Search

The temperature dependence of the magnetoelastic properties of a series of germanium/cobalt co-substituted cobalt ferrite samples has been measured. Magnetostriction loops of the compositions [CO.sub.1+x][Ge.sub.x][Fe.sub.2-2x][O.sub.4] (x = 0.1, 0.3, and 0.6) were measured over a temperature range of 250-400 K. Germanium/cobalt co-substitution was found to change the Curie temperature, anisotropy, and the magnetostriction coefficients in the crystal directions of [100] and [111]. Both magnetostriction and strain sensitivity were seen to decrease in magnitude with increasing temperature. [Co.sub.1.1][Ge.sub.0.1][Fe.sub.1.8][O.sub.4] was found to have a maximum sensitivity [(d[lambda]/dH).sub.[sigma]] of 2.6 x [10.sup.-9] [A.sup.-1] x m, which is twice the value obtained for pure cobalt ferrite, without any decrease in maximum magnetostriction in the linear region. Index Terms--Cobalt ferrite, germanium substitution, magnetostriction, stress sensor.

Sputtered FeSiAI(N) films were studied using magnetic force microscopy, to determine magnetization reversal.

The temperature dependence of the magnetic anisotropy of magnetoelastic chromium-substituted cobalt ferrites (Co[Cr.sub.x][Fe.sub.2-x][O.sub.4] with 0 [less than or equal to] x [less than or equal to] 0.8) was investigated over the temperature range 10-400 K. The first-order cubic anisotropy coefficient [K.sub.1] was calculated by fitting the high-field regimes of the major hysteresis loops to the law of approach to saturation, which is based on the assumption that at sufficiently high field only rotational processes remain. It was found that anisotropy increases substantially with decreasing temperature from 400 to 150 K. Below 150 K, it appears that even under a maximum applied field of 5 T, the anisotropy of Co[Fe.sub.2][O.sub.04] and Co[Cr.0.2] [Fe.sub.1.8][O.sub.4] is so high that it prevents complete approach to saturation. In general, at fixed temperature the magnetic anisotropy decreases with increasing Cr content, which could be explained based on the one-ion model and the results of Mossbauer studies: According to the one-ion anisotropy model, [Co.sup.2+] ions in octahedral sites of the spinel structure are responsible for the high anisotropy of cobalt ferrite, and substitution of [Cr.sup.3+] for [Fe.sup.3+] appears to displace [Co.sup.2+] ions from octahedral sites to tetrahedral sites. Index Terms--Cobalt ferrite, ferrite, law of approach (LoA), magnetic anisotropy.

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 [degrees]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 x [10.sup.-6] to -252 x [10.sup.-6] after annealing. The maximum strain derivative [(d[lambda]/dH).sub.max], which is related to stress sensitivity, increased from 1.5 x [10.sup.-9] [A.sup.-1]m to 3.9 x [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. Index Terms--Induced anisotropy, magnetic annealing, magnetostrictive cobalt ferrite, magnetoelastic stress sensor.

The thermal expansion and magnetostriction of polycrystalline and single-crystal [Pr.sub.5] [Ni.sub.2] [Si.sub.3] were investigated over the temperature range 5-300 K. The results show two magnetic phase transitions, one corresponding to the Curie temperature and the other at a lower temperature exhibiting characteristics of a spin reorientation transition. In the temperature range below the Curie temperature, the thermal expansion in single-crystal samples exhibited a temperature dependent anisotropy. Specifically negative and positive magnetic contributions to the thermal expansion were observed along the directions parallel and perpendicular to the c axis respectively. Index Terms--Magnetostriction, rare earth, spin reorientation, single crystal, thermal expansion.

For the first time, both the magnetoelectronic properties and the magnetic domain structure of a magnetic tunnel junction (MTJ) have been characterized simultaneously. In situ tunneling magnetoresistanee measurements for a spin-dependent tunnel junction were directly compared to magnetic domain behavior imaged using magnetic force microscopy (MFM) under the same applied fields, allowing for detailed investigation of electronic behavior relative to magnetic behavior. From the results, it was found that the resistance in the hysteretic field range (less than the coercivity of Hc = 800 A/m or 10 Oe) corresponded with the complex magnetic domain behavior observed in the MFM images and qualitatively matched micromagnetically modeled junctions. Index Terms--Domain structures, magnetic force microscopy (MFM), magnetoresistance, spin-dependent transport.

