Conventional Magnetic Materials Research Articles

Magneto-optic effects of monolayer transition metal dichalcogenides induced by Rashba spin-orbit coupling

The nontrivial magnetoopitcal effects are predicted to arise at the visible frequency range for monolayer transition metal dichalcogenides upon the application of magnetic exchange interactions. However, their magnetoopitcal rotation angles at the terahertz frequency range are always low, which hinders their practical applications in the terahertz magnetoopitcal devices. In this letter, we theoretically demonstrate that the magnetoopitcal effects of monolayer transition metal dichalcogenides at the terahertz frequency range can be enhanced and controlled via introducing the Rashba spin orbit coupling, whose rotation angles are comparable with those of conventional bulky magnetic materials. Such an effect originates from the change of spin-up and spin-down bands in the band structures of monolayer transition metal dichalcogenides when the Rashba spin orbit coupling and magnetic exchange interaction coexist, which enables the spin-flip intraband optical transitions and thus generates the terahertz magnetoopitcal responses. Our results open up possibilities for the use of Rashba spin orbit coupling to generate as well as control the magnetoopitcal effects in two-dimensional layered materials.

Giant, low-loss magnetic responses and ultraslow magnetic solitons via plasmon-induced transparency

The realization of advanced materials with strong, low-loss, and pure magnetic responses to radiation fields both in linear and nonlinear regimes is an important and long-standing goal for fundamental physics and practical applications. Here, we propose a physical scheme for obtaining such responses by using a metamaterial constructed by an array of unit cells consisting of two coupled varactor-loaded split-ring resonators working under the condition of plasmon-induced transparency (PIT). We show that the PIT in such metamaterial not only significantly suppresses radiation absorption but also greatly enhances magnetic Kerr nonlinearity, which may be many orders of magnitude larger than that obtained by conventional magnetic materials reported up to now. Based on such a nonlinear metamaterial, we further show that stable magnetic solitons with ultraslow propagation velocity and very low generation power can be created. Our research opens a route for designing novel metamaterial devices with strong, low-loss, pure, and actively tunable magnetic responses and for obtaining stable and low-power nonlinear magnetic pulses, which are promising for applications in information processing and transmission.

Butterfly-shaped magnetoresistance in triangular-lattice antiferromagnet Ag2CrO2

Spintronic devices using antiferromagnets (AFMs) are promising candidates for future applications. Recently, many interesting physical properties have been reported with AFM-based devices. Here we report a butterfly-shaped magnetoresistance (MR) in a micrometer-sized triangular-lattice antiferromagnet Ag2CrO2. The material consists of two-dimensional triangular-lattice CrO2 layers with antiferromagnetically coupled S = 3/2 spins and Ag2 layers with high electrical conductivity. The butterfly-shaped MR appears only when the magnetic field is applied perpendicularly to the CrO2 plane with the maximum MR ratio (≈15%) at the magnetic ordering temperature. These features are distinct from those observed in conventional magnetic materials. We propose a theoretical model where fluctuations of partially disordered spins with the Ising anisotropy play an essential role in the butterfly-shaped MR in Ag2CrO2.

Magnetic nanoparticles for hyperthermia in cancer treatment: an emerging tool.

Cancer remains as the major cause of death worldwide. The main reason why available therapies fail is that a vicious cycle in established which initiates multiple pathways and recurrence after metastasis. Hyperthermic treatment, which involves heating tumor tissues to a moderate temperature of 40-43°C, has emerged as an effective strategy for treating tumors. This method is highly efficient at destroying tumor cells and does not induce the side effects of conventional cancer treatments. On the other hand, hyperthermic treatment method can be co-administered with conventional treatments. Nanotechnology had created huge opportunities in almost all areas of research, including the field of hyperthermic treatment. The utilization of magnetic nanoparticles (MNPs) offers functionalities not possible using conventional magnetic materials. In this review, we detail recent developments and applications of MNPs for hyperthermic treatment and discuss future possibilities.

Space-Confined Synthesis of Core-Shell BaTiO3@Carbon Microspheres as a High-Performance Binary Dielectric System for Microwave Absorption.

