Scientists report the fabrication of a nonreciprocal optical resonator with a small length footprint of 290 µm on a silicon-on-insulator substrate. The device achieves unidirectional optical transmission with an isolation ratio of up to 19.5 dB near the telecommunications wavelength of 1,550 nm in a homogeneous external magnetic field.

https://udel.edu/~hujuejun/My%20Papers/Journal%20Papers/On-chip%20optical%20isolation%20in%20monolithically%20integrated%20non-reciprocal%20optical%20resonators.pdf

On-chip optical isolation in monolithically integrated non-reciprocal optical resonators

Ferrimagnets having low RF loss are used in passive microwave components such as isolators, circulators, phase shifters, and miniature antennas operating in a wide range of frequencies (1–100 GHz) and as magnetic recording media owing to their novel physical properties. Frequency tuning of these components has so far been obtained by external magnetic fields provided by a permanent magnet or by passing current through coils. However, for high frequency operation the permanent part of magnetic bias should be as high as possible, which requires large permanent magnets resulting in relatively large size and high cost microwave passive components. A promising approach to circumvent this problem is to use hexaferrites, such as BaFe12O19 and SrFe12O19, which have high effective internal magnetic anisotropy that also contributes to the permanent bias. Such a self-biased material remains magnetized even after removing the external applied magnetic field, and thus, may not even require an external permanent magnet. In garnet and spinel ferrites, such as Y3Fe5O12 (YIG) and MgFe2O4, however, the uniaxial anisotropy is much smaller, and one would need to apply huge magnetic fields to achieve such high frequencies. In Part 1 of this review of microwave ferrites a brief discussion of fundamentals of magnetism, particularly ferrimagnetism, and chemical, structural, and magnetic properties of ferrites of interest as they pertain to net magnetization, especially to self biasing, are presented. Operational principles of microwave passive components and electrical tuning of magnetization using magnetoelectric coupling are discussed in Part 2.

/pdf/microwave-ferrites-part-1-fundamental-properties-124qghgxm4.pdf

Microwave ferrites, part 1: fundamental properties

BaFe12O19 hexaferrite nanoparticles, containing cetyltrimethylammonium chloride (CTAC), n-hexanol, and cyclohexane, were synthesized by a reverse microemulsion technique with a combination of (NH4)2CO3 and NH3·H2O as precipitator. Barium ferrite nanoparticles with 30 nm diameter and uniform flaky structure were proved to be single magnetic domains, which have magnetic properties comparable to some of the best ever reported for fine barium ferrite powders by chemical methods. Heat-treatment conditions can significantly influence the formation of pure BaFe12O19 hexaferrite phase, where quenching and nonprecalcination would produce intermediates of α-Fe2O3 and BaFe2O4, as detected by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) analyses, resulting in lower magnetic properties. High magnetocrystalline anisotropy constant K and energy barrier EA calculated from Stoner−Wohlfarth theory may also account for the high coercivity for pure BaFe12O19. The variation of electrical conductivity during ...

Synthesis and Magnetic Properties of BaFe12O19 Hexaferrite Nanoparticles by a Reverse Microemulsion Technique

This research develops a nonlocal couple stress theory to investigate static stability and free vibration characteristics of functionally graded (FG) nanobeams. The theory introduces two parameters based on nonlocal elasticity theory and modified couple stress theory to capture the size effects much accurately. Therefore, a nonlocal stress field parameter and a material length scale parameter are used to involve both stiffness-softening and stiffness-hardening effects on responses of FG nanobeams. The FG nanobeam is modeled via a higher order refined beam theory in which shear deformation effect is verified needless of shear correction factor. A power-law distribution is used to describe the graded material properties. The governing equations and the related boundary conditions are derived by Hamilton’s principle and they are solved applying Chebyshev–Ritz method which satisfies various boundary conditions. A comparison study is performed to verify the present formulation with the provided data in the literature and a good agreement is observed. The parametric study covered in this paper includes several parameters such as nonlocal and length scale parameters, power-law exponent, slenderness ratio, shear deformation and various boundary conditions on natural frequencies and buckling loads of FG nanobeams in detail.

