The angular distribution of the magnetization in magnetic recording tapes

Unique Magnetic Properties of Single Crystal γ-Fe2O3 Nanowires Synthesized by Flame Vapor Deposition

“…The individual FVD γ-Fe 2 O 3 NWs should be nearly perfect single-domain structures at the remanent state because their diameters are much smaller than the critical size below which domain wall formation becomes energetically unfavorable (166 nm for spherical particles of γ-Fe 2 O 3 , and much larger for particles with considerable shape anisotropy). 18 In fact, based on the reduced diameter S = 2.67 and the large aspect ratio (S = d/d 0 , where d = 40 nm is the average NW diameter determined by XRD, and d 0 = 15 nm is twice the exchange length in γ-Fe 2 O 3 ), 32 the isolated NWs are expected to have an "in plane flower" remanent state, which is a highly uniform single domain state of magnetization in which almost all of the magnetic moments point along the NW axis and M r /M s ≈ 1. 33,34 On the other hand, because of geometry, a collection of noninteracting single domain particles with spherically random distribution should have a low temperature M r /M s ratio of 0.5 (or slightly larger, due to intrinsic cubic magnetocrystalline anisotropy 35 ).…”

“…The coherent magnetization reversal coercivity of single, isolated γ-Fe 2 O 3 NWs should be 2320 Oe (2260 Oe due to demagnetization shape anisotropy energy caused by the large aspect ratio, and 60 Oe due to the uniaxial component of intrinsic magnetocrystalline energy). 32,39,40 If the magnetization is reversed by the coherent rotation of moments, the lowtemperature coercivity of this collection of γ-Fe 2 O 3 NWs with spherically random orientations would be 1110 Oe. 36 However, since the NW diameters (40À60 nm) are considerably larger than d 0 = 15 nm, which is twice the exchange length in γ-Fe 2 O 3 , 32 their magnetization reversal will not occur by coherent rotation of the magnetic moments.…”

“…Most likely, the reversal occurs by curling, which is the noncoherent rotation of moments away from the NW axis direction in the plane perpendicular to the radius at each point, in an amount that is a function of radius only 41 because curling has the lowest energy barrier among all reversal modes for NWs in this size range. 32 To verify whether the reversal does in fact occur by curling, we compared the coercivity predicted by the micromagnetic curling model to our measured coercivity of 380 Oe at 30 K. For these γ-Fe 2 O 3 NWs with reduced diameter S = 2.67, the curling model predicts a coercivity that is 0.169 times that of coherent rotation (2320 Oe), or approximately 390 Oe, for a collection of noninteracting NWs with spherically random orientation at low temperature. 42 This coercivity is only slightly larger than the measured value of 380 Oe at 30 K and strongly suggests that the reversal of these NWs occurs by curling.…”

Principles of Magnetic Recording

“…Measurements performed on a single particle [21] seem to confirm that these non-uniform rotation mechanisms playa dominant role in magnetization reversal.…”

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

“…Chains of magnetosomes produced by magnetotactic bacteria come close to this ideal (see hysteresis curve MV1H in the work by Dunlop and Carter‐Stiglitz [2006], for example) but as part of a living organism, the linearity of the chain can change [ Kobayashi et al , 2006], greatly reducing the coercivity [ Hendriksen et al , 1994]. Magnetic recording particles also have relatively narrow coercivity spectra but the alignment of their particles is typically not as good as that of our iron particles [ Luborsky and Paine , 1960; Knowles et al , 1980].…”