High Microwave Permeability Research Articles

Influence of Si/Cr Ratio on Flake Morphology and Magnetic Properties of Fe-Si-Al-Cr Particles

Fe-Si-Al-Cr flaky particles, as soft magnetic materials with high microwave permeability, can be used for microwave absorption. In order to enhance the permeability of the materials, some Si atoms were replaced by Cr atoms. Fe 75 Si 15-x Al 10 Cr x (x = 0, 2, 4, 6, 8) flake-shaped particles were prepared by crushing ingots by ball milling. X-ray diffraction shows that the phase of all alloys is that of a α-Fe matrix. Scanning electron microscopy shows that the particles were transformed into a flaky shape, and the average aspect ratio (width/thickness) increased with x. The mean diameters of the flaky particles increased from 31.1 μm to 230 μm. Mossbauer spectroscopy shows that the peak intensity ratio of I 2,5 /I 3,4 of a sextet increases from 2.90 to 3.3 when x increases from 0 to 6, since Fe-Si-Al-Cr flaky particles have a stronger tendency to possess a planar anisotropy than Fe-Si-Al particles, which increases magnetic permeability. Saturation magnetization (M s ) of the particles increased from 127 emu/g to 144 emu/g with x increasing from 0 to 2, but there is no significant change of M s when x increases from 2 to 6. The microwave permeability dispersion spectra were measured in the frequency range between 0.5 and 8 GHz. Thus, high permeability of Fe-Si-Al-Cr alloys can be obtained when some Si atoms are replaced by Cr atoms, which leads to a change of planar anisotropy and M s .

Influence of Sputtering Power on Static and Dynamic Magnetic Properties of FeCoN Films

A series of sputtered FeCoN thin films were deposited on Si (100) at different power levels. Films sputtered with 150 W of power exhibited almost in-plane isotropy accompanied with relatively high coercivity of about 50 Oe due to weak perpendicular anisotropy. When the deposition power was 250 W and above, a well-defined in-plane anisotropy emerged with an anisotropy field in the range of 41 to 109 Oe; the coervivity was significantly reduced to a few oersteds. Dynamic magnetic properties were measured with the shorted microstrip transmission-line perturbation method. Controllable and high microwave permeability was obtained for films deposited under high sputtering power. The differences in soft magnetic properties and magnetization dynamics originate from the changes in film composition and residual stress.

A laminate of ferromagnetic films with high effective permeability at high frequencies

The paper reports on development of magnetodielectric material with high microwave permeability. The material is a laminate of multi-layer permalloy films deposited onto a thin mylar substrate by magnetron sputtering. The deposited films are arranged into a stack and glued together under pressure to obtain the laminate. With the content of ferromagnetic component in the laminate being 22 % vol., its measured quasistatic permeability is 60. The peak value of imaginary permeability attains 50 and the peak is located near 1 GHz. As compared with the multi-layer films, which the laminate is made of, it exhibits lower magnetic loss tangent at frequencies below the magnetic loss peak and may therefore be useful for many technical applications. Lower low-frequency loss may be attributed to pressing of the glued sample. This rectifies wrinkling appearing due to sputtering of rigid multi-layer film onto flexible mylar substrate and, therefore, makes the magnetic structure of the film more uniform.

Ni3Zn ferrite octahedral nanoparticles with high microwave permeability and high magnetic loss tangent

Ni3Zn ferrite octahedral nanoparticles with grain size of about 40nm were synthesized via a low temperature hydrothermal route. The saturation magnetization (Ms) and coercive force (Hc) of the sample are 57.5emu/g and 197Oe, respectively. In 50MHz–8GHz frequency range, the complex permeability and complex permittivity for Ni3Zn ferrite/wax composites containing 60wt% ferrite powders were measured by a vector network analyzer. The real part μr' of the permeability exhibits high values between 4 and 1.65 in 50MHz–2GHz range, and the imaginary part μr'' of the permeability presents a broad peak with a maximum value of 1.53 at 2.1GHz, the calculated reflection loss of ferrite/wax coating with the thickness of 8mm reaches −13dB at 2.1GHz. The results indicate the as-prepared Ni3Zn ferrite octahedral nanoparticles can have applications in biomedicine, and microwave absorption and electric devices.

High Microwave Magnetic Permeability of Composites with Submicron Iron Flakes

An effective method for the development of magnetic materials with high microwave permeability on the basis of carbonyl iron powders has been proposed. This method consists in the modification of particle morphology from the spherical shape into flaky one with subsequent silica coating of the flakes by sol-gel method. Composites with submicron flaky iron powders (thickness < 1 μm) exhibit remarkable improvement in the microwave magnetic spectra compared to the spherical powder based composites. Even a thin silica overlayer provides a sufficient insulation of the metal particle and decreases the permittivity values.

Mössbauer study of the field induced uniaxial anisotropy in electro-deposited FeCo alloy films

Thin ferromagnetic films with in-plane magnetic anisotropy are promising materials for obtaining high microwave permeability. The paper reports a Mössbauer study of the field induced in-plane uniaxial anisotropy in electro-deposited FeCo alloy films. The FeCo alloy films were prepared by the electro-deposition method with and without an external magnetic field applied parallel to the film plane during deposition. Vibrating sample magnetometry and Mössbauer spectroscopy measurements at room temperature indicate that the film deposited in external field shows an in-plane uniaxial anisotropy with an easy direction coinciding with the external field direction and a hard direction perpendicular to the field direction, whereas the film deposited without external field does not show any in-plane anisotropy. Mössbauer spectra taken in three geometric arrangements show that the magnetic moments are almost constrained in the film plane for the film deposited with applied magnetic field. Also, the magnetic moments tend to align in the direction of the applied external magnetic field during deposition, indicating that the observed anisotropy should be attributed to directional ordering of atomic pairs.

