Magneto-optical Devices Research Articles

Growth parameters of Bi0.1Y2.9Fe5O12 thin films for high frequency applications

• Growth parameters are reported for Bi 0.1 Y 2.9 Fe 5 O 12 thin films on Gd 3 Ga 5 O 12 . • Effect of Bi ions (at dodecahedral site) in Bi 0.1 Y 2.9 Fe 5 O 12 (111)–orientation shown. • Increase in ferromagnetic resonance linewidth in (111)–orientation is observed. • Thickness dependent magnetization dynamics is studied. • Four–fold anisotropy is seen in in–plane angular dependent study. The growth and characterization of Bismuth (Bi) substituted YIG (Bi–YIG, Bi 0.1 Y 2.9 Fe 5 O 12 ) thin films are reported. Pulsed laser deposited (PLD) films with thicknesses ranging from 20 to 150 nm were grown on Gadolinium Gallium Garnet substrates. Two substrate orientations of (100) and (111) were considered. The enhanced distribution of Bi 3+ ions at dodecahedral site along (111) is observed to lead to an increment in lattice constant from 12.379 Å in (100) to 12.415 Å in (111) oriented films. Atomic force microscopy images showed decreasing roughness with increasing film thickness. Compared to (100) grown films, (111) oriented films showed an increase in ferromagnetic resonance linewidth and consequent increase in Gilbert damping. The lowest Gilbert damping values are found to be (1.06±0.12) × 10 −4 for (100) and (2.30±0.36) × 10 −4 for (111) oriented films with thickness of ≈150 nm. The observed values of extrinsic linewidth, effective magnetization, and anisotropic field are related to thickness of the films and substrate orientation. In addition, the in–plane angular variation established four–fold symmetry for the (100) deposited films unlike the case of (111) deposited films. This study prescribes growth conditions for PLD grown single–crystalline Bi–YIG films towards desired high frequency and magneto–optical device applications.

Emergence of low-temperature glassy dynamics in Ru substituted non-magnetic insulator CaHfO3

Non-magnetic insulators/semiconductors with induced magnetism introduced via transition metal substitution are one of the promising materials in the field of spintronics, magnetoelectronics and magneto-optical devices. In this context, here, we focus on magnetism induced in a non-magnetic insulator CaHfO3, by the substitution of 4d element Ru, at Hf-site. Structural investigations indicate that substitution of Ru4+ (up to 50%) does not affect the original crystal structure of the parent compound. Magnetic studies divulge a crossover from a diamagnetic to paramagnetic state with 20% Ru substitution. Further replacement of Hf results in a glassy magnetic state in CaHf1−x Ru x O3 (0.3 ⩽ x ⩽ 0.5). The nature of the low temperature glassiness (below 20 K) in these compositions is confirmed through Vogel–Fulcher and Power law, along with, magnetic memory effect and relaxation dynamics. The observed glassiness is explained through the phenomenological ‘hierarchical model’. Our studies indicate that the presence of competing short range interactions among randomly arranged Ru cations in non-magnetic insulator CaHfO3 are responsible for the observed low temperature magnetic state in this series with compositions >0.25.

First- and second-order magneto-optical effects and intrinsically anomalous transport in the two-dimensional van der Waals layered magnets CrXY (X=S,Se,Te;Y=Cl,Br,I)

