Thulium Iron Garnet Research Articles

Temperature-Dependent Surface Anisotropy in (110) Epitaxial Rare Earth Iron Garnet Films.

Ferrimagnetic oxide thin films are important material platforms for spintronic devices. Films grown on low symmetry orientations such as (110) exhibit complex anisotropy landscapes that can provide insight into novel phenomena such as spin-torque auto-oscillation and spin superfluidity. Using spin-Hall magnetoresistance measurements, the in-plane (IP) and out-of-plane (OOP) uniaxial anisotropy energies are determined for a thickness series (5-50nm) of europium iron garnet (EuIG) and thulium iron garnet (TmIG) films epitaxially grown on a gadolinium gallium substrate with (110) orientation and capped with Pt. Pt/EuIG/GGG exhibits an (001) easy plane of magnetization perpendicular to the substrate, whereas Pt/TmIG/GGG exhibits an (001) hard plane of magnetization perpendicular to the substrate with an IP easy axis. Both IP and OOP surface anisotropy energies comparable in magnitude to the bulk anisotropy are observed. The temperature dependence of the surface anisotropies is consistent with first-order predictions of a simplified Néel surface anisotropy model. By taking advantage of the thickness and temperature dependence demonstrated in these ferrimagnetic oxides grown on the low symmetry (110) orientations, the complex anisotropy landscapes can be tuned to act as a platform to explore rich spin textures and dynamics.

Large-scale fabrication of thulium iron garnet film with perpendicular magnetic anisotropy using RF magnetron sputtering

Large-scale fabrication of thulium iron garnet (TmIG) films on gadolinium gallium garnet (GGG) substrates, with a total area of 25 cm2, has been demonstrated by rotating substrate holders during on-axis sputtering. By optimizing the growth parameters based on the pressure and flow rate of the oxygen ratio, a Tm/Fe ratio of 0.65 was obtained, which is close to the stoichiometry of TmIG. The increase in post-annealing temperature has induced the growth of the TmIG structure by the strain of the lattice constant mechanism. At the highest post-annealing temperature, the crystal structure of TmIG (444) and the perpendicular magnetic anisotropy (PMA) were obtained. This result demonstrates the potential method for large-scale fabrication of TmIG film with PMA.

In-plane magnetic properties and anisotropy of scandium substituted thulium iron garnet thin films for fluxgate magnetometer

In-plane easily magnetized scandium substituted thulium iron garnet films(Tm3Fe5-xScxO12) with soft magnetic properties have recently attracted attention for fluxgate magnetometer applications. In this paper, thin Tm3Fe5-xScxO12 films were prepared on gadolinium gallium garnet substrates by liquid phase epitaxial(LPE) method. Microstructural properties, composition, magnetic properties and the in-plane anisotropy of the films were discussed. We found that the films turned from tensile stress to compressive stress as the Sc3+ substitution decreased, and the in-plane anisotropy was mainly caused by stress reducing as Sc3+ declined. By analyzing the in-plane magnetic hysteresis loops, the magnetic performance was the best when x = 0.75. Tm3Fe4.25Sc0.75O12 film had the lowest in-plane coercive field(0.006 Oe) and the highest initial magnetic permeability(378), which was more beneficial to realize fluxgate magnetometers.

Enhancement of Perpendicular Magnetic Anisotropy in Tm3Fe5O12(111) Epitaxial Films via Synergistic Stoichiometry and Strain Engineering

AbstractEstablishing a reliable control of perpendicular magnetic anisotropy (PMA) is challenging but essential for the full utilization of rare‐earth iron garnets in spintronic devices. In this study, a feasible approach to enhance the PMA of ferrimagnetic thulium iron garnet (TmIG) films is presented. This approach involves precise adjustments in cation stoichiometry and epitaxial strain state. By fine‐tuning the pre‐ablation process and oxygen partial pressure during pulsed laser deposition, a series of high‐quality TmIG films can grow with variable cation stoichiometry, i.e., the Tm/Fe molar ratio. The finding reveals that cation stoichiometry plays a crucial role in determining the magnetic properties of the TmIG films. Particularly, the stoichiometric TmIG film has the strongest PMA due to the maximized magnetostriction coefficient. Combining this stoichiometry optimization and strain engineering, an unprecedented PMA strength of ≈30 kJ m−3 for TmIG is achieved. This achievement demonstrates a simple and effective method for harnessing the magnetic properties of rare‐earth iron garnet films, paving the way for their advanced applications in next‐generation spintronic devices.

