Ferromagnetic Resonance Frequency Research Articles

Frequency fluctuations of ferromagnetic resonances at millikelvin temperatures

Unwanted fluctuations over time, in short, noise, are usually detrimental to device performance, especially for quantum coherent circuits. Recent efforts have demonstrated routes to utilizing magnon systems for quantum technologies by interfacing single magnons with superconducting qubits. However, the coupling of several components often introduces additional noise to the system, hence degrading its coherence. Researching the system's temporal behavior can help us to identify the underlying noise sources, which is a vital step toward improving coherence times and hybrid device performance. Yet, the frequency noise of the ferromagnetic resonance (FMR) has so far been unexplored at mK-temperatures. Here, we investigate such FMR frequency fluctuations of a yttrium-iron-garnet (YIG) sphere and find that these fluctuations are independent of temperature and drive power. This suggests that the measured frequency noise in YIG is dominated by undetermined noise sources, whose properties are not consistent with the conventional model of two-level systems, despite their effect on the sample linewidth. Moreover, the functional form of the FMR frequency noise power spectral density (PSD) cannot be described by a simple power law. By employing time-series analysis, we find a closed function for the PSD that fits the observations. Our results underline the necessity of coherence improvements to magnon systems for useful applications in quantum magnonics.

Microstructure and magnetic properties of FeCoHfN thin films deposited by DC reactive sputtering

The microstructure and electro-magnetic properties of FeCoHfN thin films deposited by reactive sputtering under an in-situ magnetic field have been studied as a function of nitrogen partial pressure (R(N2)). As the (R(N2)) increases, the grain size of α-FeCo phase can be effectively reduced, and the structure of the films changes from an amorphous/nanocrystalline dual-phase structure with ultrafine α-FeCo magnetic nanoparticles isolated by a nonmagnetic insulating layer to a single amorphous structure. The structural transformation and the enrichment of HfN layer improve the resistivity and static and dynamic soft magnetic properties of the films. Higher resistivity is expected to reduce eddy current losses at high frequencies. The critical (R(N2)) when the film shows indications of excellent high-frequency magnetic properties is 8%. The deposited FeCoHfN thin films have high saturation magnetization of 14.9–16.2 kGs, great permeability of 222–322 at GHz, and high ferromagnetic resonance frequency up to 3.5 GHz, showing the promise of high-frequency applications.

(Invited) Development of Anisotropic Magnetic Nanocomposites for Miniaturized RF Device Applications

The miniaturization of radio frequency (RF) components has become increasingly in-demand due to the popularity of mobile devices, such as smartphones, tablets, and other ad-hoc networking products. Many state-of-the-art RF components, due to the typical use of ferrite materials and a bulky permanent magnet that magnetizes the ferrites, are too cumbersome to be used in fully integrated monolithic microwave integrated circuits (MMICs). Thus, passive RF components that do not require external magnetic fields are needed.Here we present a class of anisotropic magnetic nanocomposites (AMNs) that are capable of working as the magnetic layer within passive RF components. Our AMNs are based on high aspect ratio (>1000) magnetic nanowires and functional matrices, and show a square shaped easy-axis M(H) loop, a large remanent magnetization, and a tunable ferromagnetic resonant frequency. With the AMNs, miniaturized passive circulators were prototyped. In this presentation, we will discuss the synthesis of AMNs and the tailoring of the AMNs magnetic and RF properties to improve circulator performance.This research was developed with funding from the Defense Advanced Research Projects Agency (DARPA). The views, opinions and/or findings expressed are those of the author and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government.Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.Distribution Statement A - Approved for Public Release, Distribution Unlimited.

Collective magnetic and plasma excitations in Josephson ψ junctions

We show that Josephson $\ensuremath{\psi}$ junctions with a half-metallic (HM) weak link coupled to superconducting (S) electrodes through ferromagnetic (F) layers host collective excitations of the magnetic moment and the Josephson phase. This results in a shift of the ferromagnetic resonance frequency, anomalies in the current-voltage characteristics, and the appearance of additional magnetic anisotropy in the F layers. In contrast to previously studied S/F/S junctions, coupling between magnetic and plasma modes emerges even in the long wavelength limit. Such coupling is shown to enable controllable magnetization reversal in the F layer governed by a dc current pulse, which provides an effective mechanism for the magnetic moment manipulation in the devices of superconducting spintronics.

