Magnetic Bias Research Articles

Evidence of two dimensional dilute antiferromagnet mediated exchange bias field and vertical shift in LaFeO3-NiO nanocomposite

Low dimensional antiferromagnetic systems, such as nanoparticles, are known to exhibit weak ferromagnetic behavior due to the uncompensated surface spins. In this report, we present a detailed magnetic study of LaFeO3/NiO nanocomposite, synthesized by propylene glycol gel method. The X-ray diffraction (XRD) based analyses, in combination with transmission electron microscopy (TEM), confirms 50:50 ratio composite with ~35–40 nm size particles. Owing to the high Néel temperatures (TN) of LaFeO3 and NiO, significant exchange bias (EB) and vertical magnetization shift (VMS) have been observed in the nanocomposite at room temperature. These features are attributed to the exchange interaction at the interface between the core and shell of the nanoparticles that are homogeneously dispersed in the nanocomposite. In our studies, the EB field and the VMS attains a value as high as 2050 Oe & 1.8 × 10-1 emu/g at 5 K and 75 Oe & 1.7 × 10-2 emu/g at 300 K, respectively. The thermoremanent magnetization (TRM) and isothermoremanent magnetization (IRM) analyses are done to identify the two-dimensional (2D) diluted antiferromagnet (DAFF) behavior of the surface shell of nanoparticles. We propose a phenomenological antiferromagnetic-(2D-DAFF)-antiferromagnetic magnetic structure model, to explain all the rich features and the observed magnetic properties of this nanocomposite.

Magnetization reversal and sign reversal exchange bias field in polycrystalline Ni5.33Ta0.67B2O10

The sign reversal of both magnetization and exchange bias field was studied in the polycrystalline Ni5.33Ta0.67B2O10. The crystal structure of Ni5.33Ta0.67B2O10 is quasi–low dimensional due to NiO6/TaO6 octahedra forming two dimensional infinite layers. The antiferromagnetic/ferrimagnetic phase transition was observed at TN = 165 K. The negative exchange bias effect was found in the 30–90 K temperature range, whereas the positive exchange bias effect was observed at temperatures below 30 K. We assume that the exchange bias effect is due to different types of magnetic ordering of Ni2+ magnetic moments in two layers.

Magnetocaloric properties of the spin-S (S ≥ 1) Ising model driven by a time dependent oscillating magnetic field

By means of mean field approximation, we have investigated magnetocaloric properties of the spin-S Ising model for some spin values S=1,3/2,2,5/2,3 and 7/2 on a simple cubic lattice. The considered system is driven by a time dependent oscillating magnetic field including a bias field. Effects of the spin magnitude, oscillating magnetic field amplitude (h0/J) and bias field (hb/J) on the dynamic critical behavior, isothermal entropy change (|ΔSM|) and cooling capacity (q) of the system have been elucidated in detail. Our numerical outcomes indicate that both bias and oscillating magnetic fields prominently affect the |ΔSM| and q values of the system. When hb/J is increased, both |ΔSM| and q quantities get larger for all considered spin magnitudes, in the absence of oscillating field. It is found that there is a linear relationship between q and hb/J such that it will obey the law: q∝γ(S)hb/J for all spin magnitudes. Moreover, the isothermal entropy change varies in such manner that it exhibits an exponential behavior, |ΔSM|∝eαh0/J for a fixed value of bias field. These results show that the magnetocaloric properties of a material can be adjusted by changing the oscillating magnetic field components as well as bias field.

Strong electric field tuning of magnetism in self-biased multiferroic structures

A new type of multiferroic heterostructure has been proposed in this work with strong electric field tuning of magnetism. It is composed of a self-biased magnetic layered structure with perpendicular magnetic anisotropy (PMA) and one piezoelectric substrate. Two configurations were investigated by a modeling approach, Ni/Ni/Ni/PMN-PT with Cu as spacer and Terfenol-D/CoFeB/Ni/PMN-PT. Magnetic multilayers at their resonance exhibit multiple absorption peaks from acoustic and optical modes of spin interaction between adjacent magnetic layers. A piezoelectric substrate transfers electric field induced strain to adjacent magnetic layer and thus shifts resonance frequencies of the multiferroic structure by tuning magnetic effective fields through magnetoelastic coupling. It has been demonstrated computationally that the resonance frequencies for the simulated structures could be up to 76 GHz under zero magnetic bias field. A larger tunability (> 100%) is achieved with applied electric field to the PMN-PT [011] substrate. Resonance mode selectivity is present in the configuration Terfenol-D/CoFeB/Ni/PMN-PT wherein one desired mode exhibits a much higher tunability compared to other modes. This enables the total mode number to be tuned by merging or diverging different modes under E-field.

