J. A. Paulsen

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.

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...

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.

Magnetic force microscopy characterization of a first-order transition: Magnetic-martensitic phase transformation in Gd5(SixGe1−x)4

The magnetic-martensitic phase transformation of Gd5(SixGe1−x)4 (x≈0.5), which occurs close to room temperature, has been observed for the first time using a magnetic force microscope (MFM) equipped with a heating–cooling stage. MFM images obtained from a polycrystalline Gd5(Si2.09Ge1.91) sample and single crystal Gd5(Si1.95Ge2.05) and Gd5(Si2Ge2) samples showed transition to a domain structure at low temperatures indicative of a ferromagnetic phase. Some samples exhibited complex domain structures, suggesting that Gd5(SixGe1−x)4 (x≈0.5) has a strong magnetic anisotropy. As the sample temperature increased the domain structure diminished, reflecting the transformation from ferromagnetic to paramagnetic state. On cooling the sample the domain structure reappeared, but at a lower transformation temperature than on heating. This magnetic phase transformation is highly unusual because it is an “order–disorder” phase transition, which is normally second order, but in this case the “order–disorder” (ferromagnet...

Thermal expansion of single-crystal Gd/sub 5/(Si/sub 1.95/Ge/sub 2.05/) showing unusual first-order transformation

Measurements of thermal expansion of single-crystal Gd/sub 5/(Si/sub 1.95/Ge/sub 2.05/) during cooling and heating were conducted for the first time. A very steep change in strain with temperature was observed when the material underwent a phase transformation. This was an unusual simultaneous magnetic and structural phase transformation from a ferromagnet with an orthorhombic crystal structure below the transition temperature T/sub c/ to a paramagnet with a monoclinic crystal structure above T/sub c/. This transition temperature T/sub c/ was found to depend on the magnetic field, and to exhibit hysteresis depending on whether the material was being cooled or heated. In the absence of a magnetic field, T/sub c/ was 267 K on cooling and 269 K on heating. However, when the material was subjected to a magnetic induction B in the range 0-2.5 tesla (T), the transition temperatures, on both cooling and heating, were found to increase linearly with temperature by about 4.8 K/T. This rate of change of transition temperature with magnetic field was in good agreement with calculations based on the assumption that the additional energy due to the magnetic field can suppress the thermal vibration of Gd atoms and that the additional thermal energy per Gd atom needed to cause the phase transition to occur is equal to the additional magnetic energy of each Gd atom caused by the magnetic field.

A magnetic imaging system for evaluation of material conditions using magnetoresistive devices

In this paper, we develop a new magnetic imaging system for non-destructive evaluation of structural and mechanical conditions of materials. This produces a multi-parameter contour plot of the spatial variations of variety of magnetic properties such as coercivity, remanence, permeability and Barkhausen effect signal. Also we have developed an integrated sensor probe capable of measuring both magnetic hysteresis and BE signals using magnetoresistive devices.

Quantitative evaluation of stress distribution in magnetic materials by Barkhausen effect and magnetic hysteresis measurements

The feasibility of determining surface stress distribution in magnetic materials by magnetic measurements has been studied using a newly developed magnetic imaging system. The results indicate that magnetic measurements can be usefully used for detecting stress concentration in magnetic materials nondestructively. The system measured hysteresis loop and Barkhausen effect signal using a surface sensor, and converted the data into a two-dimensional image showing spatial variations in magnetic properties. The sample used in this study was a nickel plate machined into a shear stress load beam configuration. When compressive stresses were applied along the neutral axis of the sample, the image of magnetic properties such as coercivity exhibited patterns which were similar to the stress distributions calculated using finite element model (FEM). For direct comparison with FEM results, stress distributions were determined empirically from the measured coercivity values using experimental calibration of the stress dependence of coercivity. The stress patterns derived from the measured magnetic properties were found to closely resemble those calculated using FEM.

Development of a Magnetic NDE Imaging System Using Magnetoresistive Devices

A magnetic imaging system has been developed for non‐destructive evaluation of structural and mechanical conditions of materials using magnetic hysteresis and Barkhausen effect (BE) measurements. The system can be used to measure magnetic properties while scanning the surface of a material, and to convert the data into an image showing variations in the material conditions from one location to another. An integrated sensor probe capable of measuring both magnetic hysteresis and BE signals was developed using magnetoresistive devices. The imaging system has been found useful in detecting surface and sub‐surface notches as well as thickness variations in steel plates.

Thermal expansion studies on the unusual first order transition of Gd5Si2.09Ge1.91: effects of purity of Gd

Two polycrystalline samples were made by using high purity Gd and commercial Gd, respectively, but with Si and Ge starting materials of the same purity in both cases. Thermal expansion results showed that both samples exhibited a first order phase transformation, with a discontinuity in thermally-induced strain and with hysteresis in the Curie temperature. Magnetic force microscopy has been used to demonstrate the magnetic phase transformation process from paramagnetic to ferromagnetic upon cooling. It was found that the Curie temperature was lower and the thermally-induced strain higher, in the sample made from lower purity level Gd starting materials compared with the sample made from high purity Gd metal. These results indicate that the impurities (mainly C, O, N, and F) in the Gd starting material can significantly alter the strain and Curie temperature of Gd5(SixGe1−x)4 alloys.