Ferromagnetic Wires Research Articles

Multifunctional Sensor Based on a Hybrid Ferromagnetic/Sol–Gel TiO2 Coating Nanostructure

Development of a multifunction sensor for detection of environmental parameters such as temperature and relative humidity is described in this work. The multifunctional sensor is based on the combination in the same sensor unit of a magnetoinductive temperature sensor (soft magnetic FeCrSiBCuNb nanocrystalline wire) and a capacitive ceramic element (TiO2 microscale coating) for humidity detection. The sensing principle is based on the changes of electrical impedance of both the inner ferromagnetic wire (self-inductance, L) and the ceramic TiO2 coating (capacitance, C). In the magnetoinductive temperature element, the sensor span can be suitably selected through control of the Curie temperature of the residual amorphous phase (i.e., Cr content of the residual amorphous phase). With respect to the TiO2 coating, its doping with alkali elements (K) improves the humidity sensor sensibility. The designed cylindrical metal/TiO2/metal capacitive structures display resistive switching phenomena (memresistive behav...

Circulating Tumor Cell Microseparator Based on Lateral Magnetophoresis and Immunomagnetic Nanobeads

This paper presents a circulating tumor cell (CTC) microseparator for isolation of CTCs from human peripheral blood using immunomagnetic nanobeads with bound antiepithelial cell adhesive molecule (EpCAM) antibodies that specifically bind to epithelial cancer cells. The isolation is performed through lateral magnetophoresis, which is induced by high-gradient magnetic separation technology, involving a ferromagnetic wire array inlaid in the bottom substrate of a microchannel. Experimental results showed that the CTC microseparator isolates about 90% of spiked CTCs in human peripheral blood at a flow rate of up to 5 mL/h and purifies to approximately 97%. The overall isolation procedure was completed within 15 min for 200 μL of peripheral blood. CTCs from peripheral blood of patients with breast and lung cancers were isolated with the CTC microseparator, and the results were compared with those of healthy donors. Using a fluorescence-based viability assay, the viability of CTCs isolated from peripheral blood of patients with cancer was observed. In addition, the usefulness of the CTC microseparator for subsequent genetic assay was confirmed by reverse-transcriptase polymerase chain reaction (RT-PCR) amplification of cancer-specific genes using CTCs isolated from patients with cancer.

Stationary Josephson effect in a short multiterminal junction

We study the dc Josephson effect and the density of states in a multiterminal structure of cross-type geometry that consists of four superconducting electrodes connected by one-dimensional normal or ferromagnetic wires. We find that the Josephson current ${I}_{Jz}$ has a sinusoidal dependence on the phase difference ${\ensuremath{\phi}}_{z}$ between the superconductors in the horizontal wire with a critical current ${I}_{c}$ that can be varied by changing the phase difference ${\ensuremath{\phi}}_{y}$ between the superconductors in the vertical wire. The period of the function ${I}_{Jz}({\ensuremath{\phi}}_{z})$ depends on the ratio of the interface resistances ${R}_{Sn,z}/{R}_{Sn,y}$ being equal to $2\ensuremath{\pi}$ if this ratio is small and equal to $4\ensuremath{\pi}$ in the opposite limit. We also calculate the amplitudes of both the singlet and the odd-frequency triplet components in the system under consideration. The triplet component amplitude may be significantly larger than the amplitude of the singlet component.

Orthogonal Fields Controlled Fluxgate with Ferrofluid

The work presents an active magnetic field transducer which makes use of ferrofluid as a non-linear magnetic medium. The ferrofluid is subjected to the action of two orthogonal magnetic fields: the magnetic field used to explore the magnetization characteristics, also known as exciting or pumping field, and the field whose intensity and direction (polarity) must be determined. The orthogonal control of the magnetic devices, especially of the magnetic field transducers with ferromagnetic wires excited by an alternative current, is also known as Procopiu effect.

