Ordering intermetallic alloys by ion irradiation: a way to tailor magnetic media.

We show how, combining He ion irradiation and thermal mobility below 600 K, the transformation from chemical disorder to order in thin films of an intermetallic ferromagnet (FePd) may be triggered and controlled. Kinetic Monte Carlo simulations show that the initial directional short range order determines the transformation. Magnetic ordering perpendicular to the film plane was achieved, promoting the initially weak magnetic anisotropy to the highest values known for FePd films. Applications to ultrahigh density magnetic recording are suggested.

Analysis of the high-spin states of the 2[4Fe-4Se]+ ferredoxin from Clostridium pasteurianum by Mössbauer spectroscopy.

A Mössbauer study of the three spin states of the reduced selenium-substituted 2[4Fe-4Se]+ ferredoxin from Clostridium pasteurianum was carried out at T = 1.6 K, in perpendicular and parallel applied magnetic fields of up to 10 T. In low (0.1 T) applied fields, the spectra of the classical S = 1/2 state exhibit magnetic hyperfine patterns arising from two pairs of iron atoms with opposite hyperfine fields. The S = 7/2 state is in the slow relaxation limit at T less than or equal to 4 K in the absence of applied field. The corresponding Mössbauer spectrum can be understood in terms of anti-parallel coupling between one high-spin Fe3+ ion (Hhf = +21.2 T) and three high-spin Fe2+ ions (Hhf,xy = -25.5 T, Hhf,z = -28.5 T). For the S = 3/2 spin state, the use of high (8-10 T) applied fields was necessary to ensure the validity of the high-field approximation and overcome the intercluster spin-spin interaction. The spectral data obtained under such conditions allowed the determination of the hyperfine field (-4.2 T) and a tentative estimation of the zero-field splitting (D approximately less than 3 cm-1).

High-multiplicity spin states of 2[4Fe-4Se]+ clostridial ferredoxins.

The electron paramagnetic resonance (EPR) spectra of the reduced selenium-substituted 2-[4Fe-4Se]+ ferredoxins from three bacteria of the Clostridium genus display low-field signals at g = 5.17, g = 10.11, and g = 12.76. The positions, shapes, and temperature dependencies of these signals have allowed their assignments to the three excited states of an S = 7/2 spin multiplet, the fundamental state of which is observed as unusual features in low-temperature (T less than or equal to 20 K) Mössbauer spectra. The S = 7/2 spin state is present in 2[4Fe-4Se]+ clostridial ferredoxins together with the classical S = 1/2 state and with a S = 3/2 state, the fundamental doublet of which is observed as a broad signal in the g = 3-4 region. The relative intensities of the EPR signals corresponding to these spin states depend on the species of Clostridium that the ferredoxin is extracted from. In contrast with clostridial ferredoxins, the reduced selenium-substituted ferredoxin from Bacillus stearothermophilus, which differs significantly from the clostridial proteins by its primary structure and by its containing only one tetranuclear cluster, displays only the S = 1/2 state. Thus, the high-multiplicity spin states arise from a specific interaction between the clostridial ferredoxin polypeptide chain and the reduced [4Fe-4Se]+ clusters.

Unusual features in EPR and Mössbauer spectra of the 2[4Fe-4Se]+ ferredoxin from Clostridium pasteurianum.

The electronic and magnetic properties of the selenium-substituted 2[4Fe-4Se]2+/+ ferredoxin (Fd) from Clostridium pasteurianum have been investigated by EPR and Mössbauer spectroscopy. The [4Fe-4Se]2+ clusters of oxidized Fd are diamagnetic and the Mössbauer spectra are nearly identical to those of oxidized 2[4Fe-4S]2+ Fd. The addition of 2e- per molecule of Se-substituted Fd causes the simultaneous appearance of three EPR signals: one (g1,2,3 = 2.103, 1.940, 1.888) is reminiscent of [4Fe-4S]+ EPR spectra and accounts for 0.7 to 0.8 spin/molecule. The two others consist of a broad signal with g = 4.5, 3.5, and approximately 2 (0.7 to 0.8 spin/molecule) and of a narrow peak at g = 5.172 which is observed up to 60 K. Peculiar features are also present in the Mössbauer spectra of 2[4Fe-4Se]+ Fd below 20 K: a subcomponent with lines near to +/- 4 mm/s and accounting for 20% of the total iron corresponds to two antiferromagnetically coupled sites in approximately a 3:1 ratio and displays fully developed paramagnetic hyperfine interactions at 4.2 K without any applied field. At 77 K, however, the reduced Se-substituted Fd yields a Mössbauer spectrum similar to that of 2[4Fe-4S]+ Fd. The new EPR and Mössbauer spectroscopic features of the 2[4Fe-4Se]+ Fd are attributed to S = 3/2 and S = 7/2 spin states which accompany the classical S = 1/2 state of [4Fe-4X]+ (X = S, Se) structures.