Recently, the two-dimensional magnetic semiconductor CrSBr has attracted considerable attention due to its excellent air-stable property and high magnetic critical temperature. Here, we systematically investigate the electronic structure, magnetocrystalline anisotropy energy, first-order magneto-optical effects (Kerr and Faraday effects), and second-order magneto-optical effects (Sch\"afer-Hubert and Voigt effects) as well as intrinsically anomalous transport properties (anomalous Hall, anomalous Nernst, and anomalous thermal Hall effects) of two-dimensional van der Waals layered magnets $\mathrm{Cr}\mathit{XY} (X=\mathrm{S},\mathrm{Se},\mathrm{Te};Y=\mathrm{Cl},\mathrm{Br},\mathrm{I})$ by using first-principles calculations. Our results show that monolayer and bilayer $\mathrm{Cr}\mathit{XY} (X=\mathrm{S},\mathrm{Se})$ are narrow-band-gap semiconductors, whereas monolayer and bilayer $\mathrm{CrTe}Y$ are multiband metals. The magnetic ground states of bilayer $\mathrm{Cr}\mathit{XY}$ and the easy magnetization axis of monolayer and bilayer $\mathrm{Cr}\mathit{XY}$ are confirmed by the magnetocrystalline anisotropy energy calculations. Utilizing magnetic group theory analysis, the first-order magneto-optical effects as well as anomalous Hall, anomalous Nernst, and anomalous thermal Hall effects are identified to exist in a ferromagnetic state with out-of-plane magnetization. The second-order magneto-optical effects are not restricted by the above symmetry requirements, and therefore they can arise in ferromagnetic and antiferromagnetic states with in-plane magnetization. The calculated results are compared with the available theoretical and experimental data of other two-dimensional magnets and some conventional ferromagnets. The present work reveals that monolayer and bilayer $\mathrm{Cr}\mathit{XY}$ with superior magneto-optical responses and anomalous transport properties provide an excellent material platform for the promising applications of magneto-optical devices, spintronics, and spin caloritronics.

Structural, Optical, and Magnetic Properties of Erbium-Substituted Yttrium Iron Garnets.

We synthesized a series of slightly erbium-substituted yttrium iron garnets (Er:YIG), Y3–xErxFe5O12 at different Er concentrations (x = 0, 0.01, 0.05, 0.10, and 0.20) using a solid-state reaction and investigated their structural, magnetic, and optical properties as a function of Er concentration. The volume of the unit cell slightly increased with Er concentration and Er atoms predominately replaced Y atoms in the dodecahedrons of YIG. The optical properties exhibited certain decreases in reflectance in the 1500–1600 nm wavelength range due to the presence of Er3+. Despite the many unpaired 4f electrons in Er3+, the total magnetic moments of Er:YIG showed similar trends with temperatures and magnetic fields above 30 K. An X-ray magnetic circular dichroism study confirmed the robust Fe 3d magnetic moments. However, the magnetic moments suddenly decreased to below 30 K with Er substitution, and the residual magnetism (MR) and coercive field (HC) in the magnetic hysteresis loops decreased to below 30 K with Er substitution. This implies that Er substitution in YIG has a negligible effect on magnetic properties over a wide temperature range except below 30 K where the Er 4f spins are coupled antiparallel to the majority Fe 3d spins. Our studies demonstrated that above 30 K the magnetic properties of YIG are retained even with Er substitution, which is evidence that the Er doping scheme is applicable for YIG-based magneto-optical devices in the mid-infrared regime.

Tunable Faraday effect in Graphene/SiO2/Ta2O5 layered structure using a pump He-Ne laser light

Faraday rotation spectrum of a Graphene/SiO2/Ta2O5 layered structure under an external magnetic field and a pump light of a He-Ne laser with the wavelength of 632.8 nm is investigated in the THz frequency region. The results showed that, when the pump light is turned off and the magnetic field strength is increased, the maximum Faraday rotation angle frequency shifts toward the higher frequencies, as well as, the value of the maximum Faraday rotation angle increases. Furthermore, for a constant external magnetic field, by increasing the pump light power, the value of Faraday rotation increases, and the maximum Faraday rotation frequency shifts toward the lower frequencies. However, the noticeable changes in the transmission spectrum were not observed by changing the magnetic field strength and the pump light power. These results would have potential applications in designing the magneto-optical devices with tuning property of the Faraday rotation using a pump He-Ne laser.

Magneto-optical Kerr effect in surface engineered 2D hexagonal boron nitride

Magnetism in atomically thin functional materials can be an important phenomenon for exploring two-dimensional magneto-optics. Magneto-optical experimental data have revealed significant Kerr signals in insulator thin films. Here, the magneto-optical Kerr effect of oxygen functionalized and doped hexagonal boron nitride (hBN) has been investigated by performing first-principles calculations. We calculated Kerr angle and Kerr ellipticity for functionalized hBN as an attention-drawn material. Moreover, increasing of oxygen doping percentage leads to the introduction of surface plasmon to hBN. Our findings show that the functionalized hBN can tolerate high-temperature conditions, keeping oxygen atoms bridge-bonded. These giant opto/magnetic responses of insulating 2D materials provide a platform for the potential designing of magneto-optical devices.

