Ferromagnetic Resonance Response Research Articles

Tailoring structural and magnetic properties of NiCu nanowires by electrodeposition

In this work, we investigated the tailoring of structural and magnetic properties of NiCu nanowires through electrodeposition. Continuous (S1) and composition-modulated (S2) wires were fabricated by electrodeposition using porous alumina membranes as a template. Morphological characterization revealed that the total length of the wires was 8 ± 3 µm in both S1 and S2. For the composition-modulated wires, the length of the segments with the lowest and highest Cu concentrations was 1.2 ± 0.4 µm and 226 ± 65 nm, respectively. Mapping by energy dispersive spectroscopy (EDS) revealed that the concentration of copper and nickel varied along the length of the composition-modulated nanowires, while the continuous nanowires contained a relatively constant concentration of both metals. It is demonstrated that the change in Cu concentration along the wire modifies the lattice parameter, average crystallite size (D) and lattice strain (ε) of Ni. This result is pivotal for understanding the magnetic properties of the wires, as nickel is primarily responsible for the magnetic behavior of the wires. From the ferromagnetic resonance (FMR) results, the linewidth and resonance field values for samples S1 and S2 were determined. It was demonstrated that the greater deformation in the nickel lattice in NiCu nanowires increases the angular dependence of the resonance field. Furthermore, the smaller nickel crystallite size was shown to increase spin dispersion and magnetic damping, leading to complex behavior in FMR responses. Finally, it was demonstrated how Cu can influence the magnetic properties such as coercivity (HC) and squareness (MR/MS) of the wires. Overall, this work contributes to understanding the tailoring of structural and magnetic properties of NiCu nanowires through electrodeposition.

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.

The Ferris ferromagnetic resonance technique: Principles and applications

Measurements of ferromagnetic resonance (FMR) are pivotal to modern magnetism and spintronics. Recently, we reported on the Ferris FMR technique, which relies on large-amplitude modulation of the externally applied magnetic field. It was shown to benefit from high sensitivity while being broadband. The Ferris FMR also expanded the resonance linewidth such that the sensitivity to spin currents was enhanced as well. Eventually, the spin Hall angle (θSH) was measurable even in wafer-level measurements that require low current densities to reduce the Joule heating. Despite the various advantages, analysis of the Ferris FMR response is limited to numerical modeling, where the linewidth depends on multiple factors such as the field modulation profile and the magnetization saturation. Here, we describe, in detail, the basic principles of operation of the Ferris FMR and discuss its applicability and engineering considerations. We demonstrated these principles in a measurement of the orbital Hall effect taking place in Cu using an Au layer as the orbital-to-spin current converter. This illustrates the potential of the Ferris FMR for the future development of spintronics technology.

Application of a Microfabricated Microwave Resonator in a Co - Pd –Based Magnetic Hydrogen-Gas Sensor

We investigate the ferromagnetic resonance (FMR) response of microfabricated microwave resonators loaded with small ${\mathrm{Co}}_{16}{\mathrm{Pd}}_{84}$ alloy rectangles. A major increase in the FMR signal-to-noise ratio is achieved by employing the microwave-resonator structure. A FMR peak shift similar to that of ${\mathrm{Co}}_{16}{\mathrm{Pd}}_{84}$ continuous films is measured in the presence of hydrogen gas in the sample environment. We show that the very high sensitivity of the FMR signal of the ${\mathrm{Co}}_{16}{\mathrm{Pd}}_{84}$ alloy rectangle to hydrogen exposure can be used to measure relatively small hydrogen-concentration steps near 100% ${\mathrm{H}}_{2}$. Additionally, we also demonstrate that this structure can measure hydrogen over a concentration range from 3% to 100% ${\mathrm{H}}_{2}$ in ${\mathrm{N}}_{2}$. In time-dependent FMR measurements, we discover a temperature dependence of the FMR signal, which we relate to intrinsic temperature-dependent changes in saturation magnetization and the magnetic anisotropy of the $\mathrm{Co}$-$\mathrm{Pd}$ alloy.