Binary dielectric composites are viewed as a kind of promising candidate for conventional magnetic materials in the field of microwave absorption. Herein, we demonstrate the successful fabrication of core-shell BaTiO3@carbon microspheres through a space-confined strategy. The electromagnetic properties of BaTiO3@carbon microspheres can be easily tailored by manipulating the relative content of carbon shells. It is confirmed that dielectric loss of these composites mainly benefits from conductivity loss, dipole orientation polarization, and interfacial polarization, and the core-shell configuration shows its positive contribution to the reinforcement of interfacial polarization. When the content of carbon shells is optimized, the as-obtained composite will display excellent microwave-absorption performance due to decent attenuation and well-matched impedance. The strongest reflection loss can reach up to -88.5 dB at 6.9 GHz with the absorber thickness of 3.0 mm, and the qualified bandwidth below -10.0 dB covers 9.0-12.0 GHz, when the thickness is designated at 2.0 mm. Such a performance in the X band is superior to those of most typical binary dielectric systems. More importantly, these BaTiO3@carbon microspheres maintain good performance after being treated under high-temperature and acidic conditions for a long time, manifesting their promising prospect for practical application. It is believed that these results may be helpful for the development of multicomponent dielectric systems as high-performance microwave absorbing materials.

Analysis of magnetic losses in Fe-polymer composites

Abstract The most important properties of soft magnetic materials are peak induction and magnetic losses, determining the size and efficiency of electric devices. Conventional soft magnetic materials are not suitable for the construction of miniaturized magnetic cores. Soft magnetic composites meet miniaturization requirements of electric and electronic devices. In this paper, magnetic losses in self-developed Fe-polymer composites are analyzed. The frequency dependencies of magnetic losses are measured at different level of maximum induction. The influence of Fe-grain size on magnetic losses is also discussed.

Cavity‐Induced Enhancement of Magneto‐Optic Effects in Monolayer Transition Metal Dichalcogenides

AbstractMonolayer transition metal dichalcogenides represent a class of 2D direct‐bandgap semiconductors, which do not support magneto‐optical (MO) effects in the absence of magnetic field due to time reversal symmetry. Magnetic exchange field (MEF) has been demonstrated to be able to generate MO effects in monolayer transition metal dichalcogenides, causing Faraday rotation of several tenths of degree without the need of magnetic field. Here, the possibility of enhancing the MO effects of monolayer transition metal dichalcogenides upon MEF using a cavity structure is investigated. It is identified that the Faraday rotation (FR) angle can be boosted by choosing the proper thickness of optical cavity due to its resonant characteristics. In particular, unlike the conventional bulky magnetic materials always with fixed MO effects, the FR angle here can be manipulated by the chemical potential, enabling a route of engineering the MO effects in monolayer transition metal dichalcogenides. The results suggest 2D layered semiconductors being the possible building blocks in nonreciprocal MO devices.

Interface Chemistry of Contact Metals and Ferromagnets on the Topological Insulator Bi2Se3

The interface between the topological insulator (TI) Bi2Se3 and deposited metal films is investigated using X-ray photoelectron spectroscopy including conventional contact metals (Au, Pd, Cr, and Ir) and magnetic materials (Co, Fe, Ni, Co0.8Fe0.2, and Ni0.8Fe0.2). Au is the only metal to show little or no interaction with the Bi2Se3, with no interfacial layer between the metal and the surface of the TI. The other metals show a range of reaction behaviors with the relative strength of reaction (obtained from the amount of Bi2Se3 consumed during reaction) ordered as Au < Pd < Ir < Co ≤ CoFe < Ni < Cr < NiFe < Fe, in approximate agreement with the behavior expected from the Gibbs free energies of formation for the alloys formed. Post metallization anneals at 300 °C in vacuum were also performed for each interface. Several of the metal films were not stable upon anneal and desorbed from the surface (Au, Pd, Ni, and Ni0.8Fe0.2), while Cr, Fe, Co, and Co0.8Fe0.2 showed accelerated reactions with the underlying ...

Tunable Curie temperature around room temperature and magnetocaloric effect in ternary Ce–Fe–B amorphous ribbons

Ce13−xFe81+xB6 (x = 0, 0.5, 1, 1.5, and 2) amorphous magnets were prepared by melt-spinning method. These magnets are magnetically soft at low temperature, and undergo a second-order phase transition from ferromagnetic to paramagnetic state near room temperature with a broad temperature span. The phase-transition temperature is tunable by the variation of the Ce/Fe atomic ratio, which is mainly due to the change of the coordination number of Fe atoms in these ternary Ce–Fe–B amorphous magnets. Though the entropy change is low, the refrigeration capacities are in the ranges of 116–150 J kg−1 and 319–420 J kg−1, respectively, for the magnetic field changes of 0–2 T and 0–5 T, which is comparable with those of conventional magnetic materials for room-temperature refrigeration. Given the low cost of Fe and Ce, Ce–Fe–B amorphous magnets are attractive magnetic refrigerant candidates.

