Sub-terahertz/terahertz electron resonances in hard ferrimagnets

Gorbachev E.A., Trusov L.A., Alyabyeva L.N., Roslyakov I.V., Lebedev V.A., Kozlyakova E.S., Magdysyuk O.V., Sobolev A.V., Glazkova I.S., Beloshapkin S.A., Gorshunov B.P., Kazin P.E.

Herein, we demonstrate for the first time compact ferrite ceramics with giant coercivity. The materials are manufactured via sintering single-domain Sr0.67Ca0.33Fe8Al4O19 particles synthesized by a citrate-nitrate auto-combustion method. The obtained ceramics show coercivities up to 22.5 kOe and natural ferromagnetic resonance frequencies (NFMR) in a sub-THz range of 160-282 GHz. At a maximum density of 95%, the sample displays coercivity of 18.5 kOe, which is the highest value among dense ferrite materials reported so far. In addition, we report an unusual blueshift of the NFMR frequency from 160 to 200 GHz, which occurs during material sintering.

Kim S.K., Beach G.S., Lee K., Ono T., Rasing T., Yang H.

Ferrimagnets composed of multiple and antiferromagnetically coupled magnetic elements have attracted much attention recently as a material platform for spintronics. They offer the combined advantages of both ferromagnets and antiferromagnets, namely the easy control and detection of their net magnetization by an external field, antiferromagnetic-like dynamics faster than ferromagnetic dynamics and the potential for high-density devices. This Review summarizes recent progress in ferrimagnetic spintronics, with particular attention to the most-promising functionalities of ferrimagnets, which include their spin transport, spin texture dynamics and all-optical switching. Ferrimagnets offer the combined advantages of both ferromagnets and antiferromagnets for spintronics applications. This Review surveys recent progress.

Onoda H., Sukegawa H., Inoue J., Yanagihara H.

Perpendicular magnetic anisotropy (PMA) energy up to $K_{\mathrm{u}}=6.1\pm0.8$ MJ m$^{-3}$ is demonstrated in this study by inducing large lattice-distortion exceeding 3% at room temperature in epitaxially distorted cobalt ferrite Co$ _{x} $Fe$ _{3-x} $O$ _{4} $ (x = 0.72) (001) thin films. Although the thin film materials include no rare-earth elements or noble metals, the observed $ K_{u} $ is larger than that of the neodymium-iron-boron compounds for high-performance permanent magnets. The large PMA is attributed to the significantly enhanced magneto-elastic effects, which are pronounced in distorted films with epitaxial lattice structures upon introducing a distortion control layer of composition Mg$ _{2-x} $Sn$_{1+x}$O$ _{4} $. Surprisingly, the induced $ K_{u} $ can be quantitatively explained in terms of the agreement between the local crystal field of Co$ ^{2+} $ and the phenomenological magneto-elastic model, indicating that the linear response of induced $K_u$ is sufficiently valid even under lattice distortions as large as 3.2%. Controlling tetragonal lattice deformation using a non-magnetic spinel layer for ferrites could be a promising protocol for developing materials with large magnetic anisotropies.

Gorbachev E.A., Trusov L.A., Kovalenko A.D., Morozov A.V., Kazin P.E.

Exchange-coupled hard/soft ferrite nanoparticles are prospective to squeeze out a part of expensive magnets based on rare-earth elements. However, the known exchange-coupled composite ferrite nanoparticles often suffer from the lack of a powerful enough hard magnetic core, high defectivity of magnetic phases, and a poor interface between them. Herein, we demonstrate the first efficient synthesis of sandwiched nanomagnets, which exhibit a pronounced exchange-coupling effect. This work is featured by the use of individual highly coercive strontium hexaferrite nanoplates prepared by a borate glass crystallization method as cores for the composite particles. The high crystal quality of the hexaferrite cores as the substrate promotes the epitaxial growth of CoFe2O4 layers on the 001 facets from an organic high-boiling solvent and results in the enhancement of the remanent magnetization and maximum energy product of the composite material. The results of this work open new prospects for the fabrication of multilayer oxide heterostructures with synergetic performance, which expands the applications of exchange-coupled composites.

Gorbachev E.A., Kozlyakova E.S., Trusov L.A., Sleptsova A.E., Zykin M.A., Kazin P.E.