Field-Free Switching of Perpendicular Magnetization in an Ultrathin Epitaxial Magnetic Insulator.

For energy-efficient magnetic memories, switching of perpendicular magnetization by spin-orbit torque (SOT) appears to be a promising solution. This SOT switching requires the assistance of an in-plane magnetic field to break the symmetry. Here, we demonstrate the field-free SOT switching of a perpendicularly magnetized thulium iron garnet (Tm3Fe5O12, TmIG). The polarity of the switching loops, clockwise or counterclockwise, is determined by the direction of the initial current pulses, in contrast with field-assisted switching where the polarity is controlled by the direction of the magnetic field. From Brillouin light scattering, we determined the Dzyaloshinskii-Moriya interaction (DMI) induced by the Pt-TmIG interface. We will discuss the possible origins of field-free switching and the roles of the interfacial DMI and cubic magnetic anisotropy of TmIG. This discussion is substantiated by magnetotransport, Kerr microscopy, and micromagnetic simulations. Our observation of field-free electrical switching of a magnetic insulator is an important milestone for low-power spintronic devices.

Damping and Interfacial Dzyaloshinskii-Moriya Interaction in Thulium Iron Garnet/Bismuth-Substituted Yttrium Iron Garnet Bilayers.

Thin films of ferrimagnetic iron garnets can exhibit useful magnetic properties, including perpendicular magnetic anisotropy (PMA) and high domain wall velocities. In particular, bismuth-substituted yttrium iron garnet (BiYIG) films grown on garnet substrates have a low Gilbert damping but zero Dzyaloshinskii-Moriya interaction (DMI), whereas thulium iron garnet (TmIG) films have higher damping but a nonzero DMI. We report the damping and DMI of thulium-substituted BiYIG (BiYTmIG) and TmIG|BiYIG bilayer thin films deposited on (111) substituted gadolinium gallium garnet and neodymium gallium garnet (NGG) substrates. The films are epitaxial and exhibit PMA. BiYIG|TmIG bilayers have a damping value that is an order of magnitude lower than that of TmIG, and BiYIG|TmIG|NGG have DMI of 0.0145 ± 0.0011 mJ/m2, similar to that of TmIG|NGG. The bilayer therefore provides a combination of DMI and moderate damping, useful for the development of high-speed spin orbit torque-driven devices.

On-axis sputtering fabrication of Tm3Fe5O12 film with perpendicular magnetic anisotropy

Thulium iron garnet, Tm3Fe5O12 with perpendicular magnetic anisotropy is fabricated using an on-axis sputtering technique followed by annealing, whereas previous reports have used unusual off-axis geometries. Stoichiometric Tm3Fe5O12 is obtained after the modification of the deposition conditions involving the position of the substrate relative to the cathode, which affects both the chemical and structural properties. The effective perpendicular magnetic anisotropy of 8.6 kJ/m3 is well in line with the results of previous studies using pulse laser deposition and off-axis sputtering. A maze domain pattern is observed, and the domain-wall energy is evaluated as 0.69 mJ/m2.

Mapping of Spin‐Wave Transport in Thulium Iron Garnet Thin Films Using Diamond Quantum Microscopy

AbstractSpin waves, collective dynamic magnetic excitations, offer crucial insights into magnetic material properties. Rare‐earth iron garnets offer an ideal spin‐wave (SW) platform with long propagation length, short wavelength, gigahertz frequency, and applicability to magnon spintronic platforms. Of particular interest, thulium iron garnet (TmIG) has attracted huge interest recently due to its successful growth down to a few nanometers, observed topological Hall effect, and spin‐orbit torque‐induced switching effects. However, there is no direct spatial measurement of its SW properties. This work uses diamond nitrogen‐vacancy (NV) magnetometry in combination with SW electrical transmission spectroscopy to study SW transport properties in TmIG thin films. NV magnetometry allows probing spin waves at the sub‐micrometer scale, seen by the amplification of the local microwave magnetic field due to the coupling of NV spin qubits with the stray magnetic field produced by the microwave‐excited spin waves. By monitoring the NV spin resonances, the SW properties in TmIG thin films are measured as a function of the applied magnetic field, including their amplitude, decay length (≈50 µm), and wavelength (0.8–2 µm). These results pave the way for studying spin qubit‐magnon interactions in rare‐earth magnetic insulators, relevant to quantum magnonics applications.

