Weak External Magnetic Field Research Articles

Magnetic field remotely controlled selective biocatalysis

Many applications for medical therapy, biotechnology and biosensors rely on efficient delivery and release of active substances. Here, we demonstrate a platform that explores magnetic-field-responsive compartmentalization of biocatalytic reactions for well-controlled release of chemicals or biological materials on demand. This platform combines two different kinds of core–shell magnetic nanoparticle: one loaded with enzymes and another with substrate-bound therapeutic (bio)chemicals. Both cargos are shielded with a polymer brush structure of the nanoparticle shell, which prevents any enzyme–substrate interactions. The shield’s barrier is overcome when a relatively weak (a fraction of 1 T) external magnetic field is applied and the enzyme and the substrate are merged and forced to interact in the generated nanocompartment. The merged biocatalytic nanoparticles liberate the substrate-bound therapeutic drugs when the enzymes degrade the substrate. The developed platform provides a proof of concept for the remotely controlled release of drugs or (bio)chemicals using the energy of a non-invasive, weak magnetic field. Biocatalysis, if selective, offers great potential for the well-controlled release of drugs and other payloads. Here, Minko and co-workers separate enzymes and substrates by loading them onto individual, polymer-coated nanoparticles, and show that a magnetic field switches on the catalytic activity by merging the polymer shells.

Superconducting spin valves controlled by spiral re-orientation in B20-family magnets

We propose a superconducting spin-triplet valve, which consists of a superconductor and an itinerant magnetic material, with the magnet showing an intrinsic non-collinear order characterized by a wave vector that may be aligned in a few equivalent preferred directions under the control of a weak external magnetic field. Re-orienting the spiral direction allows one to controllably modify long-range spin-triplet superconducting correlations, leading to spin-valve switching behavior. Our results indicate that the spin-valve effect may be noticeable. This bilayer may be used as a magnetic memory element for cryogenic nanoelectronics. It has the following advantages in comparison to superconducting spin valves proposed previously: (i) it contains only one magnetic layer, which may be more easily fabricated and controlled; (ii) its ground states are separated by a potential barrier, which solves the “half-select” problem of the addressed switch of memory elements.

Spin wave propagation detected over [formula omitted] in half-metallic Heusler alloy Co2MnSi

The field of magnon spintronics offers a charge current free way of information transportation by using spin waves (SWs). Compared to forward volume spin waves for example, Damon-Eshbach (DE) SWs need a relatively weak external magnetic field which is suitable for small spintronic devices. In this work we study DE SWs in Co2MnSi, a half-metallic Heusler alloy with significant potential for magnonics. Thin films have been produced by pulsed laser deposition. Integrated coplanar waveguide (CPW) antennas with different distances between emitter and detection antenna have been prepared on a Co2MnSi film. We used a vector network analyzer to measure spin wave reflection and transmission. We observe spin wave propagation up to 100μm, a new record for half-metallic Heusler thin films.

Rashba induced spin multistability of the intersubband optical absorption in asymmetric coupled quantum wells

We study the multistable behavior of the intersubband optical absorption for InSb-based tunnel-coupled quantum wells. We consider four sublevels coming from the splitting of the two deepest levels due to the inversion asymmetry of the structure (Rashba effect), and a weak external in-plane magnetic field (Zeeman effect). Photoexcitation with an intense terahertz pump produces the redistribution of nonequilibrium electrons among the four spin sublevels. The redistribution produces a photoinduced self-consistent potential, giving rise to the renormalization of energy distance between sublevels. Depending on total electron concentration, magnetic field intensity, and pumping efficiency, we find different multistable behaviors in the intersubband optical absorption spectrum. Based on the matrix density, we describe the electron redistribution by means of a system of balance equations for electron concentrations.

Critical behavior of a monoaxial chiral helimagnet

We analyze the critical behavior of magnetically ordered phases appearing in a monoaxial chiral helimagnet in a weak external magnetic field. Using the formalism of the equations of state in the critical region, we determine the temperature dependence of the order parameters for the conical phase and the soliton-lattice phase. We calculated the critical exponents and show that they coincide with those in the three-dimensional Heisenberg model.

