Ferromagnetic Resonance Research Articles

Large out-of-plane spin–orbit torque in topological Weyl semimetal TaIrTe4

The unique electronic properties of topological quantum materials, such as protected surface states and exotic quasiparticles, can provide an out-of-plane spin-polarized current needed for external field-free magnetization switching of magnets with perpendicular magnetic anisotropy. Conventional spin–orbit torque (SOT) materials provide only an in-plane spin-polarized current, and recently explored materials with lower crystal symmetries provide very low out-of-plane spin-polarized current components, which are not suitable for energy-efficient SOT applications. Here, we demonstrate a large out-of-plane damping-like SOT at room temperature using the topological Weyl semimetal candidate TaIrTe4 with a lower crystal symmetry. We performed spin–torque ferromagnetic resonance (STFMR) and second harmonic Hall measurements on devices based on TaIrTe4/Ni80Fe20 heterostructures and observed a large out-of-plane damping-like SOT efficiency. The out-of-plane spin Hall conductivity is estimated to be (4.05 ± 0.23)×104 (ℏ ⁄ 2e) (Ωm)−1, which is an order of magnitude higher than the reported values in other materials.

Interfacial reaction behavior between ferromagnetic CoFeB and the topological insulator Sb2Te3

The successful prevention of interfacial reactions between ferromagnetic materials and topological insulators (TIs) is crucial for the realization of reliable spintronic devices using TIs. To this end, we investigated the magnetic properties and interfacial reaction behavior of a bilayer structure composed of ferromagnetic CoFeB and TI Sb2Te3. The effects of including a MgO interlayer was also investigated. Ferromagnetic resonance (FMR) studies showed a remarkably weak resonance peak for the sample annealed at 400 °C for the 2-nm-thick interlayer, and finally, no resonance peak for a film below 1 nm in thickness was observed. X-ray diffraction (XRD) results demonstrated that the Sb2Te3 peak intensities started to decrease upon annealing at 200 °C and completely disappeared for annealing temperatures >400 °C. Hard X-ray photoelectron spectroscopy (HAXPES) results also support that the core-level peaks of Sb and Te split upon annealing at temperatures >200 °C, suggesting the dissociation of Sb2Te3. These results indicate that Sb2Te3 and CoFeB react at the interface during annealing, resulting in a loss of the ferromagnetic properties of the CoFeB layer. Meanwhile, a sample containing a 3-nm-thick MgO layer retained its original Sb2Te3/MgO/CoFeB structure even after annealing at 400 °C, as evidenced by its unchanged XRD peak intensity and FMR spectrum. We expect that our findings will be highly valuable in developing TI-based spintronic devices.

A broad-band investigation of ferromagnetic resonance in exchange coupled permalloy/phthalocyanine bilayer heterostructures at room temperature

The phenomenon of ferromagnet/molecule interfacial electronic hybridization enables the possibility of modifying magnetic characteristics of downscaled systems for spintronic applications. Ferromagnetic resonance (FMR) at the nanoscale level through a coplanar waveguide (CPW) over a wide frequency range is a remarkably reliable technique for investigation of the magnetic heterostructure’s interfacial properties and their feasibility in applications as spintronic devices. Here, magnetron sputtering and organic molecular beam epitaxy methods are used to fabricate the following nanoscale heterostructures: permalloy (Py)/Fe phthalocyanine (Pc), Py/ZnPc, and Py/Cr. Magnetization dynamics of the bilayer heterostructures is investigated in a CPW by studying frequency dependence (5–50 GHz) of the resonance field and linewidth with parallel and perpendicular geometries of the static applied magnetic field, thereby providing insights into possible intrinsic and extrinsic sources of FMR damping. Planar Pc molecules at the Py layer interface are found to have the potential to contribute to magnetic stabilization based on calculated magnetic parameters. This is evidenced by the larger results of surface anisotropy of the Py/MPc bilayer heterostructures induced by exchange coupling interaction at room temperature as well as the lower values of damping constant.

