Polarization control in magnonic vortex crystals

We report the nanofabrication and characterization of x-ray transmission gratings with a high aspect ratio and a feature size of down to 65 nm. Two nanofabrication methods, the combination of electron beam and optical lithography and the combination of electron beam, x-ray, and optical lithography, are presented in detail. In the former approach, the proximity effect of electron beam lithography based on a thin membrane of low-z material was investigated, and the x-ray transmission gratings with a line density of up to 6666 lines/mm were demonstrated. In the latter approach, which is suitable for low volume production, we investigated the x-ray mask pattern correction during the electron beam lithography process and the diffraction effect between the mask and wafer during the x-ray lithography process, and we demonstrated the precise control ability of line width and vertical side-wall profile. A large number of x-ray transmission gratings with a line density of 5000 lines/mm and Au absorber thickness of up to 580 nm were fabricated. The optical characterization results of the fabricated x-ray transmission gratings were given, suggesting that these two reliable approaches also promote the development of x-ray diffractive optical elements.

This letter reports the first results on a new generation of anti-symmetric lamb-wave devices using a simple solidly mounted platform of AlN/Mo/Si and operating at very-high-to-super-high frequency range. AlN/Mo/Si-based solidly mounted resonators (SMRs) with sub-micrometer interdigitated transducers were fabricated using the electron beam lithography and liftoff process. Plasma etching was used to etch vias in ground pads to get access to the bottom electrode for grounding. By comparing the floating, grounded, and no bottom electrode device performances, it has been observed that the presented SMR design with grounded bottom electrode results in enhancement of the coupling efficiency by 500% compared with conventional SAW devices. In addition, these devices are more mechanically robust and can be monolithically integrated with a CMOS platform without requiring complex packaging processes compared with conventional released lamb-wave resonators.

Research of micro area piezoelectric properties of AlN films and fabrication of high frequency SAW devices

A systematic investigation of the magnetic domain structure is presented for epitaxial sub-micron (101~0) Co wires characterized by a strong in-plane uniaxial magneto-crystalline anisotropy whose easy axis is oriented perpendicular to the long wire axis. Wires of varying width (100 to 1000 nm) and thickness (30 to 80 nm) were patterned by electron beam lithography and lift-off process. We establish experimentally the boundaries between the ground state transverse single domain state and the open stripe structure as a function of wire thickness and width. Moreover, the stability of the transverse single domain state is investigated as a function of the magnetization history.

The magnetization process and switching field distribution (SFD) of ion beam patterned MnGa films were discussed based on the first-order reversal curve (FORC) analysis and magnetic force microscope (MFM) observations. The FORC diagrams with so-called “boomerang” shape resulting from the increase of the interaction between “hysterons” that can be associated with ferromagnetic particles and/or domains were confirmed in the patterned MnGa with dot size larger than 520 nm. On the other hand, the shape of the FORC diagram became a simple Gaussian distribution, which is typical in the media comprised of well-isolated magnetic particles, for dot size smaller than 240 nm. The variation of the shape of FORC diagram was closely linked with the domain structure observed by MFM; multidomain in the dot was confirmed for dot size larger than 520 nm while single domain for dot size less than 240 nm. The average switching field of the patterned film increased with decreasing the dot size, indicating that the dot edge damage and exchange interaction through ion-irradiated regions were negligible in the ion-beam patterned MnGa. The SFD also increased with decreasing the dot size. The SFD is considered to result from the existence of the pinning centers in the as-prepared MnGa and also from the dot size fluctuation of the resist masks fabricated by electron beam lithography. The optimization of the fabrication process will be effective to reduce SFD and sufficiently low SFD in high-density MnGa bit patterned media will be achieved by these process optimizations.

High-throughput nanogap formation is reported for simultaneous fabrication of integrated nanogap arrays. Ten series-connected nanogaps with butterfly and bottle shapes were integrated by using electromigration induced by a field emission current (“activation”). Initially, ten series-connected butterfly-shaped nickel (Ni) nanogaps were fabricated with electron-beam lithography and lift-off processes. Activation with a preset current of 300 nA reduced the separation of the gaps to <10 nm. Similar results for bottle-shaped nanogaps indicated that integration of nanogaps using activation is not dependent on nanogap shape. The activation method was also used for the mass production of 30 identical nanogaps. Overall, the distance between the Ni nanogap electrodes was completely controlled by activation.