Optical properties of He+-implanted and diamond blade-diced terbium gallium garnet crystal planar and ridge waveguides

Terbium gallium garnet (Tb3Ga5O12, TGG) crystal can be used to fabricate various magneto-optical devices due to its optimal Faraday effect. In this work, 400-keV He+ ions with a fluence of 6.0 × 1016 ions/cm2 are irradiated into the TGG crystal for the planar waveguide formation. The precise diamond blade dicing with a rotation speed of 2 × 104 rpm and a cutting velocity of 0.1 mm/s is performed on the He+-implanted TGG planar waveguide for the ridge structure. The dark-mode spectrum of the He+-implanted TGG planar waveguide is measured by the prism-coupling method, thereby obtaining the relationship between the reflected light intensity and the effective refractive index. The refractive index profile of the planar waveguide is reconstructed by the reflectivity calculation method. The near-field light intensity distribution of the planar waveguide and the ridge waveguide are recorded by the end-face coupling method. The He+-implanted and diamond blade-diced TGG crystal planar and ridge waveguides are promising candidates for integrated magneto-optical devices.

Direct investigation of the atomic structure and decreased magnetism of antiphase boundaries in garnet

The ferrimagnetic insulator iron garnets, tailored artificially with specific compositions, have been widely utilized in magneto-optical (MO) devices. The adjustment on synthesis always induces structural variation, which is underestimated due to the limited knowledge of the local structures. Here, by analyzing the structure and magnetic properties, two different antiphase boundaries (APBs) with individual interfacial structure are investigated in substituted iron garnet film. We reveal that magnetic signals decrease in the regions close to APBs, which implies degraded MO performance. In particular, the segregation of oxygen deficiencies across the APBs directly leads to reduced magnetic elements, further decreases the magnetic moment of Fe and results in a higher absorption coefficient close to the APBs. Furthermore, the formation of APBs can be eliminated by optimizing the growth rate, thus contributing to the enhanced MO performance. These analyses at the atomic scale provide important guidance for optimizing MO functional materials.

Research on the magneto-optical properties of Tb3+-Ho3+ co-doped GeO2-B2O3-P2O5-ZnO (GBPZ) Faraday glass

In this paper, 32GeO2–20B2O3–5P2O5–3ZnO-40Tb2O3-xHo2O3 (x = 0, 2, 4, 6, 8 and 10 mol%) Faraday glasses doped with different concentrations of Ho3+ ions are prepared. The structure and physical properties of the 32GeO2–20B2O3–5P2O5–3ZnO-40Tb2O3-xHo2O3 (x = 0, 2, 4, 6, 8 and 10 mol%) Faraday glass system are investigated by XRD, FT-IR and theoretical calculations. The effect of the different concentrations of Ho3+ ions, incident wavelengths and test temperatures on the Verdet constants are studied. It was found that the magneto-optical properties of 32GeO2–20B2O3–5P2O5–3ZnO-40Tb2O3-xHo2O3 (x = 0, 2, 4, 6, 8 and 10 mol%) Faraday glass has a certain dependence on the changes of incident wavelength and the test temperature. At room temperature, with the increase of Ho3+ ion doping concentrations, the values of Verdet constants increase continuously at 633 nm. 32GeO2–20B2O3–5P2O5–3ZnO-40Tb2O3–10Ho2O3 Faraday glass has the highest Verdet constant value (-172.65 rad/(T·m)), which is increased by -42.35 rad/(T·m) compared with 32GeO2–20B2O3–5P2O5–3ZnO-40Tb2O3–10Ho2O3 Faraday glass without Ho3+ ions (-130.30 rad/(T·m)). And it is much higher than that of the commercialized terbium gallium garnet crystal (-134 rad/(T·m)), indicating that the incorporation of Ho3+ ions with a large magnetic moment can significantly improve the properties of the magneto-optical glass, which has not been reported in other literatures. At the same time, it is found that the Verdet constant of 32GeO2–20B2O3–5P2O5–3ZnO-40Tb2O3–10Ho2O3 Faraday glass decreased with the increase of test temperature. These findings have important implications for the application of magneto-optical glass in the fields of optical fiber, laser, information, and magneto-optical devices.

Tunable Polarized Microcavity Characterized by Magnetic Circular Dichroism Spectrum.