Lattice-commensurate skyrmion texture in a centrosymmetric breathing kagome magnet

Skyrmion lattices (SkL) in centrosymmetric materials typically have a magnetic period on the nanometer-scale, so that the coupling between magnetic superstructures and the underlying crystal lattice cannot be neglected. We reveal the commensurate locking of a SkL to the atomic lattice in Gd3Ru4Al12 via high-resolution resonant elastic x-ray scattering (REXS). Weak easy-plane magnetic anisotropy, demonstrated here by a combination of ferromagnetic resonance and REXS, penalizes placing a skyrmion core on a site of the atomic lattice. Under these conditions, a commensurate SkL, locked to the crystal lattice, is stable at finite temperatures – but gives way to a competing incommensurate ground state upon cooling. We discuss the role of Umklapp-terms in the Hamiltonian for the formation of this lattice-locked state, its magnetic space group, and the role of slight discommensurations, or (line) defects in the magnetic texture. We also contrast our findings with the case of SkLs in noncentrosymmetric material platforms.

Exchange coupling in (Co/Pt)2/Nb/(Pt/Co)2 multilayers induced by the Yu–Shiba–Rusinov bound states

The Ruderman–Kittel–Kasuya–Yosida (RKKY) theory of oscillatory exchange interaction in synthetic antiferromagnetic multilayers results in the oscillation behavior of giant magnetoresistance ratio. In addition, the formation of Yu–Shiba–Rusinov (YSR) bound states will induce an extra antiferromagnetic interaction, which is expected to mediate stronger exchange coupling than conventional RKKY interaction. However, the YSR bound states have only been studied in some superconducting hosts with magnetic impurities. The narrow range of material systems that are not compatible with the device integration architecture limits the practical application of YSR interaction. Here, we observe the RKKY interaction and provide the evidence for YSR interaction in Pt/(Co/Pt)2/Nb/(Pt/Co)2/Pt synthetic antiferromagnetic multilayers by quantitatively determining the coupling energy via ferromagnetic resonance technique.

Modulating spin current induced effective damping in β–W/Py heterostructures by a systematic variation in resistivity of the sputtered deposited β–W films

Utilizing the spin-induced pumping from a ferromagnet (FM) into a heavy metal (HM) under the ferromagnetic resonance (FMR) condition, we report an enhancement in effective damping in β- W/Py bilayers by systematically varying resistivity (ρ W ) of β-W films. Different resistivity ranging from 100 μΩ-cm to 1400 μΩ-cm with a thickness of 8 nm can be achieved by varying the argon pressure (P Ar ) during the growth by the method of sputtering. The coefficient of effective damping α eff is observed to increase from 0.010 to 0.025 with ρ W , which can be modulated by P Ar . We observe a modest dependence of α eff on the sputtering power (p S ) while keeping the P Ar constant. α eff dependence on both P Ar and p S suggests that there exists a strong correlation between α eff and ρ W . It is thus possible to utilize ρ W as a tuning parameter to regulate the α eff , which can be advantageous for faster magnetization dynamics switching. The thickness dependence study of Py in the aforementioned bilayers manifests a higher spin mixing conductance ( geff↑↓ ) which suggests a strong spin pumping from Py into the β-W layer. The effective spin current (J S(eff)) is also evaluated by considering the spin-back flow in this process. Intrinsic spin mixing conductance ( gW↑↓ ) and spin diffusion length (λ SD ) of β-W are additionally investigated using thickness variations in β-W. Furthermore, the low-temperature study in β-W/Py reveals an intriguing temperature dependence in α eff which is quite different from α b of single Py layer and the enhancement in α eff at low temperature can be attributed to the spin-induced pumping from Py layer into β-W.

Epitaxial Fe/Rh bilayers for efficient spin-to-charge conversion

To address the spin pumping in the conventional ferromagnetic/“normal” metal systems, we fabricated 6 nm Fe/1–15 nm Rh epitaxial bilayers and determined the g-factor, magnetic anisotropy, and magnetization damping by combining both 0–40 GHz CPW-based frequency-dependent and cavity-based 9.56 GHz in-plane angular-dependent ferromagnetic resonance measurements at room temperature. Auger electron spectroscopy and low-energy electron diffraction show that Rh grows epitaxially on Fe. The epitaxial bilayers exhibit a high spin mixing conductance gmix↑↓=(2.9±0.2)×1019 m−2 and a spin diffusion length in Rhodium λsd=9.0±1.3 nm. This makes Rh comparable to Pt and Pd in terms of spin pumping and spin transport efficiency at room temperature.

