R. Coehoorn

Meltspun permanent magnet materials containing Fe3B as the main phase

Abstract A novel type of permanent magnet material was obtained by annealing amorphous melt spun flakes of the approximate composition Nd 4 Fe 78 B 18 . The annealing process consists of two steps. At first, at a temperature T 1 , the metastable compound Fe 3 B crystallizes while, at a higher temperature T 2 , the hard magnetic phase Nd 2 Fe 14 B is formed. This phase comprises only 15% of the alloy. Nevertheless, materials in which the latter reaction has taken place show remarkable isotropic hard magnetic properties. The remanence μ 0 M r is 1.2 T, while intrinsic coercive fields μ 0 H c of almost 0.4 T have been attained, with ( BH ) max =95 kJ/m 3 . In this paper the preparation of these materials, the compositional dependence of the magnetic properties and a possible explanation for the high isotropic remanence are presented.

Orientational dependence of the exchange biasing in molecular‐beam‐epitaxy‐grown Ni80Fe20/Fe50Mn50 bilayers (invited)

The exchange biasing field (Heb) and coercive field (Hc) of molecular‐beam‐epitaxy‐grown Cu/Ni80Fe20/Fe50Mn50 samples in [111], [001], and [110] orientations have been investigated by longitudinal Kerr effect measurements. Ni80Fe20 and Fe50Mn50 were deposited as orthogonal wedge‐shaped layers on single‐crystal Cu substrates in a magnetic field, enabling the study of the thickness dependence of Heb and Hc on a single sample for each orientation. A strong dependence of Heb and Hc on the growth orientation is observed. The results are interpreted in terms of the observed noncollinear spin structure of the antiferromagnet and a comparison is given with the predictions from recent theoretical models.

Disk recording beyond 100 Gb/in.2: Hybrid recording? (invited)

A new method for recording above 100 Gb/in.2 is discussed. We call this method “hybrid recording,” a form of thermally-assisted recording that combines thermo-magnetic writing and magnetic reading. In order to increase the stability of the recorded information, writing is carried out at an elevated temperature on a medium with a very high coercivity at room temperature. In our proposal write and read heads with extremely narrow trackwidths are used, so the trackwidth is not determined by the optical spot size and the written bits have a rectangular shape, in contrast to the schemes proposed by others. Preliminary experiments are shown. The applicability of today’s granular and MO type media for hybrid recording is discussed. It is calculated that hybrid recording on optimized media can give an increase of the areal density of a factor 2.9 in areal density or 7 dB (2.2×) medium SNR improvement in case of Poisson noise and 11 dB (3.4×) in case of transition noise. Practically a factor of about 2 in density ...

On the 4f-3d exchange interaction in intermetallic compounds

Abstract A review is given of the experimental values of the magnetic intersublattice-coupling constants in rare-earth (R) — transition-metal (T) intermetallics, derived from high-field magnetization measurements on single-crystalline powder particles that are free to be oriented by the applied magnetic field. The results are compared with values for the intersublattice-coupling constant obtained by other experimental methods and with results of electronic-band-structure calculations made for a few of the compounds investigated. An empirical relationship is found between the intersublattice-coupling constants and the reciprocal normalized molar volumes of the various R—T compounds, including T = Ni, Co and Fe.

Structure and magnetic properties of R2Fe17Nx compounds

Abstract We have shown that ternary rare earth nitrides of the approximate composition R2Fe17N2.5 can be prepared by the reaction of R2Fe17 with nitrogen gas at 500°C for all members of this series. The crystal structure and lattice constants of these compounds have been determined and are discussed in terms of volume increase and nitrogen site occupation. The N2 uptake was found to be accompanied by strong increases of the Curie temperature and room temperature magnetization. From the results it was derived that the N2 uptake may lead to substantial changes in the RFe coupling strength.

Exchange biasing by Ir19Mn81 : dependence on temperature, microstructure and antiferromagnetic layer thickness

We have investigated the thermal stability of the exchange biasing interaction in antiferromagnetic/ferromagnetic bilayers of Ir19Mn81 and Ni80Fe20 or Co90Fe10. The exchange-biasing field and the coercive field were found to depend strongly on the thickness of the Ir19Mn81 layer and on the crystallographic texture of the bilayer. The exchange-biasing field at room temperature has a maximum at 4 nm Ir19Mn81 layer thickness and then decreases for increasing Ir19Mn81 layer thickness. This coincides quite well with the thickness dependence of the (111) texture measured in Ir19Mn81 layers. Removing the Ta seed layer resulted in the disappearance of the (111) texture and at the same time the exchange-biasing field decreased for all Ir19Mn81 layer thicknesses. For films with a Ta seed layer and 8 nm Ir19Mn81 or more, we have found a blocking temperature of 560 K, decreasing with decreasing Ir19Mn81 layer thickness to around room temperature for 2 nm Ir19Mn81. Removing the Ta seed layer resulted in a decrease of ...

Dual-gate organic thin-film transistors

A dual-gate organic thin-film transistor is realized using solution-processed organic semiconductor and insulator layers. Electrodes are made from gold. Compared to conventional single-gate transistors, this device type has a higher on current and steeper subthreshold slope. We show that the improved performance is the result of a nonconstant threshold voltage rather than formation of a second accumulation channel. Formation of a second accumulation channel does occur but the field-effect mobility associated with this channel is a factor 104 lower than the primary channel due to the relatively rough insulator-semiconductor interface.

Molecular-scale simulation of electroluminescence in a multilayer white organic light-emitting diode

In multilayer white organic light-emitting diodes the electronic processes in the various layers--injection and motion of charges as well as generation, diffusion and radiative decay of excitons--should be concerted such that efficient, stable and colour-balanced electroluminescence can occur. Here we show that it is feasible to carry out Monte Carlo simulations including all of these molecular-scale processes for a hybrid multilayer organic light-emitting diode combining red and green phosphorescent layers with a blue fluorescent layer. The simulated current density and emission profile are shown to agree well with experiment. The experimental emission profile was obtained with nanometre resolution from the measured angle- and polarization-dependent emission spectra. The simulations elucidate the crucial role of exciton transfer from green to red and the efficiency loss due to excitons generated in the interlayer between the green and blue layers. The perpendicular and lateral confinement of the exciton generation to regions of molecular-scale dimensions revealed by this study demonstrate the necessity of molecular-scale instead of conventional continuum simulation.

Thickness scaling of the space-charge-limited current in poly(p-phenylene vinylene)

Charge transport in light-emitting diodes (LEDs) based on a poly(p-phenylene vinylene) (PPV) derivative is investigated as a function of sample thickness. Via the thickness dependence, the contributions from the electric field and charge carrier density to the mobility in space-charge-limited (SCL) diodes can be disentangled. It is demonstrated that a field-dependent mobility weakens the thickness dependence of the SCL current, whereas a carrier-density-dependent mobility gives rise to an enhanced thickness dependence. The enhanced thickness dependence of the experimental SCL current in PPV is in agreement with the predictions using a density-dependent mobility only. This observation confirms that in PPV-based LEDs, the hole transport is dominated by filling of the localized states.

On the ferromagnetic interlayer coupling in exchange-biased spin-valve multilayers

The ferromagnetic interlayer coupling in sputter-deposited permalloy/copper/permalloy exchange-biased spin valve multilayers has been measured as a function of the copper thickness. The variation with thickness may, for t/sub cu/>1.7 nm, be analyzed in terms of the Neel model for magnetostatic coupling due to correlated interface roughness, using parameters which are consistent with the observed microstructure.