Exchange bias switching in an antiferromagnet/ferromagnet bilayer driven by spin–orbit torque

The electrical manipulation of magnetization and exchange bias in antiferromagnet/ferromagnet thin films could be of use in the development of the next generation of spintronic devices. Current-controlled magnetization switching can be driven by spin–orbit torques generated in an adjacent heavy-metal layer, but these structures are difficult to integrate with exchange bias switching and tunnelling magnetoresistance measurements. Here, we report the current-induced switching of the exchange bias field in a perpendicularly magnetized IrMn/CoFeB bilayer structure using a spin–orbit torque generated in the antiferromagnetic IrMn layer. By manipulating the current direction and amplitude, independent and repeatable switching of the magnetization and exchange bias field below the blocking temperature can be achieved. The critical current density for the exchange bias switching is found to be larger than that for CoFeB magnetization reversal. X-ray magnetic circular dichroism, polarized neutron reflectometry measurements and micromagnetic simulations show that a small net magnetization within the IrMn interface plays a crucial role in these phenomena. The magnetization and exchange bias field in an IrMn/CoFeB bilayer can be independently switched using a current-controlled spin–orbit torque generated in the antiferromagnetic IrMn layer.

Effects of mobile oxygen ions in top-gated synthetic antiferromagnet structure

In this work, we study the effects of mobile oxygen ions in a synthetic antiferromagnet structure gated by a sputtered SiO2 dielectric layer for memory and logic applications. Our devices utilize electrochemical reactions between dielectric reactive species and magnetic elements to create irreversible changes in magnetization. We analyzed the dependence of ion velocity on the gate dielectric properties such as the lattice parameter, oxygen migration energy barrier, and electric field (E-field). Hall bar devices were patterned and tested to determine the interlayer exchange coupling between the CoFeB and [Co/Pd]n layers. The anomalous Hall effect (AHE) of the CoFeB layer at different gate voltages (Vg) was measured to determine the Vg dependence. A sharp change in the behavior of the CoFeB layer with respect to negative Vg results in a non-reciprocal decrease in the coercivity and magnetization and an increase in exchange bias. The observed change in exchange bias field and magnetization allows us to measure the change in the effective thickness of the CoFeB layer. This led us to conclude that the source of such behavior is the negatively charged mobile oxygen ions from the SiO2 gate.

Implementation of a unidirectional-mode ferrite disk antenna using permanent magnets and a two-stage matching network

We experimentally demonstrate a circularly polarized antenna that utilizes unidirectional surface waves that propagate on the boundary of a small ferrite disk. The ferrite disk is metalized on top and is mounted on a grounded dielectric substrate. The disk is normally biased by a magnetic field that is provided by two permanent magnets below and above the structure. A two-section feeding network is used to feed the antenna. The operation frequency of the antenna can be adjusted by changing the magnitude of the applied magnetic bias. The sense of polarization can be reversed by reversing the direction of the applied bias.

Substrate integrated waveguide filter based on the magnetized ferrite with tunable capability

AbstractIn this paper, a miniaturized substrate integrated waveguide filter on the magnetized ferrite is presented. The proposed filter has a small size, high power transmission capability, relatively good bandwidth, and tunable frequency bandwidth. This filter is tunable due to the magnetic bias applied to the ferrite substrate. Compared to the tunable filters reported in previous references based on PIN diode, varactors, and mechanical changes, the proposed filter has a relatively wide adjustment on the operation frequency. Using this method dramatically reduces the complexity of the external control system. This tunable filter is optimized using the Ansoft HFSS software and then fabricated. The measured operating frequency of the filter is tunable from 5.3 to 7.15 GHz by changing the magnetic bias from 100 Oe to 1100 Oe. In the frequency range of 5.55 to 6.95 GHz, an insertion loss of less than 1.1 dB and a return loss of better than 20 dB are achieved. A good agreement between the simulation and experimental results was achieved.

