Ferromagnetic Resonance Spectra Research Articles

Electron spin resonance (ESR) of magnetic sublattices in Sc-substituted barium hexaferrite

The partial substitution of Fe3+ by Sc3+ in barium hexaferrite has shown to be an effective method to tailor anisotropy for many novel microwave applications. Some basic studies have revealed that this substitution leads to unusual interactions among the magnetic sublattices of the ferrite. In order to investigate these interactions, samples with formula BaScxFe12-xO19 (1 ≤x ≤ 2) were prepared by sintering (1300°C, 6h). After structural characterization by x-ray diffraction, their ferromagnetic resonance spectra were measured in the X-band (9.4 GHz), in the 100-500 K temperature range. For x = 2, a single, broad resonance peak was observed at the low temperatures (103 K), exhibiting a progressive splitting into two peaks for increasing T, to finally coalesce again into a single (paramagnetic) narrow peak at 473 K. These results are interpreted in terms of a substitution of Fe3+ by Sc3+ ions in the 4fvi and 2b sublattices; the diamagnetic cations disrupt the superexchange interactions and produce a splitting of the 12k sublattice (which interacts directly with the 4fvi sublattice) into two sublattices with different canting angles, and different thermal dependence. As a result, the fraction of the 12k sublattices that are nearest neighbours of substituted 4fvi sites can behave as an independent sublattice for some temperature ranges. A similar behavior is observed for all the compositions with varying degrees of amplitude, but it is more evident for x = 2. A deconvolution of peaks has been attempted, in order to shed more light into this behavior.

Homodyne-detected ferromagnetic resonance of in-plane magnetized nanocontacts: Composite spin-wave resonances and their excitation mechanism

This work provides a detailed investigation of the measured in-plane field-swept homodyne-detected ferromagnetic resonance (FMR) spectra of an extended Co/Cu/NiFe pseudo spin valve stack using a nanocontact (NC) geometry. The magnetodynamics are generated by a pulse-modulated microwave current and the resulting rectified dc mixing voltage, which appears across the NC at resonance, is detected using a lock-in amplifier. Most notably, we find that the measured spectra of the NiFe layer are composite in nature and highly asymmetric, consistent with the broadband excitation of multiple modes. Additionally, the data must be fit with two Lorentzian functions in order to extract a reasonable value for the Gilbert damping of the NiFe. Aided by micromagnetic simulations, we conclude that (i) for in-plane fields the rf Oersted field in the vicinity of the NC plays the dominant role in generating the observed spectra, (ii) in addition to the FMR mode, exchange dominated spin waves are also generated, and (iii) the NC diameter sets the mean wavevector of the exchange dominated spin wave, in good agreement with the dispersion relation.

Magnetic anisotropy and relaxation of single Fe/FexOy core/shell- nanocubes: A ferromagnetic resonance investigation

In this work a full angle dependent Ferromagnetic Resonance (FMR) investigation on a system of 20 separated Fe/FexOy nanocubes without dipolar coupling is reported. The angular magnetic field dependence of FMR spectra of 20 single particles and 2 dimers were recorded using a microresonator setup with a sensitivity of 106 μB at X-band frequencies. We determine an effective magnetocrystalline anisotropy field of 2K4,eff/M = 50 mT ± 5 mT for selected particles, which is smaller than the one of bulk Fe due to the core shell morphology of the particles. The FMR resonances have a linewidth of 4 mT ± 1 mT, corresponding to a magnetic effective damping parameter α = 0.0045 ± 0.0005 matching the values of high quality iron thin films. Numerical calculations taking into account the different angular orientations of the 24 particles with respect to the external magnetic field yield a good agreement to the experiment.

