Ferromagnetic Resonance Experiments Research Articles

In situ ferromagnetic resonance in coupled ultrathin trilayers with perpendicularly orientedeasy axes

Ultrathin ferromagnetic trilayers with perpendicularly oriented easy axes—one filmin-plane the other out-of-plane—attracted recent interest in the study of interlayerexchange coupling, domain formation, and canted magnetization orientation. To investigateand understand the thermodynamic ground state of such a prototype trilayer we employedin situ ferromagnetic resonance (FMR) for a Ni/Cu/Co/Cu(001) trilayer. The FMR polarangular dependence has been measured and analysed in the framework of theLandau–Lifshitz equation. In addition the thickness of the Cu spacer layer has beenchanged to manipulate the strength of the interlayer exchange coupling and select betweenferromagnetic and antiferromagnetic coupling. The FMR experiment yields—unlike othertechniques—the coupling strength and anisotropy energy in absolute energy units.This gives also a full insight into all the parameters governing the ground statefree energy, the equilibrium angles of the canted magnetization, as well as theoptical and acoustical mode formed by the two uniform spin-wave excitations.

Interlayer exchange coupling and magnetic anisotropy in prototype trilayers:Ab initiotheory versus experiment

The magnetic anisotropy energy (MAE) and the interlayer exchange coupling (IEC) of prototype ${\mathrm{Cu}}_{4}{\mathrm{Ni}}_{8}{\mathrm{Cu}}_{\mathrm{N}}{\mathrm{Ni}}_{9}/\mathrm{Cu}(001)$ trilayers are calculated using an ab initio approach based on the experimental lattice spacings. The results thereof are compared to ferromagnetic resonance experiments which allow for the quantitative determination of the MAE as well as the IEC. The tetragonal distortion of the Ni films due to the pseudomorphic growth leads to a positive MAE of the inner Ni layers favoring an out-of-plane easy axis. At the Cu/Ni interfaces a negative surface anisotropy is present which is, however, reduced compared to a Ni/vacuum interface. The MAE is clearly determined by the Ni layers only, whereas the IEC is shown to result from Ni and Cu layers at the inner Cu/Ni interfaces.

Dynamic and static measurements on epitaxial Fe/Si/Fe

Strong antiferromagnetic interlayer exchange coupling across an insulating spacer is in increasing demand for high-density magnetic recording. We report here on the interlayer exchange coupling of epitaxial Fe(8 nm)/Si(t)/Fe(10 nm) trilayers as a function of Si thickness studied by ferromagnetic resonance (FMR), Brillouin light scattering, and magneto optic Kerr effect (MOKE) measurement techniques. A very strong antiferromagnetic (AFM) interlayer exchange coupling (>6 erg/cm2) was observed at a spacer Si thickness of 0.7 nm. The bilinear J1 and biquadratic J2 coupling constants were determined from (i) the fitting of the angular variation of the resonance field (Hres) in FMR experiments, (ii) the field variation of the frequencies of the Damon–Eshbach surface modes (both optic and acoustic) in BLS measurements, and (iii) the fitting of longitudinal MOKE hysteresis loops. We obtain a higher Hres along the easy axis than along the hard axis and the magnetizations of the two Fe films are canted. The eightfold-like symmetry of Hres as a function of the angle observed at room temperature is due to the competition between the Fe fourfold anisotropy and AFM interfacial coupling energy. This behavior vanishes at low temperatures due to a strong increase of AFM coupling (especially J2) in comparison to fourfold in-plane anisotropy. From the fitting of the temperature dependent FMR data, we obtain the temperature variation of the bilinear and biquadratic exchange coupling constants. We distinguish the existence of canted magnetization states at resonance by fitting the experimental Hres versus θH data to the model calculation.

