with the detailed study of BaFe12O19 electronic and magnetic properties. The photocatalytic activity and magnetic field effect were studied in the reaction of phenol degradation for TiO2/BaFe12O19 and composites of BaFe12O19 covered with a SiO2 protective layer and TiO2. The observed differences in phenol degradation are associated with electron transfer and the contribution of the magnetic field. All obtained magnetic composite materials can be easily separated in an external magnetic field, with efficiencies exceeding 95%, and recycled without significant loss of photocatalytic activity. The highest activity was observed for the composite of BaFe12O19 with TiO2 exposing {1 0 1} facets. However, to prevent electron transfer within the composite structure, this photocatalyst material was additionally coated with a protective SiO2 layer. Furthermore, TiO2 exposing {1 0 0} facets exhibited significant synergy with the BaFe12O19 magnetic field, leading to 2 times higher photocatalytic activity when ferrite was magnetized before the process. The photoluminescence emission study suggests that for this particular combination, the built-in magnetic field of the ferrite suppressed the recombination of the photogenerated charge carriers. Ultimately, possible effects of complex electro/magnetic interactions within the magnetic photocatalyst are shown and discussed for the first time, including the anisotropic properties of both phases.

Field-Assisted Regulation Induced by Cobalt-Doped ZnO for Dendrite-Free Lithium Metal Anodes.

Lithium metal batteries are deemed as an optimal candidate for the next generation of durable energy storage devices. However, the growth of lithium dendrite and significant volume expansion pose as obstacles that impede the application of lithium metal batteries. In this work, a functional copper current collector was designed by coating it with Co-doped ZnO (Co/ZnO) to enhance the lithiophilicity through local electric fields and built-in magnetic fields induced by the ferromagnetic material. The incorporation of Co not only induces a local electric field and thus accelerating electron transfer, but also imparts the ferromagnetic behavior to ZnO, resulting in an internal magnetic field to regulate the dynamic trajectory. Profiting from the above advantages, the symmetric cells have excellent cycle stability in 1 mA cm-2 and 1 mAh cm-2 , maintaining ultra-low voltage for over 2000 h. This study provides a realizable pathway for next-generation current collector of copper modification.

Significant mechanical softening of copper under coupled electric and magnetic stimuli

The mechanical reliability of metallic components is vital for the stable operation of electrical equipment, especially for those used in ultra-high voltage transmission and electromagnetic launching, where metallic materials inevitably suffer from high-density current and strong magnetic field. However, little has been known about the mechanical behavior of metals under the electric-magnetic coupling stimuli. Here, by performing quantitative electromechanical tests under the built-in magnetic field inside transmission electron microscope, we found that copper, one of most commonly used conductive metals, can be significantly softened by the coupled electrical-magnetic stimuli, manifested as notable yield strength drop by five-times and obvious creep deformation at room temperature. A plausible mechanism behind the unusual softening, local-Lorentz-force-assisted dislocations depinning, has been proposed. Our results shed new light on understanding the deformation-induced failure of metals served in electromagnetic environments, and also inspire a new mechanical processing way for reshaping metals at much lower loads and temperatures.

Effects of Fe doping on the magnetic and absorption spectrum of graphene-like ZnO monolayer from first-principles calculations

The stability, electronic structure, magnetic properties, and absorption spectrum of Fe doped ZnO monolayer have been investigated by first-principles calculation. The results show that Fe doped ZnO monolayer systems are stability. The band gap of ZnO monolayer is narrowed by Fe atom doping, and with the increase of Fe doping concentration, the band gap becomes more narrow(2.900 eV) in Zn14Fe2O16 monolayer, which may be due to the formation of the local built-in magnetic field by Fe atom doped in ZnO monolayer systems. Both of Zn15Fe1O16 and Zn14Fe2O16 monolayer systems are magnetic. The Curie temperature value has been estimated to be 916 K in figuration I of Zn14Fe2O16 monolayer systems. Moreover, the absorption spectrum has a red-shift in the wavelength range of 320–800 nm by Fe doping. These calculation results are helpful for the design of photoelectric and spintronic devices.

Augmenting Around-Device Interaction by Geomagnetic Field Built-in Sensor Utilization.

