Y. J. Tang

CoPt patterned media in anodized aluminum oxide templates

Patterned recording media consisting of a vertically aligned array of L10 phase CoPt nanowires embedded in a thin anodized aluminum oxide (AAO) template on silicon has been prepared. A sputter deposited thin film of aluminum on silicon was anodized and a CoPt magnetic alloy was electrodeposited into the pores of the AAO. The vertically aligned arrays of CoPt nanowires were about ∼100nm tall with ∼20nm average diameter. Since the CoPt nanowire array is laterally constrained by the surrounding AAO, the nanowire diameter is maintained without coarsening during the L10 phase conversion heat treatment at 700°C. After annealing and conversion to the L10 phase, the ∼20nm CoPt nanowires exhibit a large coercivity of ∼8kOe measured in the in-plane and perpendicular directions.

Enhanced room temperature ferromagnetism in Co- and Mn-ion-implanted silicon

The authors report on ferromagnetism at room temperature in cluster-free, cobalt- and manganese-ion-implanted crystalline silicon. Through magnetic and structural analysis it is shown that the ion-implanted Si consists of two layers of Co- and Mn-containing silicon: (1) an amorphous Si layer on the surface and (2) single crystalline Si beneath. The amorphous layer shows very little magnetism by itself but seems to be responsible for partially canceling out or masking the ferromagnetism in the crystalline Si. The authors also observe that etching of the amorphous Si layer dramatically enhances the measured magnetism by as much as 400%.

Magnetic domain configurations in spark-eroded ferromagnetic shape memory Ni-Mn-Ga particles

Spherical particles of the ferromagnetic shape memory material Ni51Mn29Ga20 obtained by spark erosion transform during cooling from the high-temperature Heusler L21 cubic phase into tetragonal martensite. Using the Fresnel (i.e., Lorentz) imaging mode, magnetic domains with an average width of 100nm are observed in the modulated martensitic phase in the absence of a magnetic field. The magnetization distribution within individual particles is determined using electron holography. The magnetic flux lines change direction when crossing boundaries between crystallographic twin variants. These boundaries, where the easy c-axis of the crystallographic variants rotate by 86.5°, coincide with quasi-90° magnetic domain walls, with thickness of approximately 17nm. The magnetization saturation determined by electron holography is about 0.57T.

Microstructural characterization of Ni-Mn-Ga ferromagnetic shape memory alloy powders

Powders of Ni-Mn-Ga ferromagnetic shape memory alloy (FSMA) were produced by spark erosion. Powders quenched in liquid nitrogen are primarily hollow spherical particles, whereas those quenched in liquid argon are mostly solid spheres. Electron microscopy observations of powders as-sparked in nitrogen show highly disordered nanocrystalline grains with an average size of less than 100 nm whereas those sparked in liquid argon display extensive crystallinity. Magnetic measurements indicate that the powders do not become fully ferromagnetic until after annealing at 973 K for 5 h. Investigations of annealed powders using differential scanning calorimetry reveal thermoelastic martensitic transition behavior. The annealed powders have a microtwinned martensite with many large grains showing a five-layer period modulation. These Ni-Mn-Ga powders should be suitable for the development of magnetic-field-driven FSMA powder/polymer microcomposites.

Magnetocaloric effect in NiMnGa particles produced by spark erosion

The magnetic entropy change of tetragonal and orthorhombic NiMnGa fine particles made by spark erosion was investigated in this paper. It was found that the structure and crystalline phase transformation temperatures can be strongly affected by the compositions of the particles, while Curie temperature is less sensitive to the compositions. Due to the possible distribution of the particle size and compositions in these particles, the magnetic entropy changes observed are much broader and smaller than those of bulk NiMnGa alloys. The maximum absolute value of entropy change ΔS=2JKg−1K−1 was observed for tetragonal structure NiMnGa particles at 95°C in a field of 2T.

Microstructure and exchange coupling in nanocrystalline Nd2(FeCo)14B∕α‐FeCo particles produced by spark erosion

Exchange spring magnet particles of Nd2(FeCo)14B∕α‐FeCo were prepared by spark erosion. X-ray diffraction and Mossbauer studies showed that the particles are composed of about ∼85vol% of Nd2(FeCo)14B and ∼13vol% of α‐FeCo with negligible other phases. No oxide was found in these particles. Transmission electron micrographs indicated that the grain sizes of the Nd2(FeCo)14B and α‐FeCo phases are ∼10–50nm, and are compatible with effective exchange coupling between the hard and soft phases. The intergrain exchange coupling was also observed in ΔM measurements.

Fabrication and magnetic properties of nanopatterned FePt media

A method for fabricating nanopatterned media has been investigated. A monolayer of regularly distributed Ni nanoparticles of uniform diameter (∼5nm) fabricated on the surface of a polyimide-coated Si substrate using metal film-polyamic acid interactions was utilized as a protruding template to deposit an array of FePt phase. A FePt film (5 nm thick) was sputtered onto this template with or without an underlayer of either 5-nm-thick MgO or CoO and analyzed before and after annealing. It was observed that while the FePt films sputtered on flat Si substrates with underlayers exhibited low Hc of only about 1 kOe after the L10 conversion, the FePt samples deposited onto protruding Ni nanoparticle templates showed a significant increase in Hc to ∼8.1kOe. The FePt sample with CoO underlayer exhibited a possibly strong perpendicular magnetic anisotropy. The intergranular exchange coupling of the samples was also investigated by using ΔM method.

Permeability Measurements of Amorphous Coatings Prepared with Fine Magnetic Particles

Amorphous or granular magnetic coatings formed with magnetic particles have high resistivities, which are desirable to reduce the eddy current t losses greatly. However, since magnetic particles are separated from one another, the permeabilities of magnetic coatings tend to be low, and obtaining accurate permeability spectra can be not so trivial. The measured relative permeability of FeSiB coating at 30 MHz was 21.5, while the expected value based on 4piMs and Hk values was 5.4. The modifications in calibration procedure allow more accurate measurement of permeability of magnetic films with intermediate or low permeability and hence enable the permeameter to cover the wider range of the permeability values.

Morphology, Microstructure and Magnetic Properties of Spark-eroded Ferromagnetic Ni-Mn-Ga Fine Particles

Ni-Mn-Ga alloys show large reversible linear strain of up to 9.5 % under an applied magnetic field. This behavior is attributed to the rearrangement of twin-related martensite variants, which are crystallographically equivalent but with different orientation [1]. Practical applications have some drawbacks, due to poor mechanical characteristics of single crystals, but these can be overcome by designing particle-polymer composites. Fine Ni-Mn-Ga particles suitable for use in preparing composite materials were produced by spark-erosion in liquid nitrogen and liquid argon [2]. The objective of the present investigation is to clarify the morphology, microstructure and magnetic properties of ferromagnetic Ni50.7Mn29.5Ga19.8 (at.%) spark-eroded particles.