Ferromagnetic Resonance Line Shape Research Articles

Spin-torque ferromagnetic resonance based on current-induced impedance

Spin-torque ferromagnetic resonance (ST-FMR) has been widely used for measuring damping-like spin–orbit torques in magnetic bilayers. Typically, the ratio between the damping-like and field-like spin–orbit torques are extrapolated based on the ferromagnetic resonance line shapes. However, when the field-like spin–orbit torque is unknown, the line shape analysis may lead to errors in extrapolating the damping-like spin–orbit torque. Here, we propose a modified version of the ST-FMR that allows extrapolation of both damping-like and field-like torques independently. By introducing an alternating current to the sample, the RF impedance is modulated, allowing detection via the reflected microwave. We show that the extrapolated field-like and damping-like torques in Py/Pt samples are consistent with the technique measuring current-induced linewidth and resonance field change but have much better signal-to-noise ratio. Our proposed method paves a way for more accurate measurement of spin–orbit torques.

Microstructure, ferromagnetic resonance, and electrical analysis of Ho-substituted Li-Ni nanoparticles

The substitution of rare earth ions is a novel approach to vary the dynamic magnetic properties of soft ferrites to such an extent that its electrical properties and ferromagnetic resonance (FMR) response can be set over a broad frequency range. In this work, a complete transmission electron microscopy (TEM), scanning electron microscopy (SEM), ferromagnetic resonance spectroscopy (FMR), X-ray photoelectron spectroscopy (XPS), and current-voltage (I-V) analysis of holmium (Ho) substituted lithium-nickel soft ferrites with composition Li1.2Ni0.4Fe2−xHoxO4 (x = 0, 0.03, 0.06, 0.09, 0.12, and 0.15) is carried out. TEM analysis reveals a spherical shape distribution of nanoparticles with a particle size of ∼ 50 nm. FMR study is carried out to check the role of Ho addition on magnetic energy losses, FMR line position, and shape. The shift in FMR line shape is mainly attributed to the anisotropy contribution, relaxation processes, and ferromagnetic interactions between the magnetic nanoparticles. XPS experiments confirm the presence of all compositional elements along their valence states. The Ho substitution increased the electrical resistivity, whereas the damping of A-B exchange interactions resulted in a decline of Curie temperature (Tc), which is required for high-frequency power applications.

Interface-driven spin pumping and inverse Rashba-Edelstein effect in FeGaB/Ag/BiSb multilayers

Spin-orbit-coupling (SOC) plays a critical role in spin-to-charge conversion (SCC) mechanism. The SCC process is usually investigated by injecting a spin current from ferromagnet (FM) into a large SOC normal metal (SOC-NM) and then measuring the voltage generated by inverse spin Hall effect (ISHE). Recently, a new mechanism of SCC at inversion symmetry breaking interfaces between topological insulators (TIs) and magnetically ordered materials has been introduced, which is driven by the inverse Rashba-Edelstein effect (IREE). Motivated by this result, we have investigated the IREE effect of a simple binary three-dimensional TI Bi85Sb15 (BiSb). The model system used here is a trilayer of FeGaB/Ag/BiSb with different Ag thickness made insitu with a combination of dc magnetron sputtering and pulsed laser ablation. The SCC efficiency has been evaluated by measuring ferromagnetic resonance (FMR) driven ISHE voltage and extraction of the Gilbert damping (α) and interfacial spin mixing conductance (g↑↓) from the FMR line shape analysis. It is interesting to note that the charge current (Ic) increases with Ag thickness. This observation indicated that the IREE process at the Ag/BiSb interface dominates over ISHE in the bulk of BiSb film. The conversion efficiency of IREE is determined by calculating the IREE length (λIREE), which increases with the thickness of the Ag layer. These findings offer a method to generate and detect the IREE in a room temperature deposited heterostructure using simple coplanar waveguide setup.