Selective surface magnetization of pentlandite with magnetite and magnetic separation

In this work, selective surface magnetization technology was employed to separate pentlandite from serpentine by adding fine magnetite coating on pentlandite surfaces, which was different from the conventional flotation. EDLVO (Extended Derjaguin-Landau-Verwey-Overbeek) theory calculations indicate that under a weak external magnetic field of 4.77kAm−1. Interactions between magnetite and pentlandite were obviously stronger than that between magnetite and serpentine. Therefore, fine magnetite fractions selectively adhered to pentlandite surfaces and enhanced its surfaces' magnetism. The above predication was well confirmed by the magnetic coating and magnetic separation tests with adding 3% magnetite fines, at a magnetic field intensity of 200kAm−1. SEM-EDS analyses show that magnetite particles were preferentially adsorbed onto pentlandite instead of serpentine. VSM (vibrating sample magnetometer) measurements further confirm that the magnetism of magnetic products (pentlandite) was strongly enhanced.

Spin-filter tunneling in superconducting mesa structures with a ferromagnetic manganite interlayer

We have studied the current transport and magnetism in epitaxial hybrid superconducting mesa structures consisting of a cuprate superconductor and superconducting niobium with a manganite LaMnO3 (LMO) interlayer. We have shown experimentally using magnetic resonance that the magnetization, magnetic anisotropy parameters, and transition temperature to the ferromagnetic state of the interlayer of the structures are analogous to those of an autonomous LMO film grown on a neodymium gallate substrate. The estimate of the barrier height obtained from the dependence of the characteristic resistance of mesa structures on the interlayer thickness has shown the barrier height variation with the thickness in the range of 5–30 mV. The temperature dependences of the conductivity of the mesa structure in the range between superconducting transition temperatures of the superconductors can be described in the theory taking into account the d-wave nature of the superconductivity for one of the electrodes and the spin-filtering of carriers passing through the tunnel interlayer. Spin-filtering is confirmed by the tunnel magnetoresistance and the high sensitivity of mesa structures to a weak external magnetic field in a voltage interval smaller than the gap of niobium.

Wake effects of a charged projectile flying above a magnetized metal film

This research deals with covering of a metal film on the semi-infinite dielectric in the presence of a weak external magnetic field. A charged projectile has been considered flying above the thin film. The surface wave frequencies of the system were derived by means of the quantum hydrodynamic theory through the appropriate boundary conditions. The energy loss of charged particle in the present system was also investigated. It is found that the external magnetic field modifies the distribution of electron gas density as well as the energy loss of flying charged particle.

Study about the nanoparticle agglomeration in a magnetic nanofluid by the Langevin dynamics simulation model using an effective Verlet-type algorithm

This paper presents a modelling study about the nanoparticle agglomeration in magnetic nanofluids. The colloidal magnetic nanoparticles size distribution is subjected to simultaneous translation and rotation movements under the action of conservative and dissipative forces, with their respective moments. In order to obtain the numerical solution of the coupled equations of motion, we use a Langevin dynamics stochastic method based on an effective Verlet-type algorithm. The presented model is based on an easy-to-implement integrator. We apply a number of analytical techniques to assess the performance of the method. The model has been tested on a magnetite nanoparticle-based nanofluid. Finally, the paper presents a number of structures obtained in various physical conditions, discussing the retrieved results of modelling and simulation. In weak external magnetic field, the nanoparticles form arrangements like linear chains or dense globes and rings, with magnetic moments rotating in both directions (both clockwise and counterclockwise). These arrangements, in vortex and toroidal states, are reported in actual scientific literature and open new perspectives for understanding the behaviour of nanofluids, with applications in engineering and medicine. Chains are predominant in high external magnetic, with local magnetic moments mainly orientated mainly along the direction of the applied field.