Dynamics of Two Ferromagnetic Insulators Coupled by Superconducting Spin Current.

A conventional superconductor sandwiched between two ferromagnets can maintain coherent equilibrium spin current. This spin supercurrent results from the rotation of odd-frequency spin correlations induced in the superconductor by the magnetic proximity effect. In the absence of intrinsic magnetization, the superconductor cannot maintain multiple rotations of the triplet component but instead provides a Josephson type weak link for the spin supercurrent. We determine the analog of the current-phase relation in various circumstances and show how it can be accessed in experiments on dynamic magnetization. In particular, concentrating on the magnetic hysteresis and the ferromagnetic resonance response, we show how the spin supercurrent affects the nonequilibrium dynamics of magnetization which depends on a competition between spin supercurrent mediated static exchange contribution and a dynamic spin pumping contribution. Depending on the outcome of this competition, a mode crossing in the system can either be an avoided crossing or mode locking.

Tuning of Magnetic Damping in Y3Fe5O12/Metal Bilayers for Spin-Wave Conduit Termination.

In this work, we investigate the structural and dynamic magnetic properties of yttrium iron garnet (YIG) films grown onto gadolinium gallium garnet (GGG) substrates with thin platinum, iridium, and gold spacer layers. Separation of the YIG film from the GGG substrate by a metal film strongly affects the crystalline structure of YIG and its magnetic damping. Despite the presence of structural defects, however, the YIG films exhibit a clear ferromagnetic resonance response. The ability to tune the magnetic damping without substantial changes to magnetization offers attractive prospects for the design of complex spin-wave conduits. We show that the insertion of a 1-nm-thick metal layer between YIG and GGG already increases the effective damping parameter enough to efficiently absorb spin waves. This bilayer structure can therefore be utilized for magnonic waveguide termination. Investigating the dispersionless propagation of spin-wave packets, we demonstrate that a damping unit consisting of the YIG/metal bilayers can dissipate incident spin-wave signals with reflection coefficient R < 0.1 at a distance comparable to the spatial width of the wave packet.

Linearly shifting ferromagnetic resonance response of La0.7Sr0.3MnO3 thin film for body temperature sensors.

Human body temperature not only reflects vital signs, but also affects the state of various organs through blood circulation, and even affects lifespan. Here a wireless body temperature detection scheme was presented that the temperature was extracted by investigating the out-of-plane (OP) ferromagnetic resonance (FMR) field of 10.2 nm thick La0.7Sr0.3MnO3 (LSMO) film using electron paramagnetic resonance (EPR) technique. Within the range of 34–42 °C, the OP FMR field changes linearly with the increasing or decreasing temperature, and this variation comes from the linear responses of magnetization to the fluctuant temperature. Using this method, a tiny temperature change (< 0.1 °C) of organisms can be detected accurately and sensitively, which shows great potential in body temperature monitoring for humans and mammals.

Manifestations of Spinodal Decomposition into Dilute Pd1-xFex “Phases” in Iron-Implanted Palladium Films: FMR Study

Palladium-iron alloys produced by high-dose implantation of iron ions into epitaxial palladium films were investigated with the ferromagnetic resonance (FMR) and vibrating sample magnetometry (VSM) techniques. The samples reveal distinct multiple FMR responses depending on the dose of iron ion implantation. The post-implantation annealing at 770 K does not bring the implanted films to a homogeneous solid solution state, as might be expected from the Pd-Fe phase diagram. On the contrary, the system approaches a stable state composed of several magnetic phases. FMR spectra exhibit an angular behavior specific for a stack of interacting magnetic layers. This observation, correlated with the magnetometry data, indicates that the palladium-iron binary alloy has a previously unknown tendency towards spinodal decomposition into isostructural phases with well-defined iron concentrations and, accordingly, with different temperatures of ferromagnetic ordering and saturation magnetizations.

