Large and sensitive magnetostriction in ferromagnetic composites with nanodispersive precipitates

Large and sensitive magnetostriction (large strain induced by small magnetic fields) is highly desired for applications of magnetostrictive materials. However, it is difficult to simultaneously improve magnetostriction and reduce the switching field because magnetostriction and the switching field are both proportional to the magnetocrystalline anisotropy. To solve this fundamental challenge, we report that introducing tetragonal nanoprecipitates into a cubic matrix can facilitate large and sensitive magnetostriction even in random polycrystals. As exhibited in a proof-of-principle reference, Fe–Ga alloys, the figure of merit—defined by the saturation magnetostriction over the magnetocrystalline anisotropy constant—can be enhanced by over 5-fold through optimum aging of the solution-treated precursor. On the one hand, the aging-induced nanodispersive face-centered tetragonal (FCT) precipitates create local tetragonal distortion of the body-centered cubic (BCC) matrix, substantially enhancing the saturation magnetostriction to be comparable to that of single crystal materials. On the other hand, these precipitates randomly couple with the matrix at the nanoscale, resulting in the collapse of net magnetocrystalline anisotropy. Our findings not only provide a simple and feasible approach to enhance the magnetostriction performance of random polycrystalline ferromagnets but also provide important insights toward understanding the mechanism of heterogeneous magnetostriction.

MAGNETOELASTIC ELECTROMAGNETIC ACOUSTIC TRANSFORMATION. Part 5

The analytical model of the applied and internal stresses influence on the efficiency of EMAT in the region of spontaneous magnetization vectors rotation processes (a saturation magnetostriction ls < 0) in the case of strong polarizing field is considered.It has been shown analytically and confirmed experimentally that in the presence of applied s0 or internal si stresses the region of maximum EMAT efficiency in the polarizing field changes. If the product lss0 or lssi is greater than zero then this region shifts to the weaker fields. If the product lss0 or lssi is less than zero then the maximum of EMAT efficiency shifts to the stronger fields region (magnetization process is difficult) compared to the stresses absence in the sample. The EMAT efficiency for materials with ls < 0 increases with the tensile stresses increasing and decreas with the compressive stresses increasing. It has been shown analytically and confirmed experimentally that the presence of internal stresses in the material changes the efficiency of EMAT inversely related to the magnitude of these stresses. The functional relationship between EMAT efficiency and internal stresses is demonstrated.

Microstructural Dependence of Magnetization and Magnetostriction in Fe-20at.%Ga

A combination study of magnetic and magnetostrictive properties in directionally cast and differently heat-treated Fe-20Ga(at.%) samples has been carried out at room temperature. Slow cooling leads to an increase in the occupation of [200] easy magnetic axes; however, a structural ordering of Ga atoms into a metastable D03 phase decreasesthe saturation magnetostriction (λs) and the saturation magnetization (Ms), and increases coercivity (Hc).Our results confirm the contribution of D03 ordering to magnetic and magnetostrictive properties due to their pinning effects against magnetic domain wall motions. As compared to slow cooling, water quenching suppresses the formation of metastable (D03) or stable (L12) ordered phases and preserves the A2 single phase structure down to room temperature, leading to enhanced magnetostriction and magnetization.

Stress-Induced Anisotropy and Stress Dependence of Saturation Magnetostriction in the Jiles-Atherton-Sablik Model of the Magnetoelastic Villari Effect

The paper presents the results of the magnetoelastic Villari effect modelling in high-permeability Mn0.51Zn0.44Fe2.05O4 ferrite. Modelling was performed on the basis of measurements of magnetoelastic characteristics of frame-shaped samples. For the modelling, the corrected Jiles-Atherton-Sablik model was used. On the base of modelling results, the stress dependence of parameter k and stress-induced anisotropy were estimated. As a result the changes of saturation magnetostriction of Mn0.51Zn0.44Fe2.05O4 ferrite subjected to stresses were calculated. Such changes were previously observed experimentally. However, the phenomenon of stress-induced sign change of saturation magnetostriction was never previously explained quantitatively

High Temperature Properties of CoFe/CoNi and Fe/CoNi Biphase Microwires

The objective of this work has been to analyze the high-temperature behavior of magnetically single-and biphase microwires because of its interest from fundamental and applications viewpoints. Two alloy compositions with amorphous structure covered by glass have been prepared as magnetically single phase microwires by quenching & drawing technique: CoFe-based with near zero saturation magnetostriction constant and Fe-based with positive saturation magnetostriction constant. The same wires were used as the core for magnetically biphase microwires. Second CoNi phase was deposited by electroplating. Magnitudes as saturation magnetization and hysteresis parameters are determined in the temperature range from room temperature up to 1200 K. We proceed to a comparative analysis of their magnetic behaviour at different temperatures as well as after cooling down to room temperature. Information on the Curie temperature of different phases and on the influence of heating process on the magnetic properties is thus derived.

Effect of Zr Addition on Magnetostriction of Tb-Dy-Fe Alloys Prepared by Micro-Pulling-Down Method

Effects of Zr addition and annealing on the magnetostriction of Tb-Dy-Fe alloy crystals were investigated. The (Fe1.9Tb0.27Dy0.73)1-xZrx (x: 0, 0.0125, 0.025, 0.05, 0.075) crystal fibers about 2 mm in diameter were grown by the micro-pulling-down (μ-PD) method. The grown crystals showed the low chemical segregation along longitudinal direction and the growth direction was oriented in the <311> direction. The saturation magnetostriction of the fiber increases with increasing the Zr content, annealing temperature and annealing time. The fiber-shape Tb-Dy-Fe crystals prepared by the μ-PD method are promising for the applications as sensors and actuators.