Abstract The review is devoted to compounds and materials demonstrating extremely high magnetic hardness. The recent advances in the synthesis of modern materials for permanent magnets are considered, and a range of exotic compounds interesting for fundamental research is described. The key details of chemical composition, crystal structure and magnetic microstructure responsible for the appearance of high magnetic anisotropy and giant coercivity are analyzed. The challenges of developing the title materials are noted and strategies for their solution are discussed. The bibliography includes 389 references.

Lee K., Lee D., Yang D., Mishra R., Kim D., Liu S., Xiong Q., Kim S.K., Lee K., Yang H.

Magnon-mediated angular-momentum flow in antiferromagnets may become a design element for energy-efficient, low-dissipation and high-speed spintronic devices1,2. Owing to their low energy dissipation, antiferromagnetic magnons can propagate over micrometre distances3. However, direct observation of their high-speed propagation has been elusive due to the lack of sufficiently fast probes2. Here we measure the antiferromagnetic magnon propagation in the time domain at the nanoscale (≤50 nm) with optical-driven terahertz emission. In non-magnetic-Bi2Te3/antiferromagnetic-insulator-NiO/ferromagnetic-Co trilayers, we observe a magnon velocity of ~650 km s–1 in the NiO layer. This velocity far exceeds previous estimations of the maximum magnon group velocity of ~40 km s–1, which were based on the magnon dispersion measurements of NiO using inelastic neutron scattering4,5. Our theory suggests that for magnon propagation at the nanoscale, a finite damping makes the dispersion anomalous for small magnon wavenumbers and yields a superluminal-like magnon velocity. Given the generality of finite dissipation in materials, our results strengthen the prospects of ultrafast nanodevices using antiferromagnetic magnons. Magnon-mediated angular-momentum flow in antiferromagnets may become a design element for energy-efficient, low-dissipation and high-speed spintronic devices. Here, terahertz emission measurements in magnetic multilayers unveil a superluminal-like magnon velocity of ~650 km s–1 in the antiferromagnetic insulator NiO at nanoscale distances.

Gorbachev E.A., Trusov L.A., Wu M., Vasiliev A.V., Svetogorov R.D., Alyabyeva L.N., Lebedev V.A., Sleptsova A.E., Karpov M.A., Mozharov Y.M., Gorshunov B.P., Kazin P.E.

Fine particles of SrFe12−xGaxO19 (x = 0–6) were obtained via a citrate auto-combustion route. Their magnetic and microwave absorption properties, as well as the features of the crystal structure were studied in detail.

Gorbachev E., Soshnikov M., Wu M., Alyabyeva L., Myakishev D., Kozlyakova E., Lebedev V., Anokhin E., Gorshunov B., Brylev O., Kazin P., Trusov L.

A fast method for the synthesis of ε-Fe2O3, yielding 100% pure material with a variable FMR frequency, is proposed.

Sleptsova A.E., Alyabyeva L.N., Gorbachev E.A., Kozlyakova E.S., Karpov M.A., Xinming C., Vasiliev A.V., Gorshunov B.P., Prokhorov A.S., Kazin P.E., Trusov L.A.

Single-domain particles of SrFe8Al4O19 were prepared by thermal treatment of porous products of citrate–nitrate autocombustion at 1200 °C, and the effect of synthesis time on the particle morphology and magnetic properties was estimated. The procedure allows one to obtain SrFe8Al4O19 particles with mean diameters of 100–460 nm and coercivity ranges from 14.5 to 18.4 kOe, while ferromagnetic resonance frequencies vary from 149 to 164 GHz.

Brataas A., van Wees B., Klein O., de Loubens G., Viret M.

Spin insulatronics covers efforts to generate, detect, control, and utilize high-fidelity pure spin currents and excitations inside magnetic insulators. Ultimately, the new findings may open doors for pure spin-based information and communication technologies. The aim is to replace moving charges with dynamical entities that utilize low-dissipation coherent and incoherent spin excitations in antiferromagnetic and ferromagnetic insulators. The ambition is that the new pure spin-based system will suffer reduced energy losses and operate at high frequencies. In magnetic insulators, there are no mobile charge carriers that can dissipate energy. Integration with conventional electronics is possible via interface exchange interactions and spin-orbit couplings. In this way, the free electrons in the metals couple to the localized spins in the magnetic insulators. In turn, these links facilitate spin-transfer torques and spin-orbit torques across metal-insulator interfaces and the associated phenomena of spin-pumping and charge-pumping. The interface couplings also connect the electron motion inside the metals with the spin fluctuations inside the magnetic insulators. These features imply that the system can enable unprecedented control of correlations resulting from the electron-magnon interactions. We review recent developments to realize electric and thermal generation, manipulation, detection, and control of pure spin information in insulators.