Magnetic properties of the mixed ferrites MnZnFe2O4 and MnAl-Fe2O4: A comparative study

Nanosized particles of Mn1-xZnxFe2O4, where 0 ≤ x ≤ 1, and MnAlxFe2-xO4, where 0 ≤ x ≤ 2, were synthesized by the sol–gel combustion method and the magnetic properties of these compounds were investigated and compared. The saturation magnetization of the samples of Mn1-xZnxFe2O4 was taken from the literature and the saturation magnetization of the samples of MnAlxFe2-xO4 was estimated from the cation distribution; ferromagnetic resonance spectra were used to determine the magnetocrystalline anisotropy. The results show that the saturation magnetization and the magnetocrystalline anisotropy of Zn- and Al- doped MnFe2O4 vary over a wide range, a result that could be useful for practical applications of these materials.

Temperature dependent interfacial-coupling and magnetization reversal of La2/3Sr1/3MnO3 film with inverted hysteresis features

Temperature-dependent magnetic anisotropy and interfacial coupling of La2/3Sr1/3MnO3 (LSMO) epitaxial films with inverted hysteresis (IHLs) features are investigated using a broadband ferromagnetic resonance (FMR) spectrometer. Through the dispersion relation obtained along [100], [110] and [010] axes, a simplified method is proposed to investigation in-plane magnetic anisotropy by broadband FMR in a cryostat. According to this method, the perpendicular uniaxial anisotropy, magneto-crystalline anisotropy, and interfacial coupling between the LSMO film and hard-magnetic interface layer are investigated as a whole. The measured value of interfacial coupling is −17.6 Oe at 300 K, close to the bias field found in minor loop, i.e., −18.4 Oe. The interfacial coupling is enhanced with the dropping of temperatures, and the value of interfacial coupling changed to −112 Oe at 10 K. A Stoner-Wohlfarth-like model is employed to calculate the hysteresis loops of the films based on the measured values of in-plane magnetic anisotropy and interfacial-coupling. The calculation reveals the mechanism of magnetization reversal and the origin of IHLs found in LSMO films.

Symmetry Breaking-Induced Resonance Dynamics in Bloch Point Nanospheres: Unveiling Magnetic Volume Effects and Geometric Parameters for Advanced Applications in Magnetic Sensing and Spintronics.

This study explores the impact of symmetry breaking on the ferromagnetic resonance of Bloch point (BP) nanospheres. Through standard Fourier analysis, we unveil two distinct oscillation mode groups characterized by low and high frequencies, respectively. Our findings emphasize the pivotal role of magnetic volume in shaping resonance amplitudes, providing new insights into the intricate dynamics of BP states. The investigation of geometric parameters reveals a quasi-monotonic decrease in resonance frequencies as a function of the asymmetry degree attributed to symmetry-breaking induced by geometric modifications. Spatial distribution analysis showcases unique resonance frequencies for the upper and lower BP hemispheres, highlighting the nuanced impact of the geometry on mode excitation. As the radius increases, additional modes emerge, demonstrating a compelling relationship between the magnetic volume and frequency. Phase analysis unveils coherent oscillations within each BP hemisphere, offering valuable insights into the rotational directions of the excitation poles. Beyond fundamental understanding, our study opens avenues for innovative applications, suggesting the potential use of nanospheres in advanced magnetic sensing, data storage, and nanoscale spintronic devices.