Optical and magneto‐optical properties of pulsed laser‐deposited thulium iron garnet thin films

AbstractThis work presents a combined optical and magneto‐optical spectroscopic study of thulium iron garnet (Tm3Fe5O12, TmIG) films on substituted gadolinium gallium garnet (Gd2.6Ca0.4Ga4.1Mg0.25Zr0.65O12, sGGG) substrates. Spectroscopic ellipsometry, transmission spectroscopy, magneto‐optical Kerr effect spectroscopy and Raman spectroscopy results are presented for TmIG films with a thickness in the range from 20 to 300 nm grown on sGGG by pulsed laser deposition. The complex dielectric functions of TmIG and sGGG are determined and compared with previously published results for bulk yttrium iron garnet and GGG, respectively. The magneto‐optical spectroscopy corroborated with Raman spectroscopy sheds light on strain‐induced changes as a function of TmIG film thickness.

Controlled Growth of Semiconductor Alloy/Ferromagnetic Insulator (GeBi/Bi:Thulium Iron Garnet (TmIG)) Spin Heterojunction and Development of Quantum Logic Device

Replacing heavy metal and heavy metal alloy films with a semiconductor thin film, to form a semiconductor/ferromagnetic insulator spin heterojunction has always been an international research hotspot. This article reports on a Ge1–xBix/Bi:thulium iron garnet (TmIG) spin heterojunction. It was found that Ge1–xBix (x = 0.06–0.22) semiconductor alloy films and Bi3+-doped control Tm3–yBiyFe4.3Ga0.7O12 (Bi:TmIG, y = 0.2–1.0) films can enhance the spin–orbit coupling (SOC) strength, improving the abnormal spin Hall effect (ASHE) and the spin Hall magnetoresistance (RH) switch characteristics. As x varies from 0.06 to 0.22, it is noticed that the spin mixing conductance considerably increases. Because Bi ions have the ability to control the SOC strength, the 5–10 nm GeBi semiconductor thin film with the Bi-ion-doped Bi:TmIG film can realize a spin magnetic moment reversal and then drive the heterojunction magnetic moment switching effect, and the spin currents injected across this interface lead to deterministic magnetization reversal at an ultralow current density of 1.82 × 106 mA/cm2. This has significant practical applications in the design and implementation of spin quantum logic devices based on semiconductor processes, laying a material foundation for the development of quantum logic devices.

Influence of substrate on interfacial Dzyaloshinskii-Moriya interaction in epitaxial Tm3Fe5O12 films

The interfacial Dzyaloshinskii-Moriya interaction (iDMI) promotes homochiral magnetic structures such as N\'eel-type domain walls (DWs) and skyrmions. iDMI as well as chiral magnetic structures have been demonstrated in metal/garnet heterostructures offering desirable properties for spintronic applications compared to their metallic counterparts. By measuring the motion of DWs as a function of current and in-plane magnetic field, we show that for 6.6 nm thick epitaxial thulium iron garnet films with perpendicular magnetic anisotropy grown on gallium garnet substrates, the iDMI and the DW width depend on the substrate but do not scale linearly with lattice mismatch strain. The largest iDMI was $0.007 \mathrm{mJ} {\mathrm{m}}^{\ensuremath{-}2}$, obtained for three substrates of different compositions. The total anisotropy, however, does increase linearly with lattice parameter, indicating a dominant magnetoelastic anisotropy contribution. The sensitivity of iDMI and anisotropy to substrate composition provides opportunities for engineering homochiral magnetic structures and their dynamics.

Interfacial roughness driven manipulation of magnetic anisotropy and coercivity in ultrathin thulium iron garnet films

We have investigated the magnetic properties of thulium iron garnet (Tm3Fe5O12, TmIG) thin films grown on {111} gadolinium gallium garnet (Gd3Ga5O12, GGG) substrates fabricated via the facing target sputtering method. The annealing temperature (Ta), thickness (tf), and roughness were systematically controlled in order to obtain the perfect magnetic properties of the film showing perpendicular magnetic anisotropy (PMA) and large coercive field, which are the requisites for non-volatile magnetic memory device applications. Here, we demonstrated that the Ta acts as the crucial parameter both for obtaining the high quality of the film surface and engineering the direction of the magnetic anisotropy from the in-plane to out-of-plane. Through the further optimization process with a function of the tf, we successfully achieved the best condition for the PMA films. Specifically, the sample fabricated at a tf of ≤ 20 nm and Ta of 1000 ℃ showed the perfect PMA. Furthermore, the surface roughness of the substrate mainly contributes to the enhancement of the coercive field of the perpendicular magnetic anisotropic films. We speculate that the drag force from the pinning energy induced by the interfacial roughness retarded the domain wall motion at the TmIG/GGG interface, resulting in enhanced coercivity.

