Terbium Iron Garnet Research Articles

Magneto‐Optical Bi‐Substituted Yttrium and Terbium Iron Garnets for On‐Chip Crystallization via Microheaters

AbstractFerrimagnetic iron garnets enable magnetic and magneto‐optical functionality in silicon photonics and electronics. However, garnets require high‐temperature processing for crystallization which can degrade other devices on the wafer. Here bismuth‐substituted yttrium and terbium iron garnet (Bi‐YIG and Bi‐TbIG) films are demonstrated with good magneto‐optical performance and perpendicular magnetic anisotropy (PMA) crystallized by a microheater built on a Si chip or by rapid thermal annealing. The Bi‐TbIG film crystallizes on Si at 873 K without a seed layer and exhibits good magneto‐optical properties with Faraday rotation (FR) of −1700 deg cm−1. The Bi‐YIG film also crystallizes on Si and fused SiO2 at 873 K without a seed layer. Rapidly cooled films exhibit PMA due to the tensile stress caused by the thermal expansion mismatch with the substrates, increasing the magnetoelastic anisotropy by 4 kJ m−3 versus slow‐cooled films. Annealing in the air for 15 s using the microheater yields fully crystallized Bi‐TbIG on the Si chip.

The thickness effect on the compensation temperature of rare-earth garnet thin films

The anomalous Hall effect measurements are used to probe the magnetization reversal of terbium iron garnet (TbIG) thin films at different temperatures. The compensation temperature (Tcomp) of TbIG thin films is revealed, and the film thickness effect on the Tcomp is studied. The results indicate a rise of Tcomp along with decreasing film thickness. We postulate two possible origins for the observed behavior, namely interfacial element diffusion and strain effects between TbIG films and Gd3Ga5O12 substrates. The results have implications for the study of spintronic devices based on ultrathin rare-earth iron garnet thin films.

Revealing Site Occupancy in a Complex Oxide: Terbium Iron Garnet.

Complex oxide films stabilized by epitaxial growth can exhibit large populations of point defects which have important effects on their properties. The site occupancy of pulsed laser-deposited epitaxial terbium iron garnet (TbIG) films with excess terbium (Tb) is analyzed, in which the terbium:iron (Tb:Fe)ratio is 0.86 compared to the stoichiometric value of 0.6. The magnetic properties of the TbIG are sensitive to site occupancy, exhibiting a higher compensation temperature (by 90 K) and a lower Curie temperature (by 40 K) than the bulk Tb3 Fe5 O12 garnet. Data derived from X-ray core-level spectroscopy, magnetometry, and molecular field coefficient modeling are consistent with occupancy of the dodecahedral sites by Tb3+ , the octahedral sites by Fe3+ , Tb3+ and vacancies, and the tetrahedral sites by Fe3+ and vacancies. Energy dispersive X-ray spectroscopy in a scanning transmission electron microscope provides direct evidence of TbFe antisites. A small fraction of Fe2+ is present, and oxygen vacancies are inferred to be present to maintain charge neutrality. Variation of the site occupancies provides a path to considerable manipulation of the magnetic properties of epitaxial iron garnet films and other complex oxides, which readily accommodate stoichiometries not found in their bulk counterparts.

Sputtered terbium iron garnet films with perpendicular magnetic anisotropy for spintronic applications

We report the structural, magnetic, and interfacial spin transport properties of epitaxial terbium iron garnet (TbIG) ultrathin films deposited by magnetron sputtering. High crystallinity was achieved by growing the films on gadolinium gallium garnet substrates either at high temperatures, or at room temperature followed by thermal annealing, above 750 °C in both cases. The films display large perpendicular magnetic anisotropy (PMA) induced by compressive strain, and tunable structural and magnetic properties through growth conditions or the substrate lattice parameter choice. The ferrimagnetic compensation temperature (TM) of selected TbIG films was measured through the temperature-dependent anomalous Hall effect in Pt/TbIG heterostructures. In the studied films, TM was found to be between 190 and 225 K, i.e., approximately 25-60 K lower than the bulk value, which is attributed to the combined action of Tb deficiency and oxygen vacancies in the garnet lattice evidenced by x-ray photoelectron spectroscopy measurements. Sputtered TbIG ultrathin films with large PMA and highly tunable properties reported here can provide a suitable material platform for a wide range of spintronic experiments and device applications.

