Artificial Magnetic Materials Research Articles

Magnetic plasmon resonances in nanostructured topological insulators for strongly enhanced light\u2013MoS2 interactions

Magnetic resonances not only play crucial roles in artificial magnetic materials but also offer a promising way for light control and interaction with matter. Recently, magnetic resonance effects have attracted special attention in plasmonic systems for overcoming magnetic response saturation at high frequencies and realizing high-performance optical functionalities. As novel states of matter, topological insulators (TIs) present topologically protected conducting surfaces and insulating bulks in a broad optical range, providing new building blocks for plasmonics. However, until now, high-frequency (e.g. visible range) magnetic resonances and related applications have not been demonstrated in TI systems. Herein, we report for the first time, to our knowledge, a kind of visible range magnetic plasmon resonances (MPRs) in TI structures composed of nanofabricated Sb2Te3 nanogrooves. The experimental results show that the MPR response can be tailored by adjusting the nanogroove height, width, and pitch, which agrees well with the simulations and theoretical calculations. Moreover, we innovatively integrated monolayer MoS2 onto a TI nanostructure and observed strongly reinforced light–MoS2 interactions induced by a significant MPR-induced electric field enhancement, remarkable compared with TI-based electric plasmon resonances (EPRs). The MoS2 photoluminescence can be flexibly tuned by controlling the incident light polarization. These results enrich TI optical physics and applications in highly efficient optical functionalities as well as artificial magnetic materials at high frequencies.

Hybrid perfect metamaterial absorber for microwave spin rectification applications

Metamaterials provide compelling capabilities to manipulate electromagnetic waves beyond the natural materials and can dramatically enhance both their electric and magnetic fields. The enhanced magnetic fields, however, are far less utilized than the electric counterparts, despite their great potential in spintronics. In this work, we propose and experimentally demonstrate a hybrid perfect metamaterial absorbers which combine the artificial metal/insulator/metal (MIM) metamaterial with the natural ferromagnetic material permalloy (Py) and realize remarkably larger spin rectification effect. Magnetic hot spot of the MIM metamaterial improves considerably electromagnetic coupling with spins in the embedded Py stripes. With the whole hybridized structure being optimized based on coupled-mode theory, perfect absorption condition is approached and an approximately 190-fold enhancement of spin-rectifying photovoltage is experimentally demonstrated at the ferromagnetic resonance at 7.1 GHz. Our work provides an innovative solution to harvest microwave energy for spintronic applications, and opens the door to hybridized magnetism from artificial and natural magnetic materials for emergent applications such as efficient optospintronics, magnonic metamaterials and wireless energy transfer.

Enhanced perpendicular magnetocrystalline anisotropy energy in an artificial magnetic material with bulk spin-momentum coupling

We systematically investigate the perpendicular magnetocrystalline anisotropy (MCA) in Co$-$Pt/Pd-based multilayers. Our magnetic measurement data shows that the asymmetric Co/Pd/Pt multilayer has a significantly larger perpendicular magnetic anisotropy (PMA) energy compared to the symmetric Co/Pt and Co/Pd multilayer samples. We further support this experiment by first principles calculations on the CoPt$_2$, CoPd$_2$, and CoPtPd, which are composite bulk materials that consist of three atomic layers in a unit cell, Pt/Co/Pt, Pd/Co/Pd, Pt/Co/Pd, respectively. By estimating the contribution of bulk spin-momentum coupling to the MCA energy, we show that the CoPtPd multilayer with the symmetry breaking has a significantly larger perpendicular magnetic anisotropy (PMA) energy than the other multilayers that are otherwise similar but lack the symmetry breaking. This observation thus provides an evidence of the PMA enhancement due to the structural inversion symmetry breaking and highlights the asymmetric CoPtPd as the first artificial magnetic material with bulk spin-momentum coupling, which opens a new pathway toward the design of materials with strong PMA.

Broadband metamaterial absorber with an in-band metasurface function.

Metamaterials (MMs) have received wide attention due to their strong ability to control wave propagation. Here we propose a thin MM that can realize broadband absorption, as well as in-band phase-controlled reflection. It absorbs more than 90% energy of the incident circularly polarized (CP) light between 7.94-9.76 and 10.99-12.26GHz and, meanwhile, works as a half-wave plate near 10.35GHz. The schemes based on both natural and artificial magnetic materials have been discussed to produce the anisotropic phase. A reflection beam steering metasurface is proposed using the absorption-selective MM. The metasurface-integrated MM absorber may have important applications in aerospace technologies such as radome.

