Pin Ho

Picosecond pulse generation with a cw GaAlAs laser diode

We report the generation of 20‐ps optical pulses at microwave repetition rate from a GaAlAs double‐heterostructure diode operating cw at room temperature. The diode is operated in an external optical resonator and is actively modulated at 3 GHz. The pulses are measured by autocorrelation using SHG in LiIO3. They are the shortest pulses ever reported for a cw laser diode.

Tunable room-temperature magnetic skyrmions in Ir/Fe/Co/Pt multilayers

Magnetic skyrmions are nanoscale topological spin structures offering great promise for next-generation information storage technologies. The recent discovery of sub-100-nm room-temperature (RT) skyrmions in several multilayer films has triggered vigorous efforts to modulate their physical properties for their use in devices. Here we present a tunable RT skyrmion platform based on multilayer stacks of Ir/Fe/Co/Pt, which we study using X-ray microscopy, magnetic force microscopy and Hall transport techniques. By varying the ferromagnetic layer composition, we can tailor the magnetic interactions governing skyrmion properties, thereby tuning their thermodynamic stability parameter by an order of magnitude. The skyrmions exhibit a smooth crossover between isolated (metastable) and disordered lattice configurations across samples, while their size and density can be tuned by factors of two and ten, respectively. We thus establish a platform for investigating functional sub-50-nm RT skyrmions, pointing towards the development of skyrmion-based memory devices.

Electrodeposition of granular FeCoNi films with large permeability for microwave applications

A simple methodology to fabricate soft magnetic FeCoNi granular films from cheap salt solutions viaelectrodeposition at room temperature is demonstrated. With the addition of a small quantity of organic and inorganic additives into the solutions, the FeCoNi nano-granular films possess ultra-high permeability, large resistivity, and other desirable magnetic properties for gigahertz microwave applications. Typically, the films have a coercivity of less than 10 or 20 Oe along the hard or easy axis, respectively, with a saturation flux density of up to 2.43 T. The magnetic permeability and resistivity are correspondingly up to a magnitude order of about 103 and 10−4 Ω cm. These soft FeCoNi films also show a big anisotropic field of more than 50 Oe and a very small magnetostriction of <10−5. They can be potentially applied to microwave absorption as well as other applications.

Magnetization reversal and magnetoresistance behavior of perpendicularly magnetized [Co/Pd]4/Au/[Co/Pd]2 nanowires

We present a systematic investigation of the magnetization reversal mechanism and magnetoresistance behavior of perpendicularly magnetized Co/Pd multilayer films and nanowires (NWs) as a function of Cu buffer layer thickness and temperature. The effect of interlayer coupling is studied by varying the Au spacer layer thickness in between two Co/Pd multilayer stacks in the [Co/Pd]4/Au/[Co/Pd]2 structure. We observed that compared with continuous (un-patterned) films deposited at the same time, the NWs display much stronger temperature dependent interlayer coupling due to magnetostatic interactions through the stray fields. At low temperature, the competition between the interlayer coupling and coercive field difference between the soft and hard Co/Pd multilayer stacks determines the overall magnetization reversal process and magnetoresistance behavior.

Oersted Field and Spin Current Effects on Magnetic Domains in [Co/Pd] 15 Nanowires

An out-of-plane Oersted field produced from a current-carrying Au wire is used to induce local domain formation in wires made from [Co/Pd]15 multilayers with perpendicular anisotropy. A 100 ns pulsed current of 56-110 mA injected into the Au wire created a reverse domain size of 120-290 nm in a Co/Pd nanowire on one side of the Au wire. A Biot-Savart model was used to estimate the position dependence of the Oersted field around the Au wire. The shape, size, and location of the reversed region of Co/Pd were consistent with the magnitude of the Oersted field and the switching field distribution of the unpatterned film. A current density of 6.2 × 1011 Am-2 in the Co/Pd nanowire did not translate the domain walls due to low spin transfer efficiency, but the Joule heating promoted domain growth in a field below the coercive field.

