The change in microstructure in sputtered CoNilPt Multilayers is studied as a function of the argon pressure. It is shown that as the sputter gas pressure increases the average grain size approximately stays constant, but the grains become more separated, the argon content decreases and the films tend to grow with rougher interfaces. Furthermore the increase in argon pressure also seems to lead to an increase in interface anisotropy.

It has been suggested that the reversal mechanism in highly exchange coupled systems, like Co/Pt multilayers, takes place by nucleation of a reversed domain, followed by domain wall movement. Based on magnetic force microscopy (MFM) and anomalous Hall effect (AHE) measurements, we show that this model is correct for a Co/Pt multilayer dot. Moreover we separately measured the field necessary to nucleate a reversed domain and the field necessary to “depin” the domain wall in the dot (propagation field).

Co/Pt multilayer dots with perpendicular anisotropy and with diameters of 250 and 350nm were fabricated on top of a Hall cross configuration. The angular dependence of the magnetic reversal of the individual dot was investigated by Anomalous Hall effect measurements. At near in-plane angles (85° with the magnetic easy axis) the dot switches partially into a stable two-domain state. This allows for separate analysis of the angular dependence of both the field required for nucleation of a reversed domain, and the field required for depinning of the domain wall. The angular dependence of the depinning field fits accurately to a 1/cos(θ) behavior, whereas the angular dependence of the nucleation field shows a minimum close to 45°. The latter dependency can be accurately fitted to the modified Kondorsky model proposed by Schumacher [1].

While magnetic nanowires generally have a preferential magnetization direction along the wire axis to minimize magnetostatic energy, it is shown here for epitaxial magnetic oxide nanowires that substrate-induced strain can be used to tailor the magnetic easy axis in any direction. La(0.67)Sr(0.33)MnO(3) (LSMO) nanowires were prepared by pulsed laser deposition of LSMO thin films on NdGaO(3) (NGO) substrates of two different orientations [NGO(110)(o) and NGO(010)(o)], followed by patterning into arrays of nanowires by laser interference lithography. The uniaxial compressive strain from the substrate induces a strong uniaxial magnetic anisotropy in the LSMO that dominates the anisotropy. Hence, one obtains LSMO nanowires having a magnetic easy axis that can lie in any direction, including perpendicular to the wire axis. In marked contrast, similar nanowires on SrTiO(3)(001) substrates without significant uniaxial strain exhibit the usual preferential magnetization direction along the wire axis, as dictated by magnetostatic shape anisotropy. The tunable magnetic anisotropy direction is a useful feature for applications of magnetic nanowires in magnetic memory, sensor, and logic devices.

Magnetic dot arrays with perpendicular magnetic anisotropy were fabricated by patterning Co(80)Pt(20)-alloy continuous films by means of laser interference lithography. As commonly seen in large dot arrays, there is a large difference in the switching field between dots. Here we investigate the origin of this large switching field distribution, by using the anomalous Hall effect (AHE). The high sensitivity of the AHE permits us to measure the magnetic reversal of individual dots in an array of 80 dots with a diameter of 180 nm. By taking 1000 hysteresis loops we reveal the thermally induced switching field distribution SFD(T) of individual dots inside the array. The SFD(T) of the first and last switching dots were fitted to an Arrhenius model, and a clear difference in switching volume and magnetic anisotropy was observed between dots switching at low and high fields.

Bit patterned media (BPM) which utilize each magnetic nanostructured dot as one recorded bit has attracted much interest as a promising candidate for future high-density magnetic recording. In this study, the magnetization reversal behaviors of nanostructured L1(0)-FePt, Co/Pt multilayer (ML), and CoPt/Ru dots are investigated. For Co/Pt and CoPt/Ru nanodots, the bi-stable state is maintained in a very wide size range up to several hundred nm, and the magnetization reversal is dominated by the nucleation of a small reversed nucleus with the dimension of domain wall width. On the other hand, the critical size for the bi-stability of L1(0)-FePt is about 60 nm, and its magnetization reversal proceeds via domain wall displacement even for such a small dot size. These reversal behaviors, depending on the magnetic materials, might be attributed to the difference in structural inhomogeneity, such as defects. In addition to the magnetic properties, the structural uniformity of the material could be crucial for the BPM application. (C) 2008 Elsevier B. V. All rights reserved.

Magnetic properties of Co/Pt multilayers have been extensively studied from both the practical and fundamental points of view. These multilayers have large perpendicular magnetic anisotropy, high remanent squareness and high coercivity [1]. Therefore, Co/Pt multilayer dots are possible candidates for future high-density magnetic storage media. In order to understand the magnetization process, it is essential to measure not only the properties of the array, but also of individual dots. Techniques like micro-SQUID’s can be used to measure on nano dots, however they are limited to cryogenic temperatures [2]. Alternatively, anomalous Hall effect (AHE) measurements are very sensitive and can be operated in a wide temperature range including room temperature. The AHE technique has previously been used to measure L10-FePt [3] and Co/Pt multilayer nanodots [4] and shown that the extremely high sensitivity allows detecting switching behaviour of a sub-100-nm dot. In this work we investigate the magnetic reversal of Co/Pt dots, with a focus on the switching field distribution. The arrays of dots are defined by laser interference lithography (LIL) [5]. LIL offers advantages over scanning electron-beam lithography due to its ability to define grid patterns over large, square cm, areas in a single, fast, maskless exposure.