A Ferromagnetic Resonance Temperature Measurement Method Based on Sweep Frequency Technique

The ferromagnetic resonance frequency of the ferromagnetic nanoparticles has a strong temperature dependency. The frequency sweep method is a standard method to measure frequency accurately in the available technology. Based on the free energy of the spin system of single-domain ferromagnetic nanoparticles with uniaxial anisotropy, we establish a relationship model between ferromagnetic resonance frequency and temperature under the ferromagnetic resonance condition. And this model is simulated by the frequency sweep method in the temperature range of 0–60 °C, which proves that it is practicable.

Screen-Printable, Self-Biased SrM/PDMS Composites for Integrated Magnetic Microwave Devices

This work introduces a material and method for high-volume, low-temperature, screen printing of self-biased magnetic films made from a composite of M-type strontium ferrite SrFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sub> (SrM) particles and polydimethylsiloxane (PDMS). The magnetic particles are aligned under a dc magnetic field, while the PDMS is cured at 75 °C, making this fabrication process compatible with semiconductor and printed-circuit-board substrates. Aligned composites with estimated SrM particle volume percentages of 14 vol.%, 20 vol.%, and 27 vol.% are fabricated, as well as an unaligned 14 vol.% composite for comparison. X-ray diffraction (XRD) and scanning electron microscopy (SEM) are used to examine the structure and composition of the composites. Furthermore, vibrating sample magnetometer and ferromagnetic resonance (FMR) measurements are performed to determine their dc and ac magnetic properties. Results show that the aligned composites have magnetic textures between 1.47 and 1.95, crystalline anisotropies between 1860 and 1960 kA/m, FMR frequencies of 45.5–45.9 GHz, and FMR linewidths between 3400 and 3500 Oe, indicating the potential for these materials in self-biased, non-reciprocal device applications. The analysis identifies that increased particle loading and better particle alignment may further improve these films by decreasing the linewidth.

Collimated Bidirectional Propagating Spin Wave Generated by a Nonlocal Spin-Current Nano-oscillator

Pure spin currents, generated by nonlocal spin injection and spin-orbit effects, have been widely used to control magnetization reversal and dynamics by current without charge transfer and its side effects in the device's actual working region. Here we experimentally demonstrate that a single coherent spin-wave mode can be excited by the pure spin current in a nonlocal spin-injection spin-valve device. The microwave spectra show that the observed spin-wave frequency is higher than the ferromagnetic resonance frequency, and almost does not change with the excitation current at moderate magnetic fields, indicating that the observed dynamical mode is a linear propagating spin-wave mode. Furthermore, micromagnetic simulations based on our device geometry generally reproduce the experimentally observed field-dependent and current-dependent oscillation characteristics, and provide us with the additional spatial information that the spin-wave mode exhibits collimated and bidirectional propagation paths in the direction perpendicular to the applied magnetic field. Our simulation results also show that the current-local Oersted field in our nonlocal device is much smaller than in conventional nanocontact magnetic oscillators, and has a minimal impact on the spin-wave dynamics. A near-symmetrically-collimated and bidirectional propagating spin-wave beam with magnetic field--controllable beam direction and current-independent frequency, achieved in our demonstrated nonlocal spin-current nano-oscillator, can be used as a local spin-wave source for magnonic logic devices and can be used to build daisy-chaining oscillatory neural networks with mutual synchronization.

Robust finite-temperature magnetization dynamics in ferrimagnetic Gdx(FeCo)1−x (x = 0 ∼ 0.44) nanospheres across angular-momentum and magnetization compensation points: An atomistic model simulation

We explored robust magnetization dynamics in ferrimagnetic Gdx(FeCo)1−x nanospheres of Gd compositions ranging from x = 0 to 0.44 at finite T = 0 – 1200 K temperatures using two-sublattice atomistic simulations based on the stochastic Landau-Lifshitz-Gilbert equation. It was found that the magnetization compensation and angular-momentum compensation temperatures vary remarkably with x and that the former temperature is always lower than the latter. Also, it was found that neither compensation temperature exists below a specific Gd composition of x = 0.22, but above which, both exist and increase with x. The ferromagnetic resonance (FMR) frequency remarkably increases at the angular momentum compensation composition, whereas the FMR frequency does not reach zero at the magnetization compensation composition. The significant difference in the FMR frequency between the two characteristic compensation points is ascribed to the contrasting effective gyromagnetic ratios of Gd and FeCo. Our present atomistic simulation study well reproduced the earlier experimental observations of such dynamic behaviors at both the angular-momentum and magnetization compensation points, which observations cannot be explained by the existing Kittel and Wangsness models. This work provides a better understanding of robust dynamic behaviors of ferrimagnetic nanospheres as functions of temperature and constituent sublattice composition, especially at and near the characteristic compensation points.

