F. J. A. den Broeder

Magnetic anisotropy of multilayers

Abstract The effect of growth conditions on the magnetic anisotropy of Co/Pd multilayers was studied. Epitaxial [111] multilayers grown at elevated substrate temperature show perpendicular magnetization up to 16 A Co layer thickness, due to a large interface anisotropy K s . The results indicate that K s is mainly of the Neel-type. Also Co/X multilayers show perpendicular anisotropy with X = Pt, Au and Ir, but not for X = Cu, Ag and Mo. The possible contribution of lattice misfit strain to interface anisotropy is discussed. A test for the existence of this type of anisotropy in Ni/Pd and NiFe/Pd multilayers indicates that it may not be as strong as predicted by a recent theory.

Magnetic interface anisotropy in Pd/Co and Pd/Fe multilayers

Pd/Co and Pd/Fe multilayer thin films containing ultrathin (2-12 A) Co and Fe layers were prepared by vapour deposition in ultrahigh vacuum. Their magnetization was measured at room temperature in fields parallel and perpendicular to the film plane. The Pd/Co multilayers show a transition of the preferred direction from lying in the film plane towards lying along the film normal when decreasing the Co layer thickness below 8 A. The Pd/Fe multilayers are preferably magnetized in the film plane, although the anisotropy decreases with lower Fe layer thickness. The magnetic anisotropy of both types of films can be interpreted by assuming an interface contribution which favours a perpendicular magnetization, and a volume contribution which favours an in-plane magnetization. The magnitude of the volume contribution is discussed in terms of magnetostatic, magnetocrystalline and magnetoelastic anisotropies, and equals, surprisingly, the anisotropy of the ferromagnetic elements in bulk thin film form in both cases.

Magnetic properties and structure of Pd/Co and Pd/Fe multilayers

Pd/Co and Pd/Fe multilayer films containing ultrathin Co and Fe layers were prepared by vapor deposition on substrates at room temperature. Their modulated structure, even for films containing 2‐A‐thin Co and Fe layers, was proved by x‐ray diffraction and transmission electron microscopy. Below a Co layer thickness of about 8 A, the Pd/Co multilayers acquire an easy magnetic axis perpendicular to the film, which is mainly caused by magnetic interface anisotropy. This leads for multilayers containing Co monolayers to almost rectangular hysteresis loops, by which these films may be very suitable as a perpendicular magnetic recording medium. Pd/Fe multilayers also have a perpendicular interface anisotropy, but the shape anisotropy dominates. Per unit Co volume the Pd/Co multilayers have a higher saturation magnetization than pure Co, which is attributed to an induced ferromagnetism on Pd interfacial atoms.

Interface reaction and a special form of grain boundary diffusion in the Cr-W system

The two phase diffusion between chromium and tungsten, forming b.c.c. solid solutions, has been followed right from the beginning by means of a special X-ray diffraction technique, light microscopy and electron microprobe analysis. It appears that in a first stage only tungsten diffuses into the chromium, while the tungsten remains pure. The Kirkendall-effect however indicates that chromium is the much more rapidly diffusing component. It is shown that these two observations are not contradictory. In this stage the Cr-rich alloy can dissolve more tungsten than the equilibrium value; a thermodynamic explanation of this effect is given. A second stage consists of the formation of a W-rich alloy of a distinct composition in a discontinuous way at separate sites of the interface, presumably by an interface reaction. At the junction of the interface and a tungsten grain boundary a special form of grain boundary diffusion occurs, during which the grain boundary most likely moves away from its initial position, as in the case of discontinuous decomposition. The experimental results are discussed in the form of an atomic description of the various phenomena, by taking into account the enormous difference in melting point of chromium and tungsten. This difference leads to a model of the atom and vacancy jumps at the interface right from the beginning of the diffusion; this model is based upon a much higher vacancy concentration in chromium compared to tungsten and a much greater mobility of the chromium atoms. From the considerations one might expect an “incubation time”, during which the motion of Kirkendall-markers is delayed. An analogous atomic model has been used to give a description of the mechanism, by which the special form of the diffusion of chromium along a tungsten grain boundary occurs. In this boundary, treated as a phase with a very high vacancy concentration, it is proposed that a reaction should take place, leading to the deposition of a tungsten-rich W-Cr alloy, epitaxially to the adjacent chromium crystallite or to one of the adjacent tungsten crystallites. As a result the initial grain boundary will split into two boundaries or will move away from its original position.

Orientational and structural dependence of magnetic anisotropy of Cu/Ni/Cu sandwiches: Misfit interface anisotropy

Magnetic anisotropies and misfit strain relaxations have been investigated in Cu/Ni‐wedge/Cu (100) and (111) sandwiches deposited by molecular beam epitaxy on single‐crystal Cu substrates. Our results reveal a clear distinction in the nature of the measured anisotropy at Ni thicknesses below and above the critical value tc, where the growth becomes incoherent. Below tc, coherent lattice strain modifies only the volume anisotropy, while interface anisotropy is Neel type; above tc, magnetoelastic effects are found to contribute to the interface anisotropy.

