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We have fabricated arrays of high aspect ratio nickel columns in both PMMA and SiO2, and we have characterized their structure using transmission electron microscopy and scanning electron microscopy. We have also characterized the magnetics of the columns with magnetic force microscopy. Such characterization of high aspect ratio perpendicular magnetic particles is only rarely found in the literature. This work lays a foundation for further studies to test the viability of lithographically defined arrays of magnetic particles as an alternative high density storage medium.

Numerical study for ballistic switching of magnetization in single domain particle triggered by a ferromagnetic resonance within a relaxation time limitWe present an experimental study of the thermally activated switching of the magnetization of individual isolated y-Fe903 particles. These particles are prolate ellipsoids -3000 A long and 650 ^ . .A wide. The measured angular dependence of the switching field, H,(B), is consistent at large angles with a uniform rotation, hut as H approaches zero, other modes of reversal appear possible, and most likely the mode of reversal is curling. By measuring the probability of reversal of the moment as a function of time and applied magnetic field at T-300 K, we found that the switching was thermally assisted, but couldn"t be described by hopping over a single energy barrier. Our results indicate that the dynamics of switching are described by a complex path in the energy landscape.

For a complete experimental and theoretical explanation of the magnetic processes in an interacting collection of submicron magnetic particles, a fundamental understanding of the magnetic properties of individual single-domain particles must first be achieved. We have prepared elongated Ni columns ranging in diameter from 0.15 to 1.0 m by electroplating into specially prepared Al 2 O 3 and glass channeled pore membranes. We have also prepared controlled arrays of Ni columns using e-beam lithography, subsequently electroplating into the written patterns. Using transmission electron microscopy, we have characterized the shape, size, morphology, and crystal structure of the columns. Magnetic force microscopy has been used to determine the switching field H s versus the applied field angle of the columns. Although the switching field data can be fit to the functional form for nucleation by curling in an infinite cylinder, the observed weak dependence of H s on column diameter is inconsistent with that expected for curling, particularly for columns of diameter Ͼ0.3 m.

Quantitative measurements of the switching field properties of individual single-domain particles are both scientifically and technologically important. Such data are relevant for the engineering of a patterned magnetic storage medium where a single bit of information corresponds to individual single-domain particles with sizes consistent with storage densities >50 Gbit/in2. By electroplating into Al2O3 and track-etched polycarbonate filters, we have prepared particles over a large range of radii from R=20–500 nm, and have characterized them by transmission electron microscopy. Using a magnetic force microscope with an in situ electromagnet, we have quantified the extent to which the particles can be considered “single-domain.” We have also measured the angular dependence of the switching field Hs for these particles. At zero degrees, Hs lies above the curling curve for R/R0>4, and falls below the curling curve for R/R0<2, where R0 is the exchange length (R0=20.6 nm for Ni). At intermediate radii, the angular dependence data are close to the curling curve with a crossover of the data and the curling solution at R/R0∼3.