U-slot patch antenna with low RCS based on a metaferrite substrate

Metamaterial ferrites or metaferrites are artificial magnetic materials which mimic the properties of ferrites at a certain frequency operation. Antenna engineers are therefore able to design and create artificial substrates which replicate the electrical properties of ferrites without actually using any in the construction. This is advantageous as ferrites can offer performance improvements to microstrip antennas, such as size reduction and wideband impedance matching. In this paper, a metaferrite substrate designed by the use of a genetic algorithm is presented. The metaferrite was optimized in order to obtain the magnetic responses at 9GHz, for its use as the substrate of a microstrip antenna. As an example, a U-slot patch antenna based on the metaferrite is demonstrated, which can achieve stable radiation and 14 dB radar cross section (RCS) reduction performance in the measurement.

Electrodynamics in cylindrical symmetry in the magnetic plasma state

Excited states in magnetic structures of the so-called spin-ices and in some artificial magnetic materials present a behaviour as being a magnetic neutral plasma. In this state the electromagnetic waves in confined systems (waveguides) filled with materials with magnetic charges are able to transmit information and energy. In the natural spin-ices, the difficulty is the very low temperature for which these magnetic entities appear, whose phenomenology under the electromagnetic interaction is that of solids containing magnetic charges. However, similar behaviour may be present in other compounds at higher temperatures, even at room temperature and they are named artificial spin-ice compounds. This analysis is addressed to obtain theoretical results about magnetic responses and frequency-dependent magnetricity. The key physical magnitudes are the plasmon frequency () which is related to the cut-off frequency in a wave guide and the effective inertial masses () of these magnetic charges. All properties of the electromagnetic propagation in these compounds with effective magnetic monopoles depend on and m. This is carried out including the dissipative forces among magnetic charges which give new characteristic features to the electromagnetic propagation. The main goal of this work is the analysis of these electromagnetic properties in order to find possible circuital applications of these materials to be utilized by devices.

Magnonic crystals for data processing

Magnons (the quanta of spin waves) propagating in magnetic materials with wavelengths at the nanometer-scale and carrying information in the form of an angular momentum can be used as data carriers in next-generation, nano-sized low-loss information processing systems. In this respect, artificial magnetic materials with properties periodically varied in space, known as magnonic crystals, are especially promising for controlling and manipulating magnon currents. In this article, different approaches for the realization of static, reconfigurable, and dynamic magnonic crystals are presented along with a variety of novel wave phenomena discovered in these crystals. Special attention is devoted to the utilization of magnonic crystals for processing of analog and digital information.

Miniaturized Array Antenna Using Artificial Magnetic Materials for Satellite-Based AIS System

The ground-based automatic identification system (AIS) is a coastal tracking and messaging system used by vessels for maritime traffic monitoring. The European SAT-AIS initiative aims at providing a space-based complementary system to extend the range of the existing AIS to high seas via very high frequency (VHF) satellite constellation. The AIS Miniaturized Antenna (AISMAN) activity concentrates on the development of a VHF array antenna for minisatellite platforms in low Earth orbit. Array element volume minimization and mass reduction are considered performance drivers due to the in-orbit array deployment and satellite mass requirements. Artificial magnetic materials (AMMs) have been chosen as design concept of the baseline array element due to the significant size reduction they can offer. Further technical solutions, such as slotted ground plane, combined with AMM, have allowed for an outstanding profile reduction, while preserving high radiation efficiency and low back radiation. Innovative testing methodologies have been specifically developed to handle truncation errors and echo signals while performing radiation pattern measurement of the array on a full-scale mockup platform in a hemispherical automotive near field (NF) range. This paper discusses the design of array elements, from breadboard to engineering model (TRL4), manufacturing and validation campaigns at single element and array level.

High coercivity in large exchange-bias Co/CoO-MgO nano-granular films**Project supported by the National Basic Research Program of China (Grant No. 2012CB932304), the National Natural Science Foundation of China (Grant Nos. U1232210, 91122035, 11174124, and 11374137), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 14KJB140003), and the Priority Academic Program Development of Jiangsu Higher Education Institutions,

We present a detailed study on the magnetic coercivity of Co/CoO-MgO core-shell systems, which exhibits a large exchange bias due to an increase of the uncompensated spin density at the interface between the CoO shell and the metallic Co core by replacing Co by Mg within the CoO shell. We find a large magnetic coercivity of 7120 Oe around the electrical percolation threshold of the Co/CoO core/shell particles, while samples with a smaller or larger Co metal volume fraction show a considerably smaller coercivity. Thus, this study may lead to a route to improving the magnetic properties of artificial magnetic material in view of potential applications.