Conventional and inverse magnetocaloric effect in Pr2CuSi3 and Gd2CuSi3 compounds

Magnetic properties and magnetocaloric effect (MCE) in Pr2CuSi3 and Gd2CuSi3 compounds were investigated systematically. Both Pr2CuSi3 and Gd2CuSi3 compounds experienced two phase transitions in a relatively narrow temperature range: first a paramagnet (PM)–ferromagnet (FM) second-order phase transition at 12 and 26K and then a FM–spin glass (SG) transition at 6K and 7.5K, respectively. The magnetic entropy change (ΔSM) was calculated based on Maxwell relation using the collected magnetization data. The maximum of ΔSM for Pr2CuSi3 and Gd2CuSi3 compounds was 7.6 and 5Jkg−1K−1, respectively, at the applied filed change of 0–5T. The shape of the temperature dependence of ΔSM (ΔSM–T) curve was obviously different from that of the conventional magnetic materials undergoing only one typical phase transition. In the left half part of ΔSM–T curve, ΔSM is not very sensitive to the applied field and they tend to intersect with the decrease of temperature. Both typical conventional and inverse MCE behavior were observed in Gd2CuSi3, which would be originated from the two transition features at the low temperatures.

ULTRA-WIDE-BAND MICROWAVE COMPOSITE ABSORBERS BASED ON PHASE GRADIENT METASURFACES

In this paper, we propose to realize ultra-wide-band absorber (UWBA) based on anomalous refraction/reflection of phase gradient metasurfaces (PGM). To achieve high absorption and meanwhile keep small thickness at low frequencies, PGM is incorporated into conventional magnetic materials (MM). The absorptivity is increased due to prolonged propagation length in the MM, which is produced via anomalous refraction/reflection mediated by the PGM. Three typical composite configurations of PGM-based absorbers are investigated and an UWBA design method is finally formulated. Due to small thickness and ultra-wide bandwidth, such absorbers possess great application potentials in EM protection, RCS reduction, etc..

ChemInform Abstract: Unexpected Magnetism in Nanomaterials

AbstractReview: 294 refs.

Design and realization of a magnetic-type absorber with a broadened operating frequency band

In this paper, we present an efficient method to obtain absorbers with broadened operating frequency bands. They are accomplished by using conventional magnetic absorbing materials (MAMs) in the forms of array and mesh structures, which are similar to those in the case of a frequency selective surface. The proposed approach is verified not only by simulations but also by experimental results under the normal incidence at microwave frequencies. Moreover, the wideband absorber is lighter than the conventional magnetic absorber. These results indicate that our proposed absorbing structures can be used for designing good electromagnetic absorbers.

Magnetic levitation from negative permeability materials

As left-handed materials and metamaterials are becoming more prevalent, we examine the effect of negative permeability upon levitation force. We first consider two half spaces of differing permeability and a point magnetic source, so that the method of images may be employed. We determine that the resulting force may be larger than for conventional magnetic materials. We then illustrate the inclusion of a finite sample thickness.

Molecule-based magnets

The conventional magnetic materials used in current technology, such as, Fe, Fe2O3, Cr2O3, SmCo5, Nd2Fe14B etc are all atom-based, and their preparation/processing require high temperature routes. Employing self-assembly methods, it is possible to engineer a bulk molecular material with long-range magnetic order, mainly because one can play with the weak intermolecular interactions. Since the first successful synthesis of molecular magnets in 1986, a large variety of them have been synthesized, which can be categorized on the basis of the chemical nature of the magnetic units involved: organic-, metal-based systems, heterobimetallic assemblies, or mixed organic-inorganic systems. The design of molecule-based magnets has also been extended to the design of poly-functional molecular magnets, such as those exhibiting second-order optical nonlinearity, liquid crystallinity, or chirality simultaneously with long-range magnetic order. Solubility, low density and biocompatibility are attractive features of molecular magnets. Being weakly coloured, unlike their opaque classical magnet ‘cousins’ listed above, possibilities of photomagnetic switching exist. Persistent efforts also continue to design the ever-elusive polymer magnets towards applications in industry. While providing a brief overview of the field of molecular magnetism, this article highlights some recent developments in it, with emphasis on a few studies from the author’s own lab.