Broadband ferromagnetic resonance spectrometer

The continuous wave ferromagnetic resonance (FMR) spectrometer operating in multioctave (0.05–40 GHz) frequency range has been built to investigate the magnetic properties of thin ferromagnetic films in the temperature range of 4–420 K. The spectrometer has two probeheads: one is the X-band microwave reflection cavity used to perform express room temperature measurements and the other is an in-cryostat microstrip line probe to carry out FMR experiments covering the entire frequency range offered by the microwave source. Very uniform and stable magnetic field up to 2.4 T, temperature 4 K–420 K, and continuous frequency scan performed by an HP8722D vector network analyzer provide various modes of operation. Both probe heads are equipped with two-circle high precision goniometers to ensure accurate characterization of magnetic anisotropy and magnetostatic waves spectra recording. Use of the phase sensitive detection, utilized by magnetic field modulation at audio frequency and computer triggering of the network analyzer, enables broadband spectrometer sensitivity to be as high as 1.3×1011 spins/Oe.

Theoretical and experimental aspects of chaos control by time-delayed feedback.

We review recent developments for the control of chaos by time-delayed feedback methods. While such methods are easily applied even in quite complex experimental context the theoretical analysis yields infinite-dimensional differential-difference systems which are hard to tackle. The essential ideas for a general theoretical approach are sketched and the results are compared to electronic circuits and to high power ferromagnetic resonance experiments. Our results show that the control performance can be understood on the basis of experimentally accessible quantities without resort to any model for the internal dynamics.

The change of saturation magnetization in neutron-irradiated low-alloy steel

The change of saturation magnetization by neutron irradiation has been investigated for nuclear reactor pressure vessel (RPV) steels, which differ in their steel refining process. The metallurgical and magnetic properties were investigated using transmission electron microscopy (TEM), hysteresis loop and ferromagnetic resonance (FMR) techniques. An increase of saturation magnetization of neutron-irradiated steel was found in hysteresis loop measurements. To explore the cause of this increase, FMR experiments were conducted. A large difference in resonance field between non-irradiated and irradiated steel was found in the two specimens, A and B, which is in accordance with the characteristics of their carbide morphology.

Spin wave resonance in Ga1−xMnxAs

We report ferromagnetic resonance experiments on Ga1−xMnxAs thin films. For the dc magnetic field perpendicular to the sample plane, we observe up to eight distinct resonances, which we attribute to spin wave modes. To account for the spacing of the resonances, we infer a linear gradient in the magnetic properties, which is ascribed to a linear variation of the uniaxial magnetic anisotropy with film thickness. Values of D=(1±0.4)×10−9 Oe cm2 for the spin stiffness and JMnMn≈1 meV for the exchange integral between Mn spins are obtained.

Chaotic Dynamics During Slow Relaxation Process in Magnon Systems

Experimental evidence of chaotic dynamics during a slow relaxation process with very low frequency magnetization auto-oscillations in magnon systems is presented. A ferromagnetic resonance experiment using a sphere of pure yttrium iron garnet (YIG) at the X-band pumping frequency within the coincidence regime reveals a slow relaxation process with a long tail after a stepwise drop in external pumping power. The relaxation process is found to follow a power-law with time as a transient process involving a transition of local dynamics from a quasi-periodic state to a chaotic state as the collapse of torus.

Improved growth and the spin reorientation transition of Ni on (√2×2√2)R45° reconstructed O/Cu(0 0 1)

Ni films between 1 and 20 monolayers (ML) thick are deposited at room temperature on clean and (√2×2√2)R45° reconstructed––via oxygen adsorption––Cu(0 0 1). A significant expansion of the out-of-plane Ni phase by about 5 ML is revealed by ferromagnetic resonance experiments. This shift of the spin reorientation transition is attributed to a huge change of about 90 μeV/atom in the surface anisotropy due to the presence of half a monolayer of oxygen atoms on the top of Ni. Furthermore, the growth of Ni on the preoxidized Cu surface is found to be closer to the layer-by-layer one as compared to the growth on the clean Cu(0 0 1) due to the presence of oxygen which acts as a surfactant.