Quantifying spin Hall topological Hall effect in ultrathin Tm3Fe5O12/Pt bilayers

Recent reports have shown that thulium iron garnet (TmIG) based bilayers are promising material platforms for realizing small, room-temperature skyrmions. For potential applications, it is imperative to accurately evaluate electrical readout signals of skyrmions. In this context, the topological Hall effect has been considered as a characteristic signature of skyrmion formation. Unlike previous studies that have modeled the anomalous Hall effect in ultrathin TmIG/Pt bilayers, we isolate its contribution to the electrical readout signal by directly measuring the magnetic hysteresis loops using a sensitive Sagnac magneto-optical Kerr effect technique. Our combined optical and electrical measurements reveal that the spin Hall topological Hall resistivity is considerably larger than previously estimated values. Our finding further indicates that skyrmions can exist at room-temperature and near-zero applied magnetic fields.

Strain-induced magnetic anisotropy of REIG thin films grown on YAG(111) substrates by pulsed laser deposition

Several studies have revealed using rare earth (RE) elements instead of yttrium to regulate the strain-induced magnetic anisotropy of garnet films in recent years. In this study, we used pulsed laser deposition to fabricate RE iron garnet (REIG) thin films on (111)-oriented yttrium aluminum garnet (YAG) substrates. The REIG films are 100 nm in thickness and crystalline with (111)-orientation. The out-of-plane (444)-plane spacing decreases as the ionic radius of RE metal ions decreases, while the crystallite size increases. Samarium, holmium, and yttrium iron garnet (SmIG, HoIG, and YIG) have out-of-plane compressive strain, while erbium and thulium iron garnet (ErIG and TmIG) have weak out-of-plane tensile strain. The strain of the REIG films is resulted from lattice mismatch, off-stoichiometry, and the recrystallization process by thermal annealing. SmIG has a rather rough surface because of the crystalline distortion due to the largest ionic radius of Sm and the lattice mismatch between SmIG and YAG substrates. Due to negative magnetostriction constant and out-of-plane compressive strain, SmIG and HoIG films show strong perpendicular magnetic anisotropy (PMA) in vibrating sample magnetometry and magneto-optical Faraday effect (MOFE). MOFE also revealed that the REIG films have different sensitivities at different wavelengths of light. With further tuning PMA on SmIG and HoIG by decreasing the thickness and lateral size, our findings could pave the way for high-density nano-scale magnetic information storage based on REIG thin films.

Bias-free spin-wave propagation in a micrometer-thick ferrimagnetic film with perpendicular magnetic anisotropy

The isotropic transmission of magnetostatic forward volume spin waves in magnetic films with perpendicular magnetic anisotropy (PMA) is shown to be useful in the implementation of magnon-based micro-conduits. However, to our knowledge, non-magnetic-bias-field spin-wave propagation in a PMA magnetic insulator has not been achieved yet, which constrains the development of magnonic information devices and systems. Herein, we demonstrate a robust, bias-free spin-wave transmission in an 18.5-μm-thick bismuth-doped thulium iron garnet film with PMA. This ferrimagnetic film grown by liquid phase epitaxy exhibits high quality in both its crystal structure and its chemical composition and displays a large PMA field of ∼173 mT. The bias-free and reciprocal propagation of spin waves is demonstrated by all-electrical spectroscopy and provides a group velocity of 4.90 km s−1 and a decay length of 20.5 µm at zero magnetic field. Direct imaging of the remnant state indicates that the bias-free spin waves propagate along the oppositely oriented stripe domains with Bloch-type walls, which are formed by in-plane pre-magnetization. Our work contributes to the construction of isotropic charge-free micro-circuits with high levels of integration and nonvolatility.

