Ferromagnetic Resonance Modes Research Articles

Dynamic magnetic interaction in La2/3Sr1/3MnO3/BiFeO3 heterostructure

The La2/3Sr1/3MnO3/BiFeO3 (LSMO/BFO) heterostructure is studied as probed by temperature-dependent ferromagnetic resonance (FMR). Consistent with the structure analysis, two FMR modes are found for both a single LSMO layer and a LSMO/BFO bilayer grown on DyScO3, corresponding to the strained and relaxed phases of LSMO. The FMR linewidth intensity vs temperature curve shows two maxima at 200 K and 140 K in the strained phase of LSMO/BFO bilayer, which is related to the magnetic excitations in BFO. Our results demonstrate a dynamic spin interaction between LSMO and BFO, suggesting an effective route to manipulate the magnetic properties of oxide heterostructures.

Nonlinear spin excitations in chiral magnetic solids

Comparative analysis of nonlinear spin excitations in inorganic and metal-organic magnets was performed. The properties of nonlinear magnetic excitations in molecular magnets were established: differences in the remagnetization modes of chiral and racemic crystals, effect of chirality of crystals on the soliton spin excitations, decrease in the threshold microwave power at transition to the nonlinear ferromagnetic resonance mode. It is shown that the domain wall dynamics and development of Suhl’s instability provide additional information on the microscopic mechanisms of magnetization of molecular metal-organic magnets in comparison with inorganic magnets.

Ferromagnetic and spin wave resonances in thin layer of expanded austenite phase

Abstract Four samples of austenite coatings deposited by reactive magnetron sputtering on silicon substrate at four different temperatures and pressures were investigated by ferromagnetic resonance (FMR) method at room temperature. The expanded austenite phase S (γ N) layers with thickness in the 160–273 nm range and concentration of magnetic atoms: 72 % Fe, 18 % Cr and 10 % Ni, were obtained. The coatings with nanometric size grains were strongly textured and grown mostly in [100] direction, perpendicular to the sample surface. Intense FMR spectra were recorded at various angles between the static magnetic field direction and the sample surface. A strong magnetic anisotropy of the main uniform FMR mode was observed and the effective magnetization 4πM eff determined. Spin wave resonance (SWR) modes were observed in all investigated samples in out-of-plane geometry of the magnetic field. The resonance fields of SWR modes in our samples varied linearly with the spin wave mode number. The value of the effective magnon stiffness constant was determined assuming a parabolic shape of the magnetization variation across the sample thickness.

Ferromagnetic resonance study of structure and relaxation of magnetization in NiFe/Ru superlattices

The structural properties and relaxation processes of magnetization in [Ni81Fe19(t1)/Ru(t2)]N superlattices (N=number of bilayers) were analyzed by ferromagnetic resonance (FMR) with a fixed microwave frequency. One series of samples was deposited with constant NiFe layer thickness (t1) and variable Ru layer thickness (t2); the other series, with constant t2 and variable t1. A single FMR mode was observed for t2<15Å and t1>75Å and it has been attributed to the resonance of the exchange-coupled NiFe layers across the Ru interlayers. For the other values of t1 and t2, several FMR modes appeared and they were associated to non-coupled magnetic phases with different effective magnetization formed during the multilayer growth. The FMR linewidths were analyzed as a function of the magnetic layer thickness and a strong dependence on t1−2 was observed. It was attributed to the contribution of the two-magnon scattering mechanism for the linewidth.

Dynamic coupling of magnetic resonance modes in pairs of mesoscopic rectangles

We investigate the magnetization dynamics in pairs of mesoscopic permalloy (Ni80Fe20) rectangles by means of broadband-ferromagnetic resonance measurements and micromagnetic simulations. Each pair consists of two rectangles that differ in their geometry. The local effective field at each element is significantly affected by the stray field of its neighbor for small center-to-center distances between the rectangles. In antiparallel magnetization alignment, this dynamic dipolar coupling becomes prominent and anticrossing between ferromagnetic resonance modes and higher-order spin-wave modes is observed. Combination of the experimental and the simulational findings provides a comprehensive understanding of dynamically coupled rectangles.