Fluctuation–dissipation considerations for phenomenological damping models for ferromagnetic thin films

Dissipation properties of ferromagnetic thin films are traditionally characterized by phenomenological damping models related to ferromagnetic resonance (FMR) linewidths. They may also be analyzed in terms of thermal magnetization fluctuations, the connection being made through the fluctuation–dissipation theorem. This article describes how fluctuation–dissipation relations provide a means for discriminating between alternative phenomenological magnetic damping models in ways that are not obvious using traditional uniform-magnetization descriptions appropriate to FMR experiments. In particular, it is shown that recently proposed alternatives to the well-known Gilbert damping model lead to physically untenable predictions/contradictions when analyzed in this light, unlike the traditional Gilbert model itself.

Ferromagnetic resonance of Fe–Sm–O thin films

In order to investigate the temperature and the annealing conditions dependence of magnetic properties in Fe72.1Sm5.2O22.7(at. %) thin films, such as the effective magnetization Meff, the spectroscopic splitting factor g, and the exchange stiffness constant A, we carried out ferromagnetic resonance (FMR) experiments. Fe–Sm–O thin films were deposited in an argon and oxygen mixed atmosphere using a rf magnetron sputtering apparatus at room temperature. The g is almost constant in the temperature range from 298 to 238 K, but increases with decreasing temperature in the range below 238 K. The A increases slightly with decreasing temperature to 238 K, and then increases linearly below 238 K. When the annealing time increases up to 6 h the Meff is almost constant, but the A decreases slightly with increasing annealing time.

Study of angular ferromagnetic resonance spectra of nanocrystalline ribbons

An angular ferromagnetic resonance experiment is reported on FINEMET nanocrystalline ribbons, which shows that complex resonance spectra are detected for 0<°,θ<20°, where θ is the angle between the applied field and the normal to the ribbon. The spectra comprise the mode related to the residual amorphous phase (UM) and modes related to the nanocrystallites (PM). When θ>20° only UM is detected. The variations with θ of PM are due to the nanocrystallites, which exhibit a magnetic phase transition between a superparamagnetic state and a ferromagnetic one. The interfacial exchange-coupling model is used to explain the results.

Transition of magnetic anisotropy in Ni/GaAs (0 0 1) observed by magnetization and ferromagnetic resonance

Polycrystalline thin Ni films deposited onto GaAs (0 0 1) show a transition of the magnetic anisotropy depending on its thickness. The anisotropy is perpendicular to the film plane for the thicknesses of the film ⩽12 nm. This becomes in-plane in the films having thicknesses ⩾15 nm. The films are deposited onto the n-type GaAs (0 0 1) substrate by the usual thermal evaporation method and also by the electron beam evaporation in ultra high vacuum onto a GaAs epilayer in the standard molecular beam epitaxy system. The magnetization and ferromagnetic resonance (FMR) are observed in the temperature range from 4.2 to 300 K. For the discussion of the microscopic origin of the anomalous properties in magnetization and FMR experiments, the experimental results are reviewed by introducing a uniaxial anisotropy, which is calculated from the easy-axis and hard-axis magnetization data. This calculated anisotropy is able to explain the temperature and angle dependency of the FMR spectra of the Ni films. Hence the magnetization and FMR spectra are in agreement with the type of the anisotropy and its temperature dependency. In addition to these, the temperature dependence of the in-plane magnetic anisotropy is able to explain the previously reported anomalous effect of reducing the squareness at low temperatures in Ni/GaAs.

T(3/2) dependence of the interlayer exchange coupling in ferromagnetic multilayers.

The temperature dependence of the interlayer exchange coupling in ferromagnetic films coupled across nonmagnetic spacers is determined via in situ ferromagnetic resonance experiments for various systems. Clear evidence for a T(3/2) law is found over a wide temperature regime.

Use of ferromagnetic resonance to determine the size distribution of magnetic particles

Ferromagnetic resonance (FMR) experiments were performed as a function of temperature (18–300 K) on magnesium ferrite nanoparticles precipitated in magnesium oxide, for two different annealing times. By measuring the relative intensities of the spectra due to superparamagnetic particles and the anisotropy field of the spectrum due to ferromagnetic particles, we determined the size distribution of magnesium ferrite particles. The same method can be applied to other systems of interest, such as magnetic fluids and magnetic powders.