Crystallization of high gyrotropy garnets with decreasing thermal processing budgets as analyzed by electron backscatter diffraction

Rare-earth iron garnets with large magnetic gyrotropy, made with reduced thermal budgets, are ideal magneto-optical materials for integrated isolators. However, reduced thermal budgets impact Faraday rotation by limiting crystallization, and characterization of crystallinity is limited by resolution or scannable area. Here, electron backscatter diffraction (EBSD) is used to measure crystallinity in cerium substituted yttrium- and terbium-iron garnets (CeYIG and CeTbIG) grown on planar Si, crystallized using one-step rapid thermal processes, leading to large Faraday rotations > −3500 °/cm at 1550 nm. Varying degrees of crystallinity are observed in planar Si and patterned Si waveguides, and specific dependences of crystallite size are attributed to the nucleation/growth processes of the garnets and the lateral dimensions of the waveguide. On the other hand, a low thermal budget alternative–exfoliated CeTbIG nanosheets–are fully crystalline and maintain high Faraday rotations of −3200 °/cm on par with monolithically integrated thin film garnets.

Modulation of structure and magnetic properties of terbium iron garnet by the introduction of Bi3+ and Ce3+

Modulation of structure and magnetic properties of terbium iron garnet by the introduction of Bi3+ and Ce3+

Diffusion-Driven Exfoliation of Magneto-Optical Garnet Nanosheets: Implications for Low Thermal Budget Integration in Si Photonics

Rare-earth iron garnets are instrumental in the development of integrated nonreciprocal passive devices such as isolators and circulators in silicon photonics. Unfortunately, monolithic integration of garnet on-chip requires annealing temperatures much higher than the thermal budget of a semiconductor foundry. Here, we report the mechanical exfoliation of large area (0.2 mm × 0.2 mm) nanosheets of a high-gyrotropy cerium-doped terbium iron garnet (CeTbIG) enabled by a strain-enhanced vacancy diffusion process that follows the Nabarro–Herring (lattice diffusion) model. Diffusivities calculated from the strain rate–stress data (1.13 × 10–18 m2s–1) identify iron and rare-earth cations as the rate-determining lattice diffusants. Cross-section scanning transmission electron microscopy reveals an exfoliation gap located ∼30 nm into the film, comparable to the cation diffusion length, which appears to verify the model. With a saturation magnetization of 18 emu cc–1 and a Faraday rotation of −2900°cm–1 at 1550 nm, the magnetic and optical properties of the nanosheets are comparable to their thin-film values. Diffusion-driven exfoliation will open foundry-acceptable pathways for heterogeneous integration of garnets on photonic waveguides and protect devices from the high-temperature processes used in crystallizing garnet films.

Magnetism and spin transport in platinum/scandium-substituted terbium iron garnet heterostructures

Growth, structural, magnetic, and spin transport properties of epitaxial scandium-substituted terbium iron garnet (TbScIG) thin films on gadolinium gallium garnet [GGG, (111) orientation] substrates are reported as a function of Sc content. The films are epitaxial on GGG without strain relaxation up to a thickness of 90 nm and exhibit perpendicular magnetic anisotropy. Sc, a nonmagnetic cation, occupies up to 40% of the octahedral Fe sites. Increasing Sc lowers the compensation temperature and Curie temperature and increases the room-temperature saturation magnetization from 37 kA/m for ${\mathrm{Tb}}_{2.8}{\mathrm{Fe}}_{5.2}{\mathrm{O}}_{12}$ to 59 kA/m for ${\mathrm{Tb}}_{2.8}{\mathrm{Sc}}_{0.8}{\mathrm{Fe}}_{4.4}{\mathrm{O}}_{12}$. Anomalous Hall effect-like spin Hall magnetoresistance measurements were performed to determine the effect of scandium content on spin mixing conductance of the TbScIG|Pt interface. The spin mixing conductance increased significantly with an increase in Sc content, consistent with a dependence of spin transport on the net Fe magnetization instead of the total magnetization of the garnet.