In this paper, we investigate the possibilities for augmenting interaction around the mobile device, with the aim of enabling input techniques that do not rely on typical touch-based gestures. The presented research focuses on utilizing a built-in magnetic field sensor, whose readouts are intentionally affected by moving a strong permanent magnet around a smartphone device. Different approaches for supporting magnet-based Around-Device Interaction are applied, including magnetic field fingerprinting, curve-fitting modeling, and machine learning. We implemented the corresponding proof-of-concept applications that incorporate magnet-based interaction. Namely, text entry is achieved by discrete positioning of the magnet within a keyboard mockup, and free-move pointing is enabled by monitoring the magnet’s continuous movement in real-time. The related solutions successfully expand both the interaction language and the interaction space in front of the device without altering its hardware or involving sophisticated peripherals. A controlled experiment was conducted to evaluate the provided text entry method initially. The obtained results were promising (text entry speed of nine words per minute) and served as a motivation for implementing new interaction modalities. The use of neural networks has shown to be a better approach than curve fitting to support free-move pointing. We demonstrate how neural networks with a very small number of input parameters can be used to provide highly usable pointing with an acceptable level of error (mean absolute error of 3 mm for pointer position on the smartphone display).

Magnetoresistance-enhanced electro-thermal conversion performance

Seeking novel and effective approaches to synergistically improve the electrical and thermal transport properties and electro-thermal conversion efficiency of Bi2Te3-based alloys is essential for commercial applications. Here, GdCo2/Bi0.37Sb1.63Te3 thermoelectric nanocomposites were designed and synthesized to achieve synergistic improvement of electrical and thermal properties. As confirmed by magnetoresistance measurement, the built-in magnetic field and magnetic scattering generated by magnetic nanoparticles decrease the carrier concentration and mobility, and increase the Seebeck coefficient. Meanwhile, GdCo2 nanoparticles lead to broad-frequency phonon scattering, which effectively reduces the lattice thermal conductivity of nanocomposites. Eventually, the average ZT of the nanocomposite is increased by 18% in the range of 300–500 K, and the cooling temperature difference is increased by 68% near room temperature. This work proves that the thermoelectromagnetic coupling effects generated by magnetic particles can effectively improve the electro-thermal conversion performance.

Atomic ordering and bond relaxation in optical spectra of self-organized InP/GaInP2 Wigner molecule structures

We used transmission electron microscopy, Raman, and photoluminescence spectroscopy to identify the effect of CuPt-type GaP-InP atomic ordering (AO) on the structural and emission properties of self-organized (SO) InP/GaInP2 Wigner molecule (WM) quantum dot (QD) structures. We found that the correlation of AO and SO growth results in the formation of InP/GaInP2 QD/AO-domain (QD/AOD) core-shell composites. This observation shows that intrinsic WMs in this system emerge due to a strong piezoelectric field generated by AODs, which induces QD doping and a built-in magnetic field. We found that the bond relaxation of AODs leads to a decrease in the emission energy of WMs of 80 meV. The photoluminescence spectra of single WMs having an emission energy ∼1.53 eV are presented here, the lowest one reported for this system.

Enhanced Magnetic Field Sensitivity in Magnetoelectric Composite Based on Positive Magnetostrictive/Negative Magnetostrictive/Piezoelectric Laminate Heterostructure

Resonance magnetoelectric (ME) responses are investigated for the ME laminate heterostructure consisting of positive magnetostrictive material FeCuNbSiB, negative magnetostrictive material Ni and piezoelectric material PZT-8. On one hand, both a built-in magnetic field induced by the strong magnetic interactions between positive magnetostrictive and negative magnetostrictive materials and an intrinsic anisotropic field with obvious hysteresis and remnant magnetization in Ni layer result in a large zero-biased ME effect. The corresponding zero-biased ME voltage coefficient reaches 10.12 V/Oe (126.5 V/cm Oe), which is about 3 times larger than that of traditional Ni/PZT-8 (NP) laminate. On the other hand, the additional stress produced by the high magnetic permeability FeCuNbSiB layer enhances the magnetostriction of Ni layer and ME response. The corresponding maximum ME voltage coefficient (dVME/dHac) is 12.09 V/Oe (150.87 V/cm Oe) at bias magnetic field $H_{\mathrm {dc}} = 4$ Oe. It is further found that even a step change of ac magnetic field less than 10−10 T can be clearly distinguished by the proposed ME composite. The value of ME voltage sensitivity to dc magnetic fields (dVME/dH $_{\mathrm {dc}})$ reaches ~76.7 mV/Oe at $H_{\mathrm {dc}}= 5$ Oe and $H_{\mathrm {ac}} =0.1$ Oe. Correspondingly, the proposed laminate heterostructure is able to possess both strong ac and dc magnetic field sensitivities.