Vaidya P., Morley S.A., van Tol J., Liu Y., Cheng R., Brataas A., Lederman D., del Barco E.

A spin-pumping antiferromagnet Antiferromagnets have been used in spintronics mainly as a source of the so-called exchange bias. However, they hold promise for a much more active role given that their magnetization dynamics can in principle be much faster than those in ferromagnets. For this promise to materialize, antiferromagnets must learn the tricks that come naturally to ferromagnets. Vaidya et al. observed one such phenomenon called spin pumping (see the Perspective by Hoffmann). The researchers irradiated the antiferromagnet MnF 2 with circularly polarized subterahertz light, causing the spins in this material to spring into action. These dynamics, in turn, caused the injection of spin current into a layer of platinum adjacent to MnF 2 . Science , this issue p. 160 ; see also p. 135

Svetogorov R.D., Dorovatovskii P.V., Lazarenko V.A.

Introduction of “Belok/XSA” beamline-modernization is presented here with commissioning of a modern synchrotron beamline with unique capabilities for the structural analysis of single and polycrystalline samples. The beamline is designed to perform important tasks in modern crystallography, such as solving and refining the atomic structure using the single- and polycrystal method, studying the real (defective) structure, and analyzing the distribution of electron density in crystals, it is also possible to study rapidly occurring processes, for example, temperature phase transitions. Due to the speed of data accumulation and a registration over a large solid angle, the method allows to study large numbers of samples. This beamline has become extremely demanded in the chemical, biological, material science, and archaeological community in Russia.

Li J., Wilson C.B., Cheng R., Lohmann M., Kavand M., Yuan W., Aldosary M., Agladze N., Wei P., Sherwin M.S., Shi J.

Spin dynamics in antiferromagnets has much shorter timescales than in ferromagnets, offering attractive properties for potential applications in ultrafast devices 1 – 3 . However, spin-current generation via antiferromagnetic resonance and simultaneous electrical detection by the inverse spin Hall effect in heavy metals have not yet been explicitly demonstrated 4 – 6 . Here we report sub-terahertz spin pumping in heterostructures of a uniaxial antiferromagnetic Cr 2 O 3 crystal and a heavy metal (Pt or Ta in its β phase). At 0.240 terahertz, the antiferromagnetic resonance in Cr 2 O 3 occurs at about 2.7 tesla, which excites only right-handed magnons. In the spin-canting state, another resonance occurs at 10.5 tesla from the precession of induced magnetic moments. Both resonances generate pure spin currents in the heterostructures, which are detected by the heavy metal as peaks or dips in the open-circuit voltage. The pure-spin-current nature of the electrically detected signals is unambiguously confirmed by the reversal of the voltage polarity observed under two conditions: when switching the detector metal from Pt to Ta, reversing the sign of the spin Hall angle 7 – 9 , and when flipping the magnetic-field direction, reversing the magnon chirality 4 , 5 . The temperature dependence of the electrical signals at both resonances suggests that the spin current contains both coherent and incoherent magnon contributions, which is further confirmed by measurements of the spin Seebeck effect and is well described by a phenomenological theory. These findings reveal the unique characteristics of magnon excitations in antiferromagnets and their distinctive roles in spin–charge conversion in the high-frequency regime. Pure spin currents are simultaneously generated and detected electrically through sub-terahertz magnons in the antiferromagnetic insulator Cr 2 O 3 , demonstrating the potential of magnon excitations in antiferromagnets for high-frequency spintronic devices.

Gorbachev E.A., Trusov L.A., Sleptsova A.E., Kozlyakova E.S., Alyabyeva L.N., Yegiyan S.R., Prokhorov A.S., Lebedev V.A., Roslyakov I.V., Vasiliev A.V., Kazin P.E.

