Ferromagnetic Resonance Response Research Articles

Extrinsic contributions to the ferromagnetic resonance response of ultrathin films

We develop a theory of the extrinsic contributions to the ferromagnetic resonance linewidth and frequency shift of ultrathin films. The basic mechanism is two magnon scattering by defects at surfaces and interfaces. In the presence of dipolar couplings between spins in the film, one realizes short wavelength spin waves degenerate with the ferromagnetic resonance (FMR) mode, provided the magnetization is parallel to the film surfaces. Defects on the surface or interface thus scatter the FMR mode into such short wavelength spin waves, producing a dephasing contribution to the linewidth, and a frequency shift of the resonance field. The mechanism described here is inoperative when the magnetization is perpendicular to the film.

Ferromagnetic Resonance in Very Fine Co Particles forming Granular Co-Cu Films

Fine magnetic particles have been investigated with ferromagnetic resonance (FMR) for many years [1]. With the growing interest in giant magnetoresistance in granular materials [2], FMR has been proved useful for revealing important information on the size and shape of particles [3]. There are several aspects which are important in description of FMR response in fine particle assemblies: (i) thermally activated magnetization reversal, i.e., superparamagnetism; (ii) magnetic anisotropies; (iii) interparticle interactions and (iv) particle size distribution. The essential factor which distinguishes FMR in fine magnetic particles from that in polycrystals is their superparamagnetic behaviour. If the sizes of particles are so small that thermal fluctuations of the magnetic moments temporarily disturb their equilibrium configuration, the static picture is no longer valid and a statistical approach is needed. Our experimental data will be interpreted in the framework of such an approach [4] extended to take into account dipolar interactions between particles as well as their size distribution. The description of FMR in fine particles will be limited here to the main concepts. A full discussion can be found in the original paper [4]. Let us consider a fine particle of volume v, magnetic moment μ= My and the uniaxial anisotropy Ku which is subjected to a slowly varying magnetic field

Calculations of giant magnetoimpedance and of ferromagnetic resonance response are rigorously equivalent

It is simply demonstrated that the giant magnetoimpedance (GMI) response of a plate or ribbon is rigorously equivalent to the response of the same sample in ferromagnetic resonance (FMR) experiment. Thus, all of the solutions for FMR response behavior of metals may be applied to the description of GMI. For situations which have not been studied before, the methods which have been developed over the past 40 years for theoretical description of FMR in metals may be applied to predict the GMI behavior.

Calculation of FMR in interacting fine particle systems

A computer model has been developed to simulate ferromagnetic resonance (FMR) in a randomly packed interacting fine particle system. Using Stoner-Wohlfarth particles with a random distribution of easy axes, the FMR response was calculated from the second derivatives of the equilibrium energy. The results show a qualitative agreement with available experimental data and are in agreement with previous mean field calculations. However, it is shown that the mean field theory does not predict changes in the interaction effects as the field is reduced from saturation.

High field effective linewidth and eddy current losses in moderate conductivity single crystal Zn-Y hexagonal ferrite at 10–35 GHz

The microwave losses associated with the extreme high field tail region of the ferromagnetic resonance (FMR) absorption curve for 0.08- to 1.38-mm-thick c-plane circular disks of single crystal Zn-Y hexagonal ferrite materials with planar anisotropy were investigated at 10, 19.3, and 35.3 GHz, with the static external magnetic field applied in plane. Analysis of the data in terms of magnetic losses only gave anomalous high field effective linewidth ΔHF results; this ΔHF showed a substantial increase with the field, changed with both disk thickness and radius, and was higher for an out-of-plane microwave field direction than for an in-plane direction. This linewidth did scale with the square of the disk thickness, one indication of predominant eddy current losses. The data were then analyzed in terms of eddy current losses, based on the assumption of an insulator FMR response and a high field eddy current loss absorption tail driven by that response. The predictions of the model were in good agreement with the data. Fits to the data gave reasonable and consistent values of the microwave conductivity which ranged from 0.03 to 0.05 Ω−1 cm−1 at 10 GHz to 0.06–0.07 Ω−1 cm−1 at 35 GHz, relative to a measured dc conductivity of 0.02–0.03 Ω−1 cm−1.