Effects of substrate annealing on uniaxial magnetic anisotropy and ferromagnetic resonance frequency of Ni80Fe20 films deposited on self-organized periodically rippled sapphire substrates

Ni80Fe20 (NiFe) films have been deposited on self-organized sapphire substrates with periodical surface ripples formed by 2–10 h thermal annealing at 1400°C. By extracting static and dynamic magnetic parameters from magneto-optical Kerr effect (MOKE) loops and ferromagnetic resonance (FMR) spectra, we quantified and analyzed the substrate annealing effects on the induced in-plane uniaxial magnetic anisotropy and ferromagnetic resonance frequency in these films. The study demonstrates that the uniaxial anisotropy of these NiFe films is strongly related to the annealing-time dependent substrate faceting and coarsening mechanisms. The uniaxial anisotropy field Hu changes from ~170 Oe to ~250Oe, and the maximum value of it is obtained in film on 8-h annealed substrate. The ferromagnetic resonance response and high frequency characteristics of films can be modulated by self-organized substrate is also confirmed. When external magnetic field is parallel to the direction of ripple, the ferromagnetic resonance frequency of films grown on the self-organized substrates is increased by ~0.3−1 GHz compared with that of reference film on plane substrate. This work would be helpful to design the high frequency properties of spintronic devices and micro-wave magnetic devices by utilizing self-organized substrates.

Low Voltage Reversible Manipulation of Ferromagnetic Resonance Response in CoFeB/HfO2 HeterostructuresSupported by the National Natural Science Foundation of China (Grant Nos. 51671099 and 11974149), the Open Foundation Project of Jiangsu Key Laboratory of Thin Films (Grant No. KJS1745), the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT-16R35), and the Fundamental Research Funds for the Central

We report that the ferromagnetic resonance (FMR) response of the CoFeB/HfO2 heterostructures is stabilized and reversibly manipulated by ionic gel. Ionic gel with excellent flexibility is used as a medium to form an electric field. When a 4 V gate voltage is applied, the resonance field H r and peak-to-peak linewidth ΔH pp at different angles are regulated. When θ = 20°, the H r is regulated up to 82 Oe. When θ = 70°, ΔH pp is tuned up to 75 Oe. When the gate voltage is repeatedly applied, the FMR spectra can be freely switched between the initial state and the gated state. Our study provides an effective method to manipulate the damping of the magnetic film stably and reversibly.

Effect of Hydrogen Gas on Ferromagnetic Resonance Properties of Ni–Co–Pd Ternary Alloy Films

We investigate the ferromagnetic resonance (FMR) response of thin Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> Pd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x-y</sub> alloy films. An FMR shift similar to Co-Pd alloy thin films is found in the presence of hydrogen gas in the sample environment. We find that the measured change in effective magnetization has an exponential dependence on the Pd concentration in the alloy. Additionally, we find a sudden decrease in sensitivity to hydrogen for low hydrogen concentrations.

Concentration gradient Co–Fe nanowire arrays: Microstructure to magnetic characterizations

Abstract In the present investigation, systematic method to fabricate nanowire arrays with a linear gradient of cobalt-iron composition has been reported. The nanowires were fabricated by direct current (DC) electro-deposition following a multi-step titration method into a commercial anodic aluminium oxide (AAO) template. Nearly 100% gradient of Co and Fe content in the nanowires is achieved following the present method. The gradient of Co and Fe composition along the nanowires is established by Energy-dispersive X-ray (EDX) mapping in scanning electron microscope (SEM). The results from EDX mapping are further supported by the microstructural characterization along the wire axis using high resolution transmission electron microscope (HRTEM) images and SAED pattern. The microstructural characterization reveals variation of crystalline phases that are signature of Co and Fe content along the wires. To explore the magnetization dynamics of nanowire arrays, ferromagnetic resonance (FMR) study was performed in flip-chip geometry. The FMR study shows the presence of four well-separated resonance peaks in gradient nanowires which is a unique finding in the present investigation. A detailed FMR analysis has been presented to reveal the origin of these peaks. The values of the Gilbert damping parameter, obtained from the FMR linewidth analysis establish the correlation of FMR response with the average composition of Co–Fe in nanowires, grown in each step of titration. This study highlights the necessity of the detailed investigation of microstructures and magnetization dynamics of gradient ferromagnetic nanowire arrays that are emerging in high-frequency spintronics applications, more suitable for a tunable tri-band band-pass filter.