Contact-free Interaction with Mobile Devices using Magnetic, Lighting and Infrared Sources

In the area of Human-Computer Interaction (HCI), the concept of Around Device Interaction (ADI) is gaining special attention nowadays. It provides the possibility of interacting with the device by making use of gestures made in the surrounding space, as well as by detecting certain ambient changes, thus eliminating the need of direct physical contact. Contact-free interaction can be useful for wearable and smart devices, especially in cases where operating with small buttons and touchscreens appears to be particularly inconvenient. This paper presents alternative techniques for smartphone interaction based on different sensory inputs. Namely, external sources of magnetic field, light, and infrared (IR) signals are used to manipulate device sensors' readings which are in turn interpreted as the corresponding interaction commands. Proof-of-concept mobile applications are developed and here presented, demonstrating both the potentials and utility of the contact-free interaction. Specifically, small neodymium magnet was used for smartphone text entry, typical battery flashlight served for sending information to device via Morse code, as well as for alarm triggering, and voice-enabled control device was utilized for invoking smartphone calls from a distance. The presented interaction techniques do not require smartphone hardware alterations. While the usage of built-in magnetic field and ambient light sensors is assumed, peripheral infrared receiver can be used in order to provide IR-based support.

Enhanced Magnetization of Cobalt Defect Clusters Embedded in TiO2−δ Films

High magnetizations are desirable for spintronic devices that operate by manipulating electronic states using built-in magnetic fields. However, the magnetic moment in promising dilute magnetic oxide nanocomposites is very low, typically corresponding to only fractions of a Bohr magneton for each dopant atom. In this study, we report a large magnetization formed by ion implantation of Co into amorphous TiO2-δ films, producing an inhomogeneous magnetic moment, with certain regions producing over 2.5 μB per Co, depending on the local dopant concentration. Polarized neutron reflectometry was used to depth-profile the magnetization in the Co:TiO2-δ nanocomposites, thus confirming the pivotal role of the cobalt dopant profile inside the titania layer. X-ray photoemission spectra demonstrate the dominant electronic state of the implanted species is Co0, with a minor fraction of Co2+. The detected magnetizations have seldom been reported before and lie near the upper limit set by Hund's rules for Co0, which is unusual because the transition metal's magnetic moment is usually reduced in a symmetric 3D crystal-field environment. Low-energy positron annihilation lifetime spectroscopy indicates that defect structures within the titania layer are strongly modified by the implanted Co. We propose that a clustering motif is promoted by the affinity of the positively charged implanted species to occupy microvoids native to the amorphous host. This provides a seed for subsequent doping and nucleation of nanoclusters within an unusual local environment.

Enhanced Sensitivity With Five-Phase Heterostructure Magnetoelectric Sensor at Low Magnetic Bias Field

In this paper, we have proposed a five-phase heterostructure magnetoelectric (ME) sensor with high-permeability Fe-based nanocrystalline soft-magnetic alloy FeCuNbSiB, ferromagnetic alloy with constant elasticity FeNi and piezoelectric ceramic lead zirconium titanate (PZT). A built-in magnetic field induced by a spatially varying magnetization within the heterostructure leads to a zero-biased ME response. The corresponding zero-biased ME coefficient reaches as high as 43.75 V/ $\text {cm}\cdot \textrm {Oe}$ (3.5 V/Oe). Meanwhile, an additional stress produced by FeCuNbSiB is transferred to FeNi, which enhances the effective resonant strain coefficient for FeCuNbSiB/FeNi at low magnetic bias and decreases the required optimum dc magnetic bias $H_{\mathrm{opt,dc}}$ . Hence, the maximum resonant ME coefficient is found to reach 51.5 V/ $\text {cm}\cdot \textrm {Oe}$ (4.12 V/Oe) at $H_{\mathrm{opt,dc}}$ of only 33 Oe. $H_{\mathrm{opt,dc}}$ for our proposed ME sensor is about one-tenth of that for FeNi/PZT/FeNi. Correspondingly, the five-phase heterostructure ME sensor is able to possess the strong ME sensitivity at low bias or even zero bias.