GENERIC BUILDING BLOCKS FOR CONSTRUCTION OF ARTIFICIAL MAGNETIC MEDIA

Variety of designs for artificial magnetic materials have been proposed in the literature. In most designs such as split-ring resonators, the inductive and capacitive responses of metallic inclusions are dependent since the area and perimeter of the resonators' geometry cannot be tuned independently. In this work, three generic resonators for the design of artificial magnetic materials are proposed. The resonators are called rose curve resonator, corrugated rectangular resonator, and sine oval resonator. The proposed resonators' patterns are characterized so that their areas and perimeters vary independently. Thus, the geometries are capable of satisfying any realizable combination of area and perimeter designed for an artificial magnetic material with desired properties. Numerical studies are considered showing the effectiveness of the new geometries to fulfil design specifications.

Measuring the Surface Properties of a Novel 3-D Artificial Magnetic Material

A novel three dimensional artificial magnetic material structure named here as Tunable Multi Layer Multi Dielectric High Impedance Surface (TMMD-HIS) is proposed. These three dimensional structures have periodicity along all the three dimensions and the remarkable feature is that these systems have complete band gaps and therefore propagation states are not allowed in any direction. The measurement of its surface properties is a means to understand its transmission coefficient, reflection coefficient and surface wave band gap characteristics. For analysis, numerical technique of Finite Element Method is used. The results obtained in HFSS simulation software are presented.

Magnon transistor for all-magnon data processing.

An attractive direction in next-generation information processing is the development of systems employing particles or quasiparticles other than electrons—ideally with low dissipation—as information carriers. One such candidate is the magnon: the quasiparticle associated with the eigen-excitations of magnetic materials known as spin waves. The realization of single-chip all-magnon information systems demands the development of circuits in which magnon currents can be manipulated by magnons themselves. Using a magnonic crystal—an artificial magnetic material—to enhance nonlinear magnon–magnon interactions, we have succeeded in the realization of magnon-by-magnon control, and the development of a magnon transistor. We present a proof of concept three-terminal device fabricated from an electrically insulating magnetic material. We demonstrate that the density of magnons flowing from the transistor’s source to its drain can be decreased three orders of magnitude by the injection of magnons into the transistor’s gate.

MINIATURIZED BANDSTOP FILTERS USING SLOTTED-COMPLEMENTARY RESONATORS

This paper presents a technique to miniaturize bandstop filters applicable for radiofrequency/microwave applications. The technique comprises the use of small resonant inclusions bridged with a slot, etched from the metallic ground plane underneath the filter’s microstrip line segment. Unlike artificial magnetic materials, for instance split-ring resonators (SRRs) that respond to vertical magnetic field, complementary SRRs (CSRRs) resonate upon an excitation of an axial time-varying electric field. Numerical full-wave studies are presented here to validate and proof the proposed concepts. It is found that high miniaturization factors can be achieved for the filters loaded with the proposed slotted complementary resonators in comparison to nonslotted resonators. Furthermore, sensitivity analysis is conducted to study the effect of geometrical parameters of the proposed slotted resonators as well as the effect of the choice of host media on the suppression bandwidth and rejection level of such filters. Miniaturized bandstop filters based on developed slotted-complementary spiral resonators are also proposed and numerically evaluated.

Analytical models for predicting the effective permittivity of complementary metamaterial structures

We present an analytical formula to calculate the resultant effective permittivity of a metamaterial medium composed of unit cells with complementary structures. Complementary metamaterial can be considered to be the complementary screen of artificial magnetic materials. Based on Babinet's principle and duality, complementary metamaterial provides a negative effective permittivity in the vicinity of its resonance frequency. The developed formula is general and predicts the effective permittivity for a specific topology. The derived model validated by comparison to results obtained using full-wave simulation. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1565–1569, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27655

A New Type of Transmission Line-Based Metamaterial Resonator and Its Implementation in Original Applications

In this paper, a new type of transmission line-based metamaterial resonator is proposed. The structure is based on a modified uniplanar series resonator bent to form a close square loop inclusion which contains a distributed capacitance and inductance. Many advantages of the proposed structure over conventional split ring resonators are discussed. Based on simple transmission line theory, the resonance frequency at which a deep rejection frequency band occurs with sharp cutoff rates beside the stopband, due to the presence of negative effective permeability in the dielectric slab of the host transmission line in the vicinity of resonance, is calculated and verified with simulated and measured results. Two prototype microwave devices are designed to illustrate the potentiality of the proposed inclusion. The first design represents a dual-band bandstop filter with designed center band frequencies at 1.6 and 2.2 GHz. The lumped element model of this filter contains four series-connected parallel LC resonators which are implemented using our high Q-factor artificial magnetic materials (AMMs) patterned on the backside of a coplanar waveguide (CPW) transmission line. The second device is a miniaturized hybrid branch line coupler operating at 2.4 GHz, based on a special loading technique that increases the electric length of transmission lines by patterning the ground plane under the conductor trace in microstrip lines with the complementary, dual-behavior, configuration of AMMs. A size reduction factor up to 16.95% is achieved compared to conventional couplers. For both design examples the measured and simulated responses are in very good agreement which validates our work.