The effects of pure shear stress on magnetization in nickel have been studied with a view to developing a theoretical understanding of the Matteucci effect, which is of great technological interest in the search for viable magnetic torque sensors. The effects of pure shear stress on a planar magnetic material and those due to torque applied to a rod are analogous in the sense that they can be described in terms of a pair of tensile and compressive stresses acting at 45[degrees] to the neutral axis of the plate or the rod axis. In this work the responses of nickel to shear stresses were studied under various dc applied fields. A model description was developed as a generalization of the theory of magnetomechanical effect to include shear stresses. This theory can be used as an aid to interpretation of the experimental results. Index Terms--Magnetomechanical effect, Matteucci effect, shear stress.

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[degrees]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. Index Terms--Cobalt ferrite, Curie temperature, magnetoelastic materials, stress sensors.

Single crystal [Gd.sub.5][(Si.sub.~0.5][Ge.sub.~0.5]).sub.4] was studied using thermal expansion measurements in the vicinity of the magnetic-structural phase transition. A strain gauge was attached on the a-b plane oriented to measure the strain along the b axis and a magnetic field of H = 0.8 x [10.sup.6] A/m (B = 1 Tesla) was applied along this same b axis. The sample was then rotated through 45[degrees], 90[degrees], 120[degrees], 165[degrees], and 180[degrees] from the direction of the applied magnetic field and the thermal expansion measurements were repeated. It was found that there was an unexpected variation in the temperature at which the sudden discontinuity in strain occurred. The results seem to suggest that the transition temperature changes with direction of applied field relative to the crystal lattice. This surprising result is attributed to the combined effects of the first order phase transformation and the presence of an anisotropy 'field,' which differs with angle. This combination of factors, therefore, results in a different strength of effective field being experienced by the magnetic moments on the Gd atoms depending on their orientation. Comparing this with the previously observed field dependence of the Curie temperature in this material provides a possible explanation for this highly unusual observation. Index Terms--Anisotropy field, first-order transformation, magnetocaloric effect, thermal expansion.

The effect of impurities on the magnetoelastic properties of Gd(sub 5)Si(sub 2.09)Ge(sub 1.91) system is reported through the measurement of thermal expansion. The results show that the impurities (mainly C, O, N, and F) in the Gd starting material alters the strain and Curie temperature of Gd(sub x)(Si(sub x)Ge(sub 1-x)) alloys.

The magnetization reversal processes in magnetic tunnel junctions (MTJs) are investigated through direct observation of the domain wall processes using magnetic force microscopy (MFM) with in situ applied field capability. The factors, such as the field direction, stray field from the pinned CoFe layer, anisotropy of the NiFe free layer, shape anisotropy, and magnetostatic interaction between the magnetic layers, must be included in any valid theoretical model of magnetic devices based on these types of MTJs.

Measurements of thermal expansion of single-crystal [Gd.sub.5]([Si.sub.l.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. Index Terms--First-order transformation, hysteresis, phase transitions, single crystal, thermal expansion.

[Gd.sub.5] ([Si.sub.2][Ge.sub.2]) and related compounds with similar (nearly equal Si-to-Ge ratio) composition exhibit large magnetoresponsive properties including a giant magnetocaloric effect, colossal magnetostriction, and giant magnetoresistance near a structural-magnetic phase transition that occurs close to ambient temperature [1]. Magnetic force microscopy (MFM) and vibrating sample magnetometry (VSM) measurements on single-crystal samples of these materials indicate that the easy magnetization axis is the b-axis of the orthorhombic magnetic phase--perpendicular to the slabs. In fact, the MFM image of a surface perpendicular to the b-axis is quite similar to domain patterns perpendicular to the easy axis of Co and other highly anisotropic magnetic materials. Therefore, it appears that [Gd.sub.5] [([Si.sub.x][Ge.sub.1-x]).sub.4] may require modeling similar to other multilayers and superlattices of rare-earth metals with one or more nonmagnetic constituents that exhibit long-range magnetic order across nonmagnetic layers. Many of the important phenomena of these Gd compounds could be explained by the interaction of localized Gd magnetic moments across the covalent bonding between atomic slabs, adapting models already suggested for other similar materials [2]. Index Terms--Magnetic force microscopy (MFM), magnetic materials, magnetic measurements, rare earths.

Magnetic force microscope (MFM) with a heating-cooling stage was used for examining the magnetic-martensitic phase transformation of Gd5(Si(sub x)Ge(sub 1-x))4 (x ~ 0.5). Hysteresis in temperature was observed in the order-disorder transition and the ferromagnetic phase exhibited strong magnetic anisotropy.

X-ray photoelectron spectroscopy (XPS) was used for examining the incorporation of nitrogen in FeSiAl(N) reactive sputtered films. The changes in the apparent relative sputtering yields are observed at higher N levels with considerable decline in the Fe content of the film and a modest increase in Al and Si percentages.