Zero-biased and resonant magnetoelectric effect in magnetostrictive/piezoelectric multilayered composites

Magnetoelectric (ME) effect was investigated in various multilayered composites with various magnetostrictive materials Ni, FeNi and FeCo prepared by electroless deposition. Compared with the homogeneous ME composites, Ni/PZT/Ni and FeCo/PZT/FeCo, the ME voltage coefficient αE,31 of graded ME composites with two different magnetostrictive materials exhibit significant hysteresis phenomenon as a function of the external dc magnetic field at low frequency. The zero-biased ME response has been observed and can be attributed to a built-in magnetic field. However, at longitudinal vibration mode the ME voltage coefficient has been weakened in graded ME composites. The zero bias ME coupling observed in composites with graded magnetostrictive materials can be used in magnetic sensors with high sensitivity.

Microwave-Induced DC Response of Spin Wave Resonance Driven by an Anisotropic Built-In Field in a Permalloy Thin Strip

The microwave-induced dc response has been investigated in an in-plane magnetized permalloy strip. The spin wave resonances (SWRs) are observed. The SWR signals get anomalously suppressed when the direction of the specimen or the amplitude of the microwave frequencies is changed. Such suppression occurs at frequencies of less than 11 GHz, while the resonant field is as weak as less than 200 Oe. They are finally quantitatively interpreted by introducing an anisotropic built-in magnetic field that describes the shape anisotropy of the specimen.

LocateMe

Fine-grained localization is extremely important to accurately locate a user indoors. Although innovative solutions have already been proposed, there is no solution that is universally accepted, easily implemented, user centric, and, most importantly, works in the absence of GSM coverage or WiFi availability. The advent of sensor rich smartphones has paved a way to develop a solution that can cater to these requirements. By employing a smartphone's built-in magnetic field sensor, magnetic signatures were collected inside buildings. These signatures displayed a uniqueness in their patterns due to the presence of different kinds of pillars, doors, elevators, etc., that consist of ferromagnetic materials like steel or iron. We theoretically analyze the cause of this uniqueness and then present an indoor localization solution by classifying signatures based on their patterns. However, to account for user walking speed variations so as to provide an application usable to a variety of users, we follow a dynamic time-warping-based approach that is known to work on similar signals irrespective of their variations in the time axis. Our approach resulted in localization distances of approximately 2m--6m with accuracies between 80--100% implying that it is sufficient to walk short distances across hallways to be located by the smartphone. The implementation of the application on different smartphones yielded response times of less than five secs, thereby validating the feasibility of our approach and making it a viable solution.

Hysteresis and remanence in magnetoelectric effects in functionally graded magnetostrictive-piezoelectric layered composites

The observation and theory of a large remanent magnetoelectric (ME) coefficient and coercivity in the static field $H$ dependence of the low-frequency ME effects are reported for bilayers of lead zirconate titanate (PZT) and a functionally graded ferromagnetic layer. The grading involves magnetization with the use of nickel zinc ferrite of composition Ni${}_{0.7}$Zn${}_{0.3}$Fe${}_{2}$O${}_{4}$ (NZFO) and pure Ni. In homogeneous bilayers of PZT-Ni or PZT-NZFO, the ME voltage coefficient (MEVC) vs $H$ data do not show any hysteresis or remanence. Upon grading the ferromagnetic layer, significant changes including hysteresis and remanece are observed. In PZT-Ni-NZFO, MEVC vs $H$ data show a positive remnant MEVC and a negative coercive field. When the grading is reversed, in samples of PZT-NZFO-Ni, the remnant MEVC is negative and the coercive field is positive. A theory is proposed for the low-frequency ME effects in the graded composites. According to the model, the grading in the magnetization leads to a built-in magnetic field in the ferromagnetic layer, and this field depends on the sequence of grading and the thickness of the NZFO and Ni layers. As a result, the total torque moment and flexural deformations in the composite and the bias field dependence of ME voltage coefficient becomes strongly hysteretic. Calculated MEVC vs $H$, remnant MEVC, and coercive field are in good agreement with the data.

