Artificial ferritin has been synthesized with control of both the magnetic state (antiferromagnetic or ferrimagnetic) and the particle size over an ofer of magnitude in the number of iron atoms. The magnetic properties of the artificial ferritin were compared with those of natural horse spleen ferritin in a range of temperatures (20 millikelvin to 300 kelvin) and fields (1 nanotesla to 27 tesla). In the classical regime, the blocking temperature was found to correlate with the average particle size. A correlation was also observed in the quantum regime between the resonance frequency of macroscopic quantum tunneling of the Néel vector and the particle size. At high magnetic fields (to 27 tesla), a spin flop transition with a strong dependence on orientation was seen in the natural ferritin, providing evidence of antiferromagnetism in this system.

No abstract

The magnetic susceptibility of the S = 2 quasi-linear chain Heisenberg antiferromagnet (2,2 ′ -bipyridine)trichloromanganese(III), M nCl 3 (bipy), has been measured from 1.8 to 300 K with the magnetic field, H, parallel ( ) and perpendicular (⊥) to the chains. The analyzed data yield g ≈ 2 and J ≈ 35 K. The magnetization, M , has been studied at 30 mK and 1.4 K in H up to 16 T. No evidence of long-range order is observed. Depending on crystal orientation, M ≈ 0 at 30 mK until a critical field is achieved (H c = 1.2 ± 0.2 T 1 and H c⊥ = 1.8 ± 0.2 T), where M increases continuously as H is increased. These results are interpreted as evidence of a Haldane gap. 75.50.Ee, 75.10.Jm Typeset using REVT E X

No abstract