Microwave assisted resonant domain wall nucleation in permalloy nanowires

We have designed a system to study microwave assisted domain wall nucleation in permalloy nanowires. We find a substantial decrease in the nucleation field when microwave fields are applied, in comparison to pulse fields. A clear resonance peak is observed in the frequency dependence of the nucleation field, which coincides with the uniform mode ferromagnetic resonance frequency. Owing to the well-defined nucleation process, the switching field distribution is small in contrast to previous reports. Our results show that localized microwave field provides an efficient tool for injecting domain walls into magnetic nanowires.

Exchange coupled bilayer thin films with tilted out-of-plane anisotropy easy axis

The ferromagnetic resonance (FMR) modes are worked out for the case of exchange coupled bilayer thin films where the anisotropy axis in the ferromagnetic film is tilted out of the plane. General formulas are obtained for the mode position, frequency and field linewidths, and intensity for an arbitrary tilt angle. The analysis is then applied for the in-plane, weak and strong perpendicular anisotropies. Analytical expressions for the magnetization curve and the FMR modes are derived. It will be shown how the exchange anisotropy field HE, the uniaxial anisotropy HK, and the magnetization angle are related to the FMR spectrum characteristics and how they can be found in a straightforward manner.

High-frequency properties of two-layer exchange-coupled ferromagnetic structures

A numerical study of peculiarities of ferromagnetic resonance (FMR) in a two-layer exchange-coupled ferromagnetic structure has been performed at various magnitudes and directions of an external magnetic field. The layers have a finite thickness and are characterized by an anisotropy of the easy-plane or easy-axis type. The FMR frequencies have been found to decrease relative to the frequencies calculated in the model of an infinitely thin film. It has been demonstrated that with an increase in the layer thickness the direction of the displacement of the lower FMR mode depends on the magnitude and direction of the external magnetic field and that the frequency of the higher mode decreases at any direction of the external field. The amplitudes of the resonance curves decrease with increasing magnetic field, with the ratio of the half-width of the resonance curve to the resonance frequency decreasing up to the saturation point, after which this parameter varies only slightly. The FMR frequencies have been studied as a function of the constant of the interlayer exchange interaction for fields located in the saturation region.

Magnetization dynamics and reversal mechanism of Fe filled Ni80Fe20 antidot nanostructures

We investigate the magnetization dynamics and reversal mechanism of Fe filled Ni80Fe20 antidot nanostructures. The mutual magnetostatic coupling between the two ferromagnetic nanostructures leads to a significant modification to the magnetization reversal mechanism and ferromagnetic resonance mode profiles when compared with reference Fe dots and Ni80Fe20 antidot array. Our experimental results are in qualitative agreement with both the static and dynamic micromagnetic simulations.

Study on High-Frequency 3–D Magnetization Precession Modes of Circular Magnetic Nano-Dots Using Coplanar Wave Guide Vector Network Analyzer Ferromagnetic Resonance

Ferromagnetic resonance (FMR) modes for Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">70</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">30</sub> magnetic nano-dots of 100 nm in diameter in a mono-domain state are studied under different in-plane and out-of-plane magnetic fields using a reflection measurement from a vector network analyzer with a coplanar wave guide (CPW VNA FMR) and a micromagnetics simulation. The observed real and imaginary parts of the reflection signal become a combination of symmetric and antisymmetric Lorentzian forms in FMR. The combinations are given different susceptibility components of χ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">xx</sub> and χ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">zx</sub> by changing the sample position on the CPW. The magnetic dots exhibit triple modes in FMR spectra from an influence of the demagnetization field distribution in the mono-domain state. The magnetic dots also show a different variation of FMR amplitude in χ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">xx</sub> and χ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">zx</sub> for an out-of-plane field increase.