The magnetic and magnetoresistive behaviour of circular and elliptical thin film multilayer rings is discussed. Rings are particularly interesting because they can adopt several stable and metastable magnetic states characterized by different numbers of domain walls. Electrically contacted rings made of Co/Cu/NiFe or IrMn/Co/Cu/NiFe, with diameters of ∼0.5–5 µm and widths of ∼100 nm were fabricated by electron-beam lithography, sputtering and liftoff processing. We show that multilayer rings switch their magnetization by a qualitatively different mechanism compared with that of a single-layer ring; how the magnetization circulation direction around the ring can be controlled; how the rings can be electrically contacted to show large fractional field-induced changes in resistance and how these structures may be used in magnetic random access memories or magnetic logic devices.

Spin electronics with semiconductor/ferromagnet hybrids is a topic of ongoing interest. We review developments in hybrid spintronics and give an overview of achievements in efficient spin injection from ferromagnetic metals into semiconductors. The focus of this work is on thin Heusler films grown on semiconductor substrates. Ni2MnIn films are deposited on a variety of substrates by coevaporation of nickel and the alloy MnIn. The almost perfect lattice match between Ni2MnIn and InAs qualifies this alloy for basic research in spintronics. Point-contact Andreev spectroscopy serves to quantify the spin polarization relevant to transport. Nanopatterning of Ni2MnIn electrodes with electron-beam lithography and lift-off processing is examined. In this context, the influence of post-growth annealing on the film’s morphology and crystal structure is studied in situ using transmission-electron microscopy. The electrodes are completed by a copper strip to form a lateral spin-valve. In first measurements in local geometry we have detected the spin-valve effect.

We report a simple and easy method for the simultaneous control of electrical properties of multiple Ni nanogaps. This method is based on electromigration induced by a field emission current, which is the so-called “activation.” The tuning of tunnel resistance of nanogaps was simultaneously achieved by passing a Fowler-Nordheim (F-N) field emission current through three initial Ni nanogaps connected in series. The Ni nanogaps having an asymmetrical shape with the separation of 80-110 nm were fabricated by electron-beam (EB) lithography and lift-off process. By performing the activation, current-voltage properties of series-connected nanogaps were simultaneously varied from “insulating” to “metallic” through “tunneling” properties with increasing the preset current of the activation. Furthermore, the tunnel resistance of simultaneously activated nanogaps decreased from the order of 100 TΩ to 100 kΩ with increasing the preset current from 1 nA to 30 μA. Scanning electron microscopy (SEM) images of the nanogaps after performing the activation with the final preset current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> of 30 μA clearly indicated that the separation of the gaps reduces to less than 10 nm after the activation. These results clearly imply that the electrical properties of series-connected nanogaps can be simultaneously tuned by the activation procedure.

We have fabricated single crystal cobalt and Co/sub 90/Cr/sub 10/ islands of dimension 130 nm/spl times/330 nm/spl times/20 nm by a direct-write electron beam lithography and ion-milling process. The islands have the crystalline easy axis along the short axis. These islands have been characterized by magnetic force microscope in the presence of an applied field. About 85% of the as-grown cobalt islands were single domain and rest 15% showed a bi-domain state. The as-grown Co/sub 90/Cr/sub 10/ islands were all in a single domain state. In remanence all the islands were single domain in both the pure and the alloyed films. Two modes of switching were observed. Some of the islands switched by going through a bi-domain state and the rest switched directly from a single domain state to a single domain state.

The development of national resists for electron-beam and ion-beam lithography processes is reviewed. Positive resists based on polymethyl methacrylate allow to create nanoscale structures. Moreover, the possibility of using a combination of the created resists as bilayer resist is demonstrated. The perspectives of the development of national processes of electron-beam and ion-beam lithography are also demonstrated. At the present moment own national technological processes of electron-beam and ion-beam lithography are at the stage of demonstrators. In the near future, national electron-beam and ion-beam lithography devices will be developed. It will take significantly more time to organize production of national ion-beam lithography equipment.

