G. Asti

Spin-order and magnetic properties of BaZn2Fe16O27 (Zn2-W) hexagonal ferrite

Mössbauer and magnetic measurements have been carried out both on single crystals and polycrystalline samples of BaZn2Fe16O27 (Zn2-W) hexagonal ferrite. The saturation magnetization at 0 K and at room temperature turns out to be very high, that is, 123 and 79 Gauss·cm3/g, respectively. The results have been interpreted by assuming a local reversal or a weakening of the Fe3+ magnetic moments due to the perturbing action of Zn2+ ions. The magnetic anisotropy is confirmed to be uniaxial with an anisotropy field at room temperature of 12.500 Oe.

Influence of the Cation Distribution on the Magnetization of Y-type Hexagonal Ferrites

Mössbauer absorption and magnetization measurements have been made in Mg2−Y and Co2−Y ferrites. The magnetizations of the various iron sublattices did not show any marked difference in Mg2−Y, whilst in the case of Co2−Y three different behaviours of the sublattice magnetizations have been detected.Low-temperature magnetization measurements in Mg2Y gave evidence of a noncollinear magnetic order. The different magnetic properties of these compounds have been explained on the ground of a preferential occupation by Mg2+ and Co2+ ions of the inner octahedral sites of the T crystallographic block. The presence in these sites of a non-magnetic ion such as Mg2+ is responsible for the peculiar magnetic order observed in Mg2−Y.

Magnetic Nanostructures in Modern Technology

We present a brief overview ohhe magnetization dynamics driven by a spin-polarized current in nonconfined ferromagnetic multilayers when a point-contact setup is used. The possibility to sustain persistent oscillations in these systems could be employed in the design of currentcontrolled microwave oscillators on nanometric scale. The attempt to reproduce the main experimental results by means ofboth macrospin and micromagnetic models encounters many difficulties. We have focused on the latter, summarizing the state-of-art and pointing out the stili unsolved methodological problems arising from such a modeling.

High field first order transitions in PrCo5±x: The role of the K3 anisotropy constant

Abstract A first order magnetization process (FOMP) is observed in PrCo 5± x in the range 14–16 T from 78–155 K. The measurements are performed in polycrystalline samples by the singular point detection technique. The temperature dependence of K 1 , K 2 and K 3 anisotropy constants has been deduced.

Magnetization processes in hard Co-rich Co–Pt films with perpendicular anisotropy

We present a study of the magnetic properties and magnetization processes in hard Co-Pt (Pt∼20at.%) films. Co-rich Co-Pt films, with thickness t ranging from 5nm up to 2μm, were prepared by electrodeposition on (0001)-oriented Ru underlayers. All samples displayed strong perpendicular magnetic anisotropy and high coercivity. Virgin magnetic domain structures for varying thickness were investigated by magnetic force microscopy (MFM). The observed increase of domain width with film thickness is well understood by full two-dimensional micromagnetic computations with no adjustable parameters. The easy-axis magnetization process, as observed by measuring virgin curves by magnetometry and imaging the corresponding magnetization configurations by MFM in variable field, consists of two stages separated by a well-defined critical field, marking the onset of domain wall propagation. A thorough analysis of the out-of-plane angular dependence of the switching field points out that unpinning of domain walls is the dom...

Microstructural and magnetic properties of exchange-coupled Co/Fe multilayers

Cobalt/iron multilayers with different layer thickness were electron beam evaporated in ultrahigh vacuum, keeping constant both the number of layers and the Co/Fe thickness ratio. Structural and magnetic properties are thoroughly investigated with different techniques. The multilayers have clean and sharp interfaces. All samples show single-phase magnetic behavior in the temperature range of 5–300 K, due to the strong exchange coupling between the layers. The room temperature hysteresis loops present a sharp switching at a field of ∼8 kA/m, followed by a residual hysteresis extending up to saturation, at fields of several tens of kA/m.

Magnetic characterization of Ca substituted Ba and Sr hexaferrites

Abstract A magnetic characterization has been worked out for the solid solution from Ba and Sr hexaferrites (BaFe 1 2 O 1 9 , SrFe 1 2 O 1 9 ) towards CaO- x Fe 2 O 3 (2 ⪕ x ⪕5.5). Measurements of Curie temperature, saturation magnetization, magnetic anisotropy, together with Mossbauer characterization indicate that the intrinsic properties of the studied compounds do not change appreciably with increasing Ca content. These results, together with the X-ray data, are consistent with the formation of an undistorted M-type cell with a low content of iron and oxygen vacancies.

Singular point detection in multidomain samples

In the present work the problem of singular point detection (SPD) is examined in the case of multidomain crystallites. As is known, a uniaxial polycrystalline ferromagnet displays a sharp peak in the second derivative of magnetization M with respect to magnetic field H versus H, located at the anisotropy field H=HA. This singularity was explained on the basis of the classical SPD theory which considers only single‐domain crystallites. Here this matter is analyzed in the frame of the Neel phase theory and leads to an interpretation of the SPD peaks which seems to be more realistic. A theoretical study of the magnetization curve of a rotational ellipsoid single crystal clearly shows existence of a critical field Hc separating the multiphase from the single phase mode. The analysis of the magnetization curve of a polycrystalline material is based on the assumption that the main features of the SPD peaks are only determined by the discontinuity in the susceptibility at Hc. Contrary to the classical SPD theory...

Comparison of magnetic methods for the determination of texture of permanent magnets

The singular point detection (SPD) technique as well as a least‐mean‐square procedure of fitting trial texture functions to demagnetization curves (DMC) were applied in order to determine the orientation density of the tetragonal axes of the Nd2Fe14B crystallites in sintered Nd‐Fe‐B magnets with different degrees of grain alignment. Both methods are less complicated than calculating the texture function from x‐ray‐diffraction data. The orientation densities obtained by both methods agree fairly. Analytically, they can be described as a Gaussian or a sum of cosine‐power functions of the angle between the local tetragonal axis and the texture axis. This agreement is a test for the DMC method which can also be used for materials showing much larger anisotropy fields than the maximum available measuring field. The SPD and DMC methods can also be applied to measure the temperature dependence of the anisotropy field HA of the magnetic main phase in noncompletely textured polycrystalline materials. By means of t...

Mössbauer Measurements in ZnY (Ba2Zn2Fe12O22) and Mn0.625–ZnY Ferrites

By means of the Mossbauer technique, the hyperfine magnetic fields and the electric quadrupole interactions in ZnY (Ba2Zn2Fe12O22) and Mn0.625–ZnY planar ferrites have been investigated.From the obtained spectra it is deduced that the Zn ions are randomly distributed over the tetrahedral sites of both S and T blocks and that Mn is substituting for Fe3+.The magnetic transition temperature for Mn0.625–ZnY turns out to be TC = (143±5)°C, not much different from that of ZnY, where TC = 135°±5°C.In these compounds the angle between the magnetic moment and the electric‐field gradient differs from 90° showing the presence of angles between the magnetic moments of iron ions.