Magnetic Properties and Growth‐Induced Anisotropy in Yttrium Thulium Iron Garnet Thin Films

AbstractRare‐earth iron garnets (REIG) have recently become the materials platform of choice for spintronic studies on ferrimagnetic insulators. However, thus far the materials studied have mainly been REIG with a single rare earth species such as thulium, yttrium, or terbium iron garnets. In this study, magnetometry, ferromagnetic resonance, and magneto‐optical Kerr effect imaging is used to explore the continuous variation of magnetic properties as a function of composition for YxTm3−x iron garnet (YxTm3−xIG) thin films grown by pulsed laser deposition on gadolinium gallium garnet substrates. It is reported that the tunability of the magnetic anisotropy energy, with full control achieved over the type of anisotropy (from perpendicular, to isotropic, to an in‐plane easy axis) on the same substrate. In addition, a nonmonotonic composition‐dependent anisotropy term is reported, which is ascribed to growth‐induced anisotropy similar to what is reported in garnet thin films grown by liquid‐phase epitaxy. Ferromagnetic resonance shows linear variation of the damping and the g‐factor across the composition range, consistent with prior theoretical work. Domain imaging reveals differences in reversal modes, remanant states, and domain sizes in YxTm3−x iron‐garnet thin films as a function of anisotropy.

High Figure of Merit Magneto‐Optical Ce‐ and Bi‐Substituted Terbium Iron Garnet Films Integrated on Si

AbstractFilms of polycrystalline terbium iron garnet (TbIG), cerium‐substituted TbIG (CeTbIG), and bismuth‐substituted TbIG (BiTbIG) are grown on Si substrates by pulsed laser deposition. The films grow under tensile strain due to thermal mismatch with the Si substrate, resulting in a dominant magnetoelastic anisotropy which, combined with shape anisotropy, leads to in‐plane magnetization. TbIG has a compensation temperature of 253 K which is reduced by substitution of Ce and Bi. The Faraday rotation at 1550 nm of the TbIG, Ce0.36TbIG, and Bi0.03TbIG films is 5400 ± 600° cm−1, 4500 ± 100° cm–1, and 6200 ± 300° cm−1, respectively, while Ce0.36TbIG and Bi0.03TbIG exhibit lower optical absorption than TbIG, attributed to a reduction in Fe2+ and Tb4+ absorption pathways. The high Faraday rotation of the films, and in particular the high magneto‐optical figure of merit of the Bi0.03TbIG of 720° dB−1 at 1550 nm, make these polycrystalline films valuable for applications in integrated photonics.

Interfacial and Bulk Magnetic Properties of Stoichiometric Cerium Doped Terbium Iron Garnet Polycrystalline Thin Films

AbstractOne of the best magneto‐optical claddings for optical isolators in photonic integrated circuits is sputter deposited cerium‐doped terbium iron garnet (Ce:TbIG) which has a large Faraday rotation (≈−3500° cm−1 at 1550 nm). Near‐ideal stoichiometry of Ce0.5Tb2.5Fe4.75O12 is found to have a 44 nm magnetic dead layer that can impede the interaction of propagating modes with garnet claddings. The effective anisotropy of Ce:TbIG on Si is also important, but calculations using bulk thermal mismatch overestimate the effective anisotropy. Here, X‐ray diffraction measurements yield highly accurate measurements of strain that show anisotropy favors an in‐plane magnetization in agreement with the positive magnetostriction of Ce:TbIG. Upon doping TbIG with Ce, a slight decrease in compensation temperature occurs which points to preferential rare‐earth occupation in dodecahedral sites and an absence of cation redistribution between different lattice sites. The high Faraday rotation, large remanent ratio, large coercivity, and preferential in‐plane magnetization enable Ce:TbIG to be an in‐plane latched garnet, immune to stray fields with magnetization collinear to direction of light propagation.

Magneto-optical materials and designs for integrated TE- and TM-mode planar waveguide isolators: a review [Invited

Optical isolators are unidirectional devices that employ the magneto-optical (MO) property of iron garnets to block the reflected light in almost all optical systems. Sputter deposition of either doped yttrium iron garnets (YIG) with seed-layers or seed-layer free terbium iron garnets (TIG) will help realize integrated planar isolators. Faraday rotation waveguides are designed as a viable solution to enable these monolithically integrated garnets to overcome the limitations inherent to hybrid integration of interferometric devices. In fact, small footprint Faraday rotation devices have been achieved using doped TIG for both TE/TM modes with sufficiently low loss and large isolation ratios.