Investigation of Ferromagnetic Resonance Shift in Screen-Printed Barium Ferrite/Samarium Cobalt Composites

This paper explores a method for incorporating samarium cobalt (both Sm2Co17 and SmCo4) inclusions into screen-printed barium hexaferrite (BaFe12O19) films in order to tune the ferromagnetic resonance (FMR). Using an all-low-temperature screen-printing process, >140- $\mu \text{m}$ -thick composite films are fabricated using heterogeneous mixtures of magnetic particles in a polydimethylsiloxane (PDMS) binder. RF characterization is performed using two-port S-parameter measurements on a coplanar waveguide (CPW) with external magnetic bias fields. The films exhibit a self-biased FMR response that ranges from 41 to 53 GHz (12-GHz span) by varying the relative ratio of the hexaferrite and Sm-Co particles. These effects are attributed to the embedded Sm-Co particles providing additional internal field bias acting on the barium ferrite.

Stacking order-dependent sign-change of microwave phase due to eddy currents in nanometer-scale NiFe/Cu heterostructures

In the field of spintronics, ferromagnetic/nonmagnetic metallic multilayers are core building blocks for emerging technologies. Resonance experiments using stripline transducers are commonly used to characterize and engineer these stacks for applications. Up to now in these experiments, the influence of eddy currents on the excitation of the dynamics of ferromagnetic magnetization below the skin-depth limit was most often neglected. Here, using a coplanar stripline transducer, we experimentally investigated the broadband ferromagnetic resonance response of NiFe/Cu bilayers a few nanometers thick in the sub-skin-depth regime. Asymmetry in the absorption spectrum gradually built up as the excitation frequency and Cu-layer thickness increased. Most significantly, the sign of the asymmetry depended on the stacking order. Experimental data were consistent with a quantitative analysis considering the eddy currents generated in the Cu layers and the subsequent phase shift of the feedback magnetic field generated by the eddy currents. These results extend our understanding of the impact of eddy currents below the microwave magnetic skin-depth and explain the line shape asymmetry and phase lags reported in stripline experiments.

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

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

Sensitivity of ferromagnetic resonance in PdCo alloyed films to hydrogen gas

In this work we study the ferromagnetic resonance (FMR) response of CoxPd1-x alloy samples with varying compositions (x = 0.65, 0.39, 0.24 and 0.14). We find significant differences in the FMR response of the samples to the presence of hydrogen gas in the samples’ environment. Without any special processing, the films with x = 0.39 and 0.24 demonstrated behaviour which is promising for application in hydrogen gas sensing. Using FMR in the alloy thin films, we were able to measure hydrogen gas concentration in a very broad range - from 0.05% to 100%.

De-embedding zero-field signal in high-frequency magneto-impedance measurements of soft ferromagnetic materials

Abstract Devices based on soft ferromagnetic materials are promising for high-frequency flexible electronics. Especially, wide-range tunable ferromagnetic resonance response of ferromagnetic microwires and ribbons can be functionalized in the form of magneto-impedance sensors in stretchable, bendable and twistable environments. A vector network analyzer calibrated by standard techniques is required to characterize the high-frequency properties of such devices. However, most of the calibration techniques necessitate calculations, computer simulations, and/or fabricating calibration kits in the shape of various test fixtures. Here, we employ a fixture calibration method for high-frequency (up to 4 GHz) impedance measurements of CoFeSiB microwires and melt-spun ribbons taking zero applied magnetic field as the reference. This method is based on a virtual de-embedding procedure, in which a reference signal is subtracted from all the subsequent measurements, which reduces the time for the total calibration procedure down to few minutes. Applications of the method are not limited to the cases where applied magnetic field varies. Any external parameter (strain, temperature, pressure etc.) that can change the impedance is allowed by this method.