Single-domain Sr 1− x /12 Ca x /12 Fe 12− x Al x O 19 ( x = 4–6) particles are synthesized by a simple citrate auto-combustion method. The room temperature coercivity of the materials rises with aluminum content from 21.3 kOe ( x = 4) to a maximum of 36 kOe ( x = 5.5). This value is the highest among ferrite materials to date. Moreover, the magnetic alignment of the particles leads to further coercivity improvement up to 40 kOe. Due to large magnetic anisotropy the samples demonstrate sub-terahertz electromagnetic wave absorption by natural (zero-field) ferromagnetic resonance (NFMR). The absorption lines shift with aluminum substitution from 160 GHz ( x = 4) to 250 GHz ( x = 5.5), which is the record NFMR frequency known for a magnetic material. This research paves the way for development low-cost materials with extremely high coercivity and sub-terahertz NFMR.

Tancredi P., Rivas-Rojas P.C., Moscoso-Londoño O., Muraca D., Knobel M., Socolovsky L.M.

The present work describes a synthesis and characterization strategy employed to study the magnetic anisotropic properties of a diluted nanoparticulate system. The system under analysis is composed of monodisperse and highly crystalline 16 nm Co0.5Fe2.5O4 nanoparticles (NPs), homogenously dispersed in 1-octadecene. Owing to the liquid nature of the matrix at room temperature, the relative orientation of the nanoparticle easy axis can be controlled by an external magnetic field, enabling us to measure how the magnetic properties are modified by the alignment of the particles within the sample. In turn, by employing this strategy, we have found a significant hardness and squareness enhancement of the hysteresis loop in the magnetically oriented system, with the coercive field reaching a value as high as 30.2 kOe at low temperatures. In addition, the magnetic behavior associated with the system under study was supported by additional magnetic measurements, which were ascribed to different events expected to take place throughout the sample characterization, such as the melting process of the 1-octadecene matrix or the NP relaxation under the Brownian mechanism at high temperatures.

Zeng X., Jia P., Pan D., Guo Y., Xi X., Li P., Qian L., Sun C., Li Y., Li B., Zhou J.

Gorbachev E.A., Alyabyeva L.N., Pronin A.V., Sultanovskaya A.S., Kozlyakova E.S., Magdysyuk O.V., Roslyakov I.V., Dressel M., Gorshunov B.P., Trusov L.A.

Dense ceramics of Al-substituted hexaferrites demonstrate robust, intensive, and highly magnetic field-sensitive ferromagnetic resonance absorption at 5–300 K.

Yang H., Jian Z., Li K., Chang H., Yang W., Liu S.

The forbidden bandgap energy, magnetic properties, natural ferromagnetic resonance and microwave absorption are studied on the Zn2+ and Sn4+ doped BaFe12-2xZnxSnxO19. The forbidden band gap of BaFe12-2xZnxSnxO19 increases with the increasing doping content x from 1.80 eV up to 1.97 eV. The coercive field, the anisotropy field and the magnetocrystalline constant of BaFe12-2xZnxSnxO19 decreases with the increasing doping. The natural ferromagnetic resonance is observed. The high permeability, accompanied with the ferromagnetic resonance, enables strong microwave absorption. BaFe10.4Zn0.8Sn0.8O19 has a wide effective microwave absorption bandwidth (RL≤-10dB) of 6 GHz with the material thickness below 2.5 mm in 11.4–17.4 GHz. The dielectric loss has little contribution to the excellent microwave absorption of BaFe12-2xZnxSnxO19. The BaFe12-2xZnxSnxO19/carbon nanotube (CNT) composites significantly increase the permittivity. The high dielectric loss and the high magnetic loss further improve the microwave absorption. With a very thin thickness of 1.8 mm, BaFe10.4Zn0.8Sn0.8O19/CNT has a bandwidth of 7.1 + GHz of 10.9–––18 + GHz. With the material thickness thinner than 3 mm, BaFe12-2xZnxSnxO19/CNT composites have very broad effective microwave absorption bandwidth of 11.4+, 10.7 and 10.7 GHz, with x = 0.6, 0.8 and 1.0, respectively. It illustrates that both BaFe10.4Zn0.8Sn0.8O19 and BaFe12-2xZnxSnxO19/CNT composites are very promising microwave absorbers.

Kim H., Cao Van P., Jo Y., Jeong J., Kim K.