Two-magnon frequency-pulling effect in ferromagnetic resonance

We report the experimental observation in thin films of the hybridization of the uniform ferromagnetic resonance mode with nonuniform magnons as a result of the two-magnon scattering mechanism, leading to a frequency-pulling effect on the ferromagnetic resonance. This effect, when not properly accounted for, leads to a discrepancy in the dependence of the ferromagnetic resonance field on frequency for different field orientations. The frequency-pulling effect is the complement of the broadening of the ferromagnetic resonance line shape by two-magnon scattering and can be calculated using the same parameters. By accounting for the two-magnon frequency shifts through these means, consistency is achieved in fitting data from in-plane and perpendicular-to-plane resonance conditions.

Mechanisms of FMR line broadening in CoFeB-LiNbO3 granular films in the vicinity of metal-insulator transition

Metal-insulator (CoFeB)x(LiNbO3)100−x nanocomposite films with different content of the ferromagnetic (FM) phase x are investigated by ferromagnetic resonance (FMR) technique. A strong change of the FMR line shape is observed in the vicinity of metal-insulator transition (MIT) of the film, where the hopping-type conductivity σ modifies to the regime of a strong intergranular tunnelling, characterized by a logarithmic dependence σ(T) at high temperatures. It is shown that below MIT, the FMR linewidth is mainly determined by the inhomogeneous distribution of the local anisotropy axes in the film plane. Above MIT, the contribution of this inhomogeneity to the line broadening decreases. At the same time, two-magnon magnetic relaxation processes begin to play a significant role in the formation of the linewidth. The observed behaviour indicates the critical role of interparticle exchange in the tunnelling regime above MIT of the nanocomposite.

Integration of Both Invariable and Tunable Microwave Magnetisms in a Single Flexible La0.67Sr0.33MnO3 Thin Film.

High-quality flexible magnetic oxide thin films have promoted a wide range of potential applications in spintronic devices due to their unique physical properties. To obtain the optimized microwave magnetism for future all-oxide-based spintronic applications, high-quality oxide materials with excellent epitaxial quality as well as specific bending properties related to ferromagnetic resonance are high in demand. Here, (001)-oriented La0.67Sr0.33MnO3 epitaxial thin films with different thicknesses have been grown and subsequently transferred onto flexible poly(dimethylsiloxane) substrates. The microwave magnetisms of these film samples have been investigated under various bending states. Under bending, the ferromagnetic resonance lineshape of the film gradually transits from a single mode to a superposition of multimodes, possibly because of the uneven distribution of magnetization in the bending film at X-band. This phenomenon is more apparent when the direction of the applied magnetic field goes close to the out-of-plane of the film. Hence, an integration of invariable and continuous tuning of ferromagnetic resonance field under various mechanical bending can be achieved in one same sample by just tuning the direction of the applied magnetic field, which reveals that the flexible La0.67Sr0.33MnO3 thin films have huge potential in the applications in future flexible multifunctional devices.

Microwave magnetoimpedance and ferromagnetic resonance in Pr0.6Sr0.4MnO3.

We report the magnetic field dependence of electrical impedance (magnetoimpedance) of a ferromagnetic Pr0.6Sr0.4MnO3 sample carrying alternating current (ac) of frequency f = 1 MHz to 3 GHz measured using an impedance analyzer and broad band ferromagnetic resonance (f = 2 to 18 GHz) measured using a coplanar wave guide based spectrometer. Ac magnetoresistance is much larger than dc magnetoresistance and its sign at low magnetic fields changes from negative to positive with increasing frequency of the ac current. The field dependence of ac magnetoresistance shows a peak around Hdc = 0 for low frequencies but a double peak feature emerges at Hdc = ±Hp at higher frequencies and it shifts to higher magnetic field as the frequency of ac current increases. The field derivative of microwave power absorption measured by the broad band spectrometer shows features of ferromagnetic resonance and the resonance field increases with increasing frequency of microwave radiation following Kittel's equation for ferromagnetic resonance. A close correlation is found between the ferromagnetic resonance line shape and the positive peak in the ac magnetoresistance, which suggests the possibility of electrical detection of ferromagnetic resonance using high frequency current injected into a conducting magnetic sample.