Influence of thickness, Si and SiO2/Si substrate interfaces upon magnetization dynamics in RF sputtered Fe3O4 thin films

The effect of film thickness (90–220 nm), Si and SiO2/Si substrate interfaces upon structural, micro-structural and dynamic magnetic properties of RF sputtered Fe3O4 films are investigated. Structural and micro-structural studies have confirmed the polycrystalline Fe3O4 growth with a α-Fe2O3 minority phase. A systematic grain growth with thickness suggests the presence of anti-phase boundary (APB) and corresponding competing magnetic interactions. The absence of Verwey phase transition and lower effective magnetization of 283–359 emu/cm3 are attributed to the APBs. The influence of APBs and impurity states upon the magnetization dynamics has been investigated with co-planar waveguide ferromagnetic resonance spectroscopy from 9 to 33 GHz frequency range. A higher Gilbert damping of ∼0.016 is noted for Fe3O4/Si films as compared to the lower damping of ∼0.002–0.004 for Fe3O4/SiO2/Si interfaces. Further, inhomogeneity, intrinsic Gilbert damping and two-magnon scattering contributions to the damping parameter are evaluated and discussed.

Effect of growth conditions on magnetization reversal and magnetic anisotropy in Co2Fe0.5Ti0.5Si quaternary Heusler alloy thin films

Ferromagnetic resonance and magneto-optic Kerr effect (MOKE) techniques are employed to unravel the nature of ‘in-plane’ (IP) magnetic anisotropy and magnetization reversal (MR) processes in magnetron-sputtered 100 nm Co2Fe0.5Ti0.5Si (CFTS) thin films, deposited (and subsequently annealed) at different substrate temperatures (Ts) ranging from 200 °C to 550 °C. By varying TS, the CFTS films are produced with different amounts of anti-site (AS) atomic disorder. Irrespective of the degree of AS disorder, the IP uniaxial magnetic anisotropy (UMA) is prevalent in all the CFTS films. The TS450 and TS500 films, deposited at T S = 450 °C and 500 °C, stand out as they have (i) the least AS disorder, (ii) lowest value (α = 0.0055) of the Gilbert damping constant, (iii) high saturation magnetization ( ≅770 G ) at 300 K, (iv) UMA energy density as high as ≅1.6×104erg/cc at 120 K dropping to ≅1.0×104erg/cc at 300 K, (v) spin-wave stiffness ( D ) at T = 0 K, D0≅175 meVÅ2 and, as in other CFTS films, the electron–magnon interaction is primarily responsible for the thermal renormalization of D . Furthermore, in the TS450 and TS500 films, MOKE hysteresis loops at various angles ( ψH ) that the field makes with the easy axis in the film plane, reveals two mutually exclusive UMAs, UMA1≫UMA2 , with easy axes perpendicular to each other. By contrast, only a single UMA is observed in all the remaining films. When ψH=0∘ , MOKE domain images reveal that, consistent with the uniaxial nature of magnetic anisotropy, an abrupt reversal of magnetization is accompanied by the formation of 180° domains. When ψH = 90°, due to the presence of two UMAs in TS450 and TS500 films, following the field reversal, first the reverse domains nucleate and then grow at the expense of the fundamental domains through field-induced domain wall motion.

Magnetite in the abdomen and antennae of Apis mellifera honeybees.

The detection of magnetic fields by animals is known as magnetoreception. The ferromagnetic hypothesis explains magnetoreception assuming that magnetic nanoparticles are used as magnetic field transducers. Magnetite nanoparticles in the abdomen of Apis mellifera honeybees have been proposed in the literature as the magnetic field transducer. However, studies with ants and stingless bees have shown that the whole body of the insect contain magnetic material, and that the largest magnetization is in the antennae. The aim of the present study is to investigate the magnetization of all the body parts of honeybees as has been done with ants and stingless bees. To do that, the head without antennae, antennae, thorax, and abdomen obtained from Apis mellifera honeybees were analyzed using magnetometry and Ferromagnetic Resonance (FMR) techniques. The magnetometry and FMR measurements show the presence of magnetic material in all honeybee body parts. Our results present evidence of the presence of biomineralized magnetite nanoparticles in the honeybee abdomen and, for the first time, magnetite in the antennae. FMR measurements permit to identify the magnetite in the abdomen as biomineralized. As behavioral experiments reported in the literature have shown that the abdomen is involved in magnetoreception, new experimental approaches must be done to confirm or discard the involvement of the antennae in magnetoreception.

