Ferromagnetic Resonance Results Research Articles

Spin‐Thermoelectric Generation Associated with Magnetization Dynamics in the Insulator‐Based Generators Fabricated from Liquid Phase Epitaxial Yttrium Iron Garnet, Bi‐Substituted YIG and Bi‐ and Al‐Substituted YIG Films

An insulator‐based spin‐thermoelectric (STE) generator is composed of a thin paramagnetic metal (PM: Pt) layer for generating the STE voltage VSTE via the inverse spin Hall effect and a ferrimagnetic insulator (FMI) film or slab used for producing spin‐wave spin currents due to a temperature gradient . Yttrium iron garnet (YIG) is a key material used for the FMI of STE generators. We examined in detail the ferromagnetic resonance (FMR) properties of YIG (Y3Fe5O12), Bi‐substituted YIG (Y3‐xBixFe5O12) and Bi‐ and Al‐substituted YIG (Y3‐xBixFe5‐yAlyO12) films grown by liquid phase epitaxy (LPE). Bi‐substituted YIG films exhibited the large FMR damping and high STE voltage when the films were incorporated in the STE generator. The LPE‐grown Y2.35Bi0.65Fe5O12 film exhibited the FMR linewidth ΔH of 30 mT and the VSTE of 70 μV for the ∇T of . This paper comprehensively presents the origin of the STE generation in the insulator‐based generators on the basis of the results of FMR and STE measurements. To clarify the origin of STE generation in the generators fabricated from single crystal YIG (YIG, Bi‐substituted YIG and Bi‐ and Al‐substituted YIG) films grown by LPE, three principal factors are explained: (a) thermal energy transfer from the phonon system to the spin system, which strengthens the heat excitation of spin precession, through the spin–orbit coupling enhanced by the growth‐induced magnetic anisotropy of LPE‐grown YIG films, (b) in the YIG films incorporated in the STE generators, the generation of spin‐wave spin currents owing its origin to the ferromagnetic spin exchange interaction acting between neighboring spins due to , and (c) how the spin currents pumped into the Pt layer from the YIG film at the Pt/YIG bilayer interface are affected by the magnetization processes of single crystal YIG films. It is concluded that the STE generation of insulator‐based STE generators can be explained from a viewpoint of the spin dynamics under the effect of in the Pt/YIG bilayer structure. © 2024 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.

Annealing dependence on magnetization dynamics and two-magnon scattering in Co40Fe40B20 thin films

Investigation of magnetization dynamics of low-damping ferromagnetic layers is of paramount interest for the realization of spin-based electronic devices. Here, we report the effect of surface oxidation on the structural properties, magnetization reversal and spin dynamics of Co40Fe40B20 (CFB) films. X-Ray diffraction results show that both as-deposited and annealed CFB layers up to 250 °C exhibit an amorphous state. Partial crystallization of CoFe is observed in CFB films above 350 °C. Magnetization reversal curves measured using longitudinal magneto-optic Kerr effect indicate a characteristic square loop along the easy axis for all samples. Ferromagnetic resonance (FMR) results show that the resonance field (Hr) shifts due to changes in the structural phase or microstructure. FMR linewidth (ΔH) is linear for both as-deposited and all annealed CFB above 10 GHz, whereas anomalous behaviour in ΔH as a function of frequency is observed below 10 GHz frequency for annealed thin films. The extracted damping parameter manifests extrinsic contributions in the annealed samples compared to the as-deposited. Angular-dependant FMR results show ΔH and Hr exhibit opposite trends in CFB film annealed at 450 °C due to the presence of inhomogeneities. It suggests the signature of two-magnon scattering in the 450 °C annealed CFB. Our systematic study helps in the understanding of the effect of annealing on the crystallinity and magnetization dynamics in CFB thin films.

Investigation of structural, magnetic and electronic properties of FeTa films for varying Ta concentration at different annealing temperatures

In the present work, temperature dependent magnetic, structural and electronic properties of FeTa films with increasing Ta concentration are investigated. X-ray reflectivity (XRR) studies reveal densification of the film with increasing Ta concentration. Thermal annealing also results in an increased electron density relative to as-deposited film, concurrent with increase in grain size as observed from X-ray diffraction (XRD) and results of Soft-X-ray absorption spectroscopy (SXAS). However, grain growth is inhibited with increasing Ta concentration by presence of Ta in grain boundary (GB) region. This also results in increase in magnetic coercivity (greater than150 %) as observed from magneto-optical Kerr effect (MOKE). The structural properties were further correlated with magnetic properties as observed from Polarized neutron reflectivity (PNR) and Ferromagnetic resonance (FMR). Effective magnetic moment of the samples, as observed from PNR decreases with increasing Ta concentration and increases with increasing annealing temperature, which is in agreement with FMR results. Scattering length density profiles, observed from simultaneous fitting of XRR and PNR show presence of an oxide layer at surface, as also confirmed from SXAS measurements.