Ferromagnetic resonance of CoFeB/Ta/CoFeB spin valves versus CoFeB film

In MgO/CoFeB/MgO/Ta (single layer) and MgO/CoFeB/Ta/CoFeB/MgO/Ta spin valve (bilayer), the orientation dependences of ferromagnetic resonance (FMR) have been analyzed. Magnetic anisotropy constants as well as damping constants have been extracted. The differences between the anisotropy constants in single-layer film and spin valve MgO/CoFeB/Ta/CoFeB/MgO/Ta are explained by dipole magnetic interaction between ferromagnetic layers. The main part of the linewidth corresponds to inhomogeneous widening, while smaller part is provided by interface exchange fluctuations. The FMR lineshape can be explained by standard FMR Lorenz line and nonresonant absorption provided by inverse spin Hall effect.

Temperature dependence of electric field tunable ferromagnetic resonance lineshape in multiferroic heterostructure

Herein, we experimentally investigate the effect of temperature on the electric field tunable ferromagnetic resonance (FMR) in a ferroelectric/ferromagnetic heterostructure, and demonstrate the tuning of abnormal change in FMR using the polarization of the ferroelectric layer above 200 K. The FMR was found to be almost unchanged under different electric field strength at 100 K owing to frozen polarization, which causes extremely weak magnetoelectric coupling. More interestingly, negative effective linewidth was observed when an electric field greater than 10 kV/cm was applied above 220 K. The simultaneous electrical control of magnetization and its damping via FMR based on linear magnetoelectric coupling are directly relevant to use of composite multiferroics for a wide range of devices.

Observation of thermal spin-transfer torque via ferromagnetic resonance in magnetic tunnel junctions

The thermal spin-transfer torque (TSTT) in magnetic tunneling junctions (MTJs) was systematically studied using electrical detection of ferromagnetic resonance (FMR). Evidence for the existence of TSTT in MTJs is observed. A temperature difference was applied across an MTJ acting as a TSTT on the free layer of the MTJ. The FMR of the free layer was then excited by a microwave current and electrically detected as a dc voltage. We found that the FMR line shape was changed by the TSTT, indicated by the ratio of dispersive and Lorentz components of the FMR spectra (D/L). D/L increases by increasing the temperature difference. In addition, we analyze the magnetization orientation dependence of TSTT and provide solid evidence that this dependence differs from the magnetization orientation dependence of spin-transfer torque driven by a dc bias.

Eddy current interactions in a ferromagnet-normal metal bilayer structure, and its impact on ferromagnetic resonance lineshapes

We investigate the effect of eddy currents on ferromagnetic resonance (FMR) in ferromagnet-normal metal (FM/NM) bilayer structures. Eddy-current effects are usually neglected for NM layer thicknesses below the microwave (MW) skin depth (≃800 nm for Au at 10 GHz). However, we show that in much thinner NM layers (10–100 nm of Au or Cu) they induce a phase shift in the FMR excitation when the MW driving field has a component perpendicular to the sample plane. This results in a strong asymmetry of the measured absorption lines. In contrast to typical eddy-current effects, the asymmetry is larger for thinner NM layers and is tunable through changing the sample geometry and the NM layer thickness.

Ferromagnetic Resonance Line Shapes in Permalloy Strips at Low Magnetic Fields

Spin rectification line shapes for ferromagnetic resonance in swept, low external bias fields differ significantly from widely investigated high-field resonant spectra. The anomaly in the low-field case may originate from magnetization rotation associated with magnetic anisotropy and demagnetization. The low-field, rectified voltage spectra are also sensitive to the relative phase and polarization of the microwave field. Ferromagnetic thin strips thus have the potential for use as polarization- and phase-resolved microwave detectors at low fields.