Application of Chebyshev–Ritz method for static stability and vibration analysis of nonlocal microstructure-dependent nanostructures

Atomically thin ferromagnetic (FM) films were recently prepared by mechanical exfoliation of bulk FM semiconductor ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$. They provide a platform to explore novel two-dimensional (2D) magnetic phenomena, and they offer exciting prospects for new technologies. By performing systematic ab initio density functional calculations, here we study two relativity-induced properties of these 2D materials (monolayer, bilayer, and trilayer as well as bulk), namely magnetic anisotropy energy (MAE) and magneto-optical (MO) effects. Competing contributions of both magnetocrystalline anisotropy energy (C-MAE) and magnetic dipolar anisotropy energy (D-MAE) to the MAE are computed. The calculated MAEs of these materials are large, being on the order of $\ensuremath{\sim}0.1$ meV/Cr. Interestingly, we find that out-of-plane magnetic anisotropy is preferred in all the systems except the monolayer, where in-plane magnetization is favored because here the D-MAE is larger than the C-MAE. Crucially, this explains why long-range FM order was observed in all the few-layer ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ except the monolayer because the out-of-plane magnetic anisotropy would open a spin-wave gap and thus suppress magnetic fluctuations so that long-range FM order could be stabilized at finite temperature. In the visible frequency range, large Kerr rotations up to $\ensuremath{\sim}2.{2}^{\ensuremath{\circ}}$ in these materials are predicted, and they are comparable to that observed in famous MO materials such as PtMnSb and ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$. Moreover, they are $\ensuremath{\sim}100$ times larger than that of $3d$ transition metal monolayers deposited on Au surfaces. Faraday rotation angles in these 2D materials are also large, being up to $\ensuremath{\sim}{120}^{\ensuremath{\circ}}/\ensuremath{\mu}\mathrm{m}$, and they are thus comparable to the best-known MO semiconductor ${\mathrm{Bi}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$. These findings thus suggest that with large MAE and MO effects, atomically thin ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ films would have potential applications in novel magnetic, MO, and spintronic nanodevices.

Large magneto-optical effects and magnetic anisotropy energy in two-dimensional Cr 2 Ge 2 Te 6

Magneto-optical properties of yttrium iron garnet (YIG) thin films elaborated by radio frequency sputtering

Thin films of Yttrium Iron Garnet (YIG) are grown by radio frequency magnetron non reactive sputtering system. Thin films are crystallised by heat-treatment to obtain magneto-optical properties. On quartz substrate, the network of cracks observed on the annealed samples can be explained by the difference between the thermal expansion coefficient of substrate and YIG.

The Faraday rotation of thin films is measured with a classical ellipsometric system based in transmission which allows us to obtained an accuracy of 0.01°. We studied the variation of Faraday rotation versus the applied magnetic field. The variation of the Faraday rotation is the same that this obtained by VSM (vibrating sample magnetometer) analysis. 

With a quartz substrate, maximum Faraday rotation is observed to be 1900°/cm at the wavelength of 594 nm for an annealing of 740°C. The variation of the Faraday rotation versus the wavelength is studied. The obtained values are
comparable to those of the literature for the bulk material.

In order to eliminate the stress due to the heat-treatment, we made some films on Gadolinium Gallium Garnet (GGG) which thermal expansion coefficient is near than the YIG one. The material crystallises with no cracks and the Faraday effect is equivalent.

The development of devices that combine a magnetic material with a semiconductor chip is a major focus of current research. Barium hexaferrite (BaFe12O19 or BaM) thick films are deposited here using a rf sputtering system. The films are amorphous and nonmagnetic after deposition. Postdeposition thermal annealings are employed to obtain magnetic properties. The effects of the substrate, thermal annealing and thickness of BaM on the magnetic properties are studied using a vibrating sample magnetometer. The initial results show good magnetic properties for the two subtrates studied and after thermal annealing above 800 °C. The magnetic properties of the thick films are close to the bulk (BaM) ones.

Magnetic properties of sputtered barium ferrite thick films

Barium ferrite thick films for microwave applications

This letter describes how composite material, made of maghemite (γ‐Fe2O3) nanoparticles embedded in a silica∕titania matrix, can be used to develop new magneto-optical planar waveguides. Thin film samples are coated on Pyrex™ substrates from a magnetic particles doped sol-gel preparation. M-lines spectroscopy measurements performed on these samples show that an out of plane magnetic field applied during the gelation induces a decrease of the planar waveguide phase mismatch. Free space measurements evidence the Faraday rotation of the films up to 25°∕cm at 633 nm. Improvements and leads of further works are proposed in term of magneto-optical waveguide applications.

J. J. Rousseau

Papers

Magneto-optical properties of yttrium iron garnet (YIG) thin films elaborated by radio frequency sputtering

Magneto-optical properties of yttrium iron garnet (YIG) thin films elaborated by radio frequency sputtering

Magnetic properties of sputtered barium ferrite thick films

Barium ferrite thick films for microwave applications

Magneto-optical nanoparticle-doped silica-titania planar waveguides