Metal-bonded cobalt ferrite composites are promising candidates for torque sensors and other magnetostrictive sensing and actuating applications. The temperature dependences are explained by the changes of magnetostriction, anisotropy, spontaneous magnetization, and pinning of domain walls caused by the availability of increased thermal energy.

The magnetic properties and microstructures of as-quenched and heat-treated inert gas atomized rare-earth rich (Nd, Dy)-Fe-B powders are studied. Results show that heat treatment improves coercivity and remanence, especially in samples with high Dy and Re contents. An underquenched dendritic-like structure, with majority phase RE2Fe14B and a fine network of RE-rich material between grains were found in as-quenched particles. Correlations between the microstructure changes and magnetic property changes of heat-treated particles were also observed.

The CoCr phase of CoCr thin films was characterized using quantitative thermomagnetic analysis. A model in which magnetic exchange energy was the prime cause of phase separation was presented. The resulting CoCr phase diagram was in good agreement with the theoretical Hasebe model. In addition, calculations showed that the relative amounts of the different phases ranging from 40 to 55 vol % were nonmagnetic at room temperature.

The article discusses the pulsed-laser deposition technique, which is employed for the growth of the crystalline cobalt ferrite thin films at extremely low substrate temperatures. The analyses show that these films can be easily employed for various microelectromechanical systems devices and sensor applications.

Several studies are conducted to explain the various effects of temperature variation on the different magnetorestrictive properties of the Co[Al.sub.x][Fe.sub.2- x][O.sub.4] systems. The magnetorestriction, strain sensitivity and the other properties of the compounds are shown to be completely dependent on the temperature.

[Gd.sub.5][([Si.sub.x][Ge.sub.1-x]).sub.4] has displayed a field induced first order phase transition from a monoclinic paramagnetic to an orthorhombic ferromagnetic at temperatures above its Curie temperature. The field induced first order phase transition is observed above the projected second order phase transition temperature of the orthorhombic phase, which is attributed to the fact that the applied field is so high that it causes the broadening to a wide range of higher temperatures of the second order phase transition of the orthorhombic phase.

The variations in magnetization and magnetic anisotropy of [Ge.sup.4+]/[Co.sup.2+] cosubstituted cobalt ferrite with temperature are examined for a series of compositions [Co.sub.1+x][Ge.sub.x][Fe.sub.2-2x][O.sub.4]. The use of the Law of Approach model has given values of anisotropy coefficient [K.sub.1] that are lower than the correct values and this method cannot be used for estimating anisotropy accurately.

[Gd.sub.5][([Si.sub.x][Ge.sub.1-x]).sub.4] for 0.41

The behavior of single crystal [Tb.sub.5][Si.sub.2.2][Ge.sub.1.8] in the vicinity of its order-disorder and order-order phase transition from a higher temperature paramagnetic/monoclinic state to a lower temperature ferromagnetic/orthorhombic state is described. It is shown that the shapes and slope of the strain versus magnetic field curves are different depending on whether the materials started from above the Curie temperature or started from below the Curie temperature.

Magnetic and magnetoelastic properties of a series of Ga-substituted cobalt ferrites are examined. The enhanced strain derivative has implied high stress sensitivity, suggesting that adjusting Ga content substituted into cobalt ferrite can be used for controlling critical magnetic properties of the material for stress sensor applications.

Linear magnetostriction along the 'a' axis of single crystal [Tb.sub.5][Si.sub.2.2][Ge.sub.1.8] is investigated at different temperatures over a range of 20 to 150 K. The results show that the behavior is consistent with the hypothesis that there is a phase transition from one orthorhombic ferromagnetic state at 70 K upon heating, and in the region from 100 to 116 K there is a first order phase transition more complex than that of [Gd.sub.5][Si.sub.2][Ge.sub.2].

The temperature dependence of magnetic anisotropy and coercive field of magnetoelastic manganese-substituted cobalt ferrite specimens of composition Co[Mn.sub.x][Fe.sub.(2-x)][O.sub.4] with 0 less than or equal to x less than or equal to 0.6 is investigated. The anisotropy of all Mn-substituted cobalt ferrites were found to increase substantially with decreasing temperature and decrease with increasing Mn content.

The temperature dependent magnetization of single crystal [Pr.sub.5][Ni.sub.2][Si.sub.3] was calculated using a nearest neighbor exchange interaction approximation. The results reveal that the 'corner' atoms exhibit very different behavior from that of the other Pr atoms on the 'center' or 'edge' sites.

A series of six powder samples with compositions Co(sub 1.0)Mn(sub x)Fe(sub 2-x)O4 are investigated using transmission Mossbauer spectroscopy. The results show two sets of six-line hyperfine patterns for all samples, indicating the presence of Fe in both A and B sites.