Magnetic Separation of Organic Dyes in Wastewater Using Superconducting Bulk Magnets

We propose a magnetic separation system using superconducting bulk magnets to effectively separate organic dyes in wastewater. Two key technologies are applied in this system: magnetic seeding and magnetic separation. Magnetic activated carbons (MACs) are used to adsorb organic dyes Orange II and Crystal violet, which serve as magnetic seeds. We set up a magnetic separator by placing an acrylic pipe between the magnetic poles of a face-to-face superconducting bulk magnet for high gradient magnetic separation (HGMS) with the use of a stainless steel filter, composed of ferromagnetic wire stuffed into acrylic pipe. The experimental results show that the adsorption ratios of 99% and the separation ratios of over 97% were achieved at low flow rate for the organic dyes.

Josephson current and density of states in proximity circuits withs+−superconductors

We study the emergent proximity effect in mesoscopic circuits that involve a conventional superconductor and an unconventional pnictide superconductor separated by a diffusive normal or ferromagnetic wire. The focus is placed on revealing signatures of the proposed ${s}_{+\ensuremath{-}}$ state of pnictides from the proximity-induced density of states and Josephson current. We find analytically a universal result for the density of states that exhibits both the Thouless gap at low energies and peculiar features near the superconducting gap edges at higher energies. The latter may be used to discriminate between ${s}_{+\ensuremath{-}}$ and ${s}_{++}$ symmetry scenarios in scanning tunneling spectroscopy experiments. We also calculate Josephson current-phase relationships for different junction configurations, which are found to display robust 0-$\ensuremath{\pi}$ transitions for a wide range of parameters.

Six-state, three-level, six-fold ferromagnetic wire system

Six stable states at remanence were identified in iron wire samples of 6-fold spatial symmetry using micromagnetic simulations and the finite element method. Onion and domain-wall magnetic states were tailored by sample shape and guided by an applied magnetic field with a fixed in-plane direction. Different directions of externally applied magnetic fields revealed a tendency for stability or nonstability of the considered states.

Fe-Rich Ferromagnetic Wires for Mechanical-Stress Self-Sensing Materials

The possibility of using Fe-rich wires in mechanical stress self-sensing materials is investigated. To this end, a retrieval technique aimed to characterize the high-frequency magneto-impedance effect in ferromagnetic wires under mechanical stresses is proposed. The technique is based on the measurement of the wires inside a metallic rectangular waveguide, and it is validated through numerical simulations and tested with already published experimental data. In addition, the studied Fe-rich wires are characterized by the occurrence of the natural ferromagnetic resonance, whose frequency position increases from 7 to 8.25 GHz for elongations ranging from 0 to 60 μm.

Reversal and termination of current-induced domain wall motion via magnonic spin-transfer torque

We investigate the domain wall dynamics of a ferromagnetic wire under the combined influence of a spin-polarized current and magnonic spin-transfer torque generated by an external field, taking also into account Rashba spin-orbit coupling interactions. It is demonstrated that current-induced motion of the domain wall may be completely reversed in an oscillatory fashion by applying a magnonic spin-transfer torque as long as the spin-wave velocity is sufficiently high. Moreover, we show that the motion of the domain wall may be fully terminated by means of the generation of spin-waves, suggesting the possibility to pin the domain-walls to predetermined locations. We also discuss how strong spin-orbit interactions modify these results.