Magnetism and spin transport in rare-earth-rich epitaxial terbium and europium iron garnet films

Rare-earth iron garnet thin films with perpendicular magnetic anisotropy (PMA) have recently attracted a great deal of attention for spintronic applications. Thulium iron garnet (TmIG) has been successfully grown and TmIG/Pt heterostructures have been characterized. However, TmIG is not the only rare-earth iron garnet that can be grown with PMA. We report the growth, magnetic, and spintronic properties of epitaxial terbium iron garnet (TbIG) and europium iron garnet (EuIG) thin films with PMA. Reciprocal space mapping shows the films are lattice matched to the substrate without strain relaxation, even for films up to 56 nm thick. The lattice strain and magnetostriction coefficient produce PMA in certain cases. TbIG grows on (111) gadolinium gallium garnet (GGG) with PMA due to the in-plane compressive strain, whereas TbIG on (111) substituted GGG (SGGG) is in tension and has an in-plane easy axis. EuIG grows with PMA on (100) and (111) GGG substrates, which facilitates the investigation of spintronic properties as a function of orientation. Both garnets have excess rare earth, which is believed to occupy Fe octahedral sites and in the case of TbIG is associated with an increase in the compensation temperature to 330 K, higher than the bulk value. Anomalous Hall effect (AHE) measurements of Pt/EuIG Hall crosses show that the spin mixing conductance of Pt/ (111) and (100) EuIG is similar. AHE measurements of Pt/TbIG Hall crosses reveal a sign change in the AHE amplitude at the compensation point analogous to all-metallic systems.

Sputter-deposited magneto-optical garnet for all-mode (transverse electric/transverse magnetic) Faraday rotators

Abstract

Magnetic study of the low temperature anomalies in the magnetodielectric terbium iron garnet

The anomalous magnetic properties at low temperatures of terbium iron garnet (TbIG) are analyzed and summarized using neutron powder diffraction (NPD) experiments together with high field magnetization, magnetostriction and specific heat measurements performed on single crystals. Reliable information at both microscopic and macroscopic levels is provided about the significant change of the double umbrella structure observed in the NPD results near 54 K. The positions of the observed maxima at 55–65 K in the paraprocess magnetic susceptibility along the three mean directions and paraprocess of the forced magnetostriction along the easy axis of magnetization 〈111〉 agree with the manifestations of the “low-temperature point” TB predicted by Belov at 58 K. However, the pronounced maximum at 57 K in the excess of specific heat in zero magnetic fields reveals that the Schottky effect causes anomaly at temperature close the TB point. The results are discussed and compared with previous magnetic, magneto-optical and magnetodielectric reports.

Magnetization and specific heat studies of the low temperature anomalies in terbium iron garnet Tb3Fe5O12

The macroscopic magnetic properties of the terbium iron garnet (Tb3Fe5O12) single crystals were studied at low temperature by magnetizations MT (H) in magnetic fields up to 80 kOe and applied along the easy axis <111>. Maxima are observed near ∼ 65 and ∼ 47 K, respectively in the curves of the initial magnetic susceptibility χ0(T), and the parameter |b|(T) of the bH2 term are associated with the non linear variation of the MT (H) magnetizations. From the specific heat Cp(T) measurements, a great peak is observed near 57 K in the excess contribution of the Tb3+ ions Cpex(T) to Cp(T) which vanishes in the 170–208 K temperature range. The results are discussed with respect to the simultaneous signs of the “low-temperature point” TB predicted by Belov at 58 K and the ‘Schottky anomaly’ detected experimentally by the presence in the Cpex(T) curve of a 1/T2 term for T > TB.

