M. Hwang

Fabrication of patterned media for high density magnetic storage

Arrays of discrete, lithographically patterned magnetic elements have been proposed as a new generation of ultrahigh density patterned magnetic storage media. Interferometric lithography has been used to make prototype arrays over large areas with periods of 100–200 nm. Arrays of magnetic pillars, pyramids, and dots have been made by electrodeposition, evaporation and liftoff, and etching processes, and the magnetic properties of the particles and their mutual interactions have been measured.

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.

Fabrication of large area nanostructured magnets by interferometric lithography

Patterned arrays of magnetic elements may be useful as media for high density magnetic storage applications. Interferometric lithography has been used to fabricate arrays of cobalt and nickel pillars with periods of 200 nm over areas of 5 cm/spl times/5 cm using a UV laser. This provides an economical and rapid method for manufacturing particle arrays.

Magnetic force microscopy study of interactions in 100 nm period nanomagnet arrays

The magnetic behavior of two different 100 nm period arrays of Ni pillars has been characterized using vibrating sample magnetometry and magnetic force microscopy. The samples have similar aspect ratios of approximately two but different particle dimensions, leading to differences in the strength of magnetostatic interactions. The experimental results were compared with a simulation based on an Ising-like simple interaction model.

Micromagnetic behavior of conical ferromagnetic particles

Large area arrays of cobalt and nickel particles with truncated conical shapes and diameters of 80–120 nm have been prepared using interference lithography combined with an evaporation and lift-off process. The magnetic hysteresis has been measured and the remanent states of the particles have been compared with a three-dimensional micromagnetic model. The model shows a transition from “flower” to “vortex” magnetization states as the particle size increases. The distribution of switching fields and the magnetostatic interactions between particles have been characterized. Both lead to a slow approach to saturation in the hysteresis loops. The suitability of such arrays for data storage is discussed.

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.

The effect of aspect ratio on the magnetic anisotropy of particle arrays

Arrays of evaporated nickel particles with a variety of diameters (75–122 nm) and aspect ratios are fabricated in order to study the effect of the particles’ geometry on their magnetic behavior, interactions and switching mechanism. Hysteresis loops generated by simulating single-domain particles with out-of-plane magnetization are compared to the experimentally obtained data.

Magnetic behavior of amorphous CoP cylinder arrays

Arrays of low-aspect-ratio cylindrical amorphous CoP nanomagnets with diameters near 100 nm have been fabricated using electrodeposition. The remanence of individual particles and the easy axis direction are consistent with the predictions of a micromagnetic model, but the behavior of the arrays is dominated by magnetostatic interactions because the switching fields of the particles are low compared to the magnitude of the nearest-neighbor interactions.

Modelling of hysteresis loops of arrays of 100 nm period nanomagnets

Magnetic hysteresis loops have been measured for a 100 nm period array of 35 nm diameter nickel pyramids formed by interferometric lithography. Results are compared with a computational model which describes a square array of Stoner-Wohlfarth particles. This allows the distribution of particle anisotropies and easy axis directions, and the switching mechanism, to be inferred.

Remanent state studies of truncated conical magnetic particles

The remanent state of truncated conical particles is investigated as a function of their size, aspect ratio, and anisotropy, using a micromagnetic model based on the Landau–Lifshitz–Gilbert equation. Particles with a base diameter smaller than three times the exchange length show a “flower” state, while larger particles show a “vortex” magnetization state. The critical size for this transition increases with increasing anisotropy. Small flower-state particles show abrupt reorientation from out-of-plane to in-plane magnetization at a critical aspect ratio of 0.9. For vortex-state particles, the axial remanence gradually increases as the aspect ratio increases, and high aspect ratio particles have significant remanence even at larger diameters.