Yaowu Hao

Magnetic behavior of lithographically patterned particle arrays (invited)

This article reviews recent progress in the fabrication, characterization, and analysis of large area arrays of sub-100-nm magnetic particles made by lithographic techniques. Particles are made by electrodeposition, evaporation and liftoff, or sputtering and etching, leading to a wide range of shapes, compositions, and microstructures. The remanent states, magnetic hysteresis, and uniformity of the particles and the interparticle interactions will be discussed.

Optimization of a lithographic and ion beam etching process for nanostructuring magnetoresistive thin film stacks

The patterning of multilayer thin-film stacks to create spin valves, with dimensions ∼100 nm, for magnetic-random-access memories presents novel fabrication challenges since the materials commonly used (e.g., Co, Cu, and Ni) do not form volatile compounds, and hence cannot be reactive-ion etched. The consequent necessity of using ion-beam etching (“ion milling”) demands a solution to the twin problems of faceting and redepostion of sputtered material. In addition, antireflection layers are not used during lithography because of the necessity of avoiding high-temperature curing, which would harm the spin valve characteristics. By using a thin SiOx phase-shifting layer under the resist, we obtained adequate resist profiles; and by using a 15-nm-thick W hard mask, no measurable redeposition was observed after ion milling of cobalt. Improved etch selectivity of W relative to Co is achieved by using neon as the ion-milling gas rather than argon. A simple model for the enhanced ion-milling selectivity is presented.

Synthesis of highly active and stable Au–PtCu core–shell nanoparticles for oxygen reduction reaction

Au-PtCu core-shell nanoparticles were successfully synthesized via galvanic replacement of Cu by Pt on hollow Au nano-spheres. Characterizations of the nanoparticles were conducted by X-ray diffraction (XRD), transmission electron microscopy (TEM), and electrochemical measurements. Results indicate 2-2.5 times higher specific activity and mass activity of the Au-PtCu catalysts than commercial Pt black and Pt/C in oxygen reduction reaction (ORR), measured in a rotating disk electrode system. Besides, thinner PtCu coating (25 nm thick, deposition time of 20 min) on the hollow Au nano-spheres demonstrated a pronounced CO oxidation peak shift (by 0.13 V) and long-term durability probably due to the unique core-shell structure and strong electronic coupling between the Au core and the PtCu shell.

Magnetization reversal in sub-100 nm pseudo-spin-valve element arrays

The magnetization reversal exhibited by arrays of 70-nm-wide pseudo-spin-valve (PSV) elements has been investigated by measurements of minor hysteresis loops. Samples were patterned from sputtered NiFe (6 nm)/Cu (3 and 6 nm)/Co (4 nm)/Cu (4 nm) magnetic thin film stacks. The overall room temperature magnetic behavior of the arrays can be understood by considering a distribution of switching fields for both the hard (Co) and soft (NiFe) magnetic layers. Such layers interact through exchange and magnetostatic coupling. Increasing the lengths of the elements leads to narrower switching field distributions and higher mean switching fields (particularly for the hard layer). On the other hand, decreasing the thickness of the Cu spacer leads to an increase of the switching field of the hard layer. Results obtained are well described by a model that treats each PSV as a coupled pair of rectangular single-domain films and uses the values of the interaction field between layers deduced from experimental minor loops.

Effect of deposition parameters on the CPP-GMR of NiMnSb-based spin-valve structures

Abstract We present measurements of current perpendicular magnetoresistance of NiMnSb/Cu/NiMnSb spin-valve structures based upon the predicted half-metallic (100% spin polarized) ferromagnetic alloy NiMnSb. The observed effect of 5–10% is much smaller than the complete spin-valve effect expected from a 100% spin-polarized system. The dependence of the magnetoresistance on film thickness and deposition parameters are explored to understand the factors limiting giant magnetoresistance in these structures.