Canted standing spin-wave modes of permalloy thin films observed by ferromagnetic resonance

Non-collinear spin structures in materials that combine perpendicular and in-plane magnetic anisotropies are of great technological interest for microwave and spin wave-assisted magnetization switching. [Co/Pt] multilayers are well-known perpendicular anisotropy materials that have the potential to pin the magnetization of a soft magnetic layer, such as permalloy (Py), that has in-plane anisotropy, thereby forming a magnetic exchange spring. Here we report on multilayered [Co/Pt]/Pt/Py films, where an additional ultrathin Pt spacer has been included to control the coupling between the sub-units with in-plane and perpendicular magnetic anisotropy. Vector network analyser (VNA)-ferromagnetic resonance (FMR) measurements were made to obtain a complete picture of the resonant conditions, while the dynamical response of the sub-units was probed by synchrotron-based element- and phase selective x-ray detected FMR (XFMR). For all samples, only slight pinning of the dynamic magnetization of the Py by the [Co/Pt] was noted, and the FMR results were dominated by the 50 nm thick Py layer. Out-of-plane VNA-FMR maps reveal the presence of additional modes, e.g. a perpendicular standing spin-wave (PSSW) state. However, as the magnetic field is reduced below the saturation field, the PSSW state morphs continuously through a series of canted standing spin-wave (CSSW) states into a horizontal standing spin-wave (HSSW) state. The PSSW, CSSW and HSSW states are well described using a multilayer model of the Py film. The observation of CSSW modes is of particular relevance to microwave assisted magnetic recording, where microwave excitation stimulates precession of a soft layer canted out of plane by a pulsed magnetic field.

Tailoring functional properties of Ni nanoparticles-acrylic copolymer composites with different concentrations of magnetic filler

Magnetic composites with nickel nanoparticles synthesized by the method of the electrical explosion of wire were prepared with acrylic copolymer containing 95 wt. % of butylacrylate and 5 wt. % of methacrylic acid. Two series of composites were made with Ni content covering the range from 5 to 90 wt. % and differing in the degree of aggregation. In one series of composites, the aggregates were irregularly shaped globules with dimensions varying in 1–40 μm range. In the other, chain-like fragments were observed. As expected, the saturation magnetization value of the composites showed a linear dependence on concentration but the coercive field was slightly higher for the aggregated case for a given concentration. Microwave absorption studies of the composites showed both strong zero field absorption and ferromagnetic resonance (FMR). The FMR results were different for the two types of samples; however, each set can be explained by taking into account the distribution of local magnetic fields due to random positions of the particles.

Magnetic properties of Ni-implanted ITO thin films

Abstract The magnetic properties of Ni-implanted ITO thin films have been investigated by ferromagnetic resonance (FMR) technique and vibrating sample magnetometry (VSM) techniques. Commercially available ITO thin films on fused silica substrates have been implanted with different fluences of Ni+ ions with energy of 40 keV and ion current density of 8 µA/cm2 at room temperature. The samples with three doses of 0.5×1017, 1.0×1017 and 1.5×1017 ions/cm2 have been studied. Room temperature ferromagnetism has been observed in the nickel-implanted ITO samples with fluences of 1.0×1017 and 1.5×1017 ions/cm2. The magnetic properties of the samples have been explained by the formation of Ni-nanoparticles in the implanted surface layer of the ITO films. Although the formation of a diluted magnetic oxide phase cannot be ruled out entirely, the analysis of our FMR and VSM data reveals that the metallic Ni nanoparticles, formed during high-dose implantation process, have major contribution to the magnetic properties of the Ni-implanted ITO thin films. The sizes of the Ni-nanoparticles have been calculated from the blocking temperatures obtained by the VSM measurements. The filling factor of the Ni ferromagnetic phase in the granular magnetic layer has also been estimated by the effective magnetization approach applied to the FMR results.