Cubic ideal ferromagnets at low temperature and weak magnetic field

The low-temperature series for the free energy density, pressure, magnetization and susceptibility of cubic ideal ferromagnets in weak external magnetic fields are discussed within the effective Lagrangian framework up to three loops. The structure of the simple, body-centered, and face-centered cubic lattice is taken into account explicitly. The expansion involves integer and half-integer powers of the temperature. The corresponding coefficients depend on the magnetic field and on low-energy effective constants that can be expressed in terms of microscopic quantities. Our formulas may also serve as efficiency or consistency check for other techniques like Green's function methods, where spurious terms in the low-temperature expansion have appeared. We explore the sign and magnitude of the spin-wave interaction in the pressure, magnetization and susceptibility, and emphasize that our effective field theory approach is fully systematic and rigorous.

Highly tunable bistability using an external magnetic field in photonic crystals containing graphene and magnetooptical layers

In this paper, we propose an optical bistable structure based on a single graphene layer sandwiched between two magnetooptical layers which are located between two photonic crystals. It is indicated that such structure shows optical bistability behaviors, in the near infrared range, which can be efficiently controlled with small external magnetic fields lower than 1.5 mT. Such weak external magnetic fields cannot affect the graphene layer and the dependence of bistability phenomenon on the magnetic field results from the presence of magnetooptical layers and their influences on the resonance frequency of the structure. Both switch-up and switch-down thresholds can be significantly adjusted by variation of the external magnetic field. Furthermore, the width of hysteresis loop enhances with increasing the magnetic field. To obtain a high tunability of bistability with external magnetic fields, the thickness of magnetooptical layers should be larger than a special value. It is also found that the increase of the Fermi energy level of graphene leads to the enhancement of both switch-up and switch-down thresholds as well as the width of the hysteresis loop. Finally, it is confirmed that optical bistability can be manipulated by changing the incident illumination angle.

Magnetic interlayer coupling in Fe/h-BN/Ni(111) probed by soft X-ray magnetic circular dichroism

We investigated magnetic interlayer coupling between Fe overlayers and Ni substrate through an intermediate monolayer of hexagonal boron nitride. We performed magnetic circular dichroism experiments at L2,3 absorption edges of Fe and Ni for ultrathin-layered structure of Fe/h-BN/Ni(111) grown in-situ in ultra high vacuum environment. Antiferromagnetic coupling has been revealed with a direct evidence of opposite magnetic circular dichroism signal of Fe to Ni in the absorption spectra. The antiferromagnetic coupling energy is estimated to be in order of a hundred μeV at the interface. The comparatively small coupling energy means controllability of magnetic alignment with weak external magnetic field.

SU(2) low energy quark effective couplings in weak external magnetic field

In this work corrections to the usual flavor SU(2) Nambu-Jona-Lasinio coupling due to a weak external magnetic field are calculated by considering quark polarization in a (dressed) gluon exchange mechanism for quark interactions. The quark field is splitted into two components, one that condenses and another one that is a background field for interacting quarks, being the former integrated out. The resulting determinant is expanded for relatively large quark mass and small magnetic field, $(eB_0/{M^*}^2)<1$ by resolving magnetic field dependent low energy quark effective interactions. Besides the corrections for the NJL and vector NJL effective couplings, different $B_0-$dependent effective couplings that break isospin and chiral symmetry emerge.

External driving synchronization in a superconducting quantum interference device based oscillator

We propose an external driving, self-sustained oscillator based on superconducting resonators. The dynamics of the self-sustained oscillator can be described by a Duffing–van der Pol like equation. Under external driving, the self-sustained oscillator presents synchronization phenomena. We analytically and numerically investigate the synchronization regions, and the results show that the synchronization bandwidth can be quickly adjusted in situ by the external weak magnetic field in sub-nano seconds. Moreover, the system can re-stabilize in about 10 ns with a certain sudden change of driving frequency or the critical current of the superconducting quantum interference device (SQUID). These advantages allow the potential applications of self-sustained oscillators in timing reference, microwave communication and electromagnetic sensing.