Synthesis, Characterization, and Electromagnetic Wave Absorbing Properties of M12+–M24+ Substituted M-Type Sr-Hexaferrites

Mn–Ti, Zn–Ti, Zn–Zr substituted M-type Sr-hexaferrites (SrM), SrFe12−2xM1xM2xO19 (0 ≤ x ≤ 2.0, M1 = Mn or Zn; M2 = Ti or Zr) were synthesized, and their solubility, crystalline structure, and high-frequency properties were studied. Zn–Zr substitution caused a relatively large lattice parameter change and resulted in lower solubility (x ≤ 1.0) in the M-type phase compared with Mn–Ti and Zn–Ti substitutions. However, the ferromagnetic resonance frequency (fFMR) effectively decreased with increasing x in SrFe12−2xZnxZrxO19 (Zn–Zr:SrM) (0 ≤ x ≤ 1.0) and the electromagnetic wave (EM) absorption frequency also varied according to the shift in fFMR in the 7–18 GHz range. This is attributed to a gradual decrease in the magnetocrystalline anisotropy of Zn–Zr:SrM (0 ≤ x ≤ 1.0) with an increase in x. Zn–Zr:SrM (x = 0.9)–epoxy(10 wt%) composites exhibited a high EM absorption in the X-band (8–12 GHz) with the lowest reflection loss of &lt;−45 dB. The sample with x = 0.8 showed a broad Ku band (12–18 GHz) absorption performance satisfying RL &lt;−19 dB at 11 ≤ f ≤ 18 GHz.

Computational and experimental study on cation distribution of Cobalt substituted barium hexaferrites BaFe12−xCoxO19 (x = 0, 0.3, 0.6, 0.9) for circulator applications

The anisotropic BaFe12−xCoxO19 (x = 0, 0.3, 0.6, 0.9) (BFCO) samples were synthesized. We investigated the crystal structure, microstructure, and magnetic properties of BFCO. And the potential application value at millimeter waveband was discussed. The magnetic anisotropy field is successfully decrease from 17 kOe to 10 kOe by substituting Cobalt, which lower the Ferromagnetic resonance frequency and benefit for extending the bandwidth for the circulator. With the cobalt substitution, the saturation inevitably decreases from 60 emu/g to 45 emu/g. The site preference of cobalt in five crystallographically inequivalent Fe ions was calculated using software castep. The influence of site preference on lattice parameters and magnetic saturation was calculated and discussed. The result show cobalt mainly sites 2a and 12k position.

Giant demagnetization effects induced by superconducting films

We show that a ferromagnetic (F) slab with the in-plane magnetization sandwiched between two superconducting (S) films experiences strong demagnetization effect due to the Meissner screening of the stray magnetic field by superconductors. In the extreme case, the transition of the S film from normal to the superconducting state can switch the demagnetization factor from 0 to 1, which is in a sharp contrast with the S/F bilayers where such transition affects the magnetic field inside the F film only slightly. The giant demagnetization effect is shown to be qualitatively robust against the decreasing superconducting film thickness and may provide a hint toward the explanation of the anomalously large ferromagnetic resonance frequency shift recently observed for the S/F/S structures [Golovchanskiy et al., Phys. Rev. Appl. 14, 024086 (2020)].

Tuning of electromagnetic wave absorbing properties in Fe-deficient SrFe9.6-xCo1.2Ti1.2O19 hexaferrite-epoxy composites

We report the tunable electromagnetic (EM) wave absorption properties of Fe-deficient SrFe9.6-xCo1.2Ti1.2O19 hexaferrite–epoxy composites. SrFe9.6-xCo1.2Ti1.2O19 hexaferrite powders were prepared via solid-state reaction routes. It was observed that Sr–Ti-rich second phases were formed as x increased, i.e., the Fe content decreased. The ferromagnetic resonance (FMR) frequency of the composites gradually decreased from 8.8 GHz to 4.8 GHz with increasing x, and accordingly, the EM absorption frequency range also gradually changed. The gradual FMR frequency shift was attributed to the compositional shift in the mother phase. It is predicted that the Fe deficiency caused a decrease in the magnetocrystalline anisotropy, and in turn, it shifted the FMR frequency and modified the corresponding EM absorbing properties. All the samples demonstrated a high EM absorption performance with the lowest reflection loss of < −40 dB at the optimized frequency and thickness.