Epitaxial Electrodeposition of Cobalt on a Pt(111) Electrode Covered with a Cu(111) Film

Electrodeposition is an inexpensive alternative to the conventional molecular beam epitaxy technique used to fabricate artificial magnetic materials, such as cobalt thin film. Reported here is a scanning tunneling microscopy (STM) study on the electrodeposition of Co on a Pt(111) single-crystal electrode precoated with a Cu thin film in 0.1 M KClO4 + 1 mM HCl + 0.04 M CoCl2 (pH 3). Deposition of Co started with the nucleation of nanometer-sized clusters preferentially at pits on the Cu support, followed by lateral expansion and coalescence of Co nuclei to form a uniform Co layer. Normally a few Co layers would grow simultaneously to produce a smooth Co deposit until the 12th layer. Cobalt grew in three dimensions afterward. Atomic-resolution STM imaging showed that the first Co layer assumed a double-lined pattern, which was lifted by the deposition of another layer of Co. The second Co layer exhibited a hollow-ring pattern, which transformed into a moiré pattern and triangular pits at the third Co layer. The moiré pattern gained prominence at the expense of the triangular pits as the Co deposit thickened. The amplitude of the intensity modulation of the moiré pattern decreased with the thickness of the Co deposit and eventually became indiscernible at the 12th layer. These restructuring events resulted from a gradual release of the stress at the Co/Cu interface. Since the morphology of the copper substrate was hardly changed by the deposition of cobalt, mixing at the Co/Cu interface seems to be negligible. Similar to the deposition process, dissolution of Co deposit proceeded in layers also.

Nth order Rose curve as a generic candidate for RF artificial magnetic material

Recent work demonstrated that the permeability and magnetic loss tangent of artificial magnetic material inclusions can be represented simply in terms of the circumference and area of the inclusion. While such a representation makes direct use of the inclusion’s circuit model, the fact that the magnetic properties can be described in terms of the perimeter and area of the inclusion facilitates designing inclusions that achieve specific constraints without the need for intensive full-wave trial and error simulations. Because of such flexibility, we propose a new set of generic curves described as nth order Rose curves to be candidates for AMMs. In fact, the new curves not only provide significant design flexibility, but provide features not present in traditional topologies, most pronouncedly a wider band over which negative permeability is achieved with low dispersion.

Artificial ice goes thermal

Artificial magnetic materials may lead to studies of the thermodynamics of arbitrarily designed lattices. Unfortunately, none of the proposed materials has achieved its ground state through thermodynamic equilibrium as real materials do — until now.

Experimental verification of metamaterial based subwavelength microwave absorbers

We designed, implemented, and experimentally characterized electrically thin microwave absorbers by using the metamaterial concept. The absorbers consist of (i) a metal back plate and an artificial magnetic material layer; (ii) metamaterial back plate and a resistive sheet layer. We investigated absorber performance in terms of absorbance, fractional bandwidth, and electrical thickness, all of which depend on the dimensions of the metamaterial unit cell and the distance between the back plate and metamaterial layer. As a proof of concept, we demonstrated a λ/4.7 thick absorber of type I, with a 99.8% absorption peak along with a 8% fractional bandwidth. We have shown that as the electrical size of the metamaterial unit cell decreases, the absorber electrical thickness can further be reduced. We investigated this concept by using two different magnetic metamaterial inclusions: the split-ring resonator (SRR) and multiple SSR (MSRR). We have also demonstrated experimentally a λ/4.7 and a λ/4.2 thick absorbers of type II, based on SRR and MSRR magnetic metamaterial back plates, respectively. The absorption peak of the SRR layout is 97.4%, while for the MSRR one the absorption peak is 98.4%. The 10 dB bandwidths were 9.9% and 9.6% for the SRR and MSRR cases, respectively.

Artificial Magnetic Materials Using Fractal Hilbert Curves

Novel configurations based on Fractal Hilbert curves are proposed for realizing artificial magnetic materials. It is shown that the proposed configuration gives significant rise to miniaturization of artificial unit cells which in turn results in higher homogeneity in the material, and reduction in the profile of the artificial substrate. Analytical formulas are proposed for design and optimization of the presented structures, and are verified through full wave numerical characterization. The electromagnetic properties of the proposed structures are studied in detail and compared to square spiral from the point of view of size reduction, maximum value of the resultant permeability, magnetic loss, and frequency dispersion. To validate the analytical model and the numerical simulation results, an artificial substrate containing second-order Fractal Hilbert curve is fabricated and experimentally characterized using a microstrip-based characterization method.