Iron Garnet Films Research Articles

Routes to Low-Energy Magnetic Electronics

Various routes to low energy consumption magnetic electronics and magnetic memory are reviewed including field effect spintronics, magnetoelectric magnonics and straintronics. The new idea of biologically inspired straintronic image processing scheme as well as the new experimental results on electric field induced magnetic anisotropy modulation in iron garnet films are presented.

Morphology, Crystal Structure and Ferromagnetic Resonance Properties of Submicron-Thick Yttrium Iron Garnet Films Prepared by Pulsed Laser Deposition

Submicron-thick yttrium iron garnet films were grown on a gadolinium gallium garnet substrate by pulsed laser deposition. The morphology and crystal structure results show that the submicron-thick film has a flat surface, low surface roughness of ∼ 0.5 ± 0.05 nm and good epitaxy with the (444) plane. Magnetic hysteresis loops also indicate that the submicron-thick film has great soft magnetic properties. However, the ferromagnetic resonance (FMR) spectrum shows that the submicron-thick film has multiple resonance peaks. The angular dependence of the FMR response of the 10-nm-thick, 100-nm-thick and 525-nm-thick films shows that peaks 1 (at low field) and 2 (at high field) observed in the 525-nm-thick film spectrum originate from the internal layer and the outer layer of the films, respectively. Lastly, the angular dependence of the FMR linewidth shows that the lowest FMR linewidth occurred at an angle between the direction of the static field and film normal of 50°.

Magnetic resonance in defect spins mediated by spin waves

In search of two level quantum systems that implement a qubit, the nitrogen-vacancy (NV) center in diamond has been intensively studied for years. Despite favorable properties such as remarkable defect spin coherence times, the addressability of NV centers raises some technical issues. The coupling of a single NV center to an external driving field is limited to short distances, since an efficient coupling requires the NV to be separated by only a few microns away from the source. As a way to overcome this problem, an enhancement of coherent coupling between NV centers and a microwave field has recently been experimentally demonstrated using spin waves propagating in an adjacent yttrium iron garnet (YIG) film [1]. In this paper we analyze the optically detected magnetic resonance spectra that arise when an NV center is placed on top of a YIG film for a geometry similar to the one in the experiment. We analytically calculate the oscillating magnetic field of the spin wave on top of the YIG surface to determine the coupling of spin waves to the NV center. We compare this coupling to the case when the spin waves are absent and the NV center is driven only with the antenna field and show that the calculated coupling enhancement is dramatic and agrees well with the one obtained in the recent experiment.

Faraday rotation in iron garnet films beyond elemental substitutions

In previous decades significant efforts have been devoted to increasing the magneto-optical efficiency of iron garnet materials for the miniaturization of nonreciprocal devices such as isolators and circulators. Elemental substitutions or proper nano-structuring to benefit from optical resonances have been pursued. However, all these approaches still require film thicknesses of at least several tens of microns to deliver useful device applications, and suffer from narrow bandwidths in the case of optical resonance effects. This Letter reports on a newly discovered enhancement of the Faraday Effect observed experimentally in nanoscale bismuth-substituted iron garnet films. It is shown here that this enhancement is not due to elemental substitution or compositional variations. Nor is it due to photon trapping or resonance effects. Comprehensive experimental and theoretical analysis of the Faraday rotation reveals a dramatic seven-fold amplification in the magneto-optic gyrotropy within only 2 nm of the air-surface interface, corresponding to just a couple of atomic monolayers as a result of symmetry-breaking at the air-film interface. This finding opens up an avenue to the application of monolayer magnetic garnets for the control of light.