Ferromagnetic resonance evidence of spinodal decomposition of Nix Cu1 − x (0.5 < x < 1) alloy films

Films of diluted ferromagnetic (FM) Nix Cu1−x alloys with 0.5<x<1 of about 80nm in thickness were deposited at room temperature on glass–ceramic substrates using electron-beam co-deposition from Cu and Ni sources. The temperature dependence of the ferromagnetic resonance (FMR) spectra was investigated in the 140<T<500K temperature range. The FMR spectra for films of composition 0.75≤x≤0.89 show two absorption peaks, which can be ascribed to a formation of two different FM phases as a result of phase separation in the alloy. The observed unusual temperature dependence of the resonance magnetic fields is interpreted as potentially magnetostrictive in nature. The results are discussed and compared to those for similar Ni–Cu films obtained using magnetron sputtering.

Low Gilbert damping and in-plane magnetic anisotropy in Ni–Mn–Sn thin film with high L21 order

Mn-rich off-stoichiometric Ni–Mn–Sn Heusler alloy thin film of thickness of 1000 nm was deposited on Si (100) substrate at ambient temperature by dc magnetron sputtering and then annealed ex situ at 550 °C for 1 h under high vacuum. X-ray diffraction and atomic force microscopy analyses show that the annealed film has high L21 crystalline ordering with a lattice constant 5.96 A and average surface roughness of 1.8 nm. Annealed film exhibited ferromagnetism at room temperature with high saturation magnetization of 162 emu/cm3, low retentivity of 22 emu/cm3 and easy axis of magnetization along the plane of the film. Magneto-dynamic analysis using micro-strip ferromagnetic resonance spectra shows the presence of small in-plane anisotropy K u = 1.4 × 104 erg/cm3 with 4πM s = 2075 ± 10 Oe and g-factor of 2.1. Line width analysis yields a low intrinsic damping constant (α = 0.008). A significant contribution from extrinsic two-magnon scattering to the line width of the order of Γ = 50 Oe was observed which may be due to surface roughness or/and presence of inhomogeneity and atomic site disorder in the film. The film exhibits the unique combination of low damping constant, low K u, high magnetic saturation and low retentivity.

Magnetocrystalline anisotropy of NiCoFe2O4 nanoparticles

Nanosized particles of Ni1−xCoxFe2O4 were synthesized by the sol–gel combustion method and the physical properties of these mixed ferrites were studied by X-ray diffraction and ferromagnetic resonance. The average crystallite size in samples with different Ni–Co ratios was determined from the X-ray diffractograms, whereas ferromagnetic resonance spectra were used to study the dependence of the magnetocrystalline anisotropy on the relative concentration of nickel and cobalt. According to the X-ray diffractograms, the samples are single phase and the crystallite size is between 9 and 22nm. The ferromagnetic resonance spectra showed that the magnetocrystalline anisotropy of the samples is positive, except for pure NiFe2O4, and increases monotonically with Co concentration, a result that could be of interest for practical applications.

Microwave-Assisted Magnetization Reversal in Exchange-Coupled Composite Media Using Linearly Polarized Microwave Fields

We have experimentally demonstrated a microwave-assisted magnetization reversal (MAMR) in exchange-coupled composite (ECC) media using the linearly polarized microwave fields generated by an electrically shorted coplanar waveguide. The MAMR frequency exhibiting a maximum reduction of coercive field can be decreased using 16 nm-thick ECC media without the suppression of the thermal stability of magnetization. The threshold amplitude of microwave field required for the successful MAMR is only 3% of the anisotropy field of the medium. A splitting of ferromagnetic resonance spectrum, which suggests an appearance of stochastic MAMR, can be observed after applying microwave fields. The stochastic MAMR gives rise to the threshold behavior in the condition of microwave field amplitude needed for MAMR.

Resonant switching for an in-plane magnetized L10-FePt | Ni81Fe19 bilayer under spin wave excitation

We report the magnetization dynamics and the magnetization switching mechanism under spin wave excitation in exchange-coupled bilayers. The in-plane magnetized L10-FePt and Ni81Fe19 (permalloy; Py) layers were exchange-coupled through the interface, and the spatially twisted magnetic structure was formed when the external magnetic field was applied owing to the difference in the switching fields (Hsw) between L10-FePt and Py. The ferromagnetic resonance spectra for the microfabricated bilayer element indicated that several kinds of quantized spin wave modes existed. Among the quantized spin wave modes, the perpendicular standing spin wave (PSSW) modes were amplified remarkably when the magnetization dynamics were largely excited by applying the external rf magnetic field (Hrf). Consequently, the microfabricated bilayer element showed clear Hsw reduction due to the assistance of PSSW. In addition, narrow Hsw distribution was observed for the spin wave-assisted magnetization switching. We examined the characteristic tendency of spin wave-assisted magnetization switching using pulse-like Hrf application measurement, and found that the resonant magnetization switching was mainly induced in the conditions exciting the PSSW modes but unexpected off-resonant switching also appeared at the high magnetic field.

