P. Luo

Angular dependence of the coercivity and remanence of ferromagnetic nanowire arrays

Magnetization properties of magnetic nanowire arrays are studied on various ferromagnetic materials grown in anodic alumina (alumite) and track etched polycarbonate (PCTE) membranes by pulsed electrodeposition. Magnetization curves were measured as functions of wire material, field orientation, and wire length. The coercivity (Hc) and remanent squareness (S) of the various wire arrays were derived from hysteresis loops as a function of angle (θ) between the field and wire axis. For PCTE membranes, Hc(θ) curves for CoNiFe, NiFe, and Co nanowire arrays all show an otherwise-bell-type variation, while they change shapes from the otherwise bell to bell type for Ni nanowire arrays as the wire diameter decreases to 30 nm. These characteristics can be understood based on different magnetization reversal mechanisms of small wires. The effect of magnetostatic interaction among wires on the magnetic properties was examined by changing the wire lengths in alumite membranes. It is found that the interaction reduces H...

Fabrication of a Class of Nanostructured Materials Using Carbon Nanowalls as the Templates

Well-aligned carbon nanowalls with a thickness of a few nanometers and a lateral size in the micrometer range have been grown on various types of substrates. The nanowalls exhibit a remarkably different surface morphology as compared to fullerenes and carbon nanotubes, in particular their two-dimensionality and high surface area. In this work, we focused on the second aspect and developed a templating method to fabricate a class of nanostructured materials based on the novel surface morphology of the carbon nanowalls. These structures may have potential applications in batteries, gas sensors, catalysts, and light-emission/detection, field-emission, and biomedical devices.

Magnetic and electrical transport properties of Ge1−xMnx thin films

We report on preparation and systematic study of the structural, magnetic, and electrical transport properties of Ge1−xMnx (x=25%–42%) alloy thin films grown on GaAs (001) substrates by molecular beam epitaxy. Amorphous Ge1−xMnx, and coexistence of amorphous Ge1−xMnx, Ge crystallites, and high TC secondary phases (Mn11Ge8 and Mn5Ge3) are obtained at substrate temperatures of 160 and 200–300°C, respectively. The amorphous samples are found to consist of a low-temperature highly ordered spin-glass-like phase with an ordering temperature of TC∼20K (x∼0.39), increasing with the Mn composition, and a high-temperature “clustered dopant” phase with an ordering temperature of TC*∼104K (x∼0.39), increasing with both the Mn composition and applied external field. The magnetization of the low-temperature phase is found to be coupled antiferromagnetically with that of the high-temperature phase, leading to the appearance of a negative thermal remanent magnetization. Detailed magnetic and electrical measurement reveal...

Role of oxygen exposure in different positions in the synthetic spin valves

The magnetoresistance (MR) properties of a basic Ta/NiFe/IrMn/CoFe/Ru/CoFe/Cu/CoFe/Cu/Ta synthetic spin valve (SV) system have been systematically studied by using O2-soaked layers in five different positions. It is found that about 10% of MR ratio enhancement can be achieved without sacrificing other merits of SVs when the O2-soaking dose is controlled in the vicinity of 10−3 Pa s. It is probably due to improvement of the spin-dependent scattering (increasing the spin-dependent transmission coefficient and reducing the diffusion scattering coefficient) after O2 soaking either within the pinned layers or within the spacer layer.

Low switching current flux-closed magnetoresistive random access memory

A spin valve magnetoresistive random access memory with a flux-closed structure is presented. The flux-closed structure prevents the disruption to magnetization of the recording layer and increases the thermal stability. Simulation results show that the switching field increases under the uniform external field, however, the switching field has no change under the bit current field. Only the bit current that flows mostly at the center is used to switch the cell. Experimental results show that the bit current of 10 mA is sufficient to switch the 1 μm×4 μm cell.

Multistate per-cell magnetoresistive random-access memory written at Curie point

A multistate per-cell magnetoresistive random-access memory (MRAM) that writes data by a thermally assisted technique and reads data using the angular-dependent magnetoresistance is proposed. A hard magnetic layer or pinned ferromagnetic layer (CoFe-IrMn) is used as the recording layer. The free layer serves as the read layer. Before reading, the free layers magnetization is set to the initial state. For the N states per-cell MRAM, the magnetization angle of the ith state (i=0 to N-1) between the free layer and recording layer can be set to be acos(1-2*i/(N-1)). For example, in the four-state per-cell MRAM, the magnetization angle can be set to be acos(1), acos(1/3), acos(-1/3), and acos(-1), which represent the four states, respectively. More states can be obtained if the signal-to-noise ratio is sufficient. At near Curie point, a small external field can be used to record the signal. In order to verify the idea, a spin-valve giant-magnetoresistance memory cell was fabricated using e-beam lithography and ultrahigh voltage sputtering. A 25-mA heating current and a small external field are enough to assist the writing process. A four-state per-cell memory is realized by this method.

Asymmetrically kinked hysteresis loops in exchange biased NiFe/IrMn rings

Magnetic properties of a series of NiFe/IrMn rings with inner diameters from 0.35 to 1.50 μm, while keeping the outer diameter of 2.0 μm, have been studied by magnetic force microscopy and hysteresis loop measurements at various temperatures. The enhancements in exchange bias and coercivity have been observed in the patterned ring structures. Asymmetrical, kinked hysteresis loops are present in the NiFe/IrMn rings with inner diameters less than 1.50 μm. The kinks in hysteresis loops in the increasing field branch correspond to the presence of a flux closure state. The presence of the flux closure state has been attributed to the formation of antiferromagnetic domain walls in the magnetization reversal which acts as an energy barrier and traps the flux closure state.

Dewetting observations of ultrathin metallic films

Abstract Ultrathin metallic films like CoFe, Ta, Cu, Cr, and NiFe are widely used in magnetic devices such as magnetic random access memory (MRAM) and magnetic recording heads. Dewetting corrosions were often observed after O2 plasma ashing in MRAM fabrications. The surface stability of these films was then examined. The results show that dewetting takes place when CoFe or Cu films are exposed to air after an O2 plasma process. In contrast to the dewetting reported so far in organic or metallic liquid films on solid substrates, the observed dewetting does not occur in a liquid state but in a solid state. Several in situ and ex situ process methods were examined to control the dewetting. It is found that after ashing, the immediate immersion of wafer into acetone and ultrasonic cleaning some minutes after opening chamber can greatly suppress the occurrence of dewettings. Process examinations show that the heating is unimportant for the formation of the dewetting, while moisture in air may play an important role in the formation of the dewetting, acting as a necessary catalyst. Several dewetting patterns were observed, and the pattern shape depends not only on the thickness of the film, but also on the plasma parameters. Possible mechanisms responsible for the formation of these patterns are discussed.

Spin-flop switching of the guided synthetic anti-ferromagnet MRAM

In this work, a new MRAM structure with guided SAF layers is presented. This new structure addresses the issues regarding additional read operation, reduction of saturation field, write operation margin, and unintentional switching.

Nanometer metal line fabrication using a ZEP520∕50K PMMA bilayer resist by e-beam lithography

In this work, we propose a bilayer resist system with 50K poly methylmethacrylate as the bottom layer and ZEP520 as the top layer for lift-off process. By making use of the different dissolution rates in the rinser for the top and bottom layers, it is possible to create an overhang resist pattern suitable for lift-off. In this specific study, a set of process parameters have been optimized to fabricate Cr metal lines with a width of about 70nm. The process has been used to fabricate nanometer scale magnetic sensors for data storage applications.