H.W. van Kesteren

Hysteresis, microstructure, and magneto‐optical recording in Co/Pt and Co/Pd multilayers

The factors that influence the coercive field Hc and the shape of the magnetic hysteresis loop for Co/Pt multilayers (MLs), applied as a magneto‐optical (MO) recording medium, have been investigated. The hysteresis loop of MLs, made by evaporation of Kr sputtering, became more rectangular when the total film thickness was reduced, i.e., the saturation field Hs decreased and the nucleation field Hn increased. Hs was calculated using a stripe domain model and Hn was modeled assuming that nucleation took place at a field that destabilized small vestigial magnetic domains. Both models assumed that domain walls can move in the ML film and the agreement with experiment was good. However, these models could not explain the hysteresis loops for the MLs deposited on a thick Pt base layer or made at high sputter gas pressure. These MLs showed a more particulate microstructure and the hysteresis loop was sheared, consistent with magnetic reversal by uniform rotation rather than domain‐wall motion. From thermomagneti...

Deep levels of copper in silicon

Defect impurity levels have been examined in copper‐diffused p‐and n‐type silicon using deep level transient spectroscopy. Levels at Ev+0.09, Ev+0.23, and Ev+0.42 eV have been observed in both types of material, although the deeper levels were only oberved in n‐type material after post‐diffusion annealing at 200 °C. Associated with the appearance of these levels in n‐type material was another level at Ec−0.16 eV. This may be a further charge state of the center responsible for the Ev+0.23 eV and Ev+0.42 eV levels or the two centers may be decomposition products of a thermally unstable complex. Luminescence measurements have revealed the previously reported Cu‐Cu spectrum in all the copper‐diffused samples. The occurrence of this signal could not be correlated with the presence of the levels at Ev+0.23, Ev+0.42, or Ec−0.16 eV; this leaves the center at Ev+0.09 eV as the likely origin of the signal.

Dielectric enhancement layers for a Pt/Co multilayer magneto‐optical recording medium

Transparent ZnO with an optical index of refraction n∼2 is a suitable candidate for a dielectric enhancement layer in a magneto‐optical disk. Surprisingly, sputtering a Pt/Co multilayer on a ZnO layer not only increased the optical Kerr rotation but also increased the magnetic coercivity and improved the rectangular magnetic loop shape. Sputter etching the dielectric surface prior to the metal deposition also improved magnetic properties.

Atomic abruptness in InGaAsP/InP quantum well heterointerfaces grown by low‐pressure organometallic vapor phase epitaxy

High quality quantum well structures of InGaAsP (λ=1.55 μm)/InP were grown by low‐pressure organometallic vapor phase epitaxy on 0.2° and 2° misoriented (001) InP substrates. Multiple‐line 2 K photoluminescence emission was observed for the first time from thin quantum wells of InGaAsP grown on 0.2° misoriented substrates. The multiple‐line emission is interpreted to result from half‐monolayer well width variations within one well with lateral sizes larger than the excitonic radius. Quantum wells grown on substrates with 2° misorientation showed generally wider single‐line photoluminescence emission due to well width variations within one well with lateral sizes smaller than the excitonic radius. The studied well thicknesses ranged from ≊70 A down to ≊4 A, which showed emission as short as 905 nm (1.37 eV) corresponding to a spectral upshift of 514 meV. The very high quality of the quantum wells of InGaAsP is indicated by a 2 K photoluminescence linewidth of 8.8 meV for a ≊6 A well.

Near‐field magneto‐optical imaging in scanning tunneling microscopy

Images of magnetic bits written in a Pt/Co multilayer are presented. Using photosensitive semiconducting tips in a scanning tunneling microscope, both surface topography as well as polarization‐dependent optical transmission are measured. Magnetic contrast is achieved by detection of the Faraday effect. Magneto‐optical lateral resolution of 250 nm is demonstrated.

Scanning magnetic force microscopy on Co/Pt magneto‐optical disks

High resolution images of domains written on Co/Pt magneto‐optical disks were obtained using scanning magnetic force microscopy. The key advantage of this technique compared to electron microscopy techniques is that regular disks can be used. We studied the influence of the multilayer composition and preparation conditions as well as the effect of the pregrooves and the incorporation of a Pt underlayer, on the structure of the domains on the disk. Marks were written both with laser and magnetic field modulation, and carrier and noise levels were determined. Regularly shaped and subdomain‐free marks for both writing schemes were obtained for multilayers with about 4‐A‐thick Co layers and Pt layers thinner than 15 A. The magnitude of the coercive field had only a minor influence on the domain structure. On the other hand, the preparation conditions and the structure of the base layers have a large impact on the recording conditions and domain structures.

Magneto-optical recording in Co/Pt multilayer and GdTbFe-based disks at 820, 647 and 458-nm wavelength

The magnetooptical recording performance of Co/Pt and GdTbFe-based disks was compared at three wavelengths. At 820 and 647 nm, the carrier-to-noise ratio of the GdTbFe disks was higher (2-3 dB) due to a lower noise level. At 458-nm wavelength, Co/Pt performed 3 dB better than the GdTbFe disk. The crystalline microstructure of Co/Pt did not cause a higher noise level than that of GdTbFe. >

Nature of the lowest confined electron state in GaAs/AlAs type II quantum wells as a function of AlAs thickness

The authors report on a detailed photoluminescence study of a series of GaAs/AlAs type II quantum well structures. In these structures the thickness of the GaAs layers was kept constant at 25 AA and the thickness of the AlAs layers was varied between 28 and 114 AA. As the AlAs layer thickness was increased they observed a large change in the relative strengths of the zero-phonon and phonon-assisted type II exciton recombinations. They have interpreted this phenomenon as being due to a change in the nature of the lowest lying AlAs electron state from Xxy (k perpendicular to (001)) to Xz (k parallel to (001)) as the AlAs thickness was decreased, where (001) is the growth direction. This changeover is due to the competing effects of electron confinement and strain produced by the small but finite lattice mismatch between GaAs and AlAs. They have estimated the strain splitting of the different X states to be 23 meV. Calculations using this figure along with an envelope function treatment of the effects of electron confinement predict that the lowest confined Xz and Xxy states should cross at an AlAs thickness of the order of 60 AA. For the samples with the thickest AlAs layers (94 and 114 AA) they were able to resolve two zero-phonon lines which they attribute to the different X states, the measured splittings are in good agreement with the calculations.

Structural and magnetic studies of ultrathin epitaxial Co films deposited on a Pd(111) single crystal

Abstract We have studied the structure and magnetic properties of ultrathin epitaxial Pd/Co films deposited by vapor deposition in ultrahigh vacuum on a Pd(111) single crystal. The film crystal structure was characterized by LEED and RHEED and the magnetic properties were studied by Kerr hysteresis loops, vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR) at 10 and 34GHz. Well-defined RHEED and LEED patterns were present at all thickness proving that there was always a high degree of crystal order in the films despite the large lattice mismatch of 9%. Lattice spacing measurements using RHEED revealed that the Co relaxes to the bulk in-plane spacing in the first monolayer. FMR, Kerr and VSM measurements showed that Co films thinner than ≈ 20A had an easy magnetization axis perpendicular to the film plane. We deduced that the surface anisotropy constant was (0.80 ±0.05)MJ/m 2 .

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.