Optimized Magneto-optical Isolator Designs Inspired by Seedlayer-Free Terbium Iron Garnets with Opposite Chirality

Simulations demonstrate that undoped yttrium iron garnet (YIG) seedlayers cause reduced Faraday rotation in silicon-on-insulator waveguides with Ce-doped YIG claddings. Undoped seedlayers are required for the crystallization of the magneto-optical Ce:YIG claddings, but they diminish the interaction of the Ce:YIG with the guided modes. Therefore, new magneto-optical garnets, terbium iron garnet (TIG) and bismuth-doped TIG (Bi:TIG), are introduced that can be integrated directly on Si and quartz substrates without seedlayers. The Faraday rotations of TIG and Bi:TIG films at 1550 nm were measured to be +500°/cm and −500°/cm, respectively. Simulations show that these new garnets have the potential to significantly mitigate the negative impact of the seedlayers under Ce:YIG claddings. The successful growth of TIG and Bi:TIG on low-index fused quartz inspired novel garnet-core waveguide isolator designs, simulated using finite difference time domain methods. These designs use alternating segments of positive an...

Sol–gel derived Tb3Fe5O12 and Y3Fe5O12 garnets: Synthesis, phase purity, micro-structure and improved design of morphology

Yttrium iron garnet (Y3Fe5O12; YIG) and terbium iron garnet (Tb3Fe5O12; TbIG) were prepared by an aqueous sol–gel method using two different complexing agents (1,2-ethanediol and glycerol). For the synthesis of Tb3Fe5O12 two different molar ratios of complexing agent (1:1 and 3:1) to the total metal ions were used. Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) were performed to study the thermal behaviour of the prepared Y–Fe–O and Tb–Fe–O precursor gels. The effects of synthesis parameters on the garnet phase formation were studied by X-ray powder diffraction (XRD) analysis and scanning electron microscopy (SEM). Interestingly, the XRD data indicated that single-phase Y3Fe5O12 garnet can be prepared only with 1,2-ethanediol at lowest 1000 °C temperature. While Tb3Fe5O12 garnet can be obtained at lower temperatures depending on the nature and amount of complexing agent. The SEM results showed that parameters of sol–gel processing influence the morphological features of garnets considerably. For comparison, the conventional solid-state reaction method at 1000 °C was also used for the fabrication of both Y3Fe5O12 and Tb3Fe5O12 garnets.

Mean field analysis of the high temperature magnetic properties of terbium iron garnet in strong DC fields

This paper is devoted to the description of the magnetic phase diagrams (MPD) together with a special interest to the determination of more precise values of some reliable parameters at the compensation point, Tcomp=243.5±0.5K of the terbium iron garnet, Tb3Fe5O12 or TbIG. Using isothermal magnetizations performed on single crystal in strong DC magnetic fields up to 200kOe applied along the 〈111〉, 〈110〉 and 〈100〉 directions within the temperature range 128–295K, field-induced phase transitions between collinear and canted phases are observed in the vicinity of Tcomp at critical fields, Hc2. In comparison with the measurement at zero external magnetic field, the specific heat, Cp(T) at 80kOe along 〈111〉 shows an excess around Tcomp characterized by an anomaly which has a width in the boundaries of the canted phase and a maximum at 252K, the more accurate value of the critical temperature, TC⁎ of the MPD in the (Hc2–T) plane. Better determinations of the molecular field coefficients which represent the magnetic interactions on the Tb sublattice are obtained by an improved molecular field model based on the saturation effects of the Tb sublattice and the differential susceptibility contribution due to the Fe sublattices to the total magnetic susceptibility of TbIG. The results are discussed in terms of the previous theoretical studies of the MPD predicted for weakly anisotropic ferrimagnets.

Group theory analysis of the rhombohedral magnetic structures in rare earth iron garnets revisited

The representation analysis of Bertaut is used to enumerate all possible magnetic structures in rare earth iron garnets (RE3Fe5O12) for the RE and Fe ions on the 6e, 6e' and 12f, 2b, 6d Wyckoff positions respectively in the highest rhombohedral subgroup R3̄c of the cubic space group Ia3̄d. The basis vectors of the one-dimensional irreducible representation A2g lead to noncollinear magnetic structures of the RE moments which produce a better description than those based on the models of Wolf et al. Some results of the "double umbrella" structure found in our recent neutron powder diffraction studies of terbium iron garnet are reported.