Electrodeposited thin FeCo films with highly thermal stable properties in high frequency range obtained by annealing in a strong magnetic field

The preparation of thin ferromagnetic films with thermally stable properties in the high frequency gigahertz range is crucial for applications in different advanced devices. However, it is a challenge to attain such films as the mechanism and correlation between the crystallographic structure and dynamic magnetic properties remain unclear. Herein, through an appropriate annealing process involving a temperature of 510–580 K and an applied strong magnetic field of 10 kOe along the easy-axis of electrodeposited FeCo films, magnetic properties of the films (300–450 nm) in the gigahertz range are significantly improved. Typically, the magnetic soft nanofilms display highly stable dynamic microwave properties from room temperature to 420 K, even in the presence of a high frequency of ∼1.5 GHz and a large magnetic permeability of ∼460 (the maximum value in real part). The origin of the high thermal stability is attributed to the recrystallized uniformity in crystalline orientations and larger particle sizes aft...

Effects of Edge Taper on Domain Wall Structure and Current-Driven Walker Breakdown in a Ferromagnetic Thin Film Wire

Domain walls in a ferromagnetic thin film wire with rectangular cross section adopt transverse wall (TW) or vortex wall (VW) geometries depending on the magnetic material and the wire width and thickness. However, experimentally wires can have a trapezoidal cross section if they are made by liftoff using an undercut resist profile. Micromagnetic modeling shows that tapering of the wire not only promotes the formation of a TW over a VW, but also increases the critical current value and the domain wall velocity at which Walker breakdown occurs, providing a potential route to higher speed domain wall devices.

Ultra-thin L10-FePt for perpendicular anisotropy L10-FePt/Ag/[Co/Pd]30 pseudo spin valves

Perpendicular anisotropy L10-FePt/Ag/[Co/Pd]30 pseudo spin valves (PSVs) with ultra-thin L10-FePt alloy free layer possessing high anisotropy and thermal stability have been fabricated and studied. The thickness of the L10-FePt layer was varied between 2 and 4 nm. The PSV became increasingly decoupled with reduced L10-FePt thickness due to the larger difference between the coercivity of the L10-FePt and [Co/Pd]30 films. The PSV with an ultra-thin L10-FePt free layer of 2 nm displayed a high Ku of 2.21 × 107 ergs/cm3, high thermal stability of 84 and a largest giant magnetoresistance of 0.54%.

Fabrication of ultrahigh density metal–cell–metal crossbar memory devices with only two cycles of lithography and dry-etch procedures

A novel approach to the fabrication of metal-cell-metal trilayer memory devices was demonstrated by using only two cycles of lithography and dry-etch procedures. The fabricated ultrahigh density crossbar devices can be scaled down to ≤70 nm in half-pitch without alignment issues. Depending on the different dry-etch mechanisms in transferring high and low density nanopatterns, suitable dry-etch angles and methods are studied for the transfer of high density nanopatterns. Some novel process methods have also been developed to eliminate the sidewall and other conversion obstacles for obtaining high density of uniform metallic nanopatterns. With these methods, ultrahigh density trilayer crossbar devices (~2 × 10(10) bit cm(-2)-kilobit electronic memory), which are composed of built-in practical magnetoresistive nanocells, have been achieved. This scalable process that we have developed provides the relevant industries with a cheap means to commercially fabricate three-dimensional high density metal-cell-metal nanodevices.

A simple approach to sub-100 nm resist nanopatterns with a high aspect ratio

A simple methodology to prepare sub-100?nm resist nanopatterns with a high aspect ratio for the transfer of device nanofeatures is demonstrated. The novel method is based on a two- or multi-step developing process with the incorporation of an ?4?nm thick metal film to protect the fine resist nanopatterns in the developer solution. Using this approach, sub-100?nm resist nanopatterns of different shapes were readily fabricated using the positive- and negative-tone electron-beam resists. Subsequently, fine device nanostructures could be readily converted from these fine resist nanopatterns with a high aspect ratio.