Abnormal anticrossing effect in photon-magnon coupling

We report the experimental demonstration of an abnormal, opposite anti-crossing effect in a photon-magnon-coupled system that consists of an Yttrium Iron Garnet film and an inverted pattern of split-ring resonator structure (noted as ISRR) in a planar geometry. It is found that the normal shape of anti-crossing dispersion typically observed in photon-magnon coupling is changed to its opposite anti-crossing shape just by changing the position/orientation of the ISRR's split gap with respect to the microstrip line axis along which ac microwave currents are applied. Characteristic features of the opposite anti-crossing dispersion and its linewidth evolution are analyzed with the help of analytical derivations based on electromagnetic interactions. The observed opposite anti-crossing dispersion is ascribed to the compensation of both intrinsic damping and coupling-induced damping in the magnon modes. This compensation is achievable by controlling the relative strength and phase of oscillating magnetic fields generated from the ISRR's split gap and the microstrip feeding line. The position/orientation of an ISRR's split gap provides a robust means of controlling the dispersion shape of anti-crossing and its damping in a photon-magnon coupling, thereby offering more opportunity for advanced designs of microwave devices.

Chiral excitation of spin waves in ferromagnetic films by magnetic nanowire gratings

We theoretically investigate the interlayer dipolar and exchange couplings for an array of metallic magnetic nanowires grown on top of an extended ultrathin yttrium iron garnet film. The calculated interlayer dipolar coupling agrees with observed anticrossings [Chen \emph{et al.}, Phys. Rev. Lett. \textbf{120}, 217202 (2018)], concluding that the interlayer exchange coupling is suppressed by a spacer layer between the nanowires and film. The Kittel mode in the nanowire array couples chirally to spin waves in the film, even though Damon-Eshbach surface modes do not exist. The chirality is suppressed when the interlayer exchange coupling becomes strong.

Proposal for a microwave photon to optical photon converter based on traveling magnons in thin magnetic films

Abstract In this work, we propose a concept of a microwave to optical photon converter for applications in Quantum Information (QI) that is based on travelling magnons in a thin magnetic film. The converter employs an epitaxially grown Bi-substituted yttrium iron garnet (Bi-YIG) film as the medium for propagation of travelling magnons (spin waves). The conversion is achieved through coupling of magnons to guided optical modes of the film. We evaluate the conversion efficiency for this device theoretically. Our prediction is that it will be larger by at least four orders of magnitude than experimentally obtained in a similar process exploiting a uniform magnetization precession mode in a YIG sphere. By creating an optical resonator of a large length from the film (such that the traveling magnon decays before forming a standing wave over the resonator length) one will be able to further increase the efficiency by several orders of magnitude, potentially reaching a value similar to achieved with opto-mechanical resonators. An important advantage of the suggested concept of the QI devices based on travelling spin waves is a perfectly planar geometry compatible with the optical lithography process and a possibility of implementing the device as a hybrid opto-microwave chip.

Interface-driven chiral magnetism and current-driven domain walls in insulating magnetic garnets.

Magnetic oxides exhibit rich fundamental physics1-4 and technologically desirable properties for spin-based memory, logic and signal transmission5-7. Recently, spin-orbit-induced spin transport phenomena have been realized in insulating magnetic oxides by using proximate heavy metal layers such as platinum8-10. In their metallic ferromagnet counterparts, such interfaces also give rise to a Dzyaloshinskii-Moriya interaction11-13 that can stabilize homochiral domain walls and skyrmions with efficient current-driven dynamics. However, chiral magnetism in centrosymmetric oxides has not yet been observed. Here we discover chiral magnetism that allows for pure spin-current-driven domain wall motion in the most ubiquitous class of magnetic oxides, ferrimagnetic iron garnets. We show that epitaxial rare-earth iron garnet films with perpendicular magnetic anisotropy exhibit homochiral Néel domain walls that can be propelled faster than 800 m s-1 by spin current from an adjacent platinum layer. We find that, despite the relatively small interfacial Dzyaloshinskii-Moriya interaction, very high velocities can be attained due to the antiferromagnetic spin dynamics associated with ferrimagnetic order.

