Microwave ferrites are ubiquitous in systems that send, receive, and manipulate electromagnetic signals across very high frequency to quasi-optical frequency bands. In this paper, modern microwave ferrites are reviewed including spinel, garnet, and hexaferrite systems as thin and thick films, powders and compacts, and metamaterials. Their fundamental properties and utility are examined in the context of high frequency applications ranging from the VHF to millimeter-wave bands. Perspective and outlook of advances in theory, processing, and devices occurring in the science and engineering communities since the year 2000 are presented and discussed.

Modern Microwave Ferrites

The role of cobalt ferrite magnetic nanoparticles in medical science

Here we report a facile strategy to synthesize porous FexCo3−xO4 nanocages by heating Prussian blue analogues FeyCo1−y[Co(CN)6]0.67 nH2O nanospheres with tunable size and morphology. The iron doping amount had significant influence on the final morphology and the most uniform nanocages were obtained from x = 0.8. The catalytic performance of the nanocages was thoroughly evaluated by activation of peroxymonosulfate (PMS) for removal of bisphenol A (BPA) in water. The influence of different process parameter on the BPA degradation efficiency was examined and the catalytic stability was tested. The BPA degradation pathway was proposed based on GC–MS and LC–MS results. The involved radicals were identified through radical scavenging experiments and electron paramagnetic resonance spectroscopy. Mossbauer and XPS techniques were applied to illustrate the catalytic mechanism and B-site CoII on the surface of FexCo3−xO4 nanocages was determined as the main factor for PMS activation. Results indicate that porous FexCo3−xO4 nanocages are available to serve as alternative environmentally friendly catalysts for pollutants removal by activation of PMS.

FexCo3-xO4 nanocages derived from nanoscale metal–organic frameworks for removal of bisphenol A by activation of peroxymonosulfate

Nanoparticles of CoFe2O4 and CoIn0.15Fe1.85O4 ferrites were prepared by citrate gel route and characterized to understand their structural, electrical, and magnetic properties. X-ray diffraction and Raman spectroscopy were used to confirm the formation of single phase cubic spinel structure. The average grain sizes from the Scherrer formula were below 50 nm. Microstructural features were obtained by scanning electron microscope and compositional analysis by energy dispersive spectroscopy. The hysteresis curve shows enhancement in coercivity while reduction in saturation magnetization with the substitution of In3+ ions. Enhancement of coercivity is attributed to the transition from multidomain to single domain nature. Electrical properties, such as dc resistivity as a function of temperature and ac conductivity as a function of frequency and temperature were studied for both the samples. The activation energy derived from the Arrhenius equation was found to increase in the doped sample. The dielectric cons...

Magnetic and electrical properties of In doped cobalt ferrite nanoparticles

Cobalt ferrite materials have a wide variety of technological applications that requires temperatures higher than room temperature. Thus the magnetic properties such as saturation magnetization, Ms remanent magnetization, Mr, coercivitty, Hc, and Curie temperature, Tc, of nanoparticles of CoFe2O4 were studied in a broad range of temperature varying from room temperature to 870 K. It was observed that, for temperatures 100 K above room temperature, these magnetic properties are still the same as at room temperature. The results were discussed in terms of interparticle interactions induced by the thermal fluctuations, cation distribution, and other imperfections that exert fields on Co2+ ions could increase with temperature.

High temperature magnetic properties of cobalt ferrite nanoparticles

Metal bonded cobalt ferrite composites have been shown to be promising candidate materials for use in magnetoelastic stress sensors, due to their large magnetostriction and high sensitivity of magnetization to stress. However previous results have shown that below 60 °C the cobalt ferrite material exhibits substantial magnetomechanical hysteresis. In the current study, measurements indicate that substituting Mn for some of the Fe in the cobalt ferrite can lower the Curie temperature of the material while maintaining a suitable magnetostriction for stress sensing applications. These results demonstrate the possibility of optimizing the magnetomechanical hysteresis of cobalt ferrite-based composites for stress sensor applications, through control of the Curie temperature.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1392&context=ameslab_pubs

Manganese-substituted cobalt ferrite magnetostrictive materials for magnetic stress sensor applications

The temperature variation of magnetic anisotropy and coercive field of magnetoelastic manganese-substituted cobalt ferrites (CoMnxFe2?xO4 with 0 ? x ? 0.6) was investigated. Major magnetic hysteresis loops were measured for each sample at temperatures over the range 10–400 K, using a superconducting quantum interference device magnetometer. The high-field regimes of the hysteresis loops were modeled using the law of approach to saturation equation, based on the assumption that at sufficiently high field only rotational processes remain, with an additional forced magnetization term that was linear with applied field. The cubic anisotropy constant K1 was calculated from the fitting of the data to the theoretical equation. It was found that anisotropy increases substantially with decreasing temperature from 400 to 150 K, and decreases with increasing Mn content. Below 150 K, it appears that even under a maximum applied field of 5 T, the anisotropy of CoFe2O4 and CoMn0.2Fe1.8O4 is so high as to prevent complete approach to saturation, thereby making the use of the law of approach questionable in these cases.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1100&context=mse_pubs

Temperature dependence of magnetic anisotropy in Mn-substituted cobalt ferrite

Understanding the effect of Mn substitution for Fe in Co ferrite presents a challenge because there are three different transition-metal ions distributed among two distinct crystallographic and magnetic sublattices with complicated superexchange and anisotropic interactions. In this study, a series of six powder samples with compositions Co1.0MnxFe2?xO4 were investigated using transmission Mossbauer spectroscopy. Mossbauer spectroscopy provides an excellent tool for probing the local environment of the Fe atoms present in such materials. Results show two sets of six-line hyperfine patterns for all samples, indicating the presence of Fe in both A and B sites. Identification of sites is accomplished by evidence from hyperfine distribution width, integrated intensity, and isomer-shift data. Increasing Mn concentration was found to decrease the hyperfine field strength at both sites, but at unequal rates, and to increase the distribution width. This effect is due to the relative strengths of Fe–O–X superexchange (X = Fe, Co, or Mn) and the different numbers of the next-nearest neighbors of A and B sites. Results are consistent with a model of Mn substituting into B sites and displacing Co ions onto A sites.

/pdf/mossbauer-spectroscopy-investigation-of-mn-substituted-co-3xas1scpna.pdf

Mössbauer spectroscopy investigation of Mn-substituted Co-ferrite (CoMnxFe2−xO4)

Cobalt ferrite has been shown to be an excellent candidate material for high sensitivity magnetic stress sensors due to its large magnetomechanical effect and high sensitivity to strain. However, near room temperature, the material exhibits some magnetomechanical hysteresis, which becomes negligible for temperatures of 60/spl deg/C and above. Measurements indicate that doping the cobalt ferrite with silicon lowers the Curie temperature of the material. It was also found that the Curie temperature of the material depends on the fabrication and processing procedure. These results offer the possibility of decreasing the room temperature magnetomechanical hysteresis through control of the Curie temperature, thereby altering the temperature dependence of magnetic and magnetomechanical properties.

J. A. Paulsen

Papers

Manganese-substituted cobalt ferrite magnetostrictive materials for magnetic stress sensor applications

Temperature dependence of magnetic anisotropy in Mn-substituted cobalt ferrite

Mössbauer spectroscopy investigation of Mn-substituted Co-ferrite (CoMnxFe2−xO4)

Study of the Curie temperature of cobalt ferrite based composites for stress sensor applications

Mossbauer spectroscopy investigation of Mn-substituted co-ferrite