We examine the effect of junction sizes on the magnetization reversal process and spin-transfer torque switching of the MgO-based CoFeB magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA). From the magnetic field transport measurements, it was found that the miniaturization of MTJs inherently enhances the switching asymmetry and the PMA of the soft layer. Our micromagnetic simulations confirmed that the dipolar field from the hard layer is responsible for the switching asymmetry and the increase in perpendicular shape anisotropy induces improvement of the PMA. It was further revealed that this additional anisotropy gained from the smaller MTJ sizes is not sufficient to sustain the thermal stability to meet the long-term information storage at the state-of-the-art complementary-metal–oxide semiconductor technology node. The pulsed spin-transfer torque measurements showed that a higher current density is needed to switch the magnetization of the soft layer in MTJ with smaller lateral dimensions, which is attributed to the increase in PMA.

Nanoimprint lithography (NIL) is a nonconventional lithographic technique that promises low-cost, high-throughput patterning of structures with sub-10 nm resolution. Contamination of nanoimprint stamps is one of the key obstacles to industrialize the NIL technology. Here, we report two efficient approaches for removal of typical contamination of particles and residual resist from stamps: thermal and ultraviolet (UV) imprinting cleaning-both based on the self-cleaning effect of imprinting process. The contaminated stamps were imprinted onto polymer substrates and after demolding, they were treated with an organic solvent. The images of the stamp before and after the cleaning processes show that the two cleaning approaches can effectively remove contamination from stamps without destroying the stamp structures. The contact angles of the stamp before and after the cleaning processes indicate that the cleaning methods do not significantly degrade the anti-sticking layer. The cleaning processes reported in this work could also be used for substrate cleaning.

We present the thickness effects of CoFeB free layer on tunnelling magnetoresistive (TMR), perpendicular magnetic anisotropy (PMA) and spin-transfer torque (STT) in CoFeB–MgO based magnetic tunnel junctions (MTJs). It is found that a post-annealing process could significantly improve both TMR and PMA of the MTJ systems. When the free layer thickness is reduced from 1.3 nm to 1 nm, TMR continuously decays from 80% to 20%. On the other hand, PMA is maximized for a 1.28 nm free layer, above which demagnetization becomes stronger and results in lower PMA. If the free layer thickness is very small, dead layer effect could damage interfacial perpendicular anisotropy and PMA is reduced as a consequence. For STT-induced magnetization switching, the lowest intrinsic critical switching current density (J c0) of 2.1 MA cm−2 is achieved at a free layer thickness of 1.16 nm, accompanied by a TMR of 52% and product of resistance and area (RA) of 16 Ω µm2. Further increasing the free layer thickness will first enhance J c0 and then reduce it due to the balance between PMA and the total free layer volume. STT studies suggest that the CoFeB free layer thickness should be optimized to make a trade-off among large PMA, high TMR and low switching current density in perpendicular CoFeB–MgO MTJ systems.

A simple method for replicating metallic stamp from original masters via combined nanoimprint and electroforming is presented. In this work we demonstrate the generality of the method by replicating structures with dimensions ranged from several microns to sub-100nm in large patterned area. The structures were transferred first into an intermediate polymer sheet (IPS) with good uniformity and high structural fidelity, and then this IPS was used as the galvano-template for nickel electroforming. The major advantage of the method relies on the pattern transfer occurred between a soft polymer and the hard master, thereby the risk of damaging the mold is extremely low.

In this article we examine the use of two-level hybrid-material stamps and nanoimprint lithography (NIL) of poly(dimethylsiloxane) (PDMS) on glass substrates. A silicon/SU8 stamp manufacturing process has been developed, in order to combine nanometer and micrometer structures, thus avoiding complex deep etching processes. The stamp has been test printed in polymethyl methacrylate (PMMA) to demonstrate functionality. We describe polymer flow problems for imprinting large structures and identify optimized parameters, in accordance with previously published findings. The use of PDMS as imprint polymer was examined. Imprinting works well, however, large recovery after separation shrinks the micrometer channels substantially and renders the nanochannels useless. Glass substrates in combination with silicon stamps were used, evaluated and showed to work well at low temperature.