A short-circuited coplanar waveguide to measure the permeability of magnetic thin films: Comparison with short-circuited microstrip line.

A short-circuited coplanar waveguide jig was proposed due to its simple structure and potential applications, which was successfully used to measure the permeability spectra of magnetic thin film. The result obtained from the coplanar waveguide (CPW) jig exhibits typical ferromagnetic resonance spectra, which can be well fitted by a trade-off equation because the measured permeability spectra have a bit of asymmetry because of the special intensity distribution of the microwave magnetic field in the CPW. In order to confirm the availability of the shorted CPW jig, a previous short-circuited microstrip line jig was used to measure the permeability spectra of the same sample. The dynamic permeability results obtained by using different jigs are commendably consistent, and the accurate and valid results are also confirmed by using the vector network analyzer ferromagnetic resonance method.

Electric-field induced nonlinear ferromagnetic resonance in a CoFeB/MgO magnetic tunnel junction

We investigate the rf power dependence of homodyne-detected ferromagnetic resonance (FMR) spectra of a nanoscale CoFeB/MgO magnetic tunnel junction, in which the FMR is induced by the electric-field modulation of the magnetic anisotropy. The increase of the rf power changes the spectral lineshape and decreases characteristic frequency, at which drastic change in spectrum is observed. The behavior is consistent with nonlinear magnetization precession with a large precessional angle at high powers. From the rf power dependence of FMR spectra, we determine electric-field modulation ratio of magnetic anisotropy energy density to be 78 fJ/Vm, which is in agreement with the reported values.

Static and dynamic magnetic properties of Co2FeAl-based stripe arrays

25nm to 50nm Co2FeAl (CFA) thick wire arrays with varying widths and spacing have been patterned from continuous CFA films deposited on MgO(001) using e-beam lithography and Ar ion milling. Magneto-optical Kerr effect, transverse bias initial inverse susceptibility and torque measurements reveal that the in-plane magnetic anisotropy of the wires is dominantly monitored by a uniaxial term, in contrast with the continuous films where it is governed by the superposition of a fourfold term and of a smaller uniaxial term. The microstrip ferromagnetic resonance spectra performed using a magnetic field H, applied in the plane of the studied sample along various directions, or perpendicularly to this plane, gave us access to various quantized modes originating from the patterning. In addition, Brillouin light scattering also exhibits quantized modes. A large part of the experimental observations can be quantitatively interpreted as resulting from the demagnetizing terms induced by the geometrical patterning. However, the presented model, simply built on the effect of the demagnetizing field, is not able to give account of all the quantized modes present in the resonance spectra. When H is parallel to the wires, a more complete description is used: it considers the wave-vector quantization induced by the patterning. For the magnetic modes concerned by both approaches, the correspondence between the 2 models is easily established. When H is not parallel to the wires quantitative descriptions of the behavior of the field dependence of the observed modes still can often be performed. Finally, in all the studied patterned samples, the uniform magnetic mode, termed “film mode”, relative to the parent continuous film is observed by ferromagnetic resonance: such a behavior, which has been reported previously, remains to be completely interpreted.

