MAGNETIC-PROPERTIES OF ARRAYS OF ORIENTED IRON PARTICLES AS A FUNCTION OF PARTICLE-SIZE, SHAPE AND SPACING

Hysteresis and remanence properties are reported for elongated iron particles deposited electrochemically in cylindrical pores of diameter d and spacing d(c) oriented perpendicular to the plane of an aluminium oxide substrate. Particle size, spacing and axial ratio were in the ranges d = 17-32 nm, d(c) = 26.5-64.5 nm and l/d = 3-20 in three series of samples. For d = 17 nm and l/d greater-than-or-equal-to 10, hysteresis loops measured parallel to particle axes were almost rectangular and had parameters characteristic of single-domain (SD) particles reversing incoherently by fanning: M(rs)/M(s) = 0.75-0.95, H(cr) almost-equal-to H(c) almost-equal-to 2 kOe, H(sat) almost-equal-to 3 kOe. Perpendicular to particle axes, there was little hysteresis and H(sat), almost-equal-to 6 kOe, typical of coherent reversals. Larger and less elongated particles had reduced parallel coercive forces (H(c) = 1.4-1.7 kOe) and remanence ratios (M(rs)/M(s) = 0.3-0.4). Anhysteretic remanent magnetization produced in a weak field (10 Oe) had an inflected alternating field (a.f.) demagnetization curve characteristic of SD behaviour. The a.f. decay curve for 0.9 kOe isothermal remanent magnetization (IRM) was more exponential in form. The effect of interactions was tested by varying particle spacing d(c). The distribution of critical fields for IRM acquisition was insensitive to both particle interaction and orientation of the field parallel or perpendicular to particle axes. However, each sample had a critical field distribution for a.f. demagnetization that was different from that for acquisition of IRM. For the most strongly interacting sample (d = 17 nm, d(c) = 26.5 nm), median magnetizing fields for 0.9 kOe IRM were H(parallel-to) almost-equal-to H(perpendicular-to) almost-equal-to 450 Oe, whereas median a.f. destructive fields were H(perpendicular-to) almost-equal-to 200 Oe, H(parallel-to) almost-equal-to 400 Oe. Strong negative interactions are indicated by this observed deviation from the Wohlfarth relations.