FeCuNbSiB thin films with sub-Oersted coercivity

We investigated the microstructural and magnetic properties of nanocrystalline FeCuNbSiB thin films produced through magnetron sputtering followed by heat treatment. Our research focused on film thicknesses (80 nm and 160 nm) and heat treatment temperatures (490 °C–520 °C) that resulted in samples characterized by low coercivity and high effective magnetization. Conducting a comprehensive microstructural examination, we employed X-ray diffraction and transmission electron microscopy techniques, including selected area electron diffraction, high-resolution imaging and Fourier transform analysis. Magnetic properties were investigated using an alternating gradient field magnetometer and broadband ferromagnetic resonance. Structural analysis revealed, for all our heat-treated films, a well-defined microstructure characterized by nanograins, with a Fe3Si ferromagnetic phase, embedded within an amorphous matrix. Our films exhibit sub-Oe coercivity, reaching as low as 0.51 Oe, along with an increase in effective magnetization from 1170 emu/cm3 for the amorphous phase to a maximum of 1260 emu/cm3 in the nanocrystalline phase.

Dual mode spin to charge conversion using inverse spin Hall effect in NiFe/FeMn/NiFe multilayer thin films

Microwave devices with more than one operating frequency and their ease of tunability are of great importance for high-frequency information processing. Magnetic thin films offer an unparalleled advantage of engineering different microwave bands that can be precisely tailored and reconfigured externally. Here, novel trilayer structures consisting of NiFe/FeMn/NiFe with varying antiferromagnetic FeMn layer thickness have been investigated by exploring their ferromagnetic resonance (FMR) properties and inverse spin Hall effect (ISHE) responses. Two-step magnetic hysteresis loops are observed for higher FeMn thickness (t ⩾ 12 nm), where the bottom NiFe layer shows a comparatively more significant shift due to the presence of strong interfacial exchange coupling. FMR study reveals two resonant modes associated with the two ferromagnetic layers, which are distinguishable for higher thicknesses of FeMn or at high excitation frequencies. The choice of FeMn thickness determines the operating frequencies, which can be finely tailored by optimizing the FeMn thickness. Gilbert damping parameter is found to be in the range of 0.009–0.012 where the presence of exchange bias adds to the the scattering mechanisms. Prominent ISHE responses are obtained from the bottom NiFe layer as compared to the top NiFe layer. Variation of FeMn thickness also shows a strong influence on the spin pumping ( Vsp ), and perpendicular anisotropic magnetoresistance ( VAMR⊥ ) components. The results are correlated with the efficiency of spin flow and spin-to-charge conversion of the FeMn layer. Our systems can be used as an emerging alternative for microwave detectors and microwave energy harvesters.

Giant magneto-impedance effect in amorphous CoP@Cu microstrip based device in a wide frequency range

The giant magneto-impedance effect (GMI) consists in a large variation in the electrical impedance that occurs in some magnetic materials when subjected to a variable external magnetic field. Here we present an experimental study of the GMI in a wide frequency range observed in a structure of the metallic amorphous magnetic alloy CoP electrodeposited on copper microwires by using a microstrip configuration. In the low frequency range, the magnetic field dependence of the impedance is due to the behavior of the circumferential ac susceptibility in the magnetic coating. In the microwave range the behavior of the GMI is dominated by the main dynamical magnetic phenomenon at high frequencies, namely ferromagnetic resonance (FMR). The experimental data are well fit by Kittel equation for the FMR frequency for a ferromagnetic layer with a saturation magnetization similar to the ones reported in previous works. The structural stability of the CoP alloy against aging effects was also investigated. The overall results indicate that the CoP alloy is a good candidate for applications in sensors and in microwave devices.

Generation of localized, half-frequency spin waves in micron sized ferromagnetic stripes: Experiments and simulations

Generation of localized, half-frequency spin waves in micron sized ferromagnetic stripes: Experiments and simulations

Sensing spin wave excitations by spin defects in few-layer-thick hexagonal boron nitride.