Thin film rare earth iron garnets with perpendicular magnetic anisotropy for spintronic applications

Perpendicular magnetic anisotropy (PMA) in garnet thin films is important for achieving numerous spintronic applications including spin-orbit switching. In this study, we computationally investigated how to control PMA by tuning substrate strain in Holmium Iron Garnet (HoIG) films grown on five different (111) single crystal garnet substrates of Gadolinium Gallium Garnet (GGG, Gd3Ga5O12), Yttrium Aluminum Garnet (YAG, Y3Al5O12), Terbium Gallium Garnet (TGG, Tb3Ga5O12), Substituted Gadolinium Gallium Garnet (sGGG, Gd3Sc2Ga3O12), and Neodymium Gallium Garnet (NGG, Nd3Ga5O12). The negative sign of effective anisotropy energy density, Keff < 0, and anisotropy field, Ha < 0, determines the easy magnetization axis of the film to be perpendicular to the film surface. Here, we show that magnetoelastic anisotropy energy density determines the sign of the total anisotropy and it can be manipulated by altering the lattice parameter mismatch of the film and its substrate. Based on this study, HoIG is predicted to have PMA when grown on GGG, TGG and YAG among all five substrates mentioned. Moreover, the saturation field magnitude is calculated as an order of several hundreds of Oersteds, which is feasible in practical applications to saturate rare earth iron garnets with perpendicular magnetic anisotropy.

CuPt Alloy Thin Films for Application in Spin Thermoelectrics

Spin thermoelectrics represents a new paradigm of thermoelectricity that has a potential to overcome the fundamental limitation posed by the Wiedmann-Franz law on the efficiency of conventional thermoelectric devices. A typical spin thermoelectric device consists of a bilayer of a magnetic insulator and a high spin-orbit coupling (SOC) metal coated over a non-magnetic substrate. Pt is the most commonly used metal in spin thermoelectric devices due to its strong SOC. In this paper, we found that an alloy of Cu and Pt can perform much better than Pt in spin thermoelectric devices. A series of CuPt alloy films with different Pt concentrations were deposited on yttrium iron garnet (YIG) films coated gadolinium gallium garnet (GGG) substrate. Through spin Seebeck measurements, it was found that the Cu0.4Pt0.6/YIG/GGG device shows almost 3 times higher spin Seebeck voltage compared to Pt/YIG/GGG under identical conditions. The improved performance was attributed to the higher resistivity as well as enhanced spin hall angle of the CuPt layer.

Route toward semiconductor magnonics: Light-induced spin-wave nonreciprocity in a YIG/GaAs structure

We demonstrate laser-induced nonreciprocity of spin waves in the ferromagnetic-semiconductor structure. Surface spin waves in yttrium iron garnet film grown at the top of $n$-type gallium arsenide substrate were studied by means of Brillouin light-scattering spectroscopy. It is shown that spin-wave dispersion can be modified in a controlled manner by laser radiation. We observe the difference of up to 225 MHz when comparing the frequencies of counterpropagating spin waves. We attribute this frequency shift to the mutual influence of nonreciprocal spin-wave modal profiles and differences in magnetic anisotropies at two film surfaces as the result of laser-induced conductivity variation in GaAs substrate. We propose a simple model based on analytical dipole theory to describe the induced spin-wave nonreciprocity. Our results show the possibility of integration of magnonics and semiconductor electronics on the base of YIG/GaAs structures.