Microscopic theory for a ferromagnetic nanowire/superconductor heterostructure: Transport, fluctuations, and topological superconductivity

Motivated by the recent experiment of Wang et al. [Nature Physics 6, 389 (2010)], who observed a highly unusual transport behavior of ferromagnetic Cobalt nanowires proximity-coupled to superconducting electrodes, we study proximity effect and temperature-dependent transport in such a mesoscopic hybrid structure. It is assumed that the asymmetry in the tunneling barrier gives rise to the Rashba spin-orbit-coupling in the barrier that enables induced p-wave superconductivity in the ferromagnet to exist. We first develop a microscopic theory of Andreev scattering at the spin-orbit-coupled interface, derive a set of self-consistent boundary conditions, and find an expression for the p-wave minigap in terms of the microscopic parameters of the contact. Second, we study temperature-dependence of the resistance near the superconducting transition and find that it should generally feature a fluctuation-induced peak. The upturn in resistance is related to the suppression of the single-particle density of states due to the formation of fluctuating pairs, whose tunneling is suppressed. In conclusion, we discuss this and related setups involving ferromagnetic nanowires in the context of one-dimensional topological superconductors. It is argued that to realize unpaired end Majorana modes, one does not necessarily need a half-metallic state, but a partial spin polarization may suffice. Finally, we propose yet another related class of material systems -- ferromagnetic semiconductor wires coupled to ferromagnetic superconductors -- where direct realization of Kitaev-Majorana model should be especially straightforward.

Magnonic Domain Wall Heat Conductance in Ferromagnetic Wires

We present a theoretical study of magnon-mediated heat transport in electrically insulating ferromagnetic wires containing a domain wall (DW). In the regime of validity of continuum micromagnetism, a DW is found to have no effect on the heat conductance. However, spin waves are found to be reflected by DWs with widths of a few lattice spacings, which is associated with emergence of an additional spin wave bound state. The resulting DW heat conductance should be significant for thin films of yttrium iron garnet with sharply defined magnetic domains.

Efficient inducement of bistable spin Hall effect using in-plane-magnetized V-shaped ferromagnetic wire

An advantageous device structure for the efficient electrical detection of spin Hall effect has been demonstrated by using a V-shaped ferromagnetic wire. The spin Hall voltages were evaluated by the combination between the spin absorption into the Platinum strip and the desired spin structure in the V-shaped Permalloy wire. Rectangular-shape field dependence of the spin Hall signal with bistability in the zero magnetic field reveals the validity of the developed device structure. We also showed that the domain wall chirality trapped at the corner of the V-shape wire can be determined from the sign of the spin Hall voltage.

Negative differential magnetoresistance in ferromagnetic wires with domain walls

A domain wall in a ferromagnetic one-dimensional nanowire experiences current induced motion due to its coupling with the conduction electrons. When the current is not sufficient to drive the domain wall through the wire, or it is confined to a perpendicular layer, it nonetheless experiences oscillatory motion. In turn, this oscillatory motion of the domain wall can couple resonantly with the electrons in the system affecting the transport properties further. We investigate the effect of the coupling between these domain wall modes and the current electrons on the transport properties of the system and show that such a system demonstrates negative differential magnetoresistance due to the resonant coupling with the low-lying modes of the domain wall motion.

Observation of magnetic domain-wall dynamics transition in Co/Ni multilayered nanowires

We observe a transition of domain-wall (DW) dynamics in ferromagnetic wires made of Co/Ni multilayers by use of transport measurement. As the wire width reduces, DW dynamics exhibits a transition from dendrite growth to pure DW motion. The threshold width is found to be about 300 nm and strongly depends on the relative dragging direction of the magnetic field and the current on DW: parallel (antiparallel) direction results in much smaller (larger) threshold width. It should be considered as a building block for DW-motion-based device applications.

A Comprehensive Analysis of the Absorption Spectrum of Conducting Ferromagnetic Wires

A detailed analysis of the absorption spectrum of conductive ferromagnetic wires is presented. The absorption spectrum is computed from the solution to the scattering problem, and circuit models are formulated to clarify the interplay between losses, skin effect, and wire geometry. Both infinitely long wires and the axial resonances introduced by finite-length wires are considered. The theoretical results are validated experimentally through measurements within a metallic rectangular waveguide.