Tunable resonator is a powerful building block in fields like color filtering and optical sensing. The control of its polarization characteristics can significantly expand the applications. Nevertheless, the methods for resonator dynamic tuning are limited. Here, a magnetically regulated circular polarized resonant microcavity is demonstrated with an ultrathin ferrimagnetic composite metal layer Ta/CoTb. We successfully tuned the cavity resonant frequency and polarization performance. A huge magnetic circular dichroism (MCD) signal (∼3.41%) is observed, and the microcavity valley position shifts 5.41 nm when a small magnetic field is applied. This resonant cavity has two-stable states at 0 T due to the magnetic remanence of CoTb film and can be switched using a tiny magnetic field (∼0.01 T). Our result shows that the ferrimagnetic film-based tunable microcavity can be a highly promising candidate for on-chip magneto-optical (MO) devices.

Fabrication and characterization of highly Dy3+- and Tb3+-doped germano-borate glasses for magneto-optic device applications at 1.55 μm

Highly Dy3+- and Tb3+-doped germano-borate glasses with different rare-earth ion concentrations (10−25 mol%) were fabricated and their thermal, thermo-mechanical, optical, and magneto-optic (MO) properties were investigated with respect to the rare-earth ion concentration. The glasses showed high transmission properties of > 83% in the near-infrared regions of approximately 1.5 and 2.1 μm. The MO properties of the glasses were investigated using Faraday rotation measurements at 1.5 μm. The large Verdet constants of −5.35 and −6.11 rad/(T•m) at 1.55 μm were obtained for the glasses incorporated with the highest Dy3+ and Tb3+ concentrations of 25 mol% and 16 mol%, respectively; this was due to large effective magnetic moments and electronic transition frequencies of the rare-earth ions. The results showed that the germano-borate glasses doped with high Dy3+and Tb3+ concentrations are promising MO materials for application in MO devices in the near-infrared region.

A Brief Review on Structural, Morphological, Magnetic and Dielectric Behavior of Divalent Cation-substituted Nanocrystalline Cobalt Ferrite

Abstract: Spinel ferrites nanoparticles have received much more focus due to their promising applications in various technologies, such as magnetic refrigerators, microwave devices, colour imaging, high-density recording media and magnetic fluids. Nano-crystalline cobalt ferrite has been widely studied for its distinctive properties, like cubic magneto crystalline anisotropy, high coercivity, reasonable saturation magnetization, great chemical constancy, wear resistance and electrical insulation. Thus, cobalt ferrite has its own applications in the arena of high-frequency devices, magneto-optical devices, memory cores, recording media and spintronics and also in the biomedical sector, such as MRI, drug delivery schemes and magnetic hyperthermia. Reduced dimensionality of ferrite nanoparticles is having differences in properties when compared to its bulk counterparts. Size modulation of the physico-chemical properties of cobalt ferrite gives a distinctive response in the nanosized scale, permitting for material engineering in order to meet different necessities while addressing its different applications. The present study gives a brief review of significant effects on structural, dielectric and magnetic properties of cobalt spinel ferrite with different cation substitutions, like Mg2+, Ni2+, Zn2+, Cu2+, In2+ ...etc. This study regarding the size and its dispersity, core shell design, shape, crystallinity and surface decoration with hybrid derivatives opens up a wide variety of prospects for elementary studies as well as for growth and revolution in the field of ferrites. Keywords: Spinel ferrite, Structural properties, Microstructural properties, Magnetic properties, Dielectric properties.

Shape effect of Co nanoparticles on the electric and magnetic properties of Co–SiO2 nanogranular films

Controlling the magnetic anisotropy of nanoparticles is a crucial but challenging step for developing new magnetic functions. Here, we demonstrate a simple approach to controlling the shape of Co nanoparticles in a Co-SiO2 nanogranular film from oblate to prolate spheroid by varying the substrate rotation speed during the tandem fabrication process without changing the film composition (Co:SiO2 = 3:7). Changing the nanoparticles from oblate to prolate, increasing perpendicular length of ellipsoidal nanoparticles, changes the magnetic anisotropy axis of Co–SiO2 nanogranular films from in-plane to out-of-plane, which indicates that the shape anisotropy profoundly affects the magnetic properties. Despite the small tunneling current of a few tens of nanoamperes, a maximum tunneling magnetoresistance effect of up to 2.8 % was realized under an applied magnetic field of 12 kOe in the film plane. Achieving both in-plane and perpendicular spin-dependent tunneling, the anisotropic nanogranular films imply direction controllable tunneling materials as future topological nanoarchitecture. Such high-resistivity nanogranular films with a controllable magnetic nanoparticle shape facilitate the design of new magneto-optical devices with high withstand voltages.