Magnetoimpedance effect in ferrimagnetic insulator yttrium iron garnet films capped by copper

Abstract We investigate the magnetization dynamics through magnetoimpedance effect in films of ferrimagnetic insulator yttrium iron garnet capped by copper. Thereby, we explore the magnetoimpedance in systems with magnetic insulators by varying the thickness of the magnetic material, and probe current frequency. We observe that the difference of electrical nature of the layers composing the films gives rise to the magnetoinductive effect in the whole frequency range, as well as favors the appearance of the ferromagnetic resonance effect even at low frequencies. We discuss the experimental results in terms of the structural features of the magnetic material, its magnetic properties, and the different mechanisms governing the magnetization dynamics. The results place the films of ferrimagnetic insulator yttrium iron garnet capped by copper exhibiting magnetoimpedance effect as very attractive candidates for application as probe element in magnetic sensors operating at low frequencies and with energy-efficient technologies. Further, we disclose an auspicious technique to look at the magnetization dynamics in magnetic materials and we provide a new perspective to solve questions related to the integration of insulator ferrimagnetic materials with the coplanar waveguide in broadband frequency applications.

New ferrimagnetic film could help speed up spintronic devices

The authors grew polycrystalline europium iron garnet films that have perpendicular magnetic anisotropy and potential applications in spintronics.

Phase detection of spin waves in yttrium iron garnet and metal induced nonreciprocity

We report experiments which characterize spin wave propagation in a thin (111) yttrium iron garnet film for arbitrary angles between the in-plane magnetic field and the mode wavevectors. By measuring the magnetic field evolution of the phase of the wave traveling across the film, we deduce the frequency dependence of the wavevector, the dispersion relation, from which the mode velocity follows. Additionally, we observe multiple nodes in the regime of the propagating Damon-Eshbach mode; these arise from avoided crossings associated with the higher, exchange split, standing wave modes along the film normal, the positions of which correlate with the direct absorption measurements of their positions. This information allows a determination of the exchange parameter. Using this technique, we examine the nonreciprocity in spin wave propagation that results from an adjacent metal layer.

Perpendicular magnetic anisotropy and spin mixing conductance in polycrystalline europium iron garnet thin films

Polycrystalline single-phase europium iron garnet films (EuIG, Eu3Fe5O12, a ferrimagnetic insulator), with thicknesses from 25 to 50 nm and roughness <1 nm, have been grown on various substrates using pulsed laser deposition followed by a rapid thermal anneal. The films are under strain that originates primarily from thermal mismatch and leads to a magnetoelastic anisotropy that dominates the net anisotropy. EuIG grown on quartz (0001) demonstrated perpendicular magnetic anisotropy (PMA) attributed to the in-plane (IP) compressive thermal mismatch strain, whereas films on (11 2¯ 0) quartz, Si, fused silica, and yttria-stabilized zirconia exhibited an IP easy axis due to tensile strain, consistent with the positive magnetostriction of polycrystalline EuIG. For the PMA EuIG, the saturation magnetization was close to that of bulk EuIG, and the out-of-plane coercivity ranged from 600 to 900 Oe, depending on the film thickness. Spin transport measurements on Pt/EuIG/quartz heterostructures gave an anomalous Hall effect-like spin Hall magnetoresistance similar to that of Pt/epitaxial single crystal EuIG. These results show that high quality polycrystalline garnets can be grown with PMA making them useful for applications in spintronic devices.

Direct detection of multiple backward volume modes in yttrium iron garnet at micron scale wavelengths

Using a set of wave-vector-specific multielement antennas, we have characterized the dispersion of spin waves in an yttrium iron garnet film at submicron lengths and resolved the dispersion relations of multiple backward volume modes, particularly in the region of their minima. The techniques developed now facilitate the characterization of spin waves at length scales limited only by available lithography and at a spectral resolution that generally exceeds that of Brillouin scattering. The data obtained are in excellent agreement with theoretical predictions based on a model Hamiltonian.

Time-resolved study of nonlinear three-magnon processes in yttrium iron garnet films

In this work, the temporal aspects of two nonlinear spin-wave processes, i.e., three-magnon splitting and confluence, were investigated in a yttrium iron garnet film. Three-magnon splitting involves the conversion of a magnon at the microwave pumping frequency ${f}_{p}$ into two magnons, each with a frequency near ${f}_{p}/2$, while confluence refers to the subsequent combination of two of the split magnons into one with frequency ${f}_{c}<{f}_{p}$. Time-resolved Brillouin scattering measurements confirm that these processes occur close to the driving antenna ($<1.7\phantom{\rule{0.16em}{0ex}}\mathrm{mm}$ away), which is expected. The data indicate, however, that magnons with larger group velocities are more likely to undergo confluence, which is not predicted by existing theory. Understanding the details of the splitting and confluence processes may have important implications for the use of short-wavelength spin waves for spintronic devices.

