Detlef Raasch

Wall energy and wall thickness of exchange-coupled rare-earth transition-metal triple layer stacks

The room-temperature wall energy σw=4.0×10−3 J/m2 of an exchange-coupled Tb19.6Fe74.7Co5.7/Dy28.5Fe43.2Co28.3 double layer stack can be reduced by introducing a soft magnetic intermediate layer in between both layers exhibiting a significantly smaller anisotropy compared to Tb–FeCo and Dy–FeCo. σw will decrease linearly with increasing intermediate layer thickness, dIL, until the wall is completely located within the intermediate layer for dIL⩾dw, where dw denotes the wall thickness. Thus, dw can be obtained from the plot σw versus dIL. We determined σw and dw on Gd–FeCo intermediate layers with different anisotropy behavior (perpendicular and in-plane easy axis) and compared the results with data obtained from Brillouin light-scattering measurements, where exchange stiffness, A, and uniaxial anisotropy, Ku, could be determined. With the knowledge of A and Ku, wall energy and thickness were calculated and showed an excellent agreement with the magnetic measurements. A ten times smaller perpendicular aniso...

Accelerated life tests of CRT oxide cathodes

Sufficient oxide cathode life of several years in a cathode ray tube (CRT) environment at increased load is one of the key parameters in advanced CRT development. When changes are introduced, accelerated life tests are needed in order to quantify their impact. Hence the investigation of oxide cathode life limiting effects is the basis of accelerated life predictions. Especially, three basic effects are discussed and quantified: accelerated life by increased operating temperature, accelerated life by increased continuous load, and finally life acceleration by intermittent or continuos poisoning. These mechanisms can also suitably be combined.

Magnetic and magneto‐optical properties of amorphous rare‐earth–transition‐metal alloys containing Pr, Nd, Fe, Co

Amorphous rare‐earth–transition‐metal alloys of composition RE1−xFex and RE1−xCox with RE=Pr, Nd and 0<x<1 were prepared by coevaporation. Also disks for magneto‐optical recording were fabricated using TbNd‐FeCo or TbPr‐FeCo layers. The magnetization, Curie temperature, uniaxial anisotropy, coercivity, and Faraday and Kerr rotation were investigated as a function of composition and temperature. The spectral variation of the Kerr rotation was measured for some alloys. The magnetization data indicate a strong dispersion of the Pr and Nd subnetworks and of the Fe subnetwork for Fe‐rich alloys. The latter gives rise to a maximum of the Curie temperature TC around x=0.7 and to very low TC values for Fe‐based alloys with x≳0.9. The magneto‐optical effects result from both the light rare earths and the transition‐metal alloys giving rise to significantly larger rotations in the visible as compared to the alloys containing heavy rare earths. Recording experiments on standard disks with a TbNd‐FeCo layer indeed re...

Dynamic behavior of thermionic dispenser cathodes under ion bombardment

We have investigated the surface coverage and electron emission of thermionic dispenser cathodes during 3 keV Ar+ ion bombardment, thereby simulating the bombardment of the cathodes by residual gases that takes place in cathode-ray tubes as used in television sets. During the ion bombardment at the operating temperature of 1030 °C, a dynamic equilibrium is established between the sputter removal and resupply mechanisms of the Ba and O atoms that form the dipole layer on the cathode substrate. We demonstrated that the performance of the cathodes under ion bombardment is governed by the O removal and resupply rates. It was found that the Ba resupply rate is almost an order of magnitude higher than the O resupply rate, but that the Ba can only be present on the surface bound to O atoms. Therefore, the Ba/O ratio is approximately equal to unity during the ion bombardment. Based on the investigations of the removal and resupply processes, we proposed a model that accurately describes the surface coverage and e...

Magnetic and recording properties of amorphous TbFeCo–In thin films

A disadvantage of rare‐earth–transition‐metal (TM) ‐based magneto‐optical (MO) disks is the weak intrinsic chemical stability of the MO layer. This property can be improved by the addition of several elements, but elements with partly occupied d shells show a large influence on the magnetic and recording properties of the MO layer. Indium, as a 5p metal, is known to improve the long‐term stability of TbFeCo layers. We investigated the magnetic and recording properties of TbFeCo–In layers as a function of In content. Only a small decrease of Curie temperature, compensation temperature, and anisotropy was observed. The carrier‐to‐noise ratio (CNR) of TbFeCo–In disks was determined as a function of In and TM content. A CNR≳53 dB was obtained for In contents up to 14 at. %. With 27 at. % In, the CNR is still larger than 50 dB. The aging stability of MO disks showed a significant improvement for In contents larger than 10 at. %. For MO recording, both requirements, high CNR and long‐term stability, can be obta...

Bloch wall energy in (Tb,Nd)‐(Fe,Co) thin films

The Bloch wall energy has been determined for (Tb,Nd)‐(Fe,Co) magneto‐optic disks with varying Nd content. The procedure consists of applying a magnetic field to thermomagnetically written cylindrical domains and measuring the magnetic fields for which the domains expand and collapse. The wall energy can then be calculated in terms of a bubble model where a regular wall motion is assumed. The measured Bloch wall energy decreases continuously with increasing Nd content, reaching about one third of its initial value for more than 15 at. % Nd. This effect cannot be explained by a corresponding decrease in the exchange stiffness constant A or the anisotropy constant Ku. We therefore conclude that the addition of Nd, with its tendency to a sperimagnetic coupling, disturbs the magnetic ground state to such an extent that the conventional formula for the Bloch wall energy is no longer valid.

Sputtering of DyFeCo layers with an unbalanced magnetron

Abstract DyFeCo films were d.c. sputtered from a magnetically unbalanced magnetron cathode. Their density, composition and magnetic properties were investigated systematically as a function of the sputter parameters pressure and power, and were compared with evaporated films. They exhibit a lower anisotropy constant and behave in terms of the ferrimagnetic properties like evaporated films with 5 at.% less transition metal content. Corresponding magneto-optical discs show a low recording performance. These effects are attributed to the bombardment of the growing layer with energetic particles during deposition, which probably leads to a precipitation of transition metal clusters in the amorphous matrix, possibly nanoscale superparamagnetic transition metal crystallites.

MO-Recording: The Switching Process and its Relation to the Magnetic Properties of Thin Films

Magneto-optic (MO) recording combines the advantages of optical (high storage density of about 3.107 bit/cm2, contactless writing and reading) and magnetic (unlimited rewriteability) recording.1,2