We have obtained experimental data describing the reversal properties of individual, isolated, single-domain ferromagnetic particles. Two types of particles were studied: nearly ellipsoidal $\gamma$-Fe$\sb2$O$\sb3$ particles and elongated nickel cylinders. A process in which Ni was electrochemically deposited into the pores of channeled pore membranes was developed to prepare Ni cylinders over a wide size range. Three types of membranes were employed: Nanochannel glass arrays, Al$\sb2$O$\sb3$ filter membranes, and polycarbonate track-etched membranes. High aspect ratio Ni cylinders were prepared with diameters ranging from 27-1000 nm.
Particles were deposited onto transmission electron microscopy (TEM) grids and the size, shape, morphology, crystalline structure, and isolated nature of the particles were studied by scanning electron microscopy (SEM), TEM, and electron diffraction. The magnetic properties of the electrodeposited membrane arrays were studied by vibrating sample magnetometry (VSM) and the individual particle properties were studied by Lorentz magnetometry and magnetic force microscopy (MFM).
The switching field H$\sb{\rm s}$ was measured as a function of the angle between the applied field and the particle axis. The resulting angular dependence of the $\gamma$-Fe$\sb2$O$\sb3$ particles is qualitatively similar but quantitatively smaller than that expected for coherent rotation. This is probably explained by the voids in the particle interior and/or the precise orientation of the crystalline easy axes with respect to the particle long axis and the applied field.
Quantitative measurements and the numerical simulation of a Ni cylinder with a length L = 1.4 $\mu$m and diameter D = 116 nm showed that the remanent magnetization of the particle is essentially uniform and the remanence loop is square, hence the particle can be considered single-domain. The experimental data was compared to nucleation theory and to numerical micromagnetic simulations. For large diameter, the experimental H$\sb{\rm s}$ is much larger than the theoretical nucleation field indicating that nucleation can substantially precede the reversal. For small diameter, the experimental H$\sb{\rm s}$ is significantly reduced from the expectation of nucleation theory and a comparison with numerical simulations suggest that this reduction is due mostly to the non-ellipsoidal, cylindrical shape of the Ni particles.
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