Broad-band FMR study of ferromagnetic thin films patterned with antidot lattices

Previous ferromagnetic resonance (FMR) studies of ferromagnetic (FM) thin films patterned with antidot (AD) arrays have generally avoided the low-field, hysteretic regime that is dominated by irreversible domain wall (DW) dynamics in unpatterned films. Moreover, FM vortices have not yet been identified and systematically studied in films patterned with AD lattices (ADLs). We have studied DC magnetization and broad-band FMR data for permalloy thin films of thickness t≈25nm, patterned with square lattices of square-shaped AD of width D and separation d=1000nm. We observe highly reproducible magnetic hysteresis curves and FMR spectra in the low-field reversal regime (i.e., applied magnetic fields H<HC, where HC is the coercive field), which indicates the ADL enforces a reproducible evolution of spin textures compared to the more random behavior of DW evolution in unpatterned films. The width of the reversal regime (2HC) and the field separation between observed FMR modes increases with D for a fixed separation d. Our micromagnetic simulations suggest these effects are consequences of both edge pinning of moments by individual AD, or DW pinning by the extended ADL, which involves two distinct length scales L≈d and L≫d, respectively. FM vortices are observed in our simulations, and their stability sensitively depends upon the AD size and applied magnetic field history.

Frequency-selective control of ferromagnetic resonance linewidth in magnetic multilayers

We report on a frequency-specific linewidth broadening of the ferromagnetic resonance (FMR) mode of a NiFe free layer within a magnetic multilayer stack. The FMR studies reveal a significant broadening of the FMR linewidth of the free layer at frequencies where this resonance is degenerate with FMR modes stemming from other layers within the multilayer stack. By pinning part of the magnetic multilayer to an antiferromagnet, we tailor a ferromagnetic linewidth behavior that is anisotropic for a specific frequency.

Femtosecond Laser Excitation of Spin Resonances in Amorphous FerrimagneticGd1−xCoxAlloys

Using 100 fs laser pulses, a high-frequency exchange and a low-frequency ferromagnetic resonance mode have been excited and detected in an amorphous Gd(1-x)Co(x) (78≤x≤85) ferrimagnetic thin film, on both sides of its angular momentum compensation composition. The excitation efficiency of these modes strongly depends on the amount of laser-induced heating. Ways of selectively and efficiently exciting either one or both of these two coexisting magnetic modes by adjusting laser pulse intensity are demonstrated.

Angular dependence of ferromagnetic resonance and magnetization configuration of thin film Permalloy nanoellipse arrays

In-plane and out-of-plane angular dependence of ferromagnetic resonance (FMR) and magnetization measurements were performed on arrays of 20, 40, and 60 nm thick, 520 nm long, and 250 nm wide elliptical Permalloy elements. Besides the main FMR “volume” mode resonance, a well-defined second FMR mode was observed, which exhibits a very strong angular dependence. This mode originates from localized regions where the magnetization has a strong component perpendicular to the bias field and to the volume magnetization. These regions of nonuniform magnetization may be associated with magnetization canting at the edges of the ellipses, due to the nonuniformity of the demagnetizing tensor elements, and with magnetization vortices, which are predicted by micromagnetic simulation.

Impedance of nanometer thickness ferromagnetic Co40Fe40B20 films

Nanocrystalline Co40Fe40B20 films, with film thickness tf = 100 nm, were deposited on glass substrates by the magnetron sputtering method at room temperature. During the film deposition period, a dc magnetic field, h = 40 Oe, was applied to introduce an easy axis for each film sample: one with h||L and the other with h||w, where L and w are the length and width of the film. Ferromagnetic resonance (FMR), ultrahigh frequency impedance (IM), dc electrical resistivity (ρ), and magnetic hysteresis loops (MHL) of these films were studied. From the MHL and r measurements, we obtain saturation magnetization 4πMs = 15.5 kG, anisotropy field Hk = 0.031 kG, and r = 168 mW.cm. From FMR, we can determine the Kittel mode ferromagnetic resonance (FMR-K) frequency fFMRK = 1,963 MHz. In the h||L case, IM spectra show the quasi-Kittel-mode ferromagnetic resonance (QFMR-K) at f0 and the Walker-mode ferromagnetic resonance (FMR-W) at fn, where n = 1, 2, 3, and 4. In the h||w case, IM spectra show QFMR-K at F0 and FMR-W at Fn. We find that f0 and F0 are shifted from fFMRK, respectively, and fn = Fn. The in-plane spin-wave resonances are responsible for those relative shifts.PACS No. 76.50.+q; 84.37.+q; 75.70.-i

Magnonic Crystal as a Medium with Tunable Disorder on a Periodical Lattice

We show that periodic magnetic nanostructures represent a perfect system for studying excitations on disordered periodical lattices because of the possibility of controlled variation of the degree of disorder by varying the applied magnetic field. Magnetic force microscopy images and ferromagnetic resonance (FMR) data collected inside minor hysteresis loops for a periodic array of Permalloy nanowires were used to demonstrate correlation between the type of FMR response and the degree of disorder of the magnetic ground state.