RGB Magnetophotonic Crystals for High-contrast Magnetooptical Spatial Light Modulators

Magnetooptical spatial light modulators (MOSLMs) are photonic devices that encode information in photonic waveforms by changing their amplitude and phase using magnetooptical Faraday or Kerr rotation. Despite the progress on both MO materials and switching methods, significant improvements on materials engineering and SLM design are needed for demonstrating low-power, multicolor, analog and high-contrast MOSLM devices. In this study, we present design rules and example designs for a high-contrast and large figure-of-merit MOSLM using three-color magnetophotonic crystals (MPC). We demonstrate for the first time, a three-defect MPC capable of simultaneously enhancing Faraday rotation, and high-contrast modulation at three fundamental wavelengths of red, green and blue (RGB) within the same pixel. We show using 2D finite-difference time-domain simulations that bismuth-substituted yttrium iron garnet films are promising for low-loss and high Faraday rotation MOSLM device in the visible band. Faraday rotation and loss spectra as well as figure-of-merit values are calculated for different magnetophotonic crystals of the form (H/L)p/(D/L)q/(H/L)p. After an optimization of layer thicknesses and MPC configuration, Faraday rotation values were found to be between 20–55° for losses below 20 dB in an overall thickness less than 1.5 µm including three submicron garnet defect layers. The experimental demonstration of our proposed 3-color MOSLM devices can enable bistable photonic projectors, holographic displays, indoor visible light communication devices, photonic beamforming for 5 G telecommunications and beyond.

The linear relationship of spin pumping energy in a La:YIG/Pt heterostructure used in a microwave rectifier

ABSTRACTLa3+ doped yttrium iron garnet films have been grown on (111) oriented gadolinium gallium garnet substrates via Liquid phase epitaxy technique as a basic material for ISHE device fabrication. Pt as a material with a large spin hall angle was used as a spin detection layer. We investigated the dependence of the spin pumping effect on the power and frequency of the excitation microwaves in La:YIG/Pt bilayers by measuring the ISHE voltage. We demonstrated that the area under the ISHE curve(SISHE) across a wide power range had a nearly linear correlation with the input microwave power (Pin). The parameter SISHE can be used to describe the spin current energy in a Pt layer which can be a useful parameter for a microwave rectifier.

Optically pumped Floquet states of magnetization in ferromagnets.

Floquet states have been the subject of great research interest since Zel'dovich's pioneering work on the quasienergy of a quantum system influenced by a temporally periodic action. Nowadays, periodic modulation of the system Hamiltonian is achieved mostly by microwaves, leading to novel exciting phenomena in condensed matter physics. On the other hand, nonthermal optical control of magnetization at picosecond time scales is currently a highly appealing research topic for potential applications in magnetic data storage. Here we combine these two concepts to theoretically investigate Floquet states in the system of exchange-coupled spins in a ferromagnet. Periodic perturbation of the magnetization of an iron-garnet film by circularly polarized femtosecond laser pulses is shown to establish the magnetization dynamics behaving like Floquet states. An external magnetic field allows tuning of the Floquet states, leading to pronounced increase in the precession amplitude by one order of magnitude at the center of the Brillouin zone, i.e., when the precession frequency is a multiple of the laser pulse repetition rate. Floquet states might potentially allow for parametric generation of magnetic oscillations. The observed phenomena expand the capabilities of coherent ultrafast optical control of magnetization and pave the way for their application in quantum computation or data processing.