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 10o intervals of the angle phi between the applied field and the long axes of aligned single-domain iron particles electrodeposited in regularly spaced surface pores in an Al substrate. Our three samples have identical particle diameters of 17 nm, but varying axial ratios and interparticle spacings. Measured hysteresis loops resemble "hysterons" predicted by fanning, curling and coherent rotation models of magnetization reversal, except for two features. For phi less than 60o, particle magnetizations do not reverse simultaneously at a single critical field: distributed particle coercivities produce steep but non-vertical loop segments. For phi = 80 and 90o, broadened loops indicate imperfectly aligned particles. Comparing coercive force Hc(phi) with theoretical variations for different angular dispersions of axes and Mrs/Ms data with a theoretical cos phi variation, we find a variance of 6-8.5o in particle alignment. The Hc(phi) results most closely resemble the theoretical variation for fanning rotations below 50o and coherent rotations above 50o. A predicted hump in the Hc(phi) curve at intermediate angles marking the changeover from one mechanism to the other is suppressed by particle interactions (packing factors of 0.127- 0.373). Remanent coercive force measurements Hcr(phi) show that irreversible changes, unlike reversible rotations, are incoherent at both large and small phi. Hcr continues to rise, to a high of 340-430 mT, as phi approaches 90o whereas Hc(phi) decreases over the same range. Hcr(phi) results at large phi favour fanning reversals for one sample and curling reversals for the other two. A Day plot of Mrs/Ms vs. Hcr/Hc data gives a novel and distinctive trend from which phi can determined within about 5o for aligned uniaxial particles.