Biblio

Macroscopic hysteresis loops and microscopic magnetic moment distributions have been determined by 3-D model for Co nanowire with various easy axis deviations from applied field. It is found that both the coercivity and the remanence decrease monotonously with the increase of easy axis deviation as well as the maximum magnetic product, indicating the large impact of the easy axis orientation on the magnetic performance. Moreover, the calculated angular distributions and the evolution of magnetic moments have been shown to explain the magnetic reversal process. It is demonstrated that the large demagnetization field in the two ends of the nanowire makes the occurrence of reversal domain nucleation easier, hence the magnetic reversal. In addition, the magnetic reversal was illustrated in terms of the analysis of the energy evolution.

The discovery of the interstitial rare earth nitride Sm2Fe17N3 came about seven years after the discovery of the rare earth iron boride Nd2Fe [1],[2], and the nitride initially seemed to offer intrinsic magnetic properties that were superior (Curie temperature TC, magnetocrystalline anisotropy K1 or comparable (spontaneous magnetization Ms to those of its illustrious predecessor. However, the promise of the new material to seriously challenge Nd2Fe14B was not realized. The 2:17 nitride powder, prepared by a low-temperature gas-phase interstitial modification process proved difficult to orient and worse still, it lost its nitrogen at the temperatures needed to process dense sintered magnets [3]. Attempts at explosive compaction [4] or spark sintering [5] failed to yield material with good enough coercivity. Nevertheless, work continued in Japan and China to develop a coercive powder that could be used for bonded magnets. An early realization was zinc-bonded Sm2Fe17N3 [6] with an energy product of 84 kJm3 but a rather low coercivity of 480 kAm-1, less than 5 % of the anisotropy field (Ha = 2K1/Ms ≈ 11 MAm-1). The anisotropy field of Nd2Fe14B is significantly less (6 MAm-1) yet several decades of intensive development have led to higher values and continuous improvements of the coercivity, even in unsubstituted material. Historical experience with permanent magnets shows that a long period of materials development is needed to arrive at the best composition and processing conditions for a microstructure that allows the hard magnetism to be optimized. Coercivities of about 25% of the anisotropy field are ultimately achieved. Here we compare the magnetic properties of melt-spun material. Our Nitroquench powder, produced by Daido Steel, was in the form of flakes 10 μm thick and up to 100 μm in diameter. A crystal-lite size of approximately 15 nm deduced from Scherrer broadening of the X-ray reflections. Composition was checked by EDX microprobe analysis. Hysteresis loops have been measured in applied fields of up to 14 T, at room temperature and at 4 K.The material exhibits a room-temperature coercivity of 690 kAm-1 after saturation in 14 T, with a remanence of 700 kAm-1 in zero applied field and an extrapolated saturation magnetization of 1230 kAm-1. The remanence ratio Mr/Ms of 63% when the remanence is corrected to zero internal field, is reflected in a preferred orientation seen in the X-ray powder diffraction patterns and in 57Fe Mössbauer spectra of magnetized powder. Spectra obtained after saturation of an immobilized powder absorber either in-plane or perpendicular to the sample plane exhibit distinctly different relative intensities of the ΔM=0 absorption lines. The maximum energy product for the powder, assuming full density, is 162 kJm-3. The remanence enhancement is attributed to fact that the nanocrystallite size is not much greater than the exchange length. Melt-spun Sm-Fe-N powder has superior corrosion resistance and thermal stability compared to melt-spun Nd-Fe-B. The Nitroquench powder may be used to produce polymer-bonded magnets with an energy product in excess of 100 kJm-3.

Tensile residual stresses combined with an applied tensile stress can reduce the reliability of steel components. Nondestructive evaluation of residual stress is thus important to avoid unintended fatigue or cracking. Because magnetic hysteresis properties of ferromagnetic materials are sensitive to stress, nondestructive evaluation of residual stress through magnetic properties can be expected. The spatial mapping of local magnetic hysteresis properties becomes possible by using the acoustically stimulated electromagnetic (ASEM) method and the tensile stress dependence of the hysteresis properties has been investigated in steel. It is found that the coercivity Hc and the remanent magnetization signal Vr monotonically decrease with increasing the tensile stress. In this work, we verified the detection of residual stresses through the ASEM response in a welded steel plate. Tensile stresses are intentionally introduced on the opposite side of the partially welded face by controlling welding temperatures. We found that Hc and Vr clearly decrease in the welded region, suggesting that the presence of tensile residual stresses is well detected by the hysteresis parameters.

Micromagnetic simulations of coercivity as a function of external magnetic field direction were performed for a hexagonal array of hemispherical Permalloy nanocaps. The analysis was based on hysteresis loops for arrangements of nanocaps of variable thickness (5 nm and 10 nm). The angular dependence of coercivity had a maximum at about 80° with respect to the arrangement plane. An increase in coercivity with nanocap thickness is related to the magnetization reversal mechanism, where the dipole energy of individual caps generates an effective intermediate axis, locking the magnetic moments. The coercivity has maximum values of 109 Oe for 5 nm and 156 Oe for 10 nm thickness. The remanence decreases monotonically with angle. This is associated with the influence of shape anisotropy, where the demagnetizing field in the plane of the array is much smaller than the demagnetizing field perpendicular to the plane.

Several applications adopt electromagnetic sensors, that base their principle on the presence of magnets realized with specific magnetic materials that show a rather high remanence, but low coercivity. This work concerns the production, analysis and characterization of hybrid composite materials, with the use of metal powders, which aim to reach those specific properties. In order to obtain the best coercivity and remanence characteristics various "recipes" have been used with different percentages of soft and hard magnetic materials, bonded together by a plastic binder. The goal was to find out the interdependence between the magnetic powder composition and the characteristics of the final material. Soft magnetic material (special Fe powder) has been used to obtain a low coercivity value, while hard materials were primarily used for maintaining a good induction remanence; by increasing the soft proportion a higher magnetic permeability has been also obtained. All the selected materials have been characterized and then tested; in order to verify the validity of the proposed materials two practical tests have been performed. Special magnets have been realized for a comparison with original ones (AlNiCo and ferrite) for two experimental cases: the first is consisting in an encoder realized through a toothed wheel, the second regards the special system used for the electric guitars.