High-field effective linewidth and eddy current losses in moderate conductivity single-crystal M-type barium hexagonal ferrite disks at 10–60 GHz

The losses associated with the high-field tail region of the ferromagnetic resonance (FMR) absorption curve were investigated at 10, 19, 35, and 60 GHz for 0.10–1.75-mm-thick c-plane circular disks of flux-grown single-crystal M-type barium ferrite materials. A conventional high-field effective linewidth analysis of the data yielded an effective linewidth which increased with the square of the disk thickness and linearly with frequency, dependencies which indicate a predominant eddy current loss process. Based on these results, an eddy current loss analysis of the tail region was done, based on the insulator FMR response and eddy current losses driven by the FMR response. This analysis leads to a new noninvasive technique for the determination of the microwave conductivity in moderate conductivity ferrites. One obtains the conductivity from an appropriate analysis of the FMR absorption tail in the same way that analysis of the magnetic loss tail yields a high-field effective linewidth. Based on this technique, the microwave conductivity of these flux-grown barium ferrite single-crystal materials was determined as a function of frequency and found to increase linearly from 0.033±0.004 Ω−1 cm−1 at 10 GHz to 0.10±0.02 Ω−1 cm−1 at 60 GHz. These results are consistent with a measured dc conductivity of 0.03–0.05 Ω−1 cm−1.

Magnetic properties of ultrathin epitaxial Co films on a Pd (111) single crystal

Detailed magnetic studies were made of ultrathin epitaxial Pd/Co bilayers grown by evaporation in UHV on a Pd(111) single crystal. The magnetic properties were studied by polar Kerr hysteresis loops, vibrating sample magnetometry, and ferromagnetic resonance (FMR) at 10 and 34 GHz. The 10-GHz measurements were carried out from room temperature to 1.6 K. The effective anisotropy fields were determined from the FMR. All the magnetic measurements showed that Co films thinner than ∼9 monolayers had an easy magnetization axis perpendicular to the film plane. It was deduced that the first-order uniaxial interface and volume anisotropy constants were, respectively, 0.8±0.05 mJ/m2 and (4 ± 1) × 105 J/m3 at room temperature. The dependence of the FMR lineshape on the angles of the applied fields was analyzed by calculating the FMR response. The observed effects could be explained by allowing for a spread in the magnetic anisotropy throughout the sample.<lz> <lz>

Ferromagnetic resonance studies of particulate media

Ferromagnetic resonance (FMR) was used to characterize particles dispersed in a nonmagnetic matrix. A sequence of media was prepared in which high-aspect-ratio CrO/sub 2/ particles were dispersed in a binder matrix with volumetric packing fractions (p) in the range 0.05 to 0.50. The resonant cavity FMR spectrometer operated at 33 GHz with the DC fields at resonance sufficient to magnetically saturate the specimens. No hysteretical effects were observed in the FMR spectra. The FMR spectra of the longitudinally oriented samples were characterized by a single response with an effective anisotropy field favoring in-plane orientation. The FMR spectra for the perpendicularly oriented samples were more complicated, with two observed resonances corresponding to effective anisotropy fields favoring perpendicular and in-plane orientations. As p was increased the response associated with the effective in-plane anisotropy field increased in strength. At p=0.44 the strengths of the two responses were comparable. In unoriented media the response associated with the effective anisotropy field favoring perpendicular orientation was also observed, but at very much smaller strengths. Thus the FMR technique simultaneously provides information regarding in-plane and perpendicular orientations of the media studied. The FMR response has been show to be sensitive to particle misorientation. >

Domain mode ferromagnetic resonance studies in bismuth-substituted magnetic garnet films

Bismuth-substituted magnetic garnet films, formulated so as to have a low magnetic quality factor Q, were grown from a bismuth-based melt using liquid-phase epitaxy. The lowest Q achieved was about 0.2. Such samples were observed to support parallel stripe domain structure and were theoretically predicted to have an ‘‘in-phase’’ domain mode ferromagnetic resonance response at frequencies as low as 250 MHz. Such ‘‘in-phase’’ resonances, along with the higher-frequency ‘‘out-of-phase’’ mode, were observed experimentally with a variable radio frequency/fixed dc field ferromagnetic resonance spectrometer. The data, which were taken as a function of dc in-plane field, were found to be in good agreement with the theory.