Optical Frequency Up-Conversion of the Ferromagnetic Resonance in an Ultrathin Garnet Mediated by Magnetoelastic Coupling

We perform ultrafast pump-probe measurements on a nanometer-thick crystalline Bi-doped yttrium iron garnet film with perpendicular magnetic anisotropy. Tuning the photon energy of the pump laser pulses above and below the material's band gap, we trigger ultrafast optical and spin dynamics via both one- and two-photon absorption. Contrary to the common scenario, the optically induced excitation induces an increase up to 20% of the ferromagnetic resonance frequency of the material. We explain this unexpected result in terms of a modification of the magnetic anisotropy caused by a long-lived photo-induced strain, which transiently and reversibly modifies the magnetoelastic coupling in the material. Our results disclose the possibility to optically increase the magnetic eigenfrequency in nanometer-thick magnets.

Photoinduced nonlinear magnetoelectric effect detection in Zn2Y hexaferrite

The photoinduced nonlinear magnetoelectric effect was investigated in the single-crystal Y-type Ba2Zn2Fe12O22 (Zn2Y) hexaferrite under ultraviolet radiation. We observed a downshift of the ferromagnetic resonance frequency caused by decreasing the effective saturation magnetization. The maximum observed shift of the resonance frequency was equal to 110 MHz corresponding to effective saturation magnetization decrease by 256 G. A strong nonlinear change in effective saturation magnetization was observed as the result of the photoinduced nonlinear magnetoelectric effect. The strength of the photoinduced nonlinear magnetoelectric effect was found to be up to 85% higher compared to the strength of the nonlinear magnetoelectric effect. The results demonstrate the possibility of optical tuning of the ferromagnetic resonance frequency and the mechanism of using the single-crystal Zn2Y for radiation detection.

Theoretical investigation of ferromagnetic resonance in a ferromagnetic thin film with external stress anisotropy

We use the ferromagnetic resonance (FMR) method to study the properties of ferromagnetic thin film, in which external stress anisotropy, fourfold anisotropy and uniaxial anisotropy are considered. The analytical expressions of FMR frequency, linewidth and the imaginary part of magnetic susceptibility are obtained. Our results reveal that the FMR frequency and the imaginary part of magnetic susceptibility are distinctly enhanced, and the frequency linewidth or field linewidth are broadened due to a strong external stress anisotropy field. The hard-axis and easy-axis components of magnetization can be tuned significantly by controlling the intensity and direction of stress and the in-plane uniaxial anisotropy field.

A quantum material spintronic resonator

In a spintronic resonator a radio-frequency signal excites spin dynamics that can be detected by the spin-diode effect. Such resonators are generally based on ferromagnetic metals and their responses to spin torques. New and richer functionalities can potentially be achieved with quantum materials, specifically with transition metal oxides that have phase transitions that can endow a spintronic resonator with hysteresis and memory. Here we present the spin torque ferromagnetic resonance characteristics of a hybrid metal-insulator-transition oxide/ ferromagnetic metal nanoconstriction. Our samples incorporate {mathrm {V}}_2{mathrm {O}}_3, with Ni, Permalloy ({hbox {Ni}}_{80}{hbox {Fe}}_{20}) and Pt layers patterned into a nanoconstriction geometry. The first order phase transition in {mathrm {V}}_2{mathrm {O}}_3 is shown to lead to systematic changes in the resonance response and hysteretic current control of the ferromagnetic resonance frequency. Further, the output signal can be systematically varied by locally changing the state of the {mathrm {V}}_2{mathrm {O}}_3 with a dc current. These results demonstrate new spintronic resonator functionalities of interest for neuromorphic computing.

Electromagnetic wave absorbing properties of SrFe12-2xCoxTixO19 hexaferrite–CNT–epoxy composites