Effect of the magnetization process on low-field microwave absorption by FeBN magnetic thin film

The magnetization process of a typical soft magnetic film was examined using ferromagnetic resonance (FMR) measurements on 9.84 GHz. A 500-nm-thick Fe85.9B2.6N11.5 film with in-plane uniaxial magnetic anisotropy was deposited by RF sputtering. The saturation magnetization and coercivity of the FeBN magnetic film were 17.3 kG and 2.4 Oe, respectively, at the hard direction. The microwave absorption signals showed two different absorption peaks: small signals at a near zero dc field and at a high dc field. The resonance magnetic fields for the major and minor peaks were observed at 532 and 604 Oe, respectively. The g-factor obtained was 2.15. The uniaxial magnetic anisotropy field was approximately 14 Oe by vibrating sample magnetometer, which coincided with that of the FMR results. The absorption at a low-field revealed different characteristics from those of conventional FMR absorption. The magnetic anisotropy field obtained by low-field microwave absorption was approximately 13.5 Oe, which is comparable to that of the FMR and VSM results.

Correction: Corrigendum: Effect of magnetic dipolar interactions on nanoparticle heating efficiency: Implications for cancer hyperthermia

Nanostructured magnetic systems have many applications, including potential use in cancer therapy deriving from their ability to heat in alternating magnetic fields. In this work we explore the influence of particle chain formation on the normalized heating properties, or specific loss power (SLP) of both low- (spherical) and high- (parallelepiped) anisotropy ferrite-based magnetic fluids. Analysis of ferromagnetic resonance (FMR) data shows that high particle concentrations correlate with increasing chain length producing decreasing SLP. Monte Carlo simulations corroborate the FMR results. We propose a theoretical model describing dipole interactions valid for the linear response regime to explain the observed trends. This model predicts optimum particle sizes for hyperthermia to about 30% smaller than those previously predicted, depending on the nanoparticle parameters and chain size. Also, optimum chain lengths depended on nanoparticle surface-to-surface distance. Our results might have important implications to cancer treatment and could motivate new strategies to optimize magnetic hyperthermia.

Magnetoelastic coupling in La0.7Ca0.3MnO3/BaTiO3ultrathin films

The magnetism of La0.7Ca0.3MnO3 (LCMO) epitaxial thin films grown on SrTiO3 (STO) and BaTiO3 (BTO) substrates is studied using polarized neutron reflectometry (PNR) and ferromagnetic resonance (FMR) techniques. In LCMO/BTO, PNR reveals a strongly suppressed magnetization of 300 kA/m, equivalent to a magnetic moment of 2 μB/Mn, throughout the LCMO layer, amounting to half the expected value. The largest suppression occurs near the interface with BTO, with magnetization values of 50 kA/m, equivalent to 0.3 μB/Mn. FMR is observable at 8.9 GHz only around the [110] crystallographic direction in thin LCMO/BTO. The resonance barely shifts as the applied field is rotated away from [110]. The FMR results are analyzed in terms of magnetoelastic anisotropy and compared to LCMO/STO grown under the same conditions. A two-layer magnetization model is proposed, based on strong out-of-plane anisotropy near the BTO interface and shown to qualitatively explain the main characteristics of the FMR results.

Effect of magnetic dipolar interactions on nanoparticle heating efficiency: implications for cancer hyperthermia.

Nanostructured magnetic systems have many applications, including potential use in cancer therapy deriving from their ability to heat in alternating magnetic fields. In this work we explore the influence of particle chain formation on the normalized heating properties, or specific loss power (SLP) of both low- (spherical) and high- (parallelepiped) anisotropy ferrite-based magnetic fluids. Analysis of ferromagnetic resonance (FMR) data shows that high particle concentrations correlate with increasing chain length producing decreasing SLP. Monte Carlo simulations corroborate the FMR results. We propose a theoretical model describing dipole interactions valid for the linear response regime to explain the observed trends. This model predicts optimum particle sizes for hyperthermia to about 30% smaller than those previously predicted, depending on the nanoparticle parameters and chain size. Also, optimum chain lengths depended on nanoparticle surface-to-surface distance. Our results might have important implications to cancer treatment and could motivate new strategies to optimize magnetic hyperthermia.

