C. C. H. Lo

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

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.

Temperature dependence of magnetic anisotropy in Mn-substituted cobalt ferrite

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.

The role of new materials in the development of magnetic sensors and actuators

Abstract Broadly magnetic sensors and actuators rely on only a few basic principles. These include the law of induction, for magneto-inductive devices; the Ampere force law, for magnetomechanical sensors; and changes in materials properties under the action of a magnetic field, such as magnetoresistance, magneto-optics or magnetoelasticity for sensors based on magnetoelectronics (Proceedings of the 5th Conference on Magnetic Materials, Measurements and Modeling: symposium on magnetic sensors materials and devices, Ames, Iowa, USA, May 16–17, 2002). The identification and characterization of new materials with enhanced magnetic properties is important for the development of improved sensors and actuators. In some cases the identification of new materials can open up new applications for magnetic sensors and actuators which were previously not possible.

Magnetic and magnetoelastic properties of Cr-substituted cobalt ferrite

We have investigated the magnetic and magnetoelastic properties of a series of Cr-substituted cobalt ferrite CoCrxFe2−xO4 (x = 0.0–0.79) samples. Substitution of Cr for some of the Fe in cobalt ferrite reduced the Curie temperature, and the effect is more pronounced than that observed in Mn-substituted cobalt ferrite samples. Cr substitution also caused the maximum magnetostriction to decrease at a greater rate than substitution of the same amount of Mn. The maximum of the strain derivative, dλ/dH, was reached for x = 0.38. The behavior of the Curie temperature of Cr-substituted and Mn-substituted cobalt ferrites has been analyzed using the Neel molecular field model and compared with recent Mossbauer spectroscopy results.

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

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 superexcha...

Superhard self-lubricating AlMgB14 films for microelectromechanical devices

Performance and reliability of microelectromechanical system (MEMS) components can be enhanced dramatically through the incorporation of protective thin-film coatings. Current-generation MEMS devices prepared by the lithographie-galvanoformung-abformung (LIGA) technique employ transition metals such as Ni, Cu, Fe, or alloys thereof, and hence lack stability in oxidizing, corrosive, and/or high-temperature environments. Fabrication of a superhard self-lubricating coating based on a ternary boride compound AlMgB14 described in this letter has great potential in protective coating technology for LIGA microdevices. Nanoindentation tests show that the hardness of AlMgB14 films prepared by pulsed laser deposition ranges from 45 GPa to 51 GPa, when deposited at room temperature and 573 K, respectively. Extremely low friction coefficients of 0.04–0.05, which are thought to result from a self-lubricating effect, have also been confirmed by nanoscratch tests on the AlMgB14 films. Transmission electron microscopy st...

Magnetic and magnetoelastic properties of Ga-substituted cobalt ferrite

Magnetic and magnetoelastic properties of a series of Ga-substituted cobalt ferrites, CoGaxFe2−xO4 (x=0.0–0.8), have been investigated. The Curie temperature TC and hysteresis properties were found to vary with gallium content (x), which indicates that exchange and anisotropy energies changed as a result of substitution of Ga for Fe. The maximum magnitude of magnetostriction decreased monotonically with increasing gallium content over the range x=0.0–0.8. The rate of change of magnetostriction with applied magnetic field (dλ∕dH) showed a maximum value of 3.2×10−9A−1m for x=0.2. This is the highest value among recently reported cobalt ferrite based materials. It was found that the dependence of magnetic and magnetoelastic properties on the amount of substituent (x) was different for Mn, Cr, and Ga. This is considered to be due to the differences in cation site occupancy preferences of the elements within the spinel crystal structure: Mn3+ and Cr3+ prefer the octahedral (B) sites, whereas Ga3+ prefers the tetrahedral (A) sites.Magnetic and magnetoelastic properties of a series of Ga-substituted cobalt ferrites, CoGaxFe2−xO4 (x=0.0–0.8), have been investigated. The Curie temperature TC and hysteresis properties were found to vary with gallium content (x), which indicates that exchange and anisotropy energies changed as a result of substitution of Ga for Fe. The maximum magnitude of magnetostriction decreased monotonically with increasing gallium content over the range x=0.0–0.8. The rate of change of magnetostriction with applied magnetic field (dλ∕dH) showed a maximum value of 3.2×10−9A−1m for x=0.2. This is the highest value among recently reported cobalt ferrite based materials. It was found that the dependence of magnetic and magnetoelastic properties on the amount of substituent (x) was different for Mn, Cr, and Ga. This is considered to be due to the differences in cation site occupancy preferences of the elements within the spinel crystal structure: Mn3+ and Cr3+ prefer the octahedral (B) sites, whereas Ga3+ prefers the t...

Improvement of magnetomechanical properties of cobalt ferrite by magnetic annealing

We report dramatic improvements in both magnetostriction level and strain derivative of polycrystalline cobalt ferrite as a result of magnetic annealing. Magnetostrictive cobalt ferrite composites have potential for use in advanced magnetomechanical stress and torque sensors due to their high sensitivity of magnetization to applied stresses and high levels of magnetostriction. Results show that annealing cobalt ferrite at 300/spl deg/C in air for 36 h under a dc field of 318 kA/m (4 kOe) induced a uniaxial anisotropy with the easy axis being along the annealing field direction. Under hard axis applied fields, the maximum magnetostriction measured along the hard axis at room temperature increased in magnitude from -200/spl times/10/sup -6/to -252/spl times/10/sup -6/ after annealing. The maximum strain derivative (d/spl lambda//dH)/sub max/, which is related to stress sensitivity, increased from 1.5/spl times/10/sup -9/ A/sup -1/m to 3.9/spl times/10/sup -9/ A/sup -1/m. The results can be interpreted in terms of the effects of induced uniaxial anisotropy on the domain structure and magnetization processes.

Micromagnetic modeling of the effects of stress on magnetic properties

A micromagnetic model has been developed for investigating the effect of stress on the magnetic properties of thin films. This effect has been implemented by including the magnetoelastic energy term into the Landau–Lifshitz–Gilbert equation. Magnetization curves of a nickel film were calculated under both tensile and compressive stresses of various magnitudes applied along the field direction. The modeling results show that coercivity increased with increasing compressive stress while remanence decreased with increasing tensile stress. The results are in agreement with the experimental data in the literature and can be interpreted in terms of the effects of the applied stress on the irreversible rotation of magnetic moments during magnetization reversal under an applied field.

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

In this paper, we investigate whether this temperature can be decreased by composition changes that decrease the Curie temperature of the ferrite, thus enabling operation within the temperature range of reversible magnetomechanical response.