Brillouin light scattering detection of ferromagnetic resonance in thin films

Ferromagnetic resonance (FMR) in thin films of permalloy and yttrium iron garnet (YIG) has been studied by Brillouin light scattering (BLS) techniques. The measurements were made at 9.4 GHz on 22.4 to 75-nm-thick permalloy films and on 2.1- to 12.8-μm-thick YIG films. Intensity profiles for magnon scattered light versus in-plane applied field were obtained by analyzing the forward scattered light through the films with a high-contrast Fabry–Perot interferometer. The BLS profiles show a signal-to-noise ratio of 10-100 for the permalloy and 100-1000 for the YIG films, depending on the film thickness and the microwave power level. The FMR BLS response was quantified in terms of global response function, counts/s mW versus magnon occupation number Nu. The Nu parameter relates the scattering to the uniform mode FMR response (linewidth, field, frequency, etc.), input microwave power, and active sample volume. The response for permalloy was approximately10−8 counts/s mW magnon, which translates into a limiting sample volume of 10−12 cm3.

Microstratification and stratification in Co-Cr thin films

A number of 1.4 μm thick Co <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">78</inf> Cr <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</inf> films were prepared simultaneously by deposition on glass substrates in a modified Varian DC Magnetron sputtering system. Two ferromagnetic resonance (FMR) responses were seen in the as-deposited wafers: 1) a positive anisotropy "bulk" response and 2) a negative anisotropy "transition region" response. From selected wafers a sequence of thinner films was prepared by ion milling. On the presumption that the films were magnetically stratified, the thicknesses of the bulk and transition layers were calculated from FMR and profilometer data. The data are consistent with the presence of a "transition layer", 96 nm thick, at the glass-film interface. We find that the H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Keff</inf> values of the bulk layer resonance in these films increase with thickness. The FMR line width, ΔH, also increases as film thickness increases from 0.45 μm to 1.4 μm. For films below 0.45 μm in thickness, ΔH increases as the thickness is reduced to that of the transition layer at 0.096 μm. These microstratification effects do not appear to depend critically on whether films of a particular thickness were as-deposited or were thinned down from a thicker specimen. We conjecture that the progressive enhancement of perpendicular anisotropy might be associated with possibly (a) increased c-axis texture with thickness, (b) a perpendicular stress gradient of appropriate sign, (e) a perpendicular Cr concentration gradient of appropriate sign.

On the location of acid-hydrolysable carbon in lunar soil fines

Soil fines exposed on the lunar surface accumulate small metallic iron particles and solar wind-derived carbon. In previous work, it has been suggested that an intimate association exists between one particular carbon phase, hydrolysable carbon, and very fine iron droplets, where the carbon is in solid solution in the iron. The earlier hypothesis of a constant carbon in iron concentration across a broad range of droplet sizes is testable by combining hydrolysable carbon determinations with a variety of magnetic measurements sensitive to different droplet diameters. New measurements of ferromagnetic resonance response on density and magnetic separates from size fractions of soil 12023 are interpreted as evidence that hydrolysable carbon is preferentially associated with the larger, magnetically stable single-domain iron particles rather than with the smaller superparamagnetic droplets. For the former, there is a quite uniform ratio of iron to carbon both within a series of separates from a single soil, and among soils of widely varying FeO content.

The initial stages of oxidation of thin iron films as studied by ferromagnetic resonance

A series of polycrystalline iron films (1 < t < 100 nm) was prepared and examined under ultrahigh vacuum conditions. The effect of oxygen on the ferromagnetic resonance response was investigated, and a thickness-dependent, transient increase in the perpendicular mode resonance field was attributed to the formation of Fe m+ cations in a reconstructed surface layer. A subsequent decrease in the resonance field was consistent with the loss of ∼1.5 nm of ferromagnetic iron at large oxygen exposures. Information on the structure of the films was obtained from transmission electron microscopy of ultrahigh vacuum deposited films coated in situ with silicon monoxide.