Ballistic electron transport in a ferromagnetic quantum wire with Rashba interaction at the quantum dot

The spin-dependent transport in a ferromagnetic quantum wire with quantum dot in the presence of Rashba interaction is studied with the help of the discrete Lippmann–Schwinger equation. We show that the properties of conductance are determined by the existence of resonant states corresponding to the Breit–Wigner and Fano profiles. The distance between the points of zero and maximum conductance non-monotonically depends on the spin–orbit interaction and the difference in energy between the quantum-dot levels. The values of these quantities providing the greatest range of the controlling gate voltage are found.

Simultaneous magneto-optical Kerr effect and Sixtus-Tonks method for analyzing the shape of propagating domain walls in ultrathin magnetic wires

The controlled nucleation and propagation of magnetic domain walls in ultrathin ferromagnetic wires, such as nanowires and submicrometer wires, is extremely important for the development of new high performance magnetic domain wall logic devices. Therefore, it is equally essential to possess adequate advanced experimental investigation techniques in order to be able to achieve a comprehensive in situ analysis of as many as possible parameters related to the domain wall propagation, e.g., wall shape besides wall velocity and position. In this paper, we report on a method developed specifically for the investigation of the shape of propagating magnetic domain walls in ultrathin magnetic wires, i.e., with the diameter of the magnetic wire in the range 100-950 nm. The newly developed experimental method is based on the simultaneous use of two full-fledged experimental techniques: the magneto-optical Kerr effect for analyzing the surface effects of the passing domain wall and the Sixtus-Tonks method for the investigation of the entire moving wall. The results obtained offer essential information about the shape of the propagating magnetic domain walls, being unique to this new method.

Anomalous polarization-dependent transport in nanoscale double-barrier superconductor/ferromagnet/superconductor junctions

We study the transport properties of nanoscale superconducting (S) devices in which two superconducting electrodes are bridged by two parallel ferromagnetic (F) wires, forming an SFFS junction with a separation between the two wires less than the superconducting coherence length. This allows crossed Andreev reflection to take place. We find that the resistance as a function of temperature exhibits behavior reminiscent of the re-entrant effect and, at low temperatures and excitation energies below the superconducting gap, the resistance corresponding to antiparallel alignment of the magnetization of the ferromagnetic wires is higher than that of parallel alignment, in contrast to the behavior expected from crossed Andreev reflection. We present a model based on spin-dependent interface scattering that explains this surprising result and demonstrates the sensitivity of the junction transport properties to interfacial parameters.

Lateral metallic devices made by a multiangle shadow evaporation technique

The authors report the fabrication of lateral metallic structures with multiple materials using specifically designed resist masks and multiangle shadow evaporation. The whole fabrication process is carried out without breaking vacuum, which avoids contamination and allows for highly controlled interface properties between metals deposited sequentially. The authors incorporate the thickness of the mask as a design parameter, which allows one to introduce controlled variations between multiple contacts in the same device. Using a suspended mask, it is demonstrated the fabrication of asymmetric single electron transistors with tunnel junctions with different resistances. Using a nonsuspended mask, it is illustrated the fabrication of an extended structure (a thermopile), which consists of tenths of ferromagnetic wires with a nominal width of 30 nm connected electrically in series using a nonmagnetic metal.

Sensing domain wall pinning in the longitudinal magnetoresistance of a two-dimensional electron gas

We investigate the sensing of domain wall pinning in thin Co wires positioned on top of a two-dimensional electron gas (2DEG) heterostructure by measuring the longitudinal resistance of the 2DEG as the magnetic field is swept, in an analogy to the Barkhausen effect. For comparison, we also measure the magnetoresistance of the ferromagnetic film in the same device in a subsequent sweep. Compared to the Hall measurements, the longitudinal measurement has the advantage of sensing magnetic activity over longer lengths, while compared to the measurement of the magnetoresistance in the ferromagnetic wire, it offers complementary information related to the pinning and unpinning of the domain wall, due to its sensitivity only to the out-of-plane magnetic field component.