Effect of Meissner Screening and Trapped Magnetic Flux on Magnetization Dynamics in Thick Nb/Ni80Fe20/Nb Trilayers

We investigate the influence of Meissner screening and trapped magnetic flux on magnetization dynamics for a Ni80Fe20 film sandwiched between two thick Nb layers (100 nm) using broadband (5-20 GHz) ferromagnetic resonance (FMR) spectroscopy. Below the superconducting transition Tc of Nb, significant zero-frequency line broadening (5-6 mT) and DC resonance field shift (50 mT) to a low field are both observed if the Nb thickness is comparable to the London penetration depth of Nb films (>= 100 nm). We attribute the observed peculiar behaviors to the increased incoherent precession near the Ni80Fe20/Nb interface and the effectively focused magnetic flux in the middle Ni80Fe20 caused by strong Meissner screening and (defect-)trapped flux of the thick adjacent Nb layers. This explanation is supported by static magnetic properties of the samples and comparison with FMR data on thick Nb/Ni80Fe20 bilayers. Great care should therefore be taken in the analysis of FMR response in ferromagnetic Josephson structures with thick superconductors, a fundamental property for high-frequency device applications of spin-polarized supercurrents.

Time resolved imaging of the non-linear bullet mode within an injection-locked nano-contact spin Hall nano-oscillator

Time-resolved scanning Kerr microscopy (TRSKM) has been used to image precessional magnetization dynamics excited by a DC current within a nano-contact (NC) spin Hall nano-oscillator (SHNO). Injection of a radio frequency (RF) current was used to phase lock the SHNO to TRSKM. The out of plane magnetization was detected by means of the polar magneto optical Kerr effect (MOKE). However, longitudinal MOKE images were dominated by an artifact arising from the edges of the Au NCs. Time resolved imaging revealed the simultaneous excitation of a non-linear “bullet” mode at the centre of the device, once the DC current exceeded a threshold value, and ferromagnetic resonance (FMR) induced by the RF current. However, the FMR response observed for sub-critical DC current values exhibits an amplitude minimum at the centre, which is attributed to spreading of the RF spin current due to the reactance of the device structure. This FMR response can be subtracted to yield images of the bullet mode. As the DC current is increased above threshold, the bullet mode appears to increase in size, suggesting increased translational motion. The reduced spatial overlap of the bullet and FMR modes, and this putative translational motion, may impede the injection locking and contribute to the reduced locking range observed within NC-SHNO devices. This illustrates a more general need to control the geometry of an injection-locked oscillator so that the autonomous dynamics of the oscillator exhibit strong spatial overlap with those resulting from the injected signal.

Investigating spin-transfer torques induced by thermal gradients in magnetic tunnel junctions by using micro-cavity ferromagnetic resonance

Similar to electrical currents flowing through magnetic multilayers, thermal gradients applied across the barrier of a magnetic tunnel junction may induce pure spin-currents and generate ‘thermal’ spin-transfer torques large enough to induce magnetization dynamics in the free layer. In this study, we describe a novel experimental approach to observe spin-transfer torques induced by thermal gradients in magnetic multilayers by studying their ferromagnetic resonance response in microwave cavities. Utilizing this approach allows for measuring the magnetization dynamics on micron/nano-sized samples in open-circuit conditions, i.e. without the need of electrical contacts. We performed first experiments on magnetic tunnel junctions patterned into 6 × 9 µm2 ellipses from Co2FeAl/MgO/CoFeB stacks. We conducted microresonator ferromagnetic resonance (FMR) under focused laser illumination to induce thermal gradients in the layer stack and compared them to measurements in which the sample was globally heated from the backside of the substrate. Moreover, we carried out broadband FMR measurements under global heating conditions on the same extended films the microstructures were later on prepared from. The results clearly demonstrate the effect of thermal spin-torque on the FMR response and thus show that the microresonator approach is well suited to investigate thermal spin-transfer-driven processes for small temperatures gradients, far below the gradients required for magnetic switching.