Angular variation of the magnetic properties and reversal mode of aligned single-domain iron nanoparticles

We report magnetic hysteresis and back-field demagnetization data measured at 10 degrees intervals of the angle psi between the applied field H and the long (magnetically easy) axes of aligned elongated single-domain iron particles electrodeposited in regularly spaced surface pores in an Al substrate. Our three samples have identical particle diameters, d = 17 nm, but varying axial ratios l/d and spacings d(c). Measured hysteresis loops resemble "hysterons'' predicted by fanning, curling, and coherent rotation models of magnetization reversal, except for two features. For psi = 0-60 degrees, particle magnetizations do not reverse simultaneously at a single critical field: distributed particle coercivities produce steep but nonvertical loop segments. Broadened psi = 80-90 degrees loops indicate imperfectly aligned particles. Comparing coercive force H-c(psi) with theoretical variations for different angular dispersions of axes and M-rs/M-s data with a theoretical cos psi variation, we find a variance of 6-8.5 degrees in particle alignment. The H-c(psi) results most closely resemble the theoretical variation for fanning rotations when psi <= 50 degrees and coherent rotations when psi > 50 degrees. A predicted hump in the H-c(psi) curve at intermediate angles marking the changeover from one mechanism to the other is suppressed by particle interactions (packing factors p of 0.127-0.373). Remanent coercive force measurements H-cr(y) show that irreversible changes, unlike reversible rotations, are incoherent at both large and small psi. H-cr continues to rise, to a high of 340-430 mT, as psi --> 90 degrees, whereas H-c(psi) decreases over the same range. H-cr(psi) results at large psi favor fanning reversals for one sample and curling reversals for the other two. A Day plot of M-rs/M-s versus H-cr/H-c data gives a novel and distinctive trend from which psi can determined within +/-5 degrees for aligned uniaxial particles.