Tailoring the magnetodynamic properties of nanomagnets using magnetocrystalline and shape anisotropies

Magnetodynamical properties of nanomagnets are affected by the demagnetizing fields created by the same nanoelements. In addition, magnetocrystalline anisotropy produces an effective field that also contributes to the spin dynamics. In this article we show how the dimensions of magnetic elements can be used to balance crystalline and shape anisotropies, and that this can be used to tailor the magnetodynamic properties. We study ferromagnetic ellipses patterned from a 10 nm thick epitaxial Fe film with dimensions ranging from 50 x 150 nm to 150 x 450 nm. The study combines ferromagnetic resonance (FMR) spectroscopy with analytical calculations and micromagnetic simulations, and proves that the dynamical properties can be effectively controlled by changing the size of the nanomagnets. We also show how edge defects in the samples influence the magnetization dynamics. Dynamical edge modes localized along the sample edges are strongly influenced by edge defects, and this needs to be taken into account in understanding the full FMR spectrum

Temperature dependent spin structures in Hexaferrite crystal

In this work, the Hexaferrite Ba0.5Sr1.5Zn2Fe12O22 (BSZFO) is studied due to its interesting characteristics of long-wavelength spin structure. Ferromagnetic resonance (FMR) is used to probe the magnetic states of BSZFO single crystal and its temperature dependence behavior is analyzed by decomposing the multiple lines of FMR spectra into various phases. Distinguished phase transition is observed at 110K for one line, which is assigned to the ferro(ferri)-magnetic transition from non-collinear to collinear spin state.

Preparing magnetic yttrium iron garnet nanodot arrays by ultrathin anodic alumina template on silicon substrate

Ultrahigh density periodically ordered magnetic yttrium iron garnet (Y3Fe5O12, YIG) nanodot arrays have been prepared by pulsed laser deposition through an ultrathin alumina mask (UTAM). UTAM having periodically ordered circularly shaped holes with 350 nm in diameter, 450 nm in inter-pore distance, and 700 nm in height has been prepared on silicon substrate. Furthermore, the microstructure and magnetic properties of YIG nanodot arrays have been characterized. Nanodot arrays with a sharp distribution in diameter centered at 340 nm with standard deviation of 10 nm have been fabricated. Moreover, typical hysteresis loops and ferromagnetic resonance spectra in in-plane and out-of-plane revealed that this unique structure greatly influences the magnetics properties of YIG. First, coercivity of YIG nanodot arrays in in-plane was increased about from 15 Oe of YIG films to 500 Oe. Then, the degree of uniformity about nanodot height decided that two or more resonance peaks in out-of-plane were detected in the spectra. The peak-to-peak linewidth values were about 94 Oe and 40 Oe in the parallel and perpendicular directions, respectively, which indicated that the values were larger by the two-magnon scattering. Consequently, this pattering method creates opportunities for studying physics in oxide nanomagnets and may be applied in spin-wave devices.

Magnetic Properties of Carbon-Coated Ni Nanoparticles Prepared by Solid-Phase Pyrolysis of Nickel-Phthalocyanine

Carbon-coated nickel nanoparticles with a mean diameter of 40nm were synthesized via solid-phase pyrolysis of nickel-phthalocyanine. The composition structure and morphology of samples were investigated by scanning and transmission electron microscopy, X-ray diffraction, Raman spectroscopy and energy-dispersive X-ray microanalysis. Magnetic characteristics of samples were measured with a vibrational magnetometer and a magnetic resonance spectrometer. The main mass of Ni ferromagnetic nanoparticles have a single-domain and a vortex (pseudo-single domain) structures. We also revealed superparamagnetic Ni nanoparticles (several percent) and carbon paramagnetic centers caused by defects in the carbon matrix. We determined the main magnetic characteristics of Ni nanoparticles, their temperature and field dependences as well as the parameters of ferromagnetic resonance spectra.

On the frequency and field linewidth conversion of ferromagnetic resonance spectra

Both frequency swept and field swept ferromagnetic resonance measurements have been carried out for a number of different samples with negligible, moderate and significant extrinsic frequency independent linewidth contribution to analyze the correlation between the experimentally measured frequency and field linewidths. Contrary to the belief commonly held by many researchers, it is found that the frequency and field linewidth conversion relation does not hold for all cases. Instead it holds only for samples with negligible frequency independent linewidth contributions. For samples with non-negligible frequency independent linewidth contribution, the field linewidth values converted from the measured frequency linewidth are larger than the experimentally measured field linewidth. A close examination of the literature reveals that previously reported results support our findings, with successful conversions related to samples with negligible frequency independent linewidth contributions and unsuccessful conversions related to samples with significant frequency independent linewidth. The findings are important in providing guidance in ferromagnetic resonance linewidth conversions.