Martensitic Transition, Magnetic, Microstructural and Exchange Bias Properties of Melt Spun Ribbons of Mn-Ni-Sn Shape Memory Heusler Alloy

In the present report, we have studied the structural, microstructural, magnetic, exchange bias (EB) properties and magnetoresistance (MR) in Mn rich (Mn at. ∼50%) Mn50Ni50−xSnx (x = 10) shape memory Heusler alloy ribbons, prepared using the melt spinning method. These ribbons were found to exhibit a first order structural (i.e., martensitic) transition at around 205 K from the high temperature austenite to the low temperature martensite phase. The martensitic transition occurs at comparatively lower temperatures in these ribbons than in the same bulk alloy. Curie temperature of the austenite phase (TCA) is found to be larger i.e., around room temperature than that of bulk alloy. A significant EB field (HEB) of around 960 Oe has been observed at 2 K for these ribbons, which is found to be comparable to that reported for other Heusler systems. The presence of the EB effect in these ribbons is attributed to the coexistence of FM/AFM exchange interactions in the martensite phase. They are also found to show a maximum negative MR of around 12% near the martensitic transition, for 50 kOe field change. Investigation of DC magnetization, AC susceptibility measurements and the observation of training effect (i.e., characteristic feature of EB) strongly corroborates with the coexistence of FM/AFM exchange interactions in the martensite phase of these ribbons, which results in the large EB. The effects of temperature and magnetic field on the EB properties and MR have also been studied here.

Broadband and giant nonreciprocity at the subwavelength scale in magnetoplasmonic materials

We unveil a previously overlooked wave propagation regime in magnetized plasmonic (gyrotropic) materials with comparable plasma and cyclotron frequencies, which enables a giant and broadband (nondispersive) nonreciprocal response. We show that this effect is due to a natural form of dispersion compensation that ultimately originates from the subtle implications of the principle of causality for gyrotropic plasmonic media, which allows the existence of a low-loss frequency window with anomalous nonmonotonic dispersion for the extraordinary mode. This is in stark contrast with conventional nongyrotropic passive materials, for which the frequency derivative of the permittivity dispersion function is always positive in low-loss regions. These findings pave the way for superior nonreciprocal components in terms of bandwidth of operation and compactness, with orders-of-magnitude reductions in size. As a relevant example, we consider indium antimonide (InSb) to theoretically demonstrate a deeply subwavelength, broadband, THz isolator operating under moderate magnetic bias and at room temperature.

Dependence on sensitivity patterns of bias fields for vector measurements using optically pumped vapor magnetometers

The optical light shift effect is an all-optical technique to create magnetic bias fields to determine the vector components of the magnetic field using a scalar optically pumped magnetometer. We show that the measurement of the induced bias fields is affected by the response pattern of the scalar magnetometer. The sensitivity of the magnetometer affects the minimum total field resolution as well as the angular resolution of the vector measurement. Three configurations of multiple-laser, optically pumped cesium vapor vector magnetometers are characterized for their spatial sensitivity patterns. Each configuration has a unique response level as a function of the relative magnetic field orientation. Thus, the formulations used for angular determination are dependent on the magnetometer configuration, and the angular resolution of the vector measurement is dependent on the relative angle between the magnetic field and the optical probe beam. The spatial dependence of the light shift signals is measured, and the equations describing their responses are presented for each configuration. Equations for determining the field angles from the bias field responses are derived.

Microwave Nonreciprocity

In this article, we provide a comprehensive review of past and present efforts in the field of microwave nonreciprocity with an emphasis on the most commonly used nonreciprocal devices, namely gyrators, isolators, and circulators. After discussing the origin and history of such efforts, we elaborate on the pros and cons of four distinct approaches to break reciprocity which include magnetic biasing of ferrite materials, intrinsic nonreciprocity of solid-state devices, nonlinearities with geometric asymmetries, and finally, periodic spatiotemporal variation. We subsequently pay due attention to the spatiotemporal approach and show that the recently emerging proposals to achieve magnet-free nonreciprocity with linear, periodically time-varying circuits can compete with traditional ferrite devices and even outperform them in several metrics, thus opening the door to exciting venues for miniaturized low-cost and high-performance nonreciprocal devices with numerous applications ranging from full-duplex communications and quantum computing to biomedical imaging and radar systems.