The magnetic polaron modulated luminescence bands of organic-inorganic hybrid ferroelectric anti-perovskite (C3H9N)3Cd2Cl7 doped with Mn2+

Recently, the excellent optical properties of organic-inorganic hybrid metal halides have attracted much attention in the optoelectronic field. However, their complicated preparation processes seriously influence their properties and applications. In this work, we developed a series of organic-inorganic hybrid metal halides (C3H9N)3Cd2Cl7:x%Mn2+ with an antiperovskite structure with ferroelectrics in an early report, giving tunable emissions contemporarily with different manganese (Mn)2+ concentrations via a simple mechanochemical method. Meanwhile, their single crystals were also grown by a slow thermal evaporation method. The as-grown products with Mn dopants exhibited diluted magnetic semiconductor behavior and varied emission profiles by different excitation wavelengths, which could be modified by the heat treatment. All the emission bands come from the different magnetic polarons with enhanced electron-phonon coupling or self-trapped exciton formation. Ferromagnetic coupling Mn–Mn pairs or clusters in the doped lattice favor the magnetic polaron and red emission at room temperature and even give much stronger emission above room temperature. The excitonic magnetic polaron and local excitonic magnetic polaron were detected at about 309 nm and 398 nm, respectively, with Mn doping. Without Mn2+ dopant, the weak emission band at about 398 nm can also be detected from an intrinsically bound exciton or confined exciton from the amine incorporated metal chlorides. This Mn-doped anti-perovskite Cd halides may find applications in the solid display and lighting, as well as the magneto-optical devices.

Interfacial Effect on Photo-Modulated Magnetic Properties of Core/Shell-Structured NiFe/NiFe2O4 Nanoparticles.

Photo-modulated magnetism has become an emerging method for technological applications, such as magneto-optical devices. In this work, by introducing oxygen during rapid thermal annealing, NiFe/NiFe2O4 core/shell nanoparticles were successfully fabricated by pulsed laser deposition. Obvious photo-modulated ferromagnetism was observed in core/shell nanoparticles confined in Al2O3 film. Theoretical and experimental investigations indicate much more photogenerated electrons are captured at the interface of NiFe/NiFe2O4 compared with NiFe nanoparticles due to interfacial effect, resulting in the improved ferromagnetism under light irradiation. This work provides a promising strategy for optical engineering design of optical information storage, high-speed wireless communication, and magneto-optical semiconductor devices.

Nanophotonic devices based on magneto-optical materials: recent developments and applications

Abstract Interaction between light and magnetism in magneto-optical (MO) nanophotonic devices has been actively studied in the past few years. The recent development of MO all-dielectric resonators and metasurfaces has led to the emergence of various novel MO phenomena that were not observed in their bulk counterparts. For example, a large s-polarized transverse MO Kerr effect can be observed at magnetic resonance wavelength, which cannot exist in the bare MO films. We review recent developments in nanophotonic devices based on MO materials and focus on different modes and related MO effects in nanophotonic structures with emphasis on recently discovered new MO phenomena in magnetoplasmonics and all-dielectric nanostructures, such as dark mode, all-dielectric Mie resonance and waveguide mode. Further, we discuss the potential applications of these nanostructures for biological/chemical sensing, magnetic field sensing, and magnetic field-controlled active and nonreciprocal metasurfaces.