A wideband electromagnetic (EM) wave absorber for the X-band (8–12 GHz) is designed using M-type hexaferrite, SrFe12-2xCoxTixO19 (x = 1.1, 1.2, 1.25, and 1.3)–CNT (0–3 wt%)–epoxy (10 wt%) composites. As x increases from 1.1 to 1.3, the ferromagnetic resonance (FMR) of the hexaferrite powder gradually decreases from 15.0 GHz to 6.4 GHz due to the decrease of magnetocrytalline anisotropy. The EM absorption frequency range changes in accordance with changes in the FMR frequency. Among the series of samples, the x = 1.25 sample demonstrates EM absorption properties optimized for the X-band with the lowest reflection loss (RL) of −40 dB and satisfying RL <−10 dB in the 7–13 GHz range. When 1 wt% carbon nanotubes (CNTs) are added to the composite (x = 1.25), permittivities ε′ and ε″ increased significantly owing to the CNT particle’s high conductivity, and the intensive EM absorbing region expanded to a wide frequency–thickness range. A further increase in the CNT content (2–3 wt%) weakened the overall EM absorption due to an increase in reflection. Adding 1–2 wt% of CNT to hexaferrite is very effective because it not only increases the shielding effectiveness, but also yields better EM absorption at lower thicknesses of 1.5 mm or less.

Stretching-Tunable High-Frequency Magnetic Properties of Wrinkled CoFeB Films Grown on PDMS

We demonstrated a convenient method via applying uniaxial tensile strains to continuously tune the high-frequency properties of flexible magnetic films. CoFeB films were magnetron sputtered onto prestretched polydimethylsiloxane (PDMS) membranes. They exhibit a self-assembled periodic wrinkling surface structure because of the large mismatch of Young's moduli between the elastomeric PDMS substrates and the metal layers. The wrinkling morphology and the residual tensile stress caused by the Poisson effect can be continuously tuned by a uniaxial stretching strain less than the growth prestrain, which consequently results in changes in high-frequency performance. The initial permeability and the ferromagnetic resonance frequency of flexible CoFeB thin films can be monotonously tuned in wide ranges of about hundreds and 1 GHz, respectively. A good repeatability over thousands of stretching-relaxing cycles has been demonstrated without any obvious reduced high-frequency properties. This flexible CoFeB films with excellent stretching-tunable high-frequency performances are promising for application in flexible and tunable microwave devices.

Isotropic FMR frequency enhancement in thin Py/FeMn bilayers under strong magnetic proximity effect

Exchange biasing in ferromagnet/antiferromagnet bilayers is known to enhance the material’s ferromagnetic resonance frequency and make it strongly angle dependent due to the unidirectional anisotropy induced at the interface. We observe a ten-fold enhancement in frequency and angle-independent ferromagnetic resonance in bilayers of Py/FeMn with ultrathin FeMn, accompanied by a significantly enhanced magnetic moment. The observed isotropic frequency enhancement is consistent with rotatable rather than unidirectional magnetic anisotropy and the induced magnetic moment links this anisotropy with the ferromagnet-proximity effect. The estimated effective anisotropy field acting on the proximity-induced moment in ultrathin FeMn can be as high as 0.5 T at room temperature. Our results show the potential of the ferromagnetic proximity effect combined with the inherent exchange anisotropy in antiferromagnets for high-speed spintronic applications.

Energy-efficient spin injector into semiconductors driven by elastic waves

Generation of spin imbalance in nonmagnetic semiconductors is crucial for the functioning of many spintronic devices. An attractive design of spin injectors into semiconductors is based on a spin pumping from a precessing ferromagnet, typically excited by a microwave magnetic field leading to a high power consumption of the device. Here we describe theoretically a spin injector with greatly reduced energy losses, in which the magnetic dynamics is excited by an elastic wave injected into a ferromagnet-semiconductor heterostructure. To demonstrate the efficient functioning of such an injector, we perform micromagnetoelastic simulations of the coupled elastic and magnetic dynamics in Ni films and Ni/GaAs bilayers. For thick Ni films, it is shown that a monochromatic acoustic wave generates a spin wave with the same frequency and wavelength, which propagates over distances of several micrometers at the excitation frequencies close to the frequency of ferromagnetic resonance. The simulations of Ni/GaAs bilayers with Ni thicknesses comparable to the wavelength of the injected acoustic wave demonstrate the development of a steady-state magnetization precession at the Ni/GaAs interface. The amplitude of such a precession has a maximum at Ni thickness amounting to three quarters of the wavelength of the elastic wave, which is explained by an analytical model. Using simulation data obtained for the magnetization precession at the Ni/GaAs interface, we evaluate the spin current pumped into GaAs and calculate the spin accumulation in it by solving the spin diffusion equation. Then the electrical signals resulting from the spin flow and the inverse spin Hall effect are determined via the numerical solution of the Laplace's equation. It is shown that amplitudes of these ac signals are large enough for experimental measurement, which indicates an efficient acoustically driven spin pumping into GaAs.