Optically active spin defects in wide bandgap semiconductors serve as a local sensor of multiple degrees of freedom in a variety of "hard" and "soft" condensed matter systems. Taking advantage of the recent progress on quantum sensing using van der Waals (vdW) quantum materials, here we report direct measurements of spin waves excited in magnetic insulator Y3Fe5O12 (YIG) by boron vacancy [Formula: see text] spin defects contained in few-layer-thick hexagonal boron nitride nanoflakes. We show that the ferromagnetic resonance and parametric spin excitations can be effectively detected by [Formula: see text] spin defects under various experimental conditions through optically detected magnetic resonance measurements. The off-resonant dipole interaction between YIG magnons and [Formula: see text] spin defects is mediated by multi-magnon scattering processes, which may find relevant applications in a range of emerging quantum sensing, computing, and metrology technologies. Our results also highlight the opportunities offered by quantum spin defects in layered two-dimensional vdW materials for investigating local spin dynamic behaviors in magnetic solid-state matters.

In situ electric-field control of ferromagnetic resonance in the low-loss organic-based ferrimagnet V[TCNE]x∼2

We demonstrate indirect electric-field control of ferromagnetic resonance (FMR) in devices that integrate the low-loss, molecule-based, room-temperature ferrimagnet vanadium tetracyanoethylene (V[TCNE]x∼2) mechanically coupled to PMN-PT piezoelectric transducers. Upon straining the V[TCNE]x films, the FMR frequency is tuned by more than 6 times the resonant linewidth with no change in Gilbert damping for samples with α = 6.5 × 10−5. We show this tuning effect is due to a strain-dependent magnetic anisotropy in the films and find the magnetoelastic coefficient |λs| ∼ (1–4.4) ppm, backed by theoretical predictions from density-functional theory calculations and magnetoelastic theory. Noting the rapidly expanding application space for strain-tuned FMR, we define a new metric for magnetostrictive materials, magnetostrictive agility, given by the ratio of the magnetoelastic coefficient to the FMR linewidth. This agility allows for a direct comparison between magnetostrictive materials in terms of their comparative efficacy for magnetoelectric applications requiring ultra-low loss magnetic resonance modulated by strain. With this metric, we show V[TCNE]x is competitive with other magnetostrictive materials, including YIG and Terfenol-D. This combination of ultra-narrow linewidth and magnetostriction, in a system that can be directly integrated into functional devices without requiring heterogeneous integration in a thin film geometry, promises unprecedented functionality for electric-field tuned microwave devices ranging from low-power, compact filters and circulators to emerging applications in quantum information science and technology.

Effect of annealing temperature on crystallographic texture, magnetic and microwave properties of barium ferrite thin films

Due to it has large saturation magnetization (Ms), high magnetocrystalline anisotropy field and high ferromagnetic resonance (FMR) frequency, barium ferrite (BaM) has attracted more and more attentions in the fields of magnetic recording media, permanent magnets and microwave devices. Here, BaM thin films were deposited on Pt/TiO2/SiO2/Si substrates, and the effect of annealing temperature on the microstructure, magnetic and microwave properties of barium ferrite thin films was investigated in detail. It is found that when the BaM thin film was annealed at 1035 °C, it has good properties. The XRD data provide clear evidence that the BaM thin films have high c-axis orientation, and the Lotgering factor is as high as 0.96. The AFM morphology show that BaM grains are out of the film plane, and they are hexagonal. The magnetic hysteresis curves indicated that both saturated magnetization (4πMs), remanence ratio and coercive (Hc) for out of plane increase with increasing Ta first, then decreased, and get the maximum value at 1035 °C. The ferromagnetic resonance (FMR) measurement show that the FMR linewidth is 143 Oe@50 GHz, it means that this this sample has low microwave loss in millimeter wave loss, and the FMR absorption can be tuned by applied magnetic field. These results make sure that this BaM thin film is possible use in millimeter wave devices such as filers, circulators and isolators.