Sub-micron YIG microstructure and ferromagnetic resonance linewidth

It has been shown possible to push the grain size of sintered YIG below 1 micron by using 400Å YIG microcrystals and pressure sintering. This has implications for moving up the threshold for nonlinear spinwave excitation with a microwave pump. The ferromagnetic resonance linewidth has been shown to be inversely dependent on grain size on a log-log scale.     

The energetic equilibrium in the nonlinear ferromagnetic resonance

The equilibrium between pump and scattered energy in nonlinear ferromagnetic resonance (NFMR) occurs as a result of the following mechanisms: 1) the nonlinear decrease in magnon lifetime owing to the increase of excited levels nk, and 2) the non-linear frequency shifting of the spin waveband owing to the decrease of the average magnetization of the crystal. Both of these mechanisms tend to diminish the flow of energy into the spin system, but a comparison of the efficiencies of the two, the mechanisms shows that nonlinear frequency shifting predominates over the nonlinear dissipation. In fact, the nonlinear susceptibility of ferromagnetic crystals is determined chiefly by the nonlinear frequency shifting process. This causes the NFMR to be unstable, a result of which is confirmed experimentally.     

Powder ferromagnetic resonance spectra of some mixed ferrites

The powder FMR of six ferrites at 9.1 GHz in the temperature range 25 to 290° C is reported in this paper. The effect of the internal fields on the linewidths and the resonance fields are studied. Curie temperatures obtained for some of the ferrites from FMR are compared with the values obtained from an oscillator technique. The line shapes are found to be of Lorenzian shape. Effectiveg values were calculated and a brief discussion is made about the various contributions to the total field.     

磁性纳米膜基本物性和铁磁共振研究

在磁性纳米薄膜制备过程中,改变直流溅射的功率,制备一系列相同厚度的FeB磁性纳米膜.对纳米膜饱和磁化强度(4πMs)、矫顽力(Hc)、磁损耗μ"和铁磁共振线宽(△H)随功率的变化进行研究,发现随着溅射功率增加,Hc和△H呈现明显下降趋势,而4πMs和μ"增加.这是由于溅射功率的增加导致纳米膜内部α-Fe纳米晶相的形成,这同样被退火样品的铁磁共振曲线所证明.     

Garnet ferrite films with two thermostable frequencies of ferromagnetic resonance

The problem of thermal stabilization of the frequency of magnetization autooscillations in anisotropic cubic ferrite films is considered. The variable parameters are the magnetic field direction and the crystal lattice orientation relative to the film plane. It is shown that both parameters can be chosen so as to provide the thermal stability of two ferromagnetic resonance frequencies.     

Hexaferrite materials displaying ultra-high coercivity and sub-terahertz ferromagnetic resonance frequencies

Single-domain Sr_1−x/12Ca_x/12Fe_12−xAl_xO₁₉ (x = 4–6) particles are synthesized by a simple citrate auto-combustion method. The room temperature coercivity of the materials rises with aluminum content from 21.3 kOe (x = 4) to a maximum of 36 kOe (x = 5.5). This value is the highest among ferrite materials to date. Moreover, the magnetic alignment of the particles leads to further coercivity improvement up to 40 kOe. Due to large magnetic anisotropy the samples demonstrate sub-terahertz electromagnetic wave absorption by natural (zero-field) ferromagnetic resonance (NFMR). The absorption lines shift with aluminum substitution from 160 GHz (x = 4) to 250 GHz (x = 5.5), which is the record NFMR frequency known for a magnetic material. This research paves the way for development low-cost materials with extremely high coercivity and sub-terahertz NFMR.     

Use of a microconductor with natural ferromagnetic resonance for radio-absorbing materials

Thin radio-absorbing screens with an attenuation of 30 dB at frequencies of 8–10 GHz were fabricated using a composite with microconductor sections. Pis’ma Zh. Tekh. Fiz. 24, 21–23 (July 26, 1998)     

Application of ferromagnetic resonance probes to the characterization of flaws in metals

This article presents some results obtained in the characterization of surface flaws by means of probes using ferromagnetic resonance of yttrium iron garnets (FMR probes). These experiments on artificial flaws show that FMR probes operate like eddy current probes for nonmagnetic materials and like magnetic field sensors for magnetic ones. Consequently, the working distance is larger for magnetic materials (1000–1500 µm) than for nonmagnetic ones (100–300 µm). FMR probes have good sensitivity to narrow flaws, good spatial discrimination, and are sensitive to flaw width and depth. Vector analysis allows the separation of distance and flaw effect by phase analysis on nonmagnetic materials. On magnetic materials this phase separation does not exist and another procedure is suggested. These results, and in particular those obtained on ferromagnetic materials, point to the possibility of replacing some eddy current or magnetic particle inspections by tests with ferromagnetic resonance probes.     

Magnetic imaging in thin magnetic films by local spectrometer of ferromagnetic resonance

The automatized scanning spectrometer of ferromagnetic resonance has been designed for measuring a distribution of in-plane magnetic anisotropy, effective saturation magnetization, coercivity, and other performances across the area of thin magnetic films (TMFs) in a monodomain state. The spectrometer is supplied with a set of plug-in measuring heads overlapping frequency band 0.1 /spl divide/ 6.0 GHz. The degree of locality of measurements in the range 0.3 /spl divide/ 2.5 mm is defined by the size of the measuring hole in the microstrip resonator built in the head. The signal-to-noise ratio of the heads is not less than 10 for permalloy film of 100-/spl Aring/ thickness when the locality of measurements is about 1 mm, and it can be increased considerably in the signal accumulation mode. Magnetic imaging is demonstrated with an example of purposely-stressed TMFs.     