The possibility of magnetic probe recording into a continuous perpendicular medium is discussed. By applying a current from the tip to the medium, a very localized area can be heated and bits as small as 80 nm in diameter could be written. This value is close to the calculated minimum diameter of reversed cylindrical domains in our perpendicular medium. A current can be injected directly from the tip to the medium by means of a current source, or one can use capacitive currents. We prefer the first method, since the current, and therefore the heating process, can be controlled more precisely. The energy required to write a bit is in the order of 1 nJ. Calculations show that most of the heat is dissipated at the tip end. Demagnetizing fields of the surrounding material play an important role and are so strong that bits can be written without applying an external field. By decreasing the film thickness, the demagnetizing fields are reduced, and selective overwriting of previously written bits could be demonstrated.

The dependence of the field emission effect on distance is applied for displacement sensing and high-resolution positioning. Silicon atomic force microscopy probes were used as a field emission source by applying voltages up to 400 V between this probe and a counter-electrode sample consisting of TiW sputtered on a silicon wafer. From current-voltage characteristics measured for distances varying from 50 to 950 nm, values for the field enhancement factor were determined which show a dependence on the electrode separation. This dependence can be correctly described by a model the authors developed using finite-element calculations and is determined by the emitter geometry and tip radius. Feedback to the probe position was used to maintain a constant current to apply this distance dependence for positioning. When increasing the applied voltage from 5 to 40 V for a constant current of 3 nA, the probe position is raised similar to 90 nm. The nonlinear sensitivity of this positioning method is determined by the varying field enhancement and can be fitted by the same calculated model. Using feedback, the field emitter can be positioned with high lateral resolution and scanned over a conducting surface. Increasing the bias voltage from 3 to 50 V results in an increase in the emitter-sample distance and a decrease in lateral resolution. Damage to the scanned surface has to be prevented by using a current-limiting resistor and by annealing the probe and sample under ultra high vacuum conditions before use. (C) 2008 American Vacuum Society.

Anomalous Hall Effect (AHE) measurements have previously been used to measure the magnetization of L10-FePt [1] and Co/Pt multilayer nanodots [2]. The high sensitivity allows us to measure the magnetization reversal behaviour of sub-100-nm dots. In this work, we investigate the magnetization reversal of 180 nm Co80Pt20 dots, with a focus on the switching field distribution (SFD) of individual dots in an array. Fig. 1 shows hysteresis curves of an array of Co80Pt20 dots measured by AHE. Several steps and plateaus, due to the independent reversal of individual dots, are clearly visible. By consecutively measuring several hysteresis curves, one can observe different switching field values for a single dot (inset Fig. 1). A mathematical model was derived to calculate the effect of thermal activation on this SFD, which depends mainly on the anisotropy, switching volume and the magnetization reversal mechanism of the dot. The SFD was determined from 1000 curves and coincides with the modelled distribution (Fig. 2). By investigating different dots in the array, we conclude that there is a difference in reversal mechanism between weak and strong dots in the array.

Magnetic tunnel junctions having a low-work-function Gd/Co nanolayer at the interface with an Al2O3 tunnel barrier are shown to exhibit both positive and negative values of the tunnel magnetoresistance. The sign of the tunnel spin polarization of the Gd/Co nanolayer electrode depends on the thickness of the Gd and Co layers, temperature, and applied voltage. This reflects the nature of the interaction between the conduction electrons of the rare-earth and transition metals.

Using the technique of ballistic electron magnetic microscopy, we have studied the spin-asymmetry of transmission of hot electrons in Fe, for which a recent ab initio calculation has shown that the inelastic lifetime is similar for majority and minority spin. Nevertheless, using a spin-valve structure of Ni81Fe19/Au/ Fe, we find that the attenuation length of hot electrons in Fe at 1.2–1.6 eV is a factor of 4 larger for the majority spin. We argue that this large spin-asymmetry arises from the spin dependence of the group velocity, rather than the lifetime.

The variation of the tunnel spin-polarization (TSP) with energy is determined using a magnetic tunnel transistor, allowing quantification of the energy dependent TSP separately for both ferromagnet/insulator interfaces and direct correlation with the tunnel magnetoresistance (TMR) measured in the same device. The intrinsic TSP is reduced below the Fermi level, and more strongly so for tunneling into empty states above the Fermi level. For artificially doped barriers, the low bias TMR decreases due to defect-assisted tunneling. Yet, this mechanism becomes ineffective at large bias, where instead inelastic spin scattering causes a strong TMR decay.

In this work, we present a novel concept for high resolution proximity sensing based on field emission and provide more insight in the vacuum conditions and electronics needed for stable operation. This study is focused on tip-medium distance control, and the aim is to apply this method in a vacuum probe storage device where thousands of field emission probes In this work, we present a novel concept for high resolution proximity sensing based on field emission and provide more insight in the vacuum conditions and electronics needed for stable operation. This study is focused on tip-medium distance control, and the aim is to apply this method in a vacuum probe storage device where thousands of field emission probes were individually positioned at several nanometers above a storage medium.