Visualization of hydrogen migration in solids using switchable mirrors

Switchable mirrors made of thin films of the hydrides of yttrium (YHx), lanthanum (LaHx) or rare-earth metals exhibit spectacular changes in their optical properties as x is varied from 0 to 3. For example, α-YHx <0.23 is a shiny, hexagonally close-packed metal, β-YH2±δ is a face-centred cubic metal with a blue tint in reflection and a small transparency window at red wavelengths, whereas hexagonally close-packed γ-YHx >2.85 is a yellowish transparent semiconductor. Here we show that this concentration dependence of the optical properties, coupled with the high mobility of hydrogen in metals, offers the possibility of real-time visual observation of hydrogen migration in solids. We explore changes in the optical properties of yttrium films in which hydrogen diffuses laterally owing to a large concentration gradient. The optical transmission profiles along the length of the film vary in such a way as to show that the formation of the various hydride phases is diffusion-controlled. We can also induce electromigration of hydrogen, which diffuses towards the anode when a current flows through the film. Consequently, hydrogen in insulating YH3−δ behaves as a negative ion, in agreement with recent strong-electron-correlation theories,. This ability to manipulate the hydrogen distribution (and thus the optical properties) electrically might be useful for practical applications of these switchable mirrors.

The cobalt-rich regions of the samarium-cobalt and gadolinium-cobalt phase diagrams

Abstract The boundaries of the homogeneity regions of the phases SmCo 5 , GdCo 5 and Gd 2 Co 17 have been determined by microscopic investigation. At elevated temperatures the homogeneity region of the phases SmCo 5 and GdCo 5 extends to the Co-rich as well as to the Co-deficient side whereas the phases Gd 2 Co 17 and Sm 2 Co 17 show almost no Co solubility. A tentative diagram is presented for the phase relationships involved in the transition between the two allotropic forms of Gd 2 Co 17 . The occurrence of various types of microstructures in as-cast and annealed RCo 5 samples is explained in terms of the peculiar shape of the solvus of the RCo 5 homogeneity regions at high temperatures.

A comparison of the magnetic anisotropy of [001] and [111] oriented Co/Pd Multilayers

Superlattices of [001]fcc Co/Pd with varying Co thicknesses from one to eight atomic layers per modulation period were epitaxially grown on NaCl by vapour deposition in UHV. Transmission electron diffraction indicates lattice coherence between the Co and the Pd layers for Co thicknesses up to six atomic layers. If deposited at a substrate temperatureTs=50°C, only the superlattices containing Ci-monolayers show perpendicular magnetization. By raisingTs to 200°C, the perpendicular anisotropy for Co monolayers is increased, and is also observed for Co bilayers. We suggest that this is due tolayer smoothening, which increases Néels interface anisotropy. For more than 6 atomic layers of Co a loss of coherence is observed atTs=50°C, accompanied by a structure transformation to hcp Co with a (0001)Co∥(111)Pd orientation.Non-epitaxial polycrystalline [111]-multilayers have a different anisotropy versus thickness behaviour. For such multilayers the range of Co thicknesses giving perpendicular magnetization is extended from 8 Å up to 12 Å atTs=200°C. The different behaviour of the single crystal [001] films is caused by a strong volume contribution to the anisotropy, which favours in-plane magnetization, opposing the perpendicular interface anisotropy. This easy-plane term is attributed to magneto-elastic anisotropy due to stretching of the Co layers, via a positive magnetostriction.

Coercive force and stability of SmCo5 and GdCo5

Abstract At low temperatures the compounds GdCo 5 and SmCo 5 decompose into the corresponding phases R 2 Co 7 and R 2 Co 17 . This transformation is connected with the dependence of the coercive force of powder compacts on sintering temperature.

Evidence for microstructural inhomogeneity in sputtered Co‐Cr thin films

Thin films of the perpendicular recording material Co100−xCrx(x=17–23) were ion beam sputtered from alloy targets. The saturation magnetization and uniaxial anisotropy constant, as a function of temperature, both show a strong dependence on the substrate temperature during deposition Ts, indicating composition shifts which increase with Ts. The films have a columnar structure on a scale which does not depend on Ts in contrast to the grain size. From studies of the phase relations in bulk Co‐Cr alloys and diffusion couples it is concluded that phase separation of ordered Co‐Cr phases does not occur at high Ts. Instead the composition shifts and the grain refinement at higher Ts are proposed to be caused by oxygen gettering and oxide trapping during film growth.