Effect of extended confinement on the structure of edge channels in the quantum anomalous Hall effect

Quantum anomalous Hall (QAH) effect in the films with nontrivial band structure accompanies the ferromagnetic transition in the system of magnetic dopants. Experimentally, the QAH transition manifests itself as a jump in the dependence of longitudinal resistivity on a weak external magnetic field. Microscopically, this jump originates from the emergence of a chiral edge mode on one side of the ferromagnetic transition. We study analytically the effect of an extended confinement on the structure of the edge modes. We employ the simplest model of the extended confinement in the form of potential step next to the hard wall. It is shown that, unlike the conventional quantum Hall effect, where all edge channels are chiral, in QAH effect, a complex structure of the boundary leads to nonchiral edge modes which are present on both sides of the ferromagnetic transition. Wave functions of nonchiral modes are different above and below the transition: on the "topological" side, where the chiral edge mode is supported, nonchiral modes are "repelled" from the boundary, i.e. they are much less localized than on the "trivial" side. Thus, the disorder-induced scattering into these modes will boost the extension of the chiral edge mode. The prime experimental manifestation of nonchiral modes is that, by contributing to longitudinal resistance, they smear the QAH transition.

Numerical analysis of applied magnetic field dependence in Malmberg-Penning Trap for compact simulator of energy driver in heavy ion fusion

To simulate a pulse compression process of space charge dominated beams in heavy ion fusion, we have demonstrated a multi-particle numerical simulation as an equivalent beam using the Malmberg-Penning trap device. The results show that both transverse and longitudinal velocities as a function of external magnetic field strength are increasing during the longitudinal compression. The influence of space-charge effect, which is related to the external magnetic field, was observed as the increase of high velocity particles at the weak external magnetic field.

Pionic dispersion relations in the presence of a weak magnetic field

In this work, dispersion relations of $\pi^0$ and $\pi^{\pm}$ have been studied in vacuum in the limit of weak external magnetic field using a phenomenological pion-nucleon $(\pi N)$ Lagrangian. For our purpose, we have calculated the results up to one loop order in self energy diagrams with the pseudoscalar $(PS)$ and pseudovector $(PV)$ pion-nucleon interactions. By assuming weak external magnetic field it is seen that the effective mass of pion gets explicit magnetic field dependence and it is modified significantly for the case of PS coupling. However, for the PV coupling, only a modest increase in the effective mass is observed. These modified dispersion relations due to the presence of the external field can have substantial influence in the phenomenological aspect of the mesons both in the context of neutron stars as well as relativistic heavy ion collisions.

Giant magnetoresistance of strong magnet–polymer heterostructures with a wide band gap

We present the results of studying the effect of a weak external magnetic field on the conductivity of heterostructures containing a wide-gap polymer layer. A set of experiments is singled out for switching the metal–dielectric conductivity, controlled by a weak magnetic field. Evidence of the reliability and repeatability of key experiments are reviewed, and the effect of possible artifacts on switching of the conductivity in the heterostructures under study is discussed.

Reversible Metal/Dielectric Phase Transition of Metal/Polymer Structures in Magnetic Field: Tuning the Sign of Magnetoresistance

Abrupt phase transitions in the structure of ferromagnet/ electroactive polymer/ non-magnetic metal have been found in the number of studies in magnetic field as huge magnetoresistive effects. The sign of magnetoresistance depends on the conductivity state that the polymer has possessed at the beginning of the experiment.The ways of managing of metal/ dielectric phase transition in the polymer layer with the aid of weak external magnetic field are considered. The model of local conversion of charge mobility in the conductive channels is suggested.

Numerical studies on antiferromagnetic skyrmions in nanodisks by means of a new quantum simulation approach

We employ a new quantum simulation approach to study the magnetism of nanodisks with Dzyaloshinsky–Moriya (DM) interaction. We find that a weak external magnetic field normal to the disk plane cannot obviously affect the single AFM skyrmion structure on small disk; but if it is sufficiently strong, it can destroy the AFM skyrmion completely. By increasing DM interaction, more self-organized magnetic domains appear, the average distance of the neighboring domains agrees well with a grid theory. In this case, a magnetic anisotropy normal to the disk plane can re-construct AFM skyrmion, providing a way for experimentalists to create AMF skyrmions.