We investigate the unidirectional magnetoresistance (UMR) in a GdIG(Gd3Fe5O12)/Pt bilayer. We find a suppression of the observed UMR as the magnetic field increases, suggesting a magnonic origin for the UMR effect. Furthermore, through the examination of the temperature dependence of UMR, we observe that the sign of the UMR remains unchanged at the compensation temperature. This observation indicates that the magnon contributing to the UMR is associated with the lowest magnon mode with a distinct handedness. Our results provide compelling evidence for the existence of magnonic UMR in compensated ferrimagnets, offering potential advantages for spintronic applications.

Wang X., Yan H., Zhao S., Liu S., Chang H.

The site preferences of the ions in M-type BaFe12-y-2xAlySnxMnxO19 were determined from the XRD patterns refined with the Fullprof program. Due to the preferential occupation of the nonmagnetic Sn4+ ions, the doping Sn4+ and Mn2+/Mn3+ ions decreased the saturation magnetization. The doping also decreased the anisotropic field and the coercivity as well. BaFe12-y-2xAlySnxMnxO19 had strong natural ferromagnetic resonances at 15.37, 9.96, and 6.96 GHz with x = 0.7, 0.9 and 1.1, respectively. The frequency of the natural ferromagnetic resonance fell in the frequency range between 2 and 18 GHz was closely related to the decreased anisotropic field with the Sn4+ and Mn2+/Mn3+ doping. Accompanied with the ferromagnetic resonance, a high attenuation factor was obtained. The highest attenuation factor was 240 in x = 0.7. The minimum reflection loss was at the frequency with the impedance matching being close to 1, and at the thickness satisfying the quarter wavelength mechanism. The impedance matching was good around the natural resonance frequency in each compound. These led to a strong microwave absorption in the x = 0.7, 0.9 and 1.1 compounds. As the absorber’s thickness was as thin as 1.8 mm, the x = 0.7 compound had an effective bandwidth of over 4.1 GHz in the range from 13.9 to higher than 18 GHz. At thin thickness of 2.5 -2.9 mm, the x = 0.9 compound had a broad absorption bandwidth of 8 - 14.8 GHz, which covered the full X band of 8 -12 GHz. By varying the doping content x, the microwave absorption band covered the range of 6.1 to 18 GHz with the absorber thickness thinner than 2.9 mm. The quarter wavelength mechanism played an important role on the strong microwave absorption.

Gorbachev E.A., Lebedev V.A., Kozlyakova E.S., Alyabyeva L.N., Ahmed A., Cervellino A., Trusov L.A.

Double substitution of strontium hexaferrite by calcium and aluminum leads to a tremendous rise of hard magnetic properties, such as coercivity and natural ferromagnetic resonance frequency (NFMR). However, the properties are also inextricably linked to the material microstructure (especially, particle size), to the solid solution inhomogeneity as well as aluminum ions distribution among iron sites in crystal structure. In this work, we obtained M-type hexaferrite particles Sr1-x/12Cax/12Fe12-xAlxO19 (x = 4–6) via a facile citrate-nitrate auto-combustion method and studied the influence of the annealing temperature in a broad range on the microstructure, features of crystal structure and hard magnetic properties. At low annealing temperatures (900–1000°С) hexaferrite nanoparticles with 90% of nominal Al content and a wide chemical distribution are formed. Next, with an increase in the annealing temperature the distribution significantly narrows, chemical composition becomes close to the nominal one and particles size transfer firstly to submicron, then to micron range. The aluminum distribution over iron sites is independent distinctly on the annealing temperature. For all the compositions single domain particles with the maximum coercivity values between 22.8 and 36 kOe are obtained at 1200 °C. At 900–1000 °C the samples demonstrate coercivities up to 25 kOe, while above 1300 °C, the crystallites begin to pass into a polydomain state with a reduced coercivity. The hexaferrites with narrow chemical distribution reveal resonance absorption in sub-terahertz band. The highest NFMR frequency of 270 GHz was observed for x = 5.5 sample annealed at 1400 °C.

Gorbachev E.A., Alyabyeva L., Soshnikov M., Lebedev V., Morozov A.V., Kozlyakova E., Ahmed A., Eliseev A.A., Trusov L.A.

Nanoceramics of metastable ε-Fe2O3 were obtained by sintering at 700 °C. Temperature dependencies of the static and dynamic magnetic properties were investigated before and after sintering. The nanoceramics is more hard-magnetic at low temperatures.