Magnetic anisotropy and Verwey transition of magnetosome chains in Magnetospirillum gryphiswaldense

Magnetotactic bacteria (MTB) are characterized by cellular magnetic dipoles formed by the 1-D assembly of magnetite and/or greigite particles aligned along their magnetic easy axes. This alignment creates strong interaction-induced shape anisotropy. Ferromagnetic resonance (FMR) spectroscopy is applied to study the changes in anisotropy of the MTB Magnetospirillum gryphiswaldense between room temperature and 10 K. The Verwey transition is found at about 100 K. The characteristic FMR signal of the cellular dipole at room temperature vanishes upon cooling to the isotropic point at Ti ≈ 130 K, where the magnetocrystalline anisotropy constant K1 becomes zero. Monitoring of the FMR response of intact MTB as a function of temperature is taken to discuss theoretically the reduction of the interaction-induced shape anisotropy in magnetofossils because of diagenetic processes. It is concluded that there is a similarity in the FMR response between magnetofossils at room temperature and intact MTB near Ti. This is because the critical effect of the magnetocrystalline anisotropy constant K1 and of the alignment of magnetic easy axes on the cellular dipole. Low-temperature FMR results of intact MTB can thus be used as a guideline for detecting magnetofossils in geological environments.

Annealing Effect on Structural and Magnetic Properties of Cu2MnAl Heusler Alloy Films

Cu2MnAl Heusler alloy films were grown on MgO (001) substrates by using the ion beam sputtering technique. The films were post-annealed at varying temperatures in order to investigate the influence of annealing on crystal structure and magnetic properties. The structural properties of Cu2MnAl films have been investigated by using x-ray diffraction (XRD) and magnetic properties have been investigated by both vibrating sample magnetometer (VSM) and ferromagnetic resonance (FMR) techniques. The experimental data indicates that the crystal structure of the films strongly depends on the annealing temperature. When the films were annealed at 200 °C, the saturation magnetization (Ms=250 emu/cm3) achieved its maximum and the coercive field (Hc≈7 Oe) reached its minimum with B2 ordered structure. In addition, FMR results have revealed that the Cu2MnAl film annealed 200 °C has the highest effective magnetization. The combination of structural and magnetic characterization indicates that the optimum growth temperature is 200 °C for the Cu2MnAl Heusler alloy films on MgO substrates.

Nickel nanowires for planer microwave circuit applications and characterization

The dynamic properties of uniform one-dimensional arrays of high aspect ratio nickel nanowires have been studied by ferromagnetic resonance (FMR) technique at 24GHz. The fundamental magnetic parameters such as spontaneous magnetization, gyromagnetic ratio (γ), and magnetic anisotropies of the nanowires are derived from the angular variation of resonance field Hr(θH) positions. The effective fields of the nanowire system were observed to decrease with increasing length of the nanowires. Further, the use of nanowires as a tunable stop-band notch filter in a coplanar waveguide geometry has been assessed. The stop-band frequency (fr) is observed to be tunable up to 14GHz with an applied field (H) of up to 4kOe. The kink in the fr(H) data gives the measure of the effective field (Heff) of the nanowire system. The theoretical fitting of fr(H) data to resonance relation yields values of Heff and γ which are a little higher than the conventional FMR results.

Novel Planer Microwave Circuit Applications and Characterization of Ni Nanowires

ABSTRACTArrays of Ni nanorods were electrodeposited into alumina oxide templates with various lengths (11-50 micro meter) and fixed pore diameter (150 nm). The magnetization behavior of these rods were investigated with ferromagnetic resonance (FMR) techniques; fixed frequency (conventional FMR) and swept frequency (Network Analyzer FMR). Both resonance spectra indicate the presence of strong dipolar interaction between the nanorods. The fundamental magnetic parameters like spontaneous magnetization, gyromagnetic ratio, and magnetic anisotropies of the nanorods were derived from the angular variation of resonance field data. Further, the use of nanorods as a tunable stop-band notch-filter in a coplanar waveguide geometry has been assessed. The stop-band frequency (fr) is observed to be tunable up to 24 GHz with an applied field (H) of up to 6 kOe. The theoretical fitting of fr(H)data to resonance relation yield values of effective field (Heff) and gyromagnetic ratio, which are a little higher than the conventional FMR results.