Newly Developed Electromagnetic Tracked Flexible Neuroendoscope -Technical Note-

Flexible endoscopes can be used in areas that are difficult to approach using rigid endoscopes. No current real-time navigation systems identify the tip of the flexible neuroendoscope. We have developed a flexible neuroendoscope mounted with a magnetic field sensor tip position-tracking system and evaluated the accuracy of this magnetic field neuronavigation system. Based on an existing flexible neuroendoscope, we created a prototype with a built-in magnetic field sensor in the tip. A magnetic field measurement device provides a magnetic field with a working volume of 500 × 500 × 500 mm in front of the device. The device consists of a flat field generator that creates a pulsed magnetic field, connected to a system control unit that interfaces with a computer. The magnetic field sensor (1.8 × 9 mm) was sealed in a site 0.9 mm from the endoscope tip. Accuracy of neuroendoscope tracking was measured using a three-dimensional coordinate-measuring machine that measures the position of objects along 3 axes, with an error of about 3 µm. The accuracy for this neuroendoscope with built-in magnetic field sensor was root mean square error of 1.2 mm and standard deviation of 0.5 mm. This magnetic field neuronavigation system enables real-time tracking of the tip of the flexible neuroendoscope. Application of this flexible neuroendoscope to intraoperative navigation appears promising, and may provide new advantages for minimally invasive endoscopic surgery.

Spin relaxation in lateral quantum dots: Effects of spin-orbit interaction

We report results of calculations of the effect of spin-orbit interaction on electron spin relaxation in a lateral quantum dot. Our study is motivated by puzzling results of high source-drain transport measurements of singlet-triplet transitions of two electrons in lateral and vertical devices that show a strong asymmetry as a function of the applied magnetic field. Using exact diagonalization techniques, we investigate the influence of the spin-orbit interaction on the energy levels of a two-electron droplet and we show that the spin-orbit interaction strongly affects the expectation values of the total and $z$-projection spins of the two-electron system. We then evaluate the energy relaxation rates for the two-electron droplet through the emission of longitudinal acoustic phonons and show that they are strongly dependent on the spin energy levels involved in the process. Our study shows that the spin-orbit interaction provides an effective coupling between the spin-polarized triplet states and the singlet state. However, the transition involving the spin singlet and the unpolarized triplet component is very weak even in the presence of spin-orbit interaction. The calculated scattering rates from the excited states to the ground state of the two-electron system clearly confirm this picture and reveal a microsecond time scale for the single-triplet relaxation through spin-orbit-mediated acoustic phonon emission and the relaxation mechanism presents a built-in magnetic field asymmetry, in qualitative agreement with experimental findings.

Spin–orbit interaction and spin relaxation in a lateral quantum dot

We report results of calculations of the effect of spin–orbit interaction on electron spin relaxation in a lateral quantum dot. Our study is motivated by puzzling results of high source–drain transport measurements of singlet–triplet transition of two electrons in lateral and vertical devices that show a strong asymmetry as a function of the applied magnetic field. On the low magnetic field side of the singlet–triplet transition, both the singlet ground state and the excited triplet state are experimentally resolved. Once the triplet becomes ground state, the singlet excited state is no longer observed. By exact diagonalization techniques, we evaluate the energy levels of a two-electron droplet in the presence of both Dresselhaus and Rashba contributions to the spin–orbit interaction. We then evaluate the energy relaxation rates for the two-electron droplet through the emission of LA phonons and show that they are strongly dependent on the spin of the energy levels involved in the process: transitions involving spin singlet and unpolarized triplet states remain forbidden even in the presence of spin–orbit interaction. This dephasing mechanism presents a built-in magnetic field asymmetry in qualitative agreement with experimental findings.

Change in entropy at a first-order magnetoelastic phase transition: Case study of Gd5(SixGe1−x)4 giant magnetocaloric alloys

The change in entropy, ΔS, at the first-order magnetoelastic phase transition in Gd5(SixGe1−x)4 alloys for x⩽0.5 has been measured with a high-sensitivity differential scanning calorimeter with built-in magnetic field, H. Scaling of ΔS is achieved by changing the transition temperature, Tt, with x and H from 70 to 310 K. Tt is thus the relevant parameter in determining the giant magnetocaloric effect in these alloys. The calorimetric determination of the change in entropy is also in agreement with the indirect calculation obtained from the magnetization curves measured up to 23 T using both the Clausius–Clapeyron equation and the Maxwell relation. A simple phenomenological model based on the magnetization curves accounts for these results.