Wilhelmus J. Geerts

Modeling of Plastic Deformation Effects in Ferromagnetic Thin Films

To explain the magnetic behavior of plastic deformation of thin magnetic films (Fe and permalloy) on an elastic substrate (nitinol), it is noted that unlike in the bulk, the dislocation density does not increase dramatically because of the dimensional constraint. As a result, the resulting residual stress, even though strain hardening is limited, dominates the observed magnetic behavior. Thus, with the field parallel to the stress axis, the compressive residual stress resulting from plastic deformation causes a decrease in remanence and an increase in coercivity; and with the field perpendicular to the stress axis, the resulting compressive residual stress causes an increase in remanence and a decrease in coercivity. These elements have been inserted into the model previously developed for plastic deformation in the bulk, producing the aforementioned behavior, which has been observed experimentally in the films.

Magnetic and magneto-optical properties of NiMnSb thin films

We have developed a simple method to deposit crystalline NiMnSb films from a single-composite target using low-power magnetron sputtering. In this process, the substrate temperature is kept as low as 250°C while retaining bulk-like structural and magnetic properties. Magneto-optical Kerr rotation spectra, closely related to the electronic band structure, are found to be identical over the entire energy range to those previously published for high-quality bulk alloys. This suggests that the half-metallic behavior predicted for the bulk alloys will also be found in the present films.

Ferromagnetic Properties of Spark-Processed Photoluminescing Silicon

Magnetic properties of photoluminescing spark-processed silicon (sp-Si) have been investigated for the first time. Contrary to the diamagnetic signal known for bulk silicon, sp-Si displays a paramagnetic resonance as well as a ferromagnetic hysterisis loop. The paramagnetic resonance was studied using an EPR system and showed a high concentration of at least two distinct paramagnetic centers. One center can be eliminated by annealing in Ultra-High Purity nitrogen for 30 minutes at 600 °C. Measurements utilizing a SQUID magnetometer revealed that sp-Si displays ferromagnetic ordering with a saturization magnetization occuring at low fields. This is attributed to the high density of paramagnetic centers. Temperature dependent measurements were performed to establish possible links between magnetic properties and the luminescence of sp-Si.

Thickness dependence of the magnetic hysteresis of NiFe-31% films as a function of an applied isotropic in-plane stress

The magnetic hysteresis of dc magnetron-sputtered Ni69Fe31 films that were sandwiched between titanium layers was investigated as a function of an externally applied isotropic in-plane strain. The hysteresis curves were measured with a Kerr magnetometer that monitored the longitudinal Kerr ellipticity as a function of the in-plane magnetic field. The strain was created by bending the samples in two dimensions using a pressure cell. Measurements were performed on films with different thicknesses. The magnetoelastic properties appeared to be much smaller for films with a thickness of 100 nm than for films with a thickness of 288 or 500 nm. This might be due to a change of the domain wall pinning, or a change of the domain wall density as a function of the film thickness. Measurements under compressive isotropic in-plane stress were shown to be possible by flipping the sample in the pressure holder and measuring through the glass substrate. The Faraday effect and stress-induced birefringence in the glass sub...

Anomalous H/sub c/ in CoCr-films: surface and bulk measurements

The major and minor loops of RF sputtered CoCr films in the thickness range of 20-1000 nm and the concentration range of 19-24 at.% Cr were measured by Kerr tracer and VSM. Differences between bulk and surface magnetic data are most likely due to a thickness-dependent surface chemical composition. The maximum anomality of the minor loop coercivity, which depends strongly on the film thickness, appears to be larger at the surface than in the bulk. The anomalous coercivity of the minor loop, which is related to a domain pattern change, could be caused by repulsive forces between domain walls. It can be concluded that study of the minor loop coercivities is an interesting research tool for micromagnetic behavior. >

Dry and Wet Etch Processes for NiMnSb Heusler Alloy Thin Films

A variety of plasma etching chemistries were examined for patterning NiMnSb Heusler alloy thin films and associated Al 2 O 3 barrier layers. Chemistries based on SF 6 , Cl 2 , and BCl 3 were all found to provide faster etch rates than pure Ar sputtering. In all cases the etch rates were strongly dependent on both the ion flux and ion energy. Selectivities of ≥20 for NiMnSb over Al 2 O 3 were obtained in SF 6 -based discharges, while selectivities ≤5 were typical in Cl 2 , BCl 3 , and CH 4 /H 2 plasma chemistries. Wet etch solutions of HF/H 2 O and HNO 3 /H 2 SO 4 /H 2 O were found to provide reaction-limited etching of NiMnSb that was either nonselective or selective, respectively, to Al 2 O 3 .

Magneto-optical properties of sputter-deposited NiMnSb thin films

High-quality thin-film of NiMnSb with the Clb crystal structure can be grown by low-power sputtering, at substrate temperatures as low as 250 °C. Structural and magneto-optical properties of these films are similar to those of bulk material prepared at high temperatures, which suggest that the predicted 100% spin-polarized behavior of NiMbSb may also be found in these films. The magneto-optical Kerr spectra have been measured, and they are found to be very sensitive to the preparation condition. The spectrum of the bulk alloy is closely reproduced only in the case of optimized films. This suggests that, in the optimized case, the electronic band structure is bulklike.

Elaboration and magneto-optical study of submicron magnetic structures in Au/Co/Au ultrathin films

Abstract X-ray lithography and ion beam etching processes have been optimized to realize large areas of dots patterned in ultrathin Au/Co/Au/SiO 2 /Si(100) sandwiches. The coercive force increases dramatically as the dot diameter is reduced. Spectroscopic polar magneto-optical Kerr effect measurements show a change in the sign of the Kerr rotation with magnetic dots to non-magnetic background surface ratio.

Surface chemical state of sputtered Co-Cr films

Surface oxidation states of sputtered Co80Cr20 (at%) films were studied by performing Auger Electron Spectroscopy (AES) and X-ray Photoemission Spectroscopy (XPS). Seven samples having different thicknesses (from 50 to 1000 nm) which were grown under the same conditions have been studied. It was found from these experiments that among all the films the initial oxidation occured in the Cr; the the following diffusion of the Co was observed at the top layer. Consequently the surface consists of a four-layered structure. A weak thickness dependence in the films of the oxidized region was found from the results of AES measurements. The change of the saturation magnetization through the sample was calculated using the results of AES and XPS data. These calculations show that a region with enhanced saturation magnetization can be expected under the non-ferromagnetic top layer.

Oscillation of magneto-optical Kerr effect in Co ultra-thin films

Abstract The thickness dependence of magneto-optical (MO) properties are measured in wedge-shaped Co films deposited on Au seed layers by the MBE method. It is found that Co ultra-thin film on fcc-Au(100) crystallizes in bcc structure below the thickness of 6–10 A and changes from bcc to hcp structures from the RHEED observation. In 3.0–4.7 eV, new MO Kerr rotation ( φ K ) and Kerr ellipticity ( η K ) peaks appear in Co(100) wedge sample and its shifts towards the higher energy side with increasing Co layer thickness. An oscillation of φ K and η K as a function of Co thickness was also clearly observed. Such kinds of oscillatory behavior are unclear in hcp-Co(0001) wedges samples.