Apparent sixfold configurational anisotropy and spatial confinement of ferromagnetic resonances in hexagonal magnetic antidot lattices

The static and dynamic properties of hexagonal lattices of magnetic antidots have been studied using micromagnetic simulations and semianalytical modeling. The theoretical model is based on the Landau–Lifshitz equation and is developed for the case of comparable magnitudes of the field and linewidth of the ferromagnetic resonance (FMR). It is found that the antidot pattern induces an apparent sixfold configurational anisotropy manifesting itself via an anisotropic resonant response of the patterned film. However, calculations performed with a reduced damping reveal that the resonance peak consists of three different quasiuniform modes of the magnetization dynamics, with the resonant field of each of them showing a twofold variation with respect to the in-plane orientation of the applied magnetic field. The easy axes of the resonant modes are mutually rotated by 60° and combine to yield the observed sixfold configurational anisotropy. Micromagnetic calculations of the local dynamic susceptibility allow us to attribute each absorption line to a different area of the sample. Finally, we analyze the effect of the antidot radius and the lattice period on the broadening of the uniform FMR line in comparison with the uniform FMR mode of the corresponding continuous film.

Nanoscale confined mode ferromagnetic resonance imaging of an individual Ni81Fe19 disk using magnetic resonance force microscopy (invited)

We demonstrate and characterize the localized Ferromagnetic Resonance (FMR) modes created in an individual micron-sized Ni81Fe19 disk by means of a strong inhomogeneous probe field applied anti-parallel to the saturated magnetization of the sample. Our variational calculation accurately predicts the frequencies of the localized modes in FMR spectra, and characterizes the sizes and all related internal magnetic fields of the Bessel function modes in a simple model analogous to particle wavefunctions in a quantum well. The localized modes enable FMR imaging of the nonuniform demagnetizing field inside a Py disk demonstrating a novel magnetic resonance imaging technique able to map internal fields in ferromagnets with spectroscopic accuracy.

Precessional switching by ultrashort pulse laser: Beyond room temperature ferromagnetic resonance limit

Conventional pure magnetic recording schemes have a serious and unavoidable problem known as the ferromagnetic resonance (FMR) limit. Here we demonstrate a fast precessional switching via interplay of ultrafast heating by ultrashort pulse laser and large temperature dependency of magnetic resonance. In ferrimagnetic GdFeCo, when the temperature approaches the angular momentum compensation point TA, both frequency of FMR mode and the Gilbert damping parameter increase significantly. High-speed and strongly damped precessional switching was triggered with ultrafast heating of a Gd24.5Fe66.1Co9.4 across its magnetization compensation point TM up to TA, under a static applied magnetic field. To initiate and investigate the magnetization reversal, we have used an all-optical pump-probe technique employing an amplified Ti:sapphire laser system with 90 fs pulses. In particular, following the laser excitation with low pump fluence (0.9 mJ/cm2) the magnetic system relaxes back toward initial state while the rather high pump fluence (3.3 mJ/cm2) excitation induces a metastable opposite magnetic state. After the sudden heating which causes just 30% reduction of magnetization at first breakdown, consequently magnetization was started to rotate across Mz = 0 within 6 ps during first precession and finished the high damped precessional motion within few cycles into opposite direction.

Resonance modes in arrays of interacting ferromagnetic nanowires subjected to a transverse static magnetic field

We report the measurement and modeling of two ferromagnetic resonance (FMR) modes in an array of amorphous CoFeB ferromagnetic nanowires with the applied static magnetic field transverse to the nanowire axes. The two modes are observed below saturation and are excited independently by the components of the microwave magnetic field parallel and perpendicular to the static field. We model the magnetization dynamics of the array as arising from the response of two interacting populations of wires magnetized mirror-symmetrically to each other relative to the plane of the array. The theory is well supported by experimental results obtained from vibrating sample magnetometer and shorted waveguide FMR measurements.