Numerical calculations of equilibrium state energies and local magnetic fields in planar ring-like nanoparticle structures were performed. The dipole–dipole, Zeeman and magnetic anisotropy interactions were included into the model. The result of their competition depends on the value of the external magnetic field, magnetic parameters of an individual nanoparticle, size and shape of the structures. Flux-closed vortexes, single domain, two-domain “onion”-like, “hedgehog”-like and more complex spin structures can be realized. The critical field, providing a sharp transition from the flux-closed vortex to the “onion”-like state, can be regulated by a variation of the particle magnetization and anisotropy constant, their easy directions, and particle space arrangement.

We report on the realization of a novel interdigitated cantilever array with electrostatic control of the shape of the interdigitated array. It consists of an array of SiO2/metal double-finger cantilevers in a grating configuration together with an electrical connection part. The complete grating structure is fabricated with nanoimprint lithography, UV lithography and reactive ion etching. The patterns of the cantilever arrays are defined by nanoimprint lithography. The electrical contact pads are defined and aligned with the imprinted grating pattern by UV lithography. The two steps of reactive ion etching are optimized to get vertical sidewalls of the SiO2 cantilevers and finally to release them from the Si substrate. By applying a bias, the shape of the cantilever array can be altered due to the electrostatic force. The dimensions of the cantilevers and the spacing between them are optimized to achieve the desired functional operating characteristics of the structures. Since the fabrication scheme is based on nanoimprint lithography, such electrostatically controlled periodic structures may be relatively easily and non-expensively realized in various configurations, allowing them to function as optical switching elements, electrical filters, mass sensors, etc.

We propose a rational fabrication method for nanoimprinting moulds by scanning probe lithography. By wet chemical etching, different kinds of moulds are realized on Si(110) and Si(100) surfaces according to the Si crystalline orientation. The structures have line widths of about 200 nm with a high aspect ratio. By reactive ion etching, moulds with patterns free from the limitation of Si crystalline orientation are also obtained. With closed-loop scan control of a scanning probe microscope, the length of patterned lines is more than 100 µm by integrating several steps of patterning. The fabrication process is optimized in order to produce a mould pattern with a line width about 10 nm. The structures on the mould are further duplicated into PMMA resists through the nanoimprinting process. The method of combining scanning probe lithography with wet chemical etching or reactive ion etching (RIE) provides a resistless route for the fabrication of nanoimprinting moulds.

Nanotechnology has provided methods to fabricate surface patterns with features down to a few nm. If cells or cell processes exhibit contact guidance in response to such small patterns is an interesting question and could be pertinent for many applications. In the present study we investigated if axonal outgrowth was affected by nano-printed patterns in polymethylmethacrylate (PMMA)-covered silicon chips. To this end adult mouse sympathetic and sensory ganglia were mounted in Matrigel® on the chips close to the nano-patterns. The patterns consisted of parallel grooves with depths of 300nm and varying widths of 100–400nm. The distance between two adjacent grooves was 100–1600nm. The chips were cultured in medium containing 25ng/ml of nerve growth factor to stimulate axonal outgrowth. After 1 week of incubation, axonal outgrowth was investigated by immunocytochemistry or scanning electron microscopy. Axons displayed contact guidance on all patterns. Furthermore, we found that the nerve cell processes preferred to grow on ridge edges and elevations in the patterns rather than in grooves, a seemingly claustrophobic behavior. We conclude that axons of peripheral neurons might be guided by nanopatterns on PMMA when the lateral features are 100nm or larger. The present results can be utilized for nerve regenerating scaffolds or the construction of a stable, high-resolution electronic interface to neurons, which is required for future brain machine interfaces.

This work, for the first time, demonstrates that nano-imprinted samples, with 100 nm wide polymer lines, can act as guides for molecular motors consisting of motor proteins actin and myosin. The motor protein function was characterized using fluorescence microscopy and compared to actomyosin motility on non-structured nitrocellulose surfaces. Our results open for further use of the nano-imprint technique in the production of disposable chips for bio-nanotechnological applications and miniaturized biological test systems. We discuss how the nano-imprinted motor protein assay system may be optimized and also how it compares to previously tested assay systems involving low-resolution UV-lithography and low throughput but high-resolution electron beam lithography.