Microwave and static magnetic properties of multi-layered iron-based films

Thin ferromagnetic films with in-plane magnetic anisotropy are promising materials for obtaining high microwave permeability. To produce a bulk massive sample from thin films, multi-layer films made of a number of many thin films may be used. The paper reports on an experimental study of microwave and static magnetic properties of multi-layer iron-based films and composite samples made of these. The multi-layer films under study are deposited onto a mylar substrate by a magnetron-sputtering process performed in Ar atmosphere with controlled N 2 admixture. The sputtered iron layers are alternated with SiO 2 layers. The measured static and microwave magnetic properties of the bulk sample are found to differ from the properties of constituent films. This is an evidence for strong interactions between the magnetic layers in the sample, which interact at distances exceeding greatly the distance between adjacent magnetic layers. A theoretic model is developed to account for magneto-dipole interactions between iron films in a multi-layer system. The model explains the anomalously high demagnetization field of the sample observed in the measurements.

High-frequency behavior of magnetic composites

The paper reviews recent progress in the field of microwave magnetic properties of composites. The problem under discussion is developing composites with high microwave permeability that are needed in many applications. The theory of magnetic composites is briefly sketched with the attention paid to the laws governing the magnetic frequency dispersion in magnetic materials and basic mixing rules for composites. Recent experimental reports on the microwave performance of magnetic composites, as well as data on the agreement of the mixing rules with the measured permeability of composites that are available from the literature are discussed. From the data, a conclusion is made that the validity of a mixing rule is determined by the permeability contrast in the composite, i.e., the difference between permeability of inclusions and that of the host matrix. When the contrast is low, the Maxwell Garnet mixing rule is frequently valid. When the contrast is high, which is of the most interest for obtaining high microwave permeability of a composite, no conventionally accepted theory is capable of accurately predicting the permeability of the composites. Therefore, the mixing rules do not allow the microwave properties of magnetic composites to be predicted when the permeability of inclusions is high, that is the case of the most interest. Because of that, general limitations to the microwave performance of composites are of importance. In particular, an important relation constraining the microwave permeability of composites follows from Kittel's theory of ferromagnetic resonance and analytical properties of frequency dependence of permeability. Another constraint concerning the bandwidth of electromagnetic wave absorbers follows from the Kramers–Kronig relations for the reflection coefficient. The constraints are of importance in design and analysis of electromagnetic wave absorbers and other devices that employ the microwave magnetic properties of composites, such as magnetic substrates for microwave antennas, microwave inductors, etc.

Multi-layered Fe Films for Microwave Applications

The microwave permeability of multi-layered Fe fllms is under study. The multi- layer fllms are found to possess more rigid magnetic structure and larger damping factor of ferro- magnetic resonance compared to those of single-layer fllms. Bulk materials with high microwave permeability may be produced as laminated structures of these multi-layer ferromagnetic fllms. In the paper, experimental data are presented on the microwave permeability of such laminated regular structures based on Fe fllms. Possible technical applications of the materials under study include thin wideband radar absorbers and miniaturized patch antennas. For many microwave applications, materials with high microwave permeability values are needed. The frequency dependence of microwave permeability is typically due to the ferromagnetic reso- nance. In many magnetic materials, the permeability varies slightly with frequency until an abrupt drop appears at the ferromagnetic resonance frequency. Above the resonance, the permeability takes values that are close to unity. Hence, an estimate of microwave permeability of a magnetic material may be made from its static permeability s and resonance frequency fr. The static per- meability characterizes the magnetic performance in the operating frequency band. The resonance frequency is an estimate for the cutofi frequency, above which the material is no more permeable enough and magnetic loss increases greatly. For high microwave permeability be attained, both fr and s must be as high as possible. These values often reveal a complex dependence on the material composition, magnetic and crystalline structure, features of manufacturing and processing, etc. However, s and fr are related closely to each other by well known Snoek's law: an increase of one leads to a decrease of another, with the product of these being dependent only on the saturation magnetization Ms of the material. The anisotropy fleld or other structure-dependent values are not involved in Snoek's law. For thin fllms with in-plane magnetization, s is reciprocal to squared fr in difierence to con- ventional Snoek's law (1): (s i 1)f 2 r = (∞4…Ms) 2 (1) where ∞ … 3GHz/kOe is the gyromagnetic ratio. Eq. (1), which is referred to as Acher's law in the literature, also has the right-side part depending on the saturation magnetization only. Therefore, Eq. (1) provides a constraint for the microwave permeability of magnetic fllms that is determined by the composition and is independent of structure, the same as Snoek's law does for bulk materials. When fr < ∞4…Ms, which is several GHz or several dozens GHz for typical high-permeability fllms, Eq. (1) produces far higher values of s and, therefore, higher microwave permeability compared to Snoek's law. Therefore, thin ferromagnetic fllms are promising materials for microwave applications. However, most applications require bulk samples. With the increase of fllm thickness, the microwave permeability of the fllm degrades because of the efiect of eddy currents and out-of-plane magnetization. For these efiects to be avoided, laminates of thin ferromagnetic layers are useful. Since the thickness of substrate is large compared to that of ferromagnetic fllm, multi-layer fllms may be used to obtain high volume fractions of ferromagnetic material in the laminate. Therefore, this study presents experimental data on the microwave performance of both multi-layer thin Fe fllms and laminates made of such fllms. 2. EXPERIMENT