Biblio

We report on the significant enhancement of perpendicular magnetic anisotropy of Ni/NiO multilayers after mild annealing up to 90 min at 250 °C. Transmission electron microscopy shows that after annealing, a partial crystallization of the initially amorphous NiO layers occurs. This turns out to be the source of the anisotropy enhancement. Magnetic measurements reveal that even multilayers with Ni layers as thick as 7 nm, which in the as-deposited state showed inplane anisotropy with square hysteresis loops, show reduced in-plane remanence after thermal treatment. Hysteresis loops recorded with the field in the normal-to-film-plane direction provide evidence for perpendicular magnetic anisotropy with up and down magnetic domains at remanence. A plot of effective uniaxial magnetic anisotropy constant times individual Ni layer thickness as a function of individual Ni layer thickness shows a large change in the slope of the data attributed to a drastic change of volume anisotropy. Surface anisotropy showed a small decrease because of some layer roughening introduced by annealing.

Datasheets of different commercially available integrated sensors for vector measurements of magnetic fields provide typical specifications, such as measurement range, sampling rate, resolution, and noise. Other characteristics of interest, such as linearity, cross-sensitivity, remanent magnetization, and drifts over temperature, are mostly missing. This letter presents testing results of those characteristics of integrated three-dimensional (3-D) sensors working with different sensor principles and technologies in a reproducible measuring process. The sensors are exposed to temperatures from -20 °C to 80 °C and are cycled in hysteresis loops in fields up to 2.5 mT. For applying high-accuracy magnetic fields, a calibrated 3-D Helmholtz coil setup is used. Commercially available integrated 3-D magnetic field sensors are put in operation on a printed circuit board using nonmagnetic passive components. All sensors are configured for best measurement accuracy according to their data-sheets. The results show that sensors based on anisotropic magnetoresistance have high accuracy and low offsets yet also a high degree of nonlinearity. Hall-based sensors show good linearity but also high cross-sensitivity. A magnetic remanence appears for Hall-based sensors with integrated magnetic concentrators as well as for sensors using anisotropic magnetoresistance. Nearly all sensors show remaining drifts over temperature regarding offset and sensitivity up to several percentages.

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.

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.