Angle sensing with ferromagnetic nanowire arrays

Hysteresis loops and Ferromagnetic Resonance (FMR) linewidths of Nickel ferromagnetic nanowire arrays are measured versus angle θH between the applied magnetic field angle and the common nanowire axis. Using Preisach analysis, we extract from the hysteresis loop an interaction parameter σi that strongly depends on θH. Extending the analysis to FMR lineshapes, we deduce a strong dependence of the FMR field linewidth ΔH on θH through the interaction parameter σi. Existence of a link between static (hysteresis) and dynamic (FMR) cases through θH might be exploited in contactless absolute angle sensing devices that could compete with inductive, Hall, and magnetoresistive devices.

Ferromagnetic Resonance Studies of Exchange Biased CoO/Fe Bilayer Grown on MgO Substrate

Dynamic and static magnetizations of an exchange biased bilayer system which is constructed as a proximity of a CoO layer on an Fe-layer grown on the (100) oriented MgO substrate by ion beam sputtering technique have been investigated by ferromagnetic resonance (FMR) and vibrating sample magnetometry (VSM) techniques. The room-temperature FMR measurements reveal that the Fe layer is epitaxially grown on MgO substrate with four-fold magnetocrystalline anisotropy and the hard magnetization axis of the sample is the [100] crystallographic directions of MgO substrate. We have determined the g-value, effective magnetization, magnetocrystalline anisotropy constants and contributions to FMR linewidth due to the intrinsic Gilbert damping and inhomogeneity of magnetization by using Landau–Lifshitz–Gilbert (LLG) equation. We observed an unusual FMR line shape attributed to impedance switching of resonance cavity and complex component of conductivity of sample system. The low-temperature FMR measurement shows asymmetric hysteretic behavior of resonance field related to magnetic coupling of ferromagnetic and antiferromagnetic layers. From both FMR and VSM measurements between 10–300 K, the magnetocrystalline anisotropy is observed to dominate above blocking temperature, while unidirectional anisotropy is observed to dominate below blocking temperature over internal magnetic anisotropy. FMR spectra have a comparatively small linewidth between 40–100 Oe, which indicates to a high crystallinity of the Fe film. Gilbert constant was calculated as 0.007 from the linewidth fitting of FMR spectra. This small value is a suitable for reducing the critical switching current used in magnetic tunneling junction. Detailed exchange bias studies were carried out for hard and easy axis of the sample in the temperature range of 10–300 K. From both low-temperature FMR and VSM measurements, the blocking temperature of the system was determined as ∼60 K.

Universal Method for Separating Spin Pumping from Spin Rectification Voltage of Ferromagnetic Resonance

We develop a method for universally resolving the important issue of separating spin pumping from spin rectification signals in bilayer spintronics devices. This method is based on the characteristic distinction of spin pumping and spin rectification, as revealed in their different angular and field symmetries. It applies generally for analyzing charge voltages in bilayers induced by the ferromagnetic resonance (FMR), independent of FMR line shape. Hence, it solves the outstanding problem that device-specific microwave properties restrict the universal quantification of the spin Hall angle in bilayer devices via FMR experiments. Furthermore, it paves the way for directly measuring the nonlinear evolution of spin current generated by spin pumping. The spin Hall angle in a Py/Pt bilayer is thereby directly measured as 0.021±0.015 up to a large precession cone angle of about 20°.

Distinguishing spin pumping from spin rectification in a Pt/Py bilayer through angle dependent line shape analysis

A pure spin current driven by spin pumping is converted to a DC voltage and detected electrically in a Py/Pt bilayer sample. This DC voltage mixes with a DC voltage produced through spin rectification. The ferromagnetic resonance line shape strongly depends on the microwave magnetic h field distribution. We have systematically studied the line shapes by changing the external magnetic field orientation in plane of a Pt/Py bilayer. A method is demonstrated which allows us to calculate the microwave h field vector distribution, and distinguish spin pumping from spin rectification.