Theranostic Nanoseeds for Efficacious Internal Radiation Therapy of Unresectable Solid Tumors

Malignant tumors are considered “unresectable” if they are adhere to vital structures or the surgery would cause irreversible damages to the patients. Though a variety of cytotoxic drugs and radiation therapies are currently available in clinical practice to treat such tumor masses, these therapeutic modalities are always associated with substantial side effects. Here, we report an injectable nanoparticle-based internal radiation source that potentially offers more efficacious treatment of unresectable solid tumors without significant adverse side effects. Using a highly efficient incorporation procedure, palladium-103, a brachytherapy radioisotope in clinical practice, was coated to monodispersed hollow gold nanoparticles with a diameter about 120 nm, to form 103Pd@Au nanoseeds. The therapeutic efficacy of 103Pd@Au nanoseeds were assessed when intratumorally injected into a prostate cancer xenograft model. Five weeks after a single-dose treatment, a significant tumor burden reduction (>80%) was observed without noticeable side effects on the liver, spleen and other organs. Impressively, >95% nanoseeds were retained inside the tumors as monitored by Single Photon Emission Computed Tomography (SPECT) with the gamma emissions of 103Pd. These findings show that this nanoseed-based brachytherapy has the potential to provide a theranostic solution to unresectable solid tumors.

Fabrication and Magnetic Properties of Ordered Macroporous Nickel Structures

Here we report on the fabrication of ordered macroporous nickel structures by electrodeposition into colloidal crystal templates formed by self-assembly of polystyrene particles with diameters from 100 nm to 1 μm. The current during deposition into well-ordered colloidal crystals with low defect density exhibits periodic oscillations associated with the change in fractional area within the crystal. Magnetic hysteresis loops and magnetoresistance measurements are performed as a function of pore diameter and film thickness. The magnetic properties can be explained by modeling the ordered macroporous nickel structures as a three-dimensional network structure with magnetic particles connected by narrow ligaments.

Magnetization switching in 70-nm-wide pseudo-spin-valve nanoelements

The magnetic domain structures and magnetization reversal of patterned 70-nm-wide pseudo-spin-valve (PSV) elements were studied by magnetic force microscopy (MFM). Both magnetically soft and hard layers form single-domain states at remanence, and can be magnetized either parallel or antiparallel to each other. The switching field of each layer, and the coupling between the layers, are quantified using MFM. Individual elements show well-defined switching fields, while the ensemble has a large switching field distribution due to variability between the PSV elements.

Patterning processes for fabricating sub-100 nm pseudo-spin valve structures

Interference lithography (IL) was used to pattern sputtered Co/Cu/NiFe layers into large-area arrays of pseudo-spin valve (PSV) elements. In order to precisely control size, aspect ratio, and shape uniformity of the elements, three methods of increasing complexity were developed. Pattern transfer was achieved by reactive-ion etching and ion milling, and was found to maintain the multilayered structure of the PSV film. The switching field of the PSV elements, and the remanent state, varied with the aspect ratio as expected. Furthermore, IL was employed to fabricate magnetic random access memory-type structures. Both sense and word lines were conductive, and the buried PSV elements had similar magnetic properties to PSV elements patterned in large-area arrays.

The fabrication of short metallic nanotubes by templated electrodeposition

Template-based electrochemical synthesis has widely been used to produce metal nanowires and nanorods. Commercially available filtration membranes, such as anodic aluminum oxide (AAO) and polycarbonate track etch membranes, have commonly been utilized as hard templates for this purpose. In this process, a thick metal film is usually sputtered or vacuum evaporated onto one side of the membrane to block the pores and serve as the working electrode for the subsequent electrodeposition. Here, we show that during the deposition of the metal electrode for AAO membranes, the electrode metal diffuses into the pores and is deposited on the pore walls which leads to preferential electrodeposition of metal on the walls and therefore forms metal tubes. This phenomenon has been utilized to fabricate short nanotubes by carefully controlling the electrodeposition conditions. The process is a straightforward method for any electroplatable materials to form nanoscale tubular structures. The effects of working electrodes and electrodeposition conditions on the formation of tubular structures are discussed in detail. A new mechanism based on this simple fact is proposed to explain the formation of Ni tubes by Ni-Cu co-deposition. Also, we demonstrate how to distinguish magnetic nanotubes from nanorods by a simple magnetic measurement.