Strong Perpendicular Anisotropy and Anisotropic Landé Factor in Bismuth-Doped Thulium Garnet Thin Films

With the development of spintronics, garnet films with perpendicular magnetic anisotropy (PMA) have been attracting the attention of researchers for decades. In this work, bismuth-doped thulium iron garnet (Tm2BiFe5O12, TmBiIG) films of varying thickness having strong PMA effect were fabricated on substituted Gd3Ga5O12 (sGGG) (111) substrates using the pulsed laser deposition (PLD) technique. Crystallographic characterization and magnetic properties of TmBiIG films were investigated using high-resolution scanning transmission electron microscopy, X-ray diffraction, vibrating sample magnetometry, and broadband ferromagnetic resonance (FMR). A high perpendicular anisotropic field of H⊥ = 4,445 ± 7.5 Oe in a 10-nm-thick film and H⊥ = 4,582 ± 7.7 Oe in a 30-nm-thick film at room temperature were obtained and analyzed in detail. Surprisingly, an additional spin-wave mode was observed in the in-plane FMR spectra. The discrepancy between in-plane and the out-of-plane Landé g-factors established a correlation with the PMA effect in the TmBiIG films. The Landé g-factor of the TmBiIG films is much lower than that of free electrons, indicating that the strong spin–orbit coupling is caused by Tm and Bi heavy elements. The Gilbert damping factor α changed from 0.007 to 0.012 in various thicknesses of TmBiIG films.

Interfacial thermal transport in spin caloritronic material systems

Interfaces often govern the thermal performance of nanoscale devices and nanostructured materials. As a result, accurate knowledge of thermal interface conductance is necessary to model the temperature response of nanoscale devices or nanostructured materials to heating. Here, we report the thermal boundary conductance between metals and insulators that are commonly used in spin-caloritronic experiments. We use time-domain thermoreflectance to measure the interface conductance between metals such as Au, Pt, Ta, Cu, and Al with garnet and oxide substrates, e.g., NiO, yttrium iron garnet (YIG), thulium iron garnet (TmIG), ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$, and sapphire. We find that, at room temperature, the interface conductance in these types of material systems range from 50 to $300\phantom{\rule{0.16em}{0ex}}\mathrm{MW}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$. We also measure the interface conductance between Pt and YIG at temperatures between 80 and 350 K. At room temperature, the interface conductance of Pt/YIG is $170\phantom{\rule{0.16em}{0ex}}\mathrm{MW}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ and the Kapitza length is \ensuremath{\sim}40 nm. A Kapitza length of 40 nm means that, in the presence of a steady-state heat current, the temperature drop at the Pt/YIG interface is equal to the temperature drop across a 40-nm-thick layer of YIG. At 80 K, the interface conductance of Pt/YIG is $60\phantom{\rule{0.16em}{0ex}}\mathrm{MW}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$, corresponding to a Kapitza length of \ensuremath{\sim}300 nm.

Study of magnon–phonon non-equilibrium in a magnetic insulator—Thulium iron garnet

The non-equilibrium state between magnons and phonons is the key to understand the spin-caloric phenomena. We developed a unique optical reflectometry technique to spatially resolve Kerr angle (θK) and optical reflectance (R) in a magnetic insulator—thulium iron garnet (TmIG). The TmIG was subjected to a thermal gradient to estimate populations of thermally excited magnons and phonons through the variation of θK and R. The results showed that the spatial gradient of θK is different from that of R, indicating the non-equilibrium state between magnons and phonons. Particularly, the characteristic decay length of θK was significantly influenced by the heating power and the magnetic field, suggesting non-linear magnon scattering in a high magnon density regime. Our work not only provides a scheme to investigate the spatial profiles of magnons and phonons but also reveals the magnon–phonon non-equilibrium in TmIG. Hence, this report will stimulate further studies based on magnon–phonon non-equilibrium such as a transverse spin Seebeck effect and Bose–Einstein condensation.

Spin pumping contribution to the magnetization damping in Tm3Fe5O12/W bilayers

In this work, thulium iron garnet (Tm3Fe5O12 – TmIG (20 nm)/Tungsten(W)(t) bilayers, sputtered on top of gadolinium gallium garnet (111) substrate, were used to investigate spin pumping (SP) line broadening mechanism in Ferromagnetic Resonance (FMR). The TmIG, films prior and after tungsten cap layer deposition, were investigated employing FMR and X-ray diffraction techniques. The TmIG films showed (111) orientation and perpendicular magnetic anisotropy (PMA). Due to the interface TmIG/W, when the TmIG magnetization is in resonance a spin current is pumped out the TmIG into the W layer, increasing the damping of the magnetization. Measuring the out-of-plane angular dependence of the FMR resonance field and linewidth, we were able to obtain solely the SP contribution to the line broadening, filtering Gilbert, mosaicity, and two magnon scattering mechanisms.

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.