Study of GMI-valve characteristics in the Co-based amorphous ribbon by ferromagnetic resonance

The mechanism of the drastic increase of the giant magneto impedance (GMI) sensitivity which is enhanced to over 1000%/Oe, which shows the GMI-valve characteristic, has been studied using ferromagnetic resonance (FMR). The FMR linewidth of the annealed sample increased with the increasing annealing time in parallel and perpendicular to it. From the FMR results, it can be concluded that the GMI-valve characteristic is attributed to the surface anisotropy due to the exchanging coupling arising from the interaction between the ferromagnetic amorphous core and the antiferromagnetic oxidation layer.

Dynamics studies in magnetic bilayer Fe∕Au∕Fe(001) structures using network analyzer measurements

Fe ∕ Au ∕ Fe ( 001 ) ultrathin film structures were grown by molecular beam epitaxy on 4×6 reconstructed GaAs(001) substrates. A microwave network analyzer was used to carry out ferromagnetic resonance (FMR) measurements down to microwave frequency of 1GHz. The FMR results indicate that the crystalline ultrathin Fe films are of high crystalline quality. The FMR linewidth is mostly given by the intrinsic Gilbert damping and is only weakly affected by small local variations of the internal anisotropy fields. The study at low microwave frequency allowed one to investigate the spin-pumping mechanism in the configuration of noncollinear magnetic moments.

Ferromagnetic resonance studies in granular Co–SiO2 thin films

Properties of thin granular Co–SiO2 films have been studied by means of ferromagnetic resonance (FMR). The obtained FMR results are discussed using sample magnetization, electrical conductivity, and transmission electron microscopy analysis. Co and SiO2 were sequentially deposited for the sample preparation. The general behavior of the applied field for resonance could be described using effective out-of-plane anisotropies. A dipolar interaction model developed for magnetic heterostructures was applied to the interpretation of these anisotropies. The anisotropy terms caused by the magnetic particle shapes and by the film shape can explain the results for two metallic films close to percolation, in which the film shape is the preponderant contribution. In the case of an insulating sample, the consideration of an additional anisotropy term seems to be necessary to explain the results.

Crystallization kinetics of the (Fe 50Co 50) 73.5Ag 1Nb 3Si 13.5B 9 amorphous alloy

The crystallization kinetics of the (Fe 50Co 50) 73.5Ag 1Nb 3Si 13.5B 9 amorphous alloy was studied using ferromagnetic resonance (FMR) measurements on samples annealed at 693 K for several periods of time and differential scanning calorimetry (DSC) measurements using several heating rates between 5 and 30 K/s. The increase of the FMR linewidth with annealing time is attributed to crystallization. Assuming a linear relation between the linewidth increase and the transformed volume fraction, it is possible to determine the Avrami exponent, n, from the FMR results: n=1.83±0.10. The DSC measurements show two exothermal peaks, suggesting a crystallization process involving more than one phase. The Avrami exponents calculated from the DSC measurements are n=1.80±0.10 for the first peak and n=1.50±0.10 for the second peak; these results are consistent with a diffusion-controlled process with a decreasing nucleation rate for the first peak and with a nucleation rate close to zero for the second peak. There is a reasonable agreement between the FMR results and the DSC results for the first peak. The activation energies are 353.5±2.8 and 424.9±10.2 kJ/mol for the first and second peaks, respectively.

THE IN-PLANE ANISOTROPY OF PERMALLOY ARRAYS OF SUBMICRON RECTANGULAR ELEMENTS

We report a ferromagnetic resonance (FMR) study on the in-plane anisotropy of the patterned permalloy film with arrays of submicron rectangular elements prepared by electron beam lithography and the theoretical fitting of the FMR results, by which the in-plane anisotropy, effective magnetization, gyromagnetic ratio and g value of the samples are determined. The theoretical curves of the dependence of the resonance field on the field orientation ΦH fit well with the experimental data.

Preparation and structural characterization of nanostructured CoAg granular films

The microstructures of as-deposited and annealed CoAg granular films fabricated by ion-beam cosputtering technique were characterized using x-ray diffraction, transmission electron microscopy (TEM), and ferromagnetic resonance (FMR) methods. For the Co22Ag78 sample, the changes in size and shape of the nanoscale cobalt granules embedded in the film closely depend on the annealing temperature (TA). Real time observation in situ of TEM together with FMR spectra indicate that the size and shape of the cobalt granules evolve primarily along the film plane during the annealing process. Also, the FMR results indicate that the cobalt granules are still single domain particles embedded in the film as the sample is annealed up to 700 K.