Nonreciprocal photonic topological order driven by uniform optical pumping

Photonic topological insulators (PTIs) have raised significant interest in recent years, in part because of their protected edge propagation. Most PTI realizations have been based on reciprocal platforms, with protection only from symmetry-preserving defects. Nonreciprocal PTIs protect against arbitrary disorder, but their implementation requires breaking reciprocity either through a magnetic bias or complex spatiotemporal modulation schemes. Here, we demonstrate that nonreciprocal topological protection can be achieved by optically pumping a nonlinear photonic crystal with a uniform bias field, drastically simplifying the implementation of quantum-Hall-like topological protection in silicon-compatible platforms for classical and quantum optics.

35-GHz Barium Hexaferrite/PDMS Composite-Based Millimeter-Wave Circulators for 5G Applications

We report a self-biased, low-profile circulator operating in the Ka-band fabricated by an entirely low-temperature (70 °C) screen-printing approach on printed circuit board (PCB). The magnetic material for the circulator is a barium hexaferrite BaFe12O19 (BaM)/polydimethylsiloxane (PDMS) nanocomposite that exhibits a zero-bias ferromagnetic resonance frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FMR</sub> ) at 46.6 GHz. Using this material, a microstrip-based circulator operating at 35 GHz is designed, fabricated, and characterized. A combination of mechanical milling, screen printing, photolithography, and electroplating is used for fabricating the circulator, in which the circulator disk (diameter of 2 mm and thickness of 250 μm) is completely embedded in the PCB, realizing a packaging compatible low-profile architecture with the total device area of 33 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The measured isolation (IS) and insertion loss (IL) of the fabricated circulator at 35 GHz is 3.9 and 8.7 dB, respectively. When the additional magnetic bias is applied to the circulator using external permanent magnets, the IS and IL performance is improved to be 7.4 and 8.4 dB, respectively. The impact of the loss factors associated with dielectric loss and surface roughness on the device performance is analyzed using High Frequency Structural Simulator (HFSS).

A Novel Analytical Study of Anisotropic Multi-Layer Elliptical Structures Containing Graphene Layers

This article aims to propose a novel analytical model for anisotropic multi-layer elliptical structures incorporating graphene layers. The multi-layer structure is formed of various magnetic materials, and each one has the permittivity and permeability tensors of ε̅̅ and μ̅, respectively. An external magnetic bias has been applied in the axial direction. A graphene layer, with isotropic surface conductivity of o, has been sandwiched between the two adjacent anisotropic materials. A novel matrix representation has been derived to find the propagation parameters of the multi-layer structure. Two exemplary important cases of the proposed general structure, as waveguides, have been investigated to show, first, the validity of our proposed analytical model and second, the richness of the general structure. The analytical and simulation results show an excellent agreement. A very large value of figure of merit (FOM), e.g., FOM = 110, is achieved for the second structure for the chemical potential and external magnetic bias of μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> = 0.9 eV and B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = 1 T, respectively. Our general structure and its analytical model can be exploited to design innovative terahertz devices such as absorbers, couplers, and cloaks.

Helical magnetic structure and exchange bias across the compensation temperature of Gd/Co multilayers

Antiferromagnetic coupling between rare-earth (RE) and transition metals (TM) gives rise to various magnetic ground states in RE/TM heterostructures. Interface structure and morphology tend to play important roles in defining the magnetic properties of heterostructures of these materials. Using spin-dependent polarized neutron reflectivity (PNR), we found a distinct magnetic structure for RE/TM (Gd/Co) multilayers having different interface morphologies. Two Gd/Co multilayers, each consisting of eight bilayers of Gd and Co, were grown at different argon pressures to accomplish variation in the interface morphology. The Gd/Co multilayer, which is grown at a lower argon pressure, exhibits lower intermixing/interdiffusion at the interfaces and shows asymmetric magnetic hysteresis loops below the compensation temperature (Tcomp ∼ 140 K) and antisymmetric magnetoresistance at Tcomp. The other multilayer, grown at two different argon pressures for the top and bottom four bilayers, shows a double hysteresis loop with an exchange-bias-like shift to the field axis at temperatures above Tcomp (∼150 K). From PNR measurements across Tcomp, these behaviors were attributed to interface dependent magnetic helical structures. The PNR results indicate the formation of a twisted helical magnetic structure with planar 2π domain walls and the evolution of magnetic inhomogeneities below Tcomp. The interface dependent twisted helical magnetic structures and the development of highly correlated magnetic inhomogeneities may find applications in all-spin-based technologies.