Ferromagnetic resonance spectra in a weak stripe domain structure

The ferromagnetic resonance (FMR) spectra in magnetic films with a nonuniform magnetization configuration are known to exhibit multiple absorption peaks. In this paper, the case of ferromagnetic films supporting a weak stripe domain structure is addressed. A two-dimensional dynamic micromagnetic model is used to investigate the high-frequency response of such a magnetic structure. In a first step, the zero-field susceptibility spectra are computed. The existence of numerous resonances resulting from the excitation of surface and volume modes is predicted. The main features of spectra (number of resonances, resonance frequencies, intensities, and linewidths) strongly depend on the equilibrium spin configuration and on the rf exciting field orientation. In a second step, the susceptibility spectra in the presence of an in-plane static magnetic field applied along the stripe direction are studied in detail. The dispersion relation, frequency versus in-plane magnetic field, is computed for each magnetic excitation. These dispersion curves reveal possible mode couplings and interchange of mode characters with increasing magnetic field. The theoretical in-plane FMR spectra are then deduced from the frequency and magnetic field dependence of the dynamic susceptibility. These theoretical results are discussed in light of available experimental FMR data obtained on thin ferromagnetic films with a moderate perpendicular anisotropy.

Angular resonance and change of the magnetic state of nanocrystallites in ribbons

An angular ferromagnetic resonance experiment is reported which shows that the nanocrystallites do not possess a rigid ferromagnetic state. Following the orientation of the applied field, the nanocrystallites exhibit magnetic phase transition between a superparamagnetic and ferromagnetic state. This behavior is explained by the interfacial exchange coupling model.

Magnetic inhomogeneity in La-deficient manganate crystals

We present here, the results of an investigation of the intrinsic magnetic properties of high-quality lanthanum manganate crystals. Ferromagnetic resonance experiments show that the linewidth is small (∼100 Oe) and independent of temperature, key indications of magnetic homogeneity. The magnetization shows full alignment of the manganese spins; however, the magnetocrystalline anisotropy is not monotonic and displays an abrupt change near the ordering temperature. The results are discussed in terms of a two-phase model.

Ferromagnetic Resonance in Bulk and Microparticle Samples of Mn1.3Fe1.7O4

Ferromagnetic resonance experiments on polycrystalline manganese ferrites Mn 1.3 Fe 1.7 O 4 are presented in a systematic way. The temperature dependence of the linewidth has been studied on samples with different sizes (0.025 mm 0.15 mm. Classical contributions to the FMR linewidth according to Sparks' outline are considered in the whole range of temperature. Contributions of porosity, surface pits, anisotropy, slowly and rapidly relaxing impurities, valence-exchange and eddy-current mechanisms are analyzed. The FMR linewidth increases with decreasing sample size. This observed dependence is attributed to the decrease of the surface contribution with increasing sample size.

Perpendicular magnetic anisotropy in ultrathin yttrium iron garnet films prepared by pulsed laser deposition technique

A series of thin and ultrathin (<500 A thick) polycrystalline yttrium iron garnet (YIG) films has been prepared on quartz substrates by the pulsed laser deposition technique to study the effects of dimensional changes on the magnetic properties. The film thickness was varied from 100 to 3800 A. Saturation magnetization decreased rapidly in ultrathin films with decreasing thickness. The lattice cell of all samples was found to be slightly distorted though the elementary cell volume was one of a bulk crystal. The Curie temperature of the films did not differ from the bulk YIG value. Ferromagnetic resonance experiments show evidence of a change from in-plane magnetization to a perpendicular magnetization in films thinner than 140 A. This reorientation can be explained by the magnetoelastic anisotropy contribution related to thermally induced strain. This contribution is likely to be the principal origin of the perpendicular anisotropy.