Willem Hoving

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.

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.

Spatially resolved magneto-optical investigation of the perpendicular anisotropy in a wedge-shaped ultrathin epitaxial Co layer on Pd(111)

Abstract We have used the polar Kerr effect to measure the thickness dependence of the perpendicular anisotropy of ultrathin Co in Pd/Co/Pd(111) sandwiches in the 3–9 monolayer range. The anisotropy was determined from polar Kerr measurements of the perpendicular component of the magnetisation both as a function of the strength and the angle of the applied field. The Co layers were grown epitaxially in ultrahigh vacuum on a Pd(111) single crystal and covered by a 20 A epitaxial Pd layer. The Co layer was deposited in the form of a wedge (0–9 monolayers) so that the thickness dependence of the anisotropy could be measured on a single sample.

Local structural and polar Kerr effect measurements on an ultrathin epitaxial Co wedge grown on Pd(111)

A Pd/Co double layer has been grown epitaxially in ultrahigh vacuum on a Pd(111) single crystal, with the Co in the shape of a wedge of thickness varying from 0 to 10 monolayers (ML). The polar magneto‐optical Kerr effect has been used to make a nearly continuous set of measurements of the magnetic properties as a function of Co thickness tCo. The coercive fields Hc and nucleation fields Hn show large, well‐defined maxima at tCo∼2.3 ML. Between 4 and 9 ML, we find that Hc falls with increasing thickness according to a tCo−5/2 dependence. The Kerr ellipticity and Kerr rotation depend linearly on Co thickness above ∼2 ML Co, but show appreciable offsets when extrapolated to tCo=0. These offsets are clear evidence of the polarization of Pd near the interfaces. Extensive in situ growth studies of the structure of the films are also reported.

Magnetic properties of ultrathin epitaxial Co films on a Pd (111) single crystal

Detailed magnetic studies were made of ultrathin epitaxial Pd/Co bilayers grown by evaporation in UHV on a Pd(111) single crystal. The magnetic properties were studied by polar Kerr hysteresis loops, vibrating sample magnetometry, and ferromagnetic resonance (FMR) at 10 and 34 GHz. The 10‐GHz measurements were carried out from room temperature to 1.6 K. The effective anisotropy fields were determined from the FMR. All the magnetic measurements showed that Co films thinner than ∼9 monolayers had an easy magnetization axis perpendicular to the film plane. It was deduced that the first‐order uniaxial interface and volume anisotropy constants were, respectively, 0.8±0.05 mJ/m2 and (4 ± 1) × 105 J/m3 at room temperature. The dependence of the FMR lineshape on the angles of the applied fields was analyzed by calculating the FMR response. The observed effects could be explained by allowing for a spread in the magnetic anisotropy throughout the sample.

Co/Ni multilayers with perpendicular magnetic anisotropy: Kerr effect and thermomagnetic writing

Perpendicular magnetic anisotropy has recently been predicted and confirmed for a Co1/Ni2 multilayer [G. H. O. Daalderop, P. J. Kelly, and F. J. A. den Broeder, Phys. Rev. Lett. 68, 682 (1992)]. This letter reports on some magneto‐optical properties of this new multilayer. Its polar Kerr effect appeared to be considerably larger than for a Co1/Pd2 multilayer and comparable to a Co1/Pt2 multilayer. For certain vapor deposition conditions, the perpendicular magneto‐optic hysteresis loop of a 120 A Co1/Ni2 layer showed 100% remanence with a coercivity as high as 140 kA/m. Thermomagnetic writing was possible despite the high Curie temperature.

Accurate manipulation using laser technology

In the industrial production of electrical, optical, and micro-mechanical components, progress in miniaturization requires improved adjusting techniques. Sub-micrometer accuracy adjustment must be obtained within seconds, and the accuracy should be stable over many years. All methods that are presently applied for manipulation in sub-micron dimensions are cumbersome, time-consuming, and tedious, and require expensive equipment. A novel method, laser adjustment, is being explored in which permanent deformation of thin metal sheets are obtained by using thermo-mechanical stresses that occur when the sheets are locally heated using short, intense laser pulses. Manipulation along several degrees of freedom can be realized by both out-of-plane and in-plane laser adjustment or a combination thereof. Within the Brite-Euram project AMULET this new automated micro- manufacturing technology for mass production is developed in order to assemble components where tolerance conditions and accessibility are beyond human capability.

Growth and magnetoresistance of Fe/Cr superlattices on Ge(100)

We have studied the growth, structural properties, and magnetoresistance of Fe/Cr superlattices on Ge(100) substrates. The Fe/Cr superlattices were found to grow bcc(100) and epitaxially aligned. From Rutherford backscattering (RBS) and reflection high energy electron diffraction it is found that some intermixing occurs at the Ge–Fe interface. Using low energy electron diffraction, transmission electron microscopy, x‐ray diffraction and RBS it is shown that Fe/Cr superlattices grow epitaxially matched on Ge(100) and have flat and sharp interfaces. We investigated for Fe/Cr on Ge(100) the magnetoresistance as a function of Cr interlayer thickness tCr=8–14 A. The maximum value is found for tCr=12 A (ΔR/RS=15% at room temperature, ΔR/RS=95% at 4 K). A comparison is made with multilayers of the same composition, deposited on glass substrates.

Laser penetration spike welding: A microlaser welding technique enabling novel product designs and constructions

A novel method for laser penetration microspot welding of sheet metal is presented. With this so called “laser spike-welding,” large gap tolerances are allowed. Depending on the ratio of laser spot radius to top plate thickness, gaps of 100% of the top layer thickness and more can be bridged. With conventional keyhole penetration welding this is only 20%–40% at most. So, with spike welding the gap between the metal plates can be increased by a factor of 2.5–5. In this way a powerful and attractive welding technique for joining of thin metal plates is available. The laser spike weld method can handle larger production tolerances and will enable novel and cheap product designs and welding of dissimilar materials.

Opportunities and challenges for laser technology in microelectronics and photonics

Industrial manufacturing with the aid of laser technology has found many applications in the microelectronics industry during the last three decades. At Philips the main application fields are laser spot welding and marking. Several novel fields are strongly coming up due to the high demands in the microelectronics and (flat) display industry that cannot be met with conventional technologies. In this paper we give an overview of the present status of laser technology in electronics manufacturing and of innovative developments for new applications in microelectronics and photonics.