Copper vs. iron: Microwave magnetism in the metamaterial age

It is now well established that metamaterials based on copper patterns can exhibit significant permeability levels, but can such materials outdo conventional magnetic materials? In the case where permeability levels and bandwidth are the key figures-of-merit, it is acknowledged that copper-based metamaterials can exceed the performance of ferrites at operating frequencies above 10 GHz. As for low-frequency operation, magnetic metamaterials may also be preferred to conventional magnetic materials when requirements include excellent temperature stability or immunity to external magnetic fields. However, in many cases, metamaterials need to include certain conventional magnetic constituents in order to compete with conventional magnetic materials.

Permeability enhancement of soft magnetic films through metamaterial structures

The present article proposes a structure consisting of a single magnetic film enclosed in a metamaterial. This planar structure can be manufactured using conventional deposition and patterning techniques commonly utilized in the silicon industry. One example of such a structure was manufactured, and its permeability was measured in a wide frequency range and compared to the permeability of its core. Theoretical investigations of this structure were performed in order to establish its permeability as a function of the frequency, and an excellent agreement between the predicted and measured values was observed. Parametrical studies revealed that the metafilm exhibited two resonances; one below the gyromagnetic resonance frequency of the magnetic core, and one at a much higher frequency. Metafilms are attractive negative permeability materials since they exhibit large negative permeability levels within a wide frequency range, and since they are readily adjustable. In this respect, they combine the advantages of metamaterials and of their magnetic cores. It has been previously established that the quantity ∫ 0 F μ ″ ( f ) f d f is a good figure of merit for microwave magnetic materials. It is bounded in conventional magnetic materials by a quantity directly related to the saturation magnetization. The bounds associated with metafilms were derived, and it was shown that the ultimate limitations of metafilms with regard to this figure of merit were equivalent to those for conventional magnetic materials in the case of low-frequency operations.

Oxidation‐induced high‐Curie‐temperature ferromagnetism in CoAl(100)

AbstractIn conventional magnetic materials, oxidation is a disagreeable effect that often lowers or destroys the magnetic capabilities of those materials. By contrast, we report on the decisive paramagnetic‐ferromagnetic phase transition in CoAl(100) at room temperature, utilizing oxidation of stoichiometric CoAl. We also discuss the control and drastic increase of the coercive field by subsequent annealing of the oxidized sample. The alumina film grown by selective oxidation protects the alloy from oxidation of Co, despite the accumulation of Al vacancies and the resulting enrichment in Co of the metallic phase underneath the oxide film. As a result, a ferromagnetic thin Co‐rich phase is formed at the interface between the insulating aluminum oxide and the paramagnetic Co50Al50 bulk. The creation via simple oxidation of a ferromagnetic thin film underneath a surface insulator demonstrates a novel path to building the majority of a magnetic tunnel junction. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Nanocrystalline and Nanocomposite Magnetic Materials and Their Applications

Nanocrystalline materials can possess bulk properties quite different from those commonly associated with conventional large-grained materials. Nanocomposites, a subset of nanocrystalline materials, in addition have been found to possess magnetic properties which are similar to, but different from, the properties of the individual constituents. New magnetic phenomena, unusual property combinations, and both enhanced and diminished magnetic property values are just some of the changes observed in magnetic nanocomposites from conventional magnetic materials. Here, a description will be presented of some of the exciting new properties discovered in nanomaterials and the magnetic applications envisioned for them.

高性能圧粉磁心材料の開発

Conventional soft magnetic materials used for various applications such as high-speed rotary motors were made by laminated steels. However, when these materials were used at high frequencies (1 kHz or higher), there was a decline in the magnetic properties caused by the core loss (heat generation by eddy current).To solve these problems, we developed new soft magnetic materials made of fine magnetic iron powder coated with an insulating layer, to which ultra-trace volume of heat-resistant binder is added with high density, thus obtaining superior magnetic properties using the powder metallurgy (P/M) method.This paper discussed on the DC and AC magnetic properties and the mechanical properties of newly developed P/M soft magnetic materials.