Ferromagnetism and ferromagnetic resonance in Mn and As co-implanted Si and GaAs

Ferromagnetism persisting above 375 K and anisotropic ferromagnetic resonance (FMR) spectra have been detected for the first time in Si co-implanted with Mn and As and annealed under appropriate conditions. For comparison, semi-insulating GaAs samples have been implanted with the same ions and subsequently annealed. They also exhibit ferromagnetism with a Curie temperature well in excess of 375 K. High-resolution transmission electron microscopy (HR TEM) performed on the samples with the best magnetic characteristics has shown the presence of nanoclusters due to the segregation of the implanted species in both Si and GaAs. The angular dependence of the FMR spectra reveals the existence of magnetic entities with the hard magnetization axes aligned along the four equivalent 〈1 1 1〉 crystal axes. The spectra are very similar in Si and GaAs, indicating that hexagonal MnAs clusters might be formed in Si.     

Nonlinear ferromagnetic resonance and foldover in yttrium irongarnet thin films-inadequacy of the classical model

A thin-film resonator structure has been used for quantitative measurements of ferromagnetic resonance foldover and the associated bistable power response for yttrium iron garnet (YIG) thin films. The resonator consisted of a 1-mm by 1-mm-square, 4.9 μm-thick epitaxial YIG film on top of a 50 μm-wide, 3-mm-long microstrip transducer. A static magnetic field of 3200 Oe was applied perpendicular to the film. Low- order magnetostatic forward volume wave standing modes were excited at low power levels in the -20-dBm range and detected as resonance dips in reflected power versus frequency spectra over the range 4-5 GHz. At powers in the 0- to +15-dBm range, these dips showed foldover and bistable response characteristics for increasing and decreasing frequency or power sweeps. The use of 1-10-μs-wide pulses instead of continuous-wave (CW) excitation resulted in the consistent disappearance of the foldover and bistability characteristics. The frequency sweep pulse data at fixed power reproduced the down-sweep CW results, and the pulse data for both increasing and decreasing power at fixed frequency reproduced the increasing-power CW results. A quantitative theoretical analysis demonstrates that observed foldover and bistable response characteristics are much weaker than predicted from the classical precession foldover mechanism proposed by Anderson and Suhl, in which the decrease in the static component of the magnetization drives the response. The up-sweep and down-sweep foldover frequency jumps both occur sooner than predicted by this classical mechanism and the calculated foldover profiles are much more severe than the data show     

Excitation of hypersound by multiplication of the ferromagnetic resonance frequency in a magnetostriction transducer

With reference to the problem of increasing the frequency of hypersound excited by a magnetostriction transducer, the feasibility of multiplication of the frequency by multiplication of the eigenfrequency of the elastic resonator is considered. It is shown that efficient multiplication is possible only in odd harmonics of the excitation signal. For interpretation of the phenomenon observed, a model of nonlinear excitation due to multiplication of the values of the longitudinal and transverse magnetization components is proposed.     

Single-crystal iron garnet films with increased thermal stability of magnetization and ferromagnetic resonance field

A new method for selecting iron-yttrium garnet (IYG) film compositions is proposed and experimentally verified. Using this approach, it is possible to obtain thick IYG films with high magnetic characteristics.     

Crystallization of the amorphous alloy Fe40Ni40B20: a ferromagnetic resonance study

The ferromagnetic resonance (FMR) technique was used to study the crystallization of the metallic glass Fe₄₀Ni₄₀B₂₀ (Vitrovac 0040). The line intensity of the amorphous phase was measured for several isothermal annealing times in the temperature range 360 to 375° C. The transformed fraction, as derived from FMR data, satisfies the Johnson-Mehl-Avrami equation with the exponentn between 1.49 and 1.70. The activation energy for crystallization is estimated from the times to 25% to 75% transformation to be 364 kJ mol^–1.     

Correlation between isotropic ferromagnetic resonance field shift and rotatable anisotropy in polycrystalline NiFe/FeMn bilayers

Polycrystalline NiFe/FeMn bilayers were fabricated by DC magnetron sputtering and studied by ferromagnetic resonance and vibrating sample magnetometer. The isotropic resonance field shift and the rotatable anisotropy are correlated to each other because they both undergo the maximal values around 2.5 nm and then decrease with increasing the antiferromagnetic layer thickness. These results can be explained in terms of thermally activated transition of antiferromagnetic spins.     

A ferromagnetic resonance study of as-quenched and annealed amorphous Fe78Cr2B12Si8

Ferromagnetic resonance (FMR) studies of as-quenched and annealed amorphous Fe₇₈Cr₂B₁₂Si₈ are presented. For short annealing times (30 min) and low annealing temperatures (473 K) the resonant field,H ₁, and the FMR line width, H, are essentially constant. This can be explained in terms of a combination of effects due to magnetic anisotropy and the in homogeneous demagnetization due to stress relief in the sample. For larger annealing times or higher annealing temperatures,H ₁ and H increase due to the percipitation of crystallites in the amorphous structure. X-ray diffraction, selected area electron diffraction patterns and transmission electron micrographs are consistent with this interpretation.