Novel analytical model of anisotropic multi-layer cylindrical waveguides incorporating graphene layers

We propose a novel analytical model for anisotropic multi-layer cylindrical structures containing graphene layers. The general structure is formed by the aperiodic repetition of a three-layer sub-structure, where a graphene layer with isotropic surface conductivity of σ is sandwiched between two adjacent magnetic materials. Each anisotropic material has permittivity and permeability tensors of ε̿ and μ̿, respectively. An external magnetic bias is applied in the axial direction. The general matrix representation is obtained for our proposed analytical model to determine the dispersion relation. The relation is used to find the effective index of the structure and its other propagation parameters. Two special exemplary structures are introduced and employed to illustrate the richness of the proposed general structure in terms of the related specific plasmonic wave phenomena and effects. Several simulations were conducted to demonstrate the notable wave-guiding properties of the structure in the 10–40 THz band. Very good agreement was obtained between the analytical and simulation results. The proposed structure can be utilized to design novel plasmonic devices, such as absorbers, modulators, plasmonic sensors, and tunable antennas in the THz frequencies.

Modeling of magnetic shape memory alloy plates for pressure sensor application

This article investigates the basis for pressure sensor application based on the magnetic shape memory effect in membranes. Von Karmans nonlinear terms are considered in strain–displacement relationships of thin films, and a new method is presented for solution of large deflections of thin films with arbitrary boundary condition. In this study, the equations of motion of magnetic shape memory alloys are extended. In pressurized membranes, the complex distribution of mechanical stress can cause the martensitic reorientation, which is the underlying mechanism for sensing applications in magnetic shape memory alloys. To examine the obtained model, the governing equations of magnetic shape memory alloys are solved for clamped thin films and results are compared with experimental data. Demonstrating good agreement with experimental data, the presented model can be used for analysis of magnetic shape memory alloy–based smart structures. In this article, the deflection profile of the uniformly loaded thin films is determined with different boundary conditions. Furthermore, the center deflection of magnetic shape memory alloy membrane under different magnetic bias field is simulated. The results of simulation can be used for designing a membrane-based pressure sensor using magnetic shape memory alloys.

Age constraints on the Cambrian Mesón Group (NW Argentina) based on detrital zircons U–Pb geochronology and magnetic polarity bias

The Cambrian Mesón Group represents the first infill stages of the NW Argentina Paleozoic basin, given after the major orogenic event represented by the Tilcaric orogenic phase. This unit extends for several hundred kilometers along the Eastern Cordillera in the provinces of Salta and Jujuy, but its age is poorly constrained due to the paucity of fossil evidence and scarce geochronological data. U–Pb LA-ICP-MS geochronological studies were carried out on detrital zircons from the Lizoite, Campanario and Chalhualmayoc formations, that make up the Mesón Group, in outcrops in the Santa Victoria sub-basin in northernmost Argentina. Maximum depositional ages of 524.8 ± 4.1, 519.7 ± 2.4 and 508.6 ± 1.7 Ma were obtained for these formations, respectively. These new data, together with previous detrital zircon ages from southernmost outcrops of this Group, points to a possible diachronism in deposition, with younger ages towards the south. In our studied units, a dominant peak of Cambrian to late Ediacaran zircons is characteristic for the three formations, although a somewhat larger contribution from older crystals is observed in the Campanario and Chalhualmayoc formations. This pattern is similar to most other reported detrital zircon age spectra for the Mesón Group along the basin. Published paleomagnetic data for the Campanario Formation indicates a very strong reverse magnetic polarity bias, which allows us to infer three more likely intervals for the deposition of this formation (roughly 512–510, 509–505 and 497-493 Ma). These three alternatives are analyzed with respect to either major diachronous or roughly synchronous deposition of the Mesón Group sediments across the whole basin. Published magnetic fabric data for the Campanario Formation at three distant localities suggest possible paleocurrent directions from W to E.

Terahertz microstrip circulator based on graphene

In this paper, we present an improved wave concept iterative process (WCIP) algorithm for the analysis of anisotropic graphene. Then, we use it to investigate a novel type of terahertz microstrip circulator based on graphene. In this algorithm, we propose a new technique for modeling of magnetized graphene layers, which consists of redefining the boundary conditions of graphene in terms of waves and integrating the surface conductivity without involving volumetric discretization. Numerical examples are presented to verify the efficiency of the proposed algorithm. The numerical simulations demonstrate that the performances of the circulator can be adjusted by applying an electric or magnetic field bias.