Linear arrays of 15 nm thick, polycrystalline Co91Fe9 nanoelements have been fabricated using e-beam lithography, collimated sputtering and lift-off. The elements have widths of 0.17 and 0.25 μm, lengths from 0.5 to 4 μm, pointed ends, and center-to-center spacing of 0.5 μm. Characterization by magnetic force microscopy shows that in the virgin state these elements are single-domain and have magnetizations parallel to their long axes. The field at which an element switches its magnetization direction is shown to depend mainly on its width, and only slightly on its length. Fields of about 440 and 680 Oe are needed to switch completely the 0.25 and 0.17 μm structures, respectively. This separation in switching fields is adequate to allow elements of different widths to be switched independently. Also, magnetic force microscopy and SQUID magnetometry analysis show that a capping layer is necessary to protect the films from oxidation. Unprotected films show an altered hysteresis loop and their associated nanostructures are not consistently single-domain.

Nanostructured magnetic tunnel junction rings (MTJ) based on CoFeB free layer were fabricated to investigate the corresponding magnetization reversal processes. The dimensions of the MTJ rings were with outer diameter/linewidth of 1 mum/100 nm and 0.7 mu m/100 nm , respectively, in which the fabrication was carried out by a top-down technique in combination with electron beam lithography, ion-milling and etch-back processes. Double transition processes in magnetoresistance (MR) responses are observed in these nanostructured MTJ rings. With considering the surface roughness, an OOMMF simulation was employed to explore the magnetization evolution. The simulated hysteresis loops are consistent with the MR behaviors, unfolding that the magnetic switching process initiates from the forward to reversed onion states through an incomplete vortex state. This two-step switching behavior is reasonably assumed to be due to a pinning site resulting from the surface roughness in a region of the ring. In addition, a series of in situ magnetic force microscopy images extracted from nanostructured MTJ rings reveals the similar magnetization evolution with the simulated results.

The authors report on the integration of a hybrid lithography process for AlGaN/GaN high-electron-mobility nanoscaled, fin-based field effect transistors with an Au-free contact. The integration consists of electron beam lithography (EBL) and optical photolithography to minimize prototyping cost and time while maximizing resolution and device array density. I-line direct photolithography is performed using standard photoresist, while EBL is accomplished using 2% polymethyl methacrylate for high resolution of fin structure patterning. Au-free contact is characterized using linear transmission line method (TLM) and is deposited along with registration marks via electron beam evaporation. The registration marks and TLM structures are used for alignment in the EBL process. Details of the device fabrication process and electrical results are discussed and show possible parallel processing of GaN and Si devices.

(Co/Au)N ultrathin multilayers with perpendicular and in-plane magnetic anisotropy were studied experimentally using a combination of ferromagnetic resonance, magneto-optical magnetometry and microscopy (with both in-plane and out-of-plane magnetization-sensitive longitudinal and polar Kerr effects), and magnetic force microscopy (MFM). Three-dimensional magnetization distributions were reconstructed from micromagnetic simulations complemented by the measured magnetic parameters of the multilayers and observations by Kerr microscopy and MFM. It is shown that, in the reorientation phase transition (RPT) zone – the range of anisotropy characterized by 0 &amp;lt; Q &amp;lt; 1 (the ratio of the anisotropy energy to be gained by magnetization along the easy axis perpendicular to the sample surface, and the magnetostatic energy of a uniformly magnetized layer along the surface normal) – the three-dimensional magnetization distributions consist of alternating pairs of vortices and half-antivortices, both with in-plane magnetized cores and elliptical cross-sections. It is shown that an increase in N leads to a significant increase in the saturation field as well as changes in the hysteresis loop shape, domain structure size, and geometry. We demonstrate by simulations and prove by experimentation that, in multilayers with Q &amp;lt; 1, an increase in N induces the nucleation of out-of-plane magnetized domains, which then triggers magnetostatic interlayer coupling and RPT onset to out-of-plane magnetization states. We report on the observations of large micrometer-sized metastable domains (connected with the vortex cores) with the in-plane magnetization modulated by submicrometer-sized out-of-plane domains with sizes defined mainly by magnetostatics.