Enhanced spin-pumping efficiency in hcp Co polycrystalline films obtained by sputtering deposition with high substrate temperature

The variation of the resonance field and the Gilbert damping parameter (G) with hcp Co film thickness was evaluated by measuring the X- and Q-band ferromagnetic resonance (FMR) for buffer-layer/Co (2–16 nm)/Cu (1 nm)/Pt (15 nm) films. The two types of hcp Co films were deposited at a substrate temperature (Tsub) of either 400 °C or room temperature using a magnetron sputtering system. The G values for the Co films with Tsub = 400 °C exhibited a more rapid increase with decreasing thickness. In addition, the effective linewidth of FMR spectrum exhibited more rapid increase in the Q-band FMR for Tsub = 400 °C. The experimental results suggest that the spin-pumping is enhanced to a greater extent in the Co films fabricated at Tsub = 400 °C. Judging from the structural analysis of the Co films, it is possible that grain growth involving atomically flat surfaces of Co films might enhance the spin-pumping efficiency.

Influence of dipole-dipole interactions on the angular dependence of ferromagnetic resonance spectra in arrays of Fe/FexOy core/shell nanocubes

We present a detailed theoretical study of the role of long-range dipole-dipole interactions on the angular dependence of ferromagnetic resonance spectra in a two-dimensional array of nanocubes. Variations of polar (φ) and azimuthal (θ) angles are studied numerically and analytically to illustrate the effect of the magnetocrystalline properties and the dipole-dipole interactions, forming complex resonance bands. In addition, we show that when the static magnetic field lies in the arrays’ plane under the angle of 129° with the edge of the array or when its tilted around 15° to the plane’s normal, the spectra of absorption transform into a plateau spanning from 0.1 T to 0.4 T, which is prominent enough for experimental observation.

Temperature dependent electron paramagnetic resonance (EPR) of SrZrO3

SrZrO3 (SZO), a distorted perovskite was synthesized using gel-combustion route employing citric acid as a fuel and ammonium nitrate as oxidizer followed by characterization using X-ray diffraction (XRD) and electron paramagnetic resonance (EPR). Purity of the sample is confirmed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis. Broadening and shift of the resonance field position in EPR spectrum to the lower field was observed as the temperature is lowered; which is the characteristic of ferromagnetic resonance spectra. The value of Curie–Weiss temperature obtained for SZO particles is 8.7K. The positive sign of the Curie–Weiss temperature indicates that some of the spins are ferromagnetically coupled in this sample. Theoretical investigation using density functional theory (DFT) calculation revealed that Vacancy at zirconium site contribute maximum to the magnetic moment.

Ferromagnetic resonance in a topographically modulated permalloy film

A major focus within the field of magnonics involves the manipulation and control of spin-wave modes. This is usually done by patterning continuous soft magnetic films. Here, we report on work in which we use topographic modifications of a continuous magnetic thin film, rather than lithographic patterning techniques, to modify the ferromagnetic resonance spectrum. To demonstrate this technique we have performed in-plane, broadband, ferromagnetic resonance studies on a 100-nm-thick permalloy film sputtered onto a colloidal crystal with individual sphere diameters of 200 nm. Effects resulting from the, ideally, sixfold-symmetric underlying colloidal crystal were studied as a function of the in-plane field angle through experiment and micromagnetic modeling. Experimentally, we find two primary modes; the ratio of the intensities of these two modes exhibits a sixfold dependence. Detailed micromagnetic modeling shows that both modes are quasiuniform and nodeless in the unit cell but that they reside in different demagnetized regions of the unit cell. Our results demonstrate that topographic modification of magnetic thin films opens additional directions for manipulating ferromagnetic resonant excitations.