Photon absorption of two-dimensional nonsymmorphic Dirac semimetals

Two-dimensional Dirac semimetals have attracted much attention because of their linear energy dispersion and non-trivial Berry phase. Graphene-like 2D Dirac materials are gapless only within certain approximations, e.g., if spin-orbit coupling (SOC) is neglected. It has recently been reported that materials with nonsymmorphic crystal lattice possess symmetry-enforced Dirac-like band dispersion around certain high-symmetry momenta even in the presence of SOC. Here we calculate the optical absorption coefficient of nonsymmorphic semimetals, such as $\alpha$-bismuthene, which hosts two anisotropic Dirac cones with different Fermi velocities along $x$ and $y$ directions.We find that the optical absorption coefficient depends strongly on the anisotropy factor and the photon polarization. When a magnetic field is applied perpendicular to the plane of the material, the absorption coefficient also depends on an internal parameter we termed the mixing angle of the band structure. We further find that an in-plane magnetic field, while leaving the system gapless, can induce a Van-Hove singularity in the joint density of states: this causes a significant enhancement of the optical absorption at the frequency of the singularity for one direction of polarization but not for the orthogonal one, making the optical properties even more strongly dependent on polarization. Due to the anisotropy present in our model, the Dirac cones at two high-symmetry momenta in the Brillouin zone contribute very differently to the optical absorbance. Consequently, it might be possible to preferentially populate one valley or the other by varying photon polarization and frequency. These results suggest that nonsymmorphic 2D Dirac semimetals are excellent candidate materials for tunable magneto-optic devices.

Optical Transitions and Magnetism in Mn-Implanted Gallium Nitride for Three-Level Magnetooptic Devices

A method of preparing passive materials suitable for magnetooptic interactions is shown using manganese implantation into gallium nitride (GaN) epitaxial layers, which establishes both dilute ferromagnetism and a three-level optical system with persistence to over 300 K. A sweep of thermal anneal parameters for a high implant dose into Mg-doped p-type GaN films is presented, and the materials are tested for both their magnetization and photoluminescence (PL). The optimal anneal process at 825 °C for 5 min maintains ferromagnetism with $T_{C}$ > 305 K, confirming magnetic alignment at room temperature with a coercivity of ~100 Oe. PL and spectrophotometry of the optimally prepared materials show the effects of the mid-gap defect state on the material's optical characteristics. The anneal process returns the real part index to its baseline dispersion while retaining an onset of absorption starting at the defect level $E_{A}$ = 1.8 eV, signifying a stable mid-gap energy transition with a measured state lifetime of $τ _{PL}$ = 2.7 ns. The scalability of this process for producing three-level transition magnetic materials suggests passive optical or magnetooptic devices can be constructed that interconnect photonic and spintronic effects for emerging system designs and potential applications in quantum information.

Graphene-based magneto-optical THz modulator with 100% depth of modulation for communication purposes

In this article, we introduce a Faraday modulator based on graphene, gyroelectric (InSb), SiO2, and Si materials at 0.35 THz, considered as a band of the atmospheric window promising for the future wireless communications. The maximum value of Faraday rotation, transmission and depth of modulation are obtained at 0.35 THz for low static magnetic field B as 45 deg, 80%, and 100%, respectively. The THz signal transmission can be modified from 0 to 80% by varying applied static magnetic fields. This design opens up a new way for ultrathin magneto-optical nanophotonic devices to realize polarization modulators with high transmittance in the THz band.

Interstitial Nature of Mn2+ Doping in 2D Perovskites.

Halide perovskites doped with magnetic impurities (such as the transition metals Mn2+, Co2+, Ni2+) are being explored for a wide range of applications beyond photovoltaics, such as spintronic devices, stable light-emitting diodes, single-photon emitters, and magneto-optical devices. However, despite several recent studies, there is no consensus on whether the doped magnetic ions will predominantly replace the octahedral B-site metal via substitution or reside at interstitial defect sites. Here, by performing correlated nanoscale X-ray microscopy, spatially and temporally resolved photoluminescence measurements, and magnetic force microscopy on the inorganic 2D perovskite Cs2PbI2Cl2, we show that doping Mn2+ into the structure results in a lattice expansion. The observed lattice expansion contrasts with the predicted contraction expected to arise from the B-site metal substitution, thus implying that Mn2+ does not replace the Pb2+ sites. Photoluminescence and electron paramagnetic resonance measurements confirm the presence of Mn2+ in the lattice, while correlated nano-XRD and X-ray fluorescence track the local strain and chemical composition. Density functional theory calculations predict that Mn2+ atoms reside at the interstitial sites between two octahedra in the triangle formed by one Cl- and two I- atoms, which results in a locally expanded structure. These measurements show the fate of the transition metal dopants, the local structure, and optical emission when they are doped at dilute concentrations into a wide band gap semiconductor.