Analysis of the line shape of electrically detected ferromagnetic resonance

This work reviews and examines two particular issues related with the new technique of electrical detection of ferromagnetic resonance (FMR). This powerful technique has been broadly applied for studying magnetization and spin dynamics over the past ten years. The first issue is the relation and distinction between different mechanisms that give rise to a photovoltage via FMR in spintronic devices, and the second is the proper analysis of the FMR line shape, which has become the ``Achilles heel'' in interpreting experimental results, especially for either studying the spin pumping effect or quantifying spin Hall angles via the electrically detected FMR.

FMR and TEM Studies of Co and Ni Nanoparticles Implanted in the SiO2 Matrix

Fused silica plates have been implanted with 40 keV Co+ or Ni+ ions to high doses in the range of (0.25–1.0) × 1017 ions/cm2, and magnetic properties of the implanted samples have been studied with ferromagnetic resonance (FMR) technique supplemented by transmission electron microscopy, electron diffraction and energy dispersive X-ray spectroscopy. The high-dose implantation with 3d-ions results in the formation of cobalt and nickel metal nanoparticles in the irradiated subsurface layer of the SiO2 matrix. Co and Ni nanocrystals with hexagonal close packing and face-centered cubic structures have a spherical shape and the sizes of 4–5 nm (for cobalt) and 6–14 nm (for nickel) in diameter. Room-temperature FMR signals from ensembles of Co and Ni nanoparticles implanted in the SiO2 matrix exhibit an out-of-plane uniaxial magnetic anisotropy that is typical for thin magnetic films. The dose and temperature dependences of FMR spectra have been analyzed using the Kittel formalism, and the effective magnetization and g-factor values have been obtained for Co- and Ni-implanted samples. Nonsymmetric FMR line shapes have been fitted by a sum of two symmetrical curves. The dependences of the magnetic parameters of each curve on the implantation dose and temperature are presented.

Solenopsis ant magnetic material: statistical and seasonal studies

In this paper, we quantify the magnetic material amount in Solenopsis ants using ferromagnetic resonance (FMR) at room temperature. We sampled S. interrupta workers from several morphologically indistinguishable castes. Twenty-five oriented samples of each body part of S. interrupta (20 units each) showed that FMR line shapes are reproducible. The relative magnetic material amount was 31 ± 12% (mean ± SD) in the antennae, 27 ± 13% in the head, 21 ± 12% in the thorax and 20 ± 10% in the abdomen. In order to measure variation in the magnetic material from late summer to early winter, ants were collected each month between March and July. The amount of magnetic material was greatest in all four body parts in March and least in all four body parts in June. In addition, S. richteri majors presented more magnetic material than minor workers. Extending these findings to the genera Solenopsis, the reduction in magnetic material found in winter could be explained by our sampling fewer foraging major ants.

Ferromagnetic resonance studies on colossal magnetoresistance films: Effects of homogeneity and light illumination

Ferromagnetic resonance (FMR) in the dependence on magnetic field orientation, and effect of the laser light on the magnetic ordering were studied in colossal magnetoresistance films, La1−xSrxMnO3 and La1−xBaxMnO3. The FMR linewidth increased with the decrease in temperature in La1−xSrxMnO3 films, and was nearly temperature independent in La1−xBaxMnO3, that was attributed to the different degree of magnetic homogeneity of the films. Layered structures La1−xBaxMnO3 on silicon with intermediate layer of SrTiO3 were characterized by FMR as highly homogeneous. From the orientation and temperature dependences of the FMR spectra, the effective magnetization, and effective anisotropy fields were estimated. The laser light illumination resulted in the change of FMR line shape